Role of pH value during material synthesis and grain-grain boundary contribution on the observed semiconductor to metal like conductivity transition in Ni1.5Fe1.5O4 spinel ferrite

Ni1.5Fe1.5O4 ferrite samples were synthesized by maintaining different pH values (1-12) during chemical reaction at 80 degrees C. The as-prepared samples were annealed at 1000 degrees C to form a cubic spinel structure. The heat treated samples were used for the study of electrical conductivity and dielectric properties. In this work, we understand the mechanism of unusual metal-like state in ferrite samples, characterized by negative temperature coefficient of conductivity. We have discussed various aspects, e.g., hopping mechanism through superexchange paths (Fe3+-O2--Fe3+ and Ni2+-O2--Ni2+), charge delocalized conduction mechanism affected by the magnetic spins order in t(2g) and e(g) electronic energy levels of B sites cations, grain size variation, relaxation of charge carriers at grains and grain boundaries of the particles, for outlining the mechanism of thermal activated charge localization (semiconductor state) and delocalization (metal-like state) effect in our samples. We have carried out a detailed analysis of conductivity spectra (Jonscher's power law fit, scaling of conductivity), impedance spectra (Cole-Cole plot by incorporating constant phase element), modulus spectra (Bergman proposed KWW function), and dielectric loss and dielectric constant spectra to extract the conductivity and relaxation contributions from grains, grain boundaries and space charge polarization in the samples. (C) 2016 Elsevier B.V. All rights reserved.