Influence of the chemical environment on the electronic structure and spectroscopic properties of $Er3+$ doped $Cs3Lu2Cl9,$ $Cs3Lu2Br9,$ and $Cs3Y2I9$

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 $SLJMJ$ 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 30 000 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 $C3v$ 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.