Bi_0.5Na_0.5TiO₃-based materials have drawn much attention as a lead-free ferroelectric and piezoelectric materials. We focused on Bi_0.5Na_0.5TiO₃–NaNbO₃ and Bi_0.5Na_0.5TiO₃–NaTaO₃ solid solutions, which can be denoted as (Bi_0.5(1−x)Na_0.5(1+x))(Ti_1−xM_x)O₃ (M = Nb, Ta, x = 0.025, 0.05), and then investigated their ferroelectric and piezoelectric properties, and crystal structures using quantum beam. It was found that the ferroelectric and piezoelectric properties were deteriorated by the Nb and Ta substitutions with x = 0.05. Since the crystal structure analysis on the as-synthesized samples could not explain the change in the ferroelectric and piezoelectric properties, we analyzed the crystal structure of the samples after a polarization. It was indicated that the tilting angle of adjacent (Ti,M)O₆ increased significantly only in the case of Bi_0.5Na_0.5TiO₃, the Nb- and Ta-substituted samples with x = 0.025. It was considered that the crystal structure after the polarization played an important role for the ferroelectric and piezoelectric properties.