Inelastic neutron scattering measurements on several Tl filled skutterudites (${\mathrm{T}\mathrm{l}}_{0.5}{\mathrm{C}\mathrm{o}}_{3.5}{\mathrm{F}\mathrm{e}}_{0.5}{\mathrm{S}\mathrm{b}}_{12}$, ${\mathrm{T}\mathrm{l}}_{0.8}{\mathrm{C}\mathrm{o}}_3\mathrm{F}\mathrm{e}{\mathrm{S}\mathrm{b}}_{12}$, and ${\mathrm{T}\mathrm{l}}_{0.8}{\mathrm{C}\mathrm{o}}_4{\mathrm{S}\mathrm{b}}_{11}\mathrm{S}\mathrm{n}$) all show a sharp peak in the vibrational density of states at $57\pm 2\mathrm{K}$, which is absent in the unfilled skutterudite ${\mathrm{C}\mathrm{o}\mathrm{S}\mathrm{b}}_3$. Heat capacity measurements on ${\mathrm{T}\mathrm{l}}_{0.8}{\mathrm{C}\mathrm{o}}_4{\mathrm{S}\mathrm{b}}_{11}\mathrm{S}\mathrm{n}$ as compared to ${\mathrm{C}\mathrm{o}\mathrm{S}\mathrm{b}}_3$ are consistent with the presence of a localized vibrational mode associated with the “rattling” thallium atoms in this filled skutterudite compound. Both results are well described by a localized Einstein mode model with an Einstein temperature ${\Theta }_E$ of $53\pm 1\mathrm{K}$. These data provide perhaps the clearest example of local mode behavior in a concentrated metallic system.

• Received 20 December 2002

DOI:https://doi.org/10.1103/PhysRevLett.90.135505