XAFS study of GeO₂ glass under pressure

Using a large-volume high-pressure apparatus, Li₂O-4GeO₂ glass and pure GeO₂ gel have been compressed to 14 GPa at room temperature and their local structural changes have been investigated by an in situ XAFS (x-ray absorption fine-structure) method. On compression of Li₂O-4GeO₂ glass, the Ge-O distance gradually becomes short below 7 GPa, showing the conventional compression of the GeO₄ tetrahedron. Abrupt increase in the Ge-O distance occurs between 8 and 10 GPa, which corresponds to the coordination number (CN) changing from 4 to 6. The CN change is completed at 10 GPa. On decompression, the reverse transition occurs gradually below 10 GPa. In contrast to the case for Li₂O-4GeO₂ glass, the Ge-O distance in GeO₂ gel gradually increases over a pressure range from 2 to 12 GPa, indicating that continuous change in CN occurs. The Ge-O distance at 12 GPa is shorter than that of Li-4GeO₂ indicating that the change in CN is not completed even at this pressure. On complete release of pressure, the Ge-O distance reverts to that of the starting gel.