Oxygen Quadclusters inSiO2Glass above Megabar Pressures up to 160 GPa Revealed by X-Ray Raman Scattering

Abstract

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. Here, X-ray Raman scattering spectra (XRS) for SiO 2 glass up to 1.6 megabar reveal the evolution of heavily contracted oxygen environments characterized by a decrease in average O-O distance and the 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 possible existence of stable melts in the planetary interiors.Compressed glasses that undergo densification far above megabar pressures (i.e., pressures above 100 GPa) may consist of heavily entangled amorphous structures that are different from those at lower pressures, as well as from those observed in crystalline analogues. While the structures of major crystals in Earth's mantle at pressures greater than 100 GPa are well understood, the atomic configurations of non-crystalline silicates at the bottom of the mantle at pressures of ~130 GPa remain unclear due to experimental difficulties [1,2]. Furthermore, considering much larger radii of the super-Earths, the silicate parts in those planetary bodies are extended into much higher pressure conditions.The detailed structures of silicate melts at the pressure conditions much higher than that of Earth's lower mantle remains elusive.

Structural Transitions in MgSiO3 Glasses and Melts at the Core‐Mantle Boundary Observed via Inelastic X‐ray Scattering

“…Broad feature at ~544–545 eV emerges at ~30 GPa and persists until 130 GPa. The higher‐energy features are similar to the O K ‐edge features reported for the IXS spectra for SiO 2 glass at high pressure (Lee et al, ; Lin et al, ; Petitgirard et al, ): this feature has been attributed to the densification of the local network around oxygen in oxide glasses: suggested origins include an increase in the oxygen coordination number (i.e., formation of [2] O at low‐pressure conditions and [3] O at high‐pressure conditions) and associated changes in the medium‐range length scale as well as topological evolution (e.g., increase in Si‐O bond length and decrease in the Si‐O‐Si bond angle) (Lee et al, ). Recent ab initio simulations of crystalline MgSiO 3 indicated that the feature arises from diverse structural origins and is mainly correlated with a reduction in the O‐O distance (Yi & Lee, ).…”

“…For the MgSiO 3 glasses, Figure a presents the systematic shifts in the edge energy at the center of gravity ( E c ) values of the MgSiO 3 crystals (Yi & Lee, , ), glasses (Lee et al, ), and liquids with increasing density. As has been addressed in our studies of IXS spectra for diverse oxides at high pressure (Lee et al, , ; Yi & Lee, ), the E c of MgSiO 3 crystals and melts tend to increase linearly with bulk density. Furthermore, the E c values for MgSiO 3 glasses estimated from the experimental IXS spectra (blue circles) form a trend line showing a rapid increase up to ~50 GPa and a rather gentle increase up to ~130 GPa, while there is a slight increase in the slope at ~90–100 GPa (Figures b and c).…”

“…The simulations showed pressure‐induced increases in the covalency of the Mg‐O bonds in the melts, whereas those in the crystals remained largely unchanged (Yi & Lee, ) (Text S6; Figure S7). Recent IXS studies of SiO 2 glasses revealed the pressure‐induced shift and broadening of the O K ‐edge features, similar to the MgSiO 3 glasses and melts (Lee et al, ; Lin et al, ; Petitgirard et al, ). The calculated electronic structures for SiO 2 crystals and glasses also present substantial contributions from the oxygen d ‐orbitals (Wu et al, ; Yi & Lee, , ), in contrast to the MgSiO 3 melts, implying that an increase in the structural disorder upon the addition of Mg plays a major role in the electronic interactions for MgSiO 3 melts at high pressure.…”

“…The detailed analysis of the O K ‐edge IXS features of MgSiO 3 glasses combined with the ab initio MD simulations revealed the densification of the depolymerized silicate magmas at the lowermost mantle. Recent IXS studies on simple oxide crystals and glasses have revealed that overall shifts in the O K ‐edge features, particularly shifts in the edge onset energy serve as structural proxies of the interatomic distances and bulk densities (Lee et al, , ; Yi & Lee, , ). For the MgSiO 3 glasses, Figure a presents the systematic shifts in the edge energy at the center of gravity ( E c ) values of the MgSiO 3 crystals (Yi & Lee, , ), glasses (Lee et al, ), and liquids with increasing density.…”

“…However, the pressure condition of the earlier study was not sufficient to fully account for the changes in melt properties at CMB (Text S1). While the inefficiency of IXS poses challenges for probing structures above 100 GPa, recent experimental advances in X‐ray optics involving the postcollimation of scattered X‐rays enabled collection of oxygen K ‐edge IXS signals for simple oxides, such as B 2 O 3 and SiO 2 above megabar pressures (Chow et al, ; Lee et al, ; Lee et al, ). These studies offer opportunities to study transitions in oxygen environments in complex glasses.…”