Amorphous boron oxide at megabar pressures via inelastic X-ray scattering

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 B₂O₃ glasses at high pressure up to ∼120 GPa, where it is found that only four-coordinated boron (^[4]B) is prevalent. The reduction in the ^[4]B-O length up to 120 GPa is minor, indicating the extended stability of sp³-bonded ^[4]B. In contrast, a substantial decrease in the average O-O distance upon compression is revealed, suggesting that the densification in B₂O₃ glasses is primarily due to O-O distance reduction without the formation of ^[5]B. Together with earlier results with other archetypal oxide glasses, such as SiO₂ and GeO₂, 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.