= Sb, Bi )*

Thermoelectric (TE) materials enable direct conversion of heat into electrical energy. The conversion efficiency is inversely proportional to the thermal conductivity, which is dominated by phonons in semiconductors. Zintl compounds AMg₂X₂ constitute a class of new TE with excellent performance in n-type Mg₃(Sb,Bi)₂ alloys, with zT up to 1.6 reported so far. Mg₃X₂ exhibits very low lattice thermal conductivity (~1 W/m/K at 300K), comparable with PbTe and Bi₂Te₃, despite a much lighter average ionic mass. We report on inelastic neutron scattering (INS) and first-principles studies of the lattice dynamics of AMg₂X₂. INS provided the temperature dependent phonon density of states. Extra peaks were found at low frequency in Mg₃X₂ compared to CaMg₂X₂ or YbMg₂X₂, possibly originating from softer low-frequency TA phonons. Considerably stronger softening with temperature is also observed in Mg₃X₂. The anharmonic effects were examined with first-principles simulations, including ab initio molecular dynamics. We present our analysis of the thermal conductivity based on INS and simulations.