Novel Aeschynite-Type CeTiTa1– x Nb x O6 Orange Pigments

An orange inorganic pigment free from hazardous components conventionally used in inorganic pigments, such as Cd-, Pb-, Hg-, and Cr(VI)-based compounds, was developed based on aeschynite-type CeTiTa1–xNbxO6 (0 ≤ x ≤ 1) synthesized by a solid-state reaction method. X-ray powder diffraction, Rietveld refinement, and X-ray fluorescence analysis supported the formation of a continuous CeTiTa1–xNbxO6 solid solution over the investigated composition range, with a small amount of TiO2 impurity. UV–visible reflectance spectroscopy showed that increasing Nb5+ substitution shifted the absorption edge toward longer wavelengths, while the optical band gap energy estimated from Tauc plots decreased from 2.45 to 2.12 eV. This behavior may be associated with modification of the transition-metal d-derived conduction-band states upon Nb substitution. As a result, the sample color gradually changed from yellow to orange with increasing Nb content, and CeTiNbO6 (x = 1) exhibited a high redness value (a* = +26.4). X-ray photoelectron spectroscopy indicated that Ce3+ was the predominant Ce species in the surface region probed by XPS. CeTiNbO6 retained its orange coloration after the acid and base treatments employed in this study and after heat treatment at 800 °C for 6 h in air. These results indicate that CeTiTa1–xNbxO6 is a promising orange inorganic pigment with composition-tunable color and good color stability under the chemical and thermal conditions examined in this study.