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Heavy-element chemistry at Los Alamos

The chemistry and physics of the actinide elements (uranium, neptunium, plutonium, etc.) have always been of prime importance at Los Alamos, beginning with the early efforts to characterize the chemistry of plutonium during the Manhattan Project. This continuing institutional investment in fundamental actinide science has provided the technical basis for process and separation chemistry and metallurgy related to the national security mission of the Laboratory. As a result, Los Alamos has the expertise, capabilities, and facilities to carry out a wide range of chemical research with the actinide elements, including inorganic, organometallic, and solid-state synthesis; a wide variety of spectroscopic characterization; and electronic structure theory.

The actinide series marks the emergence of 5f electrons in the valence shell. In the pure elements, those to the left of plutonium have delocalized (bonding) electrons, while elements to the right of plutonium have localized (nonbonding) electrons. Plutonium is trapped in the middle, and for the delta-phase metal, the electrons seem to be in a unique state of being neither fully delocalized nor localized, which leads to novel electronic interactions and unusual physical and chemical behavior. The concept of localized or delocalized 5f electrons also pervades the bonding descriptions of many of the actinide molecules and compounds. In the normal nomenclature of chemistry, the delocalized electrons are those involved in covalent bonding, while the localized electrons give rise to ionic behavior. Understanding the nature of bonding in actinide materials remains a computational and experimental challenge.

A full understanding of the nature of the chemical bond in actinide systems must access the relative roles of all atomic orbitals in chemical bonding. The articles in this issue of Actinide Research Quarterly describe the Los Alamos approach to understanding covalency in actinide molecules and materials. This strategy combines synthetic chemistry, spectroscopic characterization, and theory and modeling to understand and predict the chemical and physical properties of actinide materials. This multidisciplinary approach is the strength of the Los Alamos heavy-element chemistry program and provides the scientific means to formulate rational approaches to solve complex actinide problems in a wide variety of environments.

David L. Clark and Gordon D. Jarvinen


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