Soft anharmonic phonons and ultralow thermal conductivity in Mg₃(Sb, Bi)₂ thermoelectrics.

The candidate thermoelectric compounds Mg₃Sb₂ and Mg₃Bi₂ show excellent performance near ambient temperature, enabled by an anomalously low lattice thermal conductivity (κ_l) comparable to those of much heavier PbTe or Bi₂Te₃ Contrary to common mass-trend expectations, replacing Mg with heavier Ca or Yb yields a threefold increase in κ_l in CaMg₂Sb₂ and YbMg₂Bi₂ Here, we report a comprehensive analysis of phonons in the series AMg₂X₂ (A = Mg, Ca, and Yb; X = Bi and Sb) based on inelastic neutron/x-ray scattering and first-principles simulations and show that the anomalously low κ_l of Mg₃X₂ has inherent phononic origins. We uncover a large phonon softening and flattening of low-energy transverse acoustic phonons in Mg₃X₂ compared to the ternary analogs and traced to a specific Mg-X bond, which markedly enlarges the scattering phase-space, enabling the threefold tuning in κ_l These results provide key insights for manipulating phonon scattering without the traditional reliance on heavy elements.

Duke Scholars

Author Olivier Delaire Thomas Lord Department of Mechanical Engineering and Materia ...