PROPERTIES OF ACTINIDE METALS UNDER HIGH PRESSURE
The 5f electrons of the metals Pa, U, Np and Pu are delocalized (or itinerant). This means that they are of band type, hybridize with the conduction electrons and thus contribute to the metallic bonding. In contrast, the metals Am, Cm, Bk and Cf, which follow in the actinide series, are characterized by sharp 5f levels and are said to have localized 5f electrons which do not participate in the metallic bond. Pressure favors delocalization and hybridization of f electrons. The delocalization of 5f (and 4f) electrons correlates to a decrease of the interatomic distances and to the formation of crystal structures of low symmetry (orthorhombic and monoclinic). The appearance of low symmetry structures is rationalized by a marked directionality of the hybridized orbital, though a quantitative explanation has not yet been given. Thus, the alpha-uranium type structure (orthorhombic, Cmcm) was successively observed under pressure in Ce, in Am, in Pr and very recently in Bk and Cf. In addition, monoclinic structures were observed under pressure in Ce and Am. The pressures, at which these low symmetry structures form, increase in the series: Ce about 5 GPa, Pr about 20 GPa in the lanthanide series, Am about 10 GPa, Bk about 25, Cf 41 GPa in the actinide series. In the actinide series, the volume decrease accompanying the transition increases with increasing 5f occupation: small in Am, +- 12 % in Bk, and +- 16 % in Cf. The dhcp --> fcc transition, which was known to occur under pressure in some lanthanide metals, is also observed in Am, in Bk and in Cf. These observations confirm the lanthanide like character of these heavier actinides.
Bibliographic Reference: 13TH CONFERENCE ON THE EUROPEAN HIGH PRESSURES RESEARCH GROUP, AUSSOIS (FRANCE), SEPT. 11-14, 1984 WRITE TO CEC LUXEMBOURG, DG XIII/A2, POB 1907 MENTIONING PAPER E 31852 ORA
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Record Number: 1989123013600 / Last updated on: 1987-01-01
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