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Significant effect of Mg-pressure-controlled annealing: non-stoichiometry and thermoelectric properties of Mg2-δSi1-xSbx.

Non-stoichiometry related to such an element with high vapor pressure as Mg has been difficult to control, despite its significant influence on the thermoelectric properties of the material. Here, Mg non-stoichiometry and the thermoelectric properties of Mg2-δSi1-xSbx (0 ≤ x ≤ 0.60) are investigated systematically by tuning the Mg partial pressure during annealing. The range of Mg non-stoichiometry Δδ depends on x since the Sb dopant induces Mg vacancies, which are partly filled by Mg atoms in the gas phase according to an equilibrium reaction. The defect association between the Mg vacancies and the Sb dopant decreases Δδ at high Sb contents. The maximum Δδ of 0.016, which corresponds to a carrier concentration range Δn of 4.9 × 1020 cm-3, is obtained at x = 0.10. The Seebeck coefficient shows a universal relationship with the carrier concentration, which can be explained by the DFT-calculated band structure within the rigid band approximation. The carrier mobility is reduced significantly as a result of carrier scattering at the Mg-poor grain boundaries. The vacancy formation at the Mg-site, in addition to Si-site substitution for Sb, effectively suppresses the lattice thermal conductivity. The low thermal conductivity on the Mg-poor side increases the maximum zT, while a high energy conversion efficiency is obtained on the Mg-rich side, owing to the high electrical conductivity and resultant large zT at low temperatures. Mg non-stoichiometry is a key factor in tuning the thermoelectric properties of Mg2Si-based materials and thus its control is essential.