Understanding Actinides through the Role of 5 f Electrons
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Understanding
Actinides through the Role of 5f Electrons
T. Gouder, F. Wastin, J. Rebizant, and G.H. Lander Introduction Studies of the actinide elements and compounds were (and are) motivated by the need to characterize their structural and thermodynamic properties for the development of nuclear fuels and the treatment of waste, whether it be for long-term storage or ideas involving transmutation in high-powered accelerators. For the most part, tables giving these data exist, although the data for transuranium compounds are rather sparse. A much more difficult task is to understand the data and develop theories that have predictive power in this part of the periodic table. In doing this, however, we are confronted with the extremely complicated electronic structure of the 5f shell and the great paucity of high-quality data on materials containing transuranium isotopes. Research on uranium, both the element and compounds of it, is conducted in many parts of the world because uranium is a nontoxic material (at least in the quantities used for basic research) and can be handled in university laboratories. Our lab has projects on uranium because of our collaborations with European laboratories (as well as laboratories in the United States and Japan); however, our primary focus is on transuranium materials, which cannot be investigated in many places. Because our European Commission lab is situated in Karlsruhe at an institute that specializes in transuranium materials, mostly connected with the nuclear-fuel industry in the areas of fuel development, fuel characterization, or aspects of the nuclearwaste problem, we have an infrastructure for handling transuranium isotopes (principally Np, Pu, Am, and Cm). Thus, this
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article focuses particularly on work from our own laboratory. We give a brief review of the basic research activities on actinide elements and compounds, revealing the varying nature of the 5f electrons across
this last, and least explored, row of the periodic table. We emphasize our efforts on Np, Pu, and Am, and how such studies can enhance our understanding of the complete series. We start with the importance of preparation and then describe measurement techniques including those to characterize structural, magnetic, transport, and surface properties.
Preparation and Transport Studies Our effort starts with preparation. Many different materials science techniques exist, but our laboratory has devoted a considerable effort to producing high-purity single crystals of actinide compounds,1 which these high-level physics experiments demand. Techniques such as mineralization, zone-refining and the Czochralski process for the metallic compounds, and vapor transport for the oxides, can be used to produce crystals and to increase the purity, thus assuring the stoichiometry. Some of these materials are shown in Figure 1. The crystals are then characterized by x-ray, scanning electron microscopy, and microprobe analysis techniques. A key transport property is the electrical resistivity of a material, meas
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