STRUCTURAL TRANSFORMATION IN DENSIFIED SILICA GLASS - A MOLECULAR-DYNAMICS STUDY

Pressure-induced structural transformation and the concomitant loss of intermediate-range order (IRO) in high-density SiO2 glass are investigated with the molecular-dynamics (MD) approach. The MD simulations cover a wide range of mass densities-from normal density (2.20 g/cm3) to the density corresponding to stishovite (4.28 g/cm3). This twofold increase in the density produces significant changes in the short-range order and intermediate-range order. As the density increases from 2.20 to 4.28 g/cm3, the Si-O bond length increases from 1.61 to 1.67 angstrom, the Si-O and O-O coordinations change from 4 to 5.8 and from 6 to 12, respectively, and the O-Si-O bond angle changes from 109-degrees to 90-degrees. These results provide firm evidence of structural transition from a corner-sharing Si(O1/2)4 tetrahedral network to a network of Si(O1/3)6 octahedra jointed at corners and edges. At normal density, the first sharp diffraction peak (FSDP) in the static structure factor S(q) is at 1.6 angstrom-1 whereas under pressure the height of the FSDP is considerably diminished and its position shifts to larger q values. At a density of 2.64 g/cm3, a peak in S(q) appears at 2.85 angstrom-1. The height of this peak grows as the density increases. All of these results are in agreement with the recent high-pressure x-ray measurements on SiO2 glass.