Understanding color variation with site distribution in inverse spinel structure via neutron diffraction, magnetism, and optical studies

Chromophores at different coordinations can give rise to different colors; usually, chromophores at non-centrosymmetric coordinations are preferred for intense pigments. Different solid solutions M_2-xCo_xM’O₄ (M = Mg/Zn, and M’ = Ti/Sn) with inverse spinel structure were synthesized with the goal of understanding color variation with site distribution, as the chromophore Co²⁺ in these solid solutions can occupy either the tetrahedral or octahedral sites or both depending on the composition. Another goal was to develop environmentally friendly and cheap blue pigments by reducing the carcinogenic cobalt to obtain a similar color to that of commercially available cobalt blue, which uses a significant amount of Co²⁺ (33.31 % by mass). For Mg_2-xCo_xTiO₄ series, turquoise blue hues were observed for low cobalt content, and different shades of blue were observed for Mg_2-xCo_xSnO₄ series with a color similar to cobalt blue, including just 4.90% of cobalt by mass. While for Zn_2-xCo_xTiO₄, and Zn_2-xCo_xSnO₄ series, different shades of brown and different shades of green, respectively, were observed. One of the main reasons behind the major difference in color for the Mg and Zn containing solid solutions, regardless of the same chromophore in the same structure is related to the chromophore site distribution in the system. For the Mg-containing solid solutions, different shades of blue are observed as Mg has no preference for any of the sites, Co²⁺ mostly goes to tetrahedral sites. In contrast, for the Zn-containing solid solutions, no blue shades were observed because of the strong preference of Zn for the tetrahedral sites owing to the sp³ hybridization, which in turn forces Co²⁺ to occupy the octahedral sites. Neutron refinement proves that Co²⁺ occupies mainly tetrahedral sites in the Mg-containing solid solutions and mostly octahedral sites in the Zn-containing solid solutions.