Phonon Anomalies and Electron-Lattice Coupling in Intermediate-ValenceSm0.25Y0.75S

The electronic structure and the phonon dispersion curves of EuS, SmS, ${\mathrm{Sm}}_{0.75}$ ${\mathrm{Y}}_{0.25}$ S, and YS are analyzed. A five-parameter model is deduced which describes the phonon dispersion of ${\mathrm{Sm}}_{0.75}$ ${\mathrm{Y}}_{0.25}$ S quite well. Two nearest-neighbor Sm-S forces and three deformabilities are introduced, two of which are attributed to virtual $f-d$ transitions with dipolar- and "breathing"-type symmetry while the third is a quadrupolar-type sulfur deformability. The model suggests a strongly localized form of the microscopic theory.