Mg(2)Si and Mg(2)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, Mg(2)Si(1-x)Sn(x) solid solutions must display a convergence in energy of the two conduction bands. Since Mg(2)Si(1-x)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 Mg(2)Si(1-x)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≈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.

In this study, a series of GeMnTe (x = 0-0.21) compounds were prepared by a melting-quenching-annealing process combined with spark plasma sintering (SPS). The effect of alloying MnTe into GeTe on the structure and thermoelectric properties of GeMnTe is profound. With increasing content of MnTe, the structure of the GeMnTe compounds gradually changes from rhombohedral to cubic, and the known R3m to Fm-3m phase transition temperature of GeTe moves from 700 K closer to room temperature. First-principles density functional theory calculations show that alloying MnTe into GeTe decreases the energy difference between the light and heavy valence bands in both the R3m and Fm-3m structures, enhancing a multiband character of the valence band edge that increases the hole carrier effective mass. The effect of this band convergence is a significant enhancement in the carrier effective mass from 1.44 m (GeTe) to 6.15 m (GeMnTe). In addition, alloying with MnTe decreases the phonon relaxation time by enhancing alloy scattering, reduces the phonon velocity, and increases Ge vacancies all of which result in an ultralow lattice thermal conductivity of 0.13 W m K at 823 K. Subsequent doping of the GeMnTe compositions with Sb lowers the typical very high hole carrier concentration and brings it closer to its optimal value enhancing the power factor, which combined with the ultralow thermal conductivity yields a maximum ZT value of 1.61 at 823 K (for GeMnSbTe). The average ZT value of the compound over the temperature range 400-800 K is 1.09, making it the best GeTe-based thermoelectric material.

High performance BiSbTe alloy and modules with a conversion efficiency of 5% are achieved through integrating Zn induced defect complexity with nanostructuring.

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High performance Bi2Te3-based thermoelectric material and modules with a conversion efficiency of 5.2% under a temperature gradient of 250 K were synthesized by TIFS.

Mg 2 Si 1−x Sn x compounds are low-cost and environmentally friendly thermoelectric materials expected to be applied as power generators in the intermediate temperature range. Optimization of the thermoelectric properties of Mg 2 Si 1−x Sn x compounds can be accomplished by the precise control and adjustment of the Mg content. A series of Mg 2(1+z) Si 0.5−y Sn 0.5 Sb y (0 ≤ y ≤ 0.015 and 0 ≤ z ≤ 0.15) compounds with controlled Mg content were synthesized by a two-step solid-state reaction method, followed by a spark plasma sintering technique. On the basis of optimized thermoelectric properties via doping with Sb, the effect of a variable content of Mg spanning from understoichiometry to overstoichiometry has been systematically explored. The results indicate that when the actual Mg content exceeds the stoichiometric amount, the dominant point defects in Mg 2(1+z) Si 0.49 Sn 0.5 Sb 0.01 compounds are interstitial Mg and Si/ Sn vacancies. At the same time, the electron concentration is enhanced with increasing content of Mg. However, when the actual Mg content is substoichiometric, the point defects consist mainly of Mg vacancies that tend to counteract the doping effect of Sb. Thus, the electron concentration of the nominal Mg 2 Si 0.49 Sn 0.5 Sb 0.01 compound (in reality a 2 mol % deficiency of Mg) is markedly lower compared with the Mg 2.10 Si 0.49 Sn 0.5 Sb 0.01 compound, which actually had a 2 mol % excess of Mg. Furthermore, a modest overstoichiometry of Mg enhances the power factor and improves the dimensionless figure of merit. The highest value of ZT = 1.25 at 800 K among the compounds was obtained on Mg 2.20 Si 0.49 Sn 0.5 Sb 0.01 , which had an actual Mg excess of 5.5 mol %. The study suggests that point defects, such as interstitial Mg and Si/Sn vacancies, which are created by an overstoichiometric content of Mg, have a positive effect on the electron concentration and thermoelectric properties of n-type Mg 2 Si 1−x Sn x -based compounds. This research has also established an essential foundation for further optimization of the thermoelectric properties of Mg 2 Si 1−x Sn x compounds. KEYWORDS: Mg 2(1+z) Si 0.5−y Sn 0.5 Sb y , adjustment of the Mg content, point defects, thermoelectric properties

The well-known single parabolic band (SPB) model has been useful in providing insights into the understanding of transport properties of numerous thermoelectric materials. However, the conduction and valence bands of real semiconductors are rarely truly parabolic which limits the predictive power of the SPB model. The coincidence of the band edges of two parabolic bands, a situation arising in Mg2Si1-xSnx solid solutions when x∼ 0.7, naturally makes the SPB approximation applicable to evaluate all transport parameters. We demonstrate this in the case of Bi-doped Mg2Si0.3Sn0.7 where the minima of the two conduction bands at the X-point of the Brillouin zone coincide. The combination of a large density-of-states effective mass m* ∼ 2.6 me arising from the enhanced valley degeneracy Nv, high mobility μd due to low deformation potential Ed (8.77-9.43 eV), and ultra-low alloy scattering parameter Ea (0.32-0.39 eV) leads to an outstanding power factor, PFmax∝ (m*)(3/2)μd, of up to 4.7 mW m(-1) K(-2) at around 600 K. The specification and improved understanding of scattering parameters using the SPB model are important and instructive for further optimization of the thermoelectric performance of n-type Mg2Si0.3Sn0.7.

Reduction of lattice thermal conductivity in Bi0.5Sb1.5Te3+x is absent despite dense dislocation arrays at grain boundaries.