Highly Efficient and Tunable Emission of Lead‐Free Manganese Halides toward White Light‐Emitting Diode and X‐Ray Scintillation Applications

Environmental friendly metal halides have become emerging candidates as energy downconverting emitters for lighting and X‐ray imaging applications. Herein, luminescent single crystals of tetramethylammonium manganese chloride (C 4 H 12 NMnCl 3 ) and tetraethylammonium bromide ((C 8 H 20 N) 2 MnBr 4 ) are synthesized via a facile room‐temperature evaporation method. C 4 H 12 NMnCl 3 and (C 8 H 20 N) 2 MnBr 4 with octahedrally and tetrahedrally coordinated Mn 2+ have correspondingly exhibited red and green emission peaking at 635 and 515 nm both originating from 4 T 1 – 6 A 1 transition of Mn 2+ with high photoluminescence quantum yield (PLQY) of 91.8% and 85.1% benefiting from their specific crystal structures. Thanks to their strong photoexcitation under blue light, high PLQY, tunable emission spectra, good environmental stability, the white light‐emitting diode based on blending of C 4 H 12 NMnCl 3 and (C 8 H 20 N) 2 MnBr 4 delivers an outstanding luminous efficacy of 96 lm W −1 , approaching commercial level, and shows no obvious photoluminescence intensity degradation after 3000 h under operation. In addition, manganese halides also demonstrate interesting characteristics under X‐ray excitation, C 4 H 12 NMnCl 3 and (C 8 H 20 N) 2 MnBr 4 exhibit steady‐state X‐ray light yields of 50 500 and 24 400 photons MeV −1 , low detectable limits of 36.9 and 24.2 nGy air s −1 , good radiation hardness, and X‐ray imaging demonstration with high‐resolution of 5 lp mm −1 . This work presents a new avenue for luminescent Mn‐based metal halides toward multifunctional light‐emitting applications.