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Shanmugasundaram et al. Energy Mater. 2025, 5, 500100 https://dx.doi.org/10.20517/energymater.2024.304 Page 13 of 23

Figure 5. (A) Temperature-dependent power factor (B) weighted mobility of Ag Mg 1.8-x Zn Sb (x = 0, 0.01, 0.03, and 0.05).
2
1.2
x
was confirmed, and the decreasing trend of μ due to the domination of phonon scattering. At 753 K, the μ W
W
was decreased for all the Ag-substituted samples.

Phonon transport properties and defect analysis
The total thermal conductivity (κ ) was calculated by κ = DρC , where D is thermal diffusivity, C is specific
T
p
p
T
heat capacity, and ρ is the density of as-prepared pellets. Figure 6A displays the temperature-dependent κ of
T
undoped and Ag-substituted Mg Zn Sb solid solutions, which obtained the κ is less than 1 W/mK.
1.2
1.8
2
T
Further, the point defect was introduced via substituting Ag at Mg sites, which helps to reduce the κ .
T
However, κ increases with Ag content at 303 K, which indicates the domination of electronic thermal
T
conductivity (κ ) (κ = κ + κ ). In contrast, the κ of Ag-substituted Mg Zn Sb samples was reduced at
T
T
L
e
2
1.2
e
1.8
higher temperatures. The lowest κ of 0.55 W/mK at 303 K and 0.96 W/mK at 753 K for undoped
T
Mg Zn Sb . The Ag Mg Zn Sb sample exhibits the κ of 0.693 W/mK at 753 K due to increased carrier
1.8
2
1.2
1.77
0.03
2
1.2
T
heat conduction in the lattice, which is 28% lower than the undoped sample. Figure 6B is the κ calculated
e
via the Wiedemann-Franz relationship, κ = LσT, where L is the Lorenz number, calculated by a single
e
parabolic band (SPB) model with acoustic phonon scattering assumption. The κ increases significantly with
e
the Ag content in the entire temperature range, being consistent with the change of σ. At room
temperature, the κ of 0.03 W/mK for undoped and 0.12 W/mK for Ag Mg Zn Sb sample, respectively.
e
0.05
1.75
1.2
2
The undoped Mg Zn Sb obtained the κ of 0.22 W/mK at 753 K, which is higher than that of
2
e
1.8
1.2
Ag-substituted samples.
Figure 6C illustrates the κ part of as prepared undoped and Ag-substituted Mg Zn Sb samples. The κ L
1.8
2
1.2
L
was calculated by subtraction of κ from the measured κ (κ = κ - κ ). In this present investigation, defect
L
T
e
e
T
engineering was introduced with the substitution of heavy element Ag at lighter Mg sites of Mg Zn Sb .
2
1.8
1.2