Large Harmonic Softening of the Phonon Density of States of Uranium

Abstract

Phonon density-of-states curves were obtained from inelastic neutron scattering spectra from the three crystalline phases of uranium at temperatures from 50 to 1213 K. The $\ensuremath{\alpha}$-phase showed an unusually large thermal softening of phonon frequencies. Analysis of the vibrational power spectrum showed that this phonon softening originates with the softening of a harmonic solid, as opposed to vibrations in anharmonic potentials. It follows that thermal excitations of electronic states are more significant thermodynamically than are the classical volume effects. For the $\ensuremath{\alpha}\ensuremath{-}\ensuremath{\beta}$ and $\ensuremath{\beta}\ensuremath{-}\ensuremath{\gamma}$ phase transitions, vibrational and electronic entropies were comparable.