D. Schaniel, P. Allenspach, A. Furrer, K. Krämer and H. U. Güdel
Dimer splitting of Er³⁺ in Cs₃Er₂X₉ (X = Cl, Br): mystery solved
Physica B 276-278, 690-691 (2000)                 

Abstract: Inelastic neutron scattering (INS) measurements were performed on the rare earth dimer compounds Cs₃Er₂X₉ (X=Cl,Br). The crystalline electric field (CEF) parameters were determined. The lack of any exchange splitting associated with the CEF transitions can be explained by the large single-ion anisotropy for Er. Specific heat and susceptibility data support the picture of an almost pure Ising system, which splits the lowest CEF state in the presence of the dimer interaction into a non-magnetic ground-state doublet and a magnetic excited doublet without any allowed transition between these two dimer states.