Rigid unit modes and dynamic disorder: SiO2 cristobalite and quartz

The high temperature (beta) phases of SiO2 cristobalite and quartz are stud ied bq performing molecular dynamics simulations using a model which allows easy analysis of tetrahedral motions. The dynamic nature of the disordered high-temperature phase of cristobalite is attributed to rigid unit mode (R UM) excitations, and it is found that the entire spectrum of RUMs is respon sible for the disorder. Comparisons of the results of beta-cristobalite wit h those of beta-quartz lead to the conclusion that framework structures wit h high degrees of geometric flexibility, and hence many RUMs, are free to d eform through cooperative tetrahedral rotations even in the limit of extrem ely large tetrahedral stiffnesses.