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Shanmugasundaram et al. Energy Mater. 2025, 5, 500100 https://dx.doi.org/10.20517/energymater.2024.304 Page 15 of 23

The above-mentioned defects promotes the scattering of different wavelength phonons, thus lowering the
thermal conductivity of Ag-substituted samples at elevated temperatures. Figure 6D shows the comparison
of κ with previous reports and present work, which confirms that Ag substitution in the Mg Zn Sb solid
2
1.8
1.2
L
solution significantly strengthens the various frequency phonon scattering and reduces the κ of 0.56 W/mK
L
at 753 K for the Ag Mg Zn Sb sample [68,69] . Here, it should be noted that the Ag substitution at the Mg
1.2
1.77
0.03
2
site emphasizes the significance of 0D point defects in the lattice via atomic dislocations, which strengthen
the phonon transport properties. The high-frequency phonons were scattered via point defects and the
4
relaxation time is ~ω . In this instance, it can be observed that Mg vacancy and interstitial Ag atoms
exhibit a significant capacity to diminish κ at similar point defect ratios.
L
However, the role of the bipolar effect is dominant via ionized impurity scattering which enhances the κ at
e
303 K. Due to the contribution rate of phonon-electron scattering than phonon-phonon scattering, the
enhancement of κ was obtained at 303 K. After increasing the temperature, the Ag Mg Zn Sb samples
L
x
2
1.2
1.8-x
exhibit lower κ , which is also the contribution of κ . e
L
Further, the microstructure details of Ag Mg Zn Sb sample were investigated by high-resolution TEM
1.77
1.2
2
0.03
characteristics, shown in Figure 7A-F. The intrinsic defects such as grain boundaries (long), dislocations
(mid), and stacking faults (short) have been identified using microstructural high-resolution TEM images
with Fast Fourier Transform (FFT) and IFFT patterns, respectively. In addition, the lattice strain has been
explored via geometric phase analysis (GPA) with HR-TEM results. The mass contraction between Mg and
Ag leads to lattice distortion and produces point defects. The existence of stacking faults, which change the
atomic arrangement via the scattering of short wavelength phonons and serve as phonon scattering centers
via mass and strain fluctuations, was discovered by utilizing the IFFT pattern from the HR-TEM image
[Supplementary Table 3]. The heavy element Ag substitution at lighter Mg sites of Mg Zn Sb solid
1.2
1.8
2
solution creates point defects in the form of dislocations and scatter mid-wavelength phonons,
respectively . Further, the different orientations of grains and grain boundaries scatter long wavelength
[70]
phonons near room temperature. The d-spacing values of 0.26 nm (110), 0.27 nm (110), and 0.35 nm (011)
indicate that each grain is crystallized with random orientation shown in Figure 7A. These grain boundaries
help scatter the long-wavelength phonons, which are determined by Klemens and Matthiessen’s
expression , where ν is the Poisson ratio and L is grain size. Figure 7B represents the IFFT
G
images of dislocation clusters and perfect crystal lattice, which are extracted from Figure 7C. This indicates
that Ag substitution introduces different wavelengths of defects and plays a major role in Mg Zn Sb solid
2
1.8
1.2
solution. In Figure 7D and E, the obtained dislocation indicates the heavy Ag substitution in the undoped
system leads to changes in the arrangement of atoms, which complicates the atom’s interaction and
strengthens the phonon scattering [Scheme 1A and B] . Furthermore, the presence of stacking fault
[10]
represents Ag substitution interrupting the order of stacked planes in a matrix [Figure 7F]. The obtained
defects, such as stacking faults and dislocations, scatter the mid-wavelength phonons and reduce κ of the
L
as-prepared samples [71,72] . Supplementary Figure 8 shows the calculated microstrain vs. κ , which confirms
L
the heavy element Ag at anionic Mg2 sites enhances the microstrain significantly scatters the phonons, and
reduces the κ of as-prepared samples.
L
In addition, the GPA has been performed to find the interaction of strain and displayed in Figure 7G-H1.
Figure 7G and H are the selected strain fields by GPA strain analysis from HRTEM analysis. Figure 7G1-H1
represents the presence of strain clusters and grain boundaries in the axes of ɛ , and ɛ with a scale bar of -1
xy
xx
to 1. The positive value indicates the high magnitude strain in the lattice [45,73] . This result confirms the
influence of Ag in the lattice enhancing high-magnitude lattice strains and strengthening the phonon
scattering. Thus, the obtained various defects and lattice strain significantly enhanced the phonon scattering