Dielectric properties of BiFeO3 ceramics obtained from mechanochemically synthesized nanopowders

Dielectric behaviour of BiFeO3 ceramics, obtained by hot-pressing of nanopowders produced by mechanochemical synthesis from Bi2O3 and Fe2O3 oxides (weight ratio 2:1), was studied in the temperature range 125-575 K. The ceramics was found to exhibit step-like dielectric response epsilon*(T) with high permittivity values, similar to the behaviour of materials with giant dielectric permittivity. Three overlapping relaxation processes contribute to the dielectric response: i) relaxation in the low-temperature range (220-420 K), characterized by activation energy of 0.4 eV, ii) relaxation in the temperature range 320-520 K with activation energy of 1.0 eV and iii) broad dielectric anomaly in the vicinity of 420 K, which disappears after 1 h annealing at 775 K. The low-temperature relaxation is ascribed to the carrier hopping process between Fe2+ and Fe3+ ions. The presence of mixed valence of the Fe ions was proved by X-ray photoelectron spectroscopy. Dielectric relaxation in the middle-temperature range is considered as a result of grain boundary effect and internal barrier layers related to Bi25FeO40 phase as verified by X-ray diffraction. The high-temperature dielectric anomaly we relate to short-range hopping of ordered oxygen vacancies.