Polyhedral units and network connectivity in GeO2 glass at high pressure: An X-ray total scattering investigation

Abstract

We report a pressure-induced dense tetrahedral intermediate state via Ge–O–Ge rotation formed at 3–5 GPa and the polyhedral relations in GeO2 glass up to 17.5 GPa using in situ X-ray total scattering and X-ray absorption (XAFS) techniques. It was found that the nearest-neighbor Ge-Ge correlations show a decrease reaching a minimum between 4 and 6 GPa, and exhibit negative compression behavior at 7–17.5 GPa. The Ge–Ge distance determined by XAFS shows a substantial reduction, i.e., normal compression behavior, at 7–17.5 GPa. The comparison with the theoretical g(r) function for rutile-type GeO2 (16.1 GPa) indicates that the negative compression of intermediate range order reflects the direct formation of GeO6 octahedral units. Results of coordination number analysis show that GeO2 glass undergoes a transition from tetrahedral GeO4, to GeO5 units (possibly triangular bipyramidal), and finally to octahedral GeO6 units. The present investigation provides the structural details of the polyhedral units and their relationships in GeO2 glass at high pressure.