Colossal Permittivity and Variable-Range-Hopping Conduction of Polarons in Ni0.5Zn0.5Fe2O4 Ceramic

The electrical properties of ferrites are important to their applications at high frequencies. In this paper, we investigate the colossal permittivity behavior of Ni0.5Zn0.5Fe2O4 ceramic prepared from powders synthesized by the citric acid combustion method. Its bulk conductivity is attributed to the variable-range hopping of localized polarons. These polarons exhibit universal dielectric response with frequency. They are frozen at low temperature and activated at high temperature. As temperature decreases from 235 to 130 K, their hopping energy decreases from 223 to 113 meV, while the estimated hopping range increases from 3.5 to 4.1 nm. The direct correlation of polaron conduction and colossal permittivity of NZFO is definitely established by a modified Koops’ equivalent circuit based on the internal barrier layer model, which is well confirmed by the experimental data.