Interconfiguration fluctuation valence

Core-level spectra are useful in studying mixed valence (valence instability or interconfigurational fluctuation) in rare-earth systems (e.g. SmS, Ce) which arises when Eexc = n (E -i + FJ 0 where( — , )istheenergydifferencebetweenthe4/ and 4T states and is the energy of the promoted electron. The time scale involved... 

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]. 

According to the model the period r of the interconfigurational fluctuation is given by the width, d ( hr) of the principal band. The average valence V is given by... 

Studies on concentrated lanthanide metallic systems have also made it possible to establish a correlation between nonintegral valence (observable in concentrated lanthanide systems through lattice constant, Mdssbauer isomer shift, soft X-ray absorption and X-ray photoemission spectroscopy measurements) and nonmagnetic behavior of a lanthanide ion below the characteristic temperature. This has led to the concept of valence fluctuations (or interconfiguration fluctuations) which has stimulated the current intense activity among theorists and experimentalists alike. 

The large and well-defined range of isomer shifts observed for the NGR of Eu in the di- and trivalent states of the europium ion make this isotope particularly valuable for the study of intermediate valency states in lanthanide intermetallics. Such intermediate valency states have their origin in interconfiguration fluctuations (ICF s). In the case of europium intermetallics, the unusual isomer shifts observed are interpreted in terms of fast fluctuations, on the order of 10 " sec, of an electron between a localized 4f level and the conduction band, resulting in a fluctuating charge density at the Eu nuclei. 

The pressiue dependence of the electrical resistivity of YbCuAl was investigated (Alami-Yadri et al. 1998, 1999a,b) up to 8 GPa. The resistivity at 300 K decreases with increasing pressure. At 8 GPa a dependence occurs at low temperature (Fermi-liquid behavior), and the Kondo temperature decreases with increasing pressure. The experimental setup for these measurements was presented by Jaccard et al. (1998). Furthermore, point-contact spectroscopy was used to measure the interconfigurational excitation energies and conduction-electron lifetime width of valence-fluctuating YbCuAl (Bussian et al. 1982). 

Fig. 1. Schematic energy level diagram of the in-terconfigurational fluctuation (ICF) model describing valence fluctuations between two 4f configurations (4r, 4r ), characterized by their /multiplet level structure. The basic parameters of the ICF model and denote the interconfigurational excitation energy and interconfigurational mixing width, respectively.

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