Lead-free piezoelectric ceramics, 1−x − yBi 0.5 Na 0.5 TiO 3-xBaTiO 3-yK 0.5 Na 0.5 NbO 3 0.05 x 0.07 and 0.01 y 0.03, have been synthesized by a conventional solid state sintering method. The room temperature ferroelectric and piezoelectric properties of these ceramics were studied. Based on the measured properties, the ceramics were categorized into two groups: group I compositions having dominant ferroelectric order and group II compositions displaying mixed ferroelectric and antiferroelectric properties at room temperature. A composition from group II near the boundary between these two groups exhibited a strain as large as 0.45% at an electric field of 8 kV/ mm. Polarization in this composition was not stable in that the piezoelectric coefficient d 33 at zero electric field was only about 30 pm/ V. The converse piezoelectric response becomes weaker when the composition deviated from the boundary between the groups toward either the ferroelectric or antiferroelectric compositions. These results were rationalized based on a field induced antiferroelectric-ferroelectric phase transition.

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Influence of K/Na ratio in (KxNa1−x)NbO3 on the ferroelectric stability and consequent changes in the electrical properties of 0.99(Bi0.5Na0.4K0.1)TiO3–0.01(KxNa1−x)NbO3 (BNKT–KxNN) ceramics were investigated. Results showed that change of K/Na ratio in KNN induces a phase transition from ferroelectric to ergodic relaxor phase with a significant disruption of the long‐range ferroelectric order, and correspondingly adjusts the ferroelectric–relaxor transition point TF−R to room temperature. Accordingly, giant strain of ~0.46% (corresponding to a large signal d33* of ~575 pm/V) which is comparable to that of Pb‐based antiferroelectrics is obtained at a K/Na ratio of ~1, and the emergence of large strain response induced by the change of K/Na ratio of KNN can be well explained by the correlation between the position of ferroelectric–ergodic relaxor phase boundary in the BNKT–KxNN system and the tolerance factor t of the end number (KxNN). In situ high‐energy X‐ray scattering experiments with external field reveals that the large strain response in the studied system is likely related to the electric field‐induced distortion from the pseudocubic structure.

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