Perturbing tetragonal HfO2 supercells in simulation with dopants or interstitial oxygen in the context of the ferroelectric Pca21 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 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 Hf1−xZrxO2 thin films with varying Zr content are consistent with the claim.

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A systematic simulation study of low‐energy phases of HfO2 and ZrO2, including 24‐atomic cells derived from 180° interphases, leads to a new monoclinic Pc phase (mIII) with a monoclinic angle and a polarization of 0.3 C m−2. This opens up a new transition path toward the monoclinic P2 1 /c phase (m) starting from the tetragonal P4 2 /nmc phase (t), with comparable lower‐energy barriers, where the high‐symmetric phases show the largest temperature dependence. The tetragonal P4 2 /nmc phase, the cubic (c) phase, and the less investigated cubic (cII) phase are favored.

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