高温高压下钙钛矿晶体结构变化

最近高压单晶X射线衍射实验明确显示正交晶系(Pbnm或Pnma) （钙钛矿）ABO3的高压压缩行为完全由晶体框架结构(framework)中的BO6八面体和AO12多面体的相对压缩性所确定，即对一些钙钛矿晶体，BO6较AO12多面体更易压缩时，压力导致晶体结构向高对称性结构演变；对另外一些钙钛矿，BO6较AO12多面体更不易压缩时，压力导致晶体结构向低对称性结构演变.基于价键匹配关系假设，建立多面体压缩率之间关系，由价键参数和常压晶体结构数据计算参量, 可以估计BO6和AO12多面体的相对压缩率，从而正确地预言高压压缩行为.文章最后引入高温高压钙钛矿倾斜相变的一般性规律.

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.