Cu substitutionals and defect complexes in the lead-free ferroelectric knn

The lead-free ferroelectric (K,Na)NbO3 (KNN) is a potential future substitute for lead zirconate titanate (PZT) in piezoelectric actuators and sensors. Doping, e.g. by adding CuO as a sintering aid, modifies the material properties and can cause so called hard or soft ferroelectric behavior. Using ab initio calculations based on the density functional theory, the site selectivity of Cu dopants in the KNN lattice is investigated, and the stable defect complexes of Cu dopants and lattice vacancies are determined. Cu may act as an amphoteric dopant, i.e. substitute on alkali sites as a donor or on Nb sites as an acceptor, depending on the processing conditions. On both alkali and Nb sites, defect complexes of Cu with vacancies, namely (CuK−V K), (CuNb−V O), and (V O−CuNb−V O), are stable with respect to the isolated defects. The first two defect complexes should exhibit a dipole moment and may therefore cause ferroelectric hardening.