Synthesis and optical properties of the Eu 2+ -doped alkaline-earth metal hydride chlorides AE 7 H 12 Cl 2 (AE = Ca and Sr)

Abstract

In our study of the optical properties of Eu 2+ -doped alkaline-earth metal hydride chlorides AE 7 H 12 Cl 2 (AE = Ca and Sr) we observe yellow (AE = Sr) and orange-red (AE = Ca) emission. Compared to the emission energies of Eu 2+ -doped fluorides, chlorides and mixed fluoride chlorides, this corresponds to a wide redshift of the Eu 2+ 4f 6 5d 1 –4f 7 emission. We explain this observation with the strong polarizability of the hydride anion and therefore its strong nephelauxetic effect, which shifts the 5d barycenter to lower energy. Furthermore, the redshift is even significant compared to other known hydride chlorides and hydride oxide chlorides, which is probably caused by the relatively short AE 2+ /Eu 2+ −H − distances, or it might also be caused by impurity-trapped exciton states. We have also investigated the temperature dependence of the photoluminescence lifetimes and the thermal stability. Even though the compounds are probably not ideal for phosphor application due to their low stability against air and moisture, our study underlines the large influence of the highly polarizable hydride anion on the optical properties of divalent europium and shows that Eu 2+ could be used as a probe in mixed anionic systems to determine the hydride content.