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

Abstract

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 Zn(2+) doping at the Mg(2+) site of the anionic framework (Mg2Sb2)(2-), in the series Mg3-xZnxSb2 (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 Mg3-xZnxSb2 (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 Mg3-xZnxSb2 (0 ≤ x ≤ 0.1) compound are analyzed using a single parabolic band model predicting that Mg2.9Zn0.1Sb2 exhibits a near-optimal carrier concentration for high ZT. The electronic structure of transport properties of these disordered Mg3-xZnxSb2 (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 mid-temperature thermoelectric applications.

High‐Performance N‐type Mg3Sb2 towards Thermoelectric Application near Room Temperature

“…Most of Zintl phases are intrinsically p‐type and difficult to be doped to n‐type semiconductors. While Mg 3 Sb 2 ‐based Zintl‐phase materials show outstanding n‐type TE performance by chalcogens or rare earth doping . The maximum peak ZT value of Te‐doped Mg 3+δ Sb 1.5 Bi 0.5 is higher than ≈ 1.5 at 723 K .…”

Isotropic Conduction Network and Defect Chemistry in Mg3+δSb2‐Based Layered Zintl Compounds with High Thermoelectric Performance

“…From the c direction (Figure c), we observe a tunnel‐like porous structure in the tetrahedron network; these pores likely serve as hosts for interstitial atoms as discussed later. Among thermoelectric researchers, the family of Mg 3 Sb 2 has been known as persistent p‐type thermoelectric materials with moderate performance; n‐type properties with a low carrier concentration (≈10 18 cm −3 ) have only been reported in Mn‐substituted single crystals . The persistent p‐type character has been commonly observed in CaAl 2 Si 2 ‐type compounds that belong to the same crystal structure class .…”

“…Excess Mg is required to achieve multivalley n‐type carrier transport with high thermoelectric performance. Stoichiometric Mg 3 Sb 2 is a persistent p‐type material due to negatively charged Mg vacancies that pin the Fermi level around the valence band. Thus, electron carriers are not generated only by substituting Sb with n‐type dopants (Te or Se).…”

Insights into the design of thermoelectric Mg3Sb2 and its analogs by combining theory and experiment

“…Pure Mg 3 Sb 2 is intrinsically p-type, and many attempts at doping or substituting with a series of elements result in p-type behaviors as well. [38][39][40][41][42][43][44][45][46][47][48][49][50]52,102,[126][127][128][129][130] This is because of the low formation energies of the negatively charged Mg vacancies (V 2À Mg1 and V 2À Mg2 ) (Fig. 8), which prevent the chemical potential from moving close to the conduction bands for the Sbrich condition.…”