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High-Performance Low-Cost n‑Type Se-Doped Mg3Sb2‑Based Zintl Compounds for Thermoelectric Application

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journal contribution
posted on 04.06.2017, 00:00 by Jiawei Zhang, Lirong Song, Aref Mamakhel, Mads Ry Vogel Jørgensen, Bo Brummerstedt Iversen
Thermoelectric materials, capable of converting heat directly into electricity without moving parts, provide a promising solid-state solution for waste heat harvesting. However, currently available commercial thermoelectric materials PbTe and Bi2Te3 are based on tellurium, an extremely scarce and expensive element, which prohibits large scale applications. Herein, we present a systematic study on a new low-cost Te-free material, n-type Se-doped Mg3Sb1.5Bi0.5, by combining the structure and property characterization with electronic structure and electrical transport modeling. Compared with pure Mg3Sb2, Se-doped Mg3Sb1.5Bi0.5 shows the considerably enhanced power factor as well as much lower thermal conductivity. The excellent electrical transport originates from a nontrivial near-edge conduction band with six conducting carrier pockets and a light conductivity effective mass as well as the weak contribution from a secondary conduction band with a valley degeneracy of 2. The accurate location of the conduction band minimum is revealed from the Fermi surface, which appears to be crucial for the understanding of the electronic transport properties. In addition, the total thermal conductivity is found to be reasonably low (∼0.62 W m–1 K–1 at 725 K). As a result, an optimal zT of 1.23 at 725 K is obtained in Mg3.07Sb1.5Bi0.48Se0.02. The high zT, as well as the earth-abundant constituent elements, makes the low-cost Se-doped Mg3Sb1.5Bi0.5 a promising candidate for the intermediate-temperature thermoelectric application. Moreover, the systematic electronic structure and transport modeling provide an insightful guidance for the further optimization of this material and other related Zintl compounds.