ScienceWeek

CHEMICAL PHYSICS: ON PLUTONIUM

The following points are made by Gerard H. Lander (Science 2003 301:1057):

1) Plutonium (Pu) is one of the elements best known to the general public. Its notoriety stems from the element's nuclear properties: It emits alpha particles, leading to its radiotoxicity, and is able to undergo fission with thermal neutrons, leading to its use in nuclear weapons.

2) But plutonium has another, more benign, face. Its place in the periodic table imbues it with exceptional electronic properties that still defy a clear explanation 60 years after they were first discovered.(1)

3) Plutonium is midway across the row called the actinides in which the 5f electron shell is progressively filled. In the early part of the actinide series, the 5f electron states contribute to the bonding between atoms. Here, the atomic volume dependence on electron count resembles that of a transition metal series. In contrast, the heavier actinides [americium (Am) and beyond] have larger atomic volumes that depend little on the electron count. These properties signal a rare earth-like behavior; the 5f states are localized and do not participate in the bonding.

4) In plutonium, the 5f electrons are "on the edge", and it is this unique situation that gives rise to a plethora of unusual properties (3). Plutonium goes through six different phases before it melts, more than any other element. The simple face-centered cubic (fcc) phase is stable at high temperatures and may be stabilized at room temperature by adding a small amount (less than 1 weight percent) of gallium.

5) The fact that plutonium is radioactive is a great impediment to performing experiments on this element. Despite 60 years of effort, knowledge is limited. For example, the frequencies of its elementary excitations (or phonons) are poorly known. Many elastic and thermodynamic properties depend on the phonons. These properties are known for almost all other elements and many compounds, mainly from neutron inelastic scattering. For plutonium, however, neutrons are not the answer, because the technique requires relatively large single crystals [at least 100 mm^(3)]. Furthermore, special isotopes must be used to avoid the high absorption of Pu-239.(4,5)

References (abridged):

1. X. Dai et al., Science 300, 953 (2003)

2. J. Wong et al., Science 301, 1078 (2003)

3. S. S. Hecker, MRS Bull. 26, 872 (2001)

4. H. M. Ledbetter, R. L. Moment, Acta Metallurg. 24, 891 (1976)

5. G. H. Lander et al., Adv. Phys. 43, 1 (1994)

Science http://www.sciencemag.org

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PLUTONIUM CHARACTERISTICS

Plutonium-238, with a half-life of 87.7 years, is utilized in heat sources for space application, and has been used for heart pacemakers. Plutonium-239 is used as a nuclear fuel, in the production of radioactive isotopes for research, and as the fissile agent in nuclear weapons.

Plutonium exhibits a variety of valence states in solution and in the solid state. Plutonium metal is highly electropositive. Numerous alloys of plutonium have been prepared, and a large number of intermetallic compounds have been characterized.

Reaction of the metal with hydrogen yields two hydrides. The hydrides are formed at temperatures as low as 150°C (300°F). Their decomposition above 750°C (1400°F) may be used to prepare reactive plutonium powder. The most common oxide is PuO2, which is formed by ignition of hydroxides, oxalates, peroxides, and nitrates of any oxidation state in air of 870-1200°C (1600-2200°F). A very important class of plutonium compounds are the halides and oxyhalides. Plutonium hexafluoride, the most volatile plutonium compound known, is a strong fluorinating agent. A number of other binary compounds are known. Among these are the carbides, silicides, sulfides, and selenides, which are of particular interest because of their refractory nature.

Because of its radiotoxicity, plutonium and its compounds require special handling techniques to prevent ingestion or inhalation. Therefore, all work with plutonium and its compounds must be carried out inside glove boxes. For work with plutonium and its alloys, which are attacked by moisture and by atmospheric gases, these boxes may be filled with helium or argon.

Adapted from: Fritz Weigel in: Concise Encyclopedia of Science & Technology. 3rd Edition. McGraw-Hill 1994, p.1463 More information at: http://www.amazon.com/exec/obidos/ASIN/0070455600/scienceweek

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CALCULATED PHONON SPECTRA OF PLUTONIUM AT HIGH TEMPERATURES

The following points are made by X. Dai et al (Science 2003 300:953):

1) Plutonium (Pu) is a material with very unusual solid-state properties. Despite its scientific and technological importance, many of its key properties, such as the spectrum of lattice vibrations, remain uninvestigated. It has not been possible to measure that spectrum experimentally because of Pu's extreme toxicity and radioactivity. It has not been possible to compute the spectrum theoretically, because Pu is strongly correlated, and the traditional electronic structure methods fail to describe it even qualitatively. These studies are, however, essential to be able to address the factors that govern the lattice stability of Pu, an issue that is important for Pu's storage and disposal over long time scales.

2) Pu has six crystallographic allotropes with puzzling volume variations among them (1). Starting from the low-temperature structure with 16 atoms in the elementary cell, it undergoes a series of phase transitions ending in relatively simple face-centered cubic (fcc) (delta) and body-centered cubic (bcc) (epsilon) phases at temperatures greater than 500 K. The 25% volume increase during the transition from alpha to delta is followed by a volume contraction upon further heating (2) through the delta-epsilon transition, occurring by way of an intermediate body center tetragonal delta' phase, which exists in a very narrow temperature interval. It is the unusual behavior of both the electronic and lattice degrees of freedom that determines the rich phase diagram of Pu.

3) The experimental information about the lattice dynamical properties of this element is very limited. Pu has relatively soft elastic constants and a Debye temperature near 100 K (3). Using this information, phenomenological studies of the thermodynamics of Pu have been carried out (4). However, the role phonons play in the thermodynamics of Pu is unclear because of the lack of appropriate theoretical and experimental tools to study its lattice-dynamical properties.

4) In summary: The authors constructed computer-based simulations of the lattice dynamical properties of plutonium using an electronic structure method, which incorporates correlation effects among the f-shell electrons and calculates phonon spectra at arbitrary wavelengths. The predicted spectrum for the face-centered cubic delta phase agrees well with experiments in the elastic limit and explains unusually large shear anisotropy of this material. The spectrum of the body-centered cubic phase shows an instability at zero temperature over a broad region of the wave vectors, indicating that this phase is highly anharmonic and can be stabilized at high temperatures by its phonon entropy.(5)

References (abridged):

1. For a review, see A. J. Freeman, J. B. Darby, Eds., The Actinides: Electronic Structure and Related Properties (Academic Press, New York, 1974), vols. 1 and 2

2. S. S. Hecker, L. F. Timofeeva, Los Alamos Sci. 26, 244 (2000)

3. A. Migliori, J. Baiardo, L. T. Darling, Los Alamos Sci. 26, 208 (2000)

4. D. C. Wallace, Phys. Rev. B 58, 15433 (1998)

5. S. Y. Savrasov, G. Kotliar, E. Abrahams, Nature 410, 793 (2001)

Science http://www.sciencemag.org

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PHONON DISPERSIONS OF FCC DELTA-PLUTONIUM-GALLIUM BY INELASTIC X-RAY SCATTERING

The following points are made by Joe Wong et al (Science 2003 301:1078):

1) Plutonium (Pu) is arguably the most complex metallic element known and has attracted extraordinary scientific interest since its discovery in 1941. Further, detailed understanding of the properties of Pu and its alloys is critical for the safe handling, use, and long-term storage of these important, but highly toxic, materials. However, both technical and safety issues have made experimental observations extremely difficult.

2) Pu is well known to have complex and often unique physical properties (1,2). Notably, the pure metal exhibits six solid-state phase transformations with large volume expansions and contractions between the room-temperature stable alpha-phase and the liquid state: alpha to beta to gamma to delta to delta' to epsilon to liquid. Furthermore, unalloyed Pu melts at a relatively low temperature, 640°C, to yield a liquid of higher density than the solid from which it melts. The face-centered cubic (fcc) delta-phase with density approximately 15.92 g/cm3 is 20% lower in density than the monoclinic alpha-phase (19.86 g/cm3). Pu can be retained in the highly symmetric fcc structure at room temperature by alloying with small amounts of Group III metals such as aluminum (Al) or gallium (Ga). In doing so, the metastable delta-phase field is expanded from high temperature to room temperature and below at the expense of the gamma and beta phases (3,4), suggesting very similar ground-state energies for these structures (5).

3) In summary: The authors report an experimental determination of the phonon dispersion curves in a face-centered cubic (fcc) delta-plutonium–0.6 weight % gallium alloy. Several unusual features, including a large elastic anisotropy, a small-shear elastic modulus C', a Kohn-like anomaly in the T1[011] branch, and a pronounced softening of the [111] transverse modes, are found. These features can be related to the phase transitions of plutonium and to strong coupling between the lattice structure and the 5f valence instabilities. The authors suggest their results also provide a critical test for theoretical treatments of highly correlated 5f electron systems as exemplified by recent dynamical mean field theory calculations for delta-plutonium.

References (abridged):

1. For a review, see N. G. Cooper, Ed., Los Alamos Sci. 26 (2000)

2. R. Jeanloz, Phys. Today 12, 44 (2000)

3. S. S. Hecker, L. F. Timofeeva, Los Alamos Sci. 26, 244 (2000)

4. S. S Hecker, Los Alamos Sci. 26, 290 (2000)

5. S. Y. Savrasov, G. Kotliar, E. Abrahams, Nature 410, 793 (2001)

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