SYSTEMATICS IN LANTHANIDE AND ACTINIDE SOLIDS

Generalized phase diagrams have been available far 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 d ata are lacking. The two generalized phase diagrams are examined for t heir similarities and differences. The application of systematics in l anthanide compound series has been useful in predicting crystal struct ures (phase relationships), melting points, and enthalpies and free en ergies of formation. Analyses of these data indicate that there is 4f hybridization with the other valence and/or bonding electrons. It is m ost likely that the 4f hybridization involves the empty 4f level(s) ju st above the Fermi surface and not the filled localized 4f levels. Sys tematics has also been applied to the actinide metals. Since the exper imental data are primarily limited to Th, U and Pu, and since the Vale nce changes from 3 to 7 in the early actinides, the usefulness of thes e techniques, which have been so successful for the lanthanides, has b een limited. If more experimental data were available for actinium and its alloy systems, much more progress could be made in our understand ing of actinide alloys and compounds. In addition, as a result of appl ying systematics, it is thought that the lattice parameters reported f or Ac and AcH2 are quite likely to be wrong. It is proposed that the c orrect lattice parameter for f.c.c. Ac metal is 5.670 Angstrom, about 6.7% larger than the value which has been accepted up to now.