Structural origin of negative thermal expansion in high-temperature silica polymorphs

The alpha and beta modifications of quartz and cristobalite silica have been successfully simulated using molecular dynamics simulations based on a single parametrization of a charge transfer three-body potential. The simulated alpha forms exhibit positive thermal expansion; it is almost zero for beta cristobalite up to 1500 K and slightly negative at higher temperatures, while a negative thermal expansion of beta quartz is observed immediately above the alpha-to-beta transition. A detailed analysis of atomic trajectories reveals that the origin of negative thermal expansion in the high-temperature beta forms of silica is a gradual reactivation of the same displacement mode that underlies the transformation between the alpha and beta modifications.