CALCULATED LATTICE-RELAXATION IN PU-GA

The thermodynamic behavior of the high-temperature delta (fcc) phase o f plutonium is highly anomalous. It has been well established that ab initio, local-density-approximation (LDA) calculations predict an over bonding and consequent overly large density to a much greater degree i n delta-plutonium than in any other metallic phases for which such cal culations have been reported (including (alpha-Pu). Small amounts of g allium stabilize the delta structure, at least kinetically, to below r oom 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 o rbitals method. Calculated relaxations are about one half or less of t hose observed by x-ray-absorption fine-structure spectroscopy, and thi s discrepancy is briefly discussed.