Theoretical confirmation of Ga-stabilized anti-ferromagnetism in plutonium metal
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Theoretical confirmation of Ga-stabilized anti-ferromagnetism in plutonium metal Per Söderlind1 and Alex Landa1 1 Condensed Matter and Materials Division, Physical and Life Science Directorate, Lawrence Livermore National Laboratory, Livermore, CA 94551, USA ABSTRACT The density-functional-theory model for plutonium metal is shown to be consistent with recent magnetic measurements that suggest anti-ferromagnetism in Pu-Ga alloys at low temperatures. The theoretical model predicts a stabilization of the face-centered-cubic (fcc, δ) form of plutonium in an anti-ferromagnetic configuration when alloyed with gallium. The ordered magnetic phase occurs because Ga removes the mechanical instability that exists for unalloyed δ-Pu. The cause of the Ga-induced stabilization is a combination of a lowering of the band (kinetic) and electrostatic (Coulomb) energies for the cubic relative to the tetragonal phase. INTRODUCTION Plutonium metal remains an interesting but complex and controversial material. Particularly, its face-centered cubic (fcc) δ phase has received almost as much attention as the likewise fcc phases in cerium metal (α and γ). In order to stabilize δ-Pu at lower temperatures it is alloyed with a stabilizer, often gallium, and this allows the study of the temperature dependence in a wide temperature range. Recently [1, 2], a piece of the plutonium puzzle was provided by magnetization measurements that was interpreted to show existence of an antiferromagnetic phase below a critical temperature of 30 K for the δ-Pu92Ga8 alloy and 40 K for the Pu3Ga compound. In Figure 1 we redraw the real part of the dynamic susceptibility versus temperature from Arkhipov et al. [1] that indicates the critical temperatures.
Figure 1. The real part of dynamic susceptibility versus temperature. Taken from Ref. [1].
From a modified Curie-Weiss model the authors [1, 2] were able to determine the critical (Néel) temperatures and thus identify anti-ferromagnetism in the Pu-Ga systems similar to what has been done in the past for other δ-Pu alloys [3]. In this brief report, we investigate whether anti-ferromagnetism in δ-Pu-Ga alloys, at low temperatures, is consistent with the density-functional-theory (DFT) model for Pu metal and PuGa alloys and explore possible causes for such stabilization. COMPUTATIONAL DETAILS We are focusing our first-principles modeling on a robust implementation of DFT for which consistent predictions can be made without the uncertainty of model parameters. The fullpotential linear muffin-tin orbitals (FPLMTO) method do not compromise on accuracy beyond that necessary for the electron exchange and correlation energy functional, which is chosen to be the generalized gradient approximation (GGA). Although newer varieties of this approximation have been proposed, the GGA remains the preferred choice for actinide metals [4]. Our particular FPLMTO implementation is based on an implementation that has been described in detail [5]. In addition to the choice of GGA, we have found that for actinides no geometrical appr
