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

Ramp loading enables the compression of materials to high densities corresponding to the deep interiors of Earth and extrasolar planets. GeO2 is one of a family of dioxides whose high- pressure behavior has been of significant interest as an analogue for SiO2, which is one of the plausible mineral candidates for the deep mantle of terrestrial exoplanets. Here we report on dynamic ramp compression of germanium dioxide, GeO2, to stresses up to 884 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 GPa 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 multi-hundred GPa stresses. This study provides insights into transition pathways of SiO2 under extreme conditions. Our results also 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.