Local structure and thermoelectric properties of Mg2Si0.977− x Ge x Bi0.023 (0.1⩽ x ⩽0.4)

We investigated the effect of germanium substitution for silicon in bismuth doped Mg2Si. This alloying reduces the thermal conductivity from above 7Wm−1 K−1 to 2.7Wm−1 K−1 at around 300K in part due to the added mass contrast. High resolution transmission electron microscopy (HRTEM) revealed the presence of Ge-rich domains within the Mg2(Si,Ge,Bi) particles, contributing to decreasing thermal conductivity with increasing Ge content up to 0.3 Ge per formula unit. The electrical conductivity also decreases with Ge alloying because of the increasing amount of scattering centers, while the Seebeck coefficient increased only very slightly. In total, the positive effect of Ge substitution on the thermoelectric properties of Bi doped Mg2Si resulted in a figure of merit of 0.7 at 773K for Mg2Si0.677Ge0.3Bi0.023 sample. The optimum amount of Bi seems to be 0.023 per formula unit (0.77at%), since lower Bi content resulted in electrical conductivity that is too low, and higher Bi content generated the Mg3Bi2 intermetallic phase.