Phase transitions in HfO2 probed by first-principles computations.

Ever since ferroelectricity was discovered in HfO 2 , the question of its origin remains controversial. Here, we probe this question using a combination of Landau theory of phase transitions and first-principles computations. In such an approach, the energy landscape associated with the phase transition between cubic and different experimentally demonstrated phases of HfO 2 (tetragonal, monoclinic, orthorhombic Pbca, orthorhombic Pnma, and orthorhombic Pca 2 1) is explored using density functional theory calculations. Computations revealed that stabilization of all but orthorhombic Pbca phase is driven by a single unstable zone-boundary antipolar mode X 2 −. When coupled with zone-center modes (Γ 1 + and Γ 3 +), it stabilizes the tetragonal phase. Coupling with four additional modes (Γ 5 + , X 3 − , X 5 − , X 5 +) results in the monoclinic phase, which is the ground state of the material. If, however, Γ 5 + mode is replaced with Γ 4 − mode, orthorhombic polar phase Pca 2 1 is stabilized. The application of this framework to examine the effect of electric field on the ferroelectric phase of hafnia reveals that the field of 5 MV/cm is capable of stabilizing ferroelectric phase over the monoclinic one at 0 K. [ABSTRACT FROM AUTHOR]