Shifted Homologous Relationships between the Transplutonium and Early Rare-Earth Metals

Abstract

The physico-chemical properties of the late actinide metals americium through einsteinium are compared with their rare-earth counterparts. Localization of the 5f electrons beginning at americium signals the appearance of true rare-earth-like properties, but the homologous relationship is shifted to place americium below praseodymium, einsteinium is then below europium. The comparison of crystal structures, phase transitions, vapor pressures and heats of vaporization reveals remarkable similarities, especially for Sm-Cf and Eu-Es, where the stability of the divalent metal becomes established and divalent chemistry then follows.

There is of course a major perturbation caused by the half-filled shell at curium, although it may he argued that americium is the anomaly in the so-called second rare-earth series. However, the response of americium, berkelium and californium under pressure reveals the true perturbation to be a thermodynamic one, occurring at curium.

Work performed under the asupices of the U.S. Department of Energy.