Structural, vibrational and dielectric behavior of Co1-xMxCr2O4 (M = Zn, Mg, Cu and x=0.0, 0.5) spinel chromites

Low temperature sol-gel auto combustion method is used to synthesize the spinel chromites of Co1-xMxCr2O4 (M = Zn, Mg {non-Jahn Teller (JT) ion}, Cu {JT ion}; x = 0.0, 0.5). Synchrotron, and lab x-ray diffraction pattern confirms the single-phase crystalline nature. Structural features from cubic (space group Fd3m) [CoCr2O4, Co0.5Mg0.5Cr2O4 and Co0.5Zn0.5Cr2O4] to tetragonal (space group I4(1)/amd) [Co0.5Cu0.5Cr2O4] are reported. SEM micrograph of sintered samples results in less porosity with average particle size distribution of similar to 0.2-0.3 mu m. Shifting of Raman active phonon modes is seen with doping and an additional Raman active mode is seen at 666.45 cm(-1) for Co0.5Zn0.5Cr2O4. Dielectric behavior as a function of frequency reveals that dispersion in all these chromites is attributed to hopping mechanism. Higher value of dielectric constant (epsilon') and minimum loss tangent (tan delta) for non-JT ion Co0.5Zn0.5Cr2O4 is measured inferring effective charge polarization in chromites as compare to doped JT ions. Both grains and grain boundaries are active in Co0.5Zn0.5Cr2O4 at lower frequencies as depicted from impedance analysis. Doping does not showed the presence of electric polarization in chromites. (C) 2017 Elsevier B.V. All rights reserved.