Ferroelectric and piezoelectric properties of SrTiO 3 -modified (0, 3, and 5 mol%) 0.8Bi 1/2 Na 1/2 TiO 3 -0.2Bi 1/2 K 1/2 TiO 3 leadfree piezoceramics were investigated as a model system in an attempt to lay a guideline for developing lead-free piezoelectric materials with large strains. Two guidelines, one for the choice of base composition and the other for the choice of chemical modifiers, were assumed from our current understanding of the mechanism involved. Dielectric permittivity of both poled and unpoled samples was measured and compared, leading to a conclusion that the frequency-independent anomaly (T F-R ) is the temperature at which induced-ferroelectric order converts back to relaxor state. The correlation between T F-R and depolarization temperature (T d ) was shown by the comparison with T d determined by thermally stimulated depolarization current measurements, whereas the ferroelectric-relaxor transition temperature T F-R was determined using poled samples. A large unipolar strain of 0.36% (S max /E max = 600 pm/V) at a driving field of 6 kV/mm was obtained at room temperature for a SrTiO 3 content of 5 mol%. Temperature-dependent measurements of both polarization and strain from room temperature to 200°C revealed that the origin of the large strain is due to a reversible field-induced ergodic relaxor-to-ferroelectric phase transformation.

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Lead‐free piezoelectric (1–x)(Bi0.5(Na0.75K0.25)0.5TiO3)‐xBiAlO3 (BNKT25‐xBA, x = 0–0.100) ceramics were synthesized using a conventional solid‐state reaction method. The effect of BA addition into the BNKT25 ceramics was investigated by X‐ray diffraction, dielectric and ferroelectric characterizations, and electric field‐induced strain. X‐ray diffraction revealed a phase transition from a tetragonal to a pseudocubic phase at x = 0.050. As the BA content increased, the maximum dielectric constant as well as the depolarization temperature (Td) decreased. The polarization and strain hysteresis loops indicate that the addition of BA significantly disrupts the ferroelectric order of the BNKT25 ceramics leading to a degradation of the remanent polarization and coercive field. However, the destabilization of the ferroelectric order is accompanied by a significant enhancement in the unipolar strain which peaks at x = 0.025 with a value of ~0.29%, which corresponds to a normalized strain, d*33 (=Smax/Emax) of 484 pm/V. It was observed that the unipolar strain of 0.025xBA is fairly temperature‐insensitive up to 150°C, even at 130°C the d*33 is as large as ~415 pm/V.

Lead-free 0.96[Bi1/2(Na0.84K0.16)1/2(Ti(1−x)Nbx)O3]–0.04SrTiO3 (BNKTN–ST, with x = 0–0.030) ceramics were synthesized by a conventional solid-state reaction technique. Polarization and strain hysteresis loops indicated a significant disruption of ferroelectric order accompanied by an enhanced field-induced strain (S = 0.438%) with a high normalized strain Smax/Emax of 876 pm/V at 5 kV/mm. Their reproducibility was confirmed by the fabrication of a 10-layered stack-type multilayer actuator, which demonstrated a normalized strain Smax/Emax of 720 pm/V.