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

energy (∆E ) of the Zn and Ag at Mg Sb is calculated by ∆E (ZnAg-Mg Sb ) = E( ZnAg-Mg2Sb2 ) - (E Zn-Mg3Sb2 +
form
2
2
form
3
2
E ), where Zn-Ag-Mg Sb , E( ZnAg-Mg3Sb2 ), and E denote the total energy of the Zn-Ag doped Mg Sb system,
2
Ag
Ag
3
3
2
the total energy of Zn@Mg Sb without Ag dopant, and the total energies of Ag in the bulk phase,
3
2
respectively. A plane-wave basis with a cutoff of 900 eV was used to calculate electron densities and wave
functions. Electronic structure calculations, i.e., total density of states (TDOS), projected density of states
(PDOS), band structure, and electron density analysis, were then simulated using the most stable
geometries. All optimized structures presented in this study were taken from the VESTA packages.
RESULTS AND DISCUSSIONS
Schematic illustration of the crystal structures and XRD of Ag Mg 1.8-x Zn Sb 2
1.2
x
The Mg Zn Sb samples expose the hexagonal crystal structures with a space group of p m1. Zn
1.8
2
1.2
is the suitable acceptor for boosting the density of holes, as well as enhancing the electronic transport
properties, resulting in a narrow bandgap and manipulating the band structure in the Mg Sb system. There
2
3
are two distinct crystallographic sites of Mg atoms available in Mg Sb : Mg /(Mg1-octahedral) and
2+
2
3
2-
[Mg Sb ] /(Mg2-tetrahedral). The Zn atoms are potential substitution for tetrahedral sites and proximity to
2
2
neighbouring Sb atoms assists in an overall decrease in coulombic repulsion. Specifically, zinc atoms have
lower electronegativity (χ = 1.65), shorter atomic radius (142 p.m.), and a smaller positive charge than
magnesium atoms. The electronegativity difference (Δχ) between the substitution (Mg) and foreign atom
(Zn) was a more important parameter to enhance the transport properties and produced internal defects. In
general, the Δχ was calculated by |Δχ| = |χHost - χdopant|. The calculated Δχ between Zn at the Mg site is
0.29 and at the Sb site is 0.12 [Supplementary Table 1]. In addition, the smaller positive charges of Zn atoms
are substituted at Mg sites to enhance the transport properties of Mg Sb . The differences in
2
3
electronegativity and atomic mass with host and dopant sites are shown in Supplementary Figure 1A and B.
Additionally, this contains multiple states compared to Mg Sb and is situated below the Fermi level (E )
2
3
F
which manipulates its band structure through a band convergence strategy. For further enhancing the TE
transport properties, the heavy and aliovalent element Ag was introduced at Mg sites of Mg Zn Sb .
1.2
2
1.8
Figure 1A and B illustrates the hexagonal crystal structure of undoped and Ag-substituted Mg Zn Sb . The
1.2
1.8
2
XRD image of Ag-substituted Mg Zn Sb (x = 0, 0.01, 0.03, and 0.05) samples were shown in Figure 1C.
1.2
1.8
2
The introduction of Ag in Mg Zn Sb shows a lower angle shift in the XRD result, where a secondary Sb
2
1.8
1.2
phase was observed at 42.4 (JCPDS NO: 96-901-3011). As can be seen, the occurrence of Mg vacancies
o
2-
(V ) is due to the vaporization of Mg during heat treatment. The presence of secondary Sb can confirm
Mg
the negative formation energy of low energy acceptor defect as V Mg 2- at the lattice which act as acceptor
defects (Mg = V + 2h). These defects promote a greater number of holes or absorb excess electrons, and
2-
Mg
Mg
[42]
lead to the p-type pinning behavior through the introduction of energy states that are close to the VB .
Here, the p-type materials pin the E , enabling the system in an equilibrium state via intrinsic acceptor
F
defects and the material retains its p-type nature and reduces E (If n-type materials pin the E due to
F
F
domination of intrinsic donor defects, the material remains n-type).
The ability to function as p-type TE semiconductors without any impurity substitution is caused by the
formation of an adequate number of robust cation site vacancies [43,44] . Figure 1D represents the expanded
portion of XRD analysis, which confirms the lower angle shift leads to the expansion and tensile strain. In
the Mg Sb system, Zn has a large solubility limit (70%-90%) because of its (134 pm) minimal ionic radius
2
3
compared with Mg (145 pm) [45,46] . The increment of lattice parameters confirms the replacement of Mg 2+
+
2+
ions with Zn and Ag ions at the Mg sites, which decreases the crystallite size and indicates the
contribution of strain in the lattice.