Phase stability in heavy f-electron metals from first-principles theory
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0893-JJ01-02.1
Phase stability in heavy f-electron metals from first-principles theory Per Söderlind Physics and Advanced Technologies Directorate, Lawrence Livermore National Laboratory, Livermore, CA 94550, U.S.A. ABSTRACT The structural phase stability of heavy f-electron metals is studied by means of densityfunctional theory (DFT). These include temperature-induced transitions in plutonium metal as well as pressure-induced transitions in the trans-plutonium metals Am, Cm, Bk, and Cf. The early actinides (Th-Np) display phases that could be rather well understood from the competition of a crystal-symmetry breaking mechanism (Peierls distortion) of the 5f states and electrostatic forces, while for the trans-plutonium metals (Am-Cf) the ground-state structures are governed by 6d bonding. We show in this paper that new physics is needed to understand the phases of the actinides in the volume range of about 15-30 Å3. At these volumes one would expect, from theoretical arguments made in the past, to encounter highly complex crystal phases due to a Peierls distortion. Here we argue that the symmetry reduction associated with spin polarization can make higher symmetry phases competitive. Taking this into account, DFT is shown to describe the well-known phase diagram of plutonium and also the recently discovered complex and intriguing high-pressure phase diagrams of Am and Cm. The theory is further applied to investigate the behaviors of Bk and Cf under compression.
INTRODUCTION The understanding of the actinide metals is rapidly evolving due to recent progress in experimental and theoretical research. For some time now, it has been realized that many solidstate properties in these materials are determined by the bonding characteristics of the 5f orbitals, although a detailed understanding has been lacking. Certainly, the occurrence of complex crystal structures in the actinides, not present in other elements, seems to suggest that 5f bonding is the cause. The richness of phases and their detailed description as provided by experimental studies have proved to be an excellent testing ground for theory. In fact, DFT calculations were able to reproduce the structural behaviors of the light actinides and cerium very accurately [1]. The electronic-structure calculations of the actinides suggest that the low symmetry of especially the ground-state phases of uranium, neptunium, and plutonium is caused by the fact that 5f bands are pushed to unfavorably high and often degenerate energy levels when confined to high-symmetry configurations [2]. In the heavier actinides, Am and on, the 5f-band overlap is small at ambient conditions and poses a much smaller influence on the ground-state crystal structure which instead can be explained by 6d bonding in analogy to the rare-earth metals [3]. None of these two extreme situations of 5f-band involvement in the actinide bonding can be ascribed to the expanded phases of plutonium, or the moderately compressed phases of the transplutonium metals. Here we find a range of phases from high-sy
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