As oxygen may occupy a major volume of oxides, a densification of amorphous oxides under extreme compression is dominated by reorganization of oxygen during compression. X-ray Raman scattering (XRS) spectra for ${\mathrm{SiO}}_2$ glass up to 1.6 Mbar reveal the evolution of heavily contracted oxygen environments characterized by a decrease in average O-O distance and the potential emergence of quadruply coordinated oxygen (oxygen quadcluster). Our results also reveal that the edge energies at the centers of gravity of the XRS features increase linearly with bulk density, yielding the first predictive relationship between the density and partial density of state of oxides above megabar pressures. The extreme densification paths with densified oxygen in amorphous oxides shed light upon the possible existence of stable melts in the planetary interiors.

The structural adaptation in MgSiO₃ melts under compression up to 130 GPa is the key to revealing the origins of the pronounced negative buoyancy of the melts at the core‐mantle boundary (CMB). A full understanding of the melt densification requires study of the pressure‐induced changes in the bonding configuration around oxygen at the CMB, which has proven to be difficult to measure. Here, the experimental breakthrough in O K‐edge inelastic X‐ray scattering enables collection of the spectra of MgSiO₃ glasses up to ~130 GPa, along with ab initio molecular dynamics simulations, revealing the electronic bonding transitions around heavily compressed oxygen. The spectral results indicate the emergence of denser network structures around oxygen, stemming from contractions in the Mg‐O and O‐O distances associated with flexible topological and short‐range rearrangements around Si. The results unveil the electronic structure and thus the nature of densification in dense partial melts at the CMB.

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 2p-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.

Atmospheric gases trapped in polar ice have been used to reconstruct polar and global climate changes, providing better time resolution when less diffused. Experiments have shown that gas diffusion in ice is negligible on a laboratory time scale, but its cumulative impact on old glacial ice (>1M yr) remains unclear. Here, we employ density functional theory calculations to investigate the diffusion mechanism of gases trapped in ice-Ih from the atomistic level. The results suggest that the diffusion energy barrier between interstitial sites is primarily dependent on the atomic size and charge distribution of hopping gases. The diffusion of noble gases (He, Ne, Ar, Kr, and Xe) primarily occurs via the interstitial mechanism, consistent with previous results of classical molecular dynamics simulations. In contrast, the precisely determined diffusion paths and energy barriers for CO₂, O₂, and N₂ suggest that these molecular gases prefer to hop along the hexagonal channel also via the interstitial mechanism, and the bond-breaking mechanism proposed previously to explain the diffusion of those molecular gases as fast as Ne may be unnecessary.

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 α-quartz (1 atm) to a pyrite-type structure (at ∼271 GPa), beyond the current experimental pressure limit (∼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 $\left(d_{\mathrm{O}\text{−}\mathrm{O}}\right) <\sim 2.5 \AA$ and a subsequent merging of the doublet patterns with a further decrease in $d_{\mathrm{O}\text{−}\mathrm{O}}$ down to $\sim 2.3–2.4 \AA$. Ab initio simulations of the XRS spectrum of pyrite-type ${\mathrm{SiO}}_2$ phase at $\sim 271 \mathrm{GPa}$ revealed the significant electron dispersion in $2p^{*}$ states and the pronounced electron density between edge-sharing oxygen atoms with $d_{\mathrm{O}\text{−}\mathrm{O}}$ of ∼2.1 Å, 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 $\left(E_M\right)$ and the edge energy at the center of mass of the spectrum $\left(E_C\right)$ of XRS patterns beyond multimegabar pressures. Particularly, the $E_M$ and $E_C$ increase linearly with decreasing oxygen proximity $\left(d_{\mathrm{O}\text{−}\mathrm{O}}\right)$ and bulk density up to 270 GPa. The $E_M$ and $E_C$ for ${\mathrm{MgSiO}}_3$ phases show larger changes with varying $d_{\mathrm{O}\text{−}\mathrm{O}}$ 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 $d_{\mathrm{O}\text{−}\mathrm{O}}$ (and density) up to ∼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.