Investigation of structural polymorphism in GeO2 glass by XAFS up to 60 GPa

The amorphous polymorphism of the classic network-forming tetrahedral-oxide glasses such as GeO2 and SiO2, is fundamentally important in the theories of glass structure. Our recent density measurements show that GeO2 glass exhibits a complicated compression process from tetrahedral to the octahedral forms. In this work, we report in situ x-ray absorption fine structure (XAFS) measurement for GeO2 glass in a diamond anvil cell (DAC) up to 60 GPa. GeO2 powder was put into a Pt capsule and placed inside an oven at 1390°C for 12 hrs and allowed to air-cool, resulting in a large piece of GeO2 glass. Both x-ray diffraction and Raman measurements showed the sample was amorphous. The sample was loaded into diamond anvil cells with Rhenium gasket which was preindented to about 30 μm in thickness and drilled one hole of 80 μm in diameter at center. Pressure was measured with ruby chips placed at the edge of sample chamber. Ge K-edge XAFS spectra show that Ge-O distance has a minimum at 4 GPa, and a change from slow to fast increase at 6 GPa. A shoulder at higher side of the first peak of radial structure function appears at 6 GPa and merged to one peak at 11 GPa with broad asymmetric distribution. Further compression leads to the improvement in the symmetry of radial structure function. These results present a microscopic scene for the structural changes in GeO2 glass and confirm the existence of the intermediate state located at 6-10 GPa. Acknowledgment We thank Guoyin Shen and Jiyong Zhao for helps in the experiments.