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 AMg₂ X ₂ (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 Mg₃ X ₂ has inherent phononic origins. We uncover a large phonon softening and flattening of low-energy transverse acoustic phonons in Mg₃ X ₂ 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.