Lattice dynamics of rare-earth semiconductors with unstable valence

The phonon spectra of rare-earth (RE) semiconductors with integer and mixed valence (EuS, SmS, TmSe) are calculated within the generalized charge-density distortion model. The model includes interaction of lattice vibrations with the valence shell excitations, both local (usual dipole excitations in unfilled shells of RE ions) and non-local breathing modes describing the valence fluctuations. The relative contribution of these excitations to phonon renormalization is traced along the row of RE semiconductors with rock-salt structure.