Effect of partial void filling on the transport properties of Nd_xCo₄Sb₁₂ skutterudites

Polycrystalline samples of the partially filled skutterudites Nd_xCo₄Sb₁₂ have been prepared and characterized by x-ray powder diffraction and differential thermal analysis. The saturation limit of the Nd void filling in CoSb₃ was found to be around 13%. All samples decompose incongruently at a temperature of 1149 ± 6 K. Room temperature Hall measurements show that each Nd atom donates approximately 0.8 electrons, which is significantly less than the Nd oxidation state (3+). The temperature dependence of the electrical and thermal transport properties has been measured over the range of 11–700 K. The electrical resistivity and absolute value of the Seebeck coefficient decrease with increasing Nd content and for samples with x > 0.02 the temperature dependence is typical of heavily doped semiconductors. Filling CoSb₃ with Nd causes a rapid initial decrease in the lattice thermal conductivity with a minimum at the composition Nd_0.1Co₄Sb₁₂. Nd-filled skutterudites exhibit the lowest value of the lattice thermal conductivity in comparison with other partially filled skutterudites at x < 0.1, which could be attributed to a smaller radius of Nd than that of other filling elements. At high temperature the ZT value of the Nd-filled skutterudites is limited due to intrinsic conduction caused by the relatively low carrier concentration. The effect of the partial Nd filling on the transport properties of the filled skutterudite compounds is discussed in the context of potential thermoelectric materials.