Thermal conductivity, thermoelectric power, electrical resistivity, Hall coefficient, and magnetic susceptibility of the filled skutterudites ${\mathrm{CeFe}}_{4\mathrm{-}\mathrm{x}}$ ${\mathrm{Co}}_{\mathrm{x}}$ ${\mathrm{Sb}}_{12}$ , with x=0, 0.5, 1.0, 1.5, and 2.0, have been studied from 2 to 300 K. We find that the substitution of Co at Fe sites has a dramatic effect on all transport properties. While the resistivity of ${\mathrm{CeFe}}_4$ ${\mathrm{Sb}}_{12}$ has a metallic character, substitution of Co leads to a progressively stronger activated behavior and a decrease in hole concentration. The thermopower increases with increasing Co, while the thermal conductivity is depressed, notably at low temperatures. Susceptibility data suggest the presence of large effective moments. At high temperatures Ce is nearly trivalent, but valence fluctuations prevail at low temperatures. Strong hybridization of the Ce 4f states with the Fe 3d and pnicogen p states appears to be important in understanding the physical properties of these compounds.

• Received 30 August 1996

DOI:https://doi.org/10.1103/PhysRevB.55.1476