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

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-delta(Si0.5Sn0.5)(1-x)Sb-x by Mg-pressure-controlled annealing via solid-gas equilibrium. The range of possible Mg deficiency delta, 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 Delta delta = 0.012 (0.037 <= delta <= 0.049) is obtained. Owing to Mg non-stoichiometry, Mg-rich Mg2-delta(Si0.5Sn0.5)(0.90)Sb-0.10 (d = 0.037) shows a roughly 18 times higher carrier concentration (n = 3.7 x 10(20) cm(-3)) than the Mg-poor composition ( d = 0.049 and n = 0.2 x 10(20) cm(-3)). The Mg content of Mg2-delta(Si0.5Sn0.5)(1-x)Sb-x 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 zT(max) of 1.0 at 773 K for Mg2-delta(Si0.5Sn0.5)(0.90)Sb-0.10 with an intermediate Mg content (delta = 0.042 and n = 2.2 x 10(20) cm(-3) under P-Mg = 2 Pa) that is larger than those for Mg-rich and Mg-poor compositions (zT(max) = 0.9 and 0.6 for delta = 0.037 and 0.049, respectively). (C) 2020 Elsevier B.V. All rights reserved.

Automated summary

The control of Mg content in Mg2Si-based thermoelectric materials is crucial for tuning the carrier concentration. Through Mg-pressure-controlled annealing, precise control of Mg content was achieved in Mg2-delta(Si0.5Sn0.5)(1-x)Sb-x, demonstrating the impact of Mg content on carrier concentration, Seebeck coefficient, and zT values.