Roles of interstitial Mg in improving thermoelectric properties of Sb-doped Mg2Si0.4Sn0.6 solid solutions

Abstract

While adding extra Mg is intended to compensate for the Mg loss during the synthesis, it often leads to Mg interstitials in Mg2(Si,Sn) materials and profoundly affects their thermoelectric properties. Herein we studied the electrical conductivity, Seebeck coefficient, and thermal conductivity of Mg2(1 + x)Si0.38Sn0.6Sb0.02 (0.05 ≤ x ≤ 0.12) as a function of Mg excess between 300 K and 730 K. The presence of interstitial Mg was corroborated by X-ray powder diffraction, X-ray photoelectron spectroscopy, Hall coefficient measurement, and compositional analysis. The electrical properties have been analyzed in the framework of a single parabolic band model to gain more insight on the roles of Mg interstitials. We found that increasing Mg excess content increased the carrier concentration, electronic effective mass, and electrical conductivity, while it decreased the Seebeck coefficient and led to a non-monotonic change in the lattice thermal conductivity. As a result, a maximum ZT ∼ 0.85 was attained at 700 K for Mg excess x = 0.1, a ∼60% enhancement compared to that of the sample x = 0.05. The Mg interstitials thus provide an extra tuning parameter in optimizing the thermoelectric properties of Mg2(Si,Sn)-based materials.