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 degrees C to 230 degrees 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-0 = 766K. 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.

Automated summary

This study analyzed the complex dielectric permittivity of a sintered ceramic tablet at different temperatures and found two temperature-activated relaxation processes. The first process is related to hopping processes between Fe3+ and Fe2+ induced by oxygen vacancies, while the second process is associated with dynamic rearrangements of domain boundaries. The results also revealed a decay in the ferroelectric phase transition with repeated heating cycles.