Spin Crossover of Yb2+ in CsCaX3 and CsSrX3 (X = Cl, Br, I) - A Guideline to Novel Halide-Based Scintillators

In this paper, the temperature dependence of the photoluminescence of Yb2+ doped into the halidoperovskites CsMX3 (M = Ca, Sr; X = Cl, Br, I) is presented. Yb2+ shows spin-forbidden high-spin (HS) and spin-allowed lowspin (LS) emission bands in all compounds. Upon excitation of the higher energetic LS transition, very different behaviors of the interplay of the two emissive transitions are observed. In the chlorides, the HS-based emission becomes already dominant at higher temperatures than 50 K; in the bromides, a temperature-dependent population of the HS state is observed at higher temperatures than 200 K, whereas in the iodides, the LS emission remains dominant even above 300 K. A vibrational relaxation model is attempted for the explanation of this behavior that reveals a very delicate dependence on the mode energies of the [YbX6](4-) octahedra. FIR spectra of the pure Yb2+-based compounds CsYbX3 are also presented to experimentally verify the estimated average mode energies from the model. Finally, the relevance for novel Yb2+-based scintillators is discussed.