Electron-density distribution and physical properties of plutonium–gallium alloys: Ab initio cluster calculations

A cluster model based on ab initio density-functional theory was used to model gallium-stabilized δ-plutonium alloys, and to calculate the electron-density distribution, its pressure dependence, bond lengths, elastic properties (second order and third order), and inelastic properties for Pu 12Ga (7.7 at% Ga) and Pu 18Ga (5.3 at% Ga). The electron distribution was found to contain localized, semi localized, and delocalized contributions, with the second possessing covalent character. Two of plutonium’s 8 valence electrons were found to be itinerant, consistent with a recent prediction based on an electrostatic model, with the electron configuration for plutonium being 7s 0.57p 0.55f 1 (itinerant) and 6d 15f 5 (localized), and that for gallium being 4s 14p 2. Applied hydrostatic pressure shifts the charge density toward a more localized Pu(d)-based distribution. The onset of the pressure-induced δ-Pu to α-Pu phase change is accompanied by a ∼0.2 electron increase in the localized population that may serve as a driving force for the phase change. Interior bonding within the Pu 12Ga subunits is stronger than that of the surrounding plutonium lattice, and the Pu–Ga bonds therein relax in a direction opposite to lattice strain. This study predicts covalency in metallic plutonium, both in the Pu–Ga bonding and in the Pu–Pu bonding.