Stability of δ-Pu alloys from first-principles theory

It has been proposed [Phys. Rev. B 66 (2002) 205109] that delta-Pu is stabilized at higher temperatures by magnetic interactions driving a disordered magnetic state. At lower temperatures, however, the magnetic moments are expected to align in an antiferromagnetic fashion which has been shown to destabilize delta-Pu mechanically. Consequently, delta-Pu is unstable below about 600 K but can be stabilized at lower temperatures when alloyed with a small amount of a suitable 'delta-Pu stabilizer'. Here we explain this stabilizing effect in terms of the balance between ordered and disordered magnetism and how this balance is offset by the addition of an alloying component. For this purpose we have applied density functional theory (DFT), implemented in the Korringa-Kohn-Rostocker (KKR) method within the Green's function formalism. The effect of magnetic moment disorder as well as compositional disorder was treated by means of the coherent potential approximation (CPA) implemented, in the former case, in conjunction with the disordered local moment (DLM) model. These calculations show that an alloy component larger than delta-Pu has a stabilizing effect, whereas a magnetic component that is smaller has a strongly destabilizing effect on delta-Pu. The ordered Pu3Al, Pu3Ga, Pu3In, and Pu3Tl compounds (in the Cu3Au structure) are all found to have negative heat of formation with calculated density close to experimental data. (C) 2003 Elsevier Science B.V. All rights reserved.