Ionic crystals, lanthanide ions

The f electrons within the 4fn configuration of the lanthanides have only weak interactions with the crystal field because they are shielded by outer 5s and 5p electrons. Consequently the spectra are dominated by transitions between the atomic states of the lanthanide ion. The left side of Figure 1 shows the ionic energy levels of Eu3+ in the gas phase. When the ion enters a crystal lattice, there will be additional crystal field interactions. The interactions cause small crystal field splittings on the order of 200 cm-1 that are superimposed on the atomic transitions and are easily observable. 

The coordination numbers and stereochemistry of the ions are given in Table 27-3. In both ionic crystals and in complexes, coordination numbers exceeding 6 are the general rule rather than the exception. Indeed the number of lanthanide compounds in which the coordination number of 6 has been unequivocally established is small many complexes that could have been so formulated are known to have solvent molecules bound to the metal, leading... 

The analysis of the atomic spectra of lanthanide ions has reached a high level of sophistication with remarkably good agreement between experiment and calculation. This is true for optical spectra on free ions and also ionic solids, where the same interactions apply but with the addition of the crystal-field potential. Detailed reviews of both theory and experiment may be found in Goldschmidt (1978), Morrison and Leavitt (1982) and Carnall et al. (1989). There is only space here to describe briefly those interactions to which the neutron spectra may be sensitive. 

Lanthanide ions in crystals preserve their individuality the values of free ion spin S, orbital momentum L, and total angular momentum J prove to be good quantum numbers, describing ionic states in crystals, and the splitting of the multiplet by a crystal... 

Other exponential contributions to the relaxation rates of Ce +, Nd " " ions are explained naturally from the same standpoint. It seems reasonable to associate the contributions Tj (X exp(-A /kBT) with 4 = 18-19cm with an influence of the thulium ions of the first coordination sphere on the impurity lanthanide ions. An impurity ion distorts the crystal field at a distance of 3.7 A so strongly, that the thulium doublet state is split by approximately 20cm . In fact, there already appears a purely singlet Stark structure of energy levels in the low-symmetry crystal field. The distinction of 4 values for different lanthanide ions ean easily be coimected with the distinction of differences between the ionic radii of impurities and the ionic radius of the host thulium ion. The difference is... 

---