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To develop novel inorganic red pigments without harmful elements, we focused on the band structure of Ca2(Mg, Co)WO6 and attempted to narrow its bandgap by replacing the W6+ sites in the host structure of Mo6+. Ca2Mg1−xCoxW1−yMoyO6 (0.10 ≤ x ≤ 0.30; 0.45 ≤ y ≤ 0.60) samples were synthesized by a sol-gel method using citric acids, and the crystal structure, optical properties, and color of the samples were characterized. The Ca2Mg1−xCoxW1−yMoyO6 solid solution was successfully formed, which absorbed visible light at wavelengths below 600 nm. In addition, the absorption wavelength shifted to longer wavelengths with increasing Mo6+ content. This is because a new conduction band composed of a Co3d-W5d-Mo4d hybrid orbital was formed by Mo6+ doping to reduce the bandgap energy. Thus, the color of the samples gradually changed from pale orange to dark red, with a hue angle (h°) of less than 35°. Based on the above results, the optical absorption wavelength of the Ca2Mg1−xCoxW1−yMoyO6 system can be controlled to change the color by adjusting the bandgap energy. [ABSTRACT FROM AUTHOR]

Tb4+-doped sodium zirconate samples, Na2Zr1−xTbxO3, were synthesized as novel environmentally friendly inorganic yellow pigments by a conventional solid-state reaction method. Their crystal structures, optical properties, and colors were characterized. A single-phase form was obtained for the samples in the x range of x ≤ 0.18, while impurity phases were detected for the sample with x = 0.20. All samples showed strong optical absorption in the blue light region, due to the charge transfer transition between O2− and Tb4+. As a result, the sample color became yellow, which is the complementary color of blue, and the color became more vivid with increasing Tb4+ content in the single-phase region. Among the samples, Na2Zr0.82Tb0.18O3 was the optimal composition, with the highest yellowness (b* = +67.2) and pure yellow hue (h° = 90.1). Although the b* value was lower than commercial yellow pigments such as BiVO4 and ZrSiO4:Pr, this sample had a purer yellow hue. Since Na2Zr0.82Tb0.18O3 is composed of non-toxic elements, it could be a new environmentally friendly inorganic yellow pigment. [ABSTRACT FROM AUTHOR]