Effect of Co substitution on the structural, dielectric and optical properties of KBiFe₂O₅

Cobalt (Co)-modified brownmillerite KBiFe₂O₅ (KBFO; KBiFe_2(1−x)Co_2xO₅ (x = 0, 0.05)) polycrystalline is synthesized following the solid-state reaction route. Rietveld refinement of X-ray diffraction data confirmed the phase purity of KBFO and KBiFe_1.9Co_0.1O₅ (KBFCO). The optical bandgap energy (E_g) of KBFO decreased from 1.59 to 1.51 eV because of Co substitution. The decrease in bandgap can be attributed to the tilting of the Fe–O tetrahedral structure of KBFCO. The observed room-temperature Raman peaks of KBFCO shifted by 3 cm⁻¹ toward a lower wavenumber than that of KBFO. The shift in Raman active modes can be attributed to the change in the bond angles and bond lengths of the Fe–O tetrahedral structure and modification in response to oxygen deficiency in KBFO because of Co doping. Compared with that of KBFO, the frequency-dependent dielectric constant and dielectric loss of KBFCO decrease at room temperature, which is a consequence of the reduction in oxygen migration and modification in response to vibrational modes present in the sample.