Pressure-Driven Changes in the Electronic Bonding Environment of GeO₂ Glass above Megabar Pressures

Noncrystalline oxides under pressure undergo gradual structural modifications, highlighted by the formation of a dense noncrystalline network topology. The nature of the densified networks and their electronic structures at high pressures may account for the mechanical hardening and the anomalous changes in electromagnetic properties. Despite its importance, direct probing of the electronic structures in amorphous oxides under compression above the Mbar pressure (>100 GPa) is currently lacking. Here, we report the observation of pressure-driven changes in electronic configurations and their delocalization around oxygen in glasses using inelastic X-ray scattering spectroscopy (IXS). In particular, the first O K-edge IXS spectra for compressed GeO₂ glass up to 148 GPa, the highest pressure ever reached in an experimental study of GeO₂ glass, reveal that the glass densification results from a progressive increase of oxygen proximity. While the triply coordinated oxygen ^[3]O is dominant below ∼50 GPa, the IXS spectra resolve multiple edge features that are unique to topologically disordered ^[4]O upon densification above 55 GPa. Topological compaction in GeO₂ glass above 100 GPa results in pronounced electronic delocalization, revealing the contribution from Ge d-orbitals to oxide densification. Strong correlations between the glass density and the electronic configurations beyond the Mbar conditions highlight the electronic origins of densification of heavy-metal-bearing oxide glasses. Current experimental breakthroughs shed light on the direct probing of the electronic density of states in high-Z oxides above 1 Mbar, offering prospects for studies on the pressure-driven changes in magnetism, superconductivity, and electronic transport properties in heavy-metal-bearing oxides under compression.