Ab initio and atomistic study of ferroelectricity in copper-doped potassium niobate

The ferroelectric [001], [011], and [111] instabilities in KNbO3 with up to 12.5 mol% Cu on K or Nb sites, respectively, are investigated using both ab initio density-functional theory and classical interatomic potentials. A doping level of only about 2-4 mol% Cu on K sites enhances the ferroelectric instabilities and induces a morphotropic phase boundary, while Cu on Nb sites decreases the ferroelectric energies without changing their relative order.