  The phonon dispersion curve of SmS under pressure was studied by inelastic
x-ray scattering around the pressure-induced valence transition. A significant
softening of the longitudinal acoustic modes propagating along the [111]
direction was observed spanning a wide $q$ region from
($\frac{2\pi}{3a},\frac{2\pi}{3a},\frac{2\pi}{3a}$) up to the zone boundary as
SmS becomes metallic. The largest softening occurs at the zone boundary and
stays stable up to the highest measured pressure of 80 kbar while a gradual
hardening of the low $q$ modes simultaneously appears. This phonon spectrum
indicates favorable conditions for the emergence of pressure-induced
superconductivity in SmS.
