Ultrahigh-Pressure Behavior of AO2 (A = Sn, Pb, Hf) Compounds

The high-pressure behavior of metal dioxides, AO2, is of wide interest due to their extensive polymorphism. In this study, high-pressure phase transitions in dioxides of selected group 4 and 14 elements (SnO2, PbO2, and HfO2) were examined by using in situ X-ray diffraction in the laser-heated diamond anvil cell to ∼200 GPa and theoretical density functional theory calculations to 600 GPa. The cotunnite-type phase was found to be stable up to the maximum pressure in SnO2 and PbO2. For HfO2, a transition from the cotunnite to the Fe2P-type phase was observed in experiments at pressures >125 GPa, in agreement with our theoretical computations that predict a transition pressure of 111–137 GPa. Our calculations also predict a re-entrant cotunnite phase in HfO2 above 305–314 GPa that subsequently transforms into the Ni2In-type phase at 390–469 GPa. The transition sequences predicted in these oxides are consistent among three different exchange-correlation functionals and can be explained by the energetic compe...