High thermoelectric figure of merit in p-type Mg3Si2Te6: role of multi-valley bands and high anharmonicity

Abstract

Silicon-based materials are attractive for thermoelectric applications due to their thermal stability, chemical inertness, and natural abundance of silicon. Here, using a combination of first-principles and Boltzmann transport calculations we report the thermoelectric properties of the recently synthesized compound Mg₃Si₂Te₆. Our analysis reveals that Mg₃Si₂Te₆ is a direct bandgap semiconductor with a bandgap of 1.6 eV. The combination of heavy and light valence bands, along with a high valley degeneracy, results in a large power factor under p-type doping. We also find that Mg is weakly bonded both within and between the layers, leading to low phonon group velocities. The vibrations of the Mg atoms are localized and make a significant contribution to phonon–phonon scattering. This high anharmonicity, coupled with low phonon group velocity, results in a low lattice thermal conductivity of κ_l = 0.5 W m⁻¹ K⁻¹ at room temperature, along the cross-plane direction. Combining excellent electronic transport properties and low κ_l, p-type Mg₃Si₂Te₆ achieves figure-of-merit (zT) values greater than 1 at temperatures above 600 K. Specifically, a zT of 2.0 is found at 900 K along the cross-plane direction. Our findings highlight the importance of structural complexity and chemical bonding in electronic and phonon transport, providing guiding insights for further design of Si-based thermoelectrics.