A-site dependent metastable polar phases from polarization compatible octahedral rotations in perovskite AZrO3 (A = Ca, Sr)

We investigate the electronic structures and ferroelectric properties of perovskite CaZrO₃ and SrZrO₃ using first-principles density functional theory. Both materials are in the non-polar Pnma space group with [Formula: see text] octahedral rotation pattern. However, in the absence of the octahedral rotation, a polar instability exists for both materials, which is suppressed as the magnitude of the [Formula: see text] octahedral rotation increases. This suggests that metastable polar structures can emerge by having an alternative octahedral rotation pattern different from Pnma, maintaining the polar instability. We investigate metastable polar structures in which polar distortions from unstable phonon modes are efficiently enumerated by constraining the octahedral rotation patterns. Our structural search identifies nine metastable polar structures of CaZrO₃. The ferroelectric properties of the metastable polar phase of CaZrO₃ are further investigated in which the polar R3c structure is expected as a most promising candidate material with a polarization of 53 μC/cm². In contrast, there are no metastable polar phases in SrZrO₃. We find that the increased tolerance factor suppresses the energy barriers between the structures with different octahedral rotation patterns, and regardless of the starting octahedral rotation patterns, the ionic relaxation leads to the ground state Pnma structure. Our results identify new metastable polar phases for CaZrO₃, which can be stabilized in thin film/substrate geometry as the interfacial coupling can constrain the octahedral pattern of the thin film. Moreover, we show the importance of a small A-site radius in stabilizing the metastable polar phases, providing new insight into the role of large octahedral rotation angles in inducing ferroelectricity. We believe that our work would provide useful guidelines for finding new polar materials in thin-film geometry, potentially having enhanced piezoelectricity and mechanical switching behavior.