Lattice dynamics ofα-uranium

The room-temperature lattice dynamics of $\alpha$-uranium has been investigated. The phonon dispersion curves along the orthorhombic high-symmetry directions have been measured using inelastic neutron scattering techniques. The dispersion curves in the $\left[0\mathrm{}\zeta 0\right]$ and $\left[00\mathrm{}\zeta \right]$ directions are not very unusual. However, a depression and pronounced dips are observed in the $\left[\zeta 00\right]$ direction. The slopes of the long-wavelength acoustic branches are in agreement with the slopes predicted from the elastic constants. A four-neighbor Born-von Kármán general-tensor model, a 12-neighbor axially symmetric model, a simple shell model, and a simple Heine-Abarenkov pseudopotential model are unable to satisfactorily reproduce some of the $\left[\zeta 00\right]$ branches. However, a modified form of the shell model which includes forces to six neighbors is found to reproduce most of the branches well, including those in the $\left[\zeta 00\right]$ direction. A preliminary low-temperature experiment has revealed extra elastic scattering as well as significant phonon softening along the $\left[\zeta 00\right]$ direction.