Plutonium’s Shifty Ground States

Plutonium is arguably the most complex element in the periodic table. Early actinides behave like transition metals, and late actinides behave like lanthanides. But plutonium bridges the two, with physical properties reflective of both groups. A source of plutonium’s curious and complex properties, at least for one of its six solid phases, is a superposition of electronic ground states, according to a neutron scattering study led by Marc Janoschek of Los Alamos National Laboratory (Sci. Adv. 2015, DOI: 10.1126/sciadv.1500188). To understand the basis of plutonium’s properties, Janoschek and coworkers studied the δ phase of plutonium at ambient temperature and pressure. Confirming prior theoretical work, they found that δ-Pu fluctuates among 5f⁶, 5f⁵, and 5f⁴ states by delocalizing one or two valence electrons into the conduction band. The states have lifetimes of about 0.015 picoseconds. The results explain why plutonium’s expected magnetism hasn’t been observed experimentally: The 5f⁴ and 5f⁵ states are magnetic, but 5f⁶ is nonmagnetic. Also, the various states yield different ion sizes that contribute to plutonium’s large volume changes—as much as 25%—in response to small temperature shifts.