Complex dielectric behaviours in BiFeO₃/Bi₂Fe₄O₉ ceramics

The complex dielectric permittivity of a sintered ceramic tablet consisting of 70.5% BiFeO₃, 27.7% Bi₂Fe₄O₉ and 1.8% Bi₂₅FeO₄₀ was analyzed as a function of temperature from – 120 °C to 230 °C in two separate temperature runs. The results reveal a complicated dielectric response with two temperature activated relaxation processes. The first is purely Arrhenius relaxation related to hopping processes between Fe³⁺ and Fe²⁺ traps induced by oxygen vacancies at grain boundaries. The second process is more unusual. Its characteristic relaxation time follows a quasi-Vogel–Tammann–Fulcher temperature behavior and from fitting the critical temperature point is estimated at T= 766 K. It is absent in the second temperature run. It can be related to dynamic rearrangements of domain boundaries between different crystallites in the ceramic. The results also reveal a ferroelectric phase transition that decayed with repeated heating cycles of the tablet. The ferroelectric phase transition in pure BiFeO₃ is 1098 K, whereas the current results show it at 373 K. The origin of this reduction in the critical temperature of the phase transition is traced to locally induced strains on grain boundaries mainly because of unit cell size mismatch between BiFeO₃ and Bi₂Fe₄O₉.