Study of Mechanisms of Structural Phase Transitions in HfO <sub>2</sub> Based on Soft Phonon Mode

Due to the existence of metastable polar orthorhombic (PO) Pca21 phase, surprising ferroelectricity emerges in HfO2 thin films. However, the origin and stability of the ferroelectricity in HfO2 are still unsolved problems. Due to the coexistence of multiphase system in HfO2, the major way to stabilize the metastable PO phase is to transform the nonpolar phase to the PO phase via phase transition in HfO2. In this work, we used the first-principles calculations for phonon dispersion curves to find soft modes to verify the occurrence of phase transition. Owing to the unique anti-ferrodistortive phase transition characteristic, HfO2presents totally different mechanisms of phase transition, compared with that of the traditional perovskite ferroelectrics. Through the results analyzed by soft-mode theory, we found the possible phase transition paths of HfO­­2 (i.e., cubic to tetragonal phase, tetragonal to PO phase, PO to monoclinic phase and tetragonal to monoclinic phase). To identify the specific phase transition paths, the relations between the phase transition and the soft modes in phonon dispersion curves were analyzed by applying the hydrostatic pressure to HfO2. Moreover, by controlling the hydrostatic pressure, this research finds out the different types of phase transition in HfO2 and PO phase could be made stable under the hydrostatic pressure of equal to or larger than 20 GPa.