Hibonite Blue: A New Class of Intense Inorganic Blue Colorants

Commercially available spinel cobalt blue (CoAl₂O₄) utilizes a significant amount of carcinogenic Co²⁺, which makes its synthesis more hazardous and environmentally harmful. Considerable effort has been put into developing more environmentally benign and robust blue pigments to replace cobalt blue. A new class of blue pigments with tunable hue were prepared. The solid solution series, CaAl_12–2xCo_xTi_xO₁₉ (0 < x ≤ 1), crystallizes in a hexagonal mineral hibonite (CaM₁₂O₁₉) structure with five distinct crystallographic sites for M cations (M = Al, Co, and Ti). The origin of intense blue color is attributed to a synergistic effect of allowed d–d transitions involving the chromophore Co²⁺ in both tetrahedral and trigonal bipyramidal crystal fields. Compared with commercial cobalt blue, these tunable hibonite blues possess a reddish hue that intensifies the blue color as observed in Y­(In,Mn)­O₃ (YInMn) blues, with a significant reduction of Co²⁺ concentration from 33% to as low as 4% by mass. A significant advantage of hibonite blues over cobalt blue is the substantial reduction in carcinogenic cobalt content while enhancing the color properties at a reduced cost for raw materials.