The crystal structure of hibonite with an ideal formula CaAl 12 O 19 is hexagonal P6 3 /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-x M x O 19 (A = Ca, Sr, RE 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 12-2x Ni x Ti x O 19 (x = 0  1) and Ca 1-x La x Al 12-x Ni x O 19 (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 Ni 2+. Optical measurements further verify that the observed blue color is due to d-d transitions of tetrahedral Ni 2+. The preference of Ni 2+ for the tetrahedral site is unusual because Ni 2+ prefers the octahedral site in the spinel NiAl 2 O 4. 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.

The solid solutions of chlorapatite compounds Ba 5 Mn 3-x V x O 12 Cl (x = 0-3.0) and Ba 5 Mn 3x P x O 12 Cl (x = 0-3.0) have been synthesized through solid state reactions and Pechini or sol-gel method using citric acid. The colors of the samples change from white (x = 3.0) through turquoise (x = 1.5) to dark green (x = 0) with increasing amount of manganese. Optical measurements reveal that the origin of the color is presumably a combination of d-d transitions of Mn 5+ and cation-anion charge transfer from transition metals to oxygens. Near IR reflectance measurements indicate that synthesized compounds are promising materials for "cool pigments" applications. Magnetic measurements verify that manganese has two unpaired electrons and exhibits 5+ oxidation state. The IR spectra change systematically with sample compositions and the fingerprint region (700 cm-1 to 1100 cm-1) indicates characteristic bands belonging to (MnO 4) 3-, (VO 4) 3and (PO 4) 3functional groups. Structure refinements using neutron data confirm that Mn 5+ , V 5+ and P 5+ cations occupy the tetrahedral sites in the apatite structure.

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Various oxides with the hibonite structure were synthesized and structurally analyzed using powder neutron diffraction. The structure of CaAl12O19 at 298 and 11 K shows dipoles that are apparently too dilute to order unless subjected to a suitable electric field. Magnetoplumbites, such as BaFe12O19, are isostructural with hibonite. These compounds possess ferromagnetic properties, which combined with the electric dipoles may influence multiferroic behavior. Our SrAl12O19 sample showed two distinct hexagonal phases, a major phase with the normal hibonite structure and a minor phase having a closely related structure. Our sample of the defect hibonite phase La2/3+δAl12–δO19 shows a distinctly higher δ value (0.25) vs. that reported (~0.15) for samples made from the melt. Finally, we used to advantage the negative scattering length of Ti to determine the site occupancies of Ni and Ti in CaAl10NiTiO19.