PHASE-STABILITY OF PLUTONIUM

Results of electronic-structure calculations on the experimentally observed low-temperature monoclinic alpha phase of Pu are presented and compared to calculations for fcc (delta phase) and fictitious hexagonal Pu. Energy-volume curves for fcc and hexagonal phases of Pu reveal a tendency towards more open structures, primarily due to an increased occupation of the 6d band, which has a strongly itinerant character. Bonding in these structures is not dominated by the 5f band. However, in the experimental a phase, which is computed to be the most stable, the arrangement of the atoms in the unit cell is such that 5f bonding is strongly enhanced when compared with fcc and hexagonal Pu. In addition the alpha-Pu structure benefits from a substantial charge transfer from itinerant 7s states to the bonding 6d states. A relative downward shift of the 5f band reduces the width of the valence band in alpha-Pu. The predicted narrowing should be observable in photoemission experiments on alpha- and delta-Pu.