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 00.3em0exto0.3em0ex~1500.3em0exGPa) and temperature (3000--60000.3em0exK) 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 1500.3em0exGPa. 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 Gr\"uneisen 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-Gr\"uneisen 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 40000.3em0exK---they initially increase with pressure and then decrease upon further compression. This anomalous behavior is absent at higher temperatures.