Effects of CuO addition on electrical properties of 0.6BiFeO₃-0.4(Bi_0.5K_0.5)TiO₃ lead-free piezoelectric ceramics

0.6BiFeO₃-0.4(Bi_0.5K_0.5)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.