Electronic structure, UV–VIS–NIR optical and PL performance of NiWO₄ correlated with the NI2+ electron D–D spin-allowed transitions

A single-phase NiWO 4 sample with a monoclinic structure, space group C2/m was prepared using the hydrothermal method. Detailed structure and microstructure analysis were performed utilizing a high-precision synchrotron X-ray diffraction (XRD) pattern and the Rietveld method. The sample was further characterized by applying the Fourier transform infrared (FTIR) spectroscopy technique. Results indicated that the W octahedrons exhibit smaller volume and greater distortion compared to Ni octahedrons. Diffuse reflectance measurements disclosed four absorption peaks within the ultraviolet, visible, and near-infrared regions, which could be attributed to a ligand-to-metal charge transfer and the spin-allowed d–d transitions occurring within the Ni[Formula: see text]-octahedral ions. Utilizing the Tanabe–Sugano diagram, the Racah parameter has been calculated; [Formula: see text]. The crystal-field splitting parameter, which represents E(e g )–E(t 2g ) splitting of octahedral Ni[Formula: see text] ions, is calculated as follows: [Formula: see text]. 90322[Formula: see text]eV. The direct band-gap energy for NiWO 4 was found to be 2.9[Formula: see text]eV. Meanwhile, the conduction band (E CB ) and valence band (E VB ) potentials were calculated as 0.351[Formula: see text]eV and 3.251, respectively, relative to the normal hydrogen electrode. The photoluminescence spectrum is analyzed and a schematic diagram of the primary electronic transitions within the NiWO 4 is presented. The CIE chromaticity diagrams for the NiWO 4 sample possess ([Formula: see text], [Formula: see text]) coordinates, which represent a green color.