Origin of unexpectedly low thermal conductivity in AMg₂X₂(A = Mg, Ca, Yb, X = Sb, Bi)

Thermoelectric materials enable direct conversion of waste heat into electrical energy. The conversion efficiency is inversely proportional to the thermal conductivity, which is generally dominated by phonons in semiconductors. Zintl compounds AMg₂X₂ constitute a class of new thermoelectric compounds with excellent thermoelectric performance in n-type Mg₃(Sb,Bi)₂ alloys, with zT values up to 1.6 reported so far. Mg₃Sb₂ exhibits very low lattice thermal conductivity (~1-1.5 W/m/K at 300K), comparable with PbTe and Bi₂Te₃, despite a much lighter average ionic mass. We report on neutron scattering and first-principles studies of the lattice dynamics of AMg₂X₂. Inelastic neutron scattering measurements provided the temperature dependence of the phonon density of states (DOS). Extra peaks and overall softer phonons were found at low frequency in Mg₃Sb₂ and Mg₃Bi₂ compared to CaMg₂X₂ or YbMg₂X₂. Combined with simulations, we highlight the importance of a specific soft Mg-X chemical bond that suppresses phonon group velocities and drastically enlarges the scattering phase-space, enabling the threefold suppression in thermal conductivity.