Pressure-induced changes in local electronic structures of crystalline and noncrystalline SiO2 and MgSiO3 phases in Earths interior : Insights from ab initio calculations of x-ray Raman scattering spectrum

Abstract

The potential presence of high-density SiO2- and MgSiO3-rich silicate melts has been suggested to be one of the origins of the ultralow velocity zones (ULVZ) at the lowermost mantle region. The pressure dependences of the transverse acoustic wave velocities of SiO2 and MgSiO3 glasses have been explored up to ~207 GPa to understand the elastic properties of SiO2- and MgSiO3-rich silicate melts in Earths interior. They have been suggested to be correlated with the coordination transition of Si atoms upon compression. However, because the elastic properties of amorphous oxides are determined from the short-range structures and associated electronic structures, the correlation between the densification processes of SiO2 and MgSiO3 glasses and associated changes in the electronic structures, as well as their elastic properties, should be discussed more precisely. The in situ high-pressure x-ray Raman scattering (XRS) experiments can probe the element-specific electronic bonding structures and associated local atomic structures around the target element of Earth materials at high pressures. Thus, it has been used to explore the structural changes around O atoms of SiO2 and MgSiO3 glasses from upon compression from the pressure-induced bonding transitions. Despite the efforts in the previous studies, establishing the direct correlation between the structural changes around O atoms and the evolution in O K-edge XRS features of SiO2 and MgSiO3 glasses has been experimentally challenging because of the intrinsic structural disorder in the amorphous oxides. Recent advances in the ab initio calculations have provided the opportunity to explore the electronic structures and XRS features of Earth materials at extremely high pressure that cannot be easily achieved in the current in situ high-pressure XRS experiments. Here, the l-resolved partial density of states (PDOS) and O K-edge XRS features of the SiO2 and MgSiO3 high-pressure polymorphs and the high-density noncrystalline MgSiO3 melts in a pressure range from ~0 to ~131 GPa were systematically calculated using the ab initio calculations. The pressure-induced changes in O K-edge XRS features of SiO2 and MgSiO3 high-pressure polymorphs, including an emergence of double-peak-like features of the SiO2 high-pressure polymorphs, are revealed to be correlated with the enhanced proximity between neighboring O atoms. In addition, the significant changes in O K-edge XRS features of MgSiO3 melts upon compression seem to be correlated with the decreases in interatomic distances around O atoms, primarily the O-O distances. Therefore, the pressure-induced changes in O K-edge XRS spectra of amorphous SiO2 and MgSiO3 glasses, from the in situ high-pressure XRS experiments, might be indicative of the decreases in interatomic distances around O atoms, particularly the O-O distances, rather than the coordination transition of Si and O atoms. These results suggest that the electronic structures of the crystalline and noncrystalline SiO2 and MgSiO3 phases at high pressures are strongly affected by changes in the O-O distances upon compression. Because changes in the elastic properties of amorphous oxides are induced from the short-range structural changes and associated changes in their electronic structures, the pressure dependences of transverse wave acoustic wave velocities of SiO2 and MgSiO3 glasses should be explained with changes in the nearest neighboring O-O distances rather than changes in the Si coordination environment. In further, I expect that the current study can be applied to future studies of the pressure-induced bonding transition of a wide range of crystalline and noncrystalline oxides at extremely high pressures.