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

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, Cs₂TbCl₅·6H₂O, with narrow-band green emission. In Cs₂TbCl₅·6H₂O, H₂O molecules stabilize the structure by forming [TbCl₂·6H₂O]⁺ dodecahedral units, rather than coordinating to [TbCl₆]^3- octahedral units. Under excitation, a halogen-to-lanthanide ion charge-transfer process occurs within the [TbCl₂·6H₂O]⁺ groups, resulting in emission. Bi³⁺-doping notably enhances the material's overall absorption. Through efficient energy transfer, the characteristic emission of Tb³⁺ 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.