Maja Koblar

Local atomic displacements underpin ferroelectric polarization in perovskites such as (K0.5Na0.5)NbO3 (KNN), yet experimentally observed niobium (Nb) off-centering exceeds predictions from structural models. Here, we employ atomic-resolution scanning transmission electron microscopy (STEM), quantitative four-dimensional (4D)-STEM charge-density mapping, and density functional theory (DFT) calculations to directly measure Nb displacements and identify intrinsic point defects, including oxygen and alkali vacancies. Our results show that these defects locally perturb Nb off-centering by tens of picometers, correlating with increased local polarization. Integrated differential phase contrast (iDPC) imaging confirms the presence of defects, while DFT calculations demonstrate that oxygen vacancies alone locally perturb and amplify Nb off-centering, thereby influencing polarization fields. These findings suggest that intrinsic point defects significantly contribute to the enhanced local polarization observed in KNN, providing insight into defect-stabilized polar order in lead-free ferroelectric materials.

Local atomic displacements underpin ferroelectric polarization in perovskites such as (K0.5Na0.5)NbO3 (KNN), yet experimentally observed niobium (Nb) off-centering exceeds predictions from structural models. Here, we employ atomic-resolution scanning transmission electron microscopy (STEM), quantitative four-dimensional (4D)-STEM charge-density mapping, and density functional theory (DFT) calculations to directly measure Nb displacements and identify intrinsic point defects, including oxygen and alkali vacancies. Our results show that these defects locally perturb Nb off-centering by tens of picometers, correlating with increased local polarization. Integrated differential phase contrast (iDPC) imaging confirms the presence of defects, while DFT calculations demonstrate that oxygen vacancies alone locally perturb and amplify Nb off-centering, thereby influencing polarization fields. These findings suggest that intrinsic point defects significantly contribute to the enhanced local polarization observed in KNN, providing insight into defect-stabilized polar order in lead-free ferroelectric materials.

The control of the coercive field and growth of abnormally oriented grains (AOGs) in wurtzite ferroelectric Sc _x Al _1‐x N thin films is crucial for piezoelectric and ferroelectric applications. However, elevated Sc concentrations generally result in significant AOG growth and high leakage current, degrading piezoelectric and ferroelectric properties. Here, compositionally graded Sc _x Al _1‐x N layered structures grown by sputtering are explored to effectively limit the AOG growth, compared to the 8–10% typically observed in conventional films, without requiring process optimization. Notably, a large built‐in electric field of ≈0.45 MV cm ⁻¹ , nearly 2.5 times larger than in graded PbZr _x Ti _1‐x O ₃ (PZT) and BaSr _x Ti _1‐x O ₃ (BST) systems, is achieved. This built‐in field induces unique features, including horizontal shifts in polarization‐electric field hysteresis loops, bi‐stable states in capacitance‐voltage characteristics, and highly asymmetric electrostrain behavior. Unlike other graded systems, this large built‐in field in the graded Sc _x Al _1‐x N films arises primarily from polarization and chemical gradients. These findings offer a simplified, scalable and CMOS‐compatible strategy to overcome the AOG challenges and tune ferroelectric properties of Sc _x Al _1‐x N thin films, providing the path for potential applications in advanced photonic and MEMS devices with improved performance and low‐power consumption.

Single-crystal PbZrO ₃ thin films are processed via chemical route. The 680 nm-thick films are antiferroelectric, while the 170 nm-thick films show ferroelectric-like switching, which is associated with the defect formation during the annealing.