Electronic bonding transition in compressed ${\mathrm{SiO}}_2$ glass

Knowledge of the electronic structure of amorphous and liquid silica at high pressures is essential to understanding their complex properties ranging from silica melt in magma to silica glass in optics, electronics, and material science. Here we present oxygen near $K$-edge spectra of ${\mathrm{SiO}}_2$ glass to $51\mathrm{GPa}$ obtained using x-ray Raman scattering in a diamond-anvil cell. The x-ray Raman spectra below $\sim 10\mathrm{GPa}$ are consistent with those of quartz and coesite, whereas the spectra above $\sim 22\mathrm{GPa}$ are similar to that of stishovite. This pressure-induced spectral change indicates an electronic bonding transition occurring from a fourfold quartzlike to a sixfold stishovitelike configuration in ${\mathrm{SiO}}_2$ glass between $10\mathrm{GPa}$ and $22\mathrm{GPa}$. In contrast to the irreversible densification, the electronic bonding transition is reversible upon decompression. The observed reversible bonding transition and irreversible densification call for a coherent understanding of the transformation mechanism in compressed ${\mathrm{SiO}}_2$ glass.

Figure 1

Oxygen $K$-edge spectra in compressed ${\mathrm{SiO}}_2$ glass by x-ray Raman spectroscopy. The main feature of the spectra between $1\mathrm{GPa}$ and $14\mathrm{GPa}$ is an intense peak located at $\sim 538\mathrm{eV}$ (labeled as $Q$), whereas an additional, intense peak appears at $\sim 544\mathrm{eV}$ at pressures above $19\mathrm{GPa}$ (labeled as $S$). The spectrum at $3\mathrm{GPa}$ was collected from the sample quenched from $\sim 45\mathrm{GPa}$. No prominent post edge feature above $547\mathrm{eV}$ is observed because of the low statistics resulting from the XRS experiments in a high-pressure DAC. $P$, an unidentified peak at $10.459\mathrm{keV}$ (likely due to micro x-ray diffraction); the peak intensity changed significantly when the taken-off angle ($2\theta$ of $\sim 30°$) was varied.

Figure 2

Oxygen $K$-edge spectra in ${\mathrm{SiO}}_2$ crystals (a) and glass (b) by x-ray Raman spectroscopy. These spectra were normalized by the integrated intensity from energy range between $534\mathrm{eV}$ and $547\mathrm{eV}$. The intense peak at $\sim 538\mathrm{eV}$ in $\alpha$-quartz and coesite arises from the tetrahedral bonding, whereas the most intense peak at $\sim 544\mathrm{eV}$ in stishovite is attributed to the octahedral bonding (Refs. 14, 15, 16, 17). (c) First principles, theoretically calculated oxygen $K$-edge spectra for $\alpha$-quartz at $0\mathrm{GPa}$ and stishovite at $40\mathrm{GPa}$.

Figure 3

Normalized and integrated peak intensity of the sixfold stishovitelike bonding as a function of pressure (gray circles). The XRS spectra between $534\mathrm{eV}$ and $547\mathrm{eV}$ were first normalized to 1, and the peak region resembling the octahedral bonding from $540\mathrm{eV}$ to $547\mathrm{eV}$ were then integrated after subtracting the spectrum from that of $\alpha$-quartz (solid circle) collected at ambient conditions and setting the fraction of the integrated area ratio of stishovite (solid triangle) to one. The integrated area ratio of coesite (solid diamond) is also plotted for comparison. The dashed line is a guide to the change of the integrated area ratio as a function of pressure.