Influence of the precursors on the formation and properties of the FexCr2−xO3 solid solution

A black pigment was synthesised based on FexCr2−xO3 stoichiometry, by studying the compositions with x = 0.15, 0.20, 0.50, 1.00, 1.20, 1.30, 1.50, 1.80 and 1.85 and using industrial grade reagents as raw materials. The resulting products were compared with a pigment made using chemically pure (CP grade) reagents in order to establish the most appropriate reagents for achieving minimum Cr(VI) segregation during the pigment washing stage, and comparable chromatic qualities of a standard industrial pigment. X-ray diffraction (XRD) indicates a solid solution formation based on the haematite structure; however, X-ray photoelectron spectroscopy (XPS) and Mössbauer analysis indicate the presence of a phase containing Fe(II) undetected by XRD. A smaller Cr(VI) content was found in the washing liquids when industrial grade reagents were used, but the resulting chromatic quality was poorer than that obtained for the same composition when made with CP grade raw materials.