DISTORTIONS OF FRAMEWORK STRUCTURES

We review the ''Rigid Unit Mode'' model that was developed to explain a number of features of framework silicate crystals composed of linked tetrahedral units, but which has implications also for the stability of perovskite structures if the octahedra have some degree of rigidity . The discussion includes the constraints on possible modes of deforma tion, experimental validation of the model, implications for the local structure of high-temperature phases, the local structures of defects , and the relationship to the soft-mode theory of displacive phase tra nsitions.