Mg-pressure-controlled annealing for tuning Mg content and thermoelectric properties of Mg2–δ(Si0.5Sn0.5)1–xSbx

Abstract

Control of the Mg content is essential for tuning the carrier concentration of Mg2Si-based thermoelectric materials. In this study, we achieved precise control of the Mg content in Mg2–δ(Si0.5Sn0.5)1–xSbx by Mg-pressure-controlled annealing via solid-gas equilibrium. The range of possible Mg deficiency δ, which is estimated under assumption that each Sb donates one electron and Mg vacancy compensates them, depends on the Sb content x. At x = 0.10, the widest range Δδ = 0.012 (0.037 ≤ δ ≤ 0.049) is obtained. Owing to Mg non-stoichiometry, Mg-rich Mg2–δ(Si0.5Sn0.5)0.90Sb0.10 (δ = 0.037) shows a roughly 18 times higher carrier concentration (n = 3.7 × 1020 cm−3) than the Mg-poor composition (δ = 0.049 and n = 0.2 × 1020 cm−3). The Mg content of Mg2–δ(Si0.5Sn0.5)1–xSbx shifts the Fermi level within the rigid band approximation, resulting in a consistent relationship between the Seebeck coefficient and carrier concentration in both experiment and DFT calculation. Mg-pressure-controlled annealing enables carrier concentration tuning for zT maximization and yields a zTmax of 1.0 at 773 K for Mg2–δ(Si0.5Sn0.5)0.90Sb0.10 with an intermediate Mg content (δ = 0.042 and n = 2.2 × 1020 cm−3 under PMg = 2 Pa) that is larger than those for Mg-rich and Mg-poor compositions (zTmax = 0.9 and 0.6 for δ = 0.037 and 0.049, respectively).