Single crystals of a new reduced neodymium titanate, Nd3Ti4O12 , have been obtained by slow cooling of a reduced Nd–Ti–O ceramic precursor in a molten neodymium borate flux under high vacuum. Single crystal X-ray diffraction (space group Pcmn, a = 5.420(1) Å, b = 7.610(1) Å, c = 22.015(5) Å, Z = 4, Pearson symbol oP76) showed that the structure of Nd3Ti4O12 is built from an arrangement of corner-sharing and edge-sharing TiO6-octahedra that can be considered as a regular intergrowth of the GdFeO3 and CaTa2O6 structure types. The material is nonmetallic (R(25°C) = 35 Ωcm) which may arise due to the localization of the Ti 3d-electrons in the edge-sharing pairs of TiO6-octahedra (short Ti–Ti distance of 2.760(3) Å)

Dover, R.B. van

Hessen, B.

Siegrist, T.

Sunshine, S.A.

English

Single crystals of a new reduced neodymium titanate, Nd3Ti4O12 , have been obtained by slow cooling of a reduced Nd–Ti–O ceramic precursor in a molten neodymium borate flux under high vacuum. Single crystal X-ray diffraction (space group Pcmn, a = 5.420(1) Å, b = 7.610(1) Å, c = 22.015(5) Å, Z = 4, Pearson symbol oP76) showed that the structure of Nd3Ti4O12 is built from an arrangement of corner-sharing and edge-sharing TiO6-octahedra that can be considered as a regular intergrowth of the GdFeO3 and CaTa2O6 structure types. The material is nonmetallic (R(25°C) = 35 Ωcm) which may arise due to the localization of the Ti 3d-electrons in the edge-sharing pairs of TiO6-octahedra (short Ti–Ti distance of 2.760(3) Å).

Hessen, B.,

Sunshine, S.A.,

Siegrist, T.,

Dover, R.B. van,

University of Groningen Digital Archive

A Reduced Neodymium Titanate with a New Intergrowth Structure Type

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   University of GroningenA Reduced Neodymium Titanate with a New Intergrowth Structure TypeHessen, B.; Sunshine, S.A.; Siegrist, T.; Dover, R.B. vanPublished in:Journal of Solid State ChemistryDOI:10.1006/jssc.1993.1198IMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite fromit. Please check the document version below.Document VersionPublisher's PDF, also known as Version of recordPublication date:1993Link to publication in University of Groningen/UMCG research databaseCitation for published version (APA):Hessen, B., Sunshine, S. A., Siegrist, T., & Dover, R. B. V. (1993). A Reduced Neodymium Titanate with aNew Intergrowth Structure Type. Journal of Solid State Chemistry, 105(1).https://doi.org/10.1006/jssc.1993.1198CopyrightOther than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of theauthor(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons).The publication may also be distributed here under the terms of Article 25fa of the Dutch Copyright Act, indicated by the “Taverne” license.More information can be found on the University of Groningen website: https://www.rug.nl/library/open-access/self-archiving-pure/taverne-amendment.Take-down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediatelyand investigate your claim.Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons thenumber of authors shown on this cover page is limited to 10 maximum.Download date: 21-06-2022JOURNAL OF SOLID STATE CHEMISTRY 105, 107–111 (1993) A Reduced Neodymium Titanate with a New Intergrowth Structure Type B. HESSEN, S. A. SUNSHINE, T. SIEGRIST, AND R. B. VAN DOVER AT&T Bell Laboratories, 600 Mountain Avenue, Murray Hill, New Jersey 07974 Received November 23, 1992; in revised form March 22, 1993; accepted March 24, 1993 Single crystals of a new reduced neodymium titanate, Nd3Ti4O12 , have been obtained by slow cooling of a reduced Nd–Ti–O ceramic precursor in a molten neodymium borate flux under high vacuum. Single crystal X-ray diffraction (space group Pcmn, a = 5.420(1) Å, b = 7.610(1) Å, c = 22.015(5) Å, Z = 4, Pearson symbol oP76) showed that the structure of Nd3Ti4O12 is built from an arrangement of corner-sharing and edge-sharing TiO6-octahedra that can be considered as a regular intergrowth of the GdFeO3 and CaTa2O6 structure types. The material is nonmetallic (R(25°C) = 35 Ωcm) which may arise due to the localization of the Ti 3d-electrons in the edge-sharing pairs of TiO6-octahedra (short Ti–Ti distance of 2.760(3) Å).   © 1993 Academic Press. Inc. Introduction The Ti(III) rare earth ternary oxides LnTiO3 (Ln = La, Nd, Sm, Gd) are known to have a distorted orthorhombic perovskite structure of the GdFeO3-type (1). Apart from the defect perovskite series La2/3+x TiO3 (2), few other rare-earth containing re-duced ternary titanates are known. Recently we prepared a new reduced La–Ti–O com-pound, La1.33Ti8O16 with the hollandite structure, from a LaTiO3 ceramic and a lan-thanum borate flux (3). We also investigated the use of ceramic precursors with lower rare earth content. A significant difference in behavior between the La–Ti–O and Nd-Ti-O systems is observed in that case. Here we present the synthesis and crystal structure of a new reduced neodymium ti-tanate, Nd3Ti4O12 , with a structure derived from intergrowth of the GdFeO3 and CaTa2O6 structure types. Similar attempts to prepare a La–Ti–O analogue only pro-duced crystals of the La2/3+xTiO3 defect per-ovskite. Ordered intergrowth of two different structure types has been observed in a  num- ber of systems, frequently giving rise to ho-mologous series with varying sizes of the blocks corresponding to the two structure types. Well known examples are the Ruddlesden–Popper series, with SrTiO3 and SrO layer intergrowth (4), and the in-tergrowth of BaZnF4 and GdFeO3 struc-ture types in the Ca–Na–Nb–O and Ca-La-Ti-O systems (5). It is conceivable that intergrowth of the GdFeO3 and CaTa2O6 structure types can also give rise to a homologous series. Experimental Sample Preparation Ceramic titanate precursors with nominal composition “Ln0.33Ti2O4” were prepared from pressed pellets of Ln2O3 , Ti2O3 (Alfa), and TiO2 (Aldrich) in the appropriate stoi-chiometry (9 mmole Ti/pellet). As mineral-izer Ln2O3 · 3B2O3 was added in the molar ratio of Nd : Ti : B2O3 = 1.3 : 1.0 : 1.7. The pellets were wrapped in Mo-foil and heated in vacuo to 1200°C for 24 hr. X-ray powder diffraction   showed   that    they    consisted 107 0022-4596/93 $5.00 Copyright © 1993 by Academic Press, Inc. All rights of reproduction in any form reserved. 108 HESSEN ET AL. mainly of Ti2O3 and Ln2/3+xTiO3-type perov-skites. Borates Ln2O3 · 3B2O3 were prepared from La(OH)3 (Alfa) or Nd2O3 (Aldrich) with B2O3 (Fisher) in the appropriate ratio. The mixtures were heated at 950°C in alu-mina crucibles for 24 hr. Lanthanide sesqui-oxides were fired at 1000°C in air for 12 hr before use. Mixtures of ceramic precursors of nomi-nal composition “Ln1/3Ti2O4” (Ln = La, Nd) and Ln2O3 · 3B2O3 in 1 : 3 weight ratio were placed in Mo-foil cups with Mo-foil lids. They were heated in vacuo (in a Cent-orr high-vacuum furnace) to 400°C to re-move adsorbed water, and then to 1250°C (1 hr, 10–6–10–7 Torr). The mixtures were then cooled to 1140°C at 8°C/hr. The heater was subsequently turned off, allowing the sample to reach room temperature in ap-proximately 1.5 hr. The mixtures were soaked in warm concentrated aqueous KOH for 12 hr, after which the products were mechanically broken up. For Ln = Nd the product mixture consisted of dark blue crys-talline aggregates showing in places well-developed rectangular crystal faces, to-gether with small crystals of Ti2O3 and a pink transparent crystalline flux. The blue product was determined by single crystal X-ray diffraction on several cleaved crystal fragments to be a new orthorhombic Nd–titanate, Nd3Ti4O12 . For Ln = La, a dark crystalline mass with jagged edges but with locally well-developed crystals with square faces had formed, together with small crys-tals of Ti2O3 and a colorless crystalline flux. X-ray diffraction on crystals and cleaved crystal fragments showed the dark crystal-line product to be an ordered defect perov-skite of the type La2/3+xTiO3 . A dc four-point resistivity measurement was performed on a single crystal of Nd3Ti4O12 . Silver wires were connected (with Ag/epoxy) to evaporated Au–Ge/Au strip contacts on the crystal. Structure Determinations The crystals were measured at 23°C on an Enraf-Nonius  CAD-4  diffractometer  using graphite-monochromatized MoKα radiation and the NRCCAD program package (6). All calculations were carried out on an Alliant FX/80 computer, using the NRCVAX struc-ture package (7). Lattice parameters were determined from the absolute 2θ values of reflections at a high angle (Nd3Ti4O12: 30 reflections with 49° < 2θ < 58°, La1/3+xTiO3: 28 reflections with 50° < 2θ < 56°). The metal atom positions for Nd3Ti4O12 were ob-tained by direct methods. A Gaussian ab-sorption correction was applied. Aniso-tropic  temperature factors for all atoms were  included in the refinement. Crystallo-graphic data are listed in Table I, atomic coordinates and isotropic thermal parame-ters in Table II.* Crystals of La2/3+xTiO3 appeared to be primitive cubic with a = 7.7837(3) Å. It is likely that the material is actually a tetrago-nal defect perovskite with c = 2a as de-scribed earlier for La2/3TiO2.954 (8). This also corresponds well with powder X-ray diffrac-tion on crushed crystals. The doubling in all three dimensions as observed by single crystal X-ray diffraction is likely to be pro-duced by twinning. From the observed cell parameter and comparison with the data for La2/3+xTiO3 of Kestigan and Ward (2), the composition of the crystals can be derived by interpolation to be approximately La0.73TiO3(x ≈ 0.06). Description of the Structure The structure of Nd3Ti4O12 is shown in Fig. 1,  projected along the  b-axis.  Pertinent * See NAPS document No. 05020 for 23 pages of supplementary material. Order from ASIS/NAPS. Mi-crofiche Publications, P.O. Box 3513, Grand Central Station, New York, NY 10163. Remit in advance $4.00 for microfiche copy or for photocopy, $8.65 up to 20 pages plus $.30 for each additional page. All orders must be prepaid. Institutions and Organizations may order by purchase order. However, there is a billing and handling charge for this service of $15. Foreign orders add $4.50 for postage and handling, for the first 20 pages, and $1.00 for additional 10 pages of material, $1.75 for postage of any microfiche orders.   INTERGROWTH IN NEODYMIUM TITANATE 109   Formula  Nd3Ti4O12 Space group  Pcmn  Crystal size (mm)  0.12  ×  0.18  ×  0.12  a (Å) (at 23°C)  5.4203(8)  b (Å)  7.6104(9)  c (Å)  22.105(5)  V (Å)  908.1(3)  Z  4  Dc (g/cm3)  5.971  µ (MoKα, cm-1)  202.8  Minimum transmission  0.1079  Maximum transmission  0.2513  Scan type  ω/2θ  Reflections measured  2168  Independent reflections  2099  Observed reflections  1670  (I > 2.5σ (I))   Refined parameters  98  R  0.040  Rw (w = 1/σ 2 (Fo))  0.056  Extinction length (µm)  0.29(1) interatomic distances are listed in Table III. The framework of the structure is formed by  TiO6 octahedra, arranged in puckered single and double ReO3-type chains running in the direction of the b-axis. The Ti(1)O6-octahedra make up the single chains, and share only corners with the neighboring oc-tahedra. The Ti(2)O6-octahedra form the double chains  as  edge-sharing  pairs, which TABLE II ATOMIC POSITIONAL PARAMETERS AND ISOTROPIC TEMPERATURE FACTORS FOR Nd3Ti4O12   x   y   z  Biso (Å2) Nd(1)  0.93871(15)  0.25  0.19959(3)  0.464(20) Nd(2)  0.45551(14)  0.75  0.02782(3)  0.471(19) Nd(3)  0.92754(14)  0.75  0.17866(3)  0.394(18) Ti(1)  0.4390(3)  0.49921(20)  0.17863(7)  0.38(4)  Ti(2)  0.9516(3)  0.48936(21)  0.06143(7)  0.43(4)  O(1)  0.2465(14)  0.4624(9)  0.0122(3)  0.53(19)  O(2)  0.6351(14)  0.5593(10)  0.1147(3)  0.69(21)  O(3)  0.7090(13)  0.5367(9)  0.2378(3)  0.48(19)  O(4)  0.1223(13)  0.4708(9)  0.1421(3)  0.43(19)  O(5)  0.4976(22)  0.25  0.1869(5)  0.9(4)  O(6)  0.3578(22)  0.75  0.1954(5)  0.7(3)  O(7)  0.8410(23)  0.25  0.0642(5)  0.8(3)  O(8)  0.0321(21)  0.75  0.0699(4)  0.5(3)   FIG. 1. Polyhedral representation of the structure of Nd3Ti4O12 , projected along the b-axis. Ti–O polyhedra are hatched; Nd-atoms are represented as circles. share corners in the ac-plane with the Ti(1)-octahedra. The structure can be described as a regular intergrowth of two known struc-ture types: the GdFeO3-structure, which is an orthorhombic distortion of the perovskite structure (MO6-octahedra that are corner-sharing in three dimensions) (9) and the CaTa2O6-structure, with M2O10-groups of edge-sharing octahedra linked together by corner sharing (10). All three independent Nd-atoms are located in eight-coordinate distorted bicapped-trigonal prismatic sites, as is also observed for the A-cation in the GdFeO3 and CaTa2O6 structure types. The Nd(2)-atom, located between the puckered double  ReO3-type  chains,   has   a   slightly TABLE III SELECTED INTERATOMIC DISTANCES (Å) FOR Nd3Ti4O12 WITH ESTIMATED STANDARD DEVIATIONS IN PARENTHESES  Ti(1)  –O(2)  1.822(7)   Ti(2)  –O(1)  1.942(7)    –O(3)  1.980(7)     1.979(7)     2.239(7)    –O(2)  2.145(7)    –O(4)  1.908(7)    –O(4)  2.007(7)    –O(5)  1.932(3)    –O(7)  1.919(4)    –O(6)  1.993(4)    –O(8)  2.040(3)   Nd(1)  –O(3)  2.585 (7)  2×  Nd(2)  –O(1)  2.449(7)  2×    2.649(7)  2×    2.489(7)  2×   –O(4)  2.328(7)  2×   –O(2)  2.591(7)  2×   –O(5)  2.41(1)    –O(7)  2.59(1)    –O(6)  2.35(1)    –O(8)  2.47(1)   Nd(3)  –O(2)  2.570(8)  2×  Ti(2)  –Ti(2)  2.760(3)    –O(3)  2.395(7)  2×       –O(4)  2.506(7)  2×       –O(6)  2.36(1)        –O(8)  2.461(9)       TABLE I CRYSTALLOGRAPHIC DATA FOR Nd3Ti4O12  110 HESSEN ET AL.  FIG. 2. ORTEP representation of the Ti2O10 group in Nd3Ti4O12 . Thermal ellipsoids represented at the 50% probability level. larger and more regular coordination sphere than the other Nd-atoms (average Nd(2)–O  =  2.515 Å, with maximum deviations of –0.066 and +0.076 Å, for Nd(1) 2.486 Å and  +0.163/ –0.158 Å, for Nd(3) 2.470 Å and  +0.100/ –0.110 Å respectively. The average Ti–O distances in Nd3Ti4O12 are 1.979 Å for Ti(1) and 2.005 Å for Ti(2) respectively. The average Ti–O distance for Ti(1) in the GdFeO3 type block is signifi-cantly smaller than that observed in NdTiO3 (2.022 Å (1)). It is close to the value for Ti4 + –O in rutiles (1.97 Å). The coordination geometry around Ti(1) is also considerably more distorted, with sharp O–Ti–O angles ranging from 80.3° to 94.4° (in NdTiO3 from 89.0° to 90.3° (1)). One notable feature in the structure is the distortion in the Ti2O10-groups. The Ti(2)-atoms are displaced from the center of the octahedra towards each other rather than away from each other (as might be expected from Coulomb repul-sion). The geometry of this group is shown in Fig. 2. The Ti(2)–Ti(2) distance of 2.760(3) Å is quite short for a Ti–Ti distance in a pair of edge-shared octahedra. An interesting comparison can be made with the recently published crystal structure of the Ti(4 + ) compound Nd4Ti9O24 in which edge-shared Ti2O10-pairs are also present (11). The average Ti–O distance in these groups are smaller than for Ti(2) in Nd3Ti4O12 , and typical for Ti(4 + ) (1.973/ 1.978  Å).  However,  the  Ti–Ti  distance  in the pairs (2.963 Å) is 0.20 Å longer than in Nd3Ti4O12. The stoichiometry of Nd3Ti4O12 obtained from X-ray diffraction indicates an average Ti oxidation state of 3.75 + ; i.e., there is one 3d-electron per Ti2O10-pair. These 3d-electrons are possibly localized in orbitals associated with the Ti(2)–Ti(2) pair-ing. This would imply nonmetallic behavior of the material, at least at or below the tem-perature at which the structure was deter-mined (23°C). At high temperatures a first-order transition to a metallic state may be anticipated. A low-temperature four-point resistivity measurement on a single crystal showed nonmetallic behavior (Fig. 3) with a room-temperature resistivity of 35 Ω cm. Due to the small size of the crystal, only the conductivity along the b-axis could be measured. Anisotropy in the conductivity was expected due to the low symmetry of the crystal structure, but unfortunately could  not be measured. Degradation of the very small contacts made data collection un-reliable above room temperature. The ln(ρ) vs 1/T plot is nonlinear, suggesting that the material does not behave as an intrinsic semiconductor. The high-temperature data give an activation energy of 0.26 eV, but the fit is valid only over a rather small tempera-ture range. A second fit, based on a hopping mechanism for the conduction with a ρ ∝ ρ0  FIG. 3. Plot of log(resistivity) vs 1000/T from a dc resistivity measurement on a Nd3Ti4O12 single crystal.   INTERGROWTH IN NEODYMIUM TITANATE 111 exp(T0/T)1/4, gives a critical temperature of T0 = 1.24 × 106 K. This fit extends over a larger temperature range, but at low temper-ature, the deviations are very pronounced. Further experiments would be needed to elucidate the mechanism of conductivity. Discussion It is observed that recrystallization of ceramics with nominal composition Ln0.33Ti2O4 from Ln2O3 · 3B2O3 fluxes (Ln = La, Nd) leads to two crystalline prod-ucts: Ti2O3 and a ternary compound that is  only partially reduced. The difference in ionic radius between La and Nd is relatively small (1.32 Å for La3 + , 1.26 Å for Nd3 + in eight-coordination (12)). However, this difference, under otherwise identical condi-tions, causes a drastic change in the struc-ture of the ternary product, being an ordered defect perovskite for the La–Ti–O system and Nd3Ti4O12 with the intergrowth struc-ture as described above for the Nd–Ti–O system. This suggests a delicate dependence of the thermodynamics of these ternaries on the size of the A-cation. Also notable is the apparent absence of the La1.33Ti8O16 hol-landite phase that was obtained earlier from a more La-rich mixture (3). This can corre-spond with our findings in the Sr–Nb–O sys-tem that the occurrence of disproportiona-tion reactions in ternary borate flux media is  strongly dependent on the concentration of the common ion between the flux and the ternary transition metal oxide system (13). Although currently Nd3Ti4O12 is the only member of the ordered GdFeO3/CaTa2O6 in-tergrowth type synthesized, it is possible to predict the existence of a homologous series based on this intergrowth principle. The general formula of such a series would be Nd1+nTi2+nO6+3n , of which Nd3Ti4O12 is the n = 2 member. It is based on GdFeO3-blocks expanding in the c-direction between the edge-sharing  pairs  of  octahedra  of  the CaTa2O6 structure. The end members in the Nd–Ti–O system would be defined by n = 1 (Nd2Ti3O9 , Ti4+) and n = ∞ (NdTiO3 , Ti3+). The n = even and n = odd members differ in  the orientation of the edge-sharing pairs relative to each other. In the n = even mem-bers, sequential Ti2O10-blocks along the c-direction will be shifted over a/2, and canted in the opposite direction within the ac-plane (as in Nd3Ti4O12). In the n = odd members these blocks would not be shifted, and would have the same canting direction. Presently we are exploring the possibility of accessing members of this series by ceramic routes. Initial efforts were directed to pre-pare related Ti4 + ceramics, e.g., with com-position CaNd2Ti4O12 or Nd3Ti3AlO12 . As yet, these attempts have met with little suc-cess, yielding tetragonal defect perovskite type species as main products. References 1. D. A. MACLEAN, H.-N. NG, AND J. E. GREEDAN, J. Solid State Chem. 30, 35 (1979). 2. M. KESTIGAN AND R. WARD, J. Am. Chem. Soc. 77, 6199 (1955). 3. B. HESSEN, S. A. SUNSHINE, AND T. SIEGRIST, J. Solid State Chem. 94, 306 (1991). 4. S. N. RUDDLESDEN AND P. POPPER, Acta Crys-tallogr. 10, 538 (1957); Acta Crystallogr. 11, 54 (1958). 5. M. NANOT, F. QUEYROUX, J. C. GILLES, A. CARPY, AND J. GALY, J. Solid State Chem. 11, 272 (1974). 6. Y. LEPAGE, P. S. WHITE, AND E. J. GABE, “Pro-ceedings of the American Crystallographic Associ-ation Annual Meeting, 1986, Hamilton, Canada,” AIP, New York, (1986). [Poster PA23] 7. E. J. GABE, Y. LEPAGE, J. P. CHARLAND, F. L. LEE, AND P. S. WHITE, J. Appl. Crystallogr. 22, 384 (1989). 8. M. ABE AND K. UCHINO, Mater. Res. Bull. 9, 147 (1974). 9. S. GELLER, J. Chem. Phys. 24, 1236 (1956). 10. L. JAHNBERG, Acta Chem. Scand. 71, 2548 (1963). 11. N. HÜBNER AND R. GRUEHN, Z. Anorg. Allg. Chem. 616, 86 (1992). 12. R. D. SHANNON AND C. T. PREWITT, Acta Crys-tallogr. Sect. B 26, 1046 (1970). 13. B. HESSEN, S. A. SUNSHINE, T. SIEGRIST, AND R. JIMENEZ, Mater. Res. Bull. 26, 85 (1991). 

Proceedings - University of Groningen

   University of GroningenA Reduced Neodymium Titanate with a New Intergrowth Structure TypeHessen, B.; Sunshine, S.A.; Siegrist, T.; Dover, R.B. vanPublished in:Journal of Solid State ChemistryDOI:10.1006/jssc.1993.1198IMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite fromit. Please check the document version below.Document VersionPublisher's PDF, also known as Version of recordPublication date:1993Link to publication in University of Groningen/UMCG research databaseCitation for published version (APA):Hessen, B., Sunshine, S. A., Siegrist, T., & Dover, R. B. V. (1993). A Reduced Neodymium Titanate with aNew Intergrowth Structure Type. Journal of Solid State Chemistry, 105(1).https://doi.org/10.1006/jssc.1993.1198CopyrightOther than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of theauthor(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons).The publication may also be distributed here under the terms of Article 25fa of the Dutch Copyright Act, indicated by the “Taverne” license.More information can be found on the University of Groningen website: https://www.rug.nl/library/open-access/self-archiving-pure/taverne-amendment.Take-down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediatelyand investigate your claim.Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons thenumber of authors shown on this cover page is limited to 10 maximum.Download date: 03-06-2022JOURNAL OF SOLID STATE CHEMISTRY 105, 107–111 (1993) A Reduced Neodymium Titanate with a New Intergrowth Structure Type B. HESSEN, S. A. SUNSHINE, T. SIEGRIST, AND R. B. VAN DOVER AT&T Bell Laboratories, 600 Mountain Avenue, Murray Hill, New Jersey 07974 Received November 23, 1992; in revised form March 22, 1993; accepted March 24, 1993 Single crystals of a new reduced neodymium titanate, Nd3Ti4O12 , have been obtained by slow cooling of a reduced Nd–Ti–O ceramic precursor in a molten neodymium borate flux under high vacuum. Single crystal X-ray diffraction (space group Pcmn, a = 5.420(1) Å, b = 7.610(1) Å, c = 22.015(5) Å, Z = 4, Pearson symbol oP76) showed that the structure of Nd3Ti4O12 is built from an arrangement of corner-sharing and edge-sharing TiO6-octahedra that can be considered as a regular intergrowth of the GdFeO3 and CaTa2O6 structure types. The material is nonmetallic (R(25°C) = 35 Ωcm) which may arise due to the localization of the Ti 3d-electrons in the edge-sharing pairs of TiO6-octahedra (short Ti–Ti distance of 2.760(3) Å).   © 1993 Academic Press. Inc. Introduction The Ti(III) rare earth ternary oxides LnTiO3 (Ln = La, Nd, Sm, Gd) are known to have a distorted orthorhombic perovskite structure of the GdFeO3-type (1). Apart from the defect perovskite series La2/3+x TiO3 (2), few other rare-earth containing re-duced ternary titanates are known. Recently we prepared a new reduced La–Ti–O com-pound, La1.33Ti8O16 with the hollandite structure, from a LaTiO3 ceramic and a lan-thanum borate flux (3). We also investigated the use of ceramic precursors with lower rare earth content. A significant difference in behavior between the La–Ti–O and Nd-Ti-O systems is observed in that case. Here we present the synthesis and crystal structure of a new reduced neodymium ti-tanate, Nd3Ti4O12 , with a structure derived from intergrowth of the GdFeO3 and CaTa2O6 structure types. Similar attempts to prepare a La–Ti–O analogue only pro-duced crystals of the La2/3+xTiO3 defect per-ovskite. Ordered intergrowth of two different structure types has been observed in a  num- ber of systems, frequently giving rise to ho-mologous series with varying sizes of the blocks corresponding to the two structure types. Well known examples are the Ruddlesden–Popper series, with SrTiO3 and SrO layer intergrowth (4), and the in-tergrowth of BaZnF4 and GdFeO3 struc-ture types in the Ca–Na–Nb–O and Ca-La-Ti-O systems (5). It is conceivable that intergrowth of the GdFeO3 and CaTa2O6 structure types can also give rise to a homologous series. Experimental Sample Preparation Ceramic titanate precursors with nominal composition “Ln0.33Ti2O4” were prepared from pressed pellets of Ln2O3 , Ti2O3 (Alfa), and TiO2 (Aldrich) in the appropriate stoi-chiometry (9 mmole Ti/pellet). As mineral-izer Ln2O3 · 3B2O3 was added in the molar ratio of Nd : Ti : B2O3 = 1.3 : 1.0 : 1.7. The pellets were wrapped in Mo-foil and heated in vacuo to 1200°C for 24 hr. X-ray powder diffraction   showed   that    they    consisted 107 0022-4596/93 $5.00 Copyright © 1993 by Academic Press, Inc. All rights of reproduction in any form reserved. 108 HESSEN ET AL. mainly of Ti2O3 and Ln2/3+xTiO3-type perov-skites. Borates Ln2O3 · 3B2O3 were prepared from La(OH)3 (Alfa) or Nd2O3 (Aldrich) with B2O3 (Fisher) in the appropriate ratio. The mixtures were heated at 950°C in alu-mina crucibles for 24 hr. Lanthanide sesqui-oxides were fired at 1000°C in air for 12 hr before use. Mixtures of ceramic precursors of nomi-nal composition “Ln1/3Ti2O4” (Ln = La, Nd) and Ln2O3 · 3B2O3 in 1 : 3 weight ratio were placed in Mo-foil cups with Mo-foil lids. They were heated in vacuo (in a Cent-orr high-vacuum furnace) to 400°C to re-move adsorbed water, and then to 1250°C (1 hr, 10–6–10–7 Torr). The mixtures were then cooled to 1140°C at 8°C/hr. The heater was subsequently turned off, allowing the sample to reach room temperature in ap-proximately 1.5 hr. The mixtures were soaked in warm concentrated aqueous KOH for 12 hr, after which the products were mechanically broken up. For Ln = Nd the product mixture consisted of dark blue crys-talline aggregates showing in places well-developed rectangular crystal faces, to-gether with small crystals of Ti2O3 and a pink transparent crystalline flux. The blue product was determined by single crystal X-ray diffraction on several cleaved crystal fragments to be a new orthorhombic Nd–titanate, Nd3Ti4O12 . For Ln = La, a dark crystalline mass with jagged edges but with locally well-developed crystals with square faces had formed, together with small crys-tals of Ti2O3 and a colorless crystalline flux. X-ray diffraction on crystals and cleaved crystal fragments showed the dark crystal-line product to be an ordered defect perov-skite of the type La2/3+xTiO3 . A dc four-point resistivity measurement was performed on a single crystal of Nd3Ti4O12 . Silver wires were connected (with Ag/epoxy) to evaporated Au–Ge/Au strip contacts on the crystal. Structure Determinations The crystals were measured at 23°C on an Enraf-Nonius  CAD-4  diffractometer  using graphite-monochromatized MoKα radiation and the NRCCAD program package (6). All calculations were carried out on an Alliant FX/80 computer, using the NRCVAX struc-ture package (7). Lattice parameters were determined from the absolute 2θ values of reflections at a high angle (Nd3Ti4O12: 30 reflections with 49° < 2θ < 58°, La1/3+xTiO3: 28 reflections with 50° < 2θ < 56°). The metal atom positions for Nd3Ti4O12 were ob-tained by direct methods. A Gaussian ab-sorption correction was applied. Aniso-tropic  temperature factors for all atoms were  included in the refinement. Crystallo-graphic data are listed in Table I, atomic coordinates and isotropic thermal parame-ters in Table II.* Crystals of La2/3+xTiO3 appeared to be primitive cubic with a = 7.7837(3) Å. It is likely that the material is actually a tetrago-nal defect perovskite with c = 2a as de-scribed earlier for La2/3TiO2.954 (8). This also corresponds well with powder X-ray diffrac-tion on crushed crystals. The doubling in all three dimensions as observed by single crystal X-ray diffraction is likely to be pro-duced by twinning. From the observed cell parameter and comparison with the data for La2/3+xTiO3 of Kestigan and Ward (2), the composition of the crystals can be derived by interpolation to be approximately La0.73TiO3(x ≈ 0.06). Description of the Structure The structure of Nd3Ti4O12 is shown in Fig. 1,  projected along the  b-axis.  Pertinent * See NAPS document No. 05020 for 23 pages of supplementary material. Order from ASIS/NAPS. Mi-crofiche Publications, P.O. Box 3513, Grand Central Station, New York, NY 10163. Remit in advance $4.00 for microfiche copy or for photocopy, $8.65 up to 20 pages plus $.30 for each additional page. All orders must be prepaid. Institutions and Organizations may order by purchase order. However, there is a billing and handling charge for this service of $15. Foreign orders add $4.50 for postage and handling, for the first 20 pages, and $1.00 for additional 10 pages of material, $1.75 for postage of any microfiche orders.   INTERGROWTH IN NEODYMIUM TITANATE 109   Formula  Nd3Ti4O12 Space group  Pcmn  Crystal size (mm)  0.12  ×  0.18  ×  0.12  a (Å) (at 23°C)  5.4203(8)  b (Å)  7.6104(9)  c (Å)  22.105(5)  V (Å)  908.1(3)  Z  4  Dc (g/cm3)  5.971  µ (MoKα, cm-1)  202.8  Minimum transmission  0.1079  Maximum transmission  0.2513  Scan type  ω/2θ  Reflections measured  2168  Independent reflections  2099  Observed reflections  1670  (I > 2.5σ (I))   Refined parameters  98  R  0.040  Rw (w = 1/σ 2 (Fo))  0.056  Extinction length (µm)  0.29(1) interatomic distances are listed in Table III. The framework of the structure is formed by  TiO6 octahedra, arranged in puckered single and double ReO3-type chains running in the direction of the b-axis. The Ti(1)O6-octahedra make up the single chains, and share only corners with the neighboring oc-tahedra. The Ti(2)O6-octahedra form the double chains  as  edge-sharing  pairs, which TABLE II ATOMIC POSITIONAL PARAMETERS AND ISOTROPIC TEMPERATURE FACTORS FOR Nd3Ti4O12   x   y   z  Biso (Å2) Nd(1)  0.93871(15)  0.25  0.19959(3)  0.464(20) Nd(2)  0.45551(14)  0.75  0.02782(3)  0.471(19) Nd(3)  0.92754(14)  0.75  0.17866(3)  0.394(18) Ti(1)  0.4390(3)  0.49921(20)  0.17863(7)  0.38(4)  Ti(2)  0.9516(3)  0.48936(21)  0.06143(7)  0.43(4)  O(1)  0.2465(14)  0.4624(9)  0.0122(3)  0.53(19)  O(2)  0.6351(14)  0.5593(10)  0.1147(3)  0.69(21)  O(3)  0.7090(13)  0.5367(9)  0.2378(3)  0.48(19)  O(4)  0.1223(13)  0.4708(9)  0.1421(3)  0.43(19)  O(5)  0.4976(22)  0.25  0.1869(5)  0.9(4)  O(6)  0.3578(22)  0.75  0.1954(5)  0.7(3)  O(7)  0.8410(23)  0.25  0.0642(5)  0.8(3)  O(8)  0.0321(21)  0.75  0.0699(4)  0.5(3)   FIG. 1. Polyhedral representation of the structure of Nd3Ti4O12 , projected along the b-axis. Ti–O polyhedra are hatched; Nd-atoms are represented as circles. share corners in the ac-plane with the Ti(1)-octahedra. The structure can be described as a regular intergrowth of two known struc-ture types: the GdFeO3-structure, which is an orthorhombic distortion of the perovskite structure (MO6-octahedra that are corner-sharing in three dimensions) (9) and the CaTa2O6-structure, with M2O10-groups of edge-sharing octahedra linked together by corner sharing (10). All three independent Nd-atoms are located in eight-coordinate distorted bicapped-trigonal prismatic sites, as is also observed for the A-cation in the GdFeO3 and CaTa2O6 structure types. The Nd(2)-atom, located between the puckered double  ReO3-type  chains,   has   a   slightly TABLE III SELECTED INTERATOMIC DISTANCES (Å) FOR Nd3Ti4O12 WITH ESTIMATED STANDARD DEVIATIONS IN PARENTHESES  Ti(1)  –O(2)  1.822(7)   Ti(2)  –O(1)  1.942(7)    –O(3)  1.980(7)     1.979(7)     2.239(7)    –O(2)  2.145(7)    –O(4)  1.908(7)    –O(4)  2.007(7)    –O(5)  1.932(3)    –O(7)  1.919(4)    –O(6)  1.993(4)    –O(8)  2.040(3)   Nd(1)  –O(3)  2.585 (7)  2×  Nd(2)  –O(1)  2.449(7)  2×    2.649(7)  2×    2.489(7)  2×   –O(4)  2.328(7)  2×   –O(2)  2.591(7)  2×   –O(5)  2.41(1)    –O(7)  2.59(1)    –O(6)  2.35(1)    –O(8)  2.47(1)   Nd(3)  –O(2)  2.570(8)  2×  Ti(2)  –Ti(2)  2.760(3)    –O(3)  2.395(7)  2×       –O(4)  2.506(7)  2×       –O(6)  2.36(1)        –O(8)  2.461(9)       TABLE I CRYSTALLOGRAPHIC DATA FOR Nd3Ti4O12  110 HESSEN ET AL.  FIG. 2. ORTEP representation of the Ti2O10 group in Nd3Ti4O12 . Thermal ellipsoids represented at the 50% probability level. larger and more regular coordination sphere than the other Nd-atoms (average Nd(2)–O  =  2.515 Å, with maximum deviations of –0.066 and +0.076 Å, for Nd(1) 2.486 Å and  +0.163/ –0.158 Å, for Nd(3) 2.470 Å and  +0.100/ –0.110 Å respectively. The average Ti–O distances in Nd3Ti4O12 are 1.979 Å for Ti(1) and 2.005 Å for Ti(2) respectively. The average Ti–O distance for Ti(1) in the GdFeO3 type block is signifi-cantly smaller than that observed in NdTiO3 (2.022 Å (1)). It is close to the value for Ti4 + –O in rutiles (1.97 Å). The coordination geometry around Ti(1) is also considerably more distorted, with sharp O–Ti–O angles ranging from 80.3° to 94.4° (in NdTiO3 from 89.0° to 90.3° (1)). One notable feature in the structure is the distortion in the Ti2O10-groups. The Ti(2)-atoms are displaced from the center of the octahedra towards each other rather than away from each other (as might be expected from Coulomb repul-sion). The geometry of this group is shown in Fig. 2. The Ti(2)–Ti(2) distance of 2.760(3) Å is quite short for a Ti–Ti distance in a pair of edge-shared octahedra. An interesting comparison can be made with the recently published crystal structure of the Ti(4 + ) compound Nd4Ti9O24 in which edge-shared Ti2O10-pairs are also present (11). The average Ti–O distance in these groups are smaller than for Ti(2) in Nd3Ti4O12 , and typical for Ti(4 + ) (1.973/ 1.978  Å).  However,  the  Ti–Ti  distance  in the pairs (2.963 Å) is 0.20 Å longer than in Nd3Ti4O12. The stoichiometry of Nd3Ti4O12 obtained from X-ray diffraction indicates an average Ti oxidation state of 3.75 + ; i.e., there is one 3d-electron per Ti2O10-pair. These 3d-electrons are possibly localized in orbitals associated with the Ti(2)–Ti(2) pair-ing. This would imply nonmetallic behavior of the material, at least at or below the tem-perature at which the structure was deter-mined (23°C). At high temperatures a first-order transition to a metallic state may be anticipated. A low-temperature four-point resistivity measurement on a single crystal showed nonmetallic behavior (Fig. 3) with a room-temperature resistivity of 35 Ω cm. Due to the small size of the crystal, only the conductivity along the b-axis could be measured. Anisotropy in the conductivity was expected due to the low symmetry of the crystal structure, but unfortunately could  not be measured. Degradation of the very small contacts made data collection un-reliable above room temperature. The ln(ρ) vs 1/T plot is nonlinear, suggesting that the material does not behave as an intrinsic semiconductor. The high-temperature data give an activation energy of 0.26 eV, but the fit is valid only over a rather small tempera-ture range. A second fit, based on a hopping mechanism for the conduction with a ρ ∝ ρ0  FIG. 3. Plot of log(resistivity) vs 1000/T from a dc resistivity measurement on a Nd3Ti4O12 single crystal.   INTERGROWTH IN NEODYMIUM TITANATE 111 exp(T0/T)1/4, gives a critical temperature of T0 = 1.24 × 106 K. This fit extends over a larger temperature range, but at low temper-ature, the deviations are very pronounced. Further experiments would be needed to elucidate the mechanism of conductivity. Discussion It is observed that recrystallization of ceramics with nominal composition Ln0.33Ti2O4 from Ln2O3 · 3B2O3 fluxes (Ln = La, Nd) leads to two crystalline prod-ucts: Ti2O3 and a ternary compound that is  only partially reduced. The difference in ionic radius between La and Nd is relatively small (1.32 Å for La3 + , 1.26 Å for Nd3 + in eight-coordination (12)). However, this difference, under otherwise identical condi-tions, causes a drastic change in the struc-ture of the ternary product, being an ordered defect perovskite for the La–Ti–O system and Nd3Ti4O12 with the intergrowth struc-ture as described above for the Nd–Ti–O system. This suggests a delicate dependence of the thermodynamics of these ternaries on the size of the A-cation. Also notable is the apparent absence of the La1.33Ti8O16 hol-landite phase that was obtained earlier from a more La-rich mixture (3). This can corre-spond with our findings in the Sr–Nb–O sys-tem that the occurrence of disproportiona-tion reactions in ternary borate flux media is  strongly dependent on the concentration of the common ion between the flux and the ternary transition metal oxide system (13). Although currently Nd3Ti4O12 is the only member of the ordered GdFeO3/CaTa2O6 in-tergrowth type synthesized, it is possible to predict the existence of a homologous series based on this intergrowth principle. The general formula of such a series would be Nd1+nTi2+nO6+3n , of which Nd3Ti4O12 is the n = 2 member. It is based on GdFeO3-blocks expanding in the c-direction between the edge-sharing  pairs  of  octahedra  of  the CaTa2O6 structure. The end members in the Nd–Ti–O system would be defined by n = 1 (Nd2Ti3O9 , Ti4+) and n = ∞ (NdTiO3 , Ti3+). The n = even and n = odd members differ in  the orientation of the edge-sharing pairs relative to each other. In the n = even mem-bers, sequential Ti2O10-blocks along the c-direction will be shifted over a/2, and canted in the opposite direction within the ac-plane (as in Nd3Ti4O12). In the n = odd members these blocks would not be shifted, and would have the same canting direction. Presently we are exploring the possibility of accessing members of this series by ceramic routes. Initial efforts were directed to pre-pare related Ti4 + ceramics, e.g., with com-position CaNd2Ti4O12 or Nd3Ti3AlO12 . As yet, these attempts have met with little suc-cess, yielding tetragonal defect perovskite type species as main products. References 1. D. A. MACLEAN, H.-N. NG, AND J. E. GREEDAN, J. Solid State Chem. 30, 35 (1979). 2. M. KESTIGAN AND R. WARD, J. Am. Chem. Soc. 77, 6199 (1955). 3. B. HESSEN, S. A. SUNSHINE, AND T. SIEGRIST, J. Solid State Chem. 94, 306 (1991). 4. S. N. RUDDLESDEN AND P. POPPER, Acta Crys-tallogr. 10, 538 (1957); Acta Crystallogr. 11, 54 (1958). 5. M. NANOT, F. QUEYROUX, J. C. GILLES, A. CARPY, AND J. GALY, J. Solid State Chem. 11, 272 (1974). 6. Y. LEPAGE, P. S. WHITE, AND E. J. GABE, “Pro-ceedings of the American Crystallographic Associ-ation Annual Meeting, 1986, Hamilton, Canada,” AIP, New York, (1986). [Poster PA23] 7. E. J. GABE, Y. LEPAGE, J. P. CHARLAND, F. L. LEE, AND P. S. WHITE, J. Appl. Crystallogr. 22, 384 (1989). 8. M. ABE AND K. UCHINO, Mater. Res. Bull. 9, 147 (1974). 9. S. GELLER, J. Chem. Phys. 24, 1236 (1956). 10. L. JAHNBERG, Acta Chem. Scand. 71, 2548 (1963). 11. N. HÜBNER AND R. GRUEHN, Z. Anorg. Allg. Chem. 616, 86 (1992). 12. R. D. SHANNON AND C. T. PREWITT, Acta Crys-tallogr. Sect. B 26, 1046 (1970). 13. B. HESSEN, S. A. SUNSHINE, T. SIEGRIST, AND R. JIMENEZ, Mater. Res. Bull. 26, 85 (1991). 

   University of GroningenA Reduced Neodymium Titanate with a New Intergrowth Structure TypeHessen, B.; Sunshine, S.A.; Siegrist, T.; Dover, R.B. vanPublished in:Journal of Solid State ChemistryDOI:10.1006/jssc.1993.1198IMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite fromit. Please check the document version below.Document VersionPublisher's PDF, also known as Version of recordPublication date:1993Link to publication in University of Groningen/UMCG research databaseCitation for published version (APA):Hessen, B., Sunshine, S. A., Siegrist, T., & Dover, R. B. V. (1993). A Reduced Neodymium Titanate with aNew Intergrowth Structure Type. 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For technical reasons thenumber of authors shown on this cover page is limited to 10 maximum.Download date: 09-09-2023JOURNAL OF SOLID STATE CHEMISTRY 105, 107–111 (1993) A Reduced Neodymium Titanate with a New Intergrowth Structure Type B. HESSEN, S. A. SUNSHINE, T. SIEGRIST, AND R. B. VAN DOVER AT&T Bell Laboratories, 600 Mountain Avenue, Murray Hill, New Jersey 07974 Received November 23, 1992; in revised form March 22, 1993; accepted March 24, 1993 Single crystals of a new reduced neodymium titanate, Nd3Ti4O12 , have been obtained by slow cooling of a reduced Nd–Ti–O ceramic precursor in a molten neodymium borate flux under high vacuum. Single crystal X-ray diffraction (space group Pcmn, a = 5.420(1) Å, b = 7.610(1) Å, c = 22.015(5) Å, Z = 4, Pearson symbol oP76) showed that the structure of Nd3Ti4O12 is built from an arrangement of corner-sharing and edge-sharing TiO6-octahedra that can be considered as a regular intergrowth of the GdFeO3 and CaTa2O6 structure types. The material is nonmetallic (R(25°C) = 35 Ωcm) which may arise due to the localization of the Ti 3d-electrons in the edge-sharing pairs of TiO6-octahedra (short Ti–Ti distance of 2.760(3) Å).   © 1993 Academic Press. Inc. Introduction The Ti(III) rare earth ternary oxides LnTiO3 (Ln = La, Nd, Sm, Gd) are known to have a distorted orthorhombic perovskite structure of the GdFeO3-type (1). Apart from the defect perovskite series La2/3+x TiO3 (2), few other rare-earth containing re-duced ternary titanates are known. Recently we prepared a new reduced La–Ti–O com-pound, La1.33Ti8O16 with the hollandite structure, from a LaTiO3 ceramic and a lan-thanum borate flux (3). We also investigated the use of ceramic precursors with lower rare earth content. A significant difference in behavior between the La–Ti–O and Nd-Ti-O systems is observed in that case. Here we present the synthesis and crystal structure of a new reduced neodymium ti-tanate, Nd3Ti4O12 , with a structure derived from intergrowth of the GdFeO3 and CaTa2O6 structure types. Similar attempts to prepare a La–Ti–O analogue only pro-duced crystals of the La2/3+xTiO3 defect per-ovskite. Ordered intergrowth of two different structure types has been observed in a  num- ber of systems, frequently giving rise to ho-mologous series with varying sizes of the blocks corresponding to the two structure types. Well known examples are the Ruddlesden–Popper series, with SrTiO3 and SrO layer intergrowth (4), and the in-tergrowth of BaZnF4 and GdFeO3 struc-ture types in the Ca–Na–Nb–O and Ca-La-Ti-O systems (5). It is conceivable that intergrowth of the GdFeO3 and CaTa2O6 structure types can also give rise to a homologous series. Experimental Sample Preparation Ceramic titanate precursors with nominal composition “Ln0.33Ti2O4” were prepared from pressed pellets of Ln2O3 , Ti2O3 (Alfa), and TiO2 (Aldrich) in the appropriate stoi-chiometry (9 mmole Ti/pellet). As mineral-izer Ln2O3 · 3B2O3 was added in the molar ratio of Nd : Ti : B2O3 = 1.3 : 1.0 : 1.7. The pellets were wrapped in Mo-foil and heated in vacuo to 1200°C for 24 hr. X-ray powder diffraction   showed   that    they    consisted 107 0022-4596/93 $5.00 Copyright © 1993 by Academic Press, Inc. All rights of reproduction in any form reserved. 108 HESSEN ET AL. mainly of Ti2O3 and Ln2/3+xTiO3-type perov-skites. Borates Ln2O3 · 3B2O3 were prepared from La(OH)3 (Alfa) or Nd2O3 (Aldrich) with B2O3 (Fisher) in the appropriate ratio. The mixtures were heated at 950°C in alu-mina crucibles for 24 hr. Lanthanide sesqui-oxides were fired at 1000°C in air for 12 hr before use. Mixtures of ceramic precursors of nomi-nal composition “Ln1/3Ti2O4” (Ln = La, Nd) and Ln2O3 · 3B2O3 in 1 : 3 weight ratio were placed in Mo-foil cups with Mo-foil lids. They were heated in vacuo (in a Cent-orr high-vacuum furnace) to 400°C to re-move adsorbed water, and then to 1250°C (1 hr, 10–6–10–7 Torr). The mixtures were then cooled to 1140°C at 8°C/hr. The heater was subsequently turned off, allowing the sample to reach room temperature in ap-proximately 1.5 hr. The mixtures were soaked in warm concentrated aqueous KOH for 12 hr, after which the products were mechanically broken up. For Ln = Nd the product mixture consisted of dark blue crys-talline aggregates showing in places well-developed rectangular crystal faces, to-gether with small crystals of Ti2O3 and a pink transparent crystalline flux. The blue product was determined by single crystal X-ray diffraction on several cleaved crystal fragments to be a new orthorhombic Nd–titanate, Nd3Ti4O12 . For Ln = La, a dark crystalline mass with jagged edges but with locally well-developed crystals with square faces had formed, together with small crys-tals of Ti2O3 and a colorless crystalline flux. X-ray diffraction on crystals and cleaved crystal fragments showed the dark crystal-line product to be an ordered defect perov-skite of the type La2/3+xTiO3 . A dc four-point resistivity measurement was performed on a single crystal of Nd3Ti4O12 . Silver wires were connected (with Ag/epoxy) to evaporated Au–Ge/Au strip contacts on the crystal. Structure Determinations The crystals were measured at 23°C on an Enraf-Nonius  CAD-4  diffractometer  using graphite-monochromatized MoKα radiation and the NRCCAD program package (6). All calculations were carried out on an Alliant FX/80 computer, using the NRCVAX struc-ture package (7). Lattice parameters were determined from the absolute 2θ values of reflections at a high angle (Nd3Ti4O12: 30 reflections with 49° < 2θ < 58°, La1/3+xTiO3: 28 reflections with 50° < 2θ < 56°). The metal atom positions for Nd3Ti4O12 were ob-tained by direct methods. A Gaussian ab-sorption correction was applied. Aniso-tropic  temperature factors for all atoms were  included in the refinement. Crystallo-graphic data are listed in Table I, atomic coordinates and isotropic thermal parame-ters in Table II.* Crystals of La2/3+xTiO3 appeared to be primitive cubic with a = 7.7837(3) Å. It is likely that the material is actually a tetrago-nal defect perovskite with c = 2a as de-scribed earlier for La2/3TiO2.954 (8). This also corresponds well with powder X-ray diffrac-tion on crushed crystals. The doubling in all three dimensions as observed by single crystal X-ray diffraction is likely to be pro-duced by twinning. From the observed cell parameter and comparison with the data for La2/3+xTiO3 of Kestigan and Ward (2), the composition of the crystals can be derived by interpolation to be approximately La0.73TiO3(x ≈ 0.06). Description of the Structure The structure of Nd3Ti4O12 is shown in Fig. 1,  projected along the  b-axis.  Pertinent * See NAPS document No. 05020 for 23 pages of supplementary material. Order from ASIS/NAPS. Mi-crofiche Publications, P.O. Box 3513, Grand Central Station, New York, NY 10163. Remit in advance $4.00 for microfiche copy or for photocopy, $8.65 up to 20 pages plus $.30 for each additional page. All orders must be prepaid. Institutions and Organizations may order by purchase order. However, there is a billing and handling charge for this service of $15. Foreign orders add $4.50 for postage and handling, for the first 20 pages, and $1.00 for additional 10 pages of material, $1.75 for postage of any microfiche orders.   INTERGROWTH IN NEODYMIUM TITANATE 109   Formula  Nd3Ti4O12 Space group  Pcmn  Crystal size (mm)  0.12  ×  0.18  ×  0.12  a (Å) (at 23°C)  5.4203(8)  b (Å)  7.6104(9)  c (Å)  22.105(5)  V (Å)  908.1(3)  Z  4  Dc (g/cm3)  5.971  µ (MoKα, cm-1)  202.8  Minimum transmission  0.1079  Maximum transmission  0.2513  Scan type  ω/2θ  Reflections measured  2168  Independent reflections  2099  Observed reflections  1670  (I > 2.5σ (I))   Refined parameters  98  R  0.040  Rw (w = 1/σ 2 (Fo))  0.056  Extinction length (µm)  0.29(1) interatomic distances are listed in Table III. The framework of the structure is formed by  TiO6 octahedra, arranged in puckered single and double ReO3-type chains running in the direction of the b-axis. The Ti(1)O6-octahedra make up the single chains, and share only corners with the neighboring oc-tahedra. The Ti(2)O6-octahedra form the double chains  as  edge-sharing  pairs, which TABLE II ATOMIC POSITIONAL PARAMETERS AND ISOTROPIC TEMPERATURE FACTORS FOR Nd3Ti4O12   x   y   z  Biso (Å2) Nd(1)  0.93871(15)  0.25  0.19959(3)  0.464(20) Nd(2)  0.45551(14)  0.75  0.02782(3)  0.471(19) Nd(3)  0.92754(14)  0.75  0.17866(3)  0.394(18) Ti(1)  0.4390(3)  0.49921(20)  0.17863(7)  0.38(4)  Ti(2)  0.9516(3)  0.48936(21)  0.06143(7)  0.43(4)  O(1)  0.2465(14)  0.4624(9)  0.0122(3)  0.53(19)  O(2)  0.6351(14)  0.5593(10)  0.1147(3)  0.69(21)  O(3)  0.7090(13)  0.5367(9)  0.2378(3)  0.48(19)  O(4)  0.1223(13)  0.4708(9)  0.1421(3)  0.43(19)  O(5)  0.4976(22)  0.25  0.1869(5)  0.9(4)  O(6)  0.3578(22)  0.75  0.1954(5)  0.7(3)  O(7)  0.8410(23)  0.25  0.0642(5)  0.8(3)  O(8)  0.0321(21)  0.75  0.0699(4)  0.5(3)   FIG. 1. Polyhedral representation of the structure of Nd3Ti4O12 , projected along the b-axis. Ti–O polyhedra are hatched; Nd-atoms are represented as circles. share corners in the ac-plane with the Ti(1)-octahedra. The structure can be described as a regular intergrowth of two known struc-ture types: the GdFeO3-structure, which is an orthorhombic distortion of the perovskite structure (MO6-octahedra that are corner-sharing in three dimensions) (9) and the CaTa2O6-structure, with M2O10-groups of edge-sharing octahedra linked together by corner sharing (10). All three independent Nd-atoms are located in eight-coordinate distorted bicapped-trigonal prismatic sites, as is also observed for the A-cation in the GdFeO3 and CaTa2O6 structure types. The Nd(2)-atom, located between the puckered double  ReO3-type  chains,   has   a   slightly TABLE III SELECTED INTERATOMIC DISTANCES (Å) FOR Nd3Ti4O12 WITH ESTIMATED STANDARD DEVIATIONS IN PARENTHESES  Ti(1)  –O(2)  1.822(7)   Ti(2)  –O(1)  1.942(7)    –O(3)  1.980(7)     1.979(7)     2.239(7)    –O(2)  2.145(7)    –O(4)  1.908(7)    –O(4)  2.007(7)    –O(5)  1.932(3)    –O(7)  1.919(4)    –O(6)  1.993(4)    –O(8)  2.040(3)   Nd(1)  –O(3)  2.585 (7)  2×  Nd(2)  –O(1)  2.449(7)  2×    2.649(7)  2×    2.489(7)  2×   –O(4)  2.328(7)  2×   –O(2)  2.591(7)  2×   –O(5)  2.41(1)    –O(7)  2.59(1)    –O(6)  2.35(1)    –O(8)  2.47(1)   Nd(3)  –O(2)  2.570(8)  2×  Ti(2)  –Ti(2)  2.760(3)    –O(3)  2.395(7)  2×       –O(4)  2.506(7)  2×       –O(6)  2.36(1)        –O(8)  2.461(9)       TABLE I CRYSTALLOGRAPHIC DATA FOR Nd3Ti4O12  110 HESSEN ET AL.  FIG. 2. ORTEP representation of the Ti2O10 group in Nd3Ti4O12 . Thermal ellipsoids represented at the 50% probability level. larger and more regular coordination sphere than the other Nd-atoms (average Nd(2)–O  =  2.515 Å, with maximum deviations of –0.066 and +0.076 Å, for Nd(1) 2.486 Å and  +0.163/ –0.158 Å, for Nd(3) 2.470 Å and  +0.100/ –0.110 Å respectively. The average Ti–O distances in Nd3Ti4O12 are 1.979 Å for Ti(1) and 2.005 Å for Ti(2) respectively. The average Ti–O distance for Ti(1) in the GdFeO3 type block is signifi-cantly smaller than that observed in NdTiO3 (2.022 Å (1)). It is close to the value for Ti4 + –O in rutiles (1.97 Å). The coordination geometry around Ti(1) is also considerably more distorted, with sharp O–Ti–O angles ranging from 80.3° to 94.4° (in NdTiO3 from 89.0° to 90.3° (1)). One notable feature in the structure is the distortion in the Ti2O10-groups. The Ti(2)-atoms are displaced from the center of the octahedra towards each other rather than away from each other (as might be expected from Coulomb repul-sion). The geometry of this group is shown in Fig. 2. The Ti(2)–Ti(2) distance of 2.760(3) Å is quite short for a Ti–Ti distance in a pair of edge-shared octahedra. An interesting comparison can be made with the recently published crystal structure of the Ti(4 + ) compound Nd4Ti9O24 in which edge-shared Ti2O10-pairs are also present (11). The average Ti–O distance in these groups are smaller than for Ti(2) in Nd3Ti4O12 , and typical for Ti(4 + ) (1.973/ 1.978  Å).  However,  the  Ti–Ti  distance  in the pairs (2.963 Å) is 0.20 Å longer than in Nd3Ti4O12. The stoichiometry of Nd3Ti4O12 obtained from X-ray diffraction indicates an average Ti oxidation state of 3.75 + ; i.e., there is one 3d-electron per Ti2O10-pair. These 3d-electrons are possibly localized in orbitals associated with the Ti(2)–Ti(2) pair-ing. This would imply nonmetallic behavior of the material, at least at or below the tem-perature at which the structure was deter-mined (23°C). At high temperatures a first-order transition to a metallic state may be anticipated. A low-temperature four-point resistivity measurement on a single crystal showed nonmetallic behavior (Fig. 3) with a room-temperature resistivity of 35 Ω cm. Due to the small size of the crystal, only the conductivity along the b-axis could be measured. Anisotropy in the conductivity was expected due to the low symmetry of the crystal structure, but unfortunately could  not be measured. Degradation of the very small contacts made data collection un-reliable above room temperature. The ln(ρ) vs 1/T plot is nonlinear, suggesting that the material does not behave as an intrinsic semiconductor. The high-temperature data give an activation energy of 0.26 eV, but the fit is valid only over a rather small tempera-ture range. A second fit, based on a hopping mechanism for the conduction with a ρ ∝ ρ0  FIG. 3. Plot of log(resistivity) vs 1000/T from a dc resistivity measurement on a Nd3Ti4O12 single crystal.   INTERGROWTH IN NEODYMIUM TITANATE 111 exp(T0/T)1/4, gives a critical temperature of T0 = 1.24 × 106 K. This fit extends over a larger temperature range, but at low temper-ature, the deviations are very pronounced. Further experiments would be needed to elucidate the mechanism of conductivity. Discussion It is observed that recrystallization of ceramics with nominal composition Ln0.33Ti2O4 from Ln2O3 · 3B2O3 fluxes (Ln = La, Nd) leads to two crystalline prod-ucts: Ti2O3 and a ternary compound that is  only partially reduced. The difference in ionic radius between La and Nd is relatively small (1.32 Å for La3 + , 1.26 Å for Nd3 + in eight-coordination (12)). However, this difference, under otherwise identical condi-tions, causes a drastic change in the struc-ture of the ternary product, being an ordered defect perovskite for the La–Ti–O system and Nd3Ti4O12 with the intergrowth struc-ture as described above for the Nd–Ti–O system. This suggests a delicate dependence of the thermodynamics of these ternaries on the size of the A-cation. Also notable is the apparent absence of the La1.33Ti8O16 hol-landite phase that was obtained earlier from a more La-rich mixture (3). This can corre-spond with our findings in the Sr–Nb–O sys-tem that the occurrence of disproportiona-tion reactions in ternary borate flux media is  strongly dependent on the concentration of the common ion between the flux and the ternary transition metal oxide system (13). Although currently Nd3Ti4O12 is the only member of the ordered GdFeO3/CaTa2O6 in-tergrowth type synthesized, it is possible to predict the existence of a homologous series based on this intergrowth principle. The general formula of such a series would be Nd1+nTi2+nO6+3n , of which Nd3Ti4O12 is the n = 2 member. It is based on GdFeO3-blocks expanding in the c-direction between the edge-sharing  pairs  of  octahedra  of  the CaTa2O6 structure. The end members in the Nd–Ti–O system would be defined by n = 1 (Nd2Ti3O9 , Ti4+) and n = ∞ (NdTiO3 , Ti3+). The n = even and n = odd members differ in  the orientation of the edge-sharing pairs relative to each other. In the n = even mem-bers, sequential Ti2O10-blocks along the c-direction will be shifted over a/2, and canted in the opposite direction within the ac-plane (as in Nd3Ti4O12). In the n = odd members these blocks would not be shifted, and would have the same canting direction. Presently we are exploring the possibility of accessing members of this series by ceramic routes. Initial efforts were directed to pre-pare related Ti4 + ceramics, e.g., with com-position CaNd2Ti4O12 or Nd3Ti3AlO12 . As yet, these attempts have met with little suc-cess, yielding tetragonal defect perovskite type species as main products. References 1. D. A. MACLEAN, H.-N. NG, AND J. E. GREEDAN, J. Solid State Chem. 30, 35 (1979). 2. M. KESTIGAN AND R. WARD, J. Am. Chem. Soc. 77, 6199 (1955). 3. B. HESSEN, S. A. SUNSHINE, AND T. SIEGRIST, J. Solid State Chem. 94, 306 (1991). 4. S. N. RUDDLESDEN AND P. POPPER, Acta Crys-tallogr. 10, 538 (1957); Acta Crystallogr. 11, 54 (1958). 5. M. NANOT, F. QUEYROUX, J. C. GILLES, A. CARPY, AND J. GALY, J. Solid State Chem. 11, 272 (1974). 6. Y. LEPAGE, P. S. WHITE, AND E. J. GABE, “Pro-ceedings of the American Crystallographic Associ-ation Annual Meeting, 1986, Hamilton, Canada,” AIP, New York, (1986). [Poster PA23] 7. E. J. GABE, Y. LEPAGE, J. P. CHARLAND, F. L. LEE, AND P. S. WHITE, J. Appl. Crystallogr. 22, 384 (1989). 8. M. ABE AND K. UCHINO, Mater. Res. Bull. 9, 147 (1974). 9. S. GELLER, J. Chem. Phys. 24, 1236 (1956). 10. L. JAHNBERG, Acta Chem. Scand. 71, 2548 (1963). 11. N. HÜBNER AND R. GRUEHN, Z. Anorg. Allg. Chem. 616, 86 (1992). 12. R. D. SHANNON AND C. T. PREWITT, Acta Crys-tallogr. Sect. B 26, 1046 (1970). 13. B. HESSEN, S. A. SUNSHINE, T. SIEGRIST, AND R. JIMENEZ, Mater. Res. Bull. 26, 85 (1991). 

   University of GroningenA Reduced Neodymium Titanate with a New Intergrowth Structure TypeHessen, B.; Sunshine, S.A.; Siegrist, T.; Dover, R.B. vanPublished in:Journal of Solid State ChemistryDOI:10.1006/jssc.1993.1198IMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite fromit. Please check the document version below.Document VersionPublisher's PDF, also known as Version of recordPublication date:1993Link to publication in University of Groningen/UMCG research databaseCitation for published version (APA):Hessen, B., Sunshine, S. A., Siegrist, T., & Dover, R. B. V. (1993). A Reduced Neodymium Titanate with aNew Intergrowth Structure Type. 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For technical reasons thenumber of authors shown on this cover page is limited to 10 maximum.Download date: 31-10-2023JOURNAL OF SOLID STATE CHEMISTRY 105, 107–111 (1993) A Reduced Neodymium Titanate with a New Intergrowth Structure Type B. HESSEN, S. A. SUNSHINE, T. SIEGRIST, AND R. B. VAN DOVER AT&T Bell Laboratories, 600 Mountain Avenue, Murray Hill, New Jersey 07974 Received November 23, 1992; in revised form March 22, 1993; accepted March 24, 1993 Single crystals of a new reduced neodymium titanate, Nd3Ti4O12 , have been obtained by slow cooling of a reduced Nd–Ti–O ceramic precursor in a molten neodymium borate flux under high vacuum. Single crystal X-ray diffraction (space group Pcmn, a = 5.420(1) Å, b = 7.610(1) Å, c = 22.015(5) Å, Z = 4, Pearson symbol oP76) showed that the structure of Nd3Ti4O12 is built from an arrangement of corner-sharing and edge-sharing TiO6-octahedra that can be considered as a regular intergrowth of the GdFeO3 and CaTa2O6 structure types. The material is nonmetallic (R(25°C) = 35 Ωcm) which may arise due to the localization of the Ti 3d-electrons in the edge-sharing pairs of TiO6-octahedra (short Ti–Ti distance of 2.760(3) Å).   © 1993 Academic Press. Inc. Introduction The Ti(III) rare earth ternary oxides LnTiO3 (Ln = La, Nd, Sm, Gd) are known to have a distorted orthorhombic perovskite structure of the GdFeO3-type (1). Apart from the defect perovskite series La2/3+x TiO3 (2), few other rare-earth containing re-duced ternary titanates are known. Recently we prepared a new reduced La–Ti–O com-pound, La1.33Ti8O16 with the hollandite structure, from a LaTiO3 ceramic and a lan-thanum borate flux (3). We also investigated the use of ceramic precursors with lower rare earth content. A significant difference in behavior between the La–Ti–O and Nd-Ti-O systems is observed in that case. Here we present the synthesis and crystal structure of a new reduced neodymium ti-tanate, Nd3Ti4O12 , with a structure derived from intergrowth of the GdFeO3 and CaTa2O6 structure types. Similar attempts to prepare a La–Ti–O analogue only pro-duced crystals of the La2/3+xTiO3 defect per-ovskite. Ordered intergrowth of two different structure types has been observed in a  num- ber of systems, frequently giving rise to ho-mologous series with varying sizes of the blocks corresponding to the two structure types. Well known examples are the Ruddlesden–Popper series, with SrTiO3 and SrO layer intergrowth (4), and the in-tergrowth of BaZnF4 and GdFeO3 struc-ture types in the Ca–Na–Nb–O and Ca-La-Ti-O systems (5). It is conceivable that intergrowth of the GdFeO3 and CaTa2O6 structure types can also give rise to a homologous series. Experimental Sample Preparation Ceramic titanate precursors with nominal composition “Ln0.33Ti2O4” were prepared from pressed pellets of Ln2O3 , Ti2O3 (Alfa), and TiO2 (Aldrich) in the appropriate stoi-chiometry (9 mmole Ti/pellet). As mineral-izer Ln2O3 · 3B2O3 was added in the molar ratio of Nd : Ti : B2O3 = 1.3 : 1.0 : 1.7. The pellets were wrapped in Mo-foil and heated in vacuo to 1200°C for 24 hr. X-ray powder diffraction   showed   that    they    consisted 107 0022-4596/93 $5.00 Copyright © 1993 by Academic Press, Inc. All rights of reproduction in any form reserved. 108 HESSEN ET AL. mainly of Ti2O3 and Ln2/3+xTiO3-type perov-skites. Borates Ln2O3 · 3B2O3 were prepared from La(OH)3 (Alfa) or Nd2O3 (Aldrich) with B2O3 (Fisher) in the appropriate ratio. The mixtures were heated at 950°C in alu-mina crucibles for 24 hr. Lanthanide sesqui-oxides were fired at 1000°C in air for 12 hr before use. Mixtures of ceramic precursors of nomi-nal composition “Ln1/3Ti2O4” (Ln = La, Nd) and Ln2O3 · 3B2O3 in 1 : 3 weight ratio were placed in Mo-foil cups with Mo-foil lids. They were heated in vacuo (in a Cent-orr high-vacuum furnace) to 400°C to re-move adsorbed water, and then to 1250°C (1 hr, 10–6–10–7 Torr). The mixtures were then cooled to 1140°C at 8°C/hr. The heater was subsequently turned off, allowing the sample to reach room temperature in ap-proximately 1.5 hr. The mixtures were soaked in warm concentrated aqueous KOH for 12 hr, after which the products were mechanically broken up. For Ln = Nd the product mixture consisted of dark blue crys-talline aggregates showing in places well-developed rectangular crystal faces, to-gether with small crystals of Ti2O3 and a pink transparent crystalline flux. The blue product was determined by single crystal X-ray diffraction on several cleaved crystal fragments to be a new orthorhombic Nd–titanate, Nd3Ti4O12 . For Ln = La, a dark crystalline mass with jagged edges but with locally well-developed crystals with square faces had formed, together with small crys-tals of Ti2O3 and a colorless crystalline flux. X-ray diffraction on crystals and cleaved crystal fragments showed the dark crystal-line product to be an ordered defect perov-skite of the type La2/3+xTiO3 . A dc four-point resistivity measurement was performed on a single crystal of Nd3Ti4O12 . Silver wires were connected (with Ag/epoxy) to evaporated Au–Ge/Au strip contacts on the crystal. Structure Determinations The crystals were measured at 23°C on an Enraf-Nonius  CAD-4  diffractometer  using graphite-monochromatized MoKα radiation and the NRCCAD program package (6). All calculations were carried out on an Alliant FX/80 computer, using the NRCVAX struc-ture package (7). Lattice parameters were determined from the absolute 2θ values of reflections at a high angle (Nd3Ti4O12: 30 reflections with 49° < 2θ < 58°, La1/3+xTiO3: 28 reflections with 50° < 2θ < 56°). The metal atom positions for Nd3Ti4O12 were ob-tained by direct methods. A Gaussian ab-sorption correction was applied. Aniso-tropic  temperature factors for all atoms were  included in the refinement. Crystallo-graphic data are listed in Table I, atomic coordinates and isotropic thermal parame-ters in Table II.* Crystals of La2/3+xTiO3 appeared to be primitive cubic with a = 7.7837(3) Å. It is likely that the material is actually a tetrago-nal defect perovskite with c = 2a as de-scribed earlier for La2/3TiO2.954 (8). This also corresponds well with powder X-ray diffrac-tion on crushed crystals. The doubling in all three dimensions as observed by single crystal X-ray diffraction is likely to be pro-duced by twinning. From the observed cell parameter and comparison with the data for La2/3+xTiO3 of Kestigan and Ward (2), the composition of the crystals can be derived by interpolation to be approximately La0.73TiO3(x ≈ 0.06). Description of the Structure The structure of Nd3Ti4O12 is shown in Fig. 1,  projected along the  b-axis.  Pertinent * See NAPS document No. 05020 for 23 pages of supplementary material. Order from ASIS/NAPS. Mi-crofiche Publications, P.O. Box 3513, Grand Central Station, New York, NY 10163. Remit in advance $4.00 for microfiche copy or for photocopy, $8.65 up to 20 pages plus $.30 for each additional page. All orders must be prepaid. Institutions and Organizations may order by purchase order. However, there is a billing and handling charge for this service of $15. Foreign orders add $4.50 for postage and handling, for the first 20 pages, and $1.00 for additional 10 pages of material, $1.75 for postage of any microfiche orders.   INTERGROWTH IN NEODYMIUM TITANATE 109   Formula  Nd3Ti4O12 Space group  Pcmn  Crystal size (mm)  0.12  ×  0.18  ×  0.12  a (Å) (at 23°C)  5.4203(8)  b (Å)  7.6104(9)  c (Å)  22.105(5)  V (Å)  908.1(3)  Z  4  Dc (g/cm3)  5.971  µ (MoKα, cm-1)  202.8  Minimum transmission  0.1079  Maximum transmission  0.2513  Scan type  ω/2θ  Reflections measured  2168  Independent reflections  2099  Observed reflections  1670  (I > 2.5σ (I))   Refined parameters  98  R  0.040  Rw (w = 1/σ 2 (Fo))  0.056  Extinction length (µm)  0.29(1) interatomic distances are listed in Table III. The framework of the structure is formed by  TiO6 octahedra, arranged in puckered single and double ReO3-type chains running in the direction of the b-axis. The Ti(1)O6-octahedra make up the single chains, and share only corners with the neighboring oc-tahedra. The Ti(2)O6-octahedra form the double chains  as  edge-sharing  pairs, which TABLE II ATOMIC POSITIONAL PARAMETERS AND ISOTROPIC TEMPERATURE FACTORS FOR Nd3Ti4O12   x   y   z  Biso (Å2) Nd(1)  0.93871(15)  0.25  0.19959(3)  0.464(20) Nd(2)  0.45551(14)  0.75  0.02782(3)  0.471(19) Nd(3)  0.92754(14)  0.75  0.17866(3)  0.394(18) Ti(1)  0.4390(3)  0.49921(20)  0.17863(7)  0.38(4)  Ti(2)  0.9516(3)  0.48936(21)  0.06143(7)  0.43(4)  O(1)  0.2465(14)  0.4624(9)  0.0122(3)  0.53(19)  O(2)  0.6351(14)  0.5593(10)  0.1147(3)  0.69(21)  O(3)  0.7090(13)  0.5367(9)  0.2378(3)  0.48(19)  O(4)  0.1223(13)  0.4708(9)  0.1421(3)  0.43(19)  O(5)  0.4976(22)  0.25  0.1869(5)  0.9(4)  O(6)  0.3578(22)  0.75  0.1954(5)  0.7(3)  O(7)  0.8410(23)  0.25  0.0642(5)  0.8(3)  O(8)  0.0321(21)  0.75  0.0699(4)  0.5(3)   FIG. 1. Polyhedral representation of the structure of Nd3Ti4O12 , projected along the b-axis. Ti–O polyhedra are hatched; Nd-atoms are represented as circles. share corners in the ac-plane with the Ti(1)-octahedra. The structure can be described as a regular intergrowth of two known struc-ture types: the GdFeO3-structure, which is an orthorhombic distortion of the perovskite structure (MO6-octahedra that are corner-sharing in three dimensions) (9) and the CaTa2O6-structure, with M2O10-groups of edge-sharing octahedra linked together by corner sharing (10). All three independent Nd-atoms are located in eight-coordinate distorted bicapped-trigonal prismatic sites, as is also observed for the A-cation in the GdFeO3 and CaTa2O6 structure types. The Nd(2)-atom, located between the puckered double  ReO3-type  chains,   has   a   slightly TABLE III SELECTED INTERATOMIC DISTANCES (Å) FOR Nd3Ti4O12 WITH ESTIMATED STANDARD DEVIATIONS IN PARENTHESES  Ti(1)  –O(2)  1.822(7)   Ti(2)  –O(1)  1.942(7)    –O(3)  1.980(7)     1.979(7)     2.239(7)    –O(2)  2.145(7)    –O(4)  1.908(7)    –O(4)  2.007(7)    –O(5)  1.932(3)    –O(7)  1.919(4)    –O(6)  1.993(4)    –O(8)  2.040(3)   Nd(1)  –O(3)  2.585 (7)  2×  Nd(2)  –O(1)  2.449(7)  2×    2.649(7)  2×    2.489(7)  2×   –O(4)  2.328(7)  2×   –O(2)  2.591(7)  2×   –O(5)  2.41(1)    –O(7)  2.59(1)    –O(6)  2.35(1)    –O(8)  2.47(1)   Nd(3)  –O(2)  2.570(8)  2×  Ti(2)  –Ti(2)  2.760(3)    –O(3)  2.395(7)  2×       –O(4)  2.506(7)  2×       –O(6)  2.36(1)        –O(8)  2.461(9)       TABLE I CRYSTALLOGRAPHIC DATA FOR Nd3Ti4O12  110 HESSEN ET AL.  FIG. 2. ORTEP representation of the Ti2O10 group in Nd3Ti4O12 . Thermal ellipsoids represented at the 50% probability level. larger and more regular coordination sphere than the other Nd-atoms (average Nd(2)–O  =  2.515 Å, with maximum deviations of –0.066 and +0.076 Å, for Nd(1) 2.486 Å and  +0.163/ –0.158 Å, for Nd(3) 2.470 Å and  +0.100/ –0.110 Å respectively. The average Ti–O distances in Nd3Ti4O12 are 1.979 Å for Ti(1) and 2.005 Å for Ti(2) respectively. The average Ti–O distance for Ti(1) in the GdFeO3 type block is signifi-cantly smaller than that observed in NdTiO3 (2.022 Å (1)). It is close to the value for Ti4 + –O in rutiles (1.97 Å). The coordination geometry around Ti(1) is also considerably more distorted, with sharp O–Ti–O angles ranging from 80.3° to 94.4° (in NdTiO3 from 89.0° to 90.3° (1)). One notable feature in the structure is the distortion in the Ti2O10-groups. The Ti(2)-atoms are displaced from the center of the octahedra towards each other rather than away from each other (as might be expected from Coulomb repul-sion). The geometry of this group is shown in Fig. 2. The Ti(2)–Ti(2) distance of 2.760(3) Å is quite short for a Ti–Ti distance in a pair of edge-shared octahedra. An interesting comparison can be made with the recently published crystal structure of the Ti(4 + ) compound Nd4Ti9O24 in which edge-shared Ti2O10-pairs are also present (11). The average Ti–O distance in these groups are smaller than for Ti(2) in Nd3Ti4O12 , and typical for Ti(4 + ) (1.973/ 1.978  Å).  However,  the  Ti–Ti  distance  in the pairs (2.963 Å) is 0.20 Å longer than in Nd3Ti4O12. The stoichiometry of Nd3Ti4O12 obtained from X-ray diffraction indicates an average Ti oxidation state of 3.75 + ; i.e., there is one 3d-electron per Ti2O10-pair. These 3d-electrons are possibly localized in orbitals associated with the Ti(2)–Ti(2) pair-ing. This would imply nonmetallic behavior of the material, at least at or below the tem-perature at which the structure was deter-mined (23°C). At high temperatures a first-order transition to a metallic state may be anticipated. A low-temperature four-point resistivity measurement on a single crystal showed nonmetallic behavior (Fig. 3) with a room-temperature resistivity of 35 Ω cm. Due to the small size of the crystal, only the conductivity along the b-axis could be measured. Anisotropy in the conductivity was expected due to the low symmetry of the crystal structure, but unfortunately could  not be measured. Degradation of the very small contacts made data collection un-reliable above room temperature. The ln(ρ) vs 1/T plot is nonlinear, suggesting that the material does not behave as an intrinsic semiconductor. The high-temperature data give an activation energy of 0.26 eV, but the fit is valid only over a rather small tempera-ture range. A second fit, based on a hopping mechanism for the conduction with a ρ ∝ ρ0  FIG. 3. Plot of log(resistivity) vs 1000/T from a dc resistivity measurement on a Nd3Ti4O12 single crystal.   INTERGROWTH IN NEODYMIUM TITANATE 111 exp(T0/T)1/4, gives a critical temperature of T0 = 1.24 × 106 K. This fit extends over a larger temperature range, but at low temper-ature, the deviations are very pronounced. Further experiments would be needed to elucidate the mechanism of conductivity. Discussion It is observed that recrystallization of ceramics with nominal composition Ln0.33Ti2O4 from Ln2O3 · 3B2O3 fluxes (Ln = La, Nd) leads to two crystalline prod-ucts: Ti2O3 and a ternary compound that is  only partially reduced. The difference in ionic radius between La and Nd is relatively small (1.32 Å for La3 + , 1.26 Å for Nd3 + in eight-coordination (12)). However, this difference, under otherwise identical condi-tions, causes a drastic change in the struc-ture of the ternary product, being an ordered defect perovskite for the La–Ti–O system and Nd3Ti4O12 with the intergrowth struc-ture as described above for the Nd–Ti–O system. This suggests a delicate dependence of the thermodynamics of these ternaries on the size of the A-cation. Also notable is the apparent absence of the La1.33Ti8O16 hol-landite phase that was obtained earlier from a more La-rich mixture (3). This can corre-spond with our findings in the Sr–Nb–O sys-tem that the occurrence of disproportiona-tion reactions in ternary borate flux media is  strongly dependent on the concentration of the common ion between the flux and the ternary transition metal oxide system (13). Although currently Nd3Ti4O12 is the only member of the ordered GdFeO3/CaTa2O6 in-tergrowth type synthesized, it is possible to predict the existence of a homologous series based on this intergrowth principle. The general formula of such a series would be Nd1+nTi2+nO6+3n , of which Nd3Ti4O12 is the n = 2 member. It is based on GdFeO3-blocks expanding in the c-direction between the edge-sharing  pairs  of  octahedra  of  the CaTa2O6 structure. The end members in the Nd–Ti–O system would be defined by n = 1 (Nd2Ti3O9 , Ti4+) and n = ∞ (NdTiO3 , Ti3+). The n = even and n = odd members differ in  the orientation of the edge-sharing pairs relative to each other. In the n = even mem-bers, sequential Ti2O10-blocks along the c-direction will be shifted over a/2, and canted in the opposite direction within the ac-plane (as in Nd3Ti4O12). In the n = odd members these blocks would not be shifted, and would have the same canting direction. Presently we are exploring the possibility of accessing members of this series by ceramic routes. Initial efforts were directed to pre-pare related Ti4 + ceramics, e.g., with com-position CaNd2Ti4O12 or Nd3Ti3AlO12 . As yet, these attempts have met with little suc-cess, yielding tetragonal defect perovskite type species as main products. References 1. D. A. MACLEAN, H.-N. NG, AND J. E. GREEDAN, J. Solid State Chem. 30, 35 (1979). 2. M. KESTIGAN AND R. WARD, J. Am. Chem. Soc. 77, 6199 (1955). 3. B. HESSEN, S. A. SUNSHINE, AND T. SIEGRIST, J. Solid State Chem. 94, 306 (1991). 4. S. N. RUDDLESDEN AND P. POPPER, Acta Crys-tallogr. 10, 538 (1957); Acta Crystallogr. 11, 54 (1958). 5. M. NANOT, F. QUEYROUX, J. C. GILLES, A. CARPY, AND J. GALY, J. Solid State Chem. 11, 272 (1974). 6. Y. LEPAGE, P. S. WHITE, AND E. J. GABE, “Pro-ceedings of the American Crystallographic Associ-ation Annual Meeting, 1986, Hamilton, Canada,” AIP, New York, (1986). [Poster PA23] 7. E. J. GABE, Y. LEPAGE, J. P. CHARLAND, F. L. LEE, AND P. S. WHITE, J. Appl. Crystallogr. 22, 384 (1989). 8. M. ABE AND K. UCHINO, Mater. Res. Bull. 9, 147 (1974). 9. S. GELLER, J. Chem. Phys. 24, 1236 (1956). 10. L. JAHNBERG, Acta Chem. Scand. 71, 2548 (1963). 11. N. HÜBNER AND R. GRUEHN, Z. Anorg. Allg. Chem. 616, 86 (1992). 12. R. D. SHANNON AND C. T. PREWITT, Acta Crys-tallogr. Sect. B 26, 1046 (1970). 13. B. HESSEN, S. A. SUNSHINE, T. SIEGRIST, AND R. JIMENEZ, Mater. Res. Bull. 26, 85 (1991). 

NARCIS 

University of Groningen

   University of GroningenA Reduced Neodymium Titanate with a New Intergrowth Structure TypeHessen, B.; Sunshine, S.A.; Siegrist, T.; Dover, R.B. vanPublished in:Journal of Solid State ChemistryDOI:10.1006/jssc.1993.1198IMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite fromit. Please check the document version below.Document VersionPublisher's PDF, also known as Version of recordPublication date:1993Link to publication in University of Groningen/UMCG research databaseCitation for published version (APA):Hessen, B., Sunshine, S. A., Siegrist, T., & Dover, R. B. V. (1993). A Reduced Neodymium Titanate with aNew Intergrowth Structure Type. Journal of Solid State Chemistry, 105(1).https://doi.org/10.1006/jssc.1993.1198CopyrightOther than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of theauthor(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons).Take-down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediatelyand investigate your claim.Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons thenumber of authors shown on this cover page is limited to 10 maximum.Download date: 12-11-2019JOURNAL OF SOLID STATE CHEMISTRY 105, 107–111 (1993) A Reduced Neodymium Titanate with a New Intergrowth Structure Type B. HESSEN, S. A. SUNSHINE, T. SIEGRIST, AND R. B. VAN DOVER AT&T Bell Laboratories, 600 Mountain Avenue, Murray Hill, New Jersey 07974 Received November 23, 1992; in revised form March 22, 1993; accepted March 24, 1993 Single crystals of a new reduced neodymium titanate, Nd3Ti4O12 , have been obtained by slow cooling of a reduced Nd–Ti–O ceramic precursor in a molten neodymium borate flux under high vacuum. Single crystal X-ray diffraction (space group Pcmn, a = 5.420(1) Å, b = 7.610(1) Å, c = 22.015(5) Å, Z = 4, Pearson symbol oP76) showed that the structure of Nd3Ti4O12 is built from an arrangement of corner-sharing and edge-sharing TiO6-octahedra that can be considered as a regular intergrowth of the GdFeO3 and CaTa2O6 structure types. The material is nonmetallic (R(25°C) = 35 Ωcm) which may arise due to the localization of the Ti 3d-electrons in the edge-sharing pairs of TiO6-octahedra (short Ti–Ti distance of 2.760(3) Å).   © 1993 Academic Press. Inc. Introduction The Ti(III) rare earth ternary oxides LnTiO3 (Ln = La, Nd, Sm, Gd) are known to have a distorted orthorhombic perovskite structure of the GdFeO3-type (1). Apart from the defect perovskite series La2/3+x TiO3 (2), few other rare-earth containing re-duced ternary titanates are known. Recently we prepared a new reduced La–Ti–O com-pound, La1.33Ti8O16 with the hollandite structure, from a LaTiO3 ceramic and a lan-thanum borate flux (3). We also investigated the use of ceramic precursors with lower rare earth content. A significant difference in behavior between the La–Ti–O and Nd-Ti-O systems is observed in that case. Here we present the synthesis and crystal structure of a new reduced neodymium ti-tanate, Nd3Ti4O12 , with a structure derived from intergrowth of the GdFeO3 and CaTa2O6 structure types. Similar attempts to prepare a La–Ti–O analogue only pro-duced crystals of the La2/3+xTiO3 defect per-ovskite. Ordered intergrowth of two different structure types has been observed in a  num- ber of systems, frequently giving rise to ho-mologous series with varying sizes of the blocks corresponding to the two structure types. Well known examples are the Ruddlesden–Popper series, with SrTiO3 and SrO layer intergrowth (4), and the in-tergrowth of BaZnF4 and GdFeO3 struc-ture types in the Ca–Na–Nb–O and Ca-La-Ti-O systems (5). It is conceivable that intergrowth of the GdFeO3 and CaTa2O6 structure types can also give rise to a homologous series. Experimental Sample Preparation Ceramic titanate precursors with nominal composition “Ln0.33Ti2O4” were prepared from pressed pellets of Ln2O3 , Ti2O3 (Alfa), and TiO2 (Aldrich) in the appropriate stoi-chiometry (9 mmole Ti/pellet). As mineral-izer Ln2O3 · 3B2O3 was added in the molar ratio of Nd : Ti : B2O3 = 1.3 : 1.0 : 1.7. The pellets were wrapped in Mo-foil and heated in vacuo to 1200°C for 24 hr. X-ray powder diffraction   showed   that    they    consisted 107 0022-4596/93 $5.00 Copyright © 1993 by Academic Press, Inc. All rights of reproduction in any form reserved. 108 HESSEN ET AL. mainly of Ti2O3 and Ln2/3+xTiO3-type perov-skites. Borates Ln2O3 · 3B2O3 were prepared from La(OH)3 (Alfa) or Nd2O3 (Aldrich) with B2O3 (Fisher) in the appropriate ratio. The mixtures were heated at 950°C in alu-mina crucibles for 24 hr. Lanthanide sesqui-oxides were fired at 1000°C in air for 12 hr before use. Mixtures of ceramic precursors of nomi-nal composition “Ln1/3Ti2O4” (Ln = La, Nd) and Ln2O3 · 3B2O3 in 1 : 3 weight ratio were placed in Mo-foil cups with Mo-foil lids. They were heated in vacuo (in a Cent-orr high-vacuum furnace) to 400°C to re-move adsorbed water, and then to 1250°C (1 hr, 10–6–10–7 Torr). The mixtures were then cooled to 1140°C at 8°C/hr. The heater was subsequently turned off, allowing the sample to reach room temperature in ap-proximately 1.5 hr. The mixtures were soaked in warm concentrated aqueous KOH for 12 hr, after which the products were mechanically broken up. For Ln = Nd the product mixture consisted of dark blue crys-talline aggregates showing in places well-developed rectangular crystal faces, to-gether with small crystals of Ti2O3 and a pink transparent crystalline flux. The blue product was determined by single crystal X-ray diffraction on several cleaved crystal fragments to be a new orthorhombic Nd–titanate, Nd3Ti4O12 . For Ln = La, a dark crystalline mass with jagged edges but with locally well-developed crystals with square faces had formed, together with small crys-tals of Ti2O3 and a colorless crystalline flux. X-ray diffraction on crystals and cleaved crystal fragments showed the dark crystal-line product to be an ordered defect perov-skite of the type La2/3+xTiO3 . A dc four-point resistivity measurement was performed on a single crystal of Nd3Ti4O12 . Silver wires were connected (with Ag/epoxy) to evaporated Au–Ge/Au strip contacts on the crystal. Structure Determinations The crystals were measured at 23°C on an Enraf-Nonius  CAD-4  diffractometer  using graphite-monochromatized MoKα radiation and the NRCCAD program package (6). All calculations were carried out on an Alliant FX/80 computer, using the NRCVAX struc-ture package (7). Lattice parameters were determined from the absolute 2θ values of reflections at a high angle (Nd3Ti4O12: 30 reflections with 49° < 2θ < 58°, La1/3+xTiO3: 28 reflections with 50° < 2θ < 56°). The metal atom positions for Nd3Ti4O12 were ob-tained by direct methods. A Gaussian ab-sorption correction was applied. Aniso-tropic  temperature factors for all atoms were  included in the refinement. Crystallo-graphic data are listed in Table I, atomic coordinates and isotropic thermal parame-ters in Table II.* Crystals of La2/3+xTiO3 appeared to be primitive cubic with a = 7.7837(3) Å. It is likely that the material is actually a tetrago-nal defect perovskite with c = 2a as de-scribed earlier for La2/3TiO2.954 (8). This also corresponds well with powder X-ray diffrac-tion on crushed crystals. The doubling in all three dimensions as observed by single crystal X-ray diffraction is likely to be pro-duced by twinning. From the observed cell parameter and comparison with the data for La2/3+xTiO3 of Kestigan and Ward (2), the composition of the crystals can be derived by interpolation to be approximately La0.73TiO3(x ≈ 0.06). Description of the Structure The structure of Nd3Ti4O12 is shown in Fig. 1,  projected along the  b-axis.  Pertinent * See NAPS document No. 05020 for 23 pages of supplementary material. Order from ASIS/NAPS. Mi-crofiche Publications, P.O. Box 3513, Grand Central Station, New York, NY 10163. Remit in advance $4.00 for microfiche copy or for photocopy, $8.65 up to 20 pages plus $.30 for each additional page. All orders must be prepaid. Institutions and Organizations may order by purchase order. However, there is a billing and handling charge for this service of $15. Foreign orders add $4.50 for postage and handling, for the first 20 pages, and $1.00 for additional 10 pages of material, $1.75 for postage of any microfiche orders.   INTERGROWTH IN NEODYMIUM TITANATE 109   Formula  Nd3Ti4O12 Space group  Pcmn  Crystal size (mm)  0.12  ×  0.18  ×  0.12  a (Å) (at 23°C)  5.4203(8)  b (Å)  7.6104(9)  c (Å)  22.105(5)  V (Å)  908.1(3)  Z  4  Dc (g/cm3)  5.971  µ (MoKα, cm-1)  202.8  Minimum transmission  0.1079  Maximum transmission  0.2513  Scan type  ω/2θ  Reflections measured  2168  Independent reflections  2099  Observed reflections  1670  (I > 2.5σ (I))   Refined parameters  98  R  0.040  Rw (w = 1/σ 2 (Fo))  0.056  Extinction length (µm)  0.29(1) interatomic distances are listed in Table III. The framework of the structure is formed by  TiO6 octahedra, arranged in puckered single and double ReO3-type chains running in the direction of the b-axis. The Ti(1)O6-octahedra make up the single chains, and share only corners with the neighboring oc-tahedra. The Ti(2)O6-octahedra form the double chains  as  edge-sharing  pairs, which TABLE II ATOMIC POSITIONAL PARAMETERS AND ISOTROPIC TEMPERATURE FACTORS FOR Nd3Ti4O12   x   y   z  Biso (Å2) Nd(1)  0.93871(15)  0.25  0.19959(3)  0.464(20) Nd(2)  0.45551(14)  0.75  0.02782(3)  0.471(19) Nd(3)  0.92754(14)  0.75  0.17866(3)  0.394(18) Ti(1)  0.4390(3)  0.49921(20)  0.17863(7)  0.38(4)  Ti(2)  0.9516(3)  0.48936(21)  0.06143(7)  0.43(4)  O(1)  0.2465(14)  0.4624(9)  0.0122(3)  0.53(19)  O(2)  0.6351(14)  0.5593(10)  0.1147(3)  0.69(21)  O(3)  0.7090(13)  0.5367(9)  0.2378(3)  0.48(19)  O(4)  0.1223(13)  0.4708(9)  0.1421(3)  0.43(19)  O(5)  0.4976(22)  0.25  0.1869(5)  0.9(4)  O(6)  0.3578(22)  0.75  0.1954(5)  0.7(3)  O(7)  0.8410(23)  0.25  0.0642(5)  0.8(3)  O(8)  0.0321(21)  0.75  0.0699(4)  0.5(3)   FIG. 1. Polyhedral representation of the structure of Nd3Ti4O12 , projected along the b-axis. Ti–O polyhedra are hatched; Nd-atoms are represented as circles. share corners in the ac-plane with the Ti(1)-octahedra. The structure can be described as a regular intergrowth of two known struc-ture types: the GdFeO3-structure, which is an orthorhombic distortion of the perovskite structure (MO6-octahedra that are corner-sharing in three dimensions) (9) and the CaTa2O6-structure, with M2O10-groups of edge-sharing octahedra linked together by corner sharing (10). All three independent Nd-atoms are located in eight-coordinate distorted bicapped-trigonal prismatic sites, as is also observed for the A-cation in the GdFeO3 and CaTa2O6 structure types. The Nd(2)-atom, located between the puckered double  ReO3-type  chains,   has   a   slightly TABLE III SELECTED INTERATOMIC DISTANCES (Å) FOR Nd3Ti4O12 WITH ESTIMATED STANDARD DEVIATIONS IN PARENTHESES  Ti(1)  –O(2)  1.822(7)   Ti(2)  –O(1)  1.942(7)    –O(3)  1.980(7)     1.979(7)     2.239(7)    –O(2)  2.145(7)    –O(4)  1.908(7)    –O(4)  2.007(7)    –O(5)  1.932(3)    –O(7)  1.919(4)    –O(6)  1.993(4)    –O(8)  2.040(3)   Nd(1)  –O(3)  2.585 (7)  2×  Nd(2)  –O(1)  2.449(7)  2×    2.649(7)  2×    2.489(7)  2×   –O(4)  2.328(7)  2×   –O(2)  2.591(7)  2×   –O(5)  2.41(1)    –O(7)  2.59(1)    –O(6)  2.35(1)    –O(8)  2.47(1)   Nd(3)  –O(2)  2.570(8)  2×  Ti(2)  –Ti(2)  2.760(3)    –O(3)  2.395(7)  2×       –O(4)  2.506(7)  2×       –O(6)  2.36(1)        –O(8)  2.461(9)       TABLE I CRYSTALLOGRAPHIC DATA FOR Nd3Ti4O12  110 HESSEN ET AL.  FIG. 2. ORTEP representation of the Ti2O10 group in Nd3Ti4O12 . Thermal ellipsoids represented at the 50% probability level. larger and more regular coordination sphere than the other Nd-atoms (average Nd(2)–O  =  2.515 Å, with maximum deviations of –0.066 and +0.076 Å, for Nd(1) 2.486 Å and  +0.163/ –0.158 Å, for Nd(3) 2.470 Å and  +0.100/ –0.110 Å respectively. The average Ti–O distances in Nd3Ti4O12 are 1.979 Å for Ti(1) and 2.005 Å for Ti(2) respectively. The average Ti–O distance for Ti(1) in the GdFeO3 type block is signifi-cantly smaller than that observed in NdTiO3 (2.022 Å (1)). It is close to the value for Ti4 + –O in rutiles (1.97 Å). The coordination geometry around Ti(1) is also considerably more distorted, with sharp O–Ti–O angles ranging from 80.3° to 94.4° (in NdTiO3 from 89.0° to 90.3° (1)). One notable feature in the structure is the distortion in the Ti2O10-groups. The Ti(2)-atoms are displaced from the center of the octahedra towards each other rather than away from each other (as might be expected from Coulomb repul-sion). The geometry of this group is shown in Fig. 2. The Ti(2)–Ti(2) distance of 2.760(3) Å is quite short for a Ti–Ti distance in a pair of edge-shared octahedra. An interesting comparison can be made with the recently published crystal structure of the Ti(4 + ) compound Nd4Ti9O24 in which edge-shared Ti2O10-pairs are also present (11). The average Ti–O distance in these groups are smaller than for Ti(2) in Nd3Ti4O12 , and typical for Ti(4 + ) (1.973/ 1.978  Å).  However,  the  Ti–Ti  distance  in the pairs (2.963 Å) is 0.20 Å longer than in Nd3Ti4O12. The stoichiometry of Nd3Ti4O12 obtained from X-ray diffraction indicates an average Ti oxidation state of 3.75 + ; i.e., there is one 3d-electron per Ti2O10-pair. These 3d-electrons are possibly localized in orbitals associated with the Ti(2)–Ti(2) pair-ing. This would imply nonmetallic behavior of the material, at least at or below the tem-perature at which the structure was deter-mined (23°C). At high temperatures a first-order transition to a metallic state may be anticipated. A low-temperature four-point resistivity measurement on a single crystal showed nonmetallic behavior (Fig. 3) with a room-temperature resistivity of 35 Ω cm. Due to the small size of the crystal, only the conductivity along the b-axis could be measured. Anisotropy in the conductivity was expected due to the low symmetry of the crystal structure, but unfortunately could  not be measured. Degradation of the very small contacts made data collection un-reliable above room temperature. The ln(ρ) vs 1/T plot is nonlinear, suggesting that the material does not behave as an intrinsic semiconductor. The high-temperature data give an activation energy of 0.26 eV, but the fit is valid only over a rather small tempera-ture range. A second fit, based on a hopping mechanism for the conduction with a ρ ∝ ρ0  FIG. 3. Plot of log(resistivity) vs 1000/T from a dc resistivity measurement on a Nd3Ti4O12 single crystal.   INTERGROWTH IN NEODYMIUM TITANATE 111 exp(T0/T)1/4, gives a critical temperature of T0 = 1.24 × 106 K. This fit extends over a larger temperature range, but at low temper-ature, the deviations are very pronounced. Further experiments would be needed to elucidate the mechanism of conductivity. Discussion It is observed that recrystallization of ceramics with nominal composition Ln0.33Ti2O4 from Ln2O3 · 3B2O3 fluxes (Ln = La, Nd) leads to two crystalline prod-ucts: Ti2O3 and a ternary compound that is  only partially reduced. The difference in ionic radius between La and Nd is relatively small (1.32 Å for La3 + , 1.26 Å for Nd3 + in eight-coordination (12)). However, this difference, under otherwise identical condi-tions, causes a drastic change in the struc-ture of the ternary product, being an ordered defect perovskite for the La–Ti–O system and Nd3Ti4O12 with the intergrowth struc-ture as described above for the Nd–Ti–O system. This suggests a delicate dependence of the thermodynamics of these ternaries on the size of the A-cation. Also notable is the apparent absence of the La1.33Ti8O16 hol-landite phase that was obtained earlier from a more La-rich mixture (3). This can corre-spond with our findings in the Sr–Nb–O sys-tem that the occurrence of disproportiona-tion reactions in ternary borate flux media is  strongly dependent on the concentration of the common ion between the flux and the ternary transition metal oxide system (13). Although currently Nd3Ti4O12 is the only member of the ordered GdFeO3/CaTa2O6 in-tergrowth type synthesized, it is possible to predict the existence of a homologous series based on this intergrowth principle. The general formula of such a series would be Nd1+nTi2+nO6+3n , of which Nd3Ti4O12 is the n = 2 member. It is based on GdFeO3-blocks expanding in the c-direction between the edge-sharing  pairs  of  octahedra  of  the CaTa2O6 structure. The end members in the Nd–Ti–O system would be defined by n = 1 (Nd2Ti3O9 , Ti4+) and n = ∞ (NdTiO3 , Ti3+). The n = even and n = odd members differ in  the orientation of the edge-sharing pairs relative to each other. In the n = even mem-bers, sequential Ti2O10-blocks along the c-direction will be shifted over a/2, and canted in the opposite direction within the ac-plane (as in Nd3Ti4O12). In the n = odd members these blocks would not be shifted, and would have the same canting direction. Presently we are exploring the possibility of accessing members of this series by ceramic routes. Initial efforts were directed to pre-pare related Ti4 + ceramics, e.g., with com-position CaNd2Ti4O12 or Nd3Ti3AlO12 . As yet, these attempts have met with little suc-cess, yielding tetragonal defect perovskite type species as main products. References 1. D. A. MACLEAN, H.-N. NG, AND J. E. GREEDAN, J. Solid State Chem. 30, 35 (1979). 2. M. KESTIGAN AND R. WARD, J. Am. Chem. Soc. 77, 6199 (1955). 3. B. HESSEN, S. A. SUNSHINE, AND T. SIEGRIST, J. Solid State Chem. 94, 306 (1991). 4. S. N. RUDDLESDEN AND P. POPPER, Acta Crys-tallogr. 10, 538 (1957); Acta Crystallogr. 11, 54 (1958). 5. M. NANOT, F. QUEYROUX, J. C. GILLES, A. CARPY, AND J. GALY, J. Solid State Chem. 11, 272 (1974). 6. Y. LEPAGE, P. S. WHITE, AND E. J. GABE, “Pro-ceedings of the American Crystallographic Associ-ation Annual Meeting, 1986, Hamilton, Canada,” AIP, New York, (1986). [Poster PA23] 7. E. J. GABE, Y. LEPAGE, J. P. CHARLAND, F. L. LEE, AND P. S. WHITE, J. Appl. Crystallogr. 22, 384 (1989). 8. M. ABE AND K. UCHINO, Mater. Res. Bull. 9, 147 (1974). 9. S. GELLER, J. Chem. Phys. 24, 1236 (1956). 10. L. JAHNBERG, Acta Chem. Scand. 71, 2548 (1963). 11. N. HÜBNER AND R. GRUEHN, Z. Anorg. Allg. Chem. 616, 86 (1992). 12. R. D. SHANNON AND C. T. PREWITT, Acta Crys-tallogr. Sect. B 26, 1046 (1970). 13. B. HESSEN, S. A. SUNSHINE, T. SIEGRIST, AND R. JIMENEZ, Mater. Res. Bull. 26, 85 (1991). 

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   University of GroningenA Reduced Neodymium Titanate with a New Intergrowth Structure TypeHessen, B.; Sunshine, S.A.; Siegrist, T.; Dover, R.B. vanPublished in:Journal of Solid State ChemistryDOI:10.1006/jssc.1993.1198IMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite fromit. Please check the document version below.Document VersionPublisher's PDF, also known as Version of recordPublication date:1993Link to publication in University of Groningen/UMCG research databaseCitation for published version (APA):Hessen, B., Sunshine, S. A., Siegrist, T., & Dover, R. B. V. (1993). A Reduced Neodymium Titanate with aNew Intergrowth Structure Type. 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For technical reasons thenumber of authors shown on this cover page is limited to 10 maximum.Download date: 04-06-2022JOURNAL OF SOLID STATE CHEMISTRY 105, 107–111 (1993) A Reduced Neodymium Titanate with a New Intergrowth Structure Type B. HESSEN, S. A. SUNSHINE, T. SIEGRIST, AND R. B. VAN DOVER AT&T Bell Laboratories, 600 Mountain Avenue, Murray Hill, New Jersey 07974 Received November 23, 1992; in revised form March 22, 1993; accepted March 24, 1993 Single crystals of a new reduced neodymium titanate, Nd3Ti4O12 , have been obtained by slow cooling of a reduced Nd–Ti–O ceramic precursor in a molten neodymium borate flux under high vacuum. Single crystal X-ray diffraction (space group Pcmn, a = 5.420(1) Å, b = 7.610(1) Å, c = 22.015(5) Å, Z = 4, Pearson symbol oP76) showed that the structure of Nd3Ti4O12 is built from an arrangement of corner-sharing and edge-sharing TiO6-octahedra that can be considered as a regular intergrowth of the GdFeO3 and CaTa2O6 structure types. The material is nonmetallic (R(25°C) = 35 Ωcm) which may arise due to the localization of the Ti 3d-electrons in the edge-sharing pairs of TiO6-octahedra (short Ti–Ti distance of 2.760(3) Å).   © 1993 Academic Press. Inc. Introduction The Ti(III) rare earth ternary oxides LnTiO3 (Ln = La, Nd, Sm, Gd) are known to have a distorted orthorhombic perovskite structure of the GdFeO3-type (1). Apart from the defect perovskite series La2/3+x TiO3 (2), few other rare-earth containing re-duced ternary titanates are known. Recently we prepared a new reduced La–Ti–O com-pound, La1.33Ti8O16 with the hollandite structure, from a LaTiO3 ceramic and a lan-thanum borate flux (3). We also investigated the use of ceramic precursors with lower rare earth content. A significant difference in behavior between the La–Ti–O and Nd-Ti-O systems is observed in that case. Here we present the synthesis and crystal structure of a new reduced neodymium ti-tanate, Nd3Ti4O12 , with a structure derived from intergrowth of the GdFeO3 and CaTa2O6 structure types. Similar attempts to prepare a La–Ti–O analogue only pro-duced crystals of the La2/3+xTiO3 defect per-ovskite. Ordered intergrowth of two different structure types has been observed in a  num- ber of systems, frequently giving rise to ho-mologous series with varying sizes of the blocks corresponding to the two structure types. Well known examples are the Ruddlesden–Popper series, with SrTiO3 and SrO layer intergrowth (4), and the in-tergrowth of BaZnF4 and GdFeO3 struc-ture types in the Ca–Na–Nb–O and Ca-La-Ti-O systems (5). It is conceivable that intergrowth of the GdFeO3 and CaTa2O6 structure types can also give rise to a homologous series. Experimental Sample Preparation Ceramic titanate precursors with nominal composition “Ln0.33Ti2O4” were prepared from pressed pellets of Ln2O3 , Ti2O3 (Alfa), and TiO2 (Aldrich) in the appropriate stoi-chiometry (9 mmole Ti/pellet). As mineral-izer Ln2O3 · 3B2O3 was added in the molar ratio of Nd : Ti : B2O3 = 1.3 : 1.0 : 1.7. The pellets were wrapped in Mo-foil and heated in vacuo to 1200°C for 24 hr. X-ray powder diffraction   showed   that    they    consisted 107 0022-4596/93 $5.00 Copyright © 1993 by Academic Press, Inc. All rights of reproduction in any form reserved. 108 HESSEN ET AL. mainly of Ti2O3 and Ln2/3+xTiO3-type perov-skites. Borates Ln2O3 · 3B2O3 were prepared from La(OH)3 (Alfa) or Nd2O3 (Aldrich) with B2O3 (Fisher) in the appropriate ratio. The mixtures were heated at 950°C in alu-mina crucibles for 24 hr. Lanthanide sesqui-oxides were fired at 1000°C in air for 12 hr before use. Mixtures of ceramic precursors of nomi-nal composition “Ln1/3Ti2O4” (Ln = La, Nd) and Ln2O3 · 3B2O3 in 1 : 3 weight ratio were placed in Mo-foil cups with Mo-foil lids. They were heated in vacuo (in a Cent-orr high-vacuum furnace) to 400°C to re-move adsorbed water, and then to 1250°C (1 hr, 10–6–10–7 Torr). The mixtures were then cooled to 1140°C at 8°C/hr. The heater was subsequently turned off, allowing the sample to reach room temperature in ap-proximately 1.5 hr. The mixtures were soaked in warm concentrated aqueous KOH for 12 hr, after which the products were mechanically broken up. For Ln = Nd the product mixture consisted of dark blue crys-talline aggregates showing in places well-developed rectangular crystal faces, to-gether with small crystals of Ti2O3 and a pink transparent crystalline flux. The blue product was determined by single crystal X-ray diffraction on several cleaved crystal fragments to be a new orthorhombic Nd–titanate, Nd3Ti4O12 . For Ln = La, a dark crystalline mass with jagged edges but with locally well-developed crystals with square faces had formed, together with small crys-tals of Ti2O3 and a colorless crystalline flux. X-ray diffraction on crystals and cleaved crystal fragments showed the dark crystal-line product to be an ordered defect perov-skite of the type La2/3+xTiO3 . A dc four-point resistivity measurement was performed on a single crystal of Nd3Ti4O12 . Silver wires were connected (with Ag/epoxy) to evaporated Au–Ge/Au strip contacts on the crystal. Structure Determinations The crystals were measured at 23°C on an Enraf-Nonius  CAD-4  diffractometer  using graphite-monochromatized MoKα radiation and the NRCCAD program package (6). All calculations were carried out on an Alliant FX/80 computer, using the NRCVAX struc-ture package (7). Lattice parameters were determined from the absolute 2θ values of reflections at a high angle (Nd3Ti4O12: 30 reflections with 49° < 2θ < 58°, La1/3+xTiO3: 28 reflections with 50° < 2θ < 56°). The metal atom positions for Nd3Ti4O12 were ob-tained by direct methods. A Gaussian ab-sorption correction was applied. Aniso-tropic  temperature factors for all atoms were  included in the refinement. Crystallo-graphic data are listed in Table I, atomic coordinates and isotropic thermal parame-ters in Table II.* Crystals of La2/3+xTiO3 appeared to be primitive cubic with a = 7.7837(3) Å. It is likely that the material is actually a tetrago-nal defect perovskite with c = 2a as de-scribed earlier for La2/3TiO2.954 (8). This also corresponds well with powder X-ray diffrac-tion on crushed crystals. The doubling in all three dimensions as observed by single crystal X-ray diffraction is likely to be pro-duced by twinning. From the observed cell parameter and comparison with the data for La2/3+xTiO3 of Kestigan and Ward (2), the composition of the crystals can be derived by interpolation to be approximately La0.73TiO3(x ≈ 0.06). Description of the Structure The structure of Nd3Ti4O12 is shown in Fig. 1,  projected along the  b-axis.  Pertinent * See NAPS document No. 05020 for 23 pages of supplementary material. Order from ASIS/NAPS. Mi-crofiche Publications, P.O. Box 3513, Grand Central Station, New York, NY 10163. Remit in advance $4.00 for microfiche copy or for photocopy, $8.65 up to 20 pages plus $.30 for each additional page. All orders must be prepaid. Institutions and Organizations may order by purchase order. However, there is a billing and handling charge for this service of $15. Foreign orders add $4.50 for postage and handling, for the first 20 pages, and $1.00 for additional 10 pages of material, $1.75 for postage of any microfiche orders.   INTERGROWTH IN NEODYMIUM TITANATE 109   Formula  Nd3Ti4O12 Space group  Pcmn  Crystal size (mm)  0.12  ×  0.18  ×  0.12  a (Å) (at 23°C)  5.4203(8)  b (Å)  7.6104(9)  c (Å)  22.105(5)  V (Å)  908.1(3)  Z  4  Dc (g/cm3)  5.971  µ (MoKα, cm-1)  202.8  Minimum transmission  0.1079  Maximum transmission  0.2513  Scan type  ω/2θ  Reflections measured  2168  Independent reflections  2099  Observed reflections  1670  (I > 2.5σ (I))   Refined parameters  98  R  0.040  Rw (w = 1/σ 2 (Fo))  0.056  Extinction length (µm)  0.29(1) interatomic distances are listed in Table III. The framework of the structure is formed by  TiO6 octahedra, arranged in puckered single and double ReO3-type chains running in the direction of the b-axis. The Ti(1)O6-octahedra make up the single chains, and share only corners with the neighboring oc-tahedra. The Ti(2)O6-octahedra form the double chains  as  edge-sharing  pairs, which TABLE II ATOMIC POSITIONAL PARAMETERS AND ISOTROPIC TEMPERATURE FACTORS FOR Nd3Ti4O12   x   y   z  Biso (Å2) Nd(1)  0.93871(15)  0.25  0.19959(3)  0.464(20) Nd(2)  0.45551(14)  0.75  0.02782(3)  0.471(19) Nd(3)  0.92754(14)  0.75  0.17866(3)  0.394(18) Ti(1)  0.4390(3)  0.49921(20)  0.17863(7)  0.38(4)  Ti(2)  0.9516(3)  0.48936(21)  0.06143(7)  0.43(4)  O(1)  0.2465(14)  0.4624(9)  0.0122(3)  0.53(19)  O(2)  0.6351(14)  0.5593(10)  0.1147(3)  0.69(21)  O(3)  0.7090(13)  0.5367(9)  0.2378(3)  0.48(19)  O(4)  0.1223(13)  0.4708(9)  0.1421(3)  0.43(19)  O(5)  0.4976(22)  0.25  0.1869(5)  0.9(4)  O(6)  0.3578(22)  0.75  0.1954(5)  0.7(3)  O(7)  0.8410(23)  0.25  0.0642(5)  0.8(3)  O(8)  0.0321(21)  0.75  0.0699(4)  0.5(3)   FIG. 1. Polyhedral representation of the structure of Nd3Ti4O12 , projected along the b-axis. Ti–O polyhedra are hatched; Nd-atoms are represented as circles. share corners in the ac-plane with the Ti(1)-octahedra. The structure can be described as a regular intergrowth of two known struc-ture types: the GdFeO3-structure, which is an orthorhombic distortion of the perovskite structure (MO6-octahedra that are corner-sharing in three dimensions) (9) and the CaTa2O6-structure, with M2O10-groups of edge-sharing octahedra linked together by corner sharing (10). All three independent Nd-atoms are located in eight-coordinate distorted bicapped-trigonal prismatic sites, as is also observed for the A-cation in the GdFeO3 and CaTa2O6 structure types. The Nd(2)-atom, located between the puckered double  ReO3-type  chains,   has   a   slightly TABLE III SELECTED INTERATOMIC DISTANCES (Å) FOR Nd3Ti4O12 WITH ESTIMATED STANDARD DEVIATIONS IN PARENTHESES  Ti(1)  –O(2)  1.822(7)   Ti(2)  –O(1)  1.942(7)    –O(3)  1.980(7)     1.979(7)     2.239(7)    –O(2)  2.145(7)    –O(4)  1.908(7)    –O(4)  2.007(7)    –O(5)  1.932(3)    –O(7)  1.919(4)    –O(6)  1.993(4)    –O(8)  2.040(3)   Nd(1)  –O(3)  2.585 (7)  2×  Nd(2)  –O(1)  2.449(7)  2×    2.649(7)  2×    2.489(7)  2×   –O(4)  2.328(7)  2×   –O(2)  2.591(7)  2×   –O(5)  2.41(1)    –O(7)  2.59(1)    –O(6)  2.35(1)    –O(8)  2.47(1)   Nd(3)  –O(2)  2.570(8)  2×  Ti(2)  –Ti(2)  2.760(3)    –O(3)  2.395(7)  2×       –O(4)  2.506(7)  2×       –O(6)  2.36(1)        –O(8)  2.461(9)       TABLE I CRYSTALLOGRAPHIC DATA FOR Nd3Ti4O12  110 HESSEN ET AL.  FIG. 2. ORTEP representation of the Ti2O10 group in Nd3Ti4O12 . Thermal ellipsoids represented at the 50% probability level. larger and more regular coordination sphere than the other Nd-atoms (average Nd(2)–O  =  2.515 Å, with maximum deviations of –0.066 and +0.076 Å, for Nd(1) 2.486 Å and  +0.163/ –0.158 Å, for Nd(3) 2.470 Å and  +0.100/ –0.110 Å respectively. The average Ti–O distances in Nd3Ti4O12 are 1.979 Å for Ti(1) and 2.005 Å for Ti(2) respectively. The average Ti–O distance for Ti(1) in the GdFeO3 type block is signifi-cantly smaller than that observed in NdTiO3 (2.022 Å (1)). It is close to the value for Ti4 + –O in rutiles (1.97 Å). The coordination geometry around Ti(1) is also considerably more distorted, with sharp O–Ti–O angles ranging from 80.3° to 94.4° (in NdTiO3 from 89.0° to 90.3° (1)). One notable feature in the structure is the distortion in the Ti2O10-groups. The Ti(2)-atoms are displaced from the center of the octahedra towards each other rather than away from each other (as might be expected from Coulomb repul-sion). The geometry of this group is shown in Fig. 2. The Ti(2)–Ti(2) distance of 2.760(3) Å is quite short for a Ti–Ti distance in a pair of edge-shared octahedra. An interesting comparison can be made with the recently published crystal structure of the Ti(4 + ) compound Nd4Ti9O24 in which edge-shared Ti2O10-pairs are also present (11). The average Ti–O distance in these groups are smaller than for Ti(2) in Nd3Ti4O12 , and typical for Ti(4 + ) (1.973/ 1.978  Å).  However,  the  Ti–Ti  distance  in the pairs (2.963 Å) is 0.20 Å longer than in Nd3Ti4O12. The stoichiometry of Nd3Ti4O12 obtained from X-ray diffraction indicates an average Ti oxidation state of 3.75 + ; i.e., there is one 3d-electron per Ti2O10-pair. These 3d-electrons are possibly localized in orbitals associated with the Ti(2)–Ti(2) pair-ing. This would imply nonmetallic behavior of the material, at least at or below the tem-perature at which the structure was deter-mined (23°C). At high temperatures a first-order transition to a metallic state may be anticipated. A low-temperature four-point resistivity measurement on a single crystal showed nonmetallic behavior (Fig. 3) with a room-temperature resistivity of 35 Ω cm. Due to the small size of the crystal, only the conductivity along the b-axis could be measured. Anisotropy in the conductivity was expected due to the low symmetry of the crystal structure, but unfortunately could  not be measured. Degradation of the very small contacts made data collection un-reliable above room temperature. The ln(ρ) vs 1/T plot is nonlinear, suggesting that the material does not behave as an intrinsic semiconductor. The high-temperature data give an activation energy of 0.26 eV, but the fit is valid only over a rather small tempera-ture range. A second fit, based on a hopping mechanism for the conduction with a ρ ∝ ρ0  FIG. 3. Plot of log(resistivity) vs 1000/T from a dc resistivity measurement on a Nd3Ti4O12 single crystal.   INTERGROWTH IN NEODYMIUM TITANATE 111 exp(T0/T)1/4, gives a critical temperature of T0 = 1.24 × 106 K. This fit extends over a larger temperature range, but at low temper-ature, the deviations are very pronounced. Further experiments would be needed to elucidate the mechanism of conductivity. Discussion It is observed that recrystallization of ceramics with nominal composition Ln0.33Ti2O4 from Ln2O3 · 3B2O3 fluxes (Ln = La, Nd) leads to two crystalline prod-ucts: Ti2O3 and a ternary compound that is  only partially reduced. The difference in ionic radius between La and Nd is relatively small (1.32 Å for La3 + , 1.26 Å for Nd3 + in eight-coordination (12)). However, this difference, under otherwise identical condi-tions, causes a drastic change in the struc-ture of the ternary product, being an ordered defect perovskite for the La–Ti–O system and Nd3Ti4O12 with the intergrowth struc-ture as described above for the Nd–Ti–O system. This suggests a delicate dependence of the thermodynamics of these ternaries on the size of the A-cation. Also notable is the apparent absence of the La1.33Ti8O16 hol-landite phase that was obtained earlier from a more La-rich mixture (3). This can corre-spond with our findings in the Sr–Nb–O sys-tem that the occurrence of disproportiona-tion reactions in ternary borate flux media is  strongly dependent on the concentration of the common ion between the flux and the ternary transition metal oxide system (13). Although currently Nd3Ti4O12 is the only member of the ordered GdFeO3/CaTa2O6 in-tergrowth type synthesized, it is possible to predict the existence of a homologous series based on this intergrowth principle. The general formula of such a series would be Nd1+nTi2+nO6+3n , of which Nd3Ti4O12 is the n = 2 member. It is based on GdFeO3-blocks expanding in the c-direction between the edge-sharing  pairs  of  octahedra  of  the CaTa2O6 structure. The end members in the Nd–Ti–O system would be defined by n = 1 (Nd2Ti3O9 , Ti4+) and n = ∞ (NdTiO3 , Ti3+). The n = even and n = odd members differ in  the orientation of the edge-sharing pairs relative to each other. In the n = even mem-bers, sequential Ti2O10-blocks along the c-direction will be shifted over a/2, and canted in the opposite direction within the ac-plane (as in Nd3Ti4O12). In the n = odd members these blocks would not be shifted, and would have the same canting direction. Presently we are exploring the possibility of accessing members of this series by ceramic routes. Initial efforts were directed to pre-pare related Ti4 + ceramics, e.g., with com-position CaNd2Ti4O12 or Nd3Ti3AlO12 . As yet, these attempts have met with little suc-cess, yielding tetragonal defect perovskite type species as main products. References 1. D. A. MACLEAN, H.-N. NG, AND J. E. GREEDAN, J. Solid State Chem. 30, 35 (1979). 2. M. KESTIGAN AND R. WARD, J. Am. Chem. Soc. 77, 6199 (1955). 3. B. HESSEN, S. A. SUNSHINE, AND T. SIEGRIST, J. Solid State Chem. 94, 306 (1991). 4. S. N. RUDDLESDEN AND P. POPPER, Acta Crys-tallogr. 10, 538 (1957); Acta Crystallogr. 11, 54 (1958). 5. M. NANOT, F. QUEYROUX, J. C. GILLES, A. CARPY, AND J. GALY, J. Solid State Chem. 11, 272 (1974). 6. Y. LEPAGE, P. S. WHITE, AND E. J. GABE, “Pro-ceedings of the American Crystallographic Associ-ation Annual Meeting, 1986, Hamilton, Canada,” AIP, New York, (1986). [Poster PA23] 7. E. J. GABE, Y. LEPAGE, J. P. CHARLAND, F. L. LEE, AND P. S. WHITE, J. Appl. Crystallogr. 22, 384 (1989). 8. M. ABE AND K. UCHINO, Mater. Res. Bull. 9, 147 (1974). 9. S. GELLER, J. Chem. Phys. 24, 1236 (1956). 10. L. JAHNBERG, Acta Chem. Scand. 71, 2548 (1963). 11. N. HÜBNER AND R. GRUEHN, Z. Anorg. Allg. Chem. 616, 86 (1992). 12. R. D. SHANNON AND C. T. PREWITT, Acta Crys-tallogr. Sect. B 26, 1046 (1970). 13. B. HESSEN, S. A. SUNSHINE, T. SIEGRIST, AND R. JIMENEZ, Mater. Res. Bull. 26, 85 (1991). 

   University of GroningenA Reduced Neodymium Titanate with a New Intergrowth Structure TypeHessen, B.; Sunshine, S.A.; Siegrist, T.; Dover, R.B. vanPublished in:Journal of Solid State ChemistryDOI:10.1006/jssc.1993.1198IMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite fromit. Please check the document version below.Document VersionPublisher's PDF, also known as Version of recordPublication date:1993Link to publication in University of Groningen/UMCG research databaseCitation for published version (APA):Hessen, B., Sunshine, S. A., Siegrist, T., & Dover, R. B. V. (1993). A Reduced Neodymium Titanate with aNew Intergrowth Structure Type. Journal of Solid State Chemistry, 105(1).https://doi.org/10.1006/jssc.1993.1198CopyrightOther than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of theauthor(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons).The publication may also be distributed here under the terms of Article 25fa of the Dutch Copyright Act, indicated by the “Taverne” license.More information can be found on the University of Groningen website: https://www.rug.nl/library/open-access/self-archiving-pure/taverne-amendment.Take-down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediatelyand investigate your claim.Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons thenumber of authors shown on this cover page is limited to 10 maximum.Download date: 30-10-2023JOURNAL OF SOLID STATE CHEMISTRY 105, 107–111 (1993) A Reduced Neodymium Titanate with a New Intergrowth Structure Type B. HESSEN, S. A. SUNSHINE, T. SIEGRIST, AND R. B. VAN DOVER AT&T Bell Laboratories, 600 Mountain Avenue, Murray Hill, New Jersey 07974 Received November 23, 1992; in revised form March 22, 1993; accepted March 24, 1993 Single crystals of a new reduced neodymium titanate, Nd3Ti4O12 , have been obtained by slow cooling of a reduced Nd–Ti–O ceramic precursor in a molten neodymium borate flux under high vacuum. Single crystal X-ray diffraction (space group Pcmn, a = 5.420(1) Å, b = 7.610(1) Å, c = 22.015(5) Å, Z = 4, Pearson symbol oP76) showed that the structure of Nd3Ti4O12 is built from an arrangement of corner-sharing and edge-sharing TiO6-octahedra that can be considered as a regular intergrowth of the GdFeO3 and CaTa2O6 structure types. The material is nonmetallic (R(25°C) = 35 Ωcm) which may arise due to the localization of the Ti 3d-electrons in the edge-sharing pairs of TiO6-octahedra (short Ti–Ti distance of 2.760(3) Å).   © 1993 Academic Press. Inc. Introduction The Ti(III) rare earth ternary oxides LnTiO3 (Ln = La, Nd, Sm, Gd) are known to have a distorted orthorhombic perovskite structure of the GdFeO3-type (1). Apart from the defect perovskite series La2/3+x TiO3 (2), few other rare-earth containing re-duced ternary titanates are known. Recently we prepared a new reduced La–Ti–O com-pound, La1.33Ti8O16 with the hollandite structure, from a LaTiO3 ceramic and a lan-thanum borate flux (3). We also investigated the use of ceramic precursors with lower rare earth content. A significant difference in behavior between the La–Ti–O and Nd-Ti-O systems is observed in that case. Here we present the synthesis and crystal structure of a new reduced neodymium ti-tanate, Nd3Ti4O12 , with a structure derived from intergrowth of the GdFeO3 and CaTa2O6 structure types. Similar attempts to prepare a La–Ti–O analogue only pro-duced crystals of the La2/3+xTiO3 defect per-ovskite. Ordered intergrowth of two different structure types has been observed in a  num- ber of systems, frequently giving rise to ho-mologous series with varying sizes of the blocks corresponding to the two structure types. Well known examples are the Ruddlesden–Popper series, with SrTiO3 and SrO layer intergrowth (4), and the in-tergrowth of BaZnF4 and GdFeO3 struc-ture types in the Ca–Na–Nb–O and Ca-La-Ti-O systems (5). It is conceivable that intergrowth of the GdFeO3 and CaTa2O6 structure types can also give rise to a homologous series. Experimental Sample Preparation Ceramic titanate precursors with nominal composition “Ln0.33Ti2O4” were prepared from pressed pellets of Ln2O3 , Ti2O3 (Alfa), and TiO2 (Aldrich) in the appropriate stoi-chiometry (9 mmole Ti/pellet). As mineral-izer Ln2O3 · 3B2O3 was added in the molar ratio of Nd : Ti : B2O3 = 1.3 : 1.0 : 1.7. The pellets were wrapped in Mo-foil and heated in vacuo to 1200°C for 24 hr. X-ray powder diffraction   showed   that    they    consisted 107 0022-4596/93 $5.00 Copyright © 1993 by Academic Press, Inc. All rights of reproduction in any form reserved. 108 HESSEN ET AL. mainly of Ti2O3 and Ln2/3+xTiO3-type perov-skites. Borates Ln2O3 · 3B2O3 were prepared from La(OH)3 (Alfa) or Nd2O3 (Aldrich) with B2O3 (Fisher) in the appropriate ratio. The mixtures were heated at 950°C in alu-mina crucibles for 24 hr. Lanthanide sesqui-oxides were fired at 1000°C in air for 12 hr before use. Mixtures of ceramic precursors of nomi-nal composition “Ln1/3Ti2O4” (Ln = La, Nd) and Ln2O3 · 3B2O3 in 1 : 3 weight ratio were placed in Mo-foil cups with Mo-foil lids. They were heated in vacuo (in a Cent-orr high-vacuum furnace) to 400°C to re-move adsorbed water, and then to 1250°C (1 hr, 10–6–10–7 Torr). The mixtures were then cooled to 1140°C at 8°C/hr. The heater was subsequently turned off, allowing the sample to reach room temperature in ap-proximately 1.5 hr. The mixtures were soaked in warm concentrated aqueous KOH for 12 hr, after which the products were mechanically broken up. For Ln = Nd the product mixture consisted of dark blue crys-talline aggregates showing in places well-developed rectangular crystal faces, to-gether with small crystals of Ti2O3 and a pink transparent crystalline flux. The blue product was determined by single crystal X-ray diffraction on several cleaved crystal fragments to be a new orthorhombic Nd–titanate, Nd3Ti4O12 . For Ln = La, a dark crystalline mass with jagged edges but with locally well-developed crystals with square faces had formed, together with small crys-tals of Ti2O3 and a colorless crystalline flux. X-ray diffraction on crystals and cleaved crystal fragments showed the dark crystal-line product to be an ordered defect perov-skite of the type La2/3+xTiO3 . A dc four-point resistivity measurement was performed on a single crystal of Nd3Ti4O12 . Silver wires were connected (with Ag/epoxy) to evaporated Au–Ge/Au strip contacts on the crystal. Structure Determinations The crystals were measured at 23°C on an Enraf-Nonius  CAD-4  diffractometer  using graphite-monochromatized MoKα radiation and the NRCCAD program package (6). All calculations were carried out on an Alliant FX/80 computer, using the NRCVAX struc-ture package (7). Lattice parameters were determined from the absolute 2θ values of reflections at a high angle (Nd3Ti4O12: 30 reflections with 49° < 2θ < 58°, La1/3+xTiO3: 28 reflections with 50° < 2θ < 56°). The metal atom positions for Nd3Ti4O12 were ob-tained by direct methods. A Gaussian ab-sorption correction was applied. Aniso-tropic  temperature factors for all atoms were  included in the refinement. Crystallo-graphic data are listed in Table I, atomic coordinates and isotropic thermal parame-ters in Table II.* Crystals of La2/3+xTiO3 appeared to be primitive cubic with a = 7.7837(3) Å. It is likely that the material is actually a tetrago-nal defect perovskite with c = 2a as de-scribed earlier for La2/3TiO2.954 (8). This also corresponds well with powder X-ray diffrac-tion on crushed crystals. The doubling in all three dimensions as observed by single crystal X-ray diffraction is likely to be pro-duced by twinning. From the observed cell parameter and comparison with the data for La2/3+xTiO3 of Kestigan and Ward (2), the composition of the crystals can be derived by interpolation to be approximately La0.73TiO3(x ≈ 0.06). Description of the Structure The structure of Nd3Ti4O12 is shown in Fig. 1,  projected along the  b-axis.  Pertinent * See NAPS document No. 05020 for 23 pages of supplementary material. Order from ASIS/NAPS. Mi-crofiche Publications, P.O. Box 3513, Grand Central Station, New York, NY 10163. Remit in advance $4.00 for microfiche copy or for photocopy, $8.65 up to 20 pages plus $.30 for each additional page. All orders must be prepaid. Institutions and Organizations may order by purchase order. However, there is a billing and handling charge for this service of $15. Foreign orders add $4.50 for postage and handling, for the first 20 pages, and $1.00 for additional 10 pages of material, $1.75 for postage of any microfiche orders.   INTERGROWTH IN NEODYMIUM TITANATE 109   Formula  Nd3Ti4O12 Space group  Pcmn  Crystal size (mm)  0.12  ×  0.18  ×  0.12  a (Å) (at 23°C)  5.4203(8)  b (Å)  7.6104(9)  c (Å)  22.105(5)  V (Å)  908.1(3)  Z  4  Dc (g/cm3)  5.971  µ (MoKα, cm-1)  202.8  Minimum transmission  0.1079  Maximum transmission  0.2513  Scan type  ω/2θ  Reflections measured  2168  Independent reflections  2099  Observed reflections  1670  (I > 2.5σ (I))   Refined parameters  98  R  0.040  Rw (w = 1/σ 2 (Fo))  0.056  Extinction length (µm)  0.29(1) interatomic distances are listed in Table III. The framework of the structure is formed by  TiO6 octahedra, arranged in puckered single and double ReO3-type chains running in the direction of the b-axis. The Ti(1)O6-octahedra make up the single chains, and share only corners with the neighboring oc-tahedra. The Ti(2)O6-octahedra form the double chains  as  edge-sharing  pairs, which TABLE II ATOMIC POSITIONAL PARAMETERS AND ISOTROPIC TEMPERATURE FACTORS FOR Nd3Ti4O12   x   y   z  Biso (Å2) Nd(1)  0.93871(15)  0.25  0.19959(3)  0.464(20) Nd(2)  0.45551(14)  0.75  0.02782(3)  0.471(19) Nd(3)  0.92754(14)  0.75  0.17866(3)  0.394(18) Ti(1)  0.4390(3)  0.49921(20)  0.17863(7)  0.38(4)  Ti(2)  0.9516(3)  0.48936(21)  0.06143(7)  0.43(4)  O(1)  0.2465(14)  0.4624(9)  0.0122(3)  0.53(19)  O(2)  0.6351(14)  0.5593(10)  0.1147(3)  0.69(21)  O(3)  0.7090(13)  0.5367(9)  0.2378(3)  0.48(19)  O(4)  0.1223(13)  0.4708(9)  0.1421(3)  0.43(19)  O(5)  0.4976(22)  0.25  0.1869(5)  0.9(4)  O(6)  0.3578(22)  0.75  0.1954(5)  0.7(3)  O(7)  0.8410(23)  0.25  0.0642(5)  0.8(3)  O(8)  0.0321(21)  0.75  0.0699(4)  0.5(3)   FIG. 1. Polyhedral representation of the structure of Nd3Ti4O12 , projected along the b-axis. Ti–O polyhedra are hatched; Nd-atoms are represented as circles. share corners in the ac-plane with the Ti(1)-octahedra. The structure can be described as a regular intergrowth of two known struc-ture types: the GdFeO3-structure, which is an orthorhombic distortion of the perovskite structure (MO6-octahedra that are corner-sharing in three dimensions) (9) and the CaTa2O6-structure, with M2O10-groups of edge-sharing octahedra linked together by corner sharing (10). All three independent Nd-atoms are located in eight-coordinate distorted bicapped-trigonal prismatic sites, as is also observed for the A-cation in the GdFeO3 and CaTa2O6 structure types. The Nd(2)-atom, located between the puckered double  ReO3-type  chains,   has   a   slightly TABLE III SELECTED INTERATOMIC DISTANCES (Å) FOR Nd3Ti4O12 WITH ESTIMATED STANDARD DEVIATIONS IN PARENTHESES  Ti(1)  –O(2)  1.822(7)   Ti(2)  –O(1)  1.942(7)    –O(3)  1.980(7)     1.979(7)     2.239(7)    –O(2)  2.145(7)    –O(4)  1.908(7)    –O(4)  2.007(7)    –O(5)  1.932(3)    –O(7)  1.919(4)    –O(6)  1.993(4)    –O(8)  2.040(3)   Nd(1)  –O(3)  2.585 (7)  2×  Nd(2)  –O(1)  2.449(7)  2×    2.649(7)  2×    2.489(7)  2×   –O(4)  2.328(7)  2×   –O(2)  2.591(7)  2×   –O(5)  2.41(1)    –O(7)  2.59(1)    –O(6)  2.35(1)    –O(8)  2.47(1)   Nd(3)  –O(2)  2.570(8)  2×  Ti(2)  –Ti(2)  2.760(3)    –O(3)  2.395(7)  2×       –O(4)  2.506(7)  2×       –O(6)  2.36(1)        –O(8)  2.461(9)       TABLE I CRYSTALLOGRAPHIC DATA FOR Nd3Ti4O12  110 HESSEN ET AL.  FIG. 2. ORTEP representation of the Ti2O10 group in Nd3Ti4O12 . Thermal ellipsoids represented at the 50% probability level. larger and more regular coordination sphere than the other Nd-atoms (average Nd(2)–O  =  2.515 Å, with maximum deviations of –0.066 and +0.076 Å, for Nd(1) 2.486 Å and  +0.163/ –0.158 Å, for Nd(3) 2.470 Å and  +0.100/ –0.110 Å respectively. The average Ti–O distances in Nd3Ti4O12 are 1.979 Å for Ti(1) and 2.005 Å for Ti(2) respectively. The average Ti–O distance for Ti(1) in the GdFeO3 type block is signifi-cantly smaller than that observed in NdTiO3 (2.022 Å (1)). It is close to the value for Ti4 + –O in rutiles (1.97 Å). The coordination geometry around Ti(1) is also considerably more distorted, with sharp O–Ti–O angles ranging from 80.3° to 94.4° (in NdTiO3 from 89.0° to 90.3° (1)). One notable feature in the structure is the distortion in the Ti2O10-groups. The Ti(2)-atoms are displaced from the center of the octahedra towards each other rather than away from each other (as might be expected from Coulomb repul-sion). The geometry of this group is shown in Fig. 2. The Ti(2)–Ti(2) distance of 2.760(3) Å is quite short for a Ti–Ti distance in a pair of edge-shared octahedra. An interesting comparison can be made with the recently published crystal structure of the Ti(4 + ) compound Nd4Ti9O24 in which edge-shared Ti2O10-pairs are also present (11). The average Ti–O distance in these groups are smaller than for Ti(2) in Nd3Ti4O12 , and typical for Ti(4 + ) (1.973/ 1.978  Å).  However,  the  Ti–Ti  distance  in the pairs (2.963 Å) is 0.20 Å longer than in Nd3Ti4O12. The stoichiometry of Nd3Ti4O12 obtained from X-ray diffraction indicates an average Ti oxidation state of 3.75 + ; i.e., there is one 3d-electron per Ti2O10-pair. These 3d-electrons are possibly localized in orbitals associated with the Ti(2)–Ti(2) pair-ing. This would imply nonmetallic behavior of the material, at least at or below the tem-perature at which the structure was deter-mined (23°C). At high temperatures a first-order transition to a metallic state may be anticipated. A low-temperature four-point resistivity measurement on a single crystal showed nonmetallic behavior (Fig. 3) with a room-temperature resistivity of 35 Ω cm. Due to the small size of the crystal, only the conductivity along the b-axis could be measured. Anisotropy in the conductivity was expected due to the low symmetry of the crystal structure, but unfortunately could  not be measured. Degradation of the very small contacts made data collection un-reliable above room temperature. The ln(ρ) vs 1/T plot is nonlinear, suggesting that the material does not behave as an intrinsic semiconductor. The high-temperature data give an activation energy of 0.26 eV, but the fit is valid only over a rather small tempera-ture range. A second fit, based on a hopping mechanism for the conduction with a ρ ∝ ρ0  FIG. 3. Plot of log(resistivity) vs 1000/T from a dc resistivity measurement on a Nd3Ti4O12 single crystal.   INTERGROWTH IN NEODYMIUM TITANATE 111 exp(T0/T)1/4, gives a critical temperature of T0 = 1.24 × 106 K. This fit extends over a larger temperature range, but at low temper-ature, the deviations are very pronounced. Further experiments would be needed to elucidate the mechanism of conductivity. Discussion It is observed that recrystallization of ceramics with nominal composition Ln0.33Ti2O4 from Ln2O3 · 3B2O3 fluxes (Ln = La, Nd) leads to two crystalline prod-ucts: Ti2O3 and a ternary compound that is  only partially reduced. The difference in ionic radius between La and Nd is relatively small (1.32 Å for La3 + , 1.26 Å for Nd3 + in eight-coordination (12)). However, this difference, under otherwise identical condi-tions, causes a drastic change in the struc-ture of the ternary product, being an ordered defect perovskite for the La–Ti–O system and Nd3Ti4O12 with the intergrowth struc-ture as described above for the Nd–Ti–O system. This suggests a delicate dependence of the thermodynamics of these ternaries on the size of the A-cation. Also notable is the apparent absence of the La1.33Ti8O16 hol-landite phase that was obtained earlier from a more La-rich mixture (3). This can corre-spond with our findings in the Sr–Nb–O sys-tem that the occurrence of disproportiona-tion reactions in ternary borate flux media is  strongly dependent on the concentration of the common ion between the flux and the ternary transition metal oxide system (13). Although currently Nd3Ti4O12 is the only member of the ordered GdFeO3/CaTa2O6 in-tergrowth type synthesized, it is possible to predict the existence of a homologous series based on this intergrowth principle. The general formula of such a series would be Nd1+nTi2+nO6+3n , of which Nd3Ti4O12 is the n = 2 member. It is based on GdFeO3-blocks expanding in the c-direction between the edge-sharing  pairs  of  octahedra  of  the CaTa2O6 structure. The end members in the Nd–Ti–O system would be defined by n = 1 (Nd2Ti3O9 , Ti4+) and n = ∞ (NdTiO3 , Ti3+). The n = even and n = odd members differ in  the orientation of the edge-sharing pairs relative to each other. In the n = even mem-bers, sequential Ti2O10-blocks along the c-direction will be shifted over a/2, and canted in the opposite direction within the ac-plane (as in Nd3Ti4O12). In the n = odd members these blocks would not be shifted, and would have the same canting direction. Presently we are exploring the possibility of accessing members of this series by ceramic routes. Initial efforts were directed to pre-pare related Ti4 + ceramics, e.g., with com-position CaNd2Ti4O12 or Nd3Ti3AlO12 . As yet, these attempts have met with little suc-cess, yielding tetragonal defect perovskite type species as main products. References 1. D. A. MACLEAN, H.-N. NG, AND J. E. GREEDAN, J. Solid State Chem. 30, 35 (1979). 2. M. KESTIGAN AND R. WARD, J. Am. Chem. Soc. 77, 6199 (1955). 3. B. HESSEN, S. A. SUNSHINE, AND T. SIEGRIST, J. Solid State Chem. 94, 306 (1991). 4. S. N. RUDDLESDEN AND P. POPPER, Acta Crys-tallogr. 10, 538 (1957); Acta Crystallogr. 11, 54 (1958). 5. M. NANOT, F. QUEYROUX, J. C. GILLES, A. CARPY, AND J. GALY, J. Solid State Chem. 11, 272 (1974). 6. Y. LEPAGE, P. S. WHITE, AND E. J. GABE, “Pro-ceedings of the American Crystallographic Associ-ation Annual Meeting, 1986, Hamilton, Canada,” AIP, New York, (1986). [Poster PA23] 7. E. J. GABE, Y. LEPAGE, J. P. CHARLAND, F. L. LEE, AND P. S. WHITE, J. Appl. Crystallogr. 22, 384 (1989). 8. M. ABE AND K. UCHINO, Mater. Res. Bull. 9, 147 (1974). 9. S. GELLER, J. Chem. Phys. 24, 1236 (1956). 10. L. JAHNBERG, Acta Chem. Scand. 71, 2548 (1963). 11. N. HÜBNER AND R. GRUEHN, Z. Anorg. Allg. Chem. 616, 86 (1992). 12. R. D. SHANNON AND C. T. PREWITT, Acta Crys-tallogr. Sect. B 26, 1046 (1970). 13. B. HESSEN, S. A. SUNSHINE, T. SIEGRIST, AND R. JIMENEZ, Mater. Res. Bull. 26, 85 (1991). 

   University of GroningenA Reduced Neodymium Titanate with a New Intergrowth Structure TypeHessen, B.; Sunshine, S.A.; Siegrist, T.; Dover, R.B. vanPublished in:Journal of Solid State ChemistryDOI:10.1006/jssc.1993.1198IMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite fromit. Please check the document version below.Document VersionPublisher's PDF, also known as Version of recordPublication date:1993Link to publication in University of Groningen/UMCG research databaseCitation for published version (APA):Hessen, B., Sunshine, S. A., Siegrist, T., & Dover, R. B. V. (1993). A Reduced Neodymium Titanate with aNew Intergrowth Structure Type. Journal of Solid State Chemistry, 105(1).https://doi.org/10.1006/jssc.1993.1198CopyrightOther than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of theauthor(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons).The publication may also be distributed here under the terms of Article 25fa of the Dutch Copyright Act, indicated by the “Taverne” license.More information can be found on the University of Groningen website: https://www.rug.nl/library/open-access/self-archiving-pure/taverne-amendment.Take-down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediatelyand investigate your claim.Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons thenumber of authors shown on this cover page is limited to 10 maximum.Download date: 27-04-2023JOURNAL OF SOLID STATE CHEMISTRY 105, 107–111 (1993) A Reduced Neodymium Titanate with a New Intergrowth Structure Type B. HESSEN, S. A. SUNSHINE, T. SIEGRIST, AND R. B. VAN DOVER AT&T Bell Laboratories, 600 Mountain Avenue, Murray Hill, New Jersey 07974 Received November 23, 1992; in revised form March 22, 1993; accepted March 24, 1993 Single crystals of a new reduced neodymium titanate, Nd3Ti4O12 , have been obtained by slow cooling of a reduced Nd–Ti–O ceramic precursor in a molten neodymium borate flux under high vacuum. Single crystal X-ray diffraction (space group Pcmn, a = 5.420(1) Å, b = 7.610(1) Å, c = 22.015(5) Å, Z = 4, Pearson symbol oP76) showed that the structure of Nd3Ti4O12 is built from an arrangement of corner-sharing and edge-sharing TiO6-octahedra that can be considered as a regular intergrowth of the GdFeO3 and CaTa2O6 structure types. The material is nonmetallic (R(25°C) = 35 Ωcm) which may arise due to the localization of the Ti 3d-electrons in the edge-sharing pairs of TiO6-octahedra (short Ti–Ti distance of 2.760(3) Å).   © 1993 Academic Press. Inc. Introduction The Ti(III) rare earth ternary oxides LnTiO3 (Ln = La, Nd, Sm, Gd) are known to have a distorted orthorhombic perovskite structure of the GdFeO3-type (1). Apart from the defect perovskite series La2/3+x TiO3 (2), few other rare-earth containing re-duced ternary titanates are known. Recently we prepared a new reduced La–Ti–O com-pound, La1.33Ti8O16 with the hollandite structure, from a LaTiO3 ceramic and a lan-thanum borate flux (3). We also investigated the use of ceramic precursors with lower rare earth content. A significant difference in behavior between the La–Ti–O and Nd-Ti-O systems is observed in that case. Here we present the synthesis and crystal structure of a new reduced neodymium ti-tanate, Nd3Ti4O12 , with a structure derived from intergrowth of the GdFeO3 and CaTa2O6 structure types. Similar attempts to prepare a La–Ti–O analogue only pro-duced crystals of the La2/3+xTiO3 defect per-ovskite. Ordered intergrowth of two different structure types has been observed in a  num- ber of systems, frequently giving rise to ho-mologous series with varying sizes of the blocks corresponding to the two structure types. Well known examples are the Ruddlesden–Popper series, with SrTiO3 and SrO layer intergrowth (4), and the in-tergrowth of BaZnF4 and GdFeO3 struc-ture types in the Ca–Na–Nb–O and Ca-La-Ti-O systems (5). It is conceivable that intergrowth of the GdFeO3 and CaTa2O6 structure types can also give rise to a homologous series. Experimental Sample Preparation Ceramic titanate precursors with nominal composition “Ln0.33Ti2O4” were prepared from pressed pellets of Ln2O3 , Ti2O3 (Alfa), and TiO2 (Aldrich) in the appropriate stoi-chiometry (9 mmole Ti/pellet). As mineral-izer Ln2O3 · 3B2O3 was added in the molar ratio of Nd : Ti : B2O3 = 1.3 : 1.0 : 1.7. The pellets were wrapped in Mo-foil and heated in vacuo to 1200°C for 24 hr. X-ray powder diffraction   showed   that    they    consisted 107 0022-4596/93 $5.00 Copyright © 1993 by Academic Press, Inc. All rights of reproduction in any form reserved. 108 HESSEN ET AL. mainly of Ti2O3 and Ln2/3+xTiO3-type perov-skites. Borates Ln2O3 · 3B2O3 were prepared from La(OH)3 (Alfa) or Nd2O3 (Aldrich) with B2O3 (Fisher) in the appropriate ratio. The mixtures were heated at 950°C in alu-mina crucibles for 24 hr. Lanthanide sesqui-oxides were fired at 1000°C in air for 12 hr before use. Mixtures of ceramic precursors of nomi-nal composition “Ln1/3Ti2O4” (Ln = La, Nd) and Ln2O3 · 3B2O3 in 1 : 3 weight ratio were placed in Mo-foil cups with Mo-foil lids. They were heated in vacuo (in a Cent-orr high-vacuum furnace) to 400°C to re-move adsorbed water, and then to 1250°C (1 hr, 10–6–10–7 Torr). The mixtures were then cooled to 1140°C at 8°C/hr. The heater was subsequently turned off, allowing the sample to reach room temperature in ap-proximately 1.5 hr. The mixtures were soaked in warm concentrated aqueous KOH for 12 hr, after which the products were mechanically broken up. For Ln = Nd the product mixture consisted of dark blue crys-talline aggregates showing in places well-developed rectangular crystal faces, to-gether with small crystals of Ti2O3 and a pink transparent crystalline flux. The blue product was determined by single crystal X-ray diffraction on several cleaved crystal fragments to be a new orthorhombic Nd–titanate, Nd3Ti4O12 . For Ln = La, a dark crystalline mass with jagged edges but with locally well-developed crystals with square faces had formed, together with small crys-tals of Ti2O3 and a colorless crystalline flux. X-ray diffraction on crystals and cleaved crystal fragments showed the dark crystal-line product to be an ordered defect perov-skite of the type La2/3+xTiO3 . A dc four-point resistivity measurement was performed on a single crystal of Nd3Ti4O12 . Silver wires were connected (with Ag/epoxy) to evaporated Au–Ge/Au strip contacts on the crystal. Structure Determinations The crystals were measured at 23°C on an Enraf-Nonius  CAD-4  diffractometer  using graphite-monochromatized MoKα radiation and the NRCCAD program package (6). All calculations were carried out on an Alliant FX/80 computer, using the NRCVAX struc-ture package (7). Lattice parameters were determined from the absolute 2θ values of reflections at a high angle (Nd3Ti4O12: 30 reflections with 49° < 2θ < 58°, La1/3+xTiO3: 28 reflections with 50° < 2θ < 56°). The metal atom positions for Nd3Ti4O12 were ob-tained by direct methods. A Gaussian ab-sorption correction was applied. Aniso-tropic  temperature factors for all atoms were  included in the refinement. Crystallo-graphic data are listed in Table I, atomic coordinates and isotropic thermal parame-ters in Table II.* Crystals of La2/3+xTiO3 appeared to be primitive cubic with a = 7.7837(3) Å. It is likely that the material is actually a tetrago-nal defect perovskite with c = 2a as de-scribed earlier for La2/3TiO2.954 (8). This also corresponds well with powder X-ray diffrac-tion on crushed crystals. The doubling in all three dimensions as observed by single crystal X-ray diffraction is likely to be pro-duced by twinning. From the observed cell parameter and comparison with the data for La2/3+xTiO3 of Kestigan and Ward (2), the composition of the crystals can be derived by interpolation to be approximately La0.73TiO3(x ≈ 0.06). Description of the Structure The structure of Nd3Ti4O12 is shown in Fig. 1,  projected along the  b-axis.  Pertinent * See NAPS document No. 05020 for 23 pages of supplementary material. Order from ASIS/NAPS. Mi-crofiche Publications, P.O. Box 3513, Grand Central Station, New York, NY 10163. Remit in advance $4.00 for microfiche copy or for photocopy, $8.65 up to 20 pages plus $.30 for each additional page. All orders must be prepaid. Institutions and Organizations may order by purchase order. However, there is a billing and handling charge for this service of $15. Foreign orders add $4.50 for postage and handling, for the first 20 pages, and $1.00 for additional 10 pages of material, $1.75 for postage of any microfiche orders.   INTERGROWTH IN NEODYMIUM TITANATE 109   Formula  Nd3Ti4O12 Space group  Pcmn  Crystal size (mm)  0.12  ×  0.18  ×  0.12  a (Å) (at 23°C)  5.4203(8)  b (Å)  7.6104(9)  c (Å)  22.105(5)  V (Å)  908.1(3)  Z  4  Dc (g/cm3)  5.971  µ (MoKα, cm-1)  202.8  Minimum transmission  0.1079  Maximum transmission  0.2513  Scan type  ω/2θ  Reflections measured  2168  Independent reflections  2099  Observed reflections  1670  (I > 2.5σ (I))   Refined parameters  98  R  0.040  Rw (w = 1/σ 2 (Fo))  0.056  Extinction length (µm)  0.29(1) interatomic distances are listed in Table III. The framework of the structure is formed by  TiO6 octahedra, arranged in puckered single and double ReO3-type chains running in the direction of the b-axis. The Ti(1)O6-octahedra make up the single chains, and share only corners with the neighboring oc-tahedra. The Ti(2)O6-octahedra form the double chains  as  edge-sharing  pairs, which TABLE II ATOMIC POSITIONAL PARAMETERS AND ISOTROPIC TEMPERATURE FACTORS FOR Nd3Ti4O12   x   y   z  Biso (Å2) Nd(1)  0.93871(15)  0.25  0.19959(3)  0.464(20) Nd(2)  0.45551(14)  0.75  0.02782(3)  0.471(19) Nd(3)  0.92754(14)  0.75  0.17866(3)  0.394(18) Ti(1)  0.4390(3)  0.49921(20)  0.17863(7)  0.38(4)  Ti(2)  0.9516(3)  0.48936(21)  0.06143(7)  0.43(4)  O(1)  0.2465(14)  0.4624(9)  0.0122(3)  0.53(19)  O(2)  0.6351(14)  0.5593(10)  0.1147(3)  0.69(21)  O(3)  0.7090(13)  0.5367(9)  0.2378(3)  0.48(19)  O(4)  0.1223(13)  0.4708(9)  0.1421(3)  0.43(19)  O(5)  0.4976(22)  0.25  0.1869(5)  0.9(4)  O(6)  0.3578(22)  0.75  0.1954(5)  0.7(3)  O(7)  0.8410(23)  0.25  0.0642(5)  0.8(3)  O(8)  0.0321(21)  0.75  0.0699(4)  0.5(3)   FIG. 1. Polyhedral representation of the structure of Nd3Ti4O12 , projected along the b-axis. Ti–O polyhedra are hatched; Nd-atoms are represented as circles. share corners in the ac-plane with the Ti(1)-octahedra. The structure can be described as a regular intergrowth of two known struc-ture types: the GdFeO3-structure, which is an orthorhombic distortion of the perovskite structure (MO6-octahedra that are corner-sharing in three dimensions) (9) and the CaTa2O6-structure, with M2O10-groups of edge-sharing octahedra linked together by corner sharing (10). All three independent Nd-atoms are located in eight-coordinate distorted bicapped-trigonal prismatic sites, as is also observed for the A-cation in the GdFeO3 and CaTa2O6 structure types. The Nd(2)-atom, located between the puckered double  ReO3-type  chains,   has   a   slightly TABLE III SELECTED INTERATOMIC DISTANCES (Å) FOR Nd3Ti4O12 WITH ESTIMATED STANDARD DEVIATIONS IN PARENTHESES  Ti(1)  –O(2)  1.822(7)   Ti(2)  –O(1)  1.942(7)    –O(3)  1.980(7)     1.979(7)     2.239(7)    –O(2)  2.145(7)    –O(4)  1.908(7)    –O(4)  2.007(7)    –O(5)  1.932(3)    –O(7)  1.919(4)    –O(6)  1.993(4)    –O(8)  2.040(3)   Nd(1)  –O(3)  2.585 (7)  2×  Nd(2)  –O(1)  2.449(7)  2×    2.649(7)  2×    2.489(7)  2×   –O(4)  2.328(7)  2×   –O(2)  2.591(7)  2×   –O(5)  2.41(1)    –O(7)  2.59(1)    –O(6)  2.35(1)    –O(8)  2.47(1)   Nd(3)  –O(2)  2.570(8)  2×  Ti(2)  –Ti(2)  2.760(3)    –O(3)  2.395(7)  2×       –O(4)  2.506(7)  2×       –O(6)  2.36(1)        –O(8)  2.461(9)       TABLE I CRYSTALLOGRAPHIC DATA FOR Nd3Ti4O12  110 HESSEN ET AL.  FIG. 2. ORTEP representation of the Ti2O10 group in Nd3Ti4O12 . Thermal ellipsoids represented at the 50% probability level. larger and more regular coordination sphere than the other Nd-atoms (average Nd(2)–O  =  2.515 Å, with maximum deviations of –0.066 and +0.076 Å, for Nd(1) 2.486 Å and  +0.163/ –0.158 Å, for Nd(3) 2.470 Å and  +0.100/ –0.110 Å respectively. The average Ti–O distances in Nd3Ti4O12 are 1.979 Å for Ti(1) and 2.005 Å for Ti(2) respectively. The average Ti–O distance for Ti(1) in the GdFeO3 type block is signifi-cantly smaller than that observed in NdTiO3 (2.022 Å (1)). It is close to the value for Ti4 + –O in rutiles (1.97 Å). The coordination geometry around Ti(1) is also considerably more distorted, with sharp O–Ti–O angles ranging from 80.3° to 94.4° (in NdTiO3 from 89.0° to 90.3° (1)). One notable feature in the structure is the distortion in the Ti2O10-groups. The Ti(2)-atoms are displaced from the center of the octahedra towards each other rather than away from each other (as might be expected from Coulomb repul-sion). The geometry of this group is shown in Fig. 2. The Ti(2)–Ti(2) distance of 2.760(3) Å is quite short for a Ti–Ti distance in a pair of edge-shared octahedra. An interesting comparison can be made with the recently published crystal structure of the Ti(4 + ) compound Nd4Ti9O24 in which edge-shared Ti2O10-pairs are also present (11). The average Ti–O distance in these groups are smaller than for Ti(2) in Nd3Ti4O12 , and typical for Ti(4 + ) (1.973/ 1.978  Å).  However,  the  Ti–Ti  distance  in the pairs (2.963 Å) is 0.20 Å longer than in Nd3Ti4O12. The stoichiometry of Nd3Ti4O12 obtained from X-ray diffraction indicates an average Ti oxidation state of 3.75 + ; i.e., there is one 3d-electron per Ti2O10-pair. These 3d-electrons are possibly localized in orbitals associated with the Ti(2)–Ti(2) pair-ing. This would imply nonmetallic behavior of the material, at least at or below the tem-perature at which the structure was deter-mined (23°C). At high temperatures a first-order transition to a metallic state may be anticipated. A low-temperature four-point resistivity measurement on a single crystal showed nonmetallic behavior (Fig. 3) with a room-temperature resistivity of 35 Ω cm. Due to the small size of the crystal, only the conductivity along the b-axis could be measured. Anisotropy in the conductivity was expected due to the low symmetry of the crystal structure, but unfortunately could  not be measured. Degradation of the very small contacts made data collection un-reliable above room temperature. The ln(ρ) vs 1/T plot is nonlinear, suggesting that the material does not behave as an intrinsic semiconductor. The high-temperature data give an activation energy of 0.26 eV, but the fit is valid only over a rather small tempera-ture range. A second fit, based on a hopping mechanism for the conduction with a ρ ∝ ρ0  FIG. 3. Plot of log(resistivity) vs 1000/T from a dc resistivity measurement on a Nd3Ti4O12 single crystal.   INTERGROWTH IN NEODYMIUM TITANATE 111 exp(T0/T)1/4, gives a critical temperature of T0 = 1.24 × 106 K. This fit extends over a larger temperature range, but at low temper-ature, the deviations are very pronounced. Further experiments would be needed to elucidate the mechanism of conductivity. Discussion It is observed that recrystallization of ceramics with nominal composition Ln0.33Ti2O4 from Ln2O3 · 3B2O3 fluxes (Ln = La, Nd) leads to two crystalline prod-ucts: Ti2O3 and a ternary compound that is  only partially reduced. The difference in ionic radius between La and Nd is relatively small (1.32 Å for La3 + , 1.26 Å for Nd3 + in eight-coordination (12)). However, this difference, under otherwise identical condi-tions, causes a drastic change in the struc-ture of the ternary product, being an ordered defect perovskite for the La–Ti–O system and Nd3Ti4O12 with the intergrowth struc-ture as described above for the Nd–Ti–O system. This suggests a delicate dependence of the thermodynamics of these ternaries on the size of the A-cation. Also notable is the apparent absence of the La1.33Ti8O16 hol-landite phase that was obtained earlier from a more La-rich mixture (3). This can corre-spond with our findings in the Sr–Nb–O sys-tem that the occurrence of disproportiona-tion reactions in ternary borate flux media is  strongly dependent on the concentration of the common ion between the flux and the ternary transition metal oxide system (13). Although currently Nd3Ti4O12 is the only member of the ordered GdFeO3/CaTa2O6 in-tergrowth type synthesized, it is possible to predict the existence of a homologous series based on this intergrowth principle. The general formula of such a series would be Nd1+nTi2+nO6+3n , of which Nd3Ti4O12 is the n = 2 member. It is based on GdFeO3-blocks expanding in the c-direction between the edge-sharing  pairs  of  octahedra  of  the CaTa2O6 structure. The end members in the Nd–Ti–O system would be defined by n = 1 (Nd2Ti3O9 , Ti4+) and n = ∞ (NdTiO3 , Ti3+). The n = even and n = odd members differ in  the orientation of the edge-sharing pairs relative to each other. In the n = even mem-bers, sequential Ti2O10-blocks along the c-direction will be shifted over a/2, and canted in the opposite direction within the ac-plane (as in Nd3Ti4O12). In the n = odd members these blocks would not be shifted, and would have the same canting direction. Presently we are exploring the possibility of accessing members of this series by ceramic routes. Initial efforts were directed to pre-pare related Ti4 + ceramics, e.g., with com-position CaNd2Ti4O12 or Nd3Ti3AlO12 . As yet, these attempts have met with little suc-cess, yielding tetragonal defect perovskite type species as main products. References 1. D. A. MACLEAN, H.-N. NG, AND J. E. GREEDAN, J. Solid State Chem. 30, 35 (1979). 2. M. KESTIGAN AND R. WARD, J. Am. Chem. Soc. 77, 6199 (1955). 3. B. HESSEN, S. A. SUNSHINE, AND T. SIEGRIST, J. Solid State Chem. 94, 306 (1991). 4. S. N. RUDDLESDEN AND P. POPPER, Acta Crys-tallogr. 10, 538 (1957); Acta Crystallogr. 11, 54 (1958). 5. M. NANOT, F. QUEYROUX, J. C. GILLES, A. CARPY, AND J. GALY, J. Solid State Chem. 11, 272 (1974). 6. Y. LEPAGE, P. S. WHITE, AND E. J. GABE, “Pro-ceedings of the American Crystallographic Associ-ation Annual Meeting, 1986, Hamilton, Canada,” AIP, New York, (1986). [Poster PA23] 7. E. J. GABE, Y. LEPAGE, J. P. CHARLAND, F. L. LEE, AND P. S. WHITE, J. Appl. Crystallogr. 22, 384 (1989). 8. M. ABE AND K. UCHINO, Mater. Res. Bull. 9, 147 (1974). 9. S. GELLER, J. Chem. Phys. 24, 1236 (1956). 10. L. JAHNBERG, Acta Chem. Scand. 71, 2548 (1963). 11. N. HÜBNER AND R. GRUEHN, Z. Anorg. Allg. Chem. 616, 86 (1992). 12. R. D. SHANNON AND C. T. PREWITT, Acta Crys-tallogr. Sect. B 26, 1046 (1970). 13. B. HESSEN, S. A. SUNSHINE, T. SIEGRIST, AND R. JIMENEZ, Mater. Res. Bull. 26, 85 (1991). 

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A Reduced Neodymium Titanate with a New Intergrowth Structure Type

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