Soft anharmonic phonons and ultralow thermal conductivity in Mg(3)(Sb, Bi)(2) thermoelectrics

The candidate thermoelectric compounds Mg(3)Sb(2) and Mg(3)Bi(2) show excellent performance near ambient temperature, enabled by an anomalously low lattice thermal conductivity (κ(l)) comparable to those of much heavier PbTe or Bi(2)Te(3). Contrary to common mass-trend expectations, replacing Mg with heavier Ca or Yb yields a threefold increase in κ(l) in CaMg(2)Sb(2) and YbMg(2)Bi(2). Here, we report a comprehensive analysis of phonons in the series AMg(2)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 Mg(3)X(2) has inherent phononic origins. We uncover a large phonon softening and flattening of low-energy transverse acoustic phonons in Mg(3)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.