Abstract
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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.