A Theoretical Search for Efficient Dopants in Mg2X (X = Si, Ge, Sn) Thermoelectric Materials

Intentional alloying and doping are well-established ways of improving the thermoelectric properties of Mg2X (X = Si, Ge, Sn). In this study the results of electronic structure calculations for alloying and impurity dilution are presented. We have adapted the fully self-consistent Korringa-Kohn-Rostoker method with the coherent potential approximation (KKR-CPA) to treat various types of chemical disorders and to calculate an electronic band structure with complex energy. In Mg2Si1-x Sn (x) , as the Sn content increases, the conduction and valence bands near the Fermi energy tend to overlap but do not cross each other. In contrast, in Mg2Si1-x Ge (x) , an energy gap was detected for 0 a parts per thousand currency sign x a parts per thousand currency sign 1. Moreover, the site preference of selected impurities (Al, P, Zn, Ga, Ag, Cd, In, Sb) in Mg2Si is discussed in view of total energy calculations. It was found that In, Cd, Ag, and Zn preferentially occupy the Mg site, whereas in other cases site selectivity markedly depends on impurity amount as well as chemical potentials. The sign of the thermopower in the doped systems is analyzed from the position of the Fermi level with respect to valence/conduction band edges. The Seebeck coefficient was estimated from the simplified Mott formula at standard dopant concentrations, diluted at both crystallographic sites in Mg2Si.