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 ( approximately 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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A bstractWe show that the anomalous thermal expansion behaviour of Pu±Ga alloys can be explained by an Invar e ect, which is modelled by assuming the thermal transfer of Pu atoms from a lower-energy higher-volume d 1 state to a higherenergy lower-volume d 2 state. The energy separation of these states is found to be 1400 K from a ®t to the available neutron di raction data on the lattice constants of Pu±Ga alloys. The Invar model enables a precise evaluation of the coe cient of thermal expansion and of the GruÈ neisen constant. The addition of Ga forces a stabilization of the d 2 state and suppresses the negative thermal expansion.