Ab Initio Study of the Effect of Mono-Vacancies on the Metastability of Ga-Stabilized δ-Pu

Sarah C Hernandez,Franz J Freibert

doi:10.3390/app10217628

Abstract

Most experimental studies on metallic Pu are on the room temperature monoclinic α-phase or the fcc Ga stabilized δ-phase. Stabilized δ-phase Pu-Ga alloys are metastable and exhibit a martensitic phase transformation to α’-phase at low temperatures, or applied shear, with concentrations lower than three atomic percent Ga. By using first principles, we explore the metastability of δ-phase by investigating the structural and electronic behavior induced by Ga alloying and by a mono-vacancy point defect. We find that a site substitutional Ga induces a tetragonal distortion in the lattice affected by hybridization of Ga 4p and Pu 6d states. With the addition of a mono-vacancy, a monoclinic or tetragonal distortion forms locally (dependent on its distance from Ga), and decoupling of the Pu 5f and 6d states and broadening of the 6d states occurs. This response enables hybridization of Pu 6d with the Ga 4p states affecting the mono-vacancy formation energy. Thus, stabilization of the fcc lattice correlates with hybridization of Pu 6d states with Ga 4p states, and this becomes more evident in the presence of a mono-vacancy.

Highlights

Plutonium (Pu) metal is considered one of the most complex metals in the periodic table due to 5f electronic behavior of being delocalized or localized depending on the atomic environment.Elemental plutonium undergoes five solid-state phase transformations starting with its complicated room temperature 16-atom simple monoclinic α-phase, to the 34-atom body-centered-monoclinic β-phase, and the eight-atom face-centered orthorhombic γ-phase to the highest temperature highly symmetric face-centered-cubic δ-phase [1]
Trends predicted by DFTlattice will not significantly formation for the δ-Pu-Gathat structures investigated in this study.no-spin-orbit-coupling
In a pure δ-Pu fcc lattice with no Ga, there are no structural effects locally, but the presence of a substitutional Ga induces a tetragonal distortion along the spin axis (100) plane

Summary

Introduction

Elemental plutonium undergoes five solid-state phase transformations starting with its complicated room temperature 16-atom simple monoclinic α-phase, to the 34-atom body-centered-monoclinic β-phase, and the eight-atom face-centered orthorhombic γ-phase to the highest temperature highly symmetric face-centered-cubic (fcc) δ-phase [1]. Beyond this phase is the body-centered-tetragonal δ’-phase and the body-centered-cubic ε-phase, right before the low melting temperature of 913 K [2], and the δ and δ’ phases undergo a negative thermal expansion. The stabilization of the δ-phase to room temperature is dependent on alloying solute concentration, with as little as one atomic percent (at. %) Ga may stabilize the fcc phase [3], but upon cooling or mechanical shear, this alloy will undergo a martensitic phase transformation to an expanded α-phase lattice (identified as α’-phase) retained to ambient conditions

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Conclusion