This work compared the effects of isovalent Sn 4+ and aliovalent Ta 5+ doping on Ti-sites in lead-free perovskite type Bi 1/2 (Na 0.82 K 0.18 ) 1/2 TiO 3 piezoelectric ceramics on their ferroelectric and electric field-induced strain behavior. Both impurities led to a diffuse transformation from the coexistence of ferroelectric rhombohedral and tetragonal phases to a nonpolar pseudocubic symmetry. Nevertheless, Ta doping more strongly promoted the phase transition even though the ionic size difference between Ta 5+ and Ti 4+ was smaller than that between Sn 4+ and Ti 4+ . The results evidenced that donor-induced A-site vacancies significantly contributed to the destabilization of ferroelectric phases in the Bi perovskite.