Stefan R. Lüthi, Hans U. Güdel and Markus P. Hehlen
Influence of the chemical environment on the electronic structure and spectroscopic properties of Er³⁺ doped Cs₃Lu₂Cl₉, Cs₃Lu₂Br₉, and Cs₃Y₂I₉
J. Chem. Phys. 110, 12033-12043 (1999)                 

Abstract: Energies and intensities of 114, 101, and 76 f-f absorption transitions of Er³⁺ are determined by high-resolution spectroscopy in the closely related host lattices Cs₃Lu₂Cl₉, Cs₃Lu₂Br₉, and Cs₃Y₂I₉, respectively. The observed trends in the energy-level structure reflect the increasing covalency and the length of the Er³⁺–X^– bond. The decreasing Coulomb repulsion of the 4f electrons, spin-orbit coupling, and crystal-field potential reduces the energy splittings of the SL, SLJ, and SLJM_J states by 0.5%, 0.5%, and 25%, respectively, along the series Cl–Br–I. Energy-level calculations that include crystal-field and correlation crystal-field terms in the effective Hamiltonian, reproduce most of the experimentally found trends. Root-mean-square standard deviations of 18.0, 19.2, and 21.9 cm^–1 are reached in least-squares fits to the experimental crystal-field energies. The f-f transition intensities increase along the series Cl–Br–I as a result of the decreasing energy of the f-d bands. In the iodide compound, where the first f-d bands are as low as 30000 cm^–1, this influence is especially pronounced for the f-f absorptions at higher energy. The quality of the wavefunctions obtained in the energy-level calculations is not sufficient to reliably calculate the relative absorption intensities of individual crystal-field components within a given multiplet transition. This deficiency is ascribed to small deviations of the actual coordination geometry of Er³⁺ from the C_3v point group symmetry that was assumed in the calculation. Intensities are analyzed on the level of multiplet-to-multiplet transitions using the Judd-Ofelt formalism.