The low-energy phonon modes of the intermediate-valence system CeNi and the isostructural reference compound LaNi have been measured at room temperature by inelastic neutron scattering in the scattering planes (001) and (100). In general, the frequencies of the acoustic and lower optical-phonon branches are found to be appreciably lower in CeNi. This effect is especially dramatic for the acoustic modes propagating along the $a$ and $b$ directions. Maximum softening was observed near the zone boundary, where it can exceed 20% for some of the branches. These data represent the most pronounced effect of electron-lattice coupling in a Ce-based intermediate-valence compound that we are aware of.

• Received 15 December 1997

DOI:https://doi.org/10.1103/PhysRevB.57.R8099