Soft anharmonic phonons and ultralow thermal conductivity in Mg<sub>3</sub>(Sb, Bi)<sub>2</sub> thermoelectrics.


The candidate thermoelectric compounds Mg3Sb2 and Mg3Bi2 show excellent performance near ambient temperature, enabled by an anomalously low lattice thermal conductivity (κl) 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 κl 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 κl 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 κl These results provide key insights for manipulating phonon scattering without the traditional reliance on heavy elements.