Phonon frequencies and linewidths for Sm(Y)S

The monopolar charge fluctuation model of lattice dynamics has been extended by including the relaxation in the electronic system. The extension introduces wave-vector-dependent lifetimes only in the longitudinal phonons. This phenomenological model involving eight parameters has been applied to the analysis of the neutron scattering data on the phonons of the mixed-valence compound, ${\mathrm{Sm}}_{0.75}$ ${\mathrm{Y}}_{0.25}$S. The calculated phonon frequencies and linewidths are in good agreement with the data. The relaxation time has been found to be about 4×${10}^{-14\mathrm{}}$ sec which seems to be consistent with the narrow bandwidth for the $f$ electrons in this compound.