ATOMIC-STRUCTURE OF SIO2 GLASS AND ITS RESPONSE TO PRESSURE

We describe the results of Monte Carlo simulations of SiO2 glass based on a covalent-potential model of tetrahedral Si-O bonding. The potential model has been shown to accurately reproduce the structure, compression mechanisms, and phase stability of the corresponding crystalline and liquid phases. The simulations are in good agreement with the measured equation of state of silica glass. We compare the simulated structure with experimental data directly by performing "experiments" on the simulated glass - determining the expected diffraction pattern based on our simulated structure - and find good agreement with the observed structure. We show that, unlike the case in SiO2 crystals, changes in the local structure of glass are insufficient to account for its compression. Measures of the medium-range structure, including cluster population and geometry and ring statistics, vary significantly with pressure and indicate a significant topological component to the compression of glass. We use a simple model for the effects of ring formation on density to analyze the topological changes and show that characteristic ring size increases with increasing compression, consistent with the increase in ring size with increasing density found previously in tectosilicate crystals and in simulations of SiO2 liquid. We discuss prospects for experimental verification of the predicted pressure-induced structural changes.