Effects of Bi Alloying on the Thermoelectric Properties of p-Type Mg3Sb2-xBix Solid Solutions

Bismuth is the most common alloying element for high-performance Mg3Sb2-based thermoelectric compounds. However, the electrical and thermal transport data is lacking for undoped Mg-3(Sb, Bi)(2) solid solutions. In this study, theoretical calculation results show that Bi alloying can reduce the band gap of Mg3Sb2, favoring the regulation of carrier concentration. Both Mg3Sb2 and Mg3SbBi compounds present a single band structure on the valence band maximum at Gamma of the Brillouin zone. Incorporating the Bi element can slightly increase the density of states (DOS) on the valence band maximum, which is beneficial for the Seebeck coefficient. Mg3Sb2-xBix (0 <= x <= 0.8) compounds were experimentally prepared via ball milling (BM) combined with the spark plasma sintering (SPS) technique. The carrier concentration, electrical conductivity, and power factor of the samples tend to increase with the increasing Bi content. Bismuth alloying significantly reduces the total thermal conductivity in the low-temperature range, with the lowest value of 0.65 Wm(-1)K(-1) obtained at similar to 400 K for the x=0.2 sample. The peak ZT value of 0.077 at 800 K and the average ZT of 0.051 in the temperature range of 450-800 K are obtained for the x=0.2 sample, which is 93% and 219% higher than those of the unalloyed sample, respectively. This work reveals the Bi-alloying induced transport mechanism and provides fundamental thermoelectric data regarding the undoped Mg3Sb2-xBix (0 <= x <= 0.8) solid solutions.