High‐Performance Near‐Infrared Ba ₃ MgSb ₂ O ₉ :Fe ³⁺ , Sn ⁴⁺ Phosphors with Elucidated Multifaceted Thermal Quenching Mechanisms

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.