Markus Suta

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

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 7FJ (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.

Bright Photoluminescence of [{(CptBu2)2Ce(μ-Cl)}2]: A Valuable Technique for the Determination of the Oxidation State of Cerium

The synthesis and photoluminescence properties of the bright-yellow organocerium complex [{(CptBu2 )2 Ce(μ-Cl)}2 ] (CptBu2 =1,3-di(tert-butyl)cyclopentadienyl) are presented. This coordination compound exhibits highly efficient photoluminescence within the yellow-light wavelength range, with a high internal quantum yield of 61(±2) % at room temperature. The large red shift is attributed to the delocalizing ability of the aromatic ligands, whilst its quantum yield even makes this compound competitive with Ce3+ -activated LED phosphors in terms of its photoluminescence efficiency (disregarding its thermal stability). A bridging connection between two crystallographically independent Ce3+ ions is anticipated to be the reason for the highly efficient photoluminescence, even up to room temperature. The emission spectrum is characterized by two bands in the orange-light range at both 10 K and room temperature, which are attributed to the parity-allowed transitions 5d1 (2 D3/2 )→4f1 (2 F7/2 ) and 5d1 (2 D3/2 )→4f1 (2 F5/2 ) of Ce3+ , respectively. The photoluminescence spectra were interpreted in relation to the structure and vibrational modes of the coordination compound. The spectra and optical properties indicate that trivalent cerium ions are the dominant species in the ground state, which also resolves an often-encountered ambiguity in organocerium compounds. This result shows that photoluminescence spectroscopy is a versatile tool that can help elucidate the oxidation state of Ce in such compounds.