V 3+ Doping in Cs 2 NaInCl 6 Double Perovskite for Near-Infrared-II Luminescence

V 3+ with 3 d 2 electrons is a promising near-infrared (NIR) emitter because of their d – d electronic transitions. But V 3+ is prone to further oxidation in ambient conditions, complicating the spectral features. Here we report a simple hydrothermal synthesis of V-doped Cs 2 NaInCl 6 in the reducing environment of H 3 PO 2, resulting into ambient-stable V 3+ dopants. Optical excitation and emission spectra are explained by using the Tanabe–Sugano diagram of d 2 electrons in [VCl 6 ] 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 3 T 2g → 3 T 1g d – 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 1 E g / 1 T 2g → 3 T 1g intraconfiguration spin-flip transition. The presented synthesis and spectroscopic insights will aid future development of V 3+ as an NIR-II optoelectronic material.