New view of the high-pressure behaviour of GdFeO3-type perovskites

Abstract

Recent determinations of the structures of several GdFeO₃-type orthorhombic perovskites (ABO₃) show that the octahedra in some become more tilted with increasing pressure. In others the octahedra become less tilted and the structure evolves towards a higher-symmetry configuration. This variety of behaviour can be explained in terms of the relative compressibilities of the octahedral and dodecahedral cation sites in the perovskite structure. If the BO₆ octahedra are less compressible than the AO₁₂ sites then the perovskite will become more distorted with pressure, but the perovskite will become less distorted if the BO₆ site is more compressible than the AO₁₂ site. In this contribution we use the bond-valence concept to develop a model that predicts the relative compressibilities of the cation sites in oxide perovskites. We introduce the site parameter M _i defined in terms of the coordination number N _i , average bond length at room pressure R _i , and the bond-valence parameters R ₀ and B,M_i = ({R_i N_i }/ B)\exp [({{R_0 - R_i }) / B}].M _i represents the variation in the bond-valence sum at the central cation in a polyhedral site because of the change of the average bond distance. Experimental data suggest that the pressure-induced changes in the bond-valence sums at the two cation sites within any given perovskite are equal. With this condition we show that the ratio of cation-site compressibilities is given by \beta _B /\beta _A = M_A /M_B. This model, based only upon room-pressure bond lengths and bond-valence parameters, correctly predicts the structural behaviour and some physical properties of the oxide perovskites that have been measured at high pressure.