Large harmonic softening of the phonon density of states of uranium.
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 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 alpha-beta and beta-gamma phase transitions, vibrational and electronic entropies were comparable.
Manley
M E
ME
California Institute of Technology, Pasadena 91125, USA.
Fultz
B
B
McQueeney
R J
RJ
Brown
C M
CM
Hults
W L
WL
Smith
J L
JL
Thoma
D J
DJ
Osborn
R
R
Robertson
J L
JL