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.