0.6BiFeO3-0.4(Bi0.5K0.5)TiO3 (0.6BF-0.4BKT) ceramic samples with 0.0-4.0 mol% CuO were prepared by the solid-state reaction. The CuO addition aided the densification of the samples and slightly increased the lattice constant. The relaxor-like defuse dielectric peak of 0.6BF-0.4BKT became sharper with increasing the CuO content. Polarization-electric field curve of the undoped 0.6BF-0.4BKT was a pinched loop in the as-sintered state, while that was a square hysteresis with a large remanent polarization of 48 μC/cm2 after the thermal quenching, demonstrating a strong domain wall pinning due to defect dipoles. We found that the CuO addition up to 2.0 mol% facilitates the polarization switching in the as-sintered samples to increase the remanent polarization and the piezoelectric d33 coefficient. The results of the structural and electrical investigations suggested that the copper ion acts as a donor in 0.6BF-0.4BKT by compensating the potassium vacancy created by the evaporation of K2O during the calcination and sintering processes.
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