Pressure-driven evolution in the electronic bonding properties of MgO in the super-Earth interior up to ∼4 TPa via core-electron excitation spectroscopy: Ab initio calculations

The discovery of the super-Earth bodies has increased the need to understand planet-forming materials at extremely high pressures, crucial for insights into planetary formation and interior dynamics. Recent progress in high-pressure core-electron excitation spectroscopy, such as x-ray Raman scattering (XRS) experiments, and theoretical calculations offer an opportunity to investigate the evolution of the electronic structures of various compressed materials under such extreme pressure conditions. Despite its significance in planetary and condensed matter physics, the electronic structure of MgO, a major component of super-Earths and a prototypical ionic compound, have not yet been fully studied at pressures corresponding to the deep interiors of super-Earth bodies due to the experimental challenges. Here, we present a theoretical investigation of the XRS spectra, band structures, electron localization functions (ELFs), and Bader charges of MgO at pressures up to ∼4 TPa. The XRS patterns show the pressure-driven changes in XRS spectral shape and the emergence of new peaks with different slopes (with respect to density and interatomic distances), resulting from enhanced interatomic interactions (i.e., enhanced hybridization between O $2p$ and Mg $3s\text{−}3p$ states and between adjacent O $2p$ states). The calculated results are characterized with a pressure-driven delocalization of the unoccupied O $p$ and Mg $p$ states, revealing the electronic bonding behavior of MgO under extreme compression, far beyond the current experimental limit with XRS of ∼200 GPa. The ELF analysis provides a comprehensive understanding of the factors driving the nonlinear trend of absorption edge onset in the calculated XRS patterns and the direct-to-indirect band gap transition of MgO under compression. Together with the Bader charge analysis, it also shows how MgO undergoes a pressure-induced bonding transition from ionic to mixed ionic-covalent without a structural phase transition. This transition of the bonding behavior of MgO under extreme compression contributes to our understanding of material properties under extreme conditions relevant to the interiors of super-Earths. The current result for the prototypical ionic compound under extreme densification offers insights into deepening the fundamental understanding of ionic compounds under extremely high pressures. As the extreme pressure conditions of the current simulations cannot yet be reached in experiment, our breakthrough would be useful to guide future experimental efforts aimed at uncovering metal oxides with pressure-tunable electronic properties.