Anharmonic Properties in Mg₂X (X= C, Si, Ge, Sn, Pb) from First-Principles Calculations

Thermal conductivity reduction is one of the potential routes to improve the performance of thermoelectric materials. However, detailed understanding of the thermal transport of many promising materials is still missing. In this paper, we employ electronic-structure calculations at the level of density functional theory to elucidate thermal transport properties of the Mg2X (X=C, Si, Ge, Sn, and Pb) family of compounds, which includes Mg2Si, a material already identified as a potential thermoelectric. All these materials crystallize into the same antifluorite structure. Systematic trends in the anharmonic properties of these materials are presented and examined. Our calculations indicate that the reduction in the group velocity is the main driver of the thermal conductivity trend in these materials, as the phonon lifetimes in these compounds are very similar. We also examine the limits of the applicability of perturbation theory to study the effect of point defects on thermal transport and find that it is in good agreement with experiment in a wide range of scattering parameter values. The thermal conductivity of the recently synthesized Mg2C is computed and predicted to be 34 W/mK at 300°C