Interactions of defect complexes and domain walls in CuO-doped ferroelectric (K,Na)NbO3

"Lead-free" piezoelectric sodium potassium niobate has been studied with respect to its defect structure when doping with CuO. The results indicate that two kinds of mutually compensating charged defect complexes are formed, (Cu Nb - V O ) and (V O - Cu Nb - V O ) . Concerning the interplay of these defect complexes with the piezoelectric materials properties, the trimeric (V O - Cu Nb - V O ) defect complex primarily has an elastic dipole moment and thus is proposed to impact the electromechanical properties, whereas the dimeric (Cu Nb - V O ) defect possesses an electric dipole moment in addition to an elastic distortion. Both types of defect complexes can impede domain-wall motion and may contribute to ferroelectric "hardening.