Synthesis and Color Evaluation of Ta⁵⁺-Doped Bi₂O₃

Ta⁵⁺-doped Bi₂O₃ solid solutions, (Bi_1- Ta )₂O₃₊₂ (0 ≤ x ≤ 0.20), were synthesized by a conventional solid-state reaction method. The crystal structure, optical properties, and color of the pigments were characterized. The (Bi_1- Ta )₂O₃₊₂ (x = 0, 0.03, 0.04, 0.20) samples were obtained in a single-phase form, and the crystal structure depended on the Ta content; they were monoclinic α-phase (x = 0), tetragonal β-phase (x = 0.03, 0.04), and cubic δ-phase (x = 0.20). Among them, (Bi_0.97Ta_0.03)₂O_3.06 strongly absorbed visible light at the wavelength shorter than 480 nm and exhibited the most vivid orange color. The redness value (a*) was comparable to that of commercial orange pigments, and the yellowness value (b*) was slightly larger. Although it is necessary to improve chemical stability, the (Bi_0.97Ta_0.03)₂O_3.06 pigment has potential for an environmentally friendly inorganic orange pigment.