Phonon densities of states of γ-cerium and δ-cerium measured by time-of-flight inelastic neutron scattering

Time-of-flight inelastic neutron scattering spectra were measured on cerium metal at temperatures near the fcc (γ)-to-bcc (δ) transition temperature. Phonon density-of-states (DOS) curves were extracted from data acquired over a wide range of momentum transfers. A large softening of the phonon DOS was found in going from γ-cerium to δ-cerium, and this accounts for an increase in vibrational entropy of (0.71 ± 0.05)k_B/atom. To be consistent with the latent heat of the γ-δ transition, this increase in vibrational entropy must be accompanied by a large decrease in electronic entropy. The results not only confirm the recent discovery of a significant electronic contribution to the γ-δ transition but also suggest that it may be twice that previously reported.