Absence of higher than sixfold coordination in glassy GeO₂ up to 158 GPa revealed by X-ray absorption spectroscopy

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 GeO₂ 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 (Ge-O>). Between 10 and 30 GPa, the evolution of 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 CaCl₂-type phase of GeO₂. 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 Ge-O> and the nonbonded cation-cation distance Ge…Ge> demonstrate that edge-sharing octahedra remain the main structural motifs in glassy GeO₂. Up to 100 GPa, compaction proceeds primarily via distortions of octahedral O-Ge-O bond angles, accompanied by octahedral bond shortening and symmetrization. Above 100 GPa, octahedral distortion becomes the prevailing mechanism. Compared to its crystalline analogues (α-PbO₂ and pyrite-like phase), glassy GeO₂ exhibits a slightly less efficient compaction mechanism, likely due to kinetic constraints that inhibit reconstructive lattice rearrangements.