Structural characteristics, electrical conduction and dielectric properties of gadolinium substituted cobalt ferrite

Abstract

Gadolinium (Gd) substituted cobalt ferrites (CoFe2−xGdxO4, referred to CFGO) with variable Gd content (x=0.0–0.4) have been synthesized by solid state reaction method. The crystal structure, surface morphology, chemistry, electrical conduction and dielectric properties of CFGO compounds have been evaluated. X-ray diffraction measurements indicate that CFGO crystallize in the inverse spinel phase. The CFGO compounds exhibit lattice expansion due to substitution of larger Gd ions into the crystal lattice. Gd-substitution induced smooth microstructure and particle size reduction is evident in electron microscopy analyses. Frequency dependent dielectric measurements at room temperature obey the modified Debye model with a relaxation time of ∼10−4s and a spreading factor of 0.244–0.616. The frequency (f=20Hz–1MHz) and temperature (T=30–900°C) dependent dielectric constant analyses indicate that pure CFO exhibits two dielectric relaxations in the frequency range of 1–10kHz while Gd substituted CFO compositions exhibit only single relaxation at 1kHz. The dielectric constant of CFGO is temperature independent up to ∼550°C. The dielectric constant increases with T>550°C. Dielectric constant of CoFe2−xGdxO4 ceramics is also enhanced compared to pure CoFe2O4 due to the lattice distortion upon Gd incorporation. The tanδ (loss tangent)–T data reveals the typical behavior of relaxation loses in CFGO. Activation energy of the dielectric relaxation calculated employing Arrhenius equation varies from 0.564 to 0.668 (±0.003)eV with increasing x values from 0.0 to 0.4. Thermally activated small polaron hopping mechanism is evident in temperature dependent electrical properties of CFGO. The effect of Gd-substitution in CFO is remarkable on the resistivity and, hence, activation energy; both increases with increasing Gd content. A two-layer heterogeneous model consisting of semiconducting grains separated by insulating grain boundaries was able to account for the observed temperature and frequency dependent electrical properties in CFGO ceramics. The results demonstrate that the crystal structure, microstructure, electrical and dielectric properties can be tailored by tuning Gd-content in the CFGO compounds.