Oxygen-Vacancies-Ordering Triggered Large Ferroelectric Polarization in CaTiO 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, CaTiO 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 CaTiO thin film triggered by oxygen vacancies ordering, the maximum and remanent polarizations reaching up to 68 μC·cm and to 13 μC·cm, 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 Ti along an out-of-plane direction. It raises the suppression of antiferrodistortion and brings excellent ferroelectric performance. The current breakthrough of CaTiO room-temperature ferroelectric may provide a prospect to explore nonpolar perovskites and will enrich ferroelectric families.