Condensed Matter Physics Seminar

Actinides in Solution: Disproportionation, Strong Correlations, and Emergence

Prof. Brad Marston
Brown University
Tuesday, April 11, 2006
Higgins 235, 3pm

Plutonium in acid solutions can be found in oxidation states III through VII. There is a striking near perfect degeneracy of the reduction-oxidation (redox) potentials, each being about 1 electron volt. Neptunium is the only other element that approaches this degree of degeneracy. One consequence of the redox degeneracy is a marked tendency of plutonium ions to disproportionate; up to four different oxidation states can coexist simultaneously in the same solution, greatly complicating the environmental chemistry of the element. While the degeneracy could simply be a coincidence, more likely it is a manifestation of a higher-level organizing principle at work. Other systems that exhibit disproportionation raise the possibility of an emergent "negative-U" attractive interaction. The hypothesis can be tested by combining first-principles relativistic density-functional calculations with Hubbard models of the strong correlations between the actinide 5f electrons. I comment on the implications of this work to the problem of nuclear waste disposal at locations such as Yucca Mountain.