Herein we communicate the first example of a pressure induced octahedral tilting distortion in a double perovskite phase, which was observed during the structural characterization of Ba2YTaO6 using high-pressure synchrotron X-ray powder diffraction

Lee, Yongjae

Lufaso, Michael W

Macquart, René B

Vogt, Thomas

zur Loye, Hans-Conrad

English

UNF Digital Commons

University of North FloridaUNF Digital CommonsChemistry Faculty Publications Department of Chemistry2006Pressure Induced Octahedral Tilting Distortion inBa2YTaO6Michael W. LufasoUniversity of North Florida, michael.lufaso@unf.eduRené B. MacquartYongjae LeeThomas VogtHans-Conrad zur LoyeFollow this and additional works at: http://digitalcommons.unf.edu/achm_facpubPart of the Chemistry CommonsThis Article is brought to you for free and open access by the Departmentof Chemistry at UNF Digital Commons. It has been accepted for inclusionin Chemistry Faculty Publications by an authorized administrator of UNFDigital Commons. For more information, please contact Digital Projects.© 2006 All Rights ReservedRecommended CitationLufaso, Michael W.; Macquart, René B.; Lee, Yongjae; Vogt, Thomas; and zur Loye, Hans-Conrad, "Pressure Induced OctahedralTilting Distortion in Ba2YTaO6" (2006). Chemistry Faculty Publications. 6.http://digitalcommons.unf.edu/achm_facpub/6Pressure induced octahedral tilting distortion in Ba2YTaO6{Michael W. Lufaso,a Rene´ B. Macquart,a Yongjae Lee,b Thomas Vogta and Hans-Conrad zur Loye*aReceived (in Cambridge, UK) 13th September 2005, Accepted 20th October 2005First published as an Advance Article on the web 18th November 2005DOI: 10.1039/b512861gHerein we communicate the first example of a pressure inducedoctahedral tilting distortion in a double perovskite phase, whichwas observed during the structural characterization ofBa2YTaO6 using high-pressure synchrotron X-ray powderdiffraction.Perovskites and structurally related oxides have been one of themost widely studied structure types because of their interestingphysical properties and extensive structural diversity. The perovskite oxide has the general stoichiometry ABO3 and is composedof corner sharing BO6 octahedra with the A site cation occupyingthe void created by the three dimensional octahedral framework.Although many compounds adopt the ideal cubic perovskitearistotype, most perovskites undergo a distortion away from thehigh symmetry cubic structure.1 It is these subtle structuraldistortions, such as a cooperative octahedral tilting distortion thatoccurs when the A site cation is undersized for the corner sharingnetwork, that often greatly influence the physical properties.Research on ordered double perovskites has been pursued by alarge number of researchers, in part, because of the diversephysical properties and remarkable structural chemistry observedin these materials. Octahedral tilting in cation ordered doubleperovskites has been examined in detail by several groups using anumber of techniques, including group theoretical analysis.2 Whiletemperature is the most common intensive variable utilized in thestudy of phase transitions in these materials, pressure also mayinduce a structural phase transition; an example being the firstorder rhombohedral (R3¯) to monoclinic (I2/m) transition inBa2BiTaO6 that can be induced by either lowering the temperatureor through the application of pressure.3 Here we describe the firstexample of a pressure induced octahedral tilting distortion inBa2YTaO6, observed by variable pressure synchrotron X raypowder diffraction, which demonstrated that Ba2YTaO6 undergoes a pressure induced phase transition between 4.3 and 5.6 GPafrom space group Fm3¯m to space group I4/m with the onset of anoctahedral tilting distortion.In situ high pressure synchrotron X ray powder diffractionexperiments on Ba2YTaO6{ were performed using a diamondanvil cell at the X7A beam line at the National Synchrotron LightSource at Brookhaven National Laboratory.§ The experimentalsetup and detailed procedure are described elsewhere.4 Diffractionpatterns were collected using a wavelength of 0.62245 A˚.Diffraction data analysis was performed using the EXPGUIinterface of GSAS.5 A pseudo Voigt function was used to modelthe peak shape. The angular range employed in the refinementswas from 6u to 35u 2h, and excluded the regions that containedstrong Bragg peaks from the pressure cell gasket.Inspection of the variable pressure data above 4.3 GPa indicatedthat the diffraction peaks of certain reflections began to split. Anexample of this peak splitting is the 620 peak in space group Fm3¯mnear 27.2u 2h. Fig. 1 clearly shows the splitting of the peak atpressures of 5.6 GPa and 6.3 GPa, compared to the singlereflection at 4.3 GPa. The splitting of the reflections is welldescribed by a cubic to tetragonal phase transition and isconsistent with a phase transition to either space group P4/mncor I4/m.2 Reflections that violate the body centering conditions,such as the 212 reflection shown to be indicative6 of primitivesymmetry in Ba2PrIr1 xRuxO6, were absent. Refinements wereattempted using structure models generated by the SPuDSsoftware in space groups I4/m and P4/mnc.1 The fit was superiorin space group I4/m compared to P4/mnc. A phase transition fromcubic (Fm3¯m) to tetragonal (I4/m) was also reported7 to occur at253(1) K; therefore the pressure induced phase transition fromFm3¯m to I4/m is the same as that which occurs on lowering of thetemperature. Crystal structures shown in Fig. 2 were refined fromthe 4.3 GPa and 6.3 GPa data in space groups Fm3¯m and I4/m,respectively.aDepartment of Chemistry & Biochemistry, University of SouthCarolina, 631 Sumter St. Columbia, SC 29208, USA.E mail: zurloye@mail.chem.sc.edu; Fax: +1 (803) 777 8508;Tel: +1 (803) 777 6916bDepartment of Earth System Sciences, Yonsei University, Seoul,120749, South Korea{ Electronic supplementary information (ESI) available: tables containingthe pressure dependence of the peak FWHM and refined crystalstructures. See DOI: 10.1039/b512861gFig. 1 Splitting of the 620 peak in Fm3m to the 116/332/240 peaks inI4/m at P = 4.3, 5.6 and 6.3 GPa. The bottom pattern was fitted with spacegroup Fm3m, the top and middle patterns with I4/m.COMMUNICATION www.rsc.org/chemcomm | ChemComm168 | Chem. Commun., 2006, 168 170 This journal is  The Royal Society of Chemistry 2006View OnlineThis pressure induced phase transition is most clearly understood in terms of an octahedral tilting distortion. The untilted idealcubic ordered double perovskite crystallizes in space group Fm3¯m.In space group I4/m, an antiphase octahedral tilting distortionoccurring about the c axis is clearly shown in Fig. 2. Theoctahedral tilting phase transition allows for an additional meansof compression of the structure. In perovskites, the twodominating processes during compression are bond shorteningand octahedral tilting. In the cubic form of Ba2YTaO6, compression with retention of the cubic symmetry is possible only via thesimultaneous shortening of the Y O, Ta O and Ba O bonds. Inits tetragonal form, on the other hand, both bond shortening andoctahedral tilting can take place. However, while a decrease in thec lattice parameter is the result of only the compressive shorteningof the Y O, Ta O, and Ba O bond distances, a reduction of the alattice parameter can have contributions from both bondcompression and octahedral tilting.In an ideal I4/m perovskite crystal structure with symmetricoctahedra and 0u octahedral tilting, the !2a and c latticeparameters are equal and the c/(!2a) ratio is unity. The observationof a larger relative compression of the a lattice parametercompared to the c lattice parameter, that is when c/(!2a) becomeslarger than unity, is indicative of an additional compressionmechanism, namely octahedral tilting.Octahedral tilting has a primary effect on the Ba O bondlengths, with a minimal change in the octahedral cation bondlengths. If the octahedral bond distances remain regular, then withan increase in the magnitude of octahedral tilting the a latticeparameter becomes smaller while the c lattice parameter remainsunchanged. Therefore, we can conclude that the observed increasein the c/(!2a) ratio in Ba2YTaO6 from 1.0025 to 1.0029 upon achange in the applied pressure from 5.6 to 6.3 GPa, represents agreater degree of octahedral tilting. The average tilt angle of the[YO6] and [TaO6] octahedra is 4.1u at 6.3 GPa.One cause for such a tilting transition is a difference in therelative compressibilities of the A X vs. B X bonds in an ABX3perovskite. For example, a recent study of the perovskite NaMgF3demonstrated that the compression mechanism was dominated bya shortening of the Mg F bonds below 6 GPa and controlled byan increase in octahedral tilting above 12 GPa; however, in thatexperiment no change in the type of octahedral tilting wasobserved.8 We have obtained similar results in this study ofBa2YTaO6, where bond compression is the dominant mechanismat lower pressures, while octahedral tilting becomes moreimportant at higher pressures. These results also suggest that theBa O bonds are more compressible than the Ta O and Y Obonds, which is evident in the onset of the octahedral tiltingtransition.The pressure dependence of the lattice parameters and unit cellvolume is given in Table 1. Additional information on the Rietveldrefinements in space group Fm3¯m and in space group I4/m at6.3 GPa are given in the ESI.{ Group theoretical analysis hasshown the Fm3¯m to I4/m phase transition can be continuous.2 Thepressure dependence of the lattice parameters is shown in Fig. 3. Asecond order Birch Murnaghan equation of state was fit to thepressure dependence of the unit cell volume data between 0.9 and4.3 GPa, giving a bulk modulus of 157(16) GPa and Vo601.0(9) A˚3. The bulk modulus is comparable to that found inother double perovskites, for example Ba2BiTaO6 at 126(10) GPa,3Fig. 2 Crystal structures of the Fm3m (top) and I4/m (bottom) forms ofBa2YTaO6. Large orange spheres represent Ba, green spheres Y, redspheres are Ta, and blue spheres O.Table 1 Pressure dependence of the lattice parameters and unit cellvolumePressure/GPa Space group a/A˚ c/A˚ Volume/A˚30.9 Fm3m 8.42811(8) 598.67(2)1.7 Fm3m 8.4077(2) 594.33(5)2.2 Fm3m 8.39707(8) 592.08(2)3.1 Fm3m 8.38092(9) 588.67(2)4.3 Fm3m 8.37435(9) 587.29(2)5.6 I4/m 5.8928(2) 8.3545(4) 290.11(2)6.3 I4/m 5.8851(1) 8.3474(2) 289.10(1)Fig. 3 Pressure variation of the lattice parameters. Circles represent a inFm3m , squares c in I4/m, and triangles !2a in I4/m.This journal is  The Royal Society of Chemistry 2006 Chem. Commun , 2006, 168 170 | 169View OnlineBa2PrRu0.8Ir0.2O6 at 139(10) GPa,9 and Sr2TbRu0.3Ir0.7O6 at196(10) GPa.10In conclusion, we have demonstrated that a phase transitionfrom space group Fm3¯m to I4/m occurs upon application ofpressure to Ba2YTaO6 with the onset of an octahedral tiltingdistortion about the c axis. The phase transition to space groupI4/m marks an additional compression mechanism consisting ofsimultaneous octahedral tilting and bond shortening in contrast tobond shortening only in space group Fm3¯m.Notes and references{ Polycrystalline Ba2YTaO6 was prepared using solid state techniques byreacting Y2O3 and Ta2O5 to form a phase pure YTaO4 precursor, whichwas then appropriately mixed with BaCO3 and reacted until a single phasesample of Ba2YTaO6 was obtained, as established by laboratory powderX ray diffraction. The laboratory X ray powder diffraction pattern couldbe indexed with a cubic cell, space group Fm3m, with a # 8.42 A˚.§ Work at Brookhaven is supported by the U.S. Department of Energy,Division of Materials Sciences, under Contract DE AC02 98CH10886.Financial support from the Department of Energy through grantDE FG02 04ER46122 and the National Science Foundation throughgrant DMR:0450103 is gratefully acknowledged.1 M. W. Lufaso and P. M. Woodward, Acta Crystallogr., Sect. B: Struct.Sci., 2001, 57, 725.2 P. M. Woodward, Acta Crystallogr., Sect. B: Struct. Sci., 1997, 53, 32;C. J. Howard, B. J. Kennedy and P. M. Woodward, Acta Crystallogr.,Sect. B: Struct. Sci., 2003, B59, 463.3 K. S. Wallwork, B. J. Kennedy, Q. D. Zhou, Y. Lee and T. Vogt,J. Solid State Chem., 2005, 178, 207.4 T. Vogt, G. Schneider, J. A. Hriljac, G. Yang and J. S. Abell, Phys. Rev.B: Condens. Matter, 2001, 63, 22050.5 A. C. Larson and R. B. von Dreele, General Structure Analysis System(GSAS), Los Alamos National Laboratories, 1990.6 L. Q. Li and B. J. Kennedy, J. Solid State Chem., 2004, 177, 3290.7 R. Zurmuhlen, E. Colla, D. C. Dube, J. Petzelt, I. Reaney, A. Bell andN. Setter, J. Appl. Phys., 1994, 76, 5864.8 H. Z. Liu, J. Chen, J. Hu, C. D. Martin, D. J. Weidner, D. Hausermannand H. K. Mao, Geophys. Res. Lett., 2005, 32, L03404.9 B. J. Kennedy, L. Q. Li, Y. Lee and T. Vogt, J. Phys.: Condens. Matter,2004, 16, 3295.10 Q. Zhou, B. J. Kennedy, K. S. Wallwork, M. M. Elcombe, Y. Lee andT. Vogt, J. Solid State Chem., 2005, 178, 2282.170 | Chem. Commun., 2006, 168 170 This journal is  The Royal Society of Chemistry 2006View Online

Pressure Induced Octahedral Tilting Distortion in Ba2YTaO6

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Pressure Induced Octahedral Tilting Distortion in Ba2YTaO6

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