A high thermoelectric power factor not only enables a potentially high figure of merit ZT but also leads to a large output power density, and hence it is pivotal to find an effective route to improve the power factor.

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Mg 3Àx Na x Sb 2 has been prepared successfully by mechanical alloying plus hot pressing to investigate the effects of Na doping on the thermoelectric properties. Thermoelectric properties were characterized by the Seebeck coefficient, electrical resistivity, thermal conductivity, and thermoelectric figure of merit (ZT) from 298 to 773 K. Transport measurements reveal that an optimum doping of 1.25 at.% Na on Mg achieved ZT of 0.6 at 773 K. The enhancement in ZT is attributed to increased carrier concentration and power factor. The low cost, abundance, and nontoxicity makes this material a potentially promising thermoelectric material for power generation at a heat source below 773 K.

Complex structures with versatile chemistry provide considerable chemical tunability of the transport properties. Good thermoelectric materials are generally extrinsically doped semiconductors with optimal carrier concentrations, while charged intrinsic defects (e.g., vacancies, interstitials) can also adjust the carriers, even in the compounds with no apparent deviation from a stoichiometric nominal composition. Here we report that in Zintl compounds MgSbBi, the carrier concentration can be tuned from p-type to n-type by simply altering the initial Mg concentration. The spherical-aberration-corrected (C-corrected) high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) and energy-dispersive X-ray spectroscopy (EDX) mapping analysis show that the excess Mg would form a separate Mg-rich phase after Mg vacancies have been essentially compensated. Additionally, a slight Te doping at Bi site on MgSbBi has enabled good n-type thermoelectric properties, which is comparable to the Te-doped Mg-rich sample. The actual final composition of MgSbBi analyzed by EPMA is also close to the stoichiometry MgSbBi, answering the open question whether excess Mg is prerequisite to realize exceptionally high n-type thermoelectric performance by different sample preparation methods. The motivation for this work is first to understand the important role of vacancy and then to guide for discovering more promising n-type Zintl thermoelectric materials.