Unusual semiconductor-metal-semiconductor transitions in magnetite Fe3O4 nanoparticles.

ABSTRACT

Magnetite (Fe3O4) 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 Fd3̄m. However, 57Fe Mössbauer spectroscopy suggested the presence of Fe3+ and Fe2.5+ (mixed Fe3+ and Fe2+) 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 (∼106), reduced tangent loss (∼0.2) and moderate conductivity (>10-6 S cm-1) values, making Fe3O4 nanoparticles a potential candidate for electromagnetic absorbing materials.