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 Mg3Sb1.5Bi0.5.
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