Crystal dynamics of<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>δ</mml:mi></mml:math>fcc Pu-Ga alloy by high-resolution inelastic x-ray scattering

Abstract

We have used a microbeam on large grain sample concept to carry out inelastic x-ray scattering experiments to measure the phonon dispersion curves of a fcc $\ensuremath{\delta}$-phase Pu-Ga alloy along the main symmetry directions of the cubic lattice. This approach obviates experimental difficulties with conventional inelastic neutron scattering due to the high absorption cross section of the common $^{239}\mathrm{Pu}$ isotope and the nonavailability of large (millimeter size) single crystal materials for Pu and its alloys. A classical Born-von K\'arm\'an force constant model was used to model the experimental results, and up to fourth nearest neighbor interactions had to be included to obtain sufficient agreement. Several unusual features including, a large elastic anisotropy, a small shear elastic modulus $({C}_{11}\ensuremath{-}{C}_{12})∕2$, a positive kink in the ${T}_{1}[0\ensuremath{\xi}\ensuremath{\xi}]$ branch, and a pronounced bending (toward lower energy) of the $T[\ensuremath{\xi}\ensuremath{\xi}\ensuremath{\xi}]$ branch near the $L$ point in the Brillouin zone are found. These features are discussed in light of the various phase transformations of $\ensuremath{\delta}$ plutonium. The phonon dispersion data also provide a critical test and benchmark for theoretical treatments of highly correlated $5f$ electron systems.