Optical properties ofMFe4P12filled skutterudites

Infrared reflectance spectroscopy measurements were made on four members of the $M{\mathrm{Fe}}_4{\mathrm{P}}_{12}$ family of filled skutterudites, with $M=\mathrm{La},$ Th, Ce, and U. In progressing from $M=\mathrm{La}$ to U the system undergoes a metal-insulator transition. It is shown that, although the filling atom induces such dramatic changes in the transport properties of the system, it has only a small effect on lattice dynamics. We discuss this property of the compounds in the context of their possible thermoelectric applications.