${\mathrm{Mg}}_2\mathrm{Si}$ and ${\mathrm{Mg}}_2\mathrm{Sn}$ are indirect band gap semiconductors with two low-lying conduction bands (the lower mass and higher mass bands) that have their respective band edges reversed in the two compounds. Consequently, for some composition $x$, ${\mathrm{Mg}}_2{\mathrm{Si}}_{1-x}{\mathrm{Sn}}_x$ solid solutions must display a convergence in energy of the two conduction bands. Since ${\mathrm{Mg}}_2{\mathrm{Si}}_{1-x}{\mathrm{Sn}}_x$ solid solutions are among the most prospective of the novel thermoelectric materials, we aim on exploring the influence of such a band convergence (valley degeneracy) on the Seebeck coefficient and thermoelectric properties in a series of ${\mathrm{Mg}}_2{\mathrm{Si}}_{1-x}{\mathrm{Sn}}_x$ solid solutions uniformly doped with Sb. Transport measurements carried out from 4 to 800 K reveal a progressively increasing Seebeck coefficient that peaks at $x=0.7$. At this concentration the thermoelectric figure of merit $ZT$ reaches exceptionally large values of 1.3 near 700 K. Our first principles calculations confirm that at the Sn content $x\approx 0.7$ the two conduction bands coincide in energy. We explain the high Seebeck coefficient and $ZT$ values as originating from an enhanced density-of-states effective mass brought about by the increased valley degeneracy as the two conduction bands cross over. We corroborate the increase in the density-of-states effective mass by measurements of the low temperature specific heat. The research suggests that striving to achieve band degeneracy by means of compositional variations is an effective strategy for enhancing the thermoelectric properties of these materials.

A cooler tin selenide: Thermoelectric materials can convert heat into electricity or be used as the basis of cooling devices. Qin et al . found that doping a tin selenide thermoelectric material with lead and sodium improved the room temperature thermoelectric properties, an effect created by manipulation of the electronic bands. The authors showed that the material could be used not only for power generation but also cooling. If optimal contact materials are identified, then this approach may be attractive for future applications. —BG

Defect engineering and nano-structuring are the core stratagems for improving thermoelectric properties. In bismuth telluride alloys nanosizing individual crystallites has been extensively studied in efforts to reduce the thermal conductivity, but nanostructuring with second phases has been more challenging. In this study, we demonstrate a thermoelectric figure of merit ZT of 1.4 at 400 K, realized in Zn-containing BiSbTe alloys (specifically Bi_0.46Sb_1.54Te₃) by integrating defect complexity with nanostructuring. We have succeeded in creating nanostructured BiSbTe alloys containing ZnTe nanoprecipitates. We present a melt-spinning-based synthesis that forms in situ ZnTe nanoprecipitates to produce an extremely low lattice thermal conductivity of ∼0.35 W m⁻¹ K⁻¹ at 400 K, approaching the amorphous limit in the Bi_2−xSb_xTe₃ system, while preserving the high power factor of Bi_0.46Sb_1.54Te₃. These samples show excellent repeatability and thermal stability at temperatures up to 523 K. DFT calculations and experimental results show that Zn is inclined to form dual site defects, including two substitutional defects Zn_Bi/Sb′ and a Te vacancy, to achieve full charge compensation, which was further explicitly corroborated by Positron annihilation measurement. The strong enhancement of thermoelectric properties was validated in a thermoelectric module fabricated with the melt-spun p-legs (ZnTe-nanostructured BiSbTe) and zone-melt n-legs (conventional BiTeSe) which achieved a thermoelectric conversion efficiency of 5.0% when subjected to a temperature gradient of 250 K, representing about 40% improvement compared with a commercial zone-melt-based module. The results presented here represent a significant step forward for applications in thermoelectric power generation.

Half-Heusler (HH) alloys have attracted considerable interest as promising thermoelectric (TE) materials in the temperature range around 700 K and above, which is close to the temperature range of most industrial waste heat sources. The past few years have seen nanostructuing play an important role in significantly enhancing the TE performance of several HH alloys. In this article, we briefly review the recent progress and advances in these HH nanocomposites. We begin by presenting the structure of HH alloys and the different strategies that have been utilized for improving the TE properties of HH alloys. Next, we review the details of HH nanocomposites as obtained by different techniques. Finally, the review closes by highlighting several promising strategies for further research directions in these very promising TE materials.