Electrical transport in lead-free Na_0.5Bi_0.5TiO₃ ceramics

Lead-free Na_0.5Bi_0.5TiO₃ (NBT) ceramics were prepared via a conventional oxide-mixed sintering route and their electrical transport properties were investigated. Direct current (DC, σ_DC) and alternating current (AC, σ_AC) electrical conductivity values, polarization current (first measurements) and depolarization current, current-voltage (I-U) characteristics (first measurements), and the Seebeck coefficient (α) were determined under various conditions. The mechanism of depolarization and the electrical conductivity phenomena observed for the investigated samples were found to be typical. For low voltages, the I-U characteristics were in good agreement with Ohm’s law; for higher voltages, the observed dependences were I-U², I-U⁴, and then I-U⁶. The low-frequency σ_AC followed the formula σ_AC-ω^s (ω is the angular frequency and s is the frequency exponent). The exponent s was equal to 0.18-0.77 and 0.73-0.99 in the low- and high-frequency regions, respectively, and decreased with temperature increasing. It was shown that conduction mechanisms involved the hopping of charge carriers at low temperatures, small polarons at intermediate temperatures, and oxygen vacancies at high temperatures. Based on AC conductivity data, the density of states at the Fermi-level, and the minimum hopping length were estimated. Electrical conduction was found to undergo p-n-p transitions with increasing temperature. These transitions occurred at depolarization temperature T_d, 280 ℃, and temperature of the maximum of electric permittivity T_m is as typical of NBT materials.