Cobalt and nickel aluminate spinels: Blue and cyan pigments

M2+-doped aluminate spinels (M=Co or Ni) were prepared by a polymeric route leading to pure phases for synthesis temperatures equal to 800 or 1200 °C and characterized by UV–vis–NIR spectroscopy, 27Al NMR and XRD refinements. Coloration of the synthesized pigments is clearly sensitive to the distribution of doping ions in the aluminate spinel lattice. As the synthesis temperature increased, a color shift from green to blue has been observed for Zn1−xCoxAl2O4 compound while coloration of Zn1−xNixAl2O4 compound remains greenish-gray. Hence, to improve pigment coloration and/or synthesis cost, two different strategies have been proposed: (i) the synthesis of aluminum over-stoichiometric spinel with Zn0.9Co0.1Al2.2O4+δ formal composition in order to force Co2+ to be located in tetrahedral sites and (ii) changing from ZnAl2O4 to MgAl2O4 as host lattices for Ni2+ doping ions in order to force Ni2+ to be located in octahedral sites.