Inorganic Ho-Based Metal Halide Scintillators with Narrow-Band Red Emission for X-ray Imaging

Rare-earth halide perovskites have demonstrated great potential for optoelectronic applications; however, the development of highly efficient narrow-band red-emitting materials remains insufficient, which severely limits their practical application. Herein, we successfully synthesized Cs₂NaHoCl₆ via partial substitution of Cs⁺ with Na⁺ in Cs₃HoCl₆ crystals. The incorporation of Na⁺ ions reduces the crystal symmetry, suppresses electron-phonon coupling, and inhibits nonradiative transitions. Without altering the Cl-Ho charge transfer process, this strategy indirectly enhances the probability of electronic transitions to the characteristic energy levels, resulting in an order-of-magnitude enhancement in the red emission of Ho³⁺ at 660 nm. The color purity is also significantly improved, with a full width at half maximum (fwhm) of 19.25 nm and CIE coordinates of (0.713, 0.283). When applied to X-ray detection and imaging, Cs₂NaHoCl₆ exhibits a relative light yield of 24,939 photons/MeV and a detection limit lower than that of the commercial scintillator CsI:Tl. Its performance remains stable under continuous X-ray irradiation for 30 min. Furthermore, the fabricated scintillator film outperformed a commercial GOS scintillating screen under the same testing conditions. This study not only provides fundamental insights into narrow-band deep-red-emitting rare-earth halide perovskites but also demonstrates their promising application in X-ray detection and imaging.