Vibration-dominated negative mixing entropy for C impurities in α-U

Phonon densities of states were measured on pure uranium and solutions U-0.4% C, U-1.5% Si, and U-0.91% Fe (atomic) using inelastic neutron scattering. The solute atoms stiffened the phonons, resulting in large decreases in vibrational entropy. The vibrational entropy decrease for carbon was four times the configurational entropy increase, showing that the mixing entropy is not only negative but is dominated by vibrations. Comparison with single-crystal dispersion curves indicates that the phonon stiffening for all solutes involved the transverse optic branch propagating along (001) and displacing atoms along (010). The magnitudes of the changes were too large to be explained in terms of short-range force constant models but may originate with impurity pinning of collective modes associated with α-U's charge density wave transitions.