Oxygen Quadclusters in ${\mathrm{SiO}}_2$ Glass above Megabar Pressures up to 160 GPa Revealed by X-Ray Raman Scattering

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. X-ray Raman scattering (XRS) spectra for ${\mathrm{SiO}}_2$ glass up to 1.6 Mbar reveal the evolution of heavily contracted oxygen environments characterized by a decrease in average O-O distance and the potential 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 the possible existence of stable melts in the planetary interiors.

Figure 1

Oxygen $K$-edge XRS spectra of ${\mathrm{SiO}}_2$ glass at pressures up to $\sim 160\mathrm{GPa}$ plotted as the normalized scattered photon intensity vs the energy loss (incident energy-elastic energy). See Fig. S2 in the SM [11] for the XRS spectra without smoothing in accordance with the spectral uncertainty.

Figure 2

Calculated oxygen $K$-edge XRS spectra for α-quartz (1 atm), stishovite ($\sim 29.1\mathrm{GPa}$), and $\alpha \text{−}{\mathrm{PbO}}_2$-type ${\mathrm{SiO}}_2$ ($\sim 120\mathrm{GPa}$) with the Gaussian broadening factor of 1 eV (see Sec. 7 of the SM [11] for the calculation methods). The $K$-edge XRS spectra of $a\text{−}{\mathrm{SiO}}_2$ under similar pressure conditions and that at 160 GPa are shown for comparison.

Figure 3

(a) Variation of the edge energy at the center of gravity ($E_c$) in the oxygen $K$-edge XRS spectrum for $a\text{−}{\mathrm{SiO}}_2$ (black circles) with varying pressure. The $E_c$ values for crystalline silica polymorphs are also shown (blue triangles). The pale blue symbols are based on the three polymorphs shown in Fig. 2 (see Sec. 7 of the SM [11]). (b) $E_c$ values of crystalline ${\mathrm{SiO}}_2$ phases with varying O-O distances. (c) Relationship between $E_c$ and density for crystalline and amorphous ${\mathrm{SiO}}_2$. The densities of the $a\text{−}{\mathrm{SiO}}_2$ glasses were from a recent experiment [62].

Figure 4

Changes in the average oxygen coordination number ($C_{\mathrm{O}}$) and $E_c$ of ${\mathrm{SiO}}_2$ glass up to 160 GPa. The $C_{\mathrm{O}}$ values are calculated from the average silicon coordination numbers ($C_{\mathrm{Si}}$) from previous studies [blue circle [42]; blue diamond [40]; blue triangle [41]; red circle (this Letter), based on $E_c$ in Fig. 3; see Sec. 2 of the SM [11] ]. Schematics of the oxygen configurations with varying coordination numbers (top) and ultralow velocity zone (ULVZ) at the lower mantle are shown.