Coordination environment of silicon in silica glass up to 74 GPa: An x-ray Raman scattering study at the silicon L edge

Near-edge x-ray Raman scattering (XRS) spectra of the silicon L edge have been obtained on SiO2 glass under compression in a diamond anvil cell up to 74 GPa. Partial densities of states (DOS's) of electrons, which the spectra reflect, have been calculated. The spectra for the glass do not show significant variations with pressure, whereas distinct differences are observed between quartz and stishovite, providing clear evidence for the nonexistence of sixfold-coordinated silicon by oxygen, that is stishovitelike silicon, in silica glass up to 74 GPa. A post main-edge hump, which is not seen in the XRS spectra and DOS's of quartz and stishovite, and an edge energy shift suggest that the silicon is in a different coordination environment under pressure. A compression mechanism of SiO2 glass which involves the formation of fivefold-coordinated silicon following the decrease in Si-O-Si angle is proposed to explain the observed changes up to 74 GPa.