Density of amorphous $\mathrm{Ge}{\mathrm{O}}_2$ to 133 GPa with possible pyritelike structure and stiffness at high pressure

Germanium oxide is a prototype network-forming oxide with pressure-induced structural changes similar to those found in crystals and amorphous silicate oxides at high pressure. Studying density and coordination changes in amorphous $\mathrm{Ge}{\mathrm{O}}_2$ allows for insight into structural changes in silicate oxides at very high pressure, with implications for the properties of planetary magmas. Here, we report the density of germanium oxide glass up to 133 GPa using the x-ray absorption technique, with very good agreement with previous experimental data at pressure below 40 GPa and recent calculation up to 140 GPa. Our data highlight four distinct compressibility domains, corresponding to changes of the local structure of $\mathrm{Ge}{\mathrm{O}}_2$. Above 80 GPa, our density data show a compressibility and bulk modulus similar to the counterpart crystal phase, and we propose that a compact distorted sixfold coordination, similar to the structural motif of the pyritelike crystalline $\mathrm{Ge}{\mathrm{O}}_2$ polymorph, is likely to be stable in that pressure range. Our density data point to a smooth continuous evolution of the average coordination for pressure above 20 GPa with persistent sixfold coordination, without sharp density or density slope discontinuities. These observations are in very good agreement with theoretical calculations and spectroscopic measurements, and our results indicate that glasses and melts may behave similarly to their high-pressure solid counterparts with comparable densities, compressibility, and possibly average coordination.

Figure 1

Data collection scheme inside the DAC, here in a low-pressure run with a double-polished $\mathrm{Ge}{\mathrm{O}}_2$ plate immersed in a methanol:ethanol mixture. (a) Map through the diamonds from which the sample's dimensions are determined (b). (c) Absorption map through the Be gasket. (d) A profile extracted from the map. (e) Correlation between (b) and (d) to derive the linear absorbance of the sample, further used to calculate the density at HP.

Figure 2

Density results for $\mathrm{a}\text{−}\mathrm{Ge}{\mathrm{O}}_2$ (squares, this study) compared to other experimental and modeling data. Data from Hong et al., (2007) [11] (circles) measured using the absorption method; data from x-ray tomography, Liu et al., (2013) [46] (diamonds) and triangles; data calculated with ab initio calculation, Du and Tse, (2017) [41]. Black dashed line represents the value for the density of the crystals $\mathrm{Ge}{\mathrm{O}}_2$, Dutta et al., (2018) [1].

Figure 3

(a) $f\text{−}F$ plot revealing the different compressibility domains with a domain above 80 GPa that could be linked to a sixfold distorted environment for Ge as in the pyrite structure or it may be the start of a higher coordination. (b) Equation of state fits for the different domains.