Strain enhancement in Bi1/2(Na0.82K0.18)1/2TiO3 lead-free electromechanical ceramics by co-doping with Li and Ta

► We prepared developed a Bi-based lead free ceramics with a high normalized strain of 727 pm/V, which is higher than those of other reports. ► The giant strain was induced by ferroelectric-nonpolar phase transition by Li and Ta co-doping. ► We suggest that the transition is caused by a decrease in the tolerance factor.