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Simple model for localization in δ-Pu

First-principles methods are employed to study the effect of localization of the 5f electrons in δ-Pu. First, a full-potential linear muffin–tin orbitals (FPLMTO) method was applied to a model system, , where Puloc are Pu atoms with localized (nonbonding) 5f electrons and Puit atoms with itinerant (bonding) 5f electrons. Within the FPLMTO, this system was treated as an ordered compound, either in the Cu3Au or the CuAu structure to model δ-Pu which crystallize in a face-centred-cubic structure. A more realistic alloy treatment of our model system was provided by the Korringa–Kohn–Rostocker method within Green's function formalism in which compositional disorder is treated by means of the coherent potential approximation. With these two approaches best agreement with the experimental lattice constant for δ-Pu were achieved for a 67–68% fraction of itinerant (Puit) atoms. This corresponds to a little less than four itinerant 5f electrons/atom in δ-Pu which agrees well with some proposed theoretical models, but disagree with at least an other theoretical suggestion. We show that a good lattice constant (by construction), good bulk modulus, and full mechanical stability for δ-Pu follows from our model. The main problem with the present approach and some other presented models, trying to capture localization in δ-Pu, is that the contribution to the total energy from the localized 5f electrons cannot be calculated accurately and therefore one parameter (usually the lattice constant) needs to be fitted to experiment.