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Page 12 of 23 Shanmugasundaram et al. Energy Mater. 2025, 5, 500100 https://dx.doi.org/10.20517/energymater.2024.304

Figure 4E1 shows the Zn-d states, which strongly overlap with Sb-s and Mg-s from -0.2 eV to -1.3 eV of the
VB region, implying that Zn-Sb and Zn-Mg bonding structures of Mg ZnSb . It is also observed that, the
2
2
Ag-d orbitals hybridize with the Sb-p and Mg-p near the Fermi and VB regions from -0.21 eV to -0.95 eV,
illustrating the Ag-Sb and Ag-Mg bonding states of Ag at Mg ZnSb . Furthermore, the Sb-s and Mg-p
2
2
orbitals hybridize with the Zn-d orbitals from the VB region from ~ -0.51 eV to -1.25 eV indicating the
formation of a covalent bond between Zn, Sb, and Mg. Figure 4F1 shows the bottom of the near Fermi
region, which is contributed by the intermixing of p-d orbitals of Ag-Sb hybridized bands. This difference in
the electronic DOS pattern is responsible for the enhanced m* due to the incorporation of Ag in the
Mg ZnSb structure.
2
2
Figure 5A represents the PF of undoped and Ag-doped Mg Zn Sb samples, which is calculated by
1.2
2
1.8
PF = S σ. Regardless of the temperature, compared with Mg Zn Sb , Ag-substituted samples show the
2
1.8
1.2
2
increasing trend of PF due to the combination of large S and σ, respectively. When increasing the
2
2
2
concentration of Ag (x = 0, 0.01, 0.03, and 0.05), the PF value of 167 µW/mK , 299 µW/mK , 456 µW/mK ,
and 467 µW/mK was maximized at 753 K. Specifically, the highest PF observed for Ag Mg Zn Sb of
2
1.75
0.05
1.2
2
467 µW/mK at 753 K, which is ~180% larger than the Mg Zn Sb (167 µW/mK ). This result confirms that
2
2
1.2
2
1.8
Ag-substitution at Mg sites of Mg Zn Sb yields the best control over σ and S to optimize the improved PF
1.8
1.2
2
at higher temperatures.
Figure 5B represents the μ of the as-prepared samples. In addition, the bipolar conduction plays a pivoted
W
role in TE performance at high temperatures, further it can be realized by the μ . The μ was calculated
W
W
using measured electrical resistivity and S values by the Drude-Sommerfield free electron model and
analyzed phonon scattering mechanisms, which are calculated by ,
[66]
(5)
where k /e = 86.3 µV/K, T is an absolute temperature, and ρ is electrical resistivity. This is also a widely used
B
parameter to confirm the scattering mechanisms such as grain boundary scattering at room temperature,
ionized impurity scattering (T ) at room - 450 K, acoustic phonon scattering (T ) occurring above 450 K,
-3/2
3/2
and so on. Here, compared with undoped Mg Zn Sb the μ of all the Ag-substituted samples decreases
1.8
W
1.2
2
-0.65
with increasing temperature, which confirms the domination of acoustic phonon scattering (T to T ).
-1.13
Thus, this result helps to correlate with the presence of defects at a concerning temperature. To be specific,
the Mg Zn Sb and Ag Mg Zn Sb system has strong anharmonicity which is a significant contributor
1.2
2
0.03
2
1.77
1.8
1.2
to its low thermal conductivity below ~1 W/mK. The κ of the samples decreases from ~0.73 W/mK to
L
~0.56 W/mK for Mg Zn Sb and Ag Mg Zn Sb at 753 K, which is ~23% lower than undoped sample.
1.8
1.2
2
1.2
2
1.77
0.03
Typically, the independent κ served as the key parameter for attaining elevated TE performance through
L
the incorporation of defects in the lattice, offering a novel degree of freedom for modifying the physical
parameters to improve the thermal transport properties.
This result confirms that the Ag-substituted Mg Zn Sb samples have a lower μ value than the undoped
W
1.2
2
1.8
Mg Zn Sb at room temperature [i.e., 51 to 22.5 cm /Vs]. This is due to the domination of ambient
2
1.2
1.8
2
temperature defects such as grain boundaries and ionized impurities. While increasing the measuring
temperature, all Ag-substituted samples show higher μ values because of microstructural defects such as
W
secondary phases, dislocations, and point defects, respectively. This trend also confirms the domination of
acoustic phonon scattering after 453 K. In this, the carrier scattering behavior of the as-prepared samples