(Na0.5Bi0.5)0.94Ba0.06TiO3 + 0.3 wt.% Sm2O3 (named NBT-BT) lead-free piezoelectric ceramics were prepared by normal sintering. The effect of atmosphere sintering on NBT-BT ceramics was investigated through X-ray diffraction, dielectric, ferroelectric and piezoelectric characterizations. Real (Z′) and imaginary (Z″) parts of the complex impedance of the materials were also investigated. Dielectric studies exhibit a diffuse transition phase and are characterized by a temperature and frequency dispersion of permittivity, and this relaxation has been modeled using the modified Curie–Weiss law. The variation of the imaginary part (Z″) of impedance with frequency at various temperatures shows that the Z″ values reach a maxima peak (Z″max). The appearance of single semicircle in the Nyquist plots (Z″ vs Z′) pattern at high temperatures suggests that the electrical process occurring in the material has a relaxation process possibly due to the contribution of bulk material only. It shows that the compound exhibit negative temperature coefficient of resistance (NTCR) type behavior usually found in semiconductors. The bulk resistance of the material decreases with rising temperatures similar to a semiconductor, and the Nyquist plot showed the negative temperature coefficient of resistance (NTCR) character of materials. Also, the frequency-dependent AC conductivity at different temperatures indicated that the conduction process is thermally activated process. The plots of the relaxation times τZ″ and τM″ as a function of temperature follow the Arrhenius law, where a single slope is observed with activation energy values come to near 1.70.