Berkelium metallic state

The energy difference between the trivalent and tetravalent metallic states can be investigated in exactly the same way (fig. 4) as between the di- and trivalent states. It is obvious that no lanthanide can be a tetravalent metal - not even cerium (Johansson 1974). The trivalent state is also more stable for the actinides americium, curium and berkelium. But, in the valence picture, both plutonium and neptunium have lower energies as trivalent than tetravalent metals. Hence, tetravalent ground states for Np and Pu may be eliminated. In fact, by comparing figs. 3 and 4 it is easy... 

In 1972 a hybridized non-degenerate 6d and 5f virtual-bound-states model was used to describe the properties of the actinide metals, including berkelium [138]. It accounted for the occurrence of localized magnetism in Bk metal. In 1974 a review of the understanding of the electronic properties of berkelium metal as derived from electronic band theory was published [139]. Included was the relativistic energy band structure of face-centered cubic Bk metal (5f 6d 7s ), and the conclusion was that berkelium is a rare-earth-like metal with localized (ionic) 5f electrons resulting from less hybridization with the 6d and 7s itinerant bands than occurs in the lighter actinides. 

A phenomenological model based on crystal structure, metallic radius, melting point, and enthalpy of sublimation has been used to arrive at the electronic configuration of berkelium metal [140]. An energy difference of 0.92 eV was calculated between the 5f 7s ground state and the 5f 6d 7s first excited state. The enthalpy of sublimation of trivalent Bk metal was calculated to be 2.99 eV (288 kJmol ), reflecting the fact that berkelium metal is more volatile than curium metal. It was also concluded that the metallic valence of the face-centered cubic form of berkelium metal is less than that of the double hexagonal close-packed modification [140]. 

A relativistic Hartree-Fock-Wigner-Seitz band calculation has been performed for Bk metal in order to estimate the Coulomb term U (the energy required for a 5f electron to hop from one atomic site to an adjacent one) and the 5f-electron excitation energies (143). The results for berkelium in comparison to those for the lighter actinides show increasing localization of the 5f states, i.e., the magnitude of the Coulomb term U increases through the first half of the actinide series with a concomitant decrease in the width of the 5f level. 

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