Calculated lattice relaxation in Pu-Ga

The thermodynamic behavior of the high-temperature delta (fcc) phase of plutonium is highly anomalous. It has been well established that ab initio, local-density-approximation (LDA) calculations predict an overbonding and consequent overly large density to a much greater degree in delta-plutonium than in any other metallic phases for which such calculations have been reported (including (alpha-Pu). Small amounts of gallium stabilize the delta structure, at least kinetically, to below room temperature. To investigate the relaxation of Pu neighbors of Ga, presumably related to the delta stabilization, LDA force calculations are performed with a full-potential linear combination of muffin-tin orbitals method. Calculated relaxations are about one half or less of those observed by x-ray-absorption fine-structure spectroscopy, and this discrepancy is briefly discussed.