The effect of an isovalent chemical variation in A site cations on the stability of ferroelectric order and consequent changes in the electrical properties of ferroelectric 0.93( Bi _1/2 Na _1/2) TiO ₃-0.05 BaTiO ₃-0.02( K _0.5 Na _0.5) NbO ₃ (0.93 BNT -0.05 BT -0.02 KNN ) lead-free piezoceramic at the morphotropic phase boundary (MPB) was investigated by substituting K for Na . X-ray diffraction analysis reveals that all the studied compositions undergo an irreversible phase transition from a pseudocubic to a mixture of rhombohedral and tetragonal phases during poling. In spite of the absence of an obvious structural difference, both electric-field-induced strain and polarization hysteresis loops show a clear distinction. It was found that the effect of substitution is so drastic that about 0.5 at% substitution renders the material softer and more than 1.0 at% results in a significant destabilization of the ferroelectric order. However, no notable difference in the temperature dependent permittivity measurements was detected, which suggests that the depoling of these materials has some other origin than a symmetry-breaking phase transition.