Absence of higher than 6-fold coordination in glassy $GeO_{2}$ up to 158 GPa revealed by X-ray absorption spectroscopy

Abstract

Simple binary oxide glasses can exhibit a compression behavior distinct from that of their crystalline counterparts. In this study, we employed high-pressure X-ray absorption spectroscopy coupled to the diamond anvil cell to investigate in detail local structural changes around Ge in glassy $GeO_{2}$ up to 158 GPa. We conducted four independent runs, both with and without pressure-transmitting media. Up to 30 GPa, we observed no significant influence of the pressure medium on the pressure dependence of the $Ge-O$ bond length ($<R_{Ge-O}>$). Between 10 and 30 GPa, the evolution of $<R_{Ge-O}>$ shows substantial variability across our experiments and previous works. The measured values lie close to those reported for crystalline polymorphs, including the rutile- and $CaCl_{2}$-type phase of $GeO_{2}$. This finding suggests that the amorphous structure possesses considerable flexibility to transition among different atomic configurations. From 30 GPa to 158 GPa, our results for both $<R_{Ge-O}>$ and the non-bonded cation-cation distance $<R_{Ge...Ge}>$ demonstrate that edge-sharing octahedra remain the main structural motives in glassy $GeO_{2}$. Up to 100 GPa, compaction proceeds primarily via distortions of octahedral $O-Ge-O$ bond angles accompanied by octahedral bond shortening. Above 100 GPa, octahedral distortion becomes the prevailing mechanism. Compared to its crystalline analogues ($\alpha-PbO_{2}$ and pyrite-like phase), glassy $GeO_{2}$ exhibits a slightly less efficient compaction mechanism, likely due to kinetic constraints that inhibit reconstructive lattice rearrangements.