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.