Unusual Phonon Softening in<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>δ</mml:mi></mml:math>-Phase Plutonium

Abstract

The phonon density of states and adiabatic sound velocities were measured on fcc-stabilized $^{242}\mathrm{Pu}_{0.95}{\mathrm{Al}}_{0.05}$. 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 (\ensuremath{\sim}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.