Electronic bonding transition in compressed SiO2 glass

Knowledge of the electronic structure of amorphous and liquid silica at high pressures is essential to understanding their complex properties ranging from silica melt in magma to silica glass in optics, electronics, and material science. Here we present oxygen near K-edge spectra of SiO2 glass to 51 GPa obtained using x-ray Raman scattering in a diamond-anvil cell. The x-ray Raman spectra below similar to 10 GPa are consistent with those of quartz and coesite, whereas the spectra above similar to 22 GPa are similar to that of stishovite. This pressure-induced spectral change indicates an electronic bonding transition occurring from a fourfold quartzlike to a sixfold stishovitelike configuration in SiO2 glass between 10 GPa and 22 GPa. In contrast to the irreversible densification, the electronic bonding transition is reversible upon decompression. The observed reversible bonding transition and irreversible densification call for a coherent understanding of the transformation mechanism in compressed SiO2 glass.