Significant Role of Mg Stoichiometry in Designing High Thermoelectric Performance for Mg₃(Sb,Bi)₂-Based n-Type Zintls

Abstract

Complex structures with versatile chemistry provide considerable chemical tunability of the transport properties. Good thermoelectric materials are generally extrinsically doped semiconductors with optimal carrier concentrations, while charged intrinsic defects (e.g., vacancies, interstitials) can also adjust the carriers, even in the compounds with no apparent deviation from a stoichiometric nominal composition. Here we report that in Zintl compounds MgSbBi, the carrier concentration can be tuned from p-type to n-type by simply altering the initial Mg concentration. The spherical-aberration-corrected (C-corrected) high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) and energy-dispersive X-ray spectroscopy (EDX) mapping analysis show that the excess Mg would form a separate Mg-rich phase after Mg vacancies have been essentially compensated. Additionally, a slight Te doping at Bi site on MgSbBi has enabled good n-type thermoelectric properties, which is comparable to the Te-doped Mg-rich sample. The actual final composition of MgSbBi analyzed by EPMA is also close to the stoichiometry MgSbBi, answering the open question whether excess Mg is prerequisite to realize exceptionally high n-type thermoelectric performance by different sample preparation methods. The motivation for this work is first to understand the important role of vacancy and then to guide for discovering more promising n-type Zintl thermoelectric materials.

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

“…Undoped Mg 3.2 Sb 1.5 Bi 0.5 is an n‐type semiconductor with ultralow electrical conductivity. The electronic transport properties can be obviously improved by doping on Bi site . Figure shows the temperature‐dependent a) electrical conductivity, b) Seebeck coefficient, and c) power factor for optimized n‐type Mg 3.2 Sb 1.5 Bi 0.5‐ x Se x ( x = 0.005, 0.0075, 0.01, and 0.0125).…”

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

“…Several experimental and theoretical efforts 54,60,61,63,65,121,122 reveal that a large amount of excess Mg is not needed for achieving n-type properties as claimed by Tamaki et al 55 . A tiny amount of excess Mg or even no excess Mg is required to realize n-type properties as long as great care is taken to avoid the Mg loss during the synthesis, consistent with the earlier report without excess Mg by Zhang et al 54 The samples with smaller amounts of excess or no excess Mg even show better performance and stability than those with large amounts of excess Mg. 63,122 Various reports with different amounts of excess Mg may arise from the difficulty in preparing these materials without the Mg loss due to the high reactivity, easy oxidation, and high vapor pressure of Mg. Fig.…”

“…1), [53][54][55] comparable or even superior to the commercial n-type TE materials, such as Bi 2 Te 3 and PbTe. Hence, significant research efforts [56][57][58][59][60][61][62][63][64][65][66][67][68][69][70][71][72][73] on this promising material system are currently ongoing. Theoretically, electronic structures, 20,74-77 chemical bonding, 11,12,78,79 defects, 55,60,61,80 and phonon-related properties [81][82][83][84] have been extensively studied to understand the transport properties.…”

Enhanced Thermoelectric Performance in N‐Type Mg3.2Sb1.5Bi0.5 by La or Ce Doping into Mg

“…The high volatility of Mg induces the formation of Mg vacancies, which act as acceptors and make electrons the minority carriers 9. However, n‐type Mg 3 Sb 2 was recently prepared by adding excess Mg in order to suppress the Mg vacancies 10–14. This has inspired the further enhancement of electron concentration by extrinsic doping, typically by a chalcogen 9–12,15–17.…”

“…This has inspired the further enhancement of electron concentration by extrinsic doping, typically by a chalcogen 9–12,15–17. Combined with the alloying of Mg 3 Bi 2 to decrease the lattice thermal conductivity, ZT values higher than 1.5 have been widely achieved 9,11,13,14,16,18–21. Furthermore, an enhanced room‐temperature ZT , which is close to that of the commercially available Bi 2 Te 3 , has been obtained in chalcogen‐doped Mg 3 Sb 2 ‐based materials by co‐doping with transition metals (e.g., Mn),17,18 tuning the mole ratio of Bi and Sb,17,22 and deliberately increasing the grain size,15,18,19,23 all of which result in increased carrier mobility.…”