Mixed Metal Oxide Solid Solutions of LiMgVO₄ with Co²⁺/Ni²⁺ Substitution as Potential NIR-Reflective Cool Pigments

Mixed metal oxides are increasingly investigated as functional pigments due to their structural versatility, stability, and tunable optical properties. In this work, LiMgVO₄, a lithium-based ternary vanadate, was doped with Co²⁺ and Ni²⁺ to study phase evolution, local structure, and optical behaviour for near-infrared (NIR) reflective applications. Controlled substitution induced a transformation from an orthorhombic Na₂CrO₄-type (Cmcm) lattice to an inverse spinel (Fd3̅m), with a distinct miscibility gap between the phases. Solid-state ⁷Li and ⁵¹V MAS NMR confirmed dopant incorporation, showing paramagnetic relaxation enhancement at low levels and line broadening at higher concentrations. Diffuse reflectance spectroscopy revealed spin-allowed d-d transitions, correlating with systematic color evolution from dusty blue to black (Co) and pale yellow to brown (Ni). The doped LiMgVO₄-0.02Co and LiMgVO₄-0.02Ni compositions exhibited high NIR solar reflectance of ∼82% in the wavelength range 700-2500 nm, enabling effective solar heat management. These findings establish Co/Ni-doped LiMgVO₄ as a promising mixed oxide pigment for sustainable cool coating applications.