Molecular dynamics simulations of SiO 2 melt and glass; ionic and covalent models

Computer simulations using the molecular dynamics (MD) technique have been carried out on SiO₂ melt and glass ranging from 300 to 9000 K and 0 to 175 kbar. The MD simulations used two sets of interatomic potentials—a simple ionic model with Bom-Mayer repulsion terms and a central-force-field covalent potential—both of which were derived from quantum mechanical calculations on molecular clusters. Thermodynamic, structural, and diffusion data were obtained from these model systems and compared to experimental values wherever possible.

The MD results indicate that earlier X-ray diffraction data may need to be re-interpreted with respect to the SiOSi angle distribution. Bond-length and bond-angle responses to pressure and temperature changes compare favorably with experimental and theoretical studies on the α-quartz structure. Ring-distribution analyses show that planar rings containing three silica tetrahedra are present in the simulated glass. Predictions of second-order thermodynamic properties of the system (C_v, α, β) reveal inadequacies in the ionic approximation when applied to vitreous silica.

Arrhenius plots of In D vs. temperature and pressure between 2000 and 6000 K and between 0 and 200 kbar show that at 6000 K, D increases with pressure from 1.3 × 10⁻⁵ at 40 kbar to 6.1 × 10⁵ cm²/s at 100 kbar, then decreases to 1.5 × 10⁻⁵ at 175 kbar. Detailed analysis of the diffusion mechanism at pressures less than 100 kbar indicates that it is defect-controlled with high correlations between the percentages of 3- and 5-fold silicons, nonbridging oxygens, and the diffusion rate.