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Advances in synchrotron x-ray Raman scattering (XRS) techniques enable probing of pressure-induced element-specific bonding transitions in diverse oxides beyond megabar pressures. Evolution of the electronic structures under extreme pressure involves an emergence of additional oxygen $1s$ electron excitation pattern and shifts in the electronic states toward high-energy ranges in an XRS spectrum. Structural origins of such changes and proper spectral proxy for the corresponding evolution in electronic density of states remain to be established. Here, we calculated the O $K$-edge features in the XRS spectrum for diverse crystalline ${\mathrm{SiO}}_{2}$ polymorphs, from \ensuremath{\alpha}-quartz (1 atm) to a pyrite-type structure (at \ensuremath{\sim}271 GPa), beyond the current experimental pressure limit (\ensuremath{\sim}160 GPa). The ab initio results unravel the electronic origins of the evolution in the XRS patterns, such as the emergence of the bimodal feature with decreasing oxygen-oxygen distance $({d}_{\mathrm{O}\text{\ensuremath{-}}\mathrm{O}}) l\ensuremath{\sim}2.5 \AA{}$ and a subsequent merging of the doublet patterns with a further decrease in ${d}_{\mathrm{O}\text{\ensuremath{-}}\mathrm{O}}$ down to $\ensuremath{\sim}2.3--2.4 \AA{}$. Ab initio simulations of the XRS spectrum of pyrite-type ${\mathrm{SiO}}_{2}$ phase at $\ensuremath{\sim}271 \mathrm{GPa}$ revealed the significant electron dispersion in $2{p}^{*}$ states and the pronounced electron density between edge-sharing oxygen atoms with ${d}_{\mathrm{O}\text{\ensuremath{-}}\mathrm{O}}$ of \ensuremath{\sim}2.1 \AA{}, in contrast to an earlier prediction with a negligible electron density between those oxygen atoms. The pressure-induced increase in electron interactions leads to a systematic change in the edge energy at half of the spectrum area $({E}_{M})$ and the edge energy at the center of mass of the spectrum $({E}_{C})$ of XRS patterns beyond multimegabar pressures. Particularly, the ${E}_{M}$ and ${E}_{C}$ increase linearly with decreasing oxygen proximity $({d}_{\mathrm{O}\text{\ensuremath{-}}\mathrm{O}})$ and bulk density up to 270 GPa. The ${E}_{M}$ and ${E}_{C}$ for ${\mathrm{MgSiO}}_{3}$ phases show larger changes with varying ${d}_{\mathrm{O}\text{\ensuremath{-}}\mathrm{O}}$ than those in ${\mathrm{SiO}}_{2}$ phases, demonstrating the effect of the electronic hybridization between Mg and O on the evolution of the XRS patterns. The strong linear correlation among ${E}_{M}$, ${E}_{C}$, and ${d}_{\mathrm{O}\text{\ensuremath{-}}\mathrm{O}}$ (and density) up to \ensuremath{\sim}270 GPa confirms that the ${E}_{M}$ and ${E}_{C}$ can be effective spectral proxies to quantify the densification of crystalline and noncrystalline oxides. The results with the statistical evaluations of the electronic density of states provide improved prospects for the prediction of the evolution in core-level excitation features of diverse complex, multicomponent oxides under multimegabar pressure conditions toward the deep lower mantle of Earth and other super-Earth bodies.

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.

Despite its important geophysical implications, direct probing of the local electronic structure of mantle minerals, such as MgSiO3 perovskite and post-perovskite is experimentally challenging. Recent advances in ab initio calculations have allowed us to explore the details of the local electronic bonding structure around oxygen in MgSiO3 polymorphs in Earth’s interior. Here, we calculate the O K -edge energy-loss near-edge structure (ELNES) spectra for SiO2 and MgSiO3 polymorphs (i.e., α-quartz, stishovite, enstatite, ilmenite-type MgSiO3, MgSiO3 perovskite, and post-perovskite) using ab initio calculations based on the full-potential linearized planewave (FP-LAPW) method. The calculated O K -edge ELNES spectra for SiO2 and MgSiO3 polymorphs show characteristic oxygen K -edge features caused by distinctive local atomic configurations and topology around oxygen, and are in good agreement with previous experimental O K -edge X-ray Raman scattering (XRS) results. The O K -edge ELNES spectra for α-quartz and enstatite show similar edge features at ~538 eV, which is characteristic of corner-sharing oxygen sites ([4]Si-O-[4]Si). The spectra for stishovite and ilmenite-type MgSiO3 show edge features with double peaks at ~537–538 and ~541–543 eV due to an electronic excitation from an oxygen in edge-sharing topology. The spectrum for MgSiO3 perovskite shows a broad peak spanning from ~538 to ~543 eV, which results from corner-sharing oxygen with two six-coordinated silicon ([6]Si-O-[6]Si). The calculated O K -edge ELNES spectrum for MgSiO3 post-perovskite shows a predicted main feature at ~543–545 eV, approximately 3 eV higher than that of MgSiO3 perovskite. These O K -edge features systematically shift to higher energy with increasing degree of densification in atomic arrangement in the polymorphs (from enstatite, ilmenite, perovskite, to post-perovskite), indicating an increase in the energy of unoccupied oxygen 2 p -state with pressure. The calculated O K -edge spectra also show the effect of densification on the changes in the edge features for the crystallographically distinct oxygen sites: the features for the corner-sharing oxygen move to higher energy from enstatite, perovskite, to post-perovskite. A drastic peak shift for edge-sharing O atoms in ilmenite-MgSiO3 and post-perovskite is also observed. These results confirm that the oxygen K -edge features at ~540–550 eV for MgSiO3 glass at pressures above ~20 GPa can be due to densification of the atomic configurations around oxygen in melt networks associated with enhanced proximity between oxygen atoms. The current methods also shed light on a unique opportunity to probe the pressure-induced electronic bonding transitions and topology in diverse simple and complex oxides in Earth’s interior using ab initio calculations of O K -edge ELNES spectra.

Revealing the extent of disorder in amorphous oxides is one of the remaining puzzles in physical chemistry, glass sciences, and geochemistry. Here, we report the 27Al NMR results for amorphous Al2O3 thin films obtained from two different deposition methods (i.e., physical vapor-deposition and atomic layer-deposition), revealing two distinct amorphous states defined by a fraction of five-coordinated Al ([5]Al). The fractions of [4]Al and [5]Al are dominant (∼92−95%) in both films. While the overall similarity between these two states suggests a narrow stability of available amorphous states, the fraction of [5]Al in atomic layer-deposited thin films is apparently larger and thus more disordered than that in physical vapor-deposited films. Such results require that varying extents of disorder exist in the amorphous oxides prepared under different processing conditions. As the [5]Al site (

Whereas prototypical Al(2)O(3) is not a glass former, amorphous Al(2)O(3) can be formed as thin films through vapor deposition and can serve as a structural model for the Al(2)O(3) glass. The first two-dimensional solid-state NMR experiments for amorphous Al(2)O(3) thin film reveal that four- and five-coordinated species are predominant (95%), while six-coordinated species are minor. Such a species distribution is remarkably similar to what has been predicted theoretically for Al(2)O(3) melts. Upon annealing to 800 degrees C the five-coordinated species becomes negligible, indicating the onset of crystallization of Al(2)O(3).