Mn-atomic-layered antiphase boundary enhanced ferroelectricity in KNN-based lead-free films

One preferred lead-free ferroelectric, (K,Na)NbO₃, offers prominent features of environmentally benign and excellent piezoelectricity, but suffers from poor ferroelectricity. Mn-doping has improved its electrical properties, yet its site occupancy remained unclear. In this study, Mn-atomic-layered antiphase boundaries were created in (K,Na)NbO₃-based films, revealing Mn occupying the A-site position. These boundaries stabilized ferroelectricity in these (K,Na)NbO₃-based films with a large twice remnant polarization (~72.5 μC/cm²) across a wide frequency range (20 Hz-10 kHz). High-resolution imaging shows densely arranged antiphase boundaries are grown along three crystal axes. These boundaries are Mn-atomic-enriched at a nanoscale width of a single unit cell, equilibrating the interfacial charges and clamp the interfacial strain, resulting in the highly squared hysteresis loops and high Curie temperature of ~400 °C in the films. Our results may provide a paradigm for designing high-performance lead-free ferroelectric films, unleashing their application potential for expelling lead-containing counterparts.