Nature of Localized Excitons in $\mathrm{CsMg}{X}_3$ ($X=\mathrm{Cl}$, Br, I) and Their Interactions with ${\mathrm{Eu}}^{2+}$ Ions

In this paper, the luminescence properties of self-trapped excitons (STEs) of undoped and ${\mathrm{Eu}}^{2+}$-doped perovskite-type materials $\mathrm{CsMg}{X}_3$ ($X=\mathrm{Cl}$, 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 ${\mathrm{Sr}}^{2+}$ ions instead of ${\mathrm{Eu}}^{2+}$ 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 ${\mathrm{Eu}}^{2+}$ traps, thereby inducing an energy transfer to the ${\mathrm{Eu}}^{2+}$ ions. Because of this action, an extreme increase of the intensity of the ${\mathrm{Eu}}^{2+}$-based $4f^{6}5d\text{−}4f^7$ emission at room temperature is observed, which might be a general explanation for unusual temperature-dependent emission intensities of ${\mathrm{Eu}}^{2+}$ 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.

Figure 1

Schematic view of the crystal structure of the quasi-one-dimensional halides $\mathrm{CsMg}{X}_3$ ($X=\mathrm{Cl}$, Br, I). The ${\mathrm{Cs}}^{+}$ ions are twelvefold coordinated, and the ${\mathrm{Mg}}^{2+}$ ions are sixfold coordinated by the $X^{-}$ ions.

Figure 2

Diffuse reflectance spectra of the undoped halides ${\mathrm{CsMgCl}}_3$, ${\mathrm{CsMgBr}}_3$, and ${\mathrm{CsMgI}}_3$ at room temperature.

Figure 3

Calculated band structures and electronic total as well as projected DOS of ${\mathrm{CsMgCl}}_3$ (a),(b), ${\mathrm{CsMgBr}}_3$ (c),(d), and ${\mathrm{CsMgI}}_3$ (e),(f).

Figure 4

Photoluminescence emission (solid) and excitation spectra (dashed or dotted) of STEs in undoped (a) ${\mathrm{CsMgCl}}_3$, (b) ${\mathrm{CsMgBr}}_3$, and (c) ${\mathrm{CsMgI}}_3$ at 10 K. The $\pi$ and $\sigma$ emissions are indicated in each case.

Figure 5

Emission spectra at 10 K of (a) ${\mathrm{CsMgCl}}_3$: $x\%$ ${\mathrm{Eu}}^{2+}$, $y\%$ ${\mathrm{Sr}}^{2+}$, (b) ${\mathrm{CsMgBr}}_3$: $x\%$ ${\mathrm{Eu}}^{2+}$, $y\%$ ${\mathrm{Sr}}^{2+}$, and (c) ${\mathrm{CsMgI}}_3$: $x\%$ ${\mathrm{Eu}}^{2+}$, $y\%$ ${\mathrm{Sr}}^{2+}$. The total doping concentration is always $(x+y)\%=1\%$.

Figure 6

Photoluminescence emission spectra recorded at 10 K for (a) ${\mathrm{CsMgCl}}_3:{\mathrm{Eu}}^{2+}$, (c) ${\mathrm{CsMgBr}}_3:{\mathrm{Eu}}^{2+}$, and (e) ${\mathrm{CsMgI}}_3:{\mathrm{Eu}}^{2+}$. Solid lines denote a doping concentration of 0.1 mol %, and dashed lines are a doping concentration of 1-mol % ${\mathrm{Eu}}^{2+}$. The assignments of the emissions arising from $4f^{6}5d\to 4f^7$ emissions of ${\mathrm{Eu}}^{2+}$ on ${\mathrm{Mg}}^{2+}$ sites are indicated. The lower spectra depict the $4f^{6}5d\to 4f^7$ emissions of ${\mathrm{Eu}}^{2+}$ in $\mathrm{CsSr}{X}_3$ ($X=\mathrm{Cl}$, Br, I), respectively (cf., also, Refs. [65, 66, 67]). The corresponding photoluminescence excitation spectra upon detection of the IL STE transitions are also given for (b) ${\mathrm{CsMgCl}}_3:1\%{\mathrm{Eu}}^{2+}$, (d) ${\mathrm{CsMgBr}}_3:1\%{\mathrm{Eu}}^{2+}$, and (f) ${\mathrm{CsMgI}}_3:1\%{\mathrm{Eu}}^{2+}$ at 10 K. The dashed curves relate to the $4f^{6}5d\to 4f^7$ emission of ${\mathrm{Eu}}^{2+}$ on the ${\mathrm{Mg}}^{2+}$ sites. The corresponding excitation spectra.

Figure 7

Upper panels: Photoluminescence decay curves of (a) ${\mathrm{CsMgCl}}_3:0.1\%{\mathrm{Eu}}^{2+}$ (${\lambda }_{\mathrm{ex}}=248\mathrm{nm}$), (b) ${\mathrm{CsMgBr}}_3:0.1\%{\mathrm{Eu}}^{2+}$ (${\lambda }_{\mathrm{ex}}=275\mathrm{nm}$), and (c) ${\mathrm{CsMgI}}_3:0.1\%{\mathrm{Eu}}^{2+}$ (${\lambda }_{\mathrm{ex}}=255\mathrm{nm}$) for the $\pi$ emissions of the STEs as well as the emissions of the IL STEs. Lower panels: Double-logarithmic plots of the decay curves of the IL STE transitions indicating the $t^{-n}$ dependence of the afterglow components. Fitting curves are indicated as solid lines.

Figure 8

Temperature-dependent emission spectra of (a) ${\mathrm{CsMgCl}}_3:1\%{\mathrm{Eu}}^{2+}$, (b) ${\mathrm{CsMgBr}}_3:1\%{\mathrm{Eu}}^{2+}$, and (c) ${\mathrm{CsMgI}}_3:1\%{\mathrm{Eu}}^{2+}$. In all three cases, a UV excitation wavelength is used for the purpose of an efficient excitation into STE states.