Neutron-scattering investigation of the phonons in intermediate-valenceSm0.75Y0.25S

Neutron-scattering techniques have been used to measure the phonons in intermediate-valence ${\mathrm{Sm}}_{0.75}$ ${\mathrm{Y}}_{0.25}$S. Anomalies are found in the shape of the phonon dispersion curves and in the phonon widths that are attributable to mixed-valence effects. Phonon positions and widths are shown as a function of temperature. The greatest phonon softening and the largest phonon widths are found at the valence transition that occurs at 200 K. A screened-rigid-ion model has been used to fit the phonon dispersion curves. The model reproduces the general shape of the phonon dispersion data but fails to account for the anomalies.