Effective Electron Mass of ordered AgPbmSbTe2+m clarified by ab-initio calculations

Experimental data on PbTe2-AgSbTe2-solid solutions near the metal-insulator- transition (MIT) showed controversy resulting in high [Hsu, KF, et al., 2004] or low [Kosuga, A, et al., 2005] values of the Seebeck-coefficient and could partly be explained by specimen inhomogeneties and resulting measurement difficulties [Chen, N, et al., 2005] or different ordering states [Bilc, D, et al., 2004]. The goal of this investigation is to clarify which ordering state is expected to have the higher Seebeck coefficient by estimating the effective mass from ab-initio band structure calculations, as successfully applied for Nb-doped SrTiO3 [Wunderlich, W, et al., 2006] and metallic alloys. While the effective masses for the metallic and the semiconducting compositions were less than 0.5, the effective mass near the MIT was around m*/m0 = 9 for SrNb0.2Ti0.8O2.98 [Wunderlich, W, et al., 2006]. The effective mass depends on ordering, namely on the distance of the foreign atoms. These findings give important guidelines for further improvement of this advanced thermoelectric material