Polycrystalline samples of clathrate compounds Ba8Ga16Si30, Ba8Ga16Ge30, Ba8Ga16Sn30, and Sr8Ga16Ge30 were prepared by direct melting and characterized using X-ray powder diffraction and differential thermal analysis. The Ge- and Si-based clathrates melt congruently, whereas Ba8Ga16Sn30 melts incongruently. At room temperature the Ge- and Si-based clathrates possess a moderate negative Seebeck coefficient and a high electron concentration in the range of 7×1020–9×1020 cm−3 while Ba8Ga16Sn30 exhibits substantially lower electron concentration of 2.2×1019 cm−3. The Seebeck coefficient and electrical resistivity were measured over the range 100–870 K. The temperature dependence of transport properties of the clathrates is typical for heavily doped semiconductors. The transport properties were analyzed using a standard semiconductor transport model. There is a good agreement between the assumed model and experimental temperature dependence of the Seebeck coefficient in the extrinsic conductivity range for all studied clathrates apart from Ba8Ga16Ge30. The calculated effective masses of the clathrates range from 0.9 to 3 of the free electron mass. The estimated ZT values are 0.7 for Ba8Ga16Ge30 at 700 K and 0.87 for Ba8Ga16Si30 at 870 K. The potential for thermoelectric applications of these materials is assessed.

Polycrystalline samples of the partially filled skutterudites NdxCo4Sb12 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 Nd0.1Co4Sb12. 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.

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Intermediate valence compound YbAl3 has been synthesized by the Bridgman method. Temperature dependence of the Seebeck coefficient, electrical resistivity and thermal conductivity has been measured on YbAl3 hot-pressed and single crystal samples. At temperatures below 100 K, the electrical resistivity exhibits a ρ(T) = ρ0 + AT2 dependence, while over the temperature range 300-800 K YbAl3 exhibits metallic behaviour with a linear temperature dependence of the electrical resistivity. The Seebeck coefficient exhibits a temperature dependence typical for heavy fermion compounds and attains a maximum of its absolute value of -79 µV K-1 at around 200-250 K. Combination of a large Seebeck coefficient and low residual resistivity of single crystalline YbAl3 leads to the highest ever reported electrical power factor of 340×10-6 W cm-1 K-2 at 80 K. Thermal conductivity as a function of temperature has a broad peak at 50 K, which originated mainly from the electronic contribution to thermal conductivity. At 300 K the total thermal conductivity of hot-pressed samples ranges from 0.085 to 0.179 W cm-1 K-1 depending on the density of the samples. The calculated figure of merit value attains its maximum of 1×10-3 K-1 at 100-120 K.