The effect of charge carrier and doping site on thermoelectric properties of Mg₂Sn_0.75Ge_0.25

Mg₂Sn_0.75Ge_0.25 has been recently demonstrated to be a promising thermoelectric material for power generation in the temperature range from room temperature to 723 K because of the high power factor of ∼54 μW cm⁻¹ K⁻² upon Sb doping to the Sn site. The enhanced density of states effective mass and weak electron scattering from the alloying effect are believed to be the main reasons for the high power factor (PF) and hence high figure of merit (ZT). In this study, it is shown that the right choice of carrier donor also plays an important role in obtaining high power factor. The effect of carrier donors Y and La at Mg-site and Bi and P at Sn-site in Mg₂Sn_0.75Ge_0.25 is systematically investigated. It is found that charge donors at the Sn-site are much more effective than at the Mg-site in enhancing PF and ZT. Bi doped Mg₂Sn_0.73Bi_0.02Ge_0.25 shows a peak ZT of ∼1.4 at 673 K, a peak PF of ∼54 μW cm⁻¹ K⁻² at 577 K, which resulted in an engineering figure of merit (ZT)_eng of ∼0.76 and (PF)_eng of ∼2.05 W m⁻¹ K⁻¹ for cold side fixed at 323 K and hot side at 723 K.