Highly Efficient Inorganic 2D Bismuth‐Doped Lanthanide Metal Halide Scintillators Enable High‐Resolution X‐Ray Imaging

Abstract Inorganic metal halides have shown great promise for optoelectronic applications; however, their irreversible hydrolytic degradation compromises optoelectronic performance, hindering practical applications. Herein, for the first time a novel inorganic 2D lanthanide halide structure is reported, Cs ₂ TbCl ₅ ·6H ₂ O, with narrow‐band green emission. In Cs ₂ TbCl ₅ ·6H ₂ O, H ₂ O molecules stabilize the structure by forming [TbCl ₂ ·6H ₂ O] ⁺ dodecahedral units, rather than coordinating to [TbCl ₆ ] ³⁻ octahedral units. Under excitation, a halogen‐to‐lanthanide ion charge‐transfer process occurs within the [TbCl ₂ ·6H ₂ O] ⁺ groups, resulting in emission. Bi ³⁺ ‐doping notably enhances the material's overall absorption. Through efficient energy transfer, the characteristic emission of Tb ³⁺ is clearly observed. The resultant 2D material exhibits remarkable recoverability, with its structure and optical properties fully restored upon water‐mediated recrystallization. It has a narrow emission peak and high photoluminescence quantum yield of 82.92%. Radioluminescence studies reveal a high steady‐state X‐ray relative light yield of 58 000 ± 2000 photons/MeV. The scintillator film, fabricated by in situ recrystallization of this 2D crystal within the polymer solution, achieves a high spatial resolution of 19.69 lp mm ⁻¹ . This research not only develops a new strategy for the synthesis of novel 2D lanthanide metal halides with exceptional stability and optical properties, but also demonstrates their promising application in high‐resolution X‐ray imaging.