Mn-Atomic-Layered Antiphase Boundary Induced Robust Ferroelectricity in Transparent KNN-Based Lead-Free Film

One preferred lead-free ferroelectric, (K,Na)NbO ₃ (KNN), is gifted with the prominent features of environmentally benign and excellent piezoelectricity in both its ceramic and film forms, but it suffers from poor ferroelectricity. Albeit intensive researches have demonstrated the efficiency of Mn-doping in improving the electrical properties of KNN, the occupancy of Mn (A- or B-site) remains elusive. Here, we have formed Mn-atomic-layered antiphase boundaries in KNN-based films, and observed that Mn occupied the A-site position. The Mn-inlaid antiphase boundary has been shown to induce stable ferroelectricity with robust polarization (2 P _r  ~ 72.5 µC/cm ² ) in KNN-based films. This stability is retained across a wide range of test frequencies, from as low as 20 Hz, and up to 10 kHz, which is of great significance in KNN system. The atomically resolved STEM images reveal that densely arranged antiphase boundaries are formed and grown along three crystal axes by half unit cell lattice mismatch throughout the entire thickness of KNN-based films. These boundaries are Mn-atomic-enriched at a nanoscale width of a single unit cell, which could equilibrate the interfacial charges and clamp the vertical interfacial strain, resulting in the highly squared hysteresis loops and high Curie temperature of ~ 400 ℃ in our films. Meanwhile, these films also exhibit a high dielectric constant of 1200 and optical transmittance of ~ 75%. Our results may provide a new paradigm for designing new type of high-performance KNN-based lead-free ferroelectric films, unleashing their application potential for expelling lead-containing counterparts.