Due to the large capacity of accommodating different ions at the B-site which helps in tuning dedicated properties, the double perovskite with formula A₂B’B”O₆ have attracted much research interest. Out of the different patterns of cation ordering of B’ & B”, the so-called rock-salt ordering is prevalent due to electrostatic reasons leading to a checkerboard pattern. Both Sr²⁺ at the perovskite A-site and rock-salt ordered between the B’ & B” is the focus of our recent studies investigating Co³⁺ impacts on the Sr₂GaNbO₆ matrix by UV-vis-NIR absorption spectroscopy for inorganic pigments and/or their potentiality for photocatalytic oxygen and hydrogen evolution reactions as reported in Sr₂CoTaO₆ [1] but with a significant decreases of the cobalt content.
The presentation will underline a simple and cost-effective salt synthesis route (molten-salt approach) toward refractory oxides of composition Sr₂Co_xGa_1-xNbO₆. Herein, we will report on the refinement of the set of parameters related to heat treatment to stabilize the ordered perovskite for cobalt content, x ranging from 0.02 to 0.2. The parameters studied were the type of flux, the dwell temperature, the dwell time and the cooling rate for a given cobalt precursor. Our study relied on powder X-ray diffraction, Electron Diffraction, UV-visible-NIR, Electron Paramagnetic Resonance and Nuclear Magnetic Resonance experiments allow us to better understand the room temperature electronic structure. Thereafter, a study on the effect of annealing was carried out on Sr₂Co_0.02Ga_0.98NbO₆ double perovskite, additionally by underlining the Co substitution effect in Sr₂GaNbO₆ using an undoped sample. The structural and optical properties of the refractory oxides Sr₂Co_xGa_1-xNbO₆  (x=0 and 0.02) were investigated using X-ray powder diffraction, ⁹³Nb, ⁷¹Ga and ⁵⁹Co Magic-Angle Spinning Nuclear Magnetic Resonance, Electron Paramagnetic Resonance and UV-vis-NIR absorption spectroscopic techniques. Combined spectroscopic studies along with a statistical calculation for the structure determination further supported by UV-vis-NIR absorption analysis indicated an unusual mixed spin-state of Co³⁺ at room temperature in the Sr₂Co_0.02Ga_0.98NbO₆ double perovskite. As pointed out, our results might be of interest either for pale brown inorganic pigments or for its potentiality to be photocatalysts for water splitting since a gap of 2.7 eV has been characterized [2 & 3].

1_ Idris, A. M.; Liu, T.; Hussain Shah, J.; Han, H.; Li, C. Sr₂CoTaO₆ double perovskite oxide as a novel visible-light-absorbing bifunctional photocatalyst for photocatalytic oxygen and hydrogen evolution reactions. ACS Sustainable Chem. Eng. 2020, 8, 14190– 14197,
2_ Maneesha Varghese, Mathieu Duttine, Hitesh Duggal, Nicolas Penin, Eric Lebraud, Alexandre Farges, Olivier Toulemonde. Molten salt flux synthesis of cobalt doped refractory double perovskite Sr₂Co_xGa_1-xNbO₆: A spectroscopic investigation for multifunctional materials. Journal of Solid State Chemistry 2022, 315 , 123507.
3_ Maneesha Varghese, Struan Simpson, Gaynor Lawrence, Mathieu Duttine, Paula Sanz Camacho, Manuel Gaudon, and Olivier Toulemonde 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 J. Phys. Chem. C 2022, 126, 19, 8450–8460