New view of the high-pressure behaviour of GdFeO3-type perovskites
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 Mi defined in terms of the coordination number Ni, average bond length at room pressure Ri, and the bond-valence parameters R0 and B, Mi 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 . 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.