Soft-mode behavior in the phonon dispersion of YS

We have measured the phonon dispersion of the superconducting transition-metal chalcogenide YS by inelastic neutron scattering. The longitudinal branches exhibit well-defined soft-mode regions that closely resemble those found in the structural and electronic analogs NbC and TiN. In contrast, no softening is observed in the transverseacoustic branches. The experimental dispersion curves are well reproduced by calculations based on a double-shell model. © 1978 The American Physical Society.