Abstract
The high temperature (β) phases of SiO2 cristobalite and quartz are studied by 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 (RUM) excitations, and it is found that the entire spectrum of RUMs is responsible for the disorder. Comparisons of the results of β-cristobalite with those of β-quartz lead to the conclusion that framework structures with high degrees of geometric flexibility, and hence many RUMs, are free to deform through cooperative tetrahedral rotations even in the limit of extremely large tetrahedral stiffnesses.
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