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Page 4 of 23 Shanmugasundaram et al. Energy Mater. 2025, 5, 500100 https://dx.doi.org/10.20517/energymater.2024.304

Ag at Mg site significantly increases the n and σ of 225 S/cm at 303 K. Along with that, the VB convergence
increases the S of 257 μV/K at 753 K. Simultaneously, phonon transport is enhanced through multiscale
defect scatterings, which reduces the κ to 0.56 W/mK at 753 K for Ag Mg Zn Sb . This increment in
L
2
0.03
1.2
1.77
carrier transport and strong phonon transport of Ag Mg Zn Sb synergistically achieved a peak zT of 0.5
1.77
2
0.03
1.2
at 753 K, which is ~285% higher than undoped Mg Zn Sb . This study suggests that the heavy/aliovalent
1.2
1.8
2
substitution on the Mg Zn Sb system simultaneously optimizes the carrier and phonon transport, which
1.2
1.8
2
helps to develop a potential candidate for room-to-mid-temperature TE applications.
EXPERIMENTAL
Materials preparation of p-type Mg Zn Sb and Ag Mg 1.8-x Zn Sb pellets
1.2
x
1.8
2
2
1.2
The p-type Mg Zn Sb and Ag Mg Zn Sb (x = 0, 0.01, 0.03, and 0.05) samples were prepared by a spark
1.2
1.8
2
1.2
2
1.8-x
x
plasma sintering technique. Magnesium (Mg, 99.5%, metal turnings), zinc (Zn, 99.5%, metal powder), silver
(Ag, 98.8%, metal powder), and antimony (Sb, 99.5%, metal powder) were weighed according to the
stoichiometric ratio of Ag Mg Zn Sb (x = 0, 0.01, 0.03 and 0.05). All elements of Mg, Sb, Ag, and Zn
x
1.8-x
2
1.2
metal powders were grounded using mortar and pestle. The grounded powders were loaded into a
cylindrical graphite die (inner diameter 13 mm) and subjected to sintering via spark plasma sintering
technique at 873 K under the pressure of 40 MPa for 5 min holding time and cooling rate (50 ºC/min) to
obtain dense disk-shaped pellets [Supplementary Scheme 1A and B].
Structural characterizations and TE transport property measurements
X-ray diffraction (XRD) was used for the phase composition of each sample characterized by the help of a
PANalytical multipurpose diffractometer under CuKα radiation (λ = 1.5406 Å). High-resolution
transmission electron microscopy (HR-TEM; JEOL JEM-2100 Plus with an operating voltage of 200 kV).
The microstructure and compositional analysis of the samples was performed using high-resolution
scanning electron microscopy (HR-SEM; Thermoscentific ApreoS) equipped with energy dispersive
spectroscopy (EDS). Thermal conductivity (κ) was calculated by κ = ρDC , where ρ is the sample density
p
(estimated based on the Archimedes method), D is the thermal diffusivity (measured by a laser flash
apparatus (LFA 467 HT, NETZSCH)), and C is the specific heat (determined by differential scanning
p
calorimetry thermal analyzer). The σ and S were simultaneously measured by a commercial ADVANCE
RIKO ZEM-3 system under a helium atmosphere. The room temperature Hall measurement was measured
by using ECOPIA HMS 5300. The sample’s carrier concentration (n) and mobility (μ) were calculated using
the four-probe Van der Pauw method under a magnetic field.
Computational methods
All the periodic density functional calculations were computed by using the Vienna Ab initio Simulation
Package (VASP), with the Projected Augmented Wave (PAW) method applied to account for the
electron-ion interaction terms. The VASP package utilizes the Perdew-Burke Ernzerhof (PBE)
exchange-correlation function in combination with PAW potentials. The convergence criteria of 0.03 eV/Å
in force and 10 eV in energy were adopted to optimize the supercells. The Monkhorst-Pack K-point mesh
-5
calculated the relaxation and self-consistent computations of the crystal structures . In general, the
[41]
exchange-correlation energy comprises the exchange energy (from the Pauli exclusion principle) and the
correlation energy (from Coulomb interaction effects), which are treated in terms of different
approximations, expressed as E [ρ] = T [ρ] + E [ρ] + E [ρ] + E [ρ], where T [ρ] is kinetic energy (KE) of
ext
xc
total
H
s
s
non-interacting electrons, E [ρ] is external potential energy (PE), E [ρ] is classical Hartree energy, and
ext
H
E [ρ] is exchange-correlation energy. In our work, the Monkhorst-Pack grid generates a uniform K-point
xc
grid of 3 × 3 × 2 for geometry optimization and 6 × 6 × 4 for electronic structure simulations. In addition,
the basis set is a critical factor that directly determines the accuracy and efficiency of electronic structure
prediction and is mainly based on the (I) plane wave basis set and (II) pseudopotentials. The formation