Influence of transition metal Cr2+ doping on structural, electrical and optical properties of Mg-Zn aluminates

Mg-Zn aluminates doped by Cr [Mg0.5Zn0.5-xCrxAl2O4 (x = 0.0, 0.1, 0.2, 0.3, 0.4, 0.5)] have been synthesized by sol-gel auto combustion method. X-ray diffraction showed a cubic lattice structure with space group Fd3m and Rietveld refinement confirmed the cubic structure with average particle size of about 10-14 nm. Volume of the unit cell has been found to decrease at lower doping while it increases at higher doping and corresponding behavior in density and hence in lattice parameter is observed. Dielectric studies of the samples revealed the decrease of dielectric constant with increase in frequency and attain saturated value at higher frequency. The dielectric loss has been found initially to increase with increase in frequency and reaches a maximum value and then decreases and appears to approach the minimum possible value. The ac conductivity against frequency revealed the increase in conductivity but shows the drop at higher values of frequency. UV-Vis studies have been carried out to check the optical band-gap which was found to be ≈ 4 eV for all Cr doped Mg-Zn aluminates narrowed from Eg = 5.52 eV for parent Mg0.5Zn0.5Al2O4. FT-IR studies also showed the spinel nature.