Point-defect-enhanced local polarization in (K0.5Na0.5)NbO3 revealed by atomic-scale microscopy and simulations

<jats:p>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.</jats:p>