Room-Temperature Mixed Spin State of Co³⁺ in Sr₂Co_0.02Ga_0.98NbO₆ Double Perovskites: Combined NMR and EPR Studies in a Potential Inorganic Pigment

The structural and optical properties of the refractory oxides Sr2CoxGa1–xNbO6 (x = 0 and 0.02) were investigated using powder X-ray diffraction; 93Nb, 71Ga, and 59Co magic-angle spinning nuclear magnetic resonance; electron spin resonance; and UV–vis–NIR absorption. A flux using SrCl2 versus a ball milling strategy to stabilize rock salt-ordered Sr2CoxGa1–xNbO6 (x = 0 and 0.02) at low temperature was underlined. Sr2Co0.02Ga0.98NbO6 showed a tetragonal I4/m crystalline structure with a slight lattice distortion index in the range of 0.04. Combined spectroscopic studies, further supported by UV–vis–NIR absorption analysis, definitively indicate an unusual mixed spin state of Co3+ at room temperature in the Sr2Co0.02Ga0.98NbO6 double perovskite that might be of interest for pale-brown inorganic pigments.