Improving p-type thermoelectric performance of Mg2(Ge,Sn) compounds via solid solution and Ag doping

Mg2X (X = Si, Ge, and Sn) compounds have attracted increasing attention owing to their promising thermoelectric properties and “green” constituent elements. While most studies to date have been on n-type Mg2(Si,Sn) solid solutions, we herein report Ag-doping study in Mg2(Ge,Sn) solid solutions to pursue higher p-type performance. Two series of samples, Mg2Ge1−xSnx and Mg2−yAgyGe0.4Sn0.6, were prepared by melting in evacuated Ta tubes, followed by hot pressing, and the thermoelectric properties of the solid solutions were investigated from room temperature to 723 K. An n- to p-type crossover in the Seebeck coefficient was observed for Mg2Ge0.4Sn0.6 and Mg2Ge0.2Sn0.8 at elevated temperatures. To enhance p-type performance, we doped Ag in Mg2Ge0.4Sn0.6 that exhibited lowest thermal conductivity among the Mg2(Ge,Sn) solid solutions. It was found that (1) p-type behavior has been established in Mg2−yAgyGe0.4Sn0.6 (y = 0.005, 0.01, 0.02, 0.04) samples in the entire temperature range studied; and (2) the electrical conductivity increased with increasing Ag content until reaching the solution limit ∼0.02. As a result, a dimensionless figure of merit ZT ∼0.38 is attained for Mg1.98Ge0.4Sn0.6Ag0.02 at 675 K, which is one of the highest reported values in p-type Mg2X compounds.