Thermoelectric Properties of p-Type Mg₂Si_0.25Sn_0.75 Doped with Sodium Acetate and Metallic Sodium

Abstract

We have investigated the thermoelectric properties of p-type Na-doped Mg₂ Si_0.25Sn_0.75 solid solutions prepared by liquid–solid reaction and hot-pressing methods. Na was introduced into Mg₂Si_0.25Sn_0.75 by using either sodium acetate (CH₃COONa) or metallic sodium (2 N). The samples doped with sodium acetate consisted of phases with antifluorite structure and a small amount of MgO as revealed by x-ray diffraction, whereas the sample doped with metallic sodium contained the Sn, MgO, and Mg₂SiSn phases. The hole concentrations of Mg_1.975Na_0.025Si_0.25Sn_0.75 doped by sodium acetate and metallic sodium were 1.84 × 10²⁵ m⁻³ and 1.22 × 10²⁵ m⁻³, respectively, resulting in resistivities of 4.96 × 10⁻⁵ Ω m (sodium acetate) and 1.09 × 10⁻⁵ Ω m (metallic sodium). The Seebeck coefficients were 198 μV K⁻¹ (sodium acetate) and 241 μV K⁻¹ (metallic sodium). The figures of merit for Mg_1.975Na_0.025Si_0.25Sn_0.75 were 0.40 × 10⁻³ K⁻¹ (sodium acetate) and 0.25 × 10⁻³ K⁻¹ (metallic sodium) at 400 K. Thus, sodium acetate is a suitable Na dopant for Mg₂Si_1−x Sn _x .