RIGID UNIT MODES IN FRAMEWORK SILICATES

We describe a model for framework silicates in which the SiO4 (and AlO 4) tetrahedra are treated as perfectly rigid and freely jointed. From this model we are able to identify low-energy modes of distortion of t he structure, which we call Rigid unit modes. These modes can act as s oft modes to allow easy distortions at phase transition. We discuss th ree forces that will operate at a phase transition in conjunction with the candidate soft modes to determine which of the rigid unit modes w ill actually precipitate a phase transition, and illustrate these idea s by detailed discussions of the phase transitions in quartz, leucite and cristobalite. The model can also be used to estimate the transitio n temperature, and the theory highlights an important role for the sti ffness of the tetrahedra.