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N-type Zintl phases with earth-abundant and non-toxic constituent elements have attracted intense research interest thanks to their high thermoelectric efficiencies in the mid-temperature range, exemplified by the recently discovered Mg3Sb2 material. In this study, the liquid phase is expelled from the microstructure of the optimized n-type phase Mg3+xSb1.5Bi0.49Te0.01 by applying a melt-centrifugation technique leading to the formation of lattice dislocations, grain boundary dislocations and increasing porosity. Additional phonon scattering mechanisms were introduced in the microstructure through this manufacturing method, resulting in a significant 50% reduction in the total thermal conductivity from ∼1 W m−1 K−1 to ∼0.5 W m−1 K−1 at 723 K. Combined with high power factors, this reduced heat transport leads to a dimensionless thermoelectric figure of merit, zT, value of ∼1.64 at 723 K, 43% higher than the value obtained in untreated Mg3+xSb1.5Bi0.49Te0.01 (zT ∼ 1.14 at 723 K). This peak zT value yields a predicted device ZT of 0.95, and a promising theoretical thermoelectric efficiency of about 12%. These results further underline the great potential of the lightweight Mg3Sb2 material for mid-temperature energy harvesting via thermoelectric effects.

Discovery of novel high-performance materials with earth-abundant and environmentally friendly elements is a key task for civil applications based on advanced thermoelectric technology. Advancements in this area are greatly limited by the traditional trial-and-error method, which is both time-consuming and expensive. The materials genome initiative can provide a powerful strategy to screen for potential novel materials using high-throughput calculations, materials characterization, and synthesis. In this study, we developed a modified diffusion-couple high-throughput synthesis method and an automated histogram analysis technique to quickly screen high-performance copper chalcogenide thermoelectric materials, which has been well demonstrated in the ternary Cu-Sn-S compounds. A new copper chalcogenide with the composition of Cu₇Sn₃S₁₀ was discovered. Studies on crystal structure, band gap, and electrical and thermal transport properties were performed to show that it is a promising thermoelectric material with ultralow lattice thermal conductivity, moderate band gap, and decent electrical conductivity. Via Cl doping, the thermoelectric dimensionless figure of merit zT reaches 0.8 at 750 K, being among the highest values reported in Cu-Sn-S ternary materials. The modified diffusion-couple high-throughput synthesis method and automated histogram analysis technique developed in this study also shed light on the development of other advanced thermoelectric and functional materials.

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The unique crystal structure of BaBiTe_3 containing Te···Te resonant bonds and its narrow band gap motivated the systematic study of the thermoelectric transport properties of BaBiTe_(3–x)Se_x (x = 0, 0.05, and 0.1) presented here. This study gives insight in the chemical bonding and thermoelectric transport properties of BaBiTe_3. The study shows that the presence of Te···Te resonant bonds in BaBiTe_3 is best described as a linear combination of interdigitating (Te^(1–))_2 side groups and infinite Te_n chains. Rietveld X-ray structure refinements and extrinsic defect calculations reveal that the substitution of Te by Se occurs preferentially on the Te4 and Te5 sites, which are not involved in Te···Te bonding. This work strongly suggests that both multiband effects and native defects play an important role in the transport properties of BaBiTe_(3–x)Se_x (x = 0, 0.05, and 0.1). The carrier concentration of BaBiTe_3 can be tuned via Se substitution (BaBiTe_(3–x)Se_x with x = 0, 0.05, and 0.1) to values near those needed to optimize the thermoelectric performance. The thermal conductivity of BaBiTe_(3–x)Se_x (x = 0, 0.05, and 0.1) is found to be remarkably low (ca. 0.4 Wm^(–1)K^(–1) at 600 K), reaching values close to the glass limit of BaBiSe_3 (0.34 W m^(–1) K^(–1)) and BaBiTe_3 (0.28 W m^(–1) K^(–1)). Calculations of the defect formation energies in BaBiTe_3 suggest the presence of native Bi_(Ba)^(+1) and Te_(Bi)^(+1) antisite defects, which are low in energy and likely responsible for the native n-type conduction and the high carrier concentration (ca. 10^(20) cm^(–3)) found for all samples. The analyses of the electronic structure of BaBiTe_3 and of the optical absorption spectra of BaBiTe_(3–x)Se_x (x = 0, 0.05, 0.1, and 3) strongly suggest the presence of multiple electron pockets in the conduction band (CB) in all samples. These analyses also provide a possible explanation for the two optical transitions observed for BaBiTe_3. High-temperature optical absorption measurements and thermoelectric transport analyses indicate that bands higher in the conduction band converge with the conduction band minimum (CBM) with increasing temperature and contribute to the thermoelectric transport properties of BaBiTe_3 and BaBiTe_(2.95)Se_(0.05). This multiband contribution can account for the ∼50% higher zT_(max) of BaBiTe_3 and BaBiTe_(2.95)Se_(0.05) (∼0.4 at 617 K) compared to BaBiTe_(2.9)Se_(0.1) (∼0.2 at 617 K), for which no such contribution was found. The increase in the band offset between the CBM and bands higher in the conduction band with respect to the selenium content is one possible explanation for the absence of multiband effects in the thermoelectric transport properties of BaBiTe_(2.9)Se_(0.1).

BaZn2Sn2 (space group P4/nmm, a = 4.7459(5) angstrom, c = 11.330(2) angstrom, Z = 2) crystallizes in the CaBe2Ge2 structure type with a polyanionic framework comprising alternately stacked PbO-like {ZnSn4/4} and anti-PbO-like {SnZn4/4} layers along the c-axis. BaZn2Sn2 samples were obtained by either direct solid state reaction of the elements or from a Sn-flux method in very high yield with very small amount of b-Sn as the secondary phase. The samples were characterized by powder X-ray diffraction (PXRD) and scanning electron microscopy (SEM). The chemical compositions were determined to be off-stoichiometric with Zn/Sn ratio lower than 1.0 and Sn-2 atoms in the crystal structure were found to be either loosely bonded or not bonded which might lead to an incomplete charge balance. Electrical and thermal transport measurements have been performed in the temperature range 300-773 K. BaZn2Sn2 displays the electrical resistivity of a metal (or semimetal) along with very low Seebeck coefficients and relatively high thermal conductivity. (C) 2014 Elsevier B.V. All rights reserved.

This paper aims at elucidating the origin of the high thermoelectric power factor of p-type (AgxSbTe_(x/2+1.5)15)(GeTe)_(85) (TAGS) thermoelectric materials with 0.4 ≤ x ≤ 1.2. All samples exhibit good thermoelectric figures of merit (zT) which reach 1.5 at 700 K for x = 0.6. Thermoelectric and thermomagnetic transport properties (electrical resistivity, Seebeck, Hall and transverse Nernst–Ettinghausen coefficients) are measured and used to calculate the scattering factor, the Fermi energy, the density-of-states (DOS) effective mass and hole mean free path (mfp). The DOS effective mass is very high due to the large band mass of the primary valence band and the high degeneracy of pockets in the Fermi surface from the second valence band. The highly degenerate Fermi surface increased the total DOS without decreasing mobility, which is more desirable than the high DOS that comes from a single carrier pocket. The high-temperature hole mfp approaches the Ioffe–Regel limit for band-type conduction, which validates our discussion based on band transport and is also important for TAGS alloys having high zT with heavy bands. The present results show that multiple degenerate Fermi surface pockets provide an effective way of substantially increasing the power factor of thermoelectric materials with low thermal conductivity.