Dielectric relaxations in pure, La-doped, and (La, Co)-codoped BiFeO3: Post-sintering annealing studies

Pure, La-doped, and (La,Co)-codoped BiFeO3 ceramic samples were prepared by solid-state reaction. By means of dielectric permittivity, electric modulus spectroscopy, and impedance analysis, the dielectric properties of the samples were systematically investigated in the temperature range of 140-330 K and frequency range of 20-107 Hz. Three, two, and one dielectric relaxations were found in pure, La-doped, and (La,Co)-codoped samples, respectively, in the investigated temperature window. Post-sintering annealing studies reveal that both p- and n-types carriers coexist in the samples with the p-type carriers (holes) being the major carriers acting as relaxing species. The dielectric properties are determined by the competition between the n-type carriers (electrons) and the holes. La-doping and (La,Co) dual doping enhance the role played by the holes. Impedance analysis shows that the low-temperature (high-frequency) relaxation is a polaronic relaxation resulting from the hoping motion of holes inside grains. The middle-temperature (frequency) and high-temperature (low-frequency) relaxations are Maxwell-Wagner relaxations caused by the hoping motion of holes blocked by grain boundaries and sample-electrode contacts, respectively. These results underscore the role of sample preparing conditions in the dielectric properties of BiFeO3 and suggest that optimizing the preparing procedures would be a promising strategy to achieve superior properties of BiFeO3.