Fluctuating valence compounds

L absorption edges of Ce and Yb intermediate valence compounds Non-existence of tetravalent cerium compounds, in Valence Fluctuations in Solids, eds L.M. Falicov, W. Hanke and M.B. Maple (North-HoUand, Amsterdam) p. 417. 

Sm compounds have interesting magnetic behaviors. dHvA measurements have been performed on some compounds such as Smins, SmSb, SmGa2, and SmCu. They are, however, not valence-fluctuation compoimds but f-localized ones. Therefore, the situation is similar to the Nd system. Fermi surface studies for valence-fluctuating Sm compounds are open to future studies. 

Mixed valence compounds CeTjSij with T = Co, Ni show an enhanced Pauli paramagnetism with the fluctuation temperature around 150 K for Co and Ni which was established in the magnetic measurements in the temperature range of 1.6 to 600 K. The electrical resistivity measurements show that the compounds with T = Fe, Co and Ni do not become superconducting down to 18 mK and that the Fermi liquid behavior exists in the temperature range of 40 to 70 K (Ammar-guellat et al. 1987). 

In fig. 33.2 we have plotted the lattice constants of the LnN phases versus the radii of the Ln and Ln ions, respectively. It is interesting to note the slight (ionicity-induced ) curvature in the plot which is even more pronounced in the case of the LnSb and LnBi phases. For CeN the contraction of the cell with regard to hypothetical Ce N is Ja -0.19A and about the same for CeP, Aa -0.18 A. This contraction, however, appears to be slightly less than expected for truly tetravalent CeN (and CeP(p)). The difference is a consequence of valence fluctuations, i.e. the metallic CeN and CeP(p) are expected to be homogeneous mixed-valence compounds. The special position of the Fermi level within the narrow f-peak of the density-of-states cutve (cf. recent XPS measurements of Baer and Zurcher, 1977) then accounts for the positive Seebeck coefficient. 

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. 

As is discussed in sect. 2.3, the resulting hybridization of the 4f states with the conduction band is associated with the appearence of a Kondo-type interaction (Ce compound) and valence fluctuation (Yb compound). In fig. 89 the temperature dependence of of CeCuj is given. At 4 K a phase transition to a magnetic-ordered... 

The situation above described has important consequences in the understanding of the physical properties not only of heavy actinide metals, but also of their compounds. We can expect e.g. that the occurence of valence fluctuations should be quite common in Am, Bk and Cf compounds (to limit ourselves to the sufficiently stable isotopes) as well as in Cf metal itself ... 

Many models have been postulated to account for the interconfiguration fluctuations (ICF) in rare earth intermetallic compounds. We will consider Hirst s model which assumes that the 4/ electrons are highly correlated and preserve their atomic-like features during the valence fluctuation. Both the X-ray photoelectron spectra and the magnetic susceptibility of rare earth intermetallic compounds can be successfully explained on the basis of Hirst s model [8,11]. 

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