Non-Monotonic Transition of Conduction Mechanisms in 0.95[(Bi0.5Na0.5)0.985Sm0.01](Zr0.2Ti0.8)O3-0.05BiFeO3-Based Lead-Free Ceramics

(Bi0.5Na0.5)TiO3 (BNT)-based ceramics have attracted much attention for energy storage applications, but some fundamental issues remain unclear—in particular, how the conduction mechanism changes with composition and what role oxygen vacancies play. The (0.95−x)[(Bi0.5Na0.5)(1−1.5y)Smy](Zr0.2Ti0.8)O3-0.05BiFeO3-xBa(Sn0.2Ti0.8)O3 (abbreviated as (0.95−x)BNSmZT-0.05BF-xBaSnT, x = 0.05, 0.1, 0.15, 0.2, 0.3, 0.4, y = 0.01) system was investigated, where Ba(Sn0.2Ti0.8)O3 (BaSnT) content varies from 0.05 to 0.4. A non-monotonic conduction transition is revealed by the conductivity and relaxation behavior characterized by impedance spectroscopy and dielectric temperature spectra combined with electric modulus analysis. At low BaSnT (x ≤ 0.1), residual Bi2Ti2O7 at grain boundaries blocks oxygen vacancy migration, giving high activation energy (~2.12 eV) and presenting oxygen vacancy-mediated conduction. At x = 0.15, the impurity disappears and free oxygen vacancies surge, dropping the activation energy to 1.21 eV, corresponding to oxygen-vacancy-dominated ionic conduction with increased carrier density. For x = 0.2–0.4, strong Ba-O-Bi bonds stabilize the migration barrier around 1.57 eV, resulting in mixed oxygen vacancy-mediated conduction. The optimal composition 0.75BNSmZT-0.05BF-0.2BaSnT shows the best dielectric stability and highest resistance. From modulus master curves and broadened M″ peaks, the conduction is identified as correlated hopping of oxygen vacancies, consistent with non-Debye relaxation. This work provides a clear picture of how oxygen vacancy dynamics depend on phase purity, grain boundaries, and A-site chemistry in the BNT-based lead-free ceramics.