Theoretical and Experimental Evidence for a Post-Cotunnite Phase Transition in Hafnia at High Pressures

Using first-principles density-functional theory (DFT) computations, we have predicted a new post-cotunnite (OII) phase of hafnia (HfO2) at high pressures. Our computations, using the generalized gradient approximation (GGA), predict a phase transition from OII to a Fe2P-type structure at similar to 120 GPa (similar to 140 GPa) with a slight volume collapse at the transition pressure of similar to 0.2% (similar to 0.1%) between the two phases using the second- (third-) order Birch-Murnaghan equation of state, respectively. The prediction of the new phase is consistent with recent experiments and computations performed on similar dioxides titania (TiO2) and zirconia (ZrO2) at extreme pressure-temperature (p-T) conditions. Importantly, our theoretical prediction for the OII Fe2P transition in HfO2 is experimentally supported by the re-analysis of X-ray diffraction patterns of HfO2 at extreme pressure-temperature conditions. Additionally, the equation of state and hardness of the predicted phase have been computed and show that Fe2P-type phase while less compressible than the OII phase is nearly identical in hardness, indicating that none of the HfO2 phases qualify as superhard.