Low lattice thermal conductivity and high thermoelectric figure of merit in Na2MgSn

Thermoelectric materials enables the harvest of waste heat and direct conversion into electricity. In search of high efficient thermoelectric materials, low thermal conductivity of a material is essential and critical. Here, we have theoretically investigated the lattice thermal conductivity and thermoelectric properties of layered intermetallic Na2MgSn and Na2MgPb based on the density functional theory and linearized Boltzmann equation with the single-mode relaxation-time approximation. It is found that both materials exhibit very low and anisotropic intrinsic lattice thermal conductivity. Despite the very low mass density and simple crystal structure of Na2MgSn, its lattice thermal conductivities along a and c axes are only 1.77 and 0.81 W/m K respectively at room temperatures. When Sn is replaced by the heavier element Pb, its lattice thermal conductivities decrease remarkably to 0.56 and 0.31 W/m K respectively along a and c axes at room temperature. We show that the low lattice thermal conductivities of both materials are mainly due to their short phonon lifetimes and phonon mean free path. Combined with previous experimental measurements, the metallic Na2MgPb cannot be a good thermoelectric material. However, we predict that the semiconducting Na2MgSn is a potential room-temperature thermoelectric material with a considerable ZT of 0.36 at 300 K. Our calculations not only imply that the intermetallic Na2MgSn is a potential thermoelectric material, but also can motivate more theoretical and experimental works on the thermoelectric researches in simple layered intermetallic compounds.