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 ${\mathrm{SiO}}_2$ 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/${\mathrm{cm}}^3$) to the density corresponding to stishovite (4.28 g/${\mathrm{cm}}^3$). 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/${\mathrm{cm}}^3$, the Si-O bond length increases from 1.61 to 1.67 Å, 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° to 90°. These results provide firm evidence of structural transition from a corner-sharing Si(${\mathrm{O}}_{1/2}$ ${)}_4$ tetrahedral network to a network of Si(${\mathrm{O}}_{1/3}$ ${)}_6$ 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 A${\mathrm{̊}}^{\mathrm{-}1}$ 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/${\mathrm{cm}}^3$, a peak in S(q) appears at 2.85 A${\mathrm{̊}}^{\mathrm{-}1}$. 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 ${\mathrm{SiO}}_2$ glass.