Color Controllable Inorganic Pigments with Ce³⁺ as a Color Source

The Ba₂RE_1-CeTaO₆ (RE = La, Gd, Y; 0 ≤ x ≤ 1) pigments were synthesized by a conventional solid-state reaction method to develop environmentally friendly reddish inorganic pigments. The host was the double-perovskite-type Ba₂RETaO₆ (RE: rare-earth elements), and the color source was Ce³⁺. All Ba₂RE_1-CeTaO₆ samples were obtained in a single-phase form as solid solutions. Rietveld refinement analysis of the Ba₂RETaO₆ (RE = La, Ce, Gd, and Y) samples revealed that the average bond length between RE³⁺ and O depends on the ionic radius of RE³⁺, and the shorter the RE-O length, the stronger the crystal field surrounding Ce³⁺. A photon energy at the maximum 4f-5d absorption of Ce³⁺ depended on the weighted average ionic radius (r_RE) at the RE³⁺ site (i.e., the crystal field energy around the Ce³⁺ ions). In response to this phenomenon, the sample color was changed in order to orange, red, pink, and violet with a decrease in the r_RE value, and a hue angle (h°) was roughly linearly related to that. For validation of the tendency, we demonstrated the synthesis and characterization of Ba₂La_0.5-YCe_0.5TaO₆ to obtain a more reddish color. As we exactly expected, a more reddish color was obtained while maintaining a high C value. Furthermore, the h° values for y = 0.1 and 0.2 were in good agreement with our estimation. In light of the above results, by controlling the intensity of the crystal field surrounding Ce³⁺ and/or the concentration of Ce³⁺, the optical absorption wavelength and absorption intensity of the Ba₂RE_1-CeTaO₆ system can be changed to adjust the color.