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 Zn_1−xCo_xAl₂O₄ compound while coloration of Zn_1−xNi_xAl₂O₄ 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 Zn_0.9Co_0.1Al_2.2O_4+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.