Heavy cerium compounds

TEP experiments on YbCuAl were performed down to low temperatures. By analogy with transition metals, flie low-temperature TEP of 4f instability compounds was interpreted in a crude two-band model of a heavy band and a broad d band according to the Mott formula (Mott 1936). Analysis of the YbCuAl data above 1K in the two-band model clearly reveals the electron-hole symmetry in comparison with cerium compounds (Jaccard et al. 1985, Jaccard and Sierro 1982). 

The distinction between itineracy and localization also can be seen in photoemission spectra. Typically, U-compounds (even heavy-electron compounds such as UPts) exhibit wide (1-2 eV) f bands centered on the Fermi level (Arko et al. 1988). On the other hand, lanthanide compounds and heavy actinides show localized f levels several eV below E f (Baer and Schneidner 1987). The case of Ce is intermediate, and hotly contested but for most Ce compounds the spectral weight lies mostly in the localized excitation, with a small amount at E. The exceptions are the a-like cerium compounds (e.g. CeCo2) where large weight at E (Baer and Schneidner 1987) suggests bandlike behavior. 

For the itinerant systems (light actinides and a-like cerium compounds) there is less guidance Ifom theory. As discussed above, the relationship between susceptibility x(0) and specific heat coefiBcient y is believed to be the same as for nearly localized heavy fermions, i.e., x(0)oc l/Tsf. Hence, differences between the two cases may not be... 

The electronic specific heat coefficient, y, is proportional to the DOSs at the Fermi level. In general, in pure metals, it is of the order of a few mj/ mol K. Simply thinking, the electronic DOSs at the Fermi level is proportional to the effective mass of conduction electrons. The most conspicuous and noticeable systems with respect to their electronic specific heat coefficients are heavy electron systems that include some actinide and cerium compounds to be mentioned later. For example, in the case of a typical heavy electron system CeCug, the electronic specific heat coefficient reaches 1.5 J/mol K, which is more than 1000 times larger than that of a normal metal (Satoh et al., 1989). 

The use of electricity in reactions is clean and, at least in some cases, can produce no waste. Toxic heavy metal ions need not be involved in the reaction. Hazardous or expensive reagents, if needed, can be generated in situ where contact with them will not occur. The actual oxidant is used in catalytic amounts, with its reduced form being reoxidized continuously by the electricity. In this way, 1 mol% of ruthenium(III) chloride can be used in aqueous sodium chloride to oxidize benzyl alcohol to benzaldehyde at 25°C in 80% yield. The benzaldehyde can, in turn, be oxidized to benzoic acid by the same system in 90% yield.289 The actual oxidant is ruthenium tetroxide. Naphthalene can be oxidized to naphthoquinone with 98% selectivity using a small amount of cerium salt in aqueous methanesulfonic acid when the cerium(III) that forms is reoxidized to cerium(IV) electrically.290 Substituted aromatic compounds can be oxidized to the corresponding phenols electrically with a platinum electrode in trifluoroacetic acid, tri-ethylamine, and methylene chloride.291 With ethyl benzoate, the product is a mixture of 44 34 22 o/m/fhhy-... 

This method has been considered the best of the classical separation procedures for producing individual elements in high purity. The most suitable compounds are ammonium nitrates (for La, Pr, and Nd) and double magnesium nitrates (for Sm, Eu, Gd). Manganese nitrates have also been used for separation of lanthanides of the cerium group (La-Nd). Bromates and sulphates have been used in the separation of the yttrium group (being the heavy lanthanides or HREE)... 

Yellow and orange Cadmium sulfide produces orange-yeUow or similar shades when combined with zinc or selenium, but it contains the problematic heavy metal and is mainly used in engineering applications. Lead chromate produces bright yellows, but is light-sensitive and must be treated. Lead chromate, sulfite, and molybdate combined can create a stable orange color. Newer non-toxic compounds based on rare earth elements, such as cerium sulfide, reportedly offer the same shades as toxic chromates. 

Closely related to the heavy fermions and spin fluctuators are the valence fluctuation/intermediate valence materials. The origin of this phenomenon starts with cerium and its a 7 transformation (see sections 3.3.4 and 3.7.2). Today it involves many cerium materials and also compounds of samarium, europium, thulium and ytterbium. Because of the breadth of the subject matter and space limitations in this chapter we refer the reader to the following reviews Jayaraman (1979), Lawrence et d. (1981), de Chatel (1982), Coqblin (1982), Nowik (1983), Brandt and Moshchalkov (1984), Varma (1985) and Stassis (1986). 

Today the lanthanide eontraction is still one of the most important tools available to the scientist in applying systematics to the behavior of lanthanide materials. Deviations from the lanthanide contraction established for a given compound series gives a measure of anomalous valences for cerium, samarium, europiun, thulium and ytterbium (see section 3.2) which are important in evaluating the nature of these elements in valence fluctuation, heavy fermion, and spin fluctuation behaviors (see section 4.4.4). 

Johansson (1978) and Brooks etal. (1984) utilized the energy difference (f d transitions) between trivalent (f"ds ) and tetravalent (f" M s ) cerium and other f-element metals to estimate thermochemical data for compounds. A similar approach has been used by Mikheev et al. (1986) and Spitsyn et al. (1985) to include the divalent as well as the tetravalent state. Again, a critical issue has been the behavior of the heavy actinides with respect to f- d transitions, and the need to utilize and to interpret the few experimental measurements on these actinides, which are discussed below (e.g., section 2.4.1.3). Johansson and Munck (1984) defined a function P (M) that removes the intershell multiplet coupling energy from the atomic reference state (A ,.,up,i j is the energy difference between the baricenter and the lowest level of a multiplet). For the lanthanides P (M) is significantly smoother than P(M),-except for an anomaly at Yb that may be due to an error in experimental data. Unfortunately, Johansson and Munck (1984) point out that spectroscopic data on the heavy actinides are inadequate to correct P(M) to P (M) for the actinides. 

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