Thermally Robust 0D Cs ₃ EuCl ₆ Microcrystals for X‐Ray Imaging

Abstract 0D lanthanide‐based halides (Cs ₃ LnX ₆ , X = Cl, Br, I) are emerging as a new class of lead‐free perovskites for solid‐state lighting, displays, and X‐ray detection/imaging due to their high photoluminescence quantum yield (PLQY), tunable emission wavelength, and superior color purity. However, extending the excitation wavelength into the blue region and achieving robust thermal stability in Cs ₃ LnX ₆ pose significant challenges. Here, 0D Cs ₃ EuCl ₆ microcrystals are reported that emit intense reddish‐orange light via the characteristic Eu ^{3+ 5} D ₀ → ⁷ F _1,2 transitions under 465 nm blue light excitation, precisely matching with the commercial blue LED chip. Intriguingly, exceptional thermal stability and even anti‐thermal quenching are realized in Cs ₃ EuCl ₆ , attributed to the synergistic effect of its ultra‐high exciton binding energy ( E _b ) and relaxation of parity‐forbidden 4f–4f transitions in the 0D structure. Leveraging its large Stokes shift and high E _b , the Cs ₃ EuCl ₆ scintillator achieves a high light yield of 12 877 photons MeV ⁻¹ and excellent radiation resistance. A flexible film fabricated from Cs ₃ EuCl ₆ also demonstrates promising X‐ray imaging capabilities, with a spatial resolution reaching up to 9.17 lp mm ⁻¹ .