Fluctuating valence

It has further been argued by Band and Fomichev [623] who have performed Dirac-Fock calculations of the transition from collapsed to decol-lapsed orbitals that a coexistence of states in the same atom, due to the formation of hybrid states, can occur One thus forms a picture in which it could also happen, in cases of fluctuating valence, that two different valence states would coexist on the same site [624]. 

It was remarked earlier that there is a strong correlation between systems which exhibit a mixed or fluctuating valence and systems which show Kondo-like anomalies. This connection was particularly clear in the Ce systems considered previously. A wide variety of experimental measurements has shown that in SmB6 and collapsed SmS, the Sm ions have a mixed valence (Sm -Sm ). We consider these systems not because they are classic Kondo systems (they are not), but because they provide considerable insight into the nature of the mixed valence state. These systems are also interesting with respect to the study of metal-insulator transitions, but this aspect will be discussed in ch. 20 by Jayaraman. 

The dihydride of Yb (like that of Eu) is non-metallic, since the Yb (and the Eu) ion is found in its divalent form. For higher hydrides, YbH2+j with x 0.4, fluctuating valencies seem to prevail leading to heavy-fermion behaviour and high yei-values (Drulis et al. 

Many works have been devoted to the study of compounds with fluctuating valency states (ICF systems), especially for semimetallic compounds. An international Colloquium on the metal-non-metal transitions in rare earth compounds, was held in 1976 and the results have been published in the Journal de Physique (Colloque C4, supplement au No. 10, Tome 37, October 1976). Theory and experiments concerning intermetallic ICF systems have not so far received an analogous development. 

Rare earth systems in fluctuating valence state... 

Coupling of 4f or 5f electron shells with delocalized itinerant electrons, which can result in valence instabilities (mixed-valence systems, fluctuating valence systems, heavy-electron systems). 

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