Thermoelectric properties of Bi-based Zintl compounds Ca_1-xYb_xMg₂Bi₂

Bi-based Zintl compounds, Ca_1-xYb_xMg₂Bi₂ with the structure of CaAl₂Si₂, have been successfully prepared by mechanical alloying followed by hot pressing. We found that the electrical conductivity, Seebeck coefficient, carrier concentration, and thermal conductivity can be adjusted by changing the Yb concentration. All Ca_1-xYb_xMg₂Bi₂ samples have low carrier concentrations (∼2.4 to 7.2 × 10¹⁸ cm^-3) and high Hall mobility (∼119 to 153 cm² V^-1 s^-1) near room temperature. The partial substitution of Ca with Yb causes structural disorders, which lowers the thermal conductivity. The highest figure of merit of ∼1.0 is observed in Ca_0.5Yb_0.5Mg₂Bi₂, and ∼0.8 in the unsubstituted CaMg₂Bi₂ and YbMg₂Bi₂. A small amount of free Bi was found in all the samples except YbMg₂Bi₂. By reducing the initial Bi concentration, we succeeded in obtaining phase pure samples in all compositions, which resulted in a much better thermoelectric performance, especially much higher (ZT)_eng and a conversion efficiency near 11%. Such a high efficiency makes this material competitive with half-Heuslers and skutterudites.