Study of Phonons in $\delta $-Plutonium near the $\delta -\alpha $' Structural Phase Transition by X-ray Thermal Diffuse Scattering

The 5f electrons in Pu can be either bonding or localized, depending sensitively on the temperature, pressure, and impurity doping. As a result, Pu displays a rich phase diagram involving a large number of phases with substantially different atomic volumes. In a recent report of the phonon dispersion curves of Ga-stablized $\delta $-Pu at room temperature and ambient pressure, a pronounced deepening of the TA[111] phonon branch near the L point was discovered. This phonon softening was suggested to be related to a lattice shearing mechanism that could lead to the structural phase transition from the fcc $\delta $ phase to the monoclinic $\alpha $' phase at about 170 K. Here we report our measurements of x-ray thermal diffuse scattering from a $\delta $-Pu crystal (with 0.6 wt{\%} Ga) at temperatures from 307K to 200K. The results show no further softening of the phonons near the L point as the sample temperature decreases. The implications regarding the relationship between the soft mode and the phase transition will be discussed.