A NEW VIEW OF THE HIGH-PRESSURE BEHAVIOR OF PEROVSKITES

Volume 41 of Reviews in Mineralogy & Geochemistry (Hazen, R.M. and Downs, R.T., eds.) was written to serve as a comprehensive introduction to the field of high-temperature and high-pressure crystal chemistry, both as a guide to improved techniques and also as a summary of the extensive literature on minerals at high pressure and temperature. In the chapter devoted to framework structures, the high-pressure behavior of GdFeO₃-type perovskites was reviewed and one of the important conclusions drawn was that the approximation of octahedra as rigid units in perovskites is inadequate to explain their high-pressure behaviour [1]. Since the publication of RiM Volume 41, we have completed a number of structural studies of perovskites at high pressure using single-crystal X-ray diffraction and have confirmed that the octahedra often undergo significant compression. We have developed a model that predicts the relative compressibilities of the octahedral and dodecahedral sites in GdFeO₃-type perovskites using the bond valence concept [2]. The ratio of the cation site compressibilities is calculated from site parameters defined in terms of their coordination number, average bond length at room pressure and bond valence parameters. Since the distortion of perovskites at high pressure is controlled by the relative compressibilities of the octahedral and dodecahedral sites, we can predict their high-pressure behavior. There is also a relationship between the ratio of the site parameters and the degree of pressure-induced distortion and tilting in GdFeO₃-type perovskites: perovskites with greater ratios are predicted to show a greater degree of distortion with pressure. Thus far, the model correctly predicts the structural behavior of GdFeO₃-type perovskites that have been measured at high-pressure. The model is also be useful in explaining the high-pressure behavior of other perovskites such as the rhombohedral to cubic transition in LaAlO₃ [3].

References: [1] Ross, N.L. (2000) In Rev. Min. Geochem., Vol. 41, 257-288; [2] Brown I.D. and Altermatt, D. (1985) Acta. Cryst. B41, 244- 247 [3] Bouvier, P. and Kreisel, J. (2002) J. Phys.: Condens. Matter 14, 3981- 3991.