Coordination variability and the structural components of silica glass under high pressures

The description of the structure of siliceous melts as the silicon ions transform from four- to six-fold coordination with increasing pressure is difficult because their structures range from open networks at low pressures to ones which are more nearly close-packed at pressures of 25-30 GPa. The effect of these coordination modifications on the density, diffusivity, viscosity and melting behavior of silicate liquids is of vital importance to the transport of partial melt in the mantle. Using molecular dynamics calculations, we have investigated the physical and structural characteristics of amorphous silica up to pressures of 135 GPa. The pairwise potential of S. Tsuneyuki and coworkers has previously been proven to be remarkably versatile in the simulation of several crystalline silica polymorphs. We point out some problems connected with its parameterization when used for amorphous phases at high pressures or high temperatures. We find large concentrations (30-50%) of five-fold coordinated Si atoms in square pyramidal geometry between 15 and 30 GPa. We describe some alternative interpretations of melt structure involving the Delaunay network and these are consistent with a model of liquid structure involving mixing of four-, five- and six-fold coordinated Si atoms with an associated negative excess mixing volume coupled with an ideal entropy of mixing on the order of 1-1.75 R. Neighbor cage backscattering effects are largely controlled by physical neighbor distances (those within the first radial distribution function coordination shell) and are not sensitive to the volume reduction of the surrounding Voronoi cell, but their behavior at long times may be controlled by the decreased Voronoi cell anisotropy at higher pressures.