Phonon density of states of α - aandδ -plutonium by inelastic x-ray scattering

Inelastic x-ray scattering measurements of the phonon density of states (DOS) were performed on polycrystalline samples of pure α-Pu and δ -Pu0.98 Ga0.02 at room temperature. The heat capacity of α-Pu is well reproduced by contributions calculated from the measured phonon DOS plus conventional thermal-expansion and electronic contributions, showing that α-Pu is a "well-behaved" metal in this regard. A comparison of the phonon DOS of the two phases at room temperature showed that the vibrational entropy difference between them is only a quarter of the total entropy difference expected from known thermodynamic measurements. The missing entropy is too large to be accounted for by conventional electronic entropy and evidence from the literature rules out a contribution from spin fluctuations. Possible alternative sources for the missing entropy are discussed.