Detailed studies show that the crystal lattice of most actinide metals expands with increasing temperature

and evolves to a simple cubic arrangement close to their melting temperature, similar to the lanthanide elements. (For numerical data on the thermal expan­sion, see Section 2.01.4.1) As the atoms move away from each other, the electrons in the 5f metals tend to favor a localized state. As discussed by Vohra and Holzapfel,15 this is particularly important for Np and Pu, which are on the threshold of localization/ itinerancy. The case for plutonium is much more complex, as shown in Figure 5. The crystal lattice of plutonium expands for the a-, p-, g-, and e-phases, and the g — to 8-transition has a positive expansion. The 8- and 8′-phases have negative thermal expan­sion and the 8- to 8′- and 8′- to s-transitions show a negative volume change, as is the case upon melting. Dynamic mean field calculations show that the monoclinic a-phase of Pu is metallic, whereas fcc 8 is slightly on the localized side of the localization — delocalization transition.16

Moreover, the stability of the crystalline state of the actinide metals varies significantly. The melting temperature is high for thorium, similar to that of the transition metals in group IVB, and low for Np and Pu (Figure 6).

When applying high temperature as well as high pressure to the actinides, phase changes can be sup­pressed, as is shown in Figure 7. For example, the triple point for the a—p—g equilibrium in uranium is found at about 1076 K and 31.5 kbar; above this pressure, ortho­rhombic a-U directly transforms in fcc g-U.17 In plu­tonium, the g-, 8-, and 8′-phases disappear at relatively low pressure and are replaced by a new phase desig­nated Z- In contrast to the other actinides, plutonium shows a negative slope for the liquidus down to the p-Z-liquid triple point (773 K, 27 kbar) reflecting the increase in density upon melting.17