Effect of Annealing on Microstructure and Thermoelectric Properties of Sb-Doped Mg₂Si_0.5Sn_0.5 Solid Solution

A 0.33 mol.% Sb-doped Mg₂Si_0.5Sn_0.5 solid solution was synthesized by combining a liquid–solid reaction and hot-pressing process. The effect of annealing (1068 K, 250 h) on microstructure and thermoelectric properties of the solid solution was studied by x-ray diffraction (XRD), scanning electron microscopy, electron probe microanalysis, transmission electron microscopy, and thermoelectric measurements. The successful synthesis of the solid solution with an antifluorite structure was confirmed by XRD. The as-prepared sample contained Si, Sn, and MgO inclusions tens of nanometers in size. After annealing, Si and Sn inclusions disappeared, while the MgO nanoparticles remained almost unchanged; the charge carrier concentration and electrical conductivity decreased and the lattice thermal conductivity increased. As a result, the thermoelectric figure of merit ZT ∼ 0.34 at 394 K for the as-prepared sample deteriorated to ∼0.24 at 388 K after the annealing. The results suggest the presence of a high density of point defects, such as Mg interstitials in the as-prepared sample. The density of these Mg interstitials was reduced by the annealing, thereby affecting the charge carrier concentration and electrical conductivity. The increase in the lattice thermal conductivity upon annealing is attributed to the disappearance of point defects, grain boundaries (grain growth) and Si and Sn inclusions, which all act as phonon scattering centers. Thus, point defects and nanoinclusions might be important for optimizing the thermoelectric properties of a material. This work provides new insights into the effect of annealing on the microstructure and its relationship with the thermoelectric properties of Sb-doped Mg₂Si_0.5Sn_0.5 solid solutions. It also provides hints for developing Mg₂Si_0.5Sn_0.5-based materials with superior thermoelectric properties.