Phase Segregation in High Mn-Doped Mg 3 SbBi Thermoelectric Materials

Introducing various n-type dopants into Mg3Sb2-based thermoelectric (TE) alloys significantly enhances their TE performance. However, the effect of excessive doping, particularly beyond the phase solubility limit, on the phase composition and microstructure remains underexplored. Here, we investigate the microstructure and TE performance of n-type Mg3–xMnx(Sb,Bi)2 with Mn doping concentrations of 0.01 ≤ x ≤ 1.0. With low Mn doping (x < 0.1), Mn occupies both Mg sites and interstitials, suppressing Mg vacancies and increasing both the electrical conductivity from 8.6 × 104 to 11.3 × 104 S m–1 and the power factor from 23.3 to 28.7 μW cm–1 K–2 at 300 K. At x = 0.01, a room-temperature figure of merit (zT) of 0.72 is achieved. Additionally, the lattice thermal conductivity decreases over the temperature range of 300–650 K due to enhanced phonon scattering, yielding a zT of 1.3 at 420 K and a peak zT of 1.74 at 573 K, as well as an average zT of 1.37, the highest reported for n-type Mg3(Sb,Bi)2 compounds over this temperature range. Moreover, a high conversion efficiency of 9.14% is achieved in a single-leg device at a temperature difference of 400 K, making this compound a promising candidate for low-grade waste heat recovery. With high Mn doping (x > 0.1), on the other hand, high-resolution transmission electron microscopy and selected area electron diffraction reveal the presence of Mn-rich secondary phases (e.g., Mn3Bi2, Mn2Sb, and MnSb) that act as electron-trapping centers, degrading TE performance and switching the n-type behavior to p-type. This study thus deepens our understanding of the relationships among dopant concentration, phase composition, microstructural evolution, and TE performance for Mg3Sb2-based TE alloys.