THEORETICAL ASPECTS OF THE 5F DELOCALIZATION OF AMERICIUM UNDER PRESSURE

Americium is the first element of the actinide series with localized and chemically inert 5f electrons. It has earlier been demonstrated that with applied pressure the 5f electrons in Am become itinerant, and therefore bonding. The crystal structure of americium has been experimentally determined to change with increasing pressure; it changes from double hcp --> fcc --> monoclinic or distorted fcc --> the alpha-uranium structure (orthorhombic), or a related structure. The experimentally observed volume collapse associated with 5f delocalization is very small. In contrast, previous theoretical calculations have predicted a volume collapse between 25% and 40%. We present the total energy versus volume of americium in the fcc and in the experimentally reported alpha-uranium structure obtained from fully relativistic, full-potential, linear-muffin-tin-orbital calculations based on the local-density approximation. In combination with previous calculations for the low-pressure phase, our calculations predict that the delocalization of the 5f electrons in americium is accompanied by a approximately 34% volume collapse.