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

We report a pressure-induced dense tetrahedral intermediate state via Ge-O-Ge rotation formed at 3-5 GPa and the polyhedral relations in GeO ₂ 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 GeO ₂ (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 GeO ₆ units. The present investigation provides the structural details of the polyhedral units and their relationships in GeO₂ glass at high pressure.