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 competition of stationary electronic flat bands and a pressure-induced shift of electronic states to lower energies.