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Shanmugasundaram et al. Energy Mater. 2025, 5, 500100 https://dx.doi.org/10.20517/energymater.2024.304 Page 3 of 23

engineering, and simultaneously reduces the κ by manipulation of defect engineering which will enhance
L
the phonon scattering.

[28]
In common, both n and p-type semiconductor legs are more important for device fabrication purposes .
The excellent performance of n-type Mg Sb was realized, and its outcome shows a clear understanding of
3
2
defect chemistry (Mg vacancies and Mg interstitials) and electron doping (Bi), which alters the physical and
optical properties of the materials. Therefore, the room-temperature performance of Mg Sb Bi -based
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3-x
x
materials shows a state-of-the-art zT and an alternative material for traditional Bi Te -based materials . In
[29]
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3
the group of Zintl phase, n-type Mg (Sb, Bi) -based materials are known as promising and highly efficient
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3
materials with the benefits of cost-effectiveness and more stability. The Mg Sb -based compounds exhibited
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3
a p-type semiconductor nature with a band gap of ~0.6-0.8 eV. In contrast, Mg Bi is metallic in nature, but
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2
the introduction of excess Mg leads to semiconducting behavior . According to earlier reports, a high zT of
[30]
1.51 at 773 K was achieved by the introduction of excess Mg for n-Mg Sb Bi Te . Similarly, the
1.5
0.49
0.01
3.2
excellent TE performance of n-type Mg Sb with hierarchical microstructure and band degeneracy strategies
2
3
achieved an extraordinary zT of 1.85 at 723 K due to its complex crystal structure and high valley
degeneracy (N = 6) [31,32] . Jiang et al. demonstrated a conversion efficiency of 10.6% for n-type Mg Sb -based
v
2
3
devices .
[33]
At the same time, the p-type Mg Sb achieves a low zT < 1, which is lower than its n-type counterpart
3
2
because of its low electrical transport properties (minimal n and μ) with the low valence band (VB)
degeneracy (N = 1). Its strong chemical bonding enhances κ . According to Li et al., p-type Mg Sb with
[34]
v
2
3
L
double substitution achieved a peak zT of ~1.0 at 773 K via alloy scattering . Also, simultaneous
[35]
modification of cationic and anionic sites of p-type Mg Sb achieves the highest peak zT of 0.85 at 723 K .
[36]
2
3
Furthermore, the conversion efficiency of 5.5% was obtained with uni-couple p-type
Mg Yb Na Zn Sb and n-type Mg SbBi Te 0.01 [37] . However, its low conversion efficiency prohibits the
0.99
3.2
1.2
2
0.006
1.594
0.2
implementation of p-type Mg Sb -based TE devices. Therefore, regulating the electrical transport properties
3
2
of p-type Mg Sb is essential for improving the peak zT and conversion efficiency of the TE device .
[38]
3
2
According to recent research, Zn doping at Mg sites increases the σ via band convergence [39,40] . To be
specific, the superior TE performance of Mg Zn Sb solid solutions has grabbed the curiosity of
1.2
2
1.8
researchers. The Mg Zn Sb framework has been widely studied compared to Mg Sb , which possesses an
2
1.8
3
2
1.2
anti-La O structure (space group - P m1). The Zn atom occupies up to 67% of the tetrahedra in the
3
2
Mg Zn Sb structure, maintaining an identical atomic arrangement. It is widely acknowledged that zinc is
2
3-x
x
3
preferred to fill the tetrahedral voids compared to octahedral ones. This might be due to the sp orbitals
rather than the localized d-ones which are essentially accountable for forming bonds. This structural
approach reveals that, when disregarding symmetry changes, the structure of Mg Zn Sb is remarkably
x
3-x
2
different from the frameworks of other substituted Mg Sb -based phases as XMg Sb (where X is Ba/Ca/Sr).
2
3
2
2
The Coulombic repulsion was diminished throughout the Mg Sb system through the introduction of Zn,
2
3
which produced a significantly less distance among Sb atoms and fewer positive charges. Due to lesser
electronegativity (1.65 vs. 1.31 for Mg) and smaller atomic radius (1.35 vs. 1.50 Å for Mg), ends up in a lesser
positive charge on the Zn atoms, which fulfils both prerequisites. By modulating the band structure of
p-type Mg Sb , Zn introduction at the Mg site may improve its electrical performance; however, Zn excess
3
2
will lead to a narrow band gap. Thus, this work aims to provide a tellurium-free, cost-effective, and one-step
synthesis of Mg Sb -based solid solution for room-to-mid-temperature TE applications.
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To elucidate the band and defect engineering in p-type Mg Zn Sb , we performed experimental and
1.2
1.8
2
theoretical studies of undoped and Ag-doped p-type Mg Zn Sb samples. The introduction of monovalent
1.8
1.2
2