Search

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 GeO2glass up to 148 GPa, the highest pressure ever reached in an experimental study of GeO2glass, 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 GeO2glass 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-Zoxides 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.

The structural adaptation in MgSiO3melts under compression up to 130 GPa is the key to revealing the origins of the pronounced negative buoyancy of the melts at the core‐mantle boundary (CMB). A full understanding of the melt densification requires study of the pressure‐induced changes in the bonding configuration around oxygen at the CMB, which has proven to be difficult to measure. Here, the experimental breakthrough in O K‐edge inelastic X‐ray scattering enables collection of the spectra of MgSiO3glasses up to ~130 GPa, along with ab initiomolecular dynamics simulations, revealing the electronic bonding transitions around heavily compressed oxygen. The spectral results indicate the emergence of denser network structures around oxygen, stemming from contractions in the Mg‐O and O‐O distances associated with flexible topological and short‐range rearrangements around Si. The results unveil the electronic structure and thus the nature of densification in dense partial melts at the CMB. A thin ultralow velocity zone at the bottom of mantle is characterized by reduced seismic wave velocities and enhanced density, suggesting the possible presence of molten silicates at a depth of ~2,850 km (~130 GPa of pressure). How the melts densify near the core‐mantle boundary is unclear. Because oxygen occupies the major volume fraction of silicates, the melt densification is dominated by the reorganization of oxygen during compression. However, the oxygen bonding environments in MgSiO3melt—a model mantle melt—near the core‐mantle boundary are unknown. Here, the inelastic X‐ray scattering spectra of MgSiO3glasses and liquids up to ~130 GPa revealed the electronic bonding transitions around compressed oxygen that are not observed at low pressures and substantially differ from those in the crystalline MgSiO3bridgmanite. The spectral results indicate the pressure‐induced emergence of denser network structures around oxygen, stemming from gradual contractions in the Mg‐O and O‐O distances associated with flexible topological and short‐range rearrangements around Si. The densification around oxygen near megabar pressures potentially contributes to the gravitational stabilization of the complex partial melts in the deeper part of the mantle toward ultralow velocity zone. The O K‐edge inelastic X‐ray scattering spectra of MgSiO3glass up to 130 GPa revealed the bonding transitions around oxygen at CMBThe emergence of dense oxygen networks near megabar pressure stems from contractions in the Mg‐O and O‐O distances, unique to MgSiO3meltsThe results provide insights into densification mechanism of dense partial melts anticipated to comprise the ULVZ at the CMB

Probing the pressure-induced structural changes and the extent of disorder in aluminosilicate glasses and melts at high pressure remains a challenge in modern physical and chemical sciences. With an aim of establishing a systematic relationship between pressure, composition, and glass structures, we report 27Al and 17O 3QMAS NMR spectra for sodium aluminosilicate glasses [Na2O:Al2O3:SiO2= 1.5:0.5:2nwith n= 1 (NAS150520, XSiO2= 0.5), 2 (NAS150540, XSiO2= 0.67), and 3 (NAS150560, XSiO2= 0.75)], quenched from melts at pressures up to 8 GPa. We also explore the stability of the [4]Al–O–[4]Al cluster in the highly depolymerized, NAS150520, glass at high pressure. For given glass composition, the [5,6]Al peak intensity increases with increasing pressure. The population of [5,6]Al increases linearly with XSiO2from NAS150520 (XSiO2= 0.5) to NAS150560 glass (XSiO2= 0.75) at both 6 and 8 GPa. The [5,6]Al/XSiO2ratio also tends to increase with pressure, indicating a possible relationship between [5,6]Al fraction and XSiO2that depends on pressure. The effect of pressure on the network connectivity in the sodium aluminosilicate glasses is manifested in the increase in [4]Si–O–[5,6]Al peak intensity and the decrease in the nonbridging oxygen (NBO) fraction with increasing pressure. The fraction of [4]Si–O–[5,6]Al in NAS150520 is smaller than in NAS150560. Taking into consideration the pressure-induced Al coordination transformation in the fully polymerized glass (albite, Na2O:Al2O3:SiO2= 1:1:6, NBO/T = 0), the fraction of [5,6]Al at a given pressure varies nonlinearlywith variations of NBO/T. [5,6]Al fraction at 8 GPa increases with decreasing degree of melt polymerization from ∼8% for fully polymerized albite melt (NBO/T = 0) to ∼37% for partially depolymerized melt (NAS150560, at NBO/T = 0.29). Then it gradually decreases to ∼15% for NAS150520 with further increase in NBO/T of 0.67. This observed trend in the densification behavior at a given pressure indicates competing densification mechanisms involving steric hindrance vs changes of NBO fraction in the silicate melts. The NMR results also suggest that both NBO and BO, particularly [4]Si–O–[4]Si, interact with Na+, and thus the Na+distribution is likely to be homogeneous around both NBO and BO at high pressure without spatial segregation of silica-rich and alkali-rich domains for the glass compositions studied here. The presence of the [4]Al–O–[4]Al cluster is distinct in the NMR spectrum for NAS150520 glass at both 6 and 8 GPa. A new scheme of pressure-induced structural transitions in silicate melts involving [4]Al–O–[4]Al includes the formation of [4]Al–O–[5]Al.