Atomistic origins of pressure-induced changes in the O K-edge x-ray Raman scattering features of SiO2 and MgSiO3 polymorphs: Insights from ab initio calculations

作者：Yi Yoo Soo; Lee Sung Keun^*

来源：Physical Review B, 2016, 94(9): 094110.

DOI：10.1103/PhysRevB.94.094110

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, MgSiO3 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 SiO2 and MgSiO3 high-pressure phases [E-A (eV) approximate to -10.9d(O-O)(angstrom) + 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 SiO2 and MgSiO3 at high pressures. These results can be applied to studies of the pressure-bonding transitions in a wide range of oxides under extreme compression.

• 出版日期2016-9-26