Softening phonon and relaxation mode in the filled skutterudite PrT4Sb12(T = Ru and Os)

Dynamics of rare-earth ions in filled skutterudite compounds PrT4Sb12 (T = Ru and Os) has been investigated by a neutron inelastic scattering technique. We found an optical mode located at extremely low energy around 4 meV exhibiting apparent softening behavior by 10 - 30% from 300 to 10 K. This softening mode is attributed to the motion of the filled Pr ions bound in anharmonic potential within the Sb cage. In addition, quasielastic responses near the Brillouin zone center indicate relaxation modes. These features can be explained in terms of a mode coupling scenario involving the motion of the filled Pr ions with huge amplitude.