Structural instabilities in LnGaO3 perovskites at high pressures

Anatoliy Senyshyn 1,2Jens M. Engel 1,3Iain Oswald 4,5Leonid Vasylechko 2Marek Berkowski 6

1. Technische Universität Darmstadt, Institute of Materials Science, Petersenstr. 23, Darmstadt 64287, Germany
2. Lviv Polytechnic National University, Semiconductor Electronics Department, Bandera Street, 12, Lviv 79013, Ukraine
3. Virginia Polytechnic Institute and State University, Blacksburg 24061, United States
4. European Synchrotron Radiation Facility (ESRF), Grenoble 38043, France
5. University of Edinburgh, Department of Chemistry, The King\'s Buildings, West Mains Rd, Edinburgh EH9 3JJ, United Kingdom
6. Polish Academy of Sciences, Institute of Physics, al. Lotników 32/46, Warszawa 02-668, Poland

Abstract

Crystal structure – property relation for perovskite type materials attract an attention of investigators over very long time. A purposeful tailoring of their physical and chemical properties requires a detailed and systematic knowledge of crystal structure behaviour under various environmental conditions and chemical composition.
Recent studies [1] resulted in obvious indications that the relative compressibilities of the A and B sites in ABX3 perovskite structure play an important role in determining the pressure-induced structural changes of perovskites. In order to prove and extend this assumption, perovskite type rare-earth gallates LnGaO3, whose crystal structure and its thermal evolution were systematically studied in the broad temperature and composition ranges [2], seem to be well-suited objects for high pressure diffraction studies, i.e. a good link between in-situ temperature- and pressure-dependent structural properties can be established. At ambient conditions all LnGaO3 possess distorted GdFeO3 type of structure and undergo transformation to LaAlO3 structure type at elevated temperatures, where temperature of the phase transformation is proportional to tolerance factor. It is worth to mention that the high pressure phase of the first representative LaGaO3 has the same symmetry as the phase found at high temperatures, i.e. LaAlO3 structure type.
High pressure structural studies on perovskite type rare-earth gallates LnGaO3 can resolve the set of questions collected, like:
-    is the LaAlO3 structure type occurring in other LnGaO3 at high pressures?
-    are phase transformation and elastic modules depending on perovskite tolerance factor?
-    what is the agreement between experimental data and results of semiclassical simulations [3]?
In order to clarify those problems, six samples have been chosen for high pressure powder diffraction experiments, four orthogallates LaGaO3, CeGaO3, PrGaO3, NdGaO3 and two solid solutions La0.50Pr0.50GaO3 and La0.63Nd0.37GaO3 whose magnitude of perovskite lattice deformation is close to those of cerium gallate.
Data collection was performed in a transmission mode at beamline ID27 at ESRF (Grenoble, France) using MAR345 imaging plate detector. To fulfil the quasi-hydrostatic condition, the 4:1 methanol-ethanol mixture was used as a pressure medium for LaGaO3 and La0.50Pr0.50GaO3, whereas other samples were cryogenically embedded into high purity nitrogen. More than 150 diffraction patterns were collected in a broad pressure range from 0.0001 to 40 GPa and results of such studies will be presented in the current contribution.

1. R.J. Angel, J. Zhao, N.L. Ross, Phys. Rev. Lett. 95 (2005) 025503.
2. L. Vasylechko, A. Senyshyn, U, Bismayer, in: Handbook on the Physics and Chemistry of Rare Earths, ed. K. A. Gschneidner Jr., J.-C. Bunzli, and V. Pecharsky (Elsevier, Amsterdam), Vol. 39 (2008) in print.
3. A. Senyshyn, H. Ehrenberg, L. Vasylechko, J.D. Gale, U Bismayer, J. Phys.: Condens. Matter 17 (2005) 6217.

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Presentation: Poster at 11th European Powder Diffraction Conference, Poster session, by Anatoliy Senyshyn
See On-line Journal of 11th European Powder Diffraction Conference

Submitted: 2008-05-01 00:02
Revised:   2009-06-07 00:48
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