Structural Origin of Negative Thermal Expansion in High-Temperature Silica Polymorphs

The α and β 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 α forms exhibit positive thermal expansion; it is almost zero for β cristobalite up to 1500 K and slightly negative at higher temperatures, while a negative thermal expansion of β quartz is observed immediately above the α-to-β transition. A detailed analysis of atomic trajectories reveals that the origin of negative thermal expansion in the high-temperature β forms of silica is a gradual reactivation of the same displacement mode that underlies the transformation between the α and β modifications.