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 ⁵¹V with nearly 100% natural abundance). The determined electron g-factor and hyperfine parameter values are g_⊥= 1.973, g_∥ = 1.945, A_⊥ = 180 MHz, and A_∥ = 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 O^2–, 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.