Unraveling the lattice thermal conductivity and thermoelectric properties of monolayer Mg₃Bi₂

Magnesium-based Zintl-phase compounds are outstanding among the high performance thermoelectric material candidates for their better flexibility, non-toxicity and low-cost. Recently, we have noted an experiment that synthesized a new thermoelectric material-monolayer Mg₃Bi₂-with an ultralow lattice thermal conductivity of <i>k</i>_l = 0.21 W m^-1 K^-1 at room temperature; however, the cause of this remains untraced. By employing a first-principles approach coupled with the Boltzmann transport equation, we herein present a deep understanding of the fundamental mechanisms responsible for the ultralow lattice thermal conductivity in monolayer Mg₃Bi₂, unveiling that this is attributed to the soft Mg3-Bi1 bonds that introduce flat phonons in the acoustic branch along the <i>Γ</i>-<i>M</i> direction in the first Brillouin zone, and in turn decrease the velocity of sound and strengthen anharmonicity and scattering rates of phonons. Additionally, the analysis of the electronic structure reveals the characteristics of multiple transport valleys that boost the large Seebeck coefficient of 140 μV K^-1, as observed in electrical transport calculations. Consequently, we identify the figure of merit (<i>ZT</i>) of 0.48 at 800 K in n-type monolayer Mg₃Bi₂. Our findings shed light on the microscopic origins of the lattice thermal conductivity and provide key indicators for searching for high performance thermoelectric materials in the Mg-Bi system.