P. Allenspach, A. Furrer, H. U. Güdel, N. Furer and H. Büttner
Crystal electric field and dimer splitting in Cs₃Er₂X₉ (X = Cl, Br)
Physica B 234-236, 744-745 (1997))                 

Abstract: Inelastic neutron scattering technique was used to measure the crystal electric field (CEF) splitting of Cs₃Er₂Cl₉. Besides a low lying magnetic excitation at around 2 meV all the other transitions are in agreement with CEF transitions found in optical measurements. Similar additional low-lying magnetic transitions were found in Cs₃Er₂Br₉. While excitations of the rare-earth dimers most likely play a role, the size of the splitting and its absence in optical spectra still remain a mystery.