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

Cobalt modified brownmillerite KBiFe₂O₅ [KBiFe_2(1-x)Co_2xO₅ (x= 0, 0.05)] polycrystalline is synthesized following solid-state reaction route. Rietveld refinement of X-ray diffraction (XRD) data reveals the phase purity of KBiFe₂O₅ (KBFO) and KBiFe_1.9Co_0.1O₅ (KBFCO). The optical band gap energy (E_g) of KBFO is observed to be decrease from 1.59 eV to 1.51 eV by Co substitution. The decrease in band gap attributes to the tilting in the Fe-O tetrahedral structure of KBFCO. The observed room temperature Raman peaks of KBFCO are shifted by 3 cm^-1 towards lower wavenumber in comparison with KBFO Raman peaks. The shifting of Raman active modes can be attributed to the change in the bond angles and bond lengths of Fe-O tetrahedral and modification in oxygen deficiency in KBFO due to Co doping. The frequency-dependent dielectric constant and loss of KBFCO also decrease with respect to KBFO at room temperature, which is a consequence of the reduction in oxygen migration and modification in vibrational modes present in the sample.