Influence of Zn²⁺ ions on the structural and electrical properties of Mg_1-xZn_xFeCrO₄ spinels

The polycrystalline magnesium-zinc ferrite chromate having the chemical formula Mg_1-xZn_xFeCrO₄ (0.0 ≤ x ≤ 1.0) were prepared by conventional solid phase reaction using high purity reagents. The structural and electrical properties at room temperature have been investigated by XRD, TEM, SEM, EDS, TGA, DTA, FT-IR and DC electrical resistivity techniques. The lattice parameters such as the lattice constant, average particle size, bonds length, density, porosity and volume shrinkage were found to vary with increasing Zn²⁺ content in the sample. The lattice constant "a" increases from 8.361 � for MgFeCrO₄ to 8.382 � for ZnFeCrO₄ spinels. The IR spectra confirm the presence of two main absorption bands υ₁ and υ₂ in the frequency range of 400-1000 cm^-1, arising due to the tetrahedral (A) and octahedral (B) stretching vibrations respectively. Values of υ₁ and υ₂ decrease with increasing Zn⁺² content over the whole composition range. The scanning electron microscope (SEM) and transmission electron microscope (TEM) micrographs showed aggregates of stacked large grains. Values of the room temperature DC electrical resistivity decrease as Zn²⁺ content increases. The low values of the vacancy model parameters indicate that the Mg_1-xZn_xFeCrO₄ spinels are strongly defective and the participation of the Zn²⁺ vacancies in the improvement of the electrical conductivity in these spinels. The thermo-gravimetric, TGA and differential thermal analysis, DTA show loss of mass caused by vaporization of the surface and trapped water molecules at higher temperatures.