Identifying the local defect structure in (Na0.5K0.5)NbO3: 1 mol. % CuO lead-free ceramics by x-ray absorption spectra

The local defect structure around dopant atoms in 1 mol. % CuO doped (Na0.5K0.5)NbO3 lead-free ceramics was investigated by means of x-ray absorption spectra as compared with the local structure around the host Nb site. The Cu K-edge and O K-edge x-ray absorption near-edge structure spectra demonstrate divalent Cu ions that occupy the octahedrally coordinated Nb sites and also reveal the existence of oxygen vacancy VO¨ in the nearest neighboring around the Cu atom evidently. Moreover, the Cu and Nb K-edge extended x-ray absorption fine structure clearly suggests that the oxygen vacancies should be located at two O22 sites with the two longest Cu-O22 bond lengths, thus producing trimeric bent VO¨-CuNb″′-VO¨ defect complexes with a dipole moment PD parallel to the spontaneous polarization Ps, instead of dimeric CuNb″′-VO¨ and straight VO¨-CuNb″′-VO¨ defect complexes. This kind of special defect structure is also different from that observed in conventional Pb-based perovskite ferroelectrics in which only dimeric CuTi″-VO¨ defect dipoles were observed. Finally, the influence of the defect complexes on the macroscopic properties was specially discussed by taking into account the interaction between Ps and PD.