Probing Phonons in Plutonium
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DD2.2.1
Probing Phonons in Plutonium Joe Wong1,*, Michael Krisch2, Daniel L. Farber1, Florent Occelli1, Adam J. Schwartz1, TaiC. Chiang3 , Mark Wall1, Carl Boro1, and Ruqing Xu3 1
Lawrence Livermore National Laboratory, University of California, PO Box 808, Livermore, CA 94551, USA
2
European Synchrotron Radiation Facility, BP 220, F-38043 Grenoble Cedex, France 3
Department of Physics and Frederick Seitz Materials Research Laboratory, University of Illinois at Urbana-Champaign, , Urbana, IL 61801, USA
Plutonium (Pu) is well known to have complex and unique physico-chemical properties [1]. Notably, the pure metal exhibits six solid-state phase transformations with large volume expansions and contractions along the way to the liquid state: α → β→ γ → δ→ δ’→ ε→ liquid. Unalloyed Pu melts at a relatively low temperature ~640oC to yield a higher density liquid than that of the solid from which it melts, (Figure 1). Detailed understanding of the properties of plutonium and plutonium-based alloys is critical for the safe handling, utilization, and long-term storage of these important, but highly toxic materials. However, both technical and and safety issues have made experimental observations extremely difficult.
Pu Crystal Structure
Density (g/cc)
α Simple Monoclinic
19.86
β Body-Centered Monoclinic
17.70
γ
17.14
Face-Centered Orthorhombic
δ Face-Centred Cubic
15.92
δ’ Body-Centered Tetragonal
16.00
ε Body-Centered cubic
16.51
L Liquid
16.65
Figure 1. Transformations in Pu to different crystal structures are accompanied by very large volume changes. Alloying with Ga or Al avoids the transformation to γ, β and α and stabilizes the δ phase all the way to room temperature and below [1].
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Phonon dispersion curves (PDCs) are key experimental data to the understanding of the basic properties of Pu materials such as: force constants, sound velocities, elastic constants, thermodynamics, phase stability, electron-phonon coupling, structural relaxation, etc. However, phonon dispersion curves (PDCs) in plutonium (Pu) and its alloys have defied measurement for the past few decades since the discovery of this element in 1941. This is due to a combination of the high thermal-neutron absorption cross section of plutonium and the inability to grow the large single crystals (with dimensions of a few millimeters) necessary for inelastic neutron scattering. Theoretical simulations of the Pu PDC continue to be hampered by the lack of suitable inter-atomic potentials. Thus, until recently the PDCs for Pu and its alloys have remained unknown experimentally and theoretically. The experimental limitations have recently been overcome by using a tightly focused undulator x-ray micro-beam scattered from single-grain domains in polycrystalline specimens. This experimental approach has been applied successfully to map the complete PDCs of an fcc δ-Pu-Ga alloy using the high resolution inelastic x-ray scattering (HRIXS) capability on ID28 [2]. The complete PDCs for an fcc Pu-0.6 wt% Ga alloy are plotted in Figure 2,
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