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

Abstract

Theoretical and experimental evidence for a post-cotunnite phase transition in hafnia at high pressuresUsing first-principles density-functional theory (DFT) computations, we have predicted a new post-cotunnite (OII) phase of hafnia (HfO 2 ) at high pressures. Our computations, using the generalized gradient approximation (GGA), predict a phase transition from OII to a Fe 2 P-type structure at ~ 120 GPa (~ 140 GPa) with a slight volume collapse at the transition pressure of ~ 0.2 % (~ 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 (TiO 2 ) and zirconia (ZrO 2 ) at extreme pressure-temperature conditions. Importantly, our theoretical prediction for the OII → Fe 2 P transition in HfO 2 is experimentally supported by the re-analysis of Xray diffraction patterns of HfO 2 at extreme pressure-temperature (p, T) conditions. Additionally, the equation of state and hardness of the predicted phase have been computed and show that Fe 2 P-type phase while less compressible than the OII phase is nearly identical in hardness, indicating that none of the HfO 2 phases qualify as superhard.

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

“…This is also in good agreement with another recent theoretical study which predicts a transition pressure of ∼120–140 GPa . These authors also suggest that weak peaks observed alongside strong cotunnite-type peaks in their measured diffraction pattern at ∼105 GPa may belong to the Fe 2 P-type phase . Our work provides unambiguous evidence for the existence of and complete transformation to this phase.…”

“…The Fe 2 P-type phase is predicted to become stable with respect to the cotunnite-type phase above 128, 111, and 137 GPa by using PBE, PBEsol, and SCAN functionals, respectively. This is also in good agreement with another recent theoretical study which predicts a transition pressure of ∼120–140 GPa . These authors also suggest that weak peaks observed alongside strong cotunnite-type peaks in their measured diffraction pattern at ∼105 GPa may belong to the Fe 2 P-type phase .…”

“…Our DFT calculations extend to higher pressures than existing computational studies , and reveal new features of the high-pressure behavior in these systems. We find evidence for a re-entrant cotunnite-type phase (inset Figure ) above 305–314 GPa in HfO 2 , which then undergoes a further pressure-induced transformation into the hexagonal Ni 2 In-type structure at 390–469 GPa.…”

“…Existing studies − on selected dioxides of groups 4 and 14, AO 2 (A = Sn, Pb, Hf), are limited to relatively lower pressures, and no conclusive evidence for post-cotunnite phases has been reported. Examination of compounds in these groups enables study of how phase transitions vary with electronic structure and ionic radius.…”

Phase relations, thermal conductivity and elastic properties of ZrO2 and HfO2 polymorphs at high pressures and temperatures

“…According to our data, the high-pressure P 6̄2 m modification is stable above 126 GPa, which is in excellent agreement with the results of previous experimental (>125 GPa) 24 and theoretical studies (120 and 128 GPa). 24,55 The resulting Clapeyron slope of the phase boundary (+5.7 MPa K −1 ) (Fig. 3) is similar to that found in experiments (+6.9 MPa K −1 ).…”