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Despite its important geophysical implications, direct probing of the local electronic structure of mantle minerals, such as MgSiO 3  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 MgSiO 3  polymorphs in Earth’s interior. Here, we calculate the O K-edge energy-loss near-edge structure (ELNES) spectra for SiO 2  and MgSiO 3  polymorphs (i.e., α-quartz, stishovite, enstatite, ilmenite-type MgSiO 3 , MgSiO 3  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 SiO 2  and MgSiO 3  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 MgSiO 3  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 MgSiO 3  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 MgSiO 3  post-perovskite shows a predicted main feature at ~543-545 eV, approximately 3 eV higher than that of MgSiO 3  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-MgSiO 3  and post-perovskite is also observed. These results confirm that the oxygen K-edge features at ~540-550 eV for MgSiO 3  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.

american mineralogist

Pressure-induced changes in local electronic structures of SiO 2  and MgSiO 3  polymorphs: Insights from ab initio calculations of O K-edge energy-loss near-edge structure spectroscopy