Mixed valence systems spectroscopy

The mixed valence systems are often discussed in terms trf an Anderson (1961) impurity model. The f-electron spectral function of this model has therefore been a long-standing issue. If the coupling A is very weak, it follows immediately that the spectrum has two peaks at (seen in PES) and at 6f + 1/ (seen in bremsstrahliing isochromat spectroscopy (BIS)), where U is the f-f Coulomb interaction. It was further realized that the spectrum has a Kondo resonance close to 6p=0 (Abrikosov 1965, Suhl 1965). Except for some special cases (Yamada 1975) it was, however, hard to determine even the qualitative properties of the Kondo resonance. 

The main aim of this chapter is threefold. First we discuss methods for calculating electron spectra for the Anderson model. Secondly we study to what extent the Anderson model can produce spectra similar to the results obtained from different electron spectroscopies applied to Ce compounds. Thirdly we study to what extent we can use one single set of parameters for each compound to describe many different spectroscopies. Our discussion will be focussed on Ce compounds, in particular mixed valence systems, but other lanthanide compounds will also be considered. 

To a large extent the opulent diversity of electronic and magnetic properties manifested by lanthanide materials arises from the existence of an open shell of atomic-like 4f electrons. A number of challenging theoretical problems accompanies the panoply of physical phenomena, not only for the exotic mixed valence systems but for comparitively simpler materials such as the metals as well. In this chapter we treat one of these issues, the calculation of 4f electron excitation energies in the lanthanide metals. Measurement of the excitation energies is one of the principal achievements of the high-energy spectroscopies which form the subject of the present volume. 

In the following, we shall briefly discuss three examples of studies of mixed-valency systems with Mossbauer spectroscopy. 

Electronic Raman spectroscopy has attracted increasing attention in recent years, and a theory of the resonance electronic Raman effect has now been given. Wong and Schatz have also discussed in detail the electronic Raman effect as applied to mixed-valence systems " particularly the Pt(IV)-Pt(II) linear chain compounds (following their previous study of the conventional resonance Raman spectra of these materials). 

Mixed valence phenomena, such as studied by photoelectron spectroscopy in lanthanide systems, are expected to become important especially (but not only) in the second half of the actinide series. It is to be expected that much of the photoelectron spectroscopic effort will be in the future devoted to the study of these phenomena in actinides, especially as soon as measurements on hazardous actinides will become more feasible. 

The tetraamide [U Fc(NSiBu Me2)2 2] was prepared from the potassium salt of 1,1 -ferrocenylenediamine Fc N(H)SiBu Me2 2 and Ufdthf). Its oxidation by I2 followed by treatment with NaBPh4 produced the mixed-valence (Fe /Fe ") compound [U Fc-(. SiBLi. le2)2 f2l[BPh ], in which the centre mediates the electronic communication. The model of electron transfer between Fe and I- e " via a direct U-Fe interaction was confirmed by magnetic measurements, EPR, NIR and IR spectroscopy and DFT calculations on model systems. ... 

Technetium(II) complexes are paramagnetic with the d5 low-spin configuration. A characteristic feature is the considerable number of mixed-valence halide clusters containing Tc in oxidation states of +1.5 to + 3. This area has been reviewed (42). For convenience, all complexes, except those of [Tc2]6+, are treated together here. EPR spectroscopy is particularly useful in both the detection of species in this oxidation state and the study of exchange reactions in solution. The nuclear spin of "Tc (1 = f) results in spectra of 10 lines with superimposed hyperfine splitting. The d5 low-spin system is treated as a d1 system in the hole formalism (40). 

Cua centers exist in two redox states [Cu(II)Cu(I)] and [Cu(I)Cu(I)]. The oxidized species is a fully delocalized mixed-valence pair (formally two Cu+ 1.5 ions), as revealed by EPR spectroscopy (Kroneck et al., 1988, 1990). Despite the similar coordination geometry around copper, these systems display sharper NMR lines than do the BCP due to a shorter electron relaxation time of the paramagnetic center (wlO "s) (dementi and Luchinat, 1998). NMR studies are available for the native Cua centers from the soluble fragments of the The. thermophilus, Paracoc-cus denitrificans, Paracoccus versutus, and Bacillus subtilis oxidases (Bertini et al., 1996 Dennison et al., 1995 Luchinat et al., 1997 Salgado et al., 1998a) and Pseudomonas stutzeri N2O reductase (Holz et al., 1999), as well as for engineered Cua sites in amicyanin (Dennison et al., 1997) and Escherichia coli quinol oxidase (Kolczak et al., 1999). 

Oxides. TijOa has been investigated by X-ray spectroscopy and by e.p.r. spectroscopy and magnetic susceptibility measurements - there has been an improved MO calculation of its band structure. A band model has been proposed to account for the origin of the magnetic moment and metallic transition of vanadium-doped Ti203- According to e.p.r. measurements the Ti ions in mixed valence phases of the system Ti 0 mined. 

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