Oxygen-Vacancies-Ordering Triggered Large Ferroelectric Polarization in CaTiO₃ Thin Films

Regulating the rich paraelectric materials into the ferroelectric state is currently urgent due to the scarcity of intrinsic ferroelectric materials for practical applications. As a paradigmatic perovskite, CaTiO₃ is an incipient ferroelectric but hard to induce polarity due to the strong tilting of the oxygen octahedron as the antiferrodistortion. Herein, we report a prominent room-temperature ferroelectric CaTiO₃ thin film triggered by oxygen vacancies ordering, the maximum and remanent polarizations reaching up to 68 μC·cm^-2 and to 13 μC·cm^-2, respectively, as the excellent level in perovskite incipient ferroelectrics to date. The synchrotron X-ray diffraction (XRD), reciprocal space mapping (RSM), and transmission Electron Microscope (TEM) investigations reveal that bulk-like and ferroelectric phases coexist in the film. Scanning transmission electron microscopy (STEM) images and electric diffractions clearly show that the oxygen vacancies orient along [0-11] in the ferroelectric phase. Such ordered oxygen vacancies soften the tilting of oxygen octahedron, leading to the off-center displacements of Ti⁴⁺ along an out-of-plane direction. It raises the suppression of antiferrodistortion and brings excellent ferroelectric performance. The current breakthrough of CaTiO₃ room-temperature ferroelectric may provide a prospect to explore nonpolar perovskites and will enrich ferroelectric families.