First-principles simulations of liquid silica: Structural and dynamical behavior at high pressure

We have carried out first-principles molecular dynamics simulations of silica liquid over a wide range of pressure (from 0 to similar to 150 GPa) and temperature (3000-6000 K) within density functional theory and the pseudopotential approximation. Our results show that the liquid structure is highly sensitive to compression: the average Si-O coordination number increases from 4 at zero pressure initially slowly on compression and then more rapidly after 30% compression, reaching 6.5 at 150 GPa. At low compression, nearly all Si-O coordination environments are fourfold and relatively undistorted, whereas at high compression several coordination types (five-, six-, and sevenfold) coexist and the polyhedra are significantly distorted. The heat capacity and Gruneisen parameter show little variation with compression within the low-pressure regime and vary rapidly with compression in the high-pressure regime. Results are successfully fitted to the Mie-Gruneisen equation of state and show no evidence of spinodal instability or a temperature of maximum density. The behavior of the self-diffusion coefficient is consistent with a crossover from strong to fragile liquid behavior with increasing temperature and increasing pressure. Both Si and O self-diffusion coefficients vary anomalously at 4000 K-they initially increase with pressure and then decrease upon further compression. This anomalous behavior is absent at higher temperatures.