The effect of Ta substitution on the crystal structure, ferroelectric, and piezoelectric properties of Bi 0.5 (Na 0.82 K 0.18 ) 0.5 Ti 1-x Ta x O 3 ceramics has been investigated. The Ta doping resulted in a transition from coexistence of ferroelectric tetragonal and rhombohedral phases to an electrostrictive pseudocubic phase, leading to degradations of the remnant polarization, coercive field, and piezoelectric coefficient d 33 . However, the electricfield-induced strain was significantly enhanced by the Ta substitution-induced phase transition and reached a highest value of S max /E max = 566 pm/V under an applied electric field 6 kV/mm when 2% Ta was substituted on Ti sites. The abnormal enhancement in strain was attributed to the pseudocubic phase with high electrostrictive constants.

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Effect of excess CuO on the sintering behaviour and piezoelectric properties of Bi 0.5 (Na 82 K 0.18 ) 0.5 TiO 3 ceramics was investigated. The addition of small amount of excess CuO as low as 0.01 mol was quite effective to lower the sintering temperature (T s ) of BNKT ceramics down to 975 • C which is 200 • C lower than the T s of pure BNKT (1175 • C) without significant degradation of their piezoelectric properties. From microstructure analysis, it was concluded that CuO promotes sintering via forming a liquid phase at 975 • C and partly dissolves into Ti-sites during sintering.