Control of the Thermoelectric Properties of Mg₂Sn Single Crystals via Point-Defect Engineering

Mg₂Sn is a potential thermoelectric (TE) material that can directly convert waste heat into electricity. In this study, Mg₂Sn single-crystal ingots are prepared by melting under an Ar atmosphere. The prepared ingots contain Mg vacancies (V_Mg) as point defects, which results in the formation of two regions: an Mg₂Sn single-crystal region without V_Mg (denoted as the single-crystal region) and a region containing V_Mg (denoted as the V_Mg region). The V_Mg region is embedded in the matrix of the single-crystal region. The interface between the V_Mg region and the single-crystal region is semi-coherent, which does not prevent electron carrier conduction but does increase phonon scattering. Furthermore, electron carrier concentration depends on the fraction of V_Mg, reflecting the acceptor characteristics of V_Mg. The maximum figure of merit zT_max of 1.4(1) × 10^-2 is realised for the Mg₂Sn single-crystal ingot by introducing V_Mg. These results demonstrate that the TE properties of Mg₂Sn can be optimised via point-defect engineering.