Structural evolutions of perovskites under high pressure and high temperature

Recent high-pressure single-crystal X ray diffraction experiments show that when the octahedral BO6 group is more compressible than the AO12 polyhedra in certain ABO3 (Pbnm or Pnma) perovskites (βB>βA), then the BO6 octahedra becomes less tilted and the structure evolves towards a high-symmetry configuration as the pressure increases. In other perovskites the BO6 octahedra are less compressible than the AO12 polyhedra (βB<βA),and the structure then becomes more tilted with increasing pressure. A new model, based on the assumption of bond-valence matching in both the Aand theBsites of a perovskite, provides a simple and universal rule that describes the dominant mechanism governing pressure-induced octahedral tilts and distortions in both polyhedral AO12 and BO6. By the relation βB/βA=MA/MB,where Mi is a site parameter calculated by using bond-valence valence parameters and crystal structural data under ambient conditions, the high-pressure behavior can be correctly predicted . A general law that predicts the variation in transition temperature of tilt transitions in perovskite has been deduced: in perovskites with MA/MB>1, the temperature Tc of zone-boundary phase transitions will decrease with increasing pressure, dTc/dP<0, while with MA/MB<1, dTc/dP>0.