Effect of Sn substituting for Sb on the low-temperature transport properties of ytterbium-filled skutterudites

We examine the effect of alloying Sn on the Sb site of ytterbium-filled skutterudites, a promising class of thermoelectric materials. We report measurements of the Hall effect, electrical resistivity, Seebeck coefficient, and thermal conductivity between 2 and 300 K on two series of samples having different ytterbium filling fractions: ${\mathrm{Yb}}_{0.19}{\mathrm{Co}}_4{\mathrm{Sb}}_{12-x}{\mathrm{Sn}}_x,$ with $x=0,$ 0.05, 0.1, and 0.2, and ${\mathrm{Yb}}_{0.5}{\mathrm{Co}}_4{\mathrm{Sb}}_{12-x}{\mathrm{Sn}}_x,$ with $x=0.5,$ 0.6, 0.8, 0.83, and 0.9. We find that the substitution of Sn does not lower the electron concentration of these samples, but rather gives rise to a p-type carrier. Hall measurement data for ${\mathrm{Yb}}_{0.5}{\mathrm{Co}}_4{\mathrm{Sb}}_{11.17}{\mathrm{Sn}}_{0.83}$ can be understood in the context of two-carrier electrical conduction. Although the thermal conductivity of these ytterbium-filled skutterudites is significantly suppressed from that of the unfilled ${\mathrm{CoSb}}_3,$ it is nearly independent of the Sn concentration. The role of phonon-point defect scattering and phonon resonance scattering in reducing the lattice thermal conductivity of the Yb-filled samples is assessed by a theoretical model.