The sensitivity of physical and spectral properties of silica glass to variations of interatomic potentials under high pressure

We have investigated with molecular dynamics various aspects of the structure and vibrational properties of silica glass at a temperature near 330 K to pressures of 40 GPa for three sets of interatomic potentials using 1500 atoms. The potentials were chosen from previous works. Only one of these potentials shows a transition from tetrahedral silicon coordination to octahedral silicon coordination. For this potential the percentage of five-fold coordinated silicon increases from 12% at 10 GPa to 35% at 15 GPa. The geometry of the five-coordinated complexes is that of a distorted pyramid with the silicon situated at the base, or, equivalently, it is an octahedron with one vertex missing. The shape of the OSiO distribution for the five-fold-coordinated complex is similar for all three potentials, but they are present in lesser amounts in the two which do not show the tetrahedral to octahedral transition. In the potential that shows the transition the distribution of SiOSi angles is bimodal at pressures above 30 GPa, coinciding with large populations of two-silicon rings. Analysis of the density of states reveals that two of the potentials give enhanced contributions from the frequencies in the 800-900 cm^-1 range which agrees with in situ infrared spectroscopic studies of silica glass. The major effect of pressure on the velocity autocorrelation function is to increase the amount of negatively correlated motion for silicon atoms during the approach to the first maximum.