First-principles calculations of stability of δ-Pu–Am alloys

AbstractFirst-principles methods are employed to study the ground-state properties of Pu–Am systems. Calculated density of -Pu 100−c Am c alloys( c = 30at.%) agrees well with the experimental data. The paramagnetic (PM) → antiferromagnetic (AF) transition temperature (T c )ofPu 100−c Am c alloys is calculated by the Monte Carlo technique. By introducing Am into the system, one could lower T c from ∼548K (purePu) to 372K (Pu 70 Am 30 ). We also found that, contrary to pure Pu where this transition destabilizes the -phase, Pu 3 Am compound remainsstable in the antiferromagnetic phase that correlates with recent discovery of a Curie–Weiss behavior in Pu 100−c Am c system at c ≈ 24at.%.© 2004 Elsevier B.V. All rights reserved. Keywords: -Pu–Am; Magnetism 1. IntroductionIt is strongly believed that many anomalous physical prop-erties of Pu metal, such as many allotropic forms ( , , , , and e), significant ( ∼24%) volume increase for the → transition, negative coefficient of thermal expansionof -Pu, low (∼913K) melting point, etc., are due to theparticular position of Pu in the Periodic Table. In respect toa progressive filling of the 5f sub-shell, Pu is located on theborder between the light actinides (Th–Np) with itinerant 5felectrons and the heavy actinides (Am–Lr) with 5f localizedstates. In other words, the transition from delocalized to lo-calized 5f electrons takes place within the plutonium phasediagram resulting in numerous allotropic forms.Among these phases -Pu has received a significant in-terest in the metallurgical community because of its highductility that makes it easy to machine and form. This phaseis also the most interesting for physicist because its 5f elec-trons exhibit intermediate behavior between delocalizationand localization [1]. The -Pu phase is stable at temperaturesbetween 593 and 736K, but can be stabilized at lower tem-peratures by alloying Pu with a foreign element, so called‘ -stabilizer’ (or deltagen). Among the elements known as -stabilizers, i.e., Al, Ga, Zn, Zr, Sc, In, Tl, Am, and Ce, only