Mg Deficiency in Grain Boundaries of n‐Type Mg₃Sb₂ Identified by Atom Probe Tomography

Abstract

Highly resistive grain boundaries significantly reduce the electrical conductivity that compromises the thermoelectric figure‐of‐merit zT in n‐type polycrystalline Mg3Sb2. In this work, discovered is a Mg deficiency near grain boundaries using atom‐probe tomography. Approximately 5 at% of Mg deficiency is observed uniformly in a 10 nm region along the grain boundary without any evidence of a stable secondary or impurity phase. The off‐stoichiometry can prevent n‐type dopants from providing electrons, lowering the local carrier concentration near the grain boundary and thus the local conductivity. This observation explains how nanometer scale compositional variations can dramatically determine thermoelectric zT, and provides concrete strategies to reduce grain‐boundary resistance and increase zT in Mg3Sb2‐based materials.

Metallic n‐Type Mg3Sb2 Single Crystals Demonstrate the Absence of Ionized Impurity Scattering and Enhanced Thermoelectric Performance

“…While initially attributed to ionized impurity scattering, [ 3,23–25 ] there has been growing evidence supporting that this low‐temperature behavior stems from thermally activated grain‐boundary resistance. [ 20–22,26 ] The effect is more pronounced for Sb‐rich alloys, particularly Mg 3 Sb 2 . [ 5 ] Additionally, a previous grain boundary resistance model study predicted more than 50% improvement of the room‐temperature weighted mobility if the grain boundary resistance is completely removed.…”

High-Performance Mg 3 Sb 2- x Bi x Thermoelectrics: Progress and Perspective

“…Moreover, they predicted a large improvement in the room temperature zT if the grain boundary resistance is eliminated. Later, using atom probe tomography, the grain boundary phase is identified to be Mg-deficient (approximately 5 at%) ( Figure 6(b) ), which is attributed to be responsible for the high grain boundary resistance [ 98 ]. Aiming at improving the μ and σ near RT by suppressing the grain boundary resistance, one direct way is to simply reduce the number of grain boundaries to get coarse-grained polycrystalline samples, which can be achieved by either increasing the sintering temperature [ 78 , 82 ] or post-annealing under Mg atmosphere if initial grain sizes are small [ 82 ].…”

“…Hence, a deep microstructure analysis to know the grain boundary region better with advanced techniques is highly necessary [ 184 , 185 ]. For n -type Mg 3 Sb 2 polycrystalline samples, the grain boundary phase is found to be a 10 nm region with Mg deficiency [ 98 ], resulting in the high electrical resistance. One possible indication thereby is if there are ways to make the grain boundary phase have an equal or even lower resistance than the matrix, the “anomalous” μ ( T ) and σ ( T ) near RT might vanish.…”

“…Fortunately, even near RT, n -type Mg 3 Sb 2- x Bi x also shows good zT of around 0.8 ( Figure 1(b) ), making it a promising alternative to the state-of-the-art n -type Bi 2 Te 3- x Se x for solid-state cooling application. Although only 4 years have passed since the discovery of n -type Mg 3 Sb 2- x Bi x , there are already many important advances achieved, including the improvement of TE performance [ 33 , 48 , 70 – 73 , 78 – 80 , 82 , 92 , 93 ], the understanding of the origin for good power factor [ 70 , 71 ] and intrinsically low κ [ 94 ], the revelation of the carrier scattering mechanism near RT [ 82 , 95 , 96 ], the increasing awareness of Mg defect chemistry [ 70 , 97 , 98 ], and even the successful attempt of TE module [ 33 ].…”

N-Type Mg 3 Sb 2- x Bi x Alloys as Promising Thermoelectric Materials

“…Kuo et al explored the defect compositions near the grain boundary of Mg 3.05 Sb 1.99 Te 0.01 (nominal composition) using 3D atom-probe tomography (APT) ( Figure 3(c) ), from which the planar defect is clearly noticeable (as marked by the arrow), and it is a maximum 5 at. % Mg deficiency [ 56 ]. As discussed above, a Mg deficiency could easily induce a high Mg vacancy ( V Mg 2- ) concentration in the vicinity of the boundary and result in the depletion of free n-type carriers since V Mg 2- serves as an effective electron-killing defect ( Figure 3(d) ).…”