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 Mg₂Sn_0.98-xPb_xSb_0.02 were first studied, and then Mg₂Sn_0.93-xSi_xPb_0.05Sb_0.02 and Mg₂Sn_0.93-xGe_xPb_0.05Sb_0.02 were accordingly investigated. The results showed that the formation of Mg₂Sn_0.98-xPb_xSb_0.02 solid solutions effectively reduced the lattice thermal conductivity of Mg₂Sn. The room temperature lattice thermal conductivity of Mg₂Sn_0.98Sb_0.02 is ∼5.2 W m^-1 K^-1 but only ∼2.5 W m^-1 K^-1 for Mg₂Sn_0.73Pb_0.25Sb_0.02, a reduction of ∼52%. Further alloying Mg₂Sn_0.98-xPb_xSb_0.02 with Mg₂Si or Mg₂Ge to form Mg₂Sn_0.93-xSi_xPb_0.05Sb_0.02 or Mg₂Sn_0.93-xGe_xPb_0.05Sb_0.02 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 Mg₂Sn_0.93-xSi_xPb_0.05Sb_0.02 and Mg₂Sn_0.93-xGe_xPb_0.05Sb_0.02 materials. The convergence of conduction bands led to higher PFs at lower temperatures for Mg₂Sn_0.93-xSi_xPb_0.05Sb_0.02 and Mg₂Sn_0.93-xGe_xPb_0.05Sb_0.02 materials. As a result, higher peak ZTs of ∼1.3 for Mg₂Sn_0.63Si_0.3Pb_0.05Sb_0.02 and ∼1.2 for Mg₂Sn_0.68Ge_0.25Pb_0.05Sb_0.02 were achieved.