Molecular dynamics of liquid SiO2 under high pressure

We have investigated with molecular dynamics the transport properties and velocity autocorrelation functions (VAF) for liqud SiO2 up to pressures of 20 GPa and at temperatures of 4000 K using 2521371 particles. A systematic study of the sensitivity of the self-diffusion coefficient to the number of particles in the system showed that around 800 are required for convergence. At low pressures the VAF reveals that motions of silicon and oxygen atoms are decoupled over the 5075-fs range, across the second negative peak in the VAF of oxygen. As the pressure increases, the decoupling is shifted into the 2550-fs range, over the first positive peak in the VAF of oxygen. In the spectral domain, the major feature on increasing pressure is the enhanced contribution of frequencies in the 8001000-cm-1 region. This is consistent with high-pressure infrared spectroscopic measurements on silica glass. Analysis of the distribution of coordination numbers revealed that both the fivefold- and sixfold-coordinated silicon become prevalent between 7 and 20 GPa. This accords well with recent findings from NMR studies of silicate glasses. We propose that the importance of fivefold-coordinated states in determining the self-diffusion coefficient needs to be reassessed.