Half-Heusler compounds with a valence electron count of 18, including ZrNiSn, ZrCoSb, and NbFeSb, are good thermoelectric materials owing to favorable electronic structures. Previous computational studies had predicted a high electrical power factor in another half-Heusler compound NbCoSn, but it has not been extensively investigated experimentally. Herein, the synthesis, structural characterization, and thermoelectric properties of the heavy-element Pt-substituted NbCoSn compounds are reported. Pt is found to be an effective substitute enabling the optimization of electrical power factor and simultaneously leading to a strong point defect scattering of phonons and the suppression of lattice thermal conductivity. Post-annealing significantly improves the carrier mobility, which is ascribed to the decreased grain boundary scattering of electrons. As a result, a maximum power factor of ∼3.4 mW m⁻¹ K⁻² is obtained at 600 K. In conjunction with the reduced lattice thermal conductivity, a maximum figure of merit zT of ∼0.6 is achieved at 773 K for the post-annealed NbCo_0.95Pt_0.05Sn, an increase of 100% compared to that of NbCoSn. This work highlights the important roles that the dopant element and microstructure play in the thermoelectric properties of half-Heusler compounds.

La-filled skutterudites La_xCo₄Sb₁₂ (x = 0.25 and 0.5) have been synthesized and sintered in one step under high-pressure conditions at 3.5 GPa in a piston-cylinder hydrostatic press. The structural properties of the reaction products were characterized by synchrotron X-ray powder diffraction, clearly showing an uneven filling factor of the skutterudite phases, confirmed by transmission electron microscopy. The non-homogeneous distribution of La filling atoms is adequate to produce a significant decrease in lattice thermal conductivity, mainly due to strain field scattering of high-energy phonons. Furthermore, the lanthanum filler primarily acts as an Einstein-like vibrational mode having a strong impact on the phonon scattering. Extra-low thermal conductivity values of 2.39 W m⁻¹ K⁻¹ and 1.30 W m⁻¹ K⁻¹ are measured for La_0.25Co₄Sb₁₂ and La_0.5Co₄Sb₁₂ nominal compositions at 780 K, respectively. Besides this, lanthanum atoms have contributed to increase the charge carrier concentration in the samples. In the case of La_0.25Co₄Sb₁₂, there is an enhancement of the power factor and an improvement of the thermoelectric properties.

SnSe has been recently reported as an attractive thermoelectric material, with an extraordinarily high, positive, Seebeck coefficient. Here, we describe the synthesis, structural, microscopic, and thermoelectric characterization of Sn_1–xSb_xSe intermetallic alloys prepared by a straightforward arc-melting technique. Sb-doped tin selenide was synthesized as strongly nanostructured polycrystalline pellets. Neutron diffraction studies reveal that Sb is placed at the Sn sublattice in the crystal structure, showing concentrations as high as 30%, and generates a significant number of Sn vacancies, while the increase of the interlayer distances favors the nanostructuration. The material is nanostructured both out-of-plane in nanometer-scale layers and in-plane by ∼5 nm undulations of these layers. This nanostructuring, along with an increased amount of Sn vacancies, accounts for a reduction of the thermal conductivity, which is highly desirable for thermoelectric materials. The phonon mean free path is estimated to be on the order of 2 nm from low temperature, thermal conductivity, and specific heat, in accordance with the nanostructuration observed by high-resolution transmission electron microscopy. The thermal conductivity of SnSe is characterized by three independent techniques to assure a room temperature value of Sn_0.8Sb_0.2Se of κ ∼ 0.6 W/m K. The freshly prepared Sb-doped compounds exhibit an abrupt change in the type of charge carriers, leading to large, negative Seebeck coefficients, although the arc-melt synthesized pellets remain too resistive for thermoelectric applications. Cold-pressed pellets evolve to be p-type at room temperature, but reproducibly turn n-type around 500 K, with increased electrical conductivity and maximum observed figure of merit, ZT ∼ 0.3 at 908 K.