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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Effects of BaTiO3 (BT) and BaZrO3 (BZ) modification on the crystal structure and electromechanical properties of lead-free Bi1/2(Na0.82K0.18)1/2TiO3 (BNKT) ceramics were compared within a doping range of 0–5 mol %. The differences between BT- and BZ-modification effects were clearly observed in polarization and electric field-induced strain hysteresis loops. BZ-modification induced a gradual transition from a ferroelectric to a nonpolar phase, resulting in an abnormal enhancement in the electric field-induced strain (EFIS) just after the transition. However, all BT-modified specimens showed strong ferroelectricity without abnormal enhancement in EFIS. The results suggest that Zr doping-induced B-site strains play a significant role in the ferroelectric-to-nonpolar phase transition in the Bi-perovskite.

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