Inorganic pigments based on fluorite-type oxynitrides

Zirconia oxynitride rare-earth-doped pigments were prepared by ammonolysis of the zirconium rare-earth oxides, previously synthesized using the citrate complexation/calcination route. Different coloration has been obtained, the intensity of which is a function of the nitrogen amount in the case of the oxynitrides; in the case of the oxides, both color and intensity depend on the doping amount of rare earth. The obtained phases, Zr_(1−x)Ce_xO₂, Zr_(1−x)R_xO_(2−x/2)□_x/2, with R = Eu or Er and Zr_(1−x)R_xO_{(2−x/2−3/2y)}N_y□_x/2■_y/2 (R = Ce, Eu, and Er), have been characterized by x-ray powder diffraction, scanning electron microscopy, and reflectance spectra data. These results show that the phases with minor rare-earth concentration adopt a baddeleyite-type structure, with a monoclinic symmetry, space group P2₁/c. By increasing the rare-earth doping, the obtained phases crystallize with the fluorite structure with tetragonal (P4₂/nmc) or cubic symmetry (Fm^¯₃m). On the other hand, the study of the magnetic properties of the oxides and oxynitrides indicate a paramagnetic behavior, and in the case of the cerium oxide, the nitridation process produces the reduction from Ce⁴⁺ to Ce³⁺. Diffuse reflectance data and CIE-LAB color coordinates suggest that these ceramics based on nitrogen containing zirconia are expected to be promising candidates as new ecological inorganic pigments.