Dielectric and optical properties of Fe1−xCoxNbO4 (x = 0 and 0.10) ceramics

The dielectric and optical properties of Fe 1−x Co x NbO 4 (x = 0 and 0.10) ceramics were investigated by impedance spectroscopy, electric modulus analysis, and diffuse reflectance measurements. The electrical response reveals thermally activated relaxation processes associated with grain and grain-boundary contributions, dominated by hopping conduction mediated by oxygen vacancies and localized mixed-valence states. Activation energies in the range of 0.52–0.74 eV were obtained from Arrhenius analysis. Cobalt substitution leads to higher impedance values and an enhanced dielectric constant, indicating modifications in lattice structure and defect distribution. Optical measurements exhibit strong UV-visible absorption for both compositions, while Co incorporation reduces the absorption intensity and slightly decreases the optical band gap, evidencing changes in the electronic structure and metal–oxygen interactions. These results highlight the role of Co substitution in tuning the dielectric and optical properties of FeNbO 4 -based ceramics.