Atomistic origins of pressure-induced changes in the OK-edge x-ray Raman scattering features ofSiO2andMgSiO3polymorphs: Insights fromab initiocalculations

Abstract

Despite its fundamental importance in condensed matter physics and geophysical implications, establishing the systematic and direct link between the pressure-induced structural changes in crystalline and noncrystalline low-$z$ oxides and their corresponding evolution in O $K$-edge core-electron excitation features under extreme compression has been challenging. Here we calculated the site-resolved partial density of states and O $K$-edge x-ray Raman scattering (XRS) spectra for two of the important oxide phases in the Earth's lower mantle, $\mathrm{MgSi}{\mathrm{O}}_{3}$ bridgmanite and post-bridgmanite, up to 120 GPa using ab initio calculations, revealing the electronic origins of the O $K$-edge features for oxides under compression. The absorption threshold $({E}_{A})$ and band gap increase linearly with a decrease in the O-O distance in diverse $\mathrm{Si}{\mathrm{O}}_{2}$ and $\mathrm{MgSi}{\mathrm{O}}_{3}$ high-pressure phases $[{E}_{A}(\mathrm{eV})\ensuremath{\approx}\ensuremath{-}10.9{d}_{\text{O-O}}(\AA{})+34.4]$, providing a predictive relationship between the ${E}_{A}$ and the O-O distances in the oxide at high pressure. Despite densification, upon isobaric phase transition from bridgmanite to post-bridgmanite at 120 GPa, a decrease in band gap results in a decrease in edge energy because of an increase in O-O distance. The oxygen proximity is a useful structural proxy of oxide densification upon compression, as it explains the pressure-induced changes in O $K$-edge XRS features of crystalline and amorphous $\mathrm{Si}{\mathrm{O}}_{2}$ and $\mathrm{MgSi}{\mathrm{O}}_{3}$ at high pressures. These results can be applied to studies of the pressure-bonding transitions in a wide range of oxides under extreme compression.