The non-stoichiometry of the perovskite (ABO3)-type phase in the system PbO---TiO2---Nd2O3 has been studied. Monophasic compounds of composition Pb1−αxNdxTiO3+x(1.5−α) with x  0.21 and 0.09  α  1.5 were prepared. The ferroelectric Curie temperature (Tc) shows a decrease of 18.5 K/at% Nd with increasing value of x. Tc shows an increase of 3.5 K/mol % PbO with decreasing value of α (increasing content of PbO). The observed effect of α on optical spectra can be interpreted by assuming that Nd(III) ions partly occupy B sites in compounds with α < 1.5

Bouwma, J.

Heilbron, M.A.

English

The non-stoichiometry of the perovskite (ABO3)-type phase in the system PbO---TiO2-Nd2O3 has been studied. Monophasic compounds of composition Pb1−αxNdxTiO3+x(1.5−α) with x ≤ 0.21 and 0.09 ≲ α ≲ 1.5 were prepared. The ferroelectric Curie temperature (Tc) shows a decrease of 18.5 K/at% Nd with increasing value of x. Tc shows an increase of 3.5 K/mol % PbO with decreasing value of α (increasing content of PbO). The observed effect of α on optical spectra can be interpreted by assuming that Nd(III) ions partly occupy B sites in compounds with α &lt; 1.5

University of Twente Research Information

Mat. Res. Bull. Vol. Ii, pp. 663-668, 1976. Pergamon Press, Inc. Printed in the United States. NON-STOICHIOMETRY AND OPTICAL SPECTRA OF Nd(III) SUBSTITUTED PbTiO 3 J. Bouwma and M.A. Heilbron Twente University of Technology, Department of Chemical Engi- neering, Laboratory of Inorganic Chemistry and Materials Science, P.O. Box 217, Enschede, the Netherlands (Received April 19, 1976; Communicated by G. H. Jonker) A B S T R A C T  The non-stoichiometry of the perovskite (ABO3)-type phase in the system PbO-TiO2-Nd203 has been studied. Mo- nophasic compounds of composition Pbl_~xNdxTiO3+x( 1 5_e) with x ~ 0.21 and 0.09 ~ e ~ 1.5 were prepared. The'zer- roelectric Curie temperature (T c) shows a decrease of 18.5 K/at% Nd with increasing value of x. T c shows an increase of 3.5 K/mol % PbO with decreasing value of (increasing content of PbO). The observed effect of on optical spectra can be interpreted by assuming that Nd(III) ions partly occupy B sites in compounds with < 1.5. Introduction The perovskite (ABO3)-type phases in the systems PbO-TiO2-Ln203 (Ln = La,Sm) are highly non-stoichiometric (1,2). Unit-cell di- mensions and dielectric properties of these phases depend Strong- ly on composition. We have interpreted this non-stoichiometry on the basis of a defectstructure with variable numbers of both Ln(III)-ions and vacancies distributed over A and B sites in the perovskite structure (I). A defectstructure of the same type has been proposed for the perovskite-type phase in the system SrO-TiO2-La203 which shows comparable non-stoichiometry (3). The defectstructures proposed are based on the results of X-ray dif- fraction studies. Additional evidence for these defectstructures, especially concerning the occurrence of Ln(III) at two different crystallo- graphic sites, might be obtained from optical spectra of these titanates substituted with suitable rare earth ions. Lindop and Goodwin (4) studied EPR and optical spectra of CaO.2AI203 doped with Nd(III) ions (4f 3 electronic configuration). They concluded that Nd(III) ions occupy four different sites in this compound. 663 664 3. BOUWMA and M. A. HEILBRON Vol. II, No. 6 In this paper we report results of our investigations on the homogeneity range, Curie temperature and optical spectra of Nd(III) substituted PbTiO 3. Experlmental Par t ceramic samples of composition Pbl_~xNdxTi03 +x(l . 5-u) with x = 0.10, 0.18 and 0.21, respectively, and various values of (0.8 ~ u ~ 1.5) were prepared using standard techniques (i). Sintering was performed at 1520 K. The measured densities of the products were higher than 95% of the theoretical densities. The compositions of the samples were determined by X-ray fluorescence analysis. Homogeneity and unit-cell dimensions were determined from X-ray powder diffraction patterns. Curie temperatures were deduced from DTA curves. Diffuse reflectance, normal transmission and scattered trans- mission spectra in the frequency range 6,000-25,000 cm -I at room temperature were recorded using a Varian Cary 171 spectrophotome- ter. Low-temperature (down to 32 K) transmission spectra were re- corded with a Perkin Elmer El4 spectrophotometer. Thin (~ I00 ~m) polished ceramic slices were used for transmission measurements. Homogeneity range and Curie temperature Table I gives a survey of the unit-cell dimensions and Curie temperatures of monophasic samples. For compounds with x = 0.21 no Curie temperatures could be detected from DTA measurements. The heat effects accompanying the ferroelectrlc-paraelectric transition are obviously below detection limit for this compo- sition. From the results of the investigations on homogeneity range and Curie temperature of the perovskite-type phase, the following conclusions can be drawn. i) Pure compounds with 0.9 I a ~ 1.5 could be prepared. This means that the width of the homogeneity range (expressed in units of ~) at fixed Nd/Ti ratio (x) far from the solubility TABLE I Unit-cell Parameters and Curie Temperatures for various Compounds of Composition Pb, x a(~) c(~) O.lO 0 . 1 8  0 . 2 1  -~xNdxTiO3+x~ 1 . 4 2  3 . 9 0 4 2  1 . 2 5  3 . 9 0 4 8  i . 0 9  3 . 9 0 6 5  0 . 9 9  3 . 9 0 9 5  1 . 5 4  3 . 9 0 6 4  1 . 4 2  3 . 9 0 6 6  1 . 2 0  3 . 9 1 3 9  1 . 1 0  3 . 9 1 7 5  0 . 9 9  3 . 9 1 7 7  1 . 4 8  3 . 9 0 7 4  0 . 9 5  3 . 9 2 1 1  4 . 0 5 5 5  4 . 0 5 5 9  4 . 0 5 7 9  4 . 0 6 3 7  3 . 9 7 3 3  3 . 9 7 8 8  3 . 9 8 6 6  3 . 9 9 2 3  3 . 9 9 5 8  3 . 9 5 7 3  3 . 9 7 8 6  c / a  1 . 0 3 9  1 . 0 3 9  1 . 0 3 9  1 . 0 3 9  I 017 ! . 0 1 8  1 . 0 1 9  1 . 0 1 9  1 . 0 2 0  1 . 0 1 3  1 . 0 1 5  v(~ 3) 61 . 8 2  61 . 8 4  61 . 9 2  6 2 . 1 1  6 0 . 6 3  6 0 . 7 2  61 . 07  61 . 2 7  61 . 3 3  6 0 . 4 2  61 .17  ,5-a) " w (K) C 589 591 598 603 422  433  444  454  462  Vol. II, No. 6 Nd SUBSTITUTED LEAD TITANATE 665 limit of Nd(III) in PbTiO 3 (Xma x ~ 0.4) equals almost the corresponding widths of the homogeneity ranges of La(III) and Sm(III) substituted PbTiO 3 (I). ii) An increase of the Nd/Ti ratio (x) of the phase at fixed value of e leads to a slight increase of the a-axis, and to decreasing values of c-axis, axial ratio c/a, unit-cell vo- lume V and Curie temperature T c. The decrease of T c is about 18.5 K/at.% Nd(III). The corresponding values of La(III) and Sm(III) substitutions are 19.5 and 18.0, respectively (i). iii) An increase of the PbO content (decreasing ~) at fixed Nd/Ti ratio leads to greater values of a,c,c/a,V and T c. The in- crease of T c is about 3.5 K/mol% PbO. The corresponding values for La(III) and Sm(III) substitutions are 2 and 5, respectively (i). Obviously, the effect of Nd(III) substitution on various pro- perties is intermediate between those of La(III) and Sm(III) sub- stitutions. This is in accordance with the intermediate value of the ionic radius of Nd(III). Optical spectra The spectra show absorption bands at frequencies which are in good agreement with literature data on spectra of Nd(III) ions (4,5). Figures 1 and 2 show transmitted intensity (It), in arbitrary units, vs. wave number (~) for the two compounds with x = 0.21 (Table I). In the figures and in the discussion of the spectra values of 1.5 and 1.0, respecti- vely, are used for the sake of simplicity. Fig. 1 shows the in- fluence of temperature on the shape of the absorption band around 17,000 cm -I, which is due to transitions from 419/2 qround state levels to 4G5/2 and 2G7/2 levels. Two effects can be dis- tinguished. i) Resolution becomes better with decreasing temperature. ii) The absorption peak at about 16,600 cm -I, which is present in the spectra recorded at room temperature and at 100 K, is absent in the spectrum re- corded at 32 K. This indicates that this peak is due to a transition from a ~tark level (belonging to the ~I9/2 mani- fold) which must be close (of the order of 100 cm -I) above the ground level. I i I It x lOCi:m4) \ ~s #o FIGURE | d5 Transmitted intensity vs. wave number for the absorption band around 17,000 cm -I of a compound with x = 0.21 and ~ = 1.0 at 32, I00 and 295 K, respectively. 666 J. BOUWMA and M. A. HEILBRON Vol. ii, No. 6 I I I 4F~, %,. 2H,~ I 4Fw~ • 2S3t 2 I T i t  ® 9 x D % m " )  _ = a= $5 i 1 4G~= • 2G71 ~ f r Zt ® 19 I~5 G= 1.5 9 x ~(cm'1) ?_ 1~5 1~ T zt © x 10"3k:m'1)= 2oot~ FIGURE 2 Transmitted intensity vs. wave number for two compounds with x = 0.21, and ~ = 1.5 and 1.0, respectively, at 32 K in the wave number regions llt000-14,000 cm -I (a), 16,500-18,000 cm -I (b) and 18,500-19,900 cm TM (c). The arrows indicate the additional absorp- tion phenomena in the compound with ~ = 1.0. Vol. ii, No. 6 Nd SUBSTITUTED LEAD TITANATE 667 The effect of u on the spectrum of compounds with x = 0.21 at 32 K can be seen from Fig. 2 (a,b and c), which shows different spectral regions. The spectra of the two compounds studied, viz. with ~ = 1.5 and 1.0, are very similar, except for some additio- nal small absorption peaks and shoulders in the spectrum of the compound with u = 1.0. Throughout the spectrum these additional absorptions are found at slightly lower wave numbers than those of the main absorption peaks. With decreasing electron-electron repulsion energy or internal electric field strength for a particular site, all absorption peaks of a spectrum are shifted to lower wave numbers. This indicates that a small part of the Nd(III) ions occupies a second site in this compound. From spectra, recorded at room temperature (not shown here), it can be seen that the absorption bands corresponding to the second site, distinctly increase with decreasing ~ values. How- ever, the relatively large spectral bandwidths do not allow to calculate the oscillator strengths of the individual bands. Therefore, a quantitative interpretation cannot be given. The features, which the spectra of compounds with different values of e have in common, must be due to Nd(III) ions at A sites. Merely on the basis of the spectroscopic results, it can- not be concluded yet that the second site, which Nd(III) obvi- ously occupies in compounds with u < 1.5, is indeed the B site of the perovskite structure. However, this conclusion appears to be justified by the results of our X-ray diffraction studies on La(III) substituted PbTiO~ (I) and SrTiO. (3). These results have shown that a defects~ructure with b~th La(III) ions and va- cancies distributed over A and B sites is most likely for these compounds. In the defectstructure proposed no B-site defects exist in compounds with ~ = 1.5 (Table 2). Hence, in these compounds the rare earth ions occupy A sites only, giving rise to rather simple optical spectra. The defect concentrations in compounds with < 1.5 are not fully determined by composition. Two extreme cases which are characterized by a negligible number of rare earth ions at B sites (case I), and a negligible number of B-site vacancies (case II), respectively, must be distinguished. The TABLE 2 Number of Defects per Unit Cell for Ln(lll) substituted PbTiO^ or SrTiO_ with x = 0.20 and ~ = 1.5 and 1.0, res- pectively. For ~he Compound with e = 1.0 separate Numbers are given for the two extreme Cases I and II. defect number number type ~ = 1.50 ~ = 1.00 case I case II Ln A V A Ln B V B 0 . 2 0  O . i O  0.194 0.032 0.032 0.162 0.064 0.032 668 J.A. BOUW~iA and M. A. HEILBRON Vol. ii, No. 6 defect concentrations in a compound with x = 0.20 and ~ = 1.0, are given in Table 2 for these cases. The actual defect concen- trations are intermediate (1,3). It is likely that this defectstructure also holds for Nd(III) substituted PbTiO.. Therefore, it is to be expected that ~ome 10% of the Nd(III~ ions occupy B sites in a compound with x = 0.21 and m = 1.0, giving rise to additional optical absorption phenomena. This means that the observed effect of e on optical spectra supports, at least qualitatively, the defectstructure proposed. Acknowledgements We are highly indebted to H.J.L. van der Valk of the Universi- ty of Groningen for making available the Perkin Elmer spectro- meter and for his assistance in the low-temperature measurements. We wish to thank G.C. Tiekstra, H. Kruidhof and F.J. Carelsen for their technical assistance and A.J. Burggraaf and P.J. Gel- lings for useful discussions. References 1 K.J. de Vries, K. Keizer, J. Bouwma and A.J. Burggraaf, Science of Ceramics 8, 187 (1975). 2 D. Hennings, Mat. Res. Bull. 6, 329 (1971). 3 J. Bouwma, K.J. de Vries and A.J. Burggraaf, accepted by Phys. Stat. Sol. (a) 35, (1976). 4 A.J. Lindop and D.W. Goodwin, J. Phys. C: Solid State Phys. 6, 1818 (1973). 5 G.H. Dieke, Spectra and energy levels of rare earth ions in crystals, Interscience (1968). 

Non-stoichiometry and optical spectra of Nd(III) substituted PbTiO<sub>3</sub>

accepted by Phys.

C: Solid State Phys.

Spectra and energy levels of rare earth ions in crystals, Interscience

https://doi.org/10.1016/0025-5408(76)90142-2

Non-stoichiometry and optical spectra of Nd(III) substituted PbTiO3

Abstract

Similar works

Full text

Available Versions

University of Twente Research Information