Molecular dynamics of amorphous silica at very high pressures (135 GPa): Thermodynamics and extraction of structures through analysis of Voronoi polyhedra

Using equilibrium molecular-dynamics techniques, we have investigated the equation of state and structural characteristics of amorphous silica up to 135 GPa in the temperature range 3002500 K, fully incorporating anharmonic effects within the context of the classical approximation. The thermal expansivity and constant-volume heat capacity both decrease with increasing pressure. There is a weak maximum in the thermal expansivity in the 2030-GPa range. The structure of amorphous silica is described with use of Voronoi polyhedra and the connectivity of edge-sharing coordination polyhedra. Basic coordination states for sevenfold- and eightfold-coordinated silicon atoms have been identified. The pressure-induced invasion of cubic silicon-centered Voronoi polyhedra by two oxygen atoms tends to occur on edge-sharing corners whereas invasion involving more than two oxygen atoms tends to be randomly distributed. The distribution of volumes of silicon-centered Voronoi polyhedra shows an anomalous variance near 70 GPa that is associated with a rapid rise in the connectivity of edge-sharing coordination polyhedra. This phenomenon may be related to the high-pressure enhancement of shear strength in silica glass previously observed experimentally.