Band engineering in Mg₃Sb₂ by alloying with Mg₃Bi₂ for enhanced thermoelectric performance

Mg₃Sb₂-Mg₃Bi₂ alloys show excellent thermoelectric properties. The benefit of alloying has been attributed to the reduction in lattice thermal conductivity. However, Mg₃Bi₂-alloying may also be expected to significantly change the electronic structure. By comparatively modeling the transport properties of n- and p-type Mg₃Sb₂-Mg₃Bi₂ and also Mg₃Bi₂-alloyed and non-alloyed samples, we elucidate the origin of the highest zT composition where electronic properties account for about 50% of the improvement. We find that Mg₃Bi₂ alloying increases the weighted mobility while reducing the band gap. The reduced band gap is found not to compromise the thermoelectric performance for a small amount of Mg₃Bi₂ because the peak zT in unalloyed Mg₃Sb₂ is at a temperature higher than the stable range for the material. By quantifying the electronic influence of Mg₃Bi₂ alloying, we model the optimum Mg₃Bi₂ content for thermoelectrics to be in the range of 20-30%, consistent with the most commonly reported composition Mg₃Sb_1.5Bi_0.5.