Analyzing transport properties of p-type Mg2Si-Mg2Sn solid solutions: optimization of thermoelectric performance and insight into the electronic band structure

We have synthesized the complete stoichiometric range of p-Mg2Si1-xSnx, striving to optimize the thermoelectric properties of p-type Mg-2(Si, Sn) with respect to composition. The experimental data are analyzed in the framework of a single parabolic band (SPB) model and we can show that the thermoelectric properties can be well presented if acoustic phonon scattering and alloy scattering are taken into account. We find that the maximum achievable carrier concentration and power factor increase with higher Sn content. Also, the carrier mobility increases strongly from Mg2Si to Mg2Sn due to the changing density of states effective mass for the valence band which decreases from m(D)* (Mg2Si) = 2.2 m(0) to m(D)* (Mg2Sn) = 1.1 m(0). Retrieval of the acoustic phonon scattering potential (E-Def = 9 eV) and the alloy scattering parameter (E-AS = 0.5 eV) allows for modelling the thermoelectric properties for any arbitrary composition. Hence, we can predict the optimum zT for x approximate to 0.65-0.7 and the maximum power factor for Sn-rich compositions. Furthermore, we reveal that a significant improvement of the thermoelectric properties of Si-rich compositions can be achieved by increasing the carrier concentration experimentally and that the disparity between n- and p-type Mg-2(Si,Sn) is due to the differences between the valence and the conduction bands and not the interaction potentials.