Developing Intense Blue and Magenta Colors in α-LiZnBO₃ : The Role of 3d-Metal Substitution and Coordination

We describe the synthesis, crystal structures, and optical absorption spectra/colors of 3d-transition-metal-substituted α-LiZnBO₃ derivatives: α-LiZn_1-x M^II_x BO₃ (M^II =Co^II (0II (0II (01+x Zn_1-2x M^III_x BO₃ (M^III =Mn^III (0III (03 , which is both disordered and distorted with respect to Li and Zn occupancies and coordination geometries, is largely retained in the derivatives, which gives rise to unique colors (blue for Co^II , magenta for Ni^II , violet for Cu^II ) that could be of significance for the development of new, inexpensive, and environmentally friendly pigment materials, particularly in the case of the blue pigments. Accordingly, this work identifies distorted tetrahedral MO₄ (M=Co, Ni, Cu) structural units, with a long M-O bond that results in trigonal bipyramidal geometry, as new chromophores for blue, magenta, and violet colors in a α-LiZnBO₃ host. From the L*a*b* color coordinates, we found that Co-substituted compounds have an intense blue color that is stronger than that of CoAl₂ O₄ and YIn_0.90 Mn_0.10 O₃ . The near-infrared (NIR) reflectance spectral studies indicate that these compounds exhibit a moderate IR reflectivity that could be significant for applications as "cool pigments".