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Page 2 of 23 Shanmugasundaram et al. Energy Mater. 2025, 5, 500100 https://dx.doi.org/10.20517/energymater.2024.304

convergence and that defect engineering leads to simultaneous improvement in thermoelectric transport
properties of p-type Mg Zn Sb .
1.2
1.8
2
Keywords: Mg Zn Sb , defect engineering, solid solution, band convergence, thermal conductivity
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1.8
1.2
INTRODUCTION
Thermoelectric (TE) materials have been significantly developed for heating and cooling applications. In
this, the thermal energy is directly converted into electrical energy and vice versa by TE devices .
[1,2]
Heat-to-energy conversion requires low lattice thermal conductivity (κ ) and balancing charge mobility/
L
carrier density (n) in crystalline solids. A dimensionless figure of merit (zT) represents the performance of
TE devices,

(1)

where S, σ, T, κ , and κ are denoted as the Seebeck coefficient, electrical conductivity, temperature, bipolar
e
B
and electronic thermal conductivity, respectively . Also, the zT values will be varied according to multiple
[3,4]
physical factors such as transport characteristics of carrier and phonon transport incorporated with the
materials quality factor, (β µ ), which is linked to the weighted mobility (μ ), and κ , respectively.
L
W
Higher zT values should be required to improve the conversion efficiency of the devices. Therefore, some
strategies were used to enhance the S σ via band engineering such as band convergence , band
2
[5]
[6]
[7]
sharpening , and energy filtering effect . Simultaneously, the thermal conductivity is reduced through
defect engineering , nanostructuring , microstructural defects , and suppression of the bipolar effect .
[10]
[8]
[9]
[11]
[12]
Over the past decades, the following high-performance TE materials have been reported such as Bi Te ,
2
3
[15]
[19]
PbTe , SnTe , GeTe , CoSb 3 [16] , SnSe , AgSbTe 2 [18] , and Zintl phase compounds . In this series,
[17]
[13]
[14]
traditional Bi Te -based TE materials have been well-known commercial and excellent room-temperature
3
2
TE materials for low-grade heat recovery due to their high zT. However, it contains a toxic and high-cost
element of tellurium and poor mechanical strength which limits its further development and usage in
real-time TE applications. A few disadvantages result from the strongly anisotropic transport properties
because the layered crystal structure exhibits weak Van der Waals bonds. In addition, the presence of Te in
Bi Te -based devices restricts its usage and makes it unstable above 500 K. In search of alternative materials
2
3
for low-grade heat recovery, TE materials should satisfy the conditions including being inexpensive,
non-toxic, and highly efficient performance for the next-generation modules. Zintl-based compounds have
gained considerable attention for TE applications due to their complex crystal structures and the use of
earth-abundant and inexpensive elements with decent TE performance .
[20]
Mg X (x = Bi/Sb) intermetallic compounds were introduced by Zintl and Husemann in the year 1930 and
3 2
are defined as Zintl phase compounds. Recently, Zintl phase compounds have been well-recognized as a
potential and proficient material for room-to-mid-temperature TE applications due to their excellent
performance and cost-effectiveness. In addition, Zintl materials have been potential candidates for various
[22]
[24]
applications such as batteries , photovoltaics , catalysts , hydrogen storage materials , and so on.
[21]
[23]
Especially, the TE performance of Mg Sb has been improved by using doping with a mixture of iso and
3
2
aliovalent alkali metals . This is an ideal representative of the phonon-glass and electron-crystal (PGEC)
[25]
mechanism. In recent times, p-type AB X -based Zintl has exhibited notable TE performance, in which “A”
2 2
is the Eu, Ba, Ca, Mg, Yb, “B” is the Mg, Cd, Zn, Na, and “C” is the Sb/Bi [26,27] . Compared with aliovalent
doping, dual/higher element doping plays a major role in enhancing the n/carrier mobility (μ) via band