Rational Design of Near-Infrared Reflective Pigments by Adjusting the Ni²⁺ Coordination Environment

Inorganic pigments offer excellent potential because of their resilience in a variety of environments, and high-reflectance near-infrared (NIR) pigments are a useful tactic to reduce the heat island effect. Nonetheless, the design of inorganic pigments is still dominated by serendipity, and it has never been easy to come up with logical designs for NIR reflective inorganic pigments. By modifying the coordination environment of Ni²⁺ using the generic formulas LiMg_1-<i>x</i>Ni_<i>x</i>PO₄ and LiZn_1-<i>x</i>Ni_<i>x</i>PO₄ (<i>x</i> = 0.02-0.12), yellow and violet pigments were successfully developed in this research. According to structural investigations, Ni²⁺ effectively replaced Mg²⁺ and Zn²⁺ to create [NiO₄] tetrahedra and [NiO₆] octahedra, causing an orderly alteration of the cell parameters. The diverse transition modes of Ni²⁺ in various crystal fields are the cause of the noticeable color changes caused by the solid solutions, according to the UV-vis-NIR spectra. Furthermore, both pigments have outstanding NIR solar reflectivity and coloring capabilities. Both the sample-colored coatings and the coating's surface under infrared light demonstrated lower temperatures than the commercial pigmented coatings for thermal insulation applications.