Valence fluctuations compounds

CePda is the archetypical valence fluctuation compound. It has a cubic crystal structure of CusAu-type with ao = 4.13A. Early estimates for the valence from lattice-parameter systematics by Harris et al. (1972) gave a value of 3.45. This value was revised to 3.23 by Bauchspiess et al. (1982) and to 3.15 by Perez et al. (1990) employing Lj, X-ray absorption measurements. In any case, the valence of Ce in CePda is considerably different from the integer value of -I- 3 (keeping in mind the present upper limit of 3.3 for VF Ce systems (Rohler 1987), and the obvious difficulty to define a valence in these systems at all (Baer and Schneider 1987). Thus CePdg can still be considered a typical valence fluctuator even if the f-count as deduced from the experimental data, has steadily increased over the years. 

SmRuSns (Sm8Ru8Sn24, phase I) is a valence fluctuating compound with antiferromagnetic ordering below 6K (Fukuhara et al. 1992). 

At the present time the electron-spectroscopic manifestations of 4f-hybridization in heavy lanthanide materials are scarce. (The so-called valence fluctuating compounds containing the elements Sm, Eu, Tm or Yb are discarded here (see section 7.3).) Nevertheless, indications for a coupling of the f states with band states have been found in materials where the Eu atoms, according to Mossbauer spectroscopy and the magnetic susceptibility, are in the divalent 4f configuration. The XPS 3d core level spectra of a few systems, like Eu metal, EuAlj, EuCuj, EuAgj (Schneider... 

RNi crystallizes in an orthorhombic structure. Figure 79 shows the CrB crystal structure and its Brillouin zone. The primitive cell contains two molecules of RNi. CeNi can be characterized as a valence-fluctuating compound, similar to CeSnj. Its Kondo temperature... 

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]. 

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, 9,26, 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. 

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. 

The term mixed valence is widely used in the literature to describe a phenomenon rather different from that considered here. Typically it refers to a metal (or a rare-earth alloy, or a compound such as SmS) which features a broad conduction band (formed by the overlap of s, p, or d orbitals), and a very narrow band such as an /-band, slightly above the Fermi level. This band becomes partially populated, hence the mixed, or fractional, valence. Fluctuations with time, and various degrees of localization in space, result from electron-phonon interaction. A useful review appeared recently.Many of the ideas used in this field parallel those used in other more chemical types of electron transfer. Recent articles on mixed-valence as a polaronic effect and on local polaronic effects exemplify this, and the dynamical properties have been discussed. Other recent reviews of this area deal with spectroscopic techniques and with mixed valency in rare-earth compounds. ... 

The relatively large Sommerfeld constant (0.79 J/(mol K )) of Sm3Se4 points towards the formation of heavy quasiparticles and, despite the fact that all Sm3X4 compounds are non-metallic, they have been discussed as low-carrier-density HF systems. Optical measurements on Sm3Sc4 (Batlogg et al. 1976) placed the 4f level inside the gap ( 4 eV width) about 0.7 eV below the bottom of the conduction band. Hence, conduction electrons do not play an essential role in the valence fluctuations. 

To reconcile the diverging Raman and neutron scattering results the following point of view has been taken by Giintherodt et al. (1985b) similarly to the valence fluctuations between two integral valence states (4f"< 4f" -l- e ) in IV rare-earth compounds the systems CeAlj, TmSe and Smo Yp jjS show... 

Therefore in this review we shall focus on the electron-phonon coupling in metallic R compounds. They have shifted to the center of interest in the last ten years because of new phenomena such as valence fluctuations, Kondo lattice behavior and the formation of heavy-electron bands. The varying degree of hybridization between 4f states and conduction electron states leads to low lying electronic states whose character is very different from the localized states due to many-body effects. Consequently they lead to a rich variety of electronic and elastic or vibrational anomalies at low temperatures. 

---