Sol–gel and sonochemically derived transition metal (Co, Ni, Cu, and Zn) chromites as pigments: A comparative study

Cobalt chromite based pigments Co1−xMxCr2O4 (M=Ni, Cu, and Zn) with different transition metal concentrations (0≤x≤1 with a step of 0.25) have been synthesized applying two aqueous synthesis approaches: sol–gel and sonochemical synthesis routes. The heat treatment of precursor powders was performed between 600 and 800 °C. XRD analysis of the obtained powders revealed that all samples fabricated by sol–gel method have crystallised in a spinel structure, whereas sonochemical synthesis of Ni chromite at lower calcination temperatures showed the formation of mixtures of oxides. In addition, the degree of crystallinity and shaping of sonochemically obtained compounds is lower than sol–gel derived products. The chromites with a higher nickel concentration displayed green colour, while Cu-substituted pigments were nearly black. The spinels with a higher Zn amount were yellowish green.