Actinides metals

Very Short Metal-Actinide Bonds NUIr and Isoelectronic Systems. 

Three predominant routes have been developed for the solution-grafting of organorare-earth metal /actinide complexes (i) direct grafting (route A),... 

The impact on actinide solid state physics of Zachariasen s ideas has been great. It has triggered a very fruitful effort in structure determination. In the meantime, it has underlined the importance, in the theory of actinides, of knowing exactly atomic volumes as an indicator of the complexities of the metallic bond. The same picture of the variation of atomic radii appears for compounds having partial metallic character. In Fig. 5, for instance, the variation vs. Z of the lattice parameter ao of actinide pnictides is compared with the atomic radii of metals. Actinide pnictides have an NaCl type structure. Their... 

Lanthanides Any sixth-period inner transition metal. Actinides Any seventh-period inner transition metal. 

Hydroxides. Pure and mixed metal actinide hydroxides have been studied for their potential utility in nuclear fuel processing. At the other end of the nuclear cycle, the hydroxides are important in spent fuel aging and dissolution, and environmental contamination. Tetravalent actinides hydrolyze readily, with Th more resistant and Pu more likely to undergo hydrolysis than and Np. All of these ions hydrolyze in a stepwise marmer to yield monomeric products of formula An(OH) with = 1,2,3 and 4, in addition to a number of polymeric species. The most prevalent and well characterized are the mono- and tetra-hydroxides, An(OH) and An(OH)4. Characterization of isolated bis and tri-hydroxides is frustrated by the propensity of hydroxide to bridge actinide centers to yield polymers. For example, for thorium, other hydroxides include the dimers. 

Heavy-metal actinide chemistry (expertise is being lost)... 

As already mentioned above, calcium chromium phosphates Ca3Cr3(P04)4, Ca2Cr4(P04)4, and Ca9Cr(P04)7 have been characterised. In addition, many mixed-metal actinide as well as lanthanide orthophosphates have also been characterised some of these contain condensed phosphate anions (see below) [63,64]. 

Ion chromatography gained general acceptance as fast and sensitive multielement detection method for the determination of alkali and alkaline earth metals, heavy and transition metals, actinides and lanthanides, as well as selected organic cations, allowing detection limits in the range of microgram per liter and analysis time of less than 20 min. 

The critical masses serve only as a rough guide. They can be considerably lower if, e.g., the density is increased by extremely high pressures. This is particularly so for metallic actinides in fission bombs. 

The f-block elements are the lanthanides and the actinides. Each of these series consists of 14 elements with similar properties in both atomic state and in their combination. They are situated in the 6 and 7 period of the periodic table, respectively, but are customarily separated from the main body of the table. These elements fill their 4f and their 5f orbital, respectively (Table 2.6). The lanthanides are more reactive than the d-block metals, behaving more like the alkali earth metals. Actinides are all very radioactive and therefore have high lethality. The actinides, with atomic numbers higher than 92, have been synthesized during nuclear reactions. 

It is worth making a cautionary remark at this stage. Early neutron experimenters were, of course, expecting that metallic actinide compounds would exhibit crystal-field splittings when viewed with neutron scattering. This attitude comes fixim a simple extrapolation of the situation found in lanthanides, where this technique has been such a considerable success (Fulde and Loewenhaupt 1986). Indeed, the first experiments on actinide rocksalt materials by Wedgwood (1974), in which he claimed to see nothing... 

Certainly the clearest conclusion from the examples of this chapter is the total absence of sharp features in the inelastic response function of anomalous lanthanide and metallic actinide materials. This contrasts strongly with the sharp dispersionless crystal-field excitations observed in most lanthanide compounds, in which the exchange interactions are weak (fig, 2), and with the sharp spin-wave excitations found in systems with strong exchange interactions. In many of the early studies with neutron inelastic scattering, for example of the heavy lanthanides or transition metals and their compounds, the width of the excitations was never an issue. It was almost always limited by the instrumental resolution, although it should be stressed that this resolution is relatively poor compared to that obtained by optical techniques. However, the situation is completely different in the materials discussed in this chapter. Now the dominant factor is often the width indeed in some materials the width of the over-damped response function is almost the only remaining parameter with which to characterize the response. 

A further consequence of 5f-Sf overlap and hybridization in metallic actinides is a reduction of the importance of valence as a useful concept. In most of the light actinide metallic materials there is no clear separation between Sf electrons and bonding electrons. Unlike 4f electrons, the 5f electrons may enter into the bonding. There are, however, cases where the concept of actinide valence is useful, particularly when the magnetism has a local-moment character. Many properties of each system must be examined and reconciled before simplifying concepts such as localized magnetism and valence can be used correctly. Chan and Lam [19]... 

In the metallic actinides, particularly in U, Np, and Pu, the subject of transformations from one polymorphic form to another is a complicated one, because of the unique crystallographic relationships and because of the large number of polymorphic forms that exist in a relatively narrow range of temperature. The transformations and their products have characteristics that depend on temperature, on the rate of change of temperature, on composition, and on initial structure. 

Neutron diffraction and magnetization measurements on the local-moment ferromagnet PuP show that conduction-electron polarization modifies the ordered moment. Thus, an ordered moment of 0.77 /Xg is found in bulk-tmgnetization measurements, whereas the effective moment is 1.1 /Xg. In general, attempts to find crystal field levels in metallic actinide systems by neutron inelastic scattering with single crystals have not been successful, although such levels are easily found with rare earths and the effects of crystalline fields on... 

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