Overcoming Thermal Quenching of Near‐Infrared Emissions From Re(IV) in Soft Double Perovskite Through Reduced Phonon‒Phonon Coupling

ABSTRACT Transition metal (TM) ion‐doped halide perovskites (HPs) rapidly gain prominence in various research fields due to tunable near‐infrared (NIR) emission characteristics. However, unsatisfactory thermal stability severely restricts their practical application, particularly in the NIR‐II region (1200‒1600 nm). Herein, both intra‐configurational spin‐flip narrow/broadband emissions from Re 4+ with low thermal quenching are realized in Cs 2 NaLuCl 6 populated by efficient Yb 3+ →Re 4+ process. Specially, quantum efficiency and thermal stability of predominantly broadband NIR‐II luminescence can be improved to 95% and 94.8%@423 K by F ‒ local structure engineering. Lattice and excited‐state dynamics, temperature‐dependent luminescent spectra, and theoretical calculations verify Re 4+ ion shows unchanged metal‐ligand bond lengths and vibronic energies under thermal activation, leading to self‐vibration effect by spatially and temporally decoupling to the accumulated lattice vibrations in soft 3D framework. The dominated low‐frequency T 2g phonon mode of Re 4+ gives rise to reduced intrinsic phonon anharmonicity via non‐crossover energy profile. Besides, a fundamental comprehension of high quantum efficiency is linked to low symmetry, strong spin–orbit coupling, weak, electron–phonon coupling and large Franck–Condon factor. The exceptional NIR‐II luminescence are fully exploited for high‐performance light‐emitting diodes, imaging, HF gas detection and anti‐counterfeiting. The vibration‐adaptive feature of Re 4+ offers new opportunities for potential multifunctional applications.