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The piezoelectric properties of lead-free x(Bi1/2Na1/2)TiO3-y(Bi1/2K1/2)TiO3-zBaTiO3 [BNBKy:z(x); y:z = 4:1 and 2:1, x+y+z=1] with tetragonal compositions were investigated, focusing on the secondary phase transition T 2 from the ferroelectric to antiferroelectric phases. Although the T 2 temperatures of BNBK near the MPB composition remained at about 115°C, T 2 rapidly shifted to a higher temperature of 200°C or higher for the off-MPB composition [x≤0.75 for BNBK4:1(x) and x≤0.78 for BNBK2:1(x)]. The piezoelectric activities of off-MPB compositions decreased with decreasing the amount (x) of BNT. The larger piezoelectric constant d 33=135 pC/N, the higher secondary phase transition temperature T 2=197°C, and the coupling factor k 33=0.45 with a good temperature dependence were obtained for BNBK2:1(0.78).

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