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

Article abstract of DOI:10.1016/j.jallcom.2017.07.162

Low temperature sol-gel auto combustion method is used to synthesize the spinel chromites of Co_1-xM_xCr₂O₄ (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, Co_0.5Mg_0.5Cr₂O₄ and Co_0.5Zn_0.5Cr₂O₄] to tetragonal (space group I4₁/amd) [Co_0.5Cu_0.5Cr₂O₄] are reported. SEM micrograph of sintered samples results in less porosity with average particle size distribution of ～0.2–0.3 μm. Shifting of Raman active phonon modes is seen with doping and an additional Raman active mode is seen at 666.45 cm^?1 for Co_0.5Zn_0.5Cr₂O₄. Dielectric behavior as a function of frequency reveals that dispersion in all these chromites is attributed to hopping mechanism. Higher value of dielectric constant (ε′) and minimum loss tangent (tan δ) for non-JT ion Co_0.5Zn_0.5Cr₂O₄ is measured inferring effective charge polarization in chromites as compare to doped JT ions. Both grains and grain boundaries are active in Co_0.5Zn_0.5Cr₂O₄ at lower frequencies as depicted from impedance analysis. Doping does not showed the presence of electric polarization in chromites.