Thermoelectric properties of highly efficient Bi-doped Mg2Si1−x−ySnxGey materials

In this work, Bi-doped Mg2Si1−x−ySnxGey (x = 0.4 and y = 0.05) solid solutions with the nominal Bi content of 0 ⩽ z ⩽ 0.035 were synthesized by solid state synthesis and sintering via hot pressing, and were studied in terms of structural, electronic transport and thermoelectric properties. These materials exhibit a high thermoelectric figure of merit, reaching a maximum of 1.4, which is the best among all reported Mg2Si1−xSnx compounds on the Si-rich side. Within this range of Bi concentrations, X-ray diffraction, scanning electron microscopy and transmission electron microscopy characterization provided strong evidence for the presence of a number of secondary phases with different stoichiometries, including Si-rich, Sn-rich and Ge-rich phases. Moreover, the existence of micro- and nanofeatures is evident and their correlation with the thermoelectric properties and the high figure of merit is discussed. The influence of the amount of Bi doping on the thermoelectric properties of the specific compounds was investigated. A single effective mass was not proven efficient to fit the Seebeck coefficient and carrier concentration data, especially for carrier densities larger than 1020 cm−3. The adopted synthesis process also yielded very good repeatability and regularity in obtaining enhanced thermoelectric properties.