Softening of Phonon by Filled Rare-Earth Ion Motion Common to Skutterudite with Sb Cages

We investigate phonon modes of the rare-earth filled skutterudite compounds CeRu ... Sb ... , CeOs ... Sb ... , PrRu ... Sb ... , and PrOs ... Sb ... by using an inelastic neutron scattering technique. These materials exhibit similar low-lying motions of filled ions inside the Sb cages that softens with decreasing temperature. Potentials at the rare-earth sites are considerably anharmonic. Despite that this behavior appears in all the four materials, magnitude of the softening of PrOs ... Sb ... is largest among them. This fact implies that difference of carrier state gives rise to renormalization effect on the rare-earth-ion vibrations. (ProQuest: ... denotes formulae/symbols omitted.)