Control of Optical Reflection in Ca₂MgWO₆ by Co and Mo Doping

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⁺. Ca₂Mg_1-CoW_1-MoO₆ (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 Ca₂Mg_1-CoW_1-MoO₆ 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 Co_3d-W_5d-Mo_4d 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 Ca₂Mg_1-CoW_1-MoO₆ system can be controlled to change the color by adjusting the bandgap energy.