Tuning the carrier concentration using Zintl chemistry in Mg₃Sb₂, and its implications for thermoelectric figure-of-merit

Zintl compounds are potential candidates for efficient thermoelectric materials, because typically they are small band gap semiconductors. In addition, such compounds allow fine tuning of the carrier concentration by chemical doping for the optimization of thermoelectric performance. Herein, such tunability is demonstrated in Mg₃Sb₂-based Zintl compounds via Zn²⁺ doping at the Mg²⁺ site of the anionic framework (Mg₂Sb₂)^2-, in the series Mg_3-xZn_xSb₂ (0 ≤ x ≤ 0.1). The materials have been successfully synthesized using the spark plasma sintering (SPS) technique. X-ray diffraction (XRD) analysis confirms a single solid solution phase of Mg_3-xZn_xSb₂ (0 ≤ x ≤ 0.1). The thermoelectric properties are characterized by the Seebeck coefficient, electrical conductivity, and thermal conductivity measurements from 323 K to 773 K. Isoelectronic Zn substitution at the Mg site presents the controlled variation in the carrier concentration for optimizing the high power factor and reduced thermal conductivity. These results lead to a substantial increase in ZT of 0.37 at 773 K for a composition with x = 0.10 which is ∼42% higher than undoped Mg₃Sb₂. The electronic transport data for the Mg_3-xZn_xSb₂ (0 ≤ x ≤ 0.1) compound are analyzed using a single parabolic band model predicting that Mg_2.9Zn_0.1Sb₂ exhibits a near-optimal carrier concentration for high ZT. The electronic structure of transport properties of these disordered Mg_3-xZn_xSb₂ (0 ≤ x ≤ 0.1) is also studied using density functional theory and the results obtained are in good agreement with experimental results. The low cost, lightness and non-toxicity of the constituent elements make these materials ideal for miderature thermoelectric applications.