To clarify the effect of A-site donor doping on the phase transition and the strain enhancement, we investigated the crystal structure, as well as the piezoelectric, ferroelectric and electric-field-induced strain (EFIS) properties of La-doped Bi_1/2(Na_0.82K_0.18)_1/2TiO₃ (BNKT) ceramics. Similarly to our previous studies on BNKT doped with B-site donors such as Nb and Ta, La doping was found to induce a ferroelectric-to-nonpolar (FE-NP) phase transition, leading to a large enhancement in EFIS just after the transition. The result provides strong evidence that a close relationship exists between the Goldschumidt’s tolerance factor and the FE-NP transition in BNKT, which has been observed in B-site-donor or isovalent impurity-doped BNKT.

The phase transition of La-doped [Bi_1/2(Na_0.82K_0.18)_1/2]TiO₃ (BNKT) ceramics was investigated by using high-temperature X-ray diffraction and temperature-dependent dielectric measurements. Undoped BNKT was found to be a nonergodic relaxor, which was evidenced by the presence of a depolarization temperature below which polar nanoregions were frozen. However, La-doped BNKT ceramics are believed to be composites consisting of ergodic and nonergodic relaxors. The results suggest that a nonergodic relaxor with tetragonal symmetry might be distributed in an ergodic relaxor matrix with pseudocubic symmetry.