Phonon dispersion in intermediate-valenceSm0.75Y0.25S

Phonon dispersion has been measured in an intermediate-valence-state system. The phonons of intermediate valence ${\mathrm{Sm}}_{0.75}$ ${\mathrm{Y}}_{0.25}$S are vastly different from those of the SmS and YS constituents. The LA phonons are unusually soft compared to the TA and have anomalous width, particularly for the [111] direction. The LO phonons lie below the TO. Temperature-dependent measurements show that the anomalous phonon properties change with changing valence.