The magic of plutonium: 5 f electrons and phase instability
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7/10/04
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The Magic of Plutonium: 5f Electrons and Phase Instability S.S. HECKER This article was presented at a symposium held in honor of Terence E. (Terry) Mitchell for his many contributions to materials science. Terry Mitchell has explored the magic of materials during his distinguished 40-plus year career, always going back to the fundamentals to understand the behavior of complex materials. He has covered a wide range of materials from metals and alloys, to ceramics, intermetallics, strained-layer superlattices, high-temperature superconductors, and nanostructured materials. In this tribute to the remarkable contributions of Terry Mitchell to the materials field, I will cover the magical world of plutonium—one of the few materials that has escaped his scrutiny to date.
I. INTRODUCTION
PLUTONIUM is a notoriously unstable metal—with little provocation, it can change its density by as much as 25 pct; it can be as brittle as glass or as malleable as aluminum; it expands when it solidifies; and its silvery freshly machined surface will tarnish in minutes, producing nearly every color in the rainbow. In addition, plutonium’s continuous radioactive decay causes self-irradiation damage that can fundamentally change its properties over time. Plutonium lies near the middle of the actinide series. It is of practical interest principally because the 239 isotope has attractive nuclear properties for energy production and nuclear explosives. It is plutonium’s structural properties, however, which are determined by its electronic rather than its nuclear structure, that are particularly unusual. The thermal instability of plutonium—that is, the large length (or volume) changes during heating or cooling, as shown in Figure 1—is an important consequence of the unusual properties of plutonium.[1] The huge volume changes in the solid state result primarily from structural transformations among an unprecedented six solid allotropes. The ambient-temperature phase of plutonium is a low-symmetry monoclinic crystal lattice, more typical of minerals than of metals. The high-symmetry, closepacked face-centered-cubic (fcc) phase exhibits the lowest density. The liquid is denser than the high-temperature solid phases resulting in contraction upon melting. The behavior of plutonium defies conventional metallurgical wisdom. One must turn to the peculiar electronic structure of the actinides to gain insight into plutonium. In solids, the valence electrons of the isolated atomic states occupy the conduction band and govern bonding. The actinides mark the filling of the 5f atomic subshell much like the rare earths mark the filling of the 4f subshell (Figure 2(a)). Yet, the 5f electrons of the light actinides behave more like the 5d electrons of the transition metals than the 4f electrons of the rare earths. Moreover, the 7s, 6d, and 5f electrons, all of nearly equivalent energies, form overlapping (hybridized) bands. The dominant band controls the bonding and resulting S.S. HECKER, Senior Fellow, is with the Materials
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