Unusual semiconductor-metal-semiconductor transitions in magnetite FeO nanoparticles.

Magnetite (FeO) nanoparticles were successfully prepared by a co-precipitation method. Rietveld refinement on the X-ray diffraction pattern confirmed the development of a single-phase cubic spinel structure with space group 3̄. However, Fe Mössbauer spectroscopy suggested the presence of Fe and Fe (mixed Fe and Fe) ions at the tetrahedral and octahedral sites of the inverse spinel structure, respectively. Impedance spectroscopy measurements showed a discontinues variation in the temperature dependence of the sample's resistive behavior, indicating the appearance of semiconductor-metal-semiconductor like transitions between the temperature range of 293 and 373 K. A similar dual transition was also observed from the dielectric and conductivity measurements around the same temperature regions. The observed unusual transition is explained in term of the competitive effects among the hopping of localized/delocalized and short-range/long-range charge carriers present in the sample. Moreover, the prepared sample exhibits colossal dielectric permittivity (∼10), reduced tangent loss (∼0.2) and moderate conductivity (>10 S cm) values, making FeO nanoparticles a potential candidate for electromagnetic absorbing materials.