Theory for α- and δ-Pu

摘要

During the last few years, there has been intense research, both theoretically and experimentally aimed to understand the fundamental properties of Pu and the anomalous, but technologically important, δ phase. Pu undergoes several phase transitions as a function of temperature at ambient pressure, and δ-Pu, stable between about 600 K and 740 K, is the least dense phase. This phase is quite different from the ground-state α phase, for instance: (ⅰ) δ-Pu has an atomic volume about 25% greater than α-Pu. (ⅱ) The crystal symmetry of δ-Pu is cubic, but α-Pu condenses in a very complex low-symmetry monoclinic phase, (ⅲ) The thermal expansion of δ-Pu is small and negative, but it is very large and positive for α-Pu. These remarkable inconsistencies occur for two phases of Pu separated by only a few hundred degrees and naturally suggest that the electronic structure in Pu is rapidly changing with temperature. The ambient pressure phase diagram of Pu is shown in Figure 1.