Abstract The inherent trade‐off between luminescence efficiency and thermal stability in near‐infrared (NIR) phosphors poses significant challenges for practical applications. Herein, a structural optimization strategy is demonstrated in Ba 3 MgSb 2 O 9 :Fe 3+ phosphors that achieve a huge leap in internal quantum efficiency (IQE) from 13.83% to 85.23% via heterovalent Sn 4+ co‐doping, and the phosphors also maintain excellent luminescence thermal stability (70.0%@423 K). It is found that the enhanced structural disorder and reduced local symmetry of FeO 6 octahedra contribute to relaxing the parity‐forbidden nature of Fe 3+ d–d transitions. Moreover, multi‐pronged theoretical analysis, including non‐radiative transition pathways, atomic relaxation behavior, and crystal structure rigidity, reveals the intrinsic origins of the excellent thermal stability. Finally, the optimized NIR Ba 3 MgSb 2 O 9 :Fe 3+ , Sn 4+ phosphor demonstrates NIR applications in non‐destructive testing and night vision imaging. This work opens up new ideas via for exploring high‐performance NIR luminescent materials by combining experiments and theoretical calculations.
Xu et al. (2025) studied this question.