A "generalized" phase diagram is constructed empirically for the lanthanides. This diagram makes it possible, not only in one picture, to assemble a lot of information but also to predict phase transitions not yet experimentally accessible. Further, it clearly illustrates the close relation between the members of the lanthanide group. To account for some of its features, the pseudopotential method is applied. The trend in crystal structure through the lanthanide series can thereby be qualitatively accounted for, as can the trend in crystal structure for an individual element, when compressed. A scaling procedure makes it possible to extend our treatment to elements neighboring the lanthanides in the Periodic Table. In total 25 elements are considered. An atomic parameter $f$ (relatable to the pseudopotential) is introduced, by means of which different phase transitions, both for an individual rare-earth element and intra-rare-earth alloys, can be correlated to certain critical values of this parameter. A nonmagnetic rare-earth series (Sc, Lu, Y, La, and Ac) is introduced and the occurrence of superconductivity is discussed with special emphasis on the pressure dependence of the transition temperature. This temperature can be correlated to the above-mentioned parameter $f$, both for intra-rare-earth alloys and pure elements at different pressures. The found correlation implies that actinium is a superconductor with a critical temperature which could be as high as (11-12) °K. It is also argued that the actinide series can be viewed upon as a second delayed rare-earth series. From this, several analogies between the two series are pointed out, which might throw new light upon several problems concerning the rare-earth themselves, e.g., the nature of the $\gamma -\alpha$ transition in cerium and the occurrence of the dhcp structure in the light lanthanides. Finally, valence transitions at high pressure for europium and ytterbium are discussed. Calculations give that these elements should become trivalent at a pressure of the order of 150 kbar.

• Received 8 April 1974

DOI:https://doi.org/10.1103/PhysRevB.11.2836