Spin Centers in Vanadium-Doped Cs₂NaInCl₆ Halide Double Perovskites

We provide direct evidence for a spin-active V4+ defect center, likely in the form of a VO2+ complex, predominantly introduced in single crystals of vanadium-doped Cs2NaInCl6 halide double perovskites grown by the solution-processed hydrothermal method. The defect has C-4v point group symmetry, exhibiting an electron paramagnetic resonance (EPR) spectrum arising from an effective electron spin of S = 1/2 and a nuclear spin of I = 7/2 (corresponding to V-51 with nearly 100% natural abundance). The determined electron g-factor and hyperfine parameter values are g(perpendicular to)= 1.973, g(parallel to) = 1.945, A(perpendicular to) = 180 MHz, and A(parallel to) = 504 MHz, with the principal axis z along a < 001 > crystallographic axis. The controlled growth of V-doped Cs2NaInCl6 in an oxygen-free environment is shown to suppress the V4+ EPR signal. The defect model is suggested to have a VOCl5 octahedral coordination, where one of the nearest-neighbor Cl- of V is replaced by O2-, with octahedral compression along the V-O axis. This VO complex formation competes with the isolated V3+ substitution of In3+, which in turn provides a means for the charge-state tuning of V ions. This finding calls for a better understanding and control of defect formation in solution-grown halide double perovskites, which is critical for optimizing and tailoring material design for solution-processable optoelectronics and spintronics.