Thermoelectric performance of n-type Mg₂Ge

Magnesium-based thermoelectric materials (Mg₂X, X = Si, Ge, Sn) have received considerable attention due to their availability, low toxicity, and reasonably good thermoelectric performance. The synthesis of these materials with high purity is challenging, however, due to the reactive nature and high vapour pressure of magnesium. In the current study, high purity single phase n-type Mg₂Ge has been fabricated through a one-step reaction of MgH₂ and elemental Ge, using spark plasma sintering (SPS) to reduce the formation of magnesium oxides due to the liberation of hydrogen. We have found that Bi has a very limited solubility in Mg₂Ge and results in the precipitation of Mg₂Bi₃. Bismuth doping increases the electrical conductivity of Mg₂Ge up to its solubility limit, beyond which the variation is minimal. The main improvement in the thermoelectric performance is originated from the significant phonon scattering achieved by the Mg₂Bi₃ precipitates located mainly at grain boundaries. This reduces the lattice thermal conductivity by ~50% and increases the maximum zT for n-type Mg₂Ge to 0.32, compared to previously reported maximum value of 0.2 for Sb-doped Mg₂Ge.