An entropy correlation for the 4f and 5f metals: relation of electronic properties to metallic radii, magnetic transformations and thermodynamics of vaporization

A systematic treatment of the thermodynamic properties of the actinides has proven to be a difficult, or even impossible task. The diversity of just the metals from actinium through americium is well-known; as more and more transplutonium metals have become available for measurement, there is the hope that perhaps here a simpler picture, more akin to the lanthanides, may emerge. The condensed-phase entropy for the lanthanide and many actinide metals is a predictable quantity which can be accurately calculated on the basis of metallic radius, molecular weight and magnetic configuration. Implicit in the correlation is a knowledge of the electronic structure, which also implies a relatively well-defined metallic valence. Thus, for the early actinides Pa, U, Np and Pu, in which f-electrons are involved in the metallic bond, accurate entropy values are not predictable, though their nonmagnetic state is shown. It is believed that the immense diversity of the actinide series, at least from Ac through Pu, precludes the success of any one theoretical model for the thermodynamic properties of these metals. The entropy is shown to be a property through which the diverse natures of these elements can be better understood. (Auth.)