A new EXAFS investigation of local structural changes in amorphous and crystalline GeO2 at high pressure

Structural transformations at high pressure in amorphous and quartz-like crystalline GeO2 have been investigated by using a Paris-Edinburgh press coupled to EXAFS spectroscopy. From both the germanium absorption edge position and the Ge-O distance evolution, new detailed information has been obtained about the pressure behavior of the short range order. Crystalline GeO2 undergoes a transformation from four- to six-fold coordination at about 8.5 GPa, but at least the whole 6-12 GPa pressure range should be considered as the transition region. On the other hand, amorphous GeO2 is characterized by a much more gradual structural change and the full octahedral state is not reached at 13 GPa as commonly believed. Furthermore, no support to the recently claimed fully pentahedral intermediate state can be given. EXAFS signals of glassy GeO2 beyond the first Ge-O shell qualitatively confirm the continuous breakdown of the intermediate range order up to 10 GPa.