UV/X‐Ray‐Induced Reduction of Eu 3+ to Eu 2+ in Rare‐Earth‐Based Halide Double Perovskite for Material‐Discriminative X‐Ray Imaging

Stacked scintillators for x‐ray energy discrimination require at least two emission bands with sufficient spectral separation to avoid signal crosstalk. Rare‐earth‐based double perovskites exhibit narrow‐band luminescence at multiple visible wavelengths, which is advantageous for such designs. However, an efficient narrow‐band blue emission has not yet been realized, hindering further development. Here, efficient Eu 2+ blue luminescence is achieved in Cs 2 (Li/Na/K)(Lu/Y/Gd)Cl 6 under ambient conditions via photo‐induced reduction of Eu 3+ to Eu 2+ , eliminating harsh reducing atmospheres. The reduction is driven by 254 nm UV or x‐ray irradiation, while mild heating and x‐ray‐induced lattice distortion assist carrier migration. The unique multi‐cation structure provides cation vacancies that trap holes, stabilizing the reduction. The resulting Eu 2+ emits narrow‐band blue light at 450 nm (FWHM 25 nm, PLQY 84.1%, 19 000 ph/MeV) in Cs 2 NaLuCl 6 , and coexisting Eu 3+ shows narrow‐band red emission, achieving a 144 nm spectral separation. Taking advantage of this well‐separated dual emission, a stacked scintillator device enables highly material‐discriminative x‐ray imaging.