Phonon density-of-states curves were obtained from inelastic neutron scatte
ring spectra from the three crystalline phases of uranium at temperatures f
rom 50 to 1213 K. The alpha -phase showed an unusually large thermal soften
ing of phonon frequencies. Analysis of the vibrational power spectrum showe
d that this phonon softening originates with the softening of a harmonic so
lid, as opposed to vibrations in anharmonic potentials. It follows that the
rmal excitations of electronic states are more significant thermodynamicall
y than are the classical volume effects. For the alpha-beta and beta-gamma
phase transitions, vibrational and electronic entropies were comparable.