RIGID-UNIT PHONON MODES AND STRUCTURAL PHASE-TRANSITIONS IN FRAMEWORKSILICATES

The rigid-unit mode model provides many new insights into the stabilit y and physical properties of framework silicates. In this model the Si O4 and AlO4 tetrahedra are treated as very stiff, to a first approxima tion as completely rigid, in comparison with intertetrahedral forces. In this paper we apply the model to several important examples. The mo del is reviewed by a detailed study of quartz, and it is shown that th e alpha-beta phase transition involves a rigid-unit mode that preserve s the Si-O-Si bond angle. The model is used to explain the phase trans itions in cristobalite and the different feldspar, sodalite, and leuci te structures. We also use the model to explain the nature of the high -temperature disordered phases of cristobalite and tridymite, to inter pret the observations of streaks of diffuse scattering in electron dif fraction patterns, to interpret the structures in the kalsilite-nephel ine solid solution, to explain volume anomalies in the cubic leucite s tructures, and to explain qualitatively the negative linear thermal ex pansion in cordierite. The results for the highest symmetry sodalite s tructure show that there is a rigid-unit mode at every wave vector, a finding with significant implications for the understanding of the sor ption and catalytic behavior of zeolites.