No role for phonon entropy in the fcc -> fcc volume collapse transition in Ce0.9Th0.1 at ambient pressure

Phonon densities of states (DOS) were obtained from inelastic neutron scattering measurements on Ce0.9Th0.1 at temperatures from 10 to 300 K. The alpha phase showed a significant softening of its phonon DOS when heated from 10 to 140 K. Despite the 17% volume collapse, the phonon DOS showed little change between the gamma phase at 150 K and the alpha phase at 140 K. This is supported by analysis of the magnetic spectra showing that most of the transition entropy can be accounted for with the crystal field and changes in the ground-state spin fluctuations. We argue that the anomalous behavior of the phonon DOS originates with the volume dependence of the ground-state spin fluctuations.