Advancing thermoelectric n‐type Mg3Sb2 alloys requires both high carrier concentration offered by effective doping and high carrier mobility enabled by large grains. Existing research usually involves chalcogen doping on the anion sites, and the resultant carrier concentration reaches ≈3 × 1019 cm−3 or below. This is much lower than the optimum theoretically predicted, which suggets that further improvements will be possible once a highly efficient dopant is found. Yttrium, a trivalent dopant, is shown to enable carrier concentrations up to and above ≈1 × 1020 cm−3 when it is doped on the cation site. Such carrier concentration allows for in‐depth understand of the electronic transport properties over a broad range of carrier concentrations, based on a single parabolic band approximation. As well as reasonably high carrier mobility in coarse‐grain materials sintered by hot deforming and fusing of large pieces of ingots synthesized by melting, higher thermoelectric performance than earlier experimentally reported for n‐type Mg3Sb2 is found. In particular, the thermoelectric figure of merit, zT, is even higher than that of any known n‐type thermoelectric, including Bi2Te3 alloys, within 300–500 K. This might pave the way for Mg3Sb2 alloys to become a realistic material for n‐type thermoelectrics for sustainable applications.

Development of thermoelectrics usually involves trial-and-error investigations, including time-consuming synthesis and measurements. Here, we identify the electronic quality factor BE for determining the maximum thermoelectric power factor, which can be conveniently estimated by a single measurement of Seebeck coefficient and electrical conductivity of only one sample, not necessarily optimized, at an arbitrary temperature. We demonstrate that thousands of experimental measurements in dozens of materials can all be described by a universal curve and a single material parameter BE for each class of materials. Furthermore, any deviation in BE with temperature or doping indicated new effects such as band convergence or additional scattering. This makes BE a powerful tool for evaluating and guiding the development of thermoelectrics. We demonstrate the power of BE to show both p-type GeTe alloys and n-type Mg3SbBi alloys as highly competitive materials, at near room temperature, to state-of-the-art Bi2Te3 alloys used in nearly all commercial applications.

Over the past years, thermoelectric Mg 3 Sb 2 alloys particularly in n‐type conduction, have attracted increasing attentions for thermoelectric applications, due to the multivalley conduction band, abundance of constituents, and less toxicity. However, the high vapor pressure, causticity of Mg, and the high melting point of Mg 3 Sb 2 tend to cause the inclusion in the materials of boundary phases and defects that affect the transport properties. In this work, a utilization of tantalum‐sealing for melting enables n‐type Mg 3 Sb 2 alloys to show a substantially higher mobility than ever reported, which can be attributed to the purification of phases and to the coarse grains. Importantly, the inherently high mobility successfully enables the thermoelectric figure of merit in optimal compositions to be highly competitive to that of commercially available n‐type Bi 2 Te 3 alloys and to be higher than that of other known n‐type thermoelectrics at 300–500 K. This work reveals Mg 3 Sb 2 alloys as a top candidate for near‐room‐temperature thermoelectric applications.

approaching their minimum, therefore fundamentally limits the availability for further improvements in these materials by this thermal strategy.This inspires the development of electronic strategies for zT enhancements in recent years. The strong coupling effect among S, σ, and κ e has led early strategies for electronic performance enhancement to mainly rely on the carrier concentration (n H ) optimization through chemical doping for nearly the past half of a century. [7] Till very recently, proven electronic strategies have been typified by engineering the band structure to increase the band degeneracy through band convergence [8] or nestification, [9] to decrease the band effective mass [10] and to introduce resonant states. [11] These approaches successfully decouple the strong correlation among electronic transport properties to some extent, enabling effective improvements in electronic performance for enhancing zT.The utilization of the above thermal and electronic strategies has led to a significant advancement mostly in conventional thermoelectrics such as PbTe [7,[12][13][14][15][16] and Bi 2 Te 3 , [17][18][19][20] while the toxicity and the scarcity of the constituent element are important issues limiting their large-scale application. Zintl compounds, which by strict definition requires to be valence balanced and semiconducting, [21] have shown excellent zT at high temperatures. [21][22][23][24] A recently developed class of Zintl compounds [25] having CaAl 2 Si 2 -type structure, Mg 3 Sb 2 , and its derivatives, demonstrates a great potential to address the toxicity and the scarcity issues and therefore attracts increasing number of researchers to focus on these materials. [26] The promising thermoelectric performance realized in these compounds is enabled by the richness in composition for further manipulation of both electronic and thermal properties as well as by the intrinsic low lattice thermal conductivity. The general formula for this class of Zintl compounds can be written as AB 2 C 2 , where A can be alkaline earth or rare-earth elements (e.g., Mg, [27][28][29] Ca, [30] Sr, [31,32] Ba, [33] Eu, [34] Yb [30] ), B can be transition metals or main-group elements (e.g., Cd, [35,36] Zn, [37] Mg, [38,39] Mn, [40] Al [41] ), and C can be Group IV/V elements (e.g., Sb, [29] Bi [42] ). Such a broad variety in composition enables either a very low lattice thermal conductivity [43][44][45][46] or a very high band degeneracy, [22,44,45] as well as great availabilities for further manipulating the phonon and charge transport properties through the formation of solid solution. [27,47] In this review, Mg 3 Sb 2 and its derivative (AB 2 C 2 ) compounds with demonstrated high thermoelectric performances are focused on. The origins leading to such a high performance are summarized Over the past couple of decades, thermoelectric Mg 3 Sb 2 and its derivatives have attracted increasing attention for thermoelectric applications. This is enabled by the richness in composition for manipulating both electronic and t...

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