2.01.2.1 Crystal Structure

The stable crystallographic modifications of the acti­nides at atmospheric pressure are listed in Table 1. Compared to the lanthanide series in which the hex­agonal close-packed (hcp) and the face-centered cubic (fcc) structures dominate, the actinide metals show a remarkable variation in the structural

properties at room temperature, as shown in Figure 1. Particularly, the elements Pa-Pu have unusual low symmetry (distorted) crystal structures. a-Pa is body-centered tetragonal, and a-U and a-Np are orthorhombic but with slightly different space groups. a-Pu has a monoclinic crystal structure with 16 atoms in the unit cell at room temperature. Plutonium is unique in the periodic table of the elements with six allotropes at atmospheric pressure and one more at elevated pressure.

This complexity of the structural properties of the actinides is also evident from Figure 2, which shows the variation of the molar volume of the a-phases of the actinides at room temperature and atmospheric pressure, indicating that the actinides Pa to Pu follow the trend in the (itinerant) d-transition
metals, whereas the actinides Am to Bk follow that of the (localized) 4f metals. It is generally accepted that this complex behavior is due to the active role of the f-electron in the metallic bond and the changes in temperature and pressure by which the f-electron bonding character is affected. Experimen­tal observations and electronic structure calculations have indeed shown that the bonding in the transition metals is dominated by d-electron contributions, that in the lanthanides there is a lack of f-electron contri­bution, and that the actinides fall in between.5