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During the decompression of plastically deformed glasses at room temperature, some aspects of irreversible densification may be preserved. This densification has been primarily attributed to topological changes in glass networks. The changes in short-range structures like cation coordination numbers are often assumed to be relaxed upon decompression. Here the NMR results for aluminosilicate glass upon permanent densification up to 24 GPa reveal noticeable changes in the Al coordination number under pressure conditions as low as ∼6 GPa. A drastic increase in the highly coordinated Al fraction is evident over only a relatively narrow pressure range of up to ∼12 GPa, above which the coordination change becomes negligible up to 24 GPa. In contrast, Si coordination environments do not change, highlighting preferential coordination transformation during deformation. The observed trend in the coordination environment shows a remarkable similarity to the pressure-induced changes in the residual glass density, yielding a predictive relationship between the irreversible densification and the detailed structures under extreme compression. The results open a way to access the nature of plastic deformation in complex glasses at room temperature.

Structural transition in amorphous oxides, including glasses, under extreme compression above megabar pressures (>1 million atmospheric pressure, 100 GPa) results in unique densification paths that differ from those in crystals. Experimentally verifying the atomistic origins of such densifications beyond 100 GPa remains unknown. Progress in inelastic X-ray scattering (IXS) provided insights into the pressure-induced bonding changes in oxide glasses; however, IXS has a signal intensity several orders of magnitude smaller than that of elastic X-rays, posing challenges for probing glass structures above 100 GPa near the Earth's core-mantle boundary. Here, we report megabar IXS spectra for prototypical BO glasses at high pressure up to ∼120 GPa, where it is found that only four-coordinated boron (B) is prevalent. The reduction in the B-O length up to 120 GPa is minor, indicating the extended stability of-bonded B. In contrast, a substantial decrease in the average O-O distance upon compression is revealed, suggesting that the densification in BO glasses is primarily due to O-O distance reduction without the formation of B. Together with earlier results with other archetypal oxide glasses, such as SiO and GeO, the current results confirm that the transition pressure of the formation of highly coordinated framework cations systematically increases with the decreasing atomic radius of the cations. These observations highlight a new opportunity to study the structure of oxide glass above megabar pressures, yielding the atomistic origins of densification in melts at the Earth's core-mantle boundary.

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.

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