The effect of heavy and light electronic bands on thermoelectric properties of Mg2Si1-xSnx alloys: Insights from an ab-initio study

Abstract

Mg2X (X = Si, Sn, and Ge) based compounds have attracted great attention due to their numerous advantages in thermoelectric (TE) applications. In particular, to date, ternary Mg2X based alloys have been recognized the most extensively explored thermoelectric systems, because these compounds exhibit excellent TE properties. Using the first-principles methods, we accomplished mutual alloying of Mg2X (Si, Sn) compounds in order to develop their efficiency in expanding the range of TE applications. The electrical, structural, and thermoelectric properties of Mg2Si1-xSnx were studied. With increasing Sn content in Mg2Si1-xSnx compounds, our calculations reveal a linear increase of the unit-cell constant and a decrease of the bulk modulus. Besides, this Sn content increase also leads to a decrease in the energy gap values. Moreover, taking into account the carrier concentration impact on the TE properties, we can state that the n-type Mg2Si1-xSnx alloys reveal higher TE behavior resulting from the large effective mass m* (in terms of electron mass m0) and higher DOS at the Fermi level. In particular, the supreme ZT value is about 1.4 for n-type Mg2Si0.375Sn0.625 at 700 K with n = 4x1020 cm−3, where the combination of elevated power factor and small thermal conductivity values play a key role. The supreme value of ZT is 0.9 for p-type Mg2Si at 700 K. These results suggest that Mg2Si1-xSnx alloys could be considered exceptional TE materials for practical use due to their fair TE performance.