Structural characterization, morphology, optical and colorimetric properties of NiWO4 crystals synthesized by the co-precipitation and polymeric precursor methods

Abstract In this paper, nickel tungstate (NiWO4) crystals were synthesized by the co-precipitation and polymeric precursor methods, followed by heat treatment at 1073 K for 4 h and 2 h, respectively. Their structure, morphology, optical and colorimetric behavior were characterized by different techniques. X-ray diffraction, Rietveld refinement and Fourier transform-infrared spectroscopy proved that these NiWO4 crystals present a wolframite-type monoclinic structure, space group (P2/c) and seven infrared-active (4Au and 3Bu) modes from 400 to 1,200 cm−1. A software of three-dimensional visualization for electronic and structural analysis using the Rietveld refinement analysis as input data was employed for modeling the cluster coordination and electron density mapping of the NiWO4 crystals. These data indicate that their lattice is composed of distorted octahedral [NiO6]/[WO6] clusters and inhomogeneous distribution of electronic charges between both clusters. Field emission scanning electron microscopy images were performed to observe the shape and average size of the crystals. Additionally, we investigated the optical band gap values and colorimetric properties through of ultraviolet–visible spectroscopy and colorimetry measures, respectively. Finally, our colorimeter data showed that the NiWO4 crystals obtained can be used as inorganic colored pigments, possibly in the ceramics industry.