Colored oxides with hibonite structure: A potential route to non-cobalt blue pigments

The crystal structure of hibonite with an ideal formula CaAl₁₂O₁₉ is hexagonal P6₃/mmc, isostructural with magnetoplumbite. Natural and synthetic hibonites have been widely studied for their formation, compositions, crystal structures, properties and applications. Recent increasing interest in its coloration has led to the search of inorganic pigments based on the hibonite structure. We present here the syntheses and characterization of hibonite compounds with a general formula of AAl_12−xM_xO₁₉ (A = Ca, Sr, RE (rare earths) or any combination thereof; and M = Ni or Ni coupled with one of the following: Ti, Sn, Ge, Nb, Ta, Sb). Bright sky-blue to royal-blue colors are induced in these oxides prepared by conventional solid state reactions, as demonstrated in the solid solutions of CaAl_12−2xNi_xTi_xO₁₉ (x = 0–1) and Ca_1−xLa_xAl_12−xNi_xO₁₉ (x = 0–1). The values of color coordinates L*a*b* range from 64.5, −5.3, −18.5 to 57, −11.33, −30.38. Structure refinements of neutron powder diffraction data reveal that Ni preferably occupies the tetrahedral site in the hibonite structure, and magnetic susceptibility analysis confirms that this Ni is Ni²⁺. Optical measurements further verify that the observed blue color is due to d-d transitions of tetrahedral Ni²⁺. The preference of Ni²⁺ for the tetrahedral site is unusual because Ni²⁺ prefers the octahedral site in the spinel NiAl₂O₄. We attribute this unexpected behavior to the unusually large Al−O distances for the tetrahedral site in the hibonite structure. These blue hibonites exhibit excellent thermal stability, superior acid/base durability and better near-infrared reflectance than that of the commercial cobalt blue pigment. Our results suggest a potential route to the development of inexpensive, enduring and cobalt-free blue pigments. Synthesis and characterization methods are briefly reviewed for hibonite type of oxides, especially those with blue colors.