The possibility of a post-stishovite phase transition of SiO2 occurring at very high pressures has long been of interest. Recently, a transition from rutile to CaCl2 structure was suggested from the results of high-pressure Raman spectroscopy and X-ray diffraction studies. However because of experimental difficulties at extremely high pressures, the exact structure of this new phase has not yet been confirmed.
GeO2 in an analogue material for SiO2 in which the rutile to CaCl2 transition is expected to occur at considerably lower pressures. We are investigating the possible occurrence of this phase transition in GeO2 by performing a precise X-ray diffraction structural analysis on quenched samples synthesized in the multianvil apparatus. Recent preliminary molecular dynamic calculations have predicted that the rutile to CaCl2 transition occurs in GeO2 at around 30 Gpa. We have therefore attempted synthesis experiments in the 25-30 Gpa pressure range at 600° to 800° using a 6/2.5 (octahedron edge length/truncation edge length, in mm) multianvil assembly. The transition observed in SiO2 is reversible and therefore difficult to quench. We have attempted to quench the high-pressure GeO2 phase by using very fine powder as a starting material. Using such a starting material, we were previously able to quenched a high-pressure phase of ZrO2 which also shows the characteristics of a displacive phase transition.