Large electro-strain response of La3+ and Nb5+ co-doped ternary 0.85Bi0.5Na0.5TiO3-0.11Bi0.5K0.5TiO3-0.04BaTiO3 lead-free piezoelectric ceramics

La3+ and Nb5+ co-doped 0.85Bi0.5Na0.5TiO3-0.11Bi0.5K0.5TiO3-0.04BaTiO3 (BNT-BKT-BT) samples were prepared through conventional solid oxide route, and the effects of doping ions on the phase structure, electrical properties, and electro-strain behavior were investigated. The XRD results showed that La3+ and Nb5+ co-dopant induced a phase transition from coexistence of ferroelectric tetragonal and rhombohedral phase to a pseudocubic phase. With increasing dopant content, the normal ferroelectric-to-ergodic relaxor (FE-to-ER) transition temperature (TF-R) decreased down from 120 °C to room temperature (RT), even below RT. A large unipolar strain of 0.34% (d33∗ = Smax/Emax = 680 pm/V) was achieved under 5 kV/mm and the largest electro-strain of 0.46% was achieved under an applied electric field of 7.5 kV/mm at RT for x = 0.0050. Temperature dependence of both polarization loops and bipolar strain curves from room temperature to 160 °C were also studied, and the results suggested that the origin of the large strain is attributed to a reversible nonpolar isotropic ergodic relaxor state to polar anisotropic ferroelectric phase transition.