An unexplored antipolar phase in HfO2 from first principles and implication for wake-up mechanism

Perturbing tetragonal HfO2 supercells in simulation with dopants or interstitial oxygen in the context of the ferroelectric Pca2(1) No. 29 (oIII-phase) formation frequently leads to an unexplored, lower energetic orthorhombic crystal phase with space group Pnma No. 62 (oV-phase). The crystal structure is inequivalent to cottunite oII-phase with the same space group but is closely related to the second ferroelectric Pmn2(1) No. 31 (oIV-phase) space group, as antipolar alignment. To explore the possibility of oIV- or oV-phase formation, we calculate the free energy in harmonic approximation and the energy landscape. Starting from the tetragonal P4(2)/nmc No. 137 phase (t-phase), according to energy and activation barrier, the formation of the oV-phase is more favorable than that of the oIV-phase. Furthermore, exploring the energy landscape of the oIV-oV system, we find possible antiferroelectric-like behavior in HfO2 and Hf0.5Zr0.5O2, but not in ZrO2. We propose the formation of an oV-phase fraction as a possible structural explanation for a contribution to the wake-up phenomenon in HfO2. X-ray diffraction results on 10 nm Hf1-xZrxO2 thin films with varying Zr content are consistent with the claim.