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 300-2500 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 20-30-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.