Ab initio and atomistic simulation of copper doping in the lead-free ferroelectric perovskite potassium sodium niobate

Recently lead-free ferroelectric ceramics, one of them the perovskite potassium sodium niobate (KNN), have attracted strong scientific interest for being environmentally friendly materials suitable for piezoelectric devices in applications like actuators and sensors. Various doping elements, for instance copper, have been found to improve the ceramic processing conditions and the piezoelectric properties of KNN significantly. Therefore a theoretical understanding of the effects of these impurities is important. We employed density functional theory in the local density approximation and atomistic simulations with empirical interatomic potentials to determine the preferred lattice site for copper impurities in KNN, and the influence of the impurities on the energy barriers associated with switching the ferroelectric polarization.