Rigid unit modes and dynamic disorder: SiO2 cristobalite and quartz |
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| Authors: | M Gambhir M T Dove V Heine |
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| Institution: | (1) Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EQ, United Kingdom, GB;(2) Cavendish Laboratory, University of Cambridge, Madingley Road, Cambridge CB3 0HE, United Kingdom, GB |
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| 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.
Received: 10 March 1998 / Revised, accepted 15 January 1999 |
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