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

Abstract

Mg3(Sb,Bi)2 alloys have recently been discovered as a competitive alternative to the state‐of‐the‐art n‐type Bi2(Te,Se)3 thermoelectric alloys. Previous theoretical studies predict that single crystals Mg3(Sb,Bi)2 can exhibit higher thermoelectric performance near room temperature by eliminating grain boundary resistance. However, the intrinsic Mg defect chemistry makes it challenging to grow n‐type Mg3(Sb,Bi)2 single crystals. Here, the first thermoelectric properties of n‐type Te‐doped Mg3Sb2 single crystals, synthesized by a combination of Sb‐flux method and Mg‐vapor annealing, is reported. The electrical conductivity and carrier mobility of single crystals exhibit a metallic behavior with a typical T−1.5 dependence, indicating that phonon scattering dominates the charge carrier transport. The absence of any evidence of ionized impurity scattering in Te‐doped Mg3Sb2 single crystals proves that the thermally activated mobility previously observed in polycrystalline materials is caused by grain boundary resistance. Eliminating this grain boundary resistance in the single crystals results in a large enhancement of the weighted mobility and figure of merit zT by more than 100% near room temperature. This work experimentally demonstrates the accurate understanding of charge‐carrier scattering is crucial for developing high‐performance thermoelectric materials and indicates that single‐crystalline Mg3(Sb,Bi)2 solid solutions can exhibit higher zT compared to polycrystalline samples.

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

“…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 ].…”

Establishing the carrier scattering phase diagram for ZrNiSn-based half-Heusler thermoelectric materials

“…They can be used to generate electricity based on the Seebeck effect when a thermal gradient exists or to transfer heat against temperature gradient based on the Peltier effect when an electric current is applied 1 . Many thermoelectric materials are being explored for power generation applications, such as half-Heusler 2 , PbTe [3][4][5] , silicides 6 , CoSb 3 7 , and Mg 3 Sb 2 8 . The energy conversion efficiency for TE materials depends heavily on the TE material performance.…”

Improved Thermoelectric Performance in N-Type Half-Heusler NbCoSn by Heavy-Element Pt Doping

“…This analysis shows that the sample with x = 0.05, with post-annealing after SPS, has a grain size of a few micrometres, which is almost ten times larger than that of the sample with x = 0.06. Grain boundary scattering has recently been found to hinder the mobilities and conductivities (particularly near room temperature) of Mg3Sb2 44 , half-Heusler Nb1-xTixFeSb 45 and (Zr,Hf)CoSb 46 , and elemental Te 47,48 . Herein, an increase in the grain size in post-annealed samples can support their high electrical conductivities and carrier mobilities.…”