RIGID UNIT MODES IN THE HIGH-TEMPERATURE PHASE OF SIO2 TRIDYMITE - CALCULATIONS AND ELECTRON-DIFFRACTION

Calculations of the rigid unit mode (RUM) spectrum of the high-tempera ture phase of SiO2 tridymite are used to explain the patterns of diffu se scattering seen in transmission electron microscopy experiments. Th ese results show that RUM's can occur with wave vectors on curved surf aces in reciprocal space rather than being confined to symmetry points , lines or planes. The fact that the calculations reproduce the detail seen in the diffuse scattering provides a striking nontrivial confirm ation of the validity of the rigid unit mode model.