Hibonite Blue: A New Class of Intense Inorganic Blue Colorants

[Image: see text] Commercially available spinel cobalt blue (CoAl(2)O(4)) utilizes a significant amount of carcinogenic Co(2+), 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(12–2x)Co(x)Ti(x)O(19) (0 < x ≤ 1), crystallizes in a hexagonal mineral hibonite (CaM(12)O(19)) 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 Co(2+) 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)O(3) (YInMn) blues, with a significant reduction of Co(2+) 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.