Influence of the host lattice on the blue color development in Ni2+, La3+ co-doped calcium aluminates: Choice not chance

The influence of (Ca 2+ + Al 3+ ) → (La 3+ + Ni 2+ ) substitution on the color of calcium aluminates (Ca 3 Al 2 O 6 , CaAl 2 O 4 , CaAl 4 O 7 , CaAl 12 O 19 ) obtained by combustion synthesis represented the focus of this work. The maximum temperature recorded during combustion reactions increased from 1406 °C to 1606 °C with increasing the Al 3+ /Ca 2+ ratio. The formation of solid solutions having the same Ni 2+ content (0.3 at.%) was confirmed by X-ray diffraction. Following the combustion reactions, the solid solutions derived from Ca 3 Al 2 O 6 , CaAl 2 O 4 and CaAl 4 O 7 had a gray color. After annealing at 1300 °C these samples showed undefined pastel shades. Increasing the Ni 2+ doping level to 5 at.% relative to tetrahedral Al 3+ proved that only Ni-doped hibonite crystal structure can render a blue coloration, which after annealing became slightly more intense. These results demonstrate the importance of the rational choice of the host crystal lattice: among all calcium aluminates herein investigated, hibonite was the only one that allowed obtaining blue pigments as a result of (Ca 2+ + Al 3+ ) → (La 3+ + Ni 2+ ) substitution.