Actinides metallic modifications

Fig. 10. The (r ) /R (R interatomic distance) ratio for different crystallographic modifications of actinide metals is plotted vs atomic number Z. The ratio is a rough measure of f-f overlapping. In the figure, the value for a 3 d-metal (Fe) is given for comparison (from )...

Similarly to the lanthanides, actinides in the elemental state are reactive electropositive metals and pyrophoric in finely dispersed form. Strong reducing agents are necessary to prepare the metals from their compounds, for instance reduction of the halides by Ca or Ba at 1200°C (e.g. Pup4 + 2Ca Pu -I- 2CaF2). Some properties of the actinides in the metallic state are fisted in Table 14.5. The number of metallic modifications and the densities are remarkably high for U, Np and Pu. Some modifications of these elements are of low symmetry this is an exception for metals that is explained by the influence of the f electrons. The properties of Am and the following elements correspond to those of the lanthanides. 

The known structures of the lanthanide and actinide metals are indicated in table 5.01, from which it will be seen that the structures characteristic of the true metals, and particularly the hexagonal close-packed arrangement, are common. Polymorphism, however, is of frequent occurrence among these elements, and plutonium, for example, crystallizes in no fewer than six modifications—the A1 and A2 structures indicated and four others of greater complexity. Praseodymium, neodymium and samarium are of interest in that they possess close-packed structures in which the sequence of layers is... 

Actinium and thorium have no / electrons and behave like transition metals with a body-centered cubic structure of thorium. Neptunium and plutonium have complex, low-symmetry, room-temperature crystal structures and exhibit multiple phase changes with increasing temperature due to their delocalized 5/ electrons. For plutonium metal, up to six crystalline modifications between room temperature and 915 K exist. The / electrons become localized for the heavier actinides. Americium, curium, berkelium, and californium all have room-temperature, double hexagonal, close-packed phases and high-temperature, face-centered cubic phases. Einsteinium, the heaviest actinide metal available in quantities sufficient for crystal structure studies on at least thin films, has a face-centered cubic structure as typical for a divalent metal. 

This chapter will deal with pure actinide metals, disordered alloy systems, and atomically ordered intermetallic compounds. The latter have been very useful in understanding properties related to the electronic structure of the pure metals the very large number of actinide intermetallic compounds yields several whose electronic structure is such that their properties are totally explained by well-understood phenomena. It then becomes possible to understand systems with more complicated properties as modifications of the well-understood materials. 

Table 19.1 Polymorphic modifications, transformation temperatures, and structural data for the actinide metals. 

Time-resolved emission spectroscopy is gaining importance in the study of various chemical aspects of luminescent lanthanide and actinide ions in solution. Here, the author describes the theoretical background of this analytical technique and discusses potential applications. Changes in the solution composition and/or in the metal-ion inner coordination sphere induce modifications of the spectroscopic properties of the luminescent species. Both time-resolved spectra and luminescence decays convey useful information. Several models, which are commonly used to extract physico-chemical information from the spectroscopic data, are presented and critically compared. Applications of time-resolved emission spectroscopy are numerous and range from the characterization of the... 

After a brief introduction to liquid membranes, studies of supported liquid membranes (SLM) and their applications in separations of various metal species relevant to nuclear research centers are described. Aspects of coupled transport in SLM and the transport model first proposed by Danesi are outlined. Choices of membrane material and solvent which improve membrane stability in a SLM system are discussed. Recent modifications of the SLM process are mentioned. Applications of SLMs in hydrometallurgy for the separation and concentration of actinides, lanthanides, and transition metals, are reviewed. A few pilot-scale studies of SLM are described which show the potential for large-scale utilization in the future. 

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