Role of (Bi_1/2K_1/2)TiO₃ in the dielectric relaxations of BiFeO₃-(Bi_1/2K_1/2)TiO₃ ceramics

Temperature-dependent dielectric relaxations of (1 − x)BiFeO₃-x(Bi_1/2K_1/2)TiO₃ (BF-BKT) lead-free piezoceramics (0.4 ≤ x ≤ 0.8) were investigated via impedance spectroscopic techniques. Regardless of the compositions, the dielectric maximum temperatures exhibit a frequency-dependent dispersion, originating from a Debye relaxation due to the presence of oxygen vacancies. It was also observed that there exist local dielectric maxima due to the relaxation of polar nanoregions as a shoulder on the lower temperature side. The onset temperature for the Debye-type relaxation decreased with decreasing BKT content, gradually overlapping with the low-temperature dielectric dispersion from the relaxation of polar nanoregions. It is proposed that the role of BKT in the BF-BKT system is to enhance the random fields that favor a relaxor state and to suppress the Debye-type relaxation of oxygen vacancy related dipoles.