Valence fluctuations

Lanthanides with fractional valences have II, III and IV valences, as well as mixed II/III and III/IV valences. Depending on temperature and pressure, the degree of oxidation can change. This effect may result in a change in the different properties of nanoparticles, such as the stability, heat capacity, conductivity and magnetic susceptibility [218]. Valence fluctuation phenomena have been reported to occur... 

Figure 4. Structure of RbjAsj Sen (a) showing the solvated ion complex (Rb )sAs/ with only weak additional bonds between neighboring complexes, and the proposed Metal-Supported Valence-Fluctuation (MSVF) (b) for the...

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 ... 

Falicov, L. M., Hanke, W. Maple, M. P. (eds) (1981) Valence Fluctuation in Solids, North-Holland, Amsterdam. 

The RIn3 systems crystallize in the cubic AuCu3 type of structure and are known for salient features as valence fluctuations and the presence of various magnetic structures (see e.g. 

M. Loewenhaupt and K.H. Fischer, Valence-fluctuation and heavy-fermion 4f systems 1... 

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]. 

Fine particles of metallic Eu and Yb have been shown to exhibit valence transition with critical particle sizes of 50 A for Eu and 35 A for Yb. At these critical particle sizes, there is a reduction in the lattice constant with no change in the critical fee structure. There appears to be a change in valence from +2 to +3. This effect may be due to a shift in the position of Fermi level, E, such that E xc approaches zero resulting in the ions at the configuration crossover. ICF state can in general result in lattice constant anomaly due to the fractional occupation of the 4/ shells. Thus once ICF is identified in a system, valence fluctuation can be studied by many techniques. 

Among the rare earth series, Ce, Sm, Eu, Tm, and Yb show valence fluctuation since they satisfy the condition... 

Fig. 9.22. XPE spectra of CeN in the 4d and 3d regions showing the existence of valence fluctuation in this compound. P(4d) and P(3d) are the calculated energy differences between the two valences, 4f and 4f1, of Ce in XPE spectra of the corresponding levels [50],...

Fig. 9.23, XPE spectra of Ce, CeAl2, Ceo.8Yo.2Al2 and CePd3 in the 4d region indicating valence fluctuations in the cases of Ceo.8Yo.2Al2 and CePd3, and the trivalent nature of CeAl2 [50],...

Fig, 9,26, XPE spectrum of 4f region of EuCu2Si2 (upper curve) compared with the theoretical multiplet structure of divalent and trivalent Eu (middle curve), indicating the existence of valence fluctuation in this compound. The lower spectrum (for YCu2Si2> shows the contribution of Cu and Si in the valence region. 

Superconductivity in the multiphase quaternary borocarbide system Y-Ni-B-C has been recently reported [141,142], Earlier work on valence fluctuations in ternary borides RENiaB were studied and in the case of YNi4B, a weak but reproducible signal of superconductivity at Tc of 12 K was observed. This is the first report or discovery of a ternary superconducting phase of the elements Y, Ni and B with a high Tc value (Fig. 12.27). 

The cluster compounds [Ag6M4Pi2]Gc6 with = Ge, Sn show at low temperatures a valence fluctuation of the inner core Ag6" +, which can be seen in the elastic behavior " and vibrational anharmonicity as well as in the measurements of the specific heat. The valence fluctuations generate a pronounced schottky anomaly, which can be emphasized more clearly by the comparison and therefore possible normalisation of cluster compounds. 

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