Absence of higher than sixfold 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 nonbonded cation–cation distance <R Ge…Ge > demonstrate that edge-sharing octahedra remain the main structural motifs 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 and symmetrization. Above 100 GPa, octahedral distortion becomes the prevailing mechanism. Compared to its crystalline analogues (α-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.