V³⁺ Doping in Cs₂NaInCl₆ Double Perovskite for Near-Infrared-II Luminescence

V³⁺ with 3d² electrons is a promising near-infrared (NIR) emitter because of their d-d electronic transitions. But V³⁺ is prone to further oxidation in ambient conditions, complicating the spectral features. Here we report a simple hydrothermal synthesis of V-doped Cs₂NaInCl₆ in the reducing environment of H₃PO₂, resulting into ambient-stable V³⁺ dopants. Optical excitation and emission spectra are explained by using the Tanabe-Sugano diagram of d² electrons in [VCl₆]^3- octahedral unit. At 300 K, a broad NIR-II emission at ∼1225 nm, with a lifetime of 195 μs, is observed owing to the spin-allowed but Laporte-forbidden ³T_2g → ³T_1gd-d transition. The spectral features change dramatically below 125 K, when a sharp emission at ∼1063 nm with a long lifetime of 19.26 ms is observed. The sharp emission originates from ¹E_g/¹T_2g → ³T_1g intraconfiguration spin-flip transition. The presented synthesis and spectroscopic insights will aid future development of V³⁺ as an NIR-II optoelectronic material.