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Page 2 of 18 Seo et al. Energy Mater. 2025, 5, 500123 https://dx.doi.org/10.20517/energymater.2025.38

INTRODUCTION
Recent advancements in energy research have focused on developing sustainable solutions to address the
[1]
global energy crisis . Among these, thermoelectric (TE) materials hold a great deal of promise for directly
converting waste heat into electrical energy . The efficiency of such materials is typically evaluated using
[1,2]
2
the figure-of-merit ZT, commonly defined as ZT = σS T/κ, where σ is the electrical conductivity, S is the
Seebeck coefficient, T is the absolute temperature, and κ is the thermal conductivity . As is obvious, a high
[2]
σ, large S, and low κ are necessary to obtain an overall large ZT. Within the field of potential TE materials,
Zintl phase compounds have garnered significant interest owing to their inherent semiconducting
properties, complex crystal structures, and high thermal stabilities .
[3]
Over the last decade, our research team has tried to discover new Zintl phase systems and understand the
correlation among the crystal structure, chemical compositions, and physical properties for TE applications.
These phases includes the A MSb (A = Ca, Yb; M = Al, Mg, Mn, Zn) , the AM Sb (A = Ca, Sr, Ba, Eu,
[4,5]
2
2
14
11
Yb; M = Zn, Cd) , the Ca Cd 3+x-y M Sb (M = Cu, Zn) , and the A M Sb (A = Ca, Eu, Yb; M = Mn, Zn,
[8]
[6,7]
4+x
9
9
9
x+y
9
Cd) systems [9-11] . Among them, of particular interest is the Ca Mn Bi -type A M Sb (A = Ca, Eu, Yb; M =
9
4
9
9
4+x
9
Mn, Zn, Cd) systems [9-11] . Research on this system has been widely conducted all over the world owing to its
considerably high ZT, and to enhance the ZT even further, many studies have explored various doping and
substitution strategies on this system. However, the polymorphic feature of the “9-4-9” phase often
produced some other “9-4-9” structure types rather than the targeted Ca Mn Bi -type phase , such as the
[12]
9
4
9
[15]
three orthorhombic Ca Mn 4.41(1) Sb -type , Eu Mn 2.87(4) Al Sb -type , Ca La 0.73(1) Mn Sb -type phases , and
[14]
[13]
9
4
1.13
9
9
9
8.27
9
the one hexagonal Ca Zn In Sb -type phase . To address this issue, firstly, we attempted to apply the
[16]
3.1
9
0.9
9
cation mixture using both Ca and Yb in the title system, which showed significant potential for enhancing
ZT by increasing phonon scattering and reducing lattice thermal conductivity [17,18] . Secondly, in the
[19]
Ca Mn Bi -type phase, interstitial M metal sites were found to significantly affect TE properties . This
9
4
9
suggests that adjusting the interstitial M metal content could enhance TE performance. Based on these
findings, we hypothesized that p-type doping of Cu for Zn could increase charge carrier concentration,
2+
+
improve electrical conductivity, and eventually enhance TE performance. Thus, we investigated the
synergetic effect of co-substitution using both cationic and anionic elements in the title Ca Yb Zn Cu Sb 9
4.5-y
x
9-x
y
system.
In this study, we successfully synthesized seven new compounds in the Ca Yb Zn Cu Sb (0 ≤ x ≤ 1.5,
9-x
4.5-y
y
9
x
0 ≤ y ≤ 0.15) system by the molten Pb-flux reaction method. A thorough analysis of the crystal structure
using powder X-ray diffraction (PXRD) and single-crystal X-ray diffraction (SXRD) confirmed that all
samples crystallized in the Ca Mn Bi -type phase. The distinct preference of cationic elements for one of the
4
9
9
five available sites was explained by employing the electronic-factor criterion, derived from Q-values
(QVAL) associated with individual atomic positions. Density functional theory (DFT) calculations using
two structural models of Ca Zn Sb and Ca YbZn Cu Sb proved that the given structure type was flexible
8
4
4.5
0.5
9
9
9
enough to accommodate a particular amount of p-type doping in the title system. To comprehend the
electronic structure and observed TE properties, analyses were performed using the density of states (DOS),
crystal orbital Hamilton population (COHP) curves, and electron localization function (ELF). The physical
and chemical properties of the synthesized compounds were investigated by employing energy-dispersive
X-ray spectroscopy (EDS), electron probe microanalysis (EPMA), and thermogravimetric analysis (TGA)
techniques. Measurements of electrical transport properties and thermal conductivity demonstrated that
substituting Yb for Ca effectively reduced κ , while the p-type Cu-doping for Zn also increased σ, leading to
tot
an improved ZT. As a result, the quinary title compound Ca YbZn Cu Sb finally attained the ZT of 0.81 at
8
9
0.1
4.4
843 K.