Estimation of band gap energy and detection of electronic transitions in UV–visible spectrum of Co4Nb2O9

• Seven major transitions are observed in UV–visible spectrum of Co 4 Nb 2 O 9 . • Metal-oxygen p-d and metal–metal d-d transitions are prominent. • Intermediate Co 3d state tailors the band gap to 1.78 eV. Evaluation of optical band gap and detection of various optoelectronic transitions in the absorption spectra of C o 4 N b 2 O 9 are the primary objectives of this article. Phase purity and crystallinity were checked using X-ray diffraction and Raman spectroscopy. The optical absorption study was conducted using UV–visible spectrum and the possible absorption maxima were described by density functional theory. Using the Tauc model the band gap is found to be 1.78 eV for direct allowed transition, and it is in good agreement with the ab initio band gap value. A combined study of absorption spectra and density of states shows seven major possible transitions comprising both metal–oxygen p-d transitions and metal–metal d-d transitions.