Optimization of the Thermoelectric Properties of p-Type Mg2-yLiyGe1-xSnx and Mg2-yLiyGe1-zSiz with x, z=0.1 and 0.2

According to recent investigations on p-type Mg2X (X = Si, Sn, Ge), p-type Mg2Ge is found to be far superior to the p-type binaries while having thermoelectric properties comparable to the best solid solutions of p-type Mg-2(Si,Sn). The unexpectedly good properties are supposedly due to a nonrigid band structure with a temperature-dependent interband separation. Further optimization can be expected by alloying Si or Sn into the Ge site to lower the thermal conductivity, thereby increasing the figure of merit. Here, solid solutions of p-type Mg2Ge1-xSnx and p-type Mg2Ge1-zSiz with x, z = 0.1 and 0.2 are successfully synthesized via ball milling. The thermal conductivities are significantly reduced throughout the whole temperature range by around 30% due to the alloying effect. The electronic properties of all Ge-rich samples show similar temperature-dependent behavior to p-type Mg2Ge. For the Mg-2(Ge,Sn) systems, a compensation of reduced thermal conductivity and decreased carrier mobility are found, leading to zT values comparable to p-type Mg2Ge. Whereas for the Si containing samples, a thermoelectric figure of merit zT of 0.49 +/- 0.07 at 675 K for z = 0.1 is achieved, the highest reported so far for the p-type Mg-2(Ge,Si) systems and a zT(avg) that is 30% higher than that of binary Mg2Ge.