Ramp Compression of Germanium Dioxide to Extreme Conditions: Phase Transitions in an SiO2 Analog

The high-pressure (HP) behavior of dioxides is of interest due to their extensive polymorphism and role as analogs for SiO2, a phase expected to be important in the deep mantles of Earth and terrestrial exoplanets. Here we report on dynamic ramp compression of quartz-type germanium dioxide GeO2 to stresses up to 882 GPa, a higher peak stress than previous studies by a factor of 5. X-ray diffraction data show that HP-PdF2-type GeO2 occurs under ramp loading from 154 to 440 GPa, and this phase persists to higher pressure than predicted by theory. Above 440 GPa, we observe evidence for transformation to a new phase of GeO2. Based on the diffraction data, the best candidate for this new phase is the cotunnite-type structure which has been predicted to be a stable phase of GeO2 above 300 GPa. The HP-PdF2-type and cotunnite-type structures are important phases in a wide range of AX2 compounds, including SiO2, at multihundred GPa stresses. Our results demonstrate that ramp compression can be an effective technique for synthesizing and characterizing such phases in oxides. In addition, we show that pulsed x-ray diffraction under ramp compression can be used to examine lower-symmetry phases in oxide materials.