Nature of Localized Excitons in CsMgX3 ( X = Cl , Br, I) and Their Interactions with Eu2+ Ions

International audienceIn this paper, the luminescence properties of self-trapped excitons (STEs) of undoped and Eu2+-doped perovskite-type materials CsMgX3 (X=Cl, Br, I) are presented. The three compounds crystallize isostructurally in a hexagonal crystal system that exhibits an intrinsic pseudo-one-dimensionality. This feature has a highly stabilizing effect on the localization of excitons. The similarities to the properties of STEs in alkali halides are drawn that are justified by the band-structure and density-of-states calculations. The luminescence spectra of all three halides are characterized and interpreted despite their high complexity with many emissive transitions. It is illustrated that both STEs and impurity-localized self-trapped excitons (IL STEs) are responsible for the features in the spectra. The impurity localization of the STEs is proven by doping the hosts with Sr2+ ions instead of Eu2+ ions. The decay times in the microsecond range indicate that emission predominantly occurs from a triplet state of the STEs with a prominent afterglow component for the IL STEs that ideally suits a trapping model along the one-dimensional chains of the halides. Moreover, by thermal activation, the excitons tend to annihilate at the Eu2+ traps, thereby inducing an energy transfer to the Eu2+ ions. Because of this action, an extreme increase of the intensity of the Eu2+-based 4f65d−4f7 emission at room temperature is observed, which might be a general explanation for unusual temperature-dependent emission intensities of Eu2+ ions. In general, an understanding of the basic optical properties of the STEs may give some insights into the mechanism of currently used x-ray storage phosphors as well as scintillators