Photoluminescence and Cathodoluminescence Properties of Nanocrystalline BaFCl:Sm³⁺ X-ray Storage Phosphor

Nanocrystalline BaFCl:Sm 3+ is an efficient photoluminescent X-ray storage phosphor. To gain a better understanding of the storage mechanism, the photoluminescence and cathodoluminescence properties of nanocrystalline BaFCl:Sm 3+ before and after X-irradiation were investigated. The results were compared with those obtained for microcrystalline BaFCl:Sm 3+ prepared by high-temperature sintering. Significant differences in the local coordination environment of Sm 3+ ions between the two samples were revealed by comparing the photoluminescence spectra at 2 and 293 K. From the cathodoluminescence microanalysis, it follows that the Sm 3+ ions in the as-prepared nanocrystalline BaFCl:Sm 3+ are mainly located on or close to the surface of the nanoparticles, whereas those in the microcrystalline sample are distributed homogeneously throughout the microcrystallites. For nanocrystalline BaFCl:Sm 3+, the X-irradiation-induced reduction of Sm 3+ to Sm 2+ ions and the photoionization of Sm 2+ to Sm 3+ ions during photobleaching were investigated by monitoring the photoluminescence intensities of both Sm 2+ and Sm 3+ ions. The two processes can be modeled well assuming dispersive first-order kinetics, where the rate constant of the electron transfer is given by an exponential function of the distance between the shallow electron traps and the Sm 3+ centers.