Filled skutterudite antimonides: Electron crystals and phonon glasses

Abstract

Crystallographic data, electrical and thermal transport measurements, and magnetic susceptibility values are reported for several compounds and alloys with the filled skutterudite structure, $R_{1-y}{\mathrm{Fe}}_{4-x}{\mathrm{Co}}_x{\mathrm{Sb}}_{12}$ ($R=\mathrm{La},$ Ce, or Th; $0<y<\mathrm{}1;$ $x=0,$1). Room-temperature velocity of sound data is also reported. These materials are of interest because of their potential in thermoelectric power generation and refrigeration applications. The transport properties of both filled and unfilled skutterudite compounds are analyzed using standard semiconductor transport models. Filled skutterudite antimonides appear to be a good approximation of an idealized solid with the good electrical transport properties of a crystal but the poor heat conduction characteristics of a glass. The incoherent rattling of the weakly bound rare-earth atoms in these materials lowers the thermal conductivity at room temperature to values comparable to that of vitreous silica. Relative to the analogous unfilled compounds, the filled skutterudites exhibit larger effective masses and smaller mobilities. Good overall electrical transport is maintained, however, as evidenced by values for the figure of merit $\mathrm{}\left(\mathrm{ZT}\right)\mathrm{}$ greater than 1 at elevated temperatures (700–1000 K). Above room temperature, there is very little difference in the electrical and thermal transport behavior between the La and Ce filled compounds. The effects of the hybridization caused by the proximity of the Ce $4f$ level to the Fermi energy, however, are evident at temperatures below 300 K.