Large harmonic softening of the phonon density of states of uranium

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.