The Actinide Compounds

We have particularly emphasized the metallic bond, because the peculiar properties of the 5f wavefunctions are better seen in its context. Also, in compounds, however, f-f overlapping as well as hybridization play a conspicuous role. Only, the presence of a nonactinide element introduces other factors that have to be taken into account  

All these factors have to be taken into account at the same time, if the electronic structure and the type of bonding of a compound have to be understood. 

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The free energies of formation for the actinide compounds above are given by the following table ... 

Among the actinide compounds the interest is concentrating on binary compounds of simple structure (e.g. 1 1 compounds with elements of the groups V and VI of the periodic table) for which the theoretical treatment is rather advanced, and on intermetal-lic (e.g. Laves-) phases. 

In the actinide compounds with the other group VI elements (X = S, Se, Te), the X/ An ratio varies from 1 to 3. These actinide chalcogenides show predominantly the composition AnX (cub.), AngXt (cub., orthorh.), AngXs (orthorh.), AnX2 (tetr.) and AnXg (orthorh.) which is also the order of decreasing thermal stability. 

The actinide compounds crystallizing in this structure type are listed in Table 4. 

In principle, the B1 structure should be strongly ionic in character. But in the actinide compounds having this structure, the ionic character is more or less weakened as their anions are less electronegative than the halogens which are the anions of the typical representatives of this structure type. 

The actinide compounds [Cp2ThH(/i-H)]2 (Cp = sterically encumbered Cp) react reversibly with CO to yield formyl complexes. The product of the reaction of Cpf(NpO)ThH (Np = neopentyl) was characterized spectroscopically, and the thermodynamics and kinetics of the insertion reaction were analyzed ... 

On the contrary, the valence of the actinides varies and there is some uncertainty as to the configuration in the metals. In particular, in the first half of the series, the number of 5/electrons can vary with temperature and pressure and is not an integer. Moreover, the magnetism is present in the metals only from the middle of the series (5). On the other hand, in the case of ionic and covalent compounds, a strong intra-atomic correlation between electrons from the 5/ states has been found and the experimental data clearly show that the 5/states have an atomic-like character (4) indeed, the 5/ electrons in the actinide compounds present a magnetic behaviour which is similar to that of the 4/ electrons in the rare earth compounds. 

Due to its drawbacks (difficult preparation of water-free starting material, neutron emission from ( a,n) reactions, presence of non-volatile impurities in the product), methods involving vaporisation of the actinide metal after reduction of a compound (oxide, carbide) are preferred. If the vapour pressure of the reductant and that of the actinide compound are markedly lower than that of the metal formed, the latter can be removed from the reaction mixture via the vapour phase and condensed in high purity ... 

Finally, Hay and Martin33 studied the actinide compounds UF6, NpF6, and PuFe. Also in this case the results (Table 4) show that the density-functional calculations produce results that are in good agreement with the experimental results, which also is the case for the Hartree-Fock methods. 

Polymerization also takes place when Cp 2ZrMe2 (Cp = Cp, Cp ) or Cp ZrMe3 are supported on dehydroxylated alumina. While only some 4% of Cp2ZrMe2 centers are active, about 12% of the Cp ZrMe3 is converted to cationic centers. The order of polymerization activity is Cp ZrMes > Cp2ZrMe2 Cp 2ZrMe2. Unlike the actinide compounds, Cp -ZrMes exhibits ethylene polymerization activity even on partially dehydroxylated alumina. 

Table 11.2 shows the results of an EDA analysis on the neutral ytterbium and plutonium complexes. The total bonding energy is substantially more negative for the plutonium system, even though the sum of the prerelaxation electrostatic and Pauli components is much more favorable in the lanthanide compound. The much larger orbital term in the actinide compound reflects the appreciably larger hybridization of the 5f orbitals with the [Pbjj] cage. 

The Kondo picture, however, does not apply in the case of the actinide compounds. The difficulties with this model have been discussed in Cox and Zawadowski (1999). The difference between the Ce-based heavy-fermion compounds and their U-counterparts can be seen directly from the photoemission spectra (Allen, 1992). In U-based heavy-fermion compounds, the fingerprint character of the transitions f is lost. Instead of the well-defined f-derived peaks familiar for the Ce systems, we encounter a rather broad f-derived feature. This fact shows that the f-valence in the actinide heavy-fermion systems is not close to integer value as it is the case in Ce-based compounds. In fact, the f-valence of the U ions has been discussed rather controversially. 

A microscopic picture for the strongly renormalized quasiparticles has finally emerged for the actinide compounds. The hypothesis of the dual character of the 5f-electrons is translated into a calculational scheme which reproduces both the Fermi surfaces and the effective masses determined by dHvA experiments without adjustable parameter. The method yields also a model for the residual interaction leading to the various instabilities of the normal phase. The next step will be to develop an appropriate Eliashberg-type theory. The dual model approach should also provide insight into the mysterious hidden order phases of U-compormds. 

The investigation of structure and physical properties of the actinide compounds with aluminum has been initiated in the eighties (Baran et al. 1984). In fact, the first examination of ThFe4Alg was performed in 1978 (Buschow and van der Kraan 1978) but it has had an incidental character in the whole series of the lanthanide compounds. The actinide compounds with higher concentration of transition metals are under intensive examination since the end of the eighties (see e.g. Suski et al. 1989). 

The rare-earth compounds and the actinide compounds will now be treated separately, in the present sect. 3.3 and in sect. 3.4, respectively. The reasons are that (1) the rare-earth compounds form a huge class of materials, much bigger than that of the... 

Certain of the actinide compounds, e.g., plutonium citrate complexes, are soluble, are absorbed (although even here absorption is very low) and dissociate in the body giving a relatively high fractional retention so that actinides in these initial physicochemical forms are bioavailable to a measurable extent. 

All of the actinide elements are metals with physical and chemical properties changing along the series from those typical of transition elements to those of the lanthanides. Several separation, purification, and preparation techniques have been developed considering the different properties of the actinide elements, their availability, and application. Powerful reducing agents are necessary to produce the metals from the actinide compounds. Actinide metals are produced by metallothermic reduction of halides, oxides, or carbides, followed by the evaporation in vacuum or the thermal dissociation of iodides to refine the metals. 

The 5/ to 6d bands are orbitally allowed and therefore more intense than those of the / to / transitions. They are also usually broader and often observed in the ultraviolet region. The metal to ligand charge-transfer bands are also fully allowed transitions that are broad and occur commonly in the ultraviolet region. When these bands trail into the visible region, they produce the intense colors associated with many of the actinide compounds. Metal-ligand frequencies are also observed in the infrared and Raman spectra of actinide compounds. 

There are rather few data concerning the intermetallic compounds of Si with rare earths, and there are no data with plutonium. We report in fig. 110 the enthalpies of formation of the (R, An)Si2 compounds as a function of atomic number. In fig. 111 we plot the enthalpies of formation of RsSis and An3Si2 compounds. Figure 112 shows plots of the enthalpies of formation of Ani cSijc and Gdi cSijc compounds as a function of Si content. The data are from Alcock et al. (1966) for the thorium-based alloys, have been assessed by Chiotti et al. (1981) for the uranium-based alloys, and are from Lukashenko et al. (1992) for the Gd-based alloys. The only trend which can be seen from these three figures is that the enthalpies of formation of the actinide compounds are less negative than those of the rare earth compounds. 

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