Alex Landa

Stability of δ-Pu alloys from first-principles theory

Abstract It has been proposed [Phys. Rev. B 66 (2002) 205109] that δ-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 δ-Pu mechanically. Consequently, δ-Pu is unstable below about 600 K but can be stabilized at lower temperatures when alloyed with a small amount of a suitable ‘δ-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 δ-Pu has a stabilizing effect, whereas a magnetic component that is smaller has a strongly destabilizing effect on δ-Pu. The ordered Pu 3 Al, Pu 3 Ga, Pu 3 In, and Pu 3 Tl compounds (in the Cu 3 Au structure) are all found to have negative heat of formation with calculated density close to experimental data.

Monte Carlo simulations of the stability of δ-Pu

The transition temperature (T-c) for delta-Pu has been calculated for the first time. A Monte Carlo method is employed for this purpose and the effective cluster interactions are obtained from firs ...

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 Greens 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.

Theory for δ‐Pu and δ‐Pu Based Alloys

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Electronic structure calculations of δ-Pu based alloys

First-principles methods are employed to study the ground-state properties of δ-Pu-based alloys. The calculations show that an alloy component larger than δ-Pu has a stabilizing effect. Detailed calculations have been performed for the δ-Pu 1−c Am c system. Calculated density of Pu-Am alloys agrees well with the experimental data. The paramagnetic → antiferromagnetic transition temperature ( T c ) of δ-Pu 100−c Am c alloys is calculated by the Monte-Carlo technique. By introducing Am into the system, one could lower T c from 548 K (pure Pu) to 372 K (Pu 70 Am 30 ). We also found that, contrary to pure Pu where this transition destabilizes δ-phase, Pu 3 Am compound remains stable in the antiferromagnetic phase that correlates with the recent discovery of the Curie-Weiss behavior of δ-Pu 100−c Am c alloys at c ≥ 24 at. %.