Systematics in lanthanide and actinide solids

Abstract

Generalized phase diagrams have been available for about ten years for both the lanthanides and actinides, and these have proven to be quite useful in understanding the phase relationships in the two series of elements and in predicting the existence of phases when experimental data are lacking. The two generalized phase diagrams are examined for their similarities and differences. The application of systematics in lanthanide compound series has been useful in predicting crystal structures (phase relationships), melting points, and enthalpies and free energies of formation. Analyses of these data indicate that there is 4f hybridization with the other valence and/or bonding electrons. It is most likely that the 4f hybridization involves the empty 4f level(s) just above the Fermi surface and not the filled localized 4f levels. Systematics has also been applied to the actinide metals. Since the experimental data are primarily limited to Th, U and Pu, and since the valence changes from 3 to 7 in the early actinides, the usefulness of these techniques, which have been so successful for the lanthanides, has been limited. If more experimental data were available for actinium and its alloy systems, much more progress could be made in our understanding of actinide alloys and compounds. In addition, as a result of applying systematics, it is thought that the lattice parameters reported for Ac and AcH 2 are quite likely to be wrong. It is proposed that the correct lattice parameter for f.c.c. Ac metal is 5.670 Å, about 6.7% larger than the value which has been accepted up to now.