Molecular dynamics study of cristobalite silica using a charge transfer three-body potential: Phase transformation and structural disorder

Structural and dynamic properties of cristobalite silica have been studied using molecular dynamics simulations based on a charge transfer three-body potential model. In this potential model, the directional covalent bonding of SiO2 is characterized by a charge transfer function of the interatomic distance between Si and O atoms, and in the form of Si-O-Si and O-Si-O three-body interactions. The dynamic properties such as infrared spectra and density of states at room and elevated temperatures are in excellent agreement with experiments, and are also consistent with the recently proposed rigid unit modes model. The alpha- and beta-cristobalite crystallographic structures are well reproduced in this model, and the transition between these modifications occurs reversibly and reproducibly in simulations, both as a result of changes in pressure and temperature. The thermally induced transition results in a significantly more disordered beta-cristobalite than the pressure-induced beta-cristobalite at room temperature. While simulated alpha-cristobalite exhibits a positive thermal expansion coefficient, it is almost zero for beta-cristobalite up to 2000 K and slightly negative at higher temperatures, confirming results from recent x-ray diffraction experiments and other simulations with potential models based on ab initio calculations. (C) 2003 American Institute of Physics.