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.

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.

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For decades, significant thermoelectric enhancements at high temperatures have been witnessed in many materials. However, efficient materials working well near room temperature are still restricted to Bi2Te3-based alloys in both conduction types, in which the scarcity of tellurium is a concern for large-scale applications. Recently, Mg3Sb2–Mg3Bi2 alloys, particularly in n-type conduction, have been revealed to show great potential for efficient thermoelectric applications. The corresponding challenges rely on the realization of sufficient charge carriers with a high mobility, which respectively depends on effective doping and the removal of scattering sources. In this work, both a high carrier concentration and an inherently high mobility are enabled in Sc-doped Mg3Sb2–Mg3Bi2 alloys with coarse grains, synthesized by a melting and hot-deforming technique. This eventually leads to an average figure of merit zT of 1.1 within 300–500 K in optimal compositions, which is superior to that of state-of-the-art n-type Bi2Te3 alloys. This work strongly demonstrates the potential of these alloys as eco-friendly and sustainable alternatives to conventional n-type Bi2Te3-based alloys for thermoelectric applications near room temperature.