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College Park, Maryland      June 6 - 10 , 2004

T2-D5 (2:45 PM): Unusual phonon softening in delta-phase plutonium (Invited)

R. J. McQueeney (Iowa State University/Ames Laboratory), A. C. Lawson, A. Migliori (Los Alamos National Laboratory), T. M. Kelley, B. Fultz (California Institute of Technology), J. C. Lashley (Los Alamos National Laboratory)

The phonon density-of-states and adiabatic sound velocities were measured on fcc-stabilized 242Pu0.95Al0.05. The phonon frequencies and sound velocities decrease considerably (soften) with increasing temperature despite negligible thermal expansion. The frequency softening of the transverse branch along the (111) direction is anomalously large (~ 30 %) and is very sensitive to alloy composition. The large magnitude of the phonon softening is not observed in any other fcc metals and may arise from an unusual temperature dependence of the electronic structure in this narrow 5f-band metal.

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