Enhanced hole concentration through Ga doping and excess of Mg and thermoelectric properties of p-type Mg2(1+z)(Si0.3Sn0.7)1−yGay

The influence of the doping amount of Ga as well as the excess of Mg on the phase composition and thermoelectric properties of Mg2(1+z)(Si0.3Sn0.7)1−yGay compounds are analyzed in detail. Regarding the content of Mg, a second phase of Mg is detected at grain boundaries whenever its over-stoichiometry exceeds 7%. On the other hand, XRD and EPMA analysis indicate that phase separation occurs when Mg is more than 3.5% deficient with respect to its stoichiometric amount. Thermoelectric property measurements reveal that doping with Ga, along with some over-stoichiometry of Mg, enhances the concentration of holes and electrical conductivity of Mg2(1+z)(Si0.3Sn0.7)1−yGay while it simultaneously reduces the Seebeck coefficient. However, there is little effect on the lattice thermal conductivity. The results also show that, in p-type Mg2Si0.3Sn0.7 based compounds, antisite point defects MgSi form when the content of Mg is over-stoichiometric. This leads to an enhanced concentration of holes. Mg2.10(Si0.3Sn0.7)0.95Ga0.05, having the optimized content of Ga and Mg, possesses the highest ZT value of 0.35 that is achieved at 650 K. This research reveals that both the doping of Ga and the excess of Mg do not have significant influence on the band structure of Mg2Si0.3Sn0.7, and the transport properties of p-type Mg2Si0.3Sn0.7 in the high hole concentration range can be well described by a simple, parabolic band model. The research work also establishes an important basis for further optimization of the figure of merit of p-type Mg2Si1−xSnx solid solutions by making use of over-stoichiometric amounts of Mg.