Designing Polychromatic Luminescence in Mn²⁺-Doped Cs₂NaLuCl₆ Double Perovskite Crystals via Energy Transfer Engineering for Optical Thermometry and X-ray Imaging

Metal halide double perovskites with splendid optical features have been considerably investigated for optoelectronic applications. Herein, a series of Mn²⁺-doped Cs₂NaLuCl₆ double perovskite crystals were prepared. Via adopting the density functional theory calculation, the effect of Mn²⁺ doping on the electronic structure of the Cs₂NaLuCl₆ compound was discussed. It was found that the Cs₂NaLuCl₆ crystal can emit intense blue light from the host self-trapped exciton (STE) emission. Excited by 320 nm, the STE and Mn²⁺ emissions were simultaneously observed in the resulting crystals. Owing to the efficient energy transfer from the STE to Mn²⁺, polychromatic luminescence was observed in final products. Moreover, taking advantage of the various responses of the emission intensities of the STE and Mn²⁺ to temperature, the thermometric characteristics of resulting crystals were investigated, and their maximum relative sensitivity is 0.99% K^-1, which is hardly impacted by Mn²⁺ content. Furthermore, the prepared crystals exhibit good X-ray radioluminescence properties, with a low detection limitation of 0.99 μGy_air s^-1. Additionally, utilizing the designed crystal, flexible X-ray imaging with a high resolution of 20 lp mm^-1 that can be operated in a high-temperature environment was realized. These results indicated that the Mn²⁺-doped Cs₂NaLuCl₆ double perovskite crystals are suitable for optical thermometry and X-ray imaging.