Flexoelectricity-driven giant polarization in (Bi, Na)TiO₃-based ferroelectric thin films

This study demonstrates the use of oxygen vacancy-induced planar defects to significantly enhance electrical polarization through a local flexoelectric effect. By introducing an appropriate level of aliovalent dopants, numerous local planar defects are induced in (Bi_0.5, Na_0.5)TiO₃-based thin films. These defects, identified as oxygen-deficient structures through direct visualization of oxygen atoms and oxygen vacancies using integrated differential phase-contrast microscopy, result in the formation of head-to-head domain structures. Geometric phase analysis confirms that these structures exhibit a substantial local strain gradient of up to 10⁹m^-1, contributing significantly to the flexoelectric polarization. Consequently, a giant maximum polarization (P_m) of 161 μC cm^-2 under 750 kV cm^-1 and a remanent polarization P_r = 115 μC cm^-2 along with a coercive field of 250 kV cm^-1 are achieved, allowing these (Bi_0.5, Na_0.5)TiO₃-based thin films to be used in low-power electronic applications. Crucially, the P_m and P_r of the thin films can be sustained at 133 and 98 μC cm^-2, respectively, at 230 °C. Additionally, they exhibit exceptional high-temperature fatigue endurance, with P_m and P_r demonstrating a negligible reduction of less than 9% after 10⁷ cycles under 750 kV cm^-1 at 230 °C. These values surpass those previously reported for oxide perovskite thin films at elevated temperatures, demonstrating potential applications of our thin films in high-temperature environments. Our findings offer promising avenues for advancing the application fields of ferroelectric thin films.