High-pressure EXAFS study of vitreous GeO2 up to 44 GPa

High-pressure extended x-ray absorption fine-structure measurements were performed on amorphous GeO2 over increasing and decreasing pressure cycles at pressures up to 44 GPa. Several structural models based on crystalline phases with fourfold, fivefold, and sixfold coordination were used to fit the Ge-O first shell. The Ge-O bond lengths gradually increased up to 30 GPa. Three different pressure regimes were identified in the pressure evolution of the Ge-O bond distances. Below 13 GPa, the local structure was well described by a fourfold "quartzlike" model whereas a disordered region formed by a mixture of four- and five-coordinated germanium-centered polyhedra was observed in the intermediate pressure range between 13 and 30 GPa. Above 30 GPa the structural transition to the maximum coordination could be considered complete. The present results shed light on the GeO2 densification process and on the nature of the amorphous-amorphous transition, suggesting that the transition is more gradual and continuous than what has been previously reported.