First-principles prediction of post-pyrite phase transitions in germanium dioxide

Two high-pressure phase transitions in GeO2 have been discovered through first-principles computer simulations: the first is a transition from the pyrite-type (FeS2) to cotunnite-type (alpha-PbCl2) structure predicted to occur at a pressure of similar to 300 GPa, and the second is a transition from the cotunnite-type to the hexagonal Fe2P-type structure at similar to 600 GPa. The former is accompanied by a remarkable volume reduction of 5.4%, while the latter has a distinctive but quite small volume change of 0.3%. The post-pyrite transition to the cotunnite-type structure is expected from known high-pressure behavior of other dioxides, while the post-cotunnite transition to an Fe2P-type structure is quite unexpected, with no report in any dioxides so far except for a recent study on SiO2. The Fe2P-type phase has higher effective coordination numbers of Ge atoms, which contributes to stabilizing this structure relative to cotunnite. The results obtained extend our knowledge of the ultrahigh-pressure crystallography of dioxide materials.