Divalent Sm, Eu, Tm and Yb

The elements Sm, Eu, Tm and Yb, in which An/ may be 1 or remain zero, exhibit characteristic spectroscopic features. Both divalent and trivalent compounds of Sm, Eu, Tm and Yb exist. The 3d — 4/ spectra of these compounds are found to be characteristic of the number of 4/ subshell electrons present. The change from divalent to trivalent forms is accompanied by the delocalisation of the last electron, which enters into ionic bonding of the Zd-Af multiplet with one 4/ electron removed from the 4/ subshell. 

These elements possess well-developed multiplet structure in the soft X-ray spectra of the condensed phase, which can be compared with the spectrum of the free atom by performing ah initio Dirac-Fock calculations, the actual 4/ occupancy in the solid can be deduced. An example is shown in fig. 11.4 The first point to note is that the multiplet structure of the atom survives in the solid, because of the strong localisation of the 4/ electrons (see section 5.6), so that soft X-ray spectroscopy provides a 

This technique of analysis is very useful in determining valence changes, as will be illustrated in section 12.11. Note in particular that no adjustable parameters were used in the calculation of multiplet structure, which is therefore the purely atomic result. 

The intensity of the weaker structure is observed to depend on the partner element in the intermetallic. The manifestation of intermediate valence in the 3d — 4/ XAS and other core and outer spectra of Ce intermetallics has been interpreted in the framework of the Anderson impurity model [628]. 

Such calculations are able to reconcile the solid state properties (magnetic susceptibilities, etc) with the high energy spectra by using a single set of adjusted parameters [628, 629]. 

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Ionic compounds which contain lanthanides with a valence 2 or 4 have been known for a long time. These anomalous valence states are due to the stability of certain 4f electronic configurations, namely those of 4f°, 4f and 4f . Compounds containing tetravalent Ce, Pr and Tb and divalent Sm, Eu, Tm and Yb can be prepared. Their abundance and stability are greater for those of Ce, Eu and Yb. Some semimetallic compounds of Sm(II), Tm(II) and perhaps Pr(IV) are known, but the most studied semimetallic and intermetallic compounds are those of Ce, Eu and Yb, which are described in the following. 

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