Perturbing tetragonal HfO2 supercells in simulation with dopants or interstitial oxygen in the context of the ferroelectric Pca21 No. 29 (oIIIphase)
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 Pmn21
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 P42/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 Hf1xZrxO2 thin films with varying Zr content are consistent with the claim.