In this study, 1.0 mol% CuO-added (Na x K 1 -x )NbO 3 (CN x K 1 -xN) ceramics with 0.0 ≤ x ≤ 0.5 were sintered at a relatively low temperature of 960°C. A large Q m value (≥1750) was obtained for the specimens with x ≤ 0.2 owing to the hardening effect of Cu 2+ ions, indicating that a large number of Cu 2+ ions had replaced Nb 5+ ions in these ceramics. The double polarization versus electric field (P-E) hysteresis loop was observed for the specimens with x ≤ 0.2, and was explained by the defect polarization (P d ) formed between Cu 2+ ions and O 2 -vacancies. These specimens also exhibited a sprout-shaped strain versus electric field (S-E) curves with a large strain of 0.14%-0.15% at 8.0 kV/mm because of the internal bias field produced by P d . For the CN 0.2 K 0.8 N ceramic, a double P-E hysteresis loop was maintained at 125°C without remnant polarization. Moreover, this ceramic showed a large strain of 0.175% after 10 4 cycles when a high electric field of 8.0 kV/mm was continuously applied to the specimens during each cycles. Therefore, CN 0.2 K 0.8 N is a useful lead-free piezoelectric ceramic for multilayer actuators.

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0.96(Na 0.5 K 0.5 )(Nb 1Àx Sb x )-0.04SrZrO 3 ceramics with 0.0≤x≤0.06 were well sintered at 1060°C for 6 hours without a secondary phase. Orthorhombic-tetragonal transition temperature (T O-T ) and Curie temperature (T C ) decreased with the addition of Sb 2 O 5 . The decrease in T C was considerable compared to that in T O-T , and thus the tetragonal phase zone disappeared when x exceeded 0.03. Therefore, a broad peak for orthorhombic-pseudocubic transition as opposed to that for orthorhombic-tetragonal transition appeared at 115°C-78.2°C for specimens with 0.04≤x≤0.06. An orthorhombic structure was observed for specimens with x≤0.03.However, the polymorphic phase boundary structure containing orthorhombic and pseudocubic structures was formed for the specimens 0.04≤x≤0.06. Furthermore, a specimen with x=0.055 exhibited a large piezoelectric strain constant of 325 pC/N, indicating that the coexistence of orthorhombic and pseudocubic structures could improve the piezoelectric properties of (Na 0.5 K 0.5 )NbO 3 -based leadfree piezoelectric ceramics.