The primary objective of this study is to comprehensively investigate the impact of substituting iron with chromium on the structural, optical, electrical, and dielectric properties of Mg0.6Co0.4Fe2−xCrxO4 (x = 0; 1) samples. These samples were successfully synthesized using sol–gel preparation. X-ray diffraction (XRD) analysis revealed a cubic spinel structure with a space group of Fd \(\overline{3 }\) m. Furthermore, the XRD analysis indicated a decrease in both the average crystallite size and lattice parameter resulting from the substitution of iron (Fe) with chromium (Cr). Additionally, FTIR analysis unveiled the presence of an absorption band corresponding to the stretching vibration of oxygen atoms and metal ions within the tetrahedral site. Moreover, the direct optical bandgap energy (Eg) increased, while the Urbach energy (Eu) decreased with the substitution of iron (Fe) with chromium (Cr). It should also be noted that the conductivity (σs) increases at temperatures below the TMS, but shows a decrease at temperatures above the TMS (440 K) following the iron (Fe) to chromium (Cr) substitution. We also observed a metal–semiconductor transition in both samples at a specific temperature (TMS = 440K), and it is worth noting that this transition was unaffected by the substitution of Fe3+ with Cr3+. Impedance data were analyzed using Zview software with an appropriate equivalent circuit of the type (Rg //CPEg + Rgb //CPEgb). The estimated activation energies, derived from the electrical modulus, impedance, and direct current conductivity analyses, increased after the substitution of iron (Fe) with chromium (Cr).
