Phase diagrams of plutonium metal are astonishingly complicated. The main feature is a sequence of crystallographically complex phases. Temperature, pressure and alloying shift the stability of these phases rapidly, so that pressure and alloy phase diagrams are also complicated. Another feature of the plutonium phase diagrams is the anomalously low melting point. How these two features, crystallographic complexity and low melting point, work to determine complicated phase diagrams is shown in Fig. 1. This figure, which is a composite of the relevant binary diagrams, shows that crystallographic complexity and low melting point are ,nearly exactly coincident. Such behavior is unique to the light actinides. It is natural to seek the source of this behavior in the collective properties of the 5f electrons, and in this paper we will trace some of the paths we are following in this search. Fig. 2 shows the linear thermal expansion of unalloyed Pu metal. As the temperature is raised, there is a progression of phases from low symmetry to high symmetry as the melting point is reached. The α- and β-phases are monoclinic, the γ-phase is orthorhombic, the δ-phase is FCC and the ε-phase is BCC. The δ'-phase is a transitional, tetragonal phase. A number of surprising features are apparent from this plot. First, the crystallographic complexity of the low temperature phases is unmatched by any other element: the α- and β-phases have 16 and 34 atoms per cell, respectively. This crystallographic complexity seems to be connected to the tendency of Pu to form intermetallic compounds with itself, the so-called "self-intermetallic" compounds (Lawson et al., 1996) or to another, probably equivalent, tendency toward structural complexity in narrow band materials (Soederlind et al., 1995, Soederlind, 1998, Hecker, 2000, and Baskes, 2000). Second, the FCC δ-phase has a surprising negative thermal expansion. Third, as noted, the melting point is surprisingly low, with a volume decrease at the melting point. This multiplicity of crystallographically stable phases leads to complex phase diagrams, some of which will be displayed in the next section, and a few features of interest will be pointed out. The following section will discuss some aspects of disorder (or short range order) that may be expected in Pu phases. After that, we will discuss the special case of vibrational disorder and present some measurements of the Debye-Waller factors of the various phases in unalloyed Pu. This leads to a discussion of the melting point of Pu, based on the Lindemann rule. Finally, there is a discussion of the thermodynamics of Pu, which leads us back to the topic of phase diagrams. At this point it is possible to make some contact with the topic of electron correlation.
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