S. Sassi

Assessment of the thermoelectric performance of polycrystalline p-type SnSe

We report the evaluation of the thermoelectric performance of polycrystalline p-type SnSe, a material in which unprecedented values of the thermoelectric figure of merit ZT have been recently discovered in single crystals. Besides anisotropic transport properties, our results confirm that this compound exhibits intrinsically very low thermal conductivity values. The electrical properties show trends typical of lightly doped, intrinsic semiconductors with thermopower values reaching 500 μV K−1 in a broad temperature range. An orthorhombic-to-orthorhombic transition sets in at 823 K, a temperature at which the power factor reaches its maximum value. A maximum ZT of 0.5 was obtained at 823 K, suggesting that proper optimization of the transport properties of SnSe might lead to higher ZT values. These findings indicate that this system represents an interesting experimental platform for the search of highly efficient thermoelectric materials.

Reinvestigation of the thermal properties of single-crystalline SnSe

The simple binary SnSe has been recently proposed as a prospective candidate for thermoelectric applications due to its exceptionally low lattice thermal conductivity. However, the thermal transport in single crystals was found to be significantly lower than in polycrystalline samples despite the presence of grain boundary scattering in the latter. In order to better understand the origin of this issue, we report here on a detailed characterization of the thermoelectric properties of a vertical-Bridgman-grown single-crystal of SnSe along the a, b, and c crystallographic axes in a wide range of temperatures (5–700 K). We find that the thermal conductivity features a pronounced Umklapp peak near 12 K whose magnitude depends on the crystal orientation. Unlike prior reports, our results evidence a significant anisotropy between the a, b, and c directions with lattice thermal conductivity values reaching 1.2, 2.3, and 1.7 W m−1 K−1 at 300 K, respectively. While the fundamental reasons behind these differences ...

Crystal Structure and Transport Properties of the Homologous Compounds (PbSe)5(Bi2Se3)3m (m = 2, 3)

We report on a detailed investigation of the crystal structure and transport properties in a broad temperature range (2-723 K) of the homologous compounds (PbSe)5(Bi2Se3)3m for m = 2, 3. Single-crystal X-ray diffraction data indicate that the m = 2, 3 compounds crystallize in the monoclinic space groups C2/m (No. 12) and P21/m (No. 11), respectively. In agreement with diffraction data, high-resolution transmission electron microscopy analyses carried out on single crystals show that the three-dimensional crystal structures are built from alternating Pb-Se and m Bi-Se layers stacked along the a axis in both compounds. Scanning electron microcopy and electron-probe microanalyses reveal deviations from the nominal stoichiometry, suggesting a domain of existence in the pseudo binary phase diagram at 873 K. The complex atomic-scale structures of these compounds lead to very low lattice thermal conductivities κL that approach the glassy limit at high temperatures. A comparison of the κL values across this series unveiled an unexpected increase with increasing m from m = 1 to m = 3, in contrast to the expectation that increasing the structural complexity should tend to lower the thermal transport. This result points to a decisive role played by the Pb-Se/Bi-Se interfaces in limiting κL in this series. Both compounds behave as heavily doped n-type semiconductors with relatively low electrical resistivity and thermopower values. As a result, moderate peak ZT values of 0.25 and 0.20 at 700 K were achieved in the m = 2, 3 compounds, respectively. The inherent poor ability of these structures to conduct heat suggests that these homologous compounds may show interesting thermoelectric properties when properly optimized by extrinsic dopants.

Tetrahedrites: Prospective Novel Thermoelectric Materials

Since their discovery in 1845, tetrahedrites, a class of minerals composed of relatively earth‐abundant and nontoxic elements, have been extensively studied in mineralogy and geology. Despite a large body of publications on this subject, their transport properties had not been explored in detail. The discovery of their interesting high‐temperature ther‐ moelectric properties and peculiar thermal transport has led to numerous experimental and theoretical studies over the last 4 years with the aim of better understanding the rela‐ tionships between the crystal, electronic, and thermal properties. Tetrahedrites provide a remarkable example of anharmonic system giving rise to a temperature dependence of the lattice thermal conductivity that mirrors that of amorphous compounds. Here, we review the progress of research on the transport properties of tetrahedrites, highlight‐ ing the main experimental and theoretical results that have been obtained so far and the important issues and questions that remain to be investigated.