Plutonium’s bizarre behavior explained

Plutonium, a radioactive metal best known for its proclivity to undergo nuclear fission chain reactions, is not magnetic and does not conduct electricity well. In addition, plutonium’s so-called δ phase at 600 K shows a 25% greater volume per atom than its more dense room-temperature α phase. What makes plutonium so bizarre? For starters, it is a strongly correlated material, in which the valence electrons cannot be treated as independent agents. To accurately model the system, condensed-matter theorists at Rutgers University in New Jersey combined two computational approaches to solid materials—the local density approximation and dynamical mean-field theory. (For more on DMFT, see Physics Today, March 2004, page 53 .) In the new model, plutonium’s eight outermost or valence electrons can circulate among different orbitals rather than being confined to specific ones. The physicists found that the number N _f of valence electrons in plutonium’s 5f orbital—the one with the greatest influence on its chemical properties—fluctuates. As the figure shows, five electrons are found there about 80% of the time, six about 20% of the time, and four less than 1% of the time. The new model accurately accounts for plutonium’s key properties and makes several experimentally verifiable predictions. The theorists hope to use their method to understand the chemistry of uranium oxide and plutonium oxide, two important byproducts in nuclear reactors. (J. H. Shim, K. Haule, G. Kotliar, Nature 446 , 513, 2007 http://dx.doi.org/10.1038/nature05647 .)