Dimer splitting of Er3+ in Cs3Er2X9 (X=Cl, Br). Mystery solved

Complete text of publication follows. Cs3R2X9 (R: rare earth; X: Cl, Br, I) is a compound of potential use in upconversion lasers. Structurally it forms rare earth dimers. Dimer splittings of typically up to some hundred μeV due to the exchange interaction have been observed for different rare earths in this structure [1]. For a long time the absence of any observable dimer splitting in the Er compound was a mystery. There were some measurements of a rather huge splitting (> 1 meV) [2] but these signals could be attributed to the crystal electric field (CEF) splitting of the secondary phase Cs2ErX5 present in the parent compound to percentages ranging up to 25%. Recently, it has been demonstrated that the extreme single ion anisotropy (in excess of 102 for χc/χab) for Er due to the CEF is responsible for the lack of any dimer signal. This almost pure Ising system (compared to almost perfect Heisenberg systems for the other rare earths) splits in the presence of the dimer interaction into a magnetic ground-state doublet, and a non-magnetic excited doublet without any allowed transitions between these two. (author) [1] A. Doemi et al., J. de Phys. 49, (1988) C8-1513; A. Furrer et al., PRL 62, (1989) 210; A. Furrer et al., PRL 64, (1990) 68; A. Furrer et al., Inorg. Chem. 29, (1990) 4081.; [2] P. Allenspach et al., Physica B 234-236, (1997) 744