Electrical conductivity and thermopower of (1 − x) BiFeO3 – xBi0.5K0.5TiO3 (x = 0.1, 0.2) ceramics near the ferroelectric to paraelectric phase transition

Abstract

Ferroelectric BiFeO₃ has attractive properties such as high strain and polarization, but a wide range of applications of bulk BiFeO₃ are hindered due to high leakage currents and a high coercive electric field. Here, we report on the thermal behaviour of the electrical conductivity and thermopower of BiFeO₃ substituted with 10 and 20 mol% Bi_0.5K_0.5TiO₃. A change from p-type to n-type conductivity in these semi-conducting materials was demonstrated by the change in the sign of the Seebeck coefficient and the change in the slope of the isothermal conductivity versus partial pressure of O. A minimum in the isothermal conductivity was observed at ∼10⁻² bar O₂ partial pressure for both solid solutions. The strong dependence of the conductivity on the partial pressure of O₂ was rationalized by a point defect model describing qualitatively the conductivity involving oxidation/reduction of Fe³⁺, the dominating oxidation state of Fe in stoichiometric BiFeO₃. The ferroelectric to paraelectric phase transition of 80 and 90 mol% BiFeO₃ was observed at 648 ± 15 and 723 ± 15 °C respectively by differential thermal analysis and confirmed by dielectric spectroscopy and high temperature powder X-ray diffraction.