Localization of 5f Electrons and Phase Transitions in Americium
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0893-JJ01-05.1
Localization of 5f Electrons and Phase Transitions in Americium Michel Pénicaud Commissariat à l’Energie Atomique, DAM-Île de France, Département de Physique Théorique et Appliquée, BP 12, 91680 Bruyères-le-Châtel, France ABSTRACT Density-functional electronic calculations have been used to investigate the high-pressure behavior of americium. The phase transitions calculated agree with the recent sequence obtained experimentally under pressure; double hexagonal close packed → face centered cubic → face centered orthorhombic → primitive orthorhombic. In the first three phases the 5f electrons are found localized, only in the fourth phase (Am IV) the 5f electrons are found delocalized. The localization of the 5f electrons is modeled by an anti-ferromagnetic configuration which has a lower energy than the ferromagnetic ones. In this study the complex crystal structures have been fully relaxed.
INTRODUCTION
×
unhybridized 5f
+ O
experiment delocalized 5f
i
ATOMIC RADII (bohr)
Americium belongs to the series of actinide metals. It is known that the series of actinide metals, corresponding to the progressive filling up of the 5f electronic sub-shell, must be split in two. In the first sub-series, from Pa to Pu, the 5f electrons bind in the manner of d electrons in transition metals. In the other sub-series, starting with Am, the 5f electrons are localized, similarly to electrons in deep atomic layers, and like 4f electrons of lanthanides do not take part in metallic bonding.
Ra Ac Th Pa U Np Pu Am Cm Bk Cf Es
Figure 1. Comparison of equilibrium atomic radii calculated [4,5] by the muffin orbital method in the fcc structure with experiment. (+) experimental and (o) calculated, with unhybridized 5f5/2, values for -Pu. (×) calculated value for Es with unhybridized 5f and 6d. (unhybridized means decoupled).
0893-JJ01-05.2
The localization of the 5f electrons is due to strong correlations between them that are not accurately treated by density functional calculations (DFT) with a local density approximation (LDA) and many attempts have been tested to solve this problem. In the case of Am the localization of the 5f electrons at atmospheric pressure was simulated successively, by a ferromagnetic (FM) configuration [1, 2], by putting the 5f electrons in the atomic core [3], by decoupling them from the others band electrons (figure 1) [4, 5], by the self-interaction corrected (SIC) LDA [6, 7], by a disordered local moment picture [8] within the LDA and the coherent potential approximation. With the method used in figure 1, of constrained LDA type, it is not possible to determine ab initio localized-delocalized transitions because the total energies calculated in constrained LDA and LDA are not comparable. In [1-8], except in [3], the calculations where simplified by the use of a face-centered cubic (fcc) structure instead of the double hexagonal close packed (dhcp) of ambient conditions. Moreover in [1] and [8] the spin-orbit coupling was neglected which is a rough approximation for a highly relativist
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