Alignment of ferroelectric polarization and defect complexes in copper-doped potassium niobate

Defect complexes consisting of Cu substitutionals on Nb sites and oxygen vacancies in potassium niobate, KNbO3, are investigated with respect to their contribution to ferroelectric hardening by means of density-functional theory and classical atomistic simulations. We determine the easy and hard directions for the ferroelectric polarization created by these defect complexes, the energy differences between easy and hard directions, and upper limits for the energy barriers for switching the ferroelectric polarization between these directions. The ferroelectric polarization preferentially aligns with the defect complexes, which is expected to impede polarization switching and hence to contribute to ferroelectric hardening.