Thermoelectric performance enhancement of Mg2Sn based solid solutions by band convergence and phonon scattering via Pb and Si/Ge substitution for Sn

In this study, the thermoelectric properties of Mg2Sn0.98−xPbxSb0.02 were first studied, and then Mg2Sn0.93−xSixPb0.05Sb0.02 and Mg2Sn0.93−xGexPb0.05Sb0.02 were accordingly investigated. The results showed that the formation of Mg2Sn0.98−xPbxSb0.02 solid solutions effectively reduced the lattice thermal conductivity of Mg2Sn. The room temperature lattice thermal conductivity of Mg2Sn0.98Sb0.02 is ∼5.2 W m−1 K−1 but only ∼2.5 W m−1 K−1 for Mg2Sn0.73Pb0.25Sb0.02, a reduction of ∼52%. Further alloying Mg2Sn0.98−xPbxSb0.02 with Mg2Si or Mg2Ge to form Mg2Sn0.93−xSixPb0.05Sb0.02 or Mg2Sn0.93−xGexPb0.05Sb0.02 reduced the lattice thermal conductivity significantly due to enhanced phonon scattering by point defects as well as nanoparticles. Moreover, bipolar thermal conductivities were suppressed due to the larger bandgap of Mg2Si and Mg2Ge than Mg2Sn. Furthermore, similar to the pseudo-binary Mg2Sn–Mg2Si and Mg2Sn–Mg2Ge systems, band convergence was also observed in pseudo-ternary Mg2Sn0.93−xSixPb0.05Sb0.02 and Mg2Sn0.93−xGexPb0.05Sb0.02 materials. The convergence of conduction bands led to higher PFs at lower temperatures for Mg2Sn0.93−xSixPb0.05Sb0.02 and Mg2Sn0.93−xGexPb0.05Sb0.02 materials. As a result, higher peak ZTs of ∼1.3 for Mg2Sn0.63Si0.3Pb0.05Sb0.02 and ∼1.2 for Mg2Sn0.68Ge0.25Pb0.05Sb0.02 were achieved.