Ramp Compression of Germanium Dioxide to Extreme Conditions: Phase Transitions in an SiO_{2} Analog

Abstract

The high-pressure (HP) behavior of dioxides is of interest due to their extensive polymorphism and role as analogs for SiO_{2}, 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 GeO_{2} 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-PdF_{2}-type GeO_{2} 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 GeO_{2}. 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 GeO_{2} above 300 GPa. The HP-PdF_{2}-type and cotunnite-type structures are important phases in a wide range of AX_{2} compounds, including SiO_{2}, 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.