Photoluminescence of CsMI3:Eu2+ (M = Mg, Ca, and Sr) – a spectroscopic probe on structural distortions

Abstract

The photoluminescence of Eu²⁺ ions doped in iodidoperovskites, CsMI₃ (M = Mg, Ca, Sr), was studied. Eu²⁺ occupies the alkaline earth site in each of these compounds. The emission spectra of the doped perovskites are characterized by 4f⁶5d¹ → 4f⁷ transitions of Eu²⁺ located in the yellow range for CsMgI₃ and the blue range for CsCaI₃ and CsSrI₃, respectively. All excitation spectra provide a well-resolved fine structure arising from different ⁷F_J (J = 0,…,6) levels of the 4f⁶ core of the excited state configuration, indicating a weak exchange interaction with the 5d electron. The decay times and temperature dependence of the luminescence were also analyzed. Both CsMgI₃:Eu²⁺ and CsSrI₃:Eu²⁺ show a redshifted emission with respect to CsCaI₃:Eu²⁺. This is explained by trigonal distortions of the [EuI₆]⁴⁻ octahedra in the former two compounds. The energetic order of the resulting crystal field states is justified in terms of the angular overlap model of ligand field theory. This series of Eu²⁺ doped earth alkaline iodidoperovskites may serve as a textbook example for the detailed understanding of the structure–luminescence relationships of Eu²⁺ ions, which is very important for tailoring functional materials for respective applications.