N-Type Mg 3 Sb 2- x Bi x Alloys as Promising Thermoelectric Materials

Abstract

N-type Mg3Sb2-x Bi
x
alloys have been extensively studied in recent years due to their significantly enhanced thermoelectric figure of merit (zT), thus promoting them as potential candidates for waste heat recovery and cooling applications. In this review, the effects resulting from alloying Mg3Bi2 with Mg3Sb2, including narrowed bandgap, decreased effective mass, and increased carrier mobility, are summarized. Subsequently, defect-controlled electrical properties in n-type Mg3Sb2-x Bi
x
are revealed. On one hand, manipulation of intrinsic and extrinsic defects can achieve optimal carrier concentration. On the other hand, Mg vacancies dominate carrier-scattering mechanisms (ionized impurity scattering and grain boundary scattering). Both aspects are discussed for Mg3Sb2-x Bi
x
thermoelectric materials. Finally, we review the present status of, and future outlook for, these materials in power generation and cooling applications.

Full text

Review Article
N-Type Mg
3
Sb
2-x
Bi
x
Alloys as Promising Thermoelectric Materials
Hongjing Shang,
1,2,3
Zhongxin Liang,
1
Congcong Xu,
1
Jun Mao,
1
Hongwei Gu,
2,3
Fazhu Ding ,
2,3
and Zhifeng Ren
1
1
Department of Physics and Texas Center for Superconductivity at the University of Houston (TcSUH), University of Houston,
Houston, TX 77204, USA
2
Key Laboratory of Applied Superconductivity and Institute of Electrical Engineering, Chinese Academy of Sciences,
Beijing 100190, China
3
University of Chinese Academy of Sciences, Beijing 100049, China
Correspondence should be addressed to Fazhu Ding; dingfazhu@mail.iee.ac.cn and Zhifeng Ren; zren2@central.uh.edu
Received 21 October 2020; Accepted 1 November 2020; Published 25 November 2020
Copyright © 2020 Hongjing Shang et al. Exclusive Licensee Science and Technology Review Publishing House. Distributed under a
Creative Commons Attribution License (CC BY 4.0).
N-type Mg
3
Sb
2-x
Bi
x
alloys have been extensively studied in recent years due to their significantly enhanced thermoelectric figure of
merit (zT), thus promoting them as potential candidates for waste heat recovery and cooling applications. In this review, the effects
resulting from alloying Mg
3
Bi
2
with Mg
3
Sb
2
, including narrowed bandgap, decreased effective mass, and increased carrier mobility,
are summarized. Subsequently, defect-controlled electrical properties in n-type Mg
3
Sb
2-x
Bi
x
are revealed. On one hand,
manipulation of intrinsic and extrinsic defects can achieve optimal carrier concentration. On the other hand, Mg vacancies
dominate carrier-scattering mechanisms (ionized impurity scattering and grain boundary scattering). Both aspects are discussed
for Mg
3
Sb
2-x
Bi
x
thermoelectric materials. Finally, we review the present status of, and future outlook for, these materials in
power generation and cooling applications.
1. Introduction
Thermoelectric technology, which can achieve reversible
conversion between electricity and heat, holds great potential
for alleviating the energy and environmental crises [1, 2].
However, large-scale commercialization of thermoelectric
technology has yet to be implemented, mainly due to the
low energy-conversion efficiency of existing thermoelectric
materials. The thermoelectric energy-conversion efficiency
is contingent on the materials’dimensionless figure of merit
zT =S2σT/ðκe+κlÞ, where Sis the Seebeck coefficient, σis
the electrical conductivity, Tis the absolute temperature, κe
is electronic thermal conductivity, and κlis the lattice
thermal conductivity [3–6].
Currently, advancements have been achieved in many
kinds of thermoelectric materials, such as lead chalcogenides
[7, 8], SnSe [9–11], and half-Heuslers [12, 13] at medium and
high temperatures. However, progress on near-room-
temperature materials has been sluggish. The Bi
2
Te
3
-based
compounds, discovered in the 1950s, have remained the
state-of-the-art thermoelectric materials at around room
temperature for several decades [14, 15]. However, these
materials are still not widely applied in viable thermoelec-
tric applications due to the high cost of tellurium (Te)
and some unresolved engineering issues (e.g., high contact
resistance between the contact materials and the thermo-
electric legs when nanostructured materials are considered
for making the modules).
Recently, the n-type Mg
3
Sb
2-x
Bi
x
alloys have attracted
significant attention because of their promising thermoelec-
tric performance and good mechanical properties, the abun-
dance and low cost of their constituent elements, etc. Mg
3
Sb
2
has a CaAl
2
Si
2
-type crystal structure, which consists of an
octahedrally coordinated cation Mg
2+
layer and a tetrahe-
drally coordinated anion structure (Mg
2
Sb
2
)
2-
that form a
nearly isotropic three-dimensional (3D) chemical bonding
network with an interlayer bond that is mostly ionic and par-
tially covalent (Figure 1(a)) [16]. These crystallographic
characteristics lead to decent electrical properties, intrinsi-
cally low lattice thermal conductivity, and good mechanical
properties. Actually, Mg
3
Sb
2-x
Bi
x
alloys have long been
regarded as persistent p-type semiconductors, and their
n-type counterparts were considered to be impossible to syn-
thesize, which should be attributed to the negatively charged
AAAS
Research
Volume 2020, Article ID 1219461, 8 pages
https://doi.org/10.34133/2020/1219461

Mg vacancies that pin the Fermi level around the valence
band [17–19]. This was the case until n-type Mg
3
Sb
2-x
Bi
x
with high thermoelectric performance was reported by
Tamaki et al. [17] through the addition of excess Mg and
doping with Te, although Zhang et al. [20] soon after
reported similar results with Te doping only. The extra Mg
can effectively suppress the Mg vacancies, thus rendering n-
type conduction in Mg
3
Sb
2-x
Bi
x
[17, 21, 22]. Since the discov-
ery of n-type Mg
3
Sb
2-x
Bi
x
, notable advancements have been
made, and its state-of-the-art average zT has been raised up
to ~1.1 in the range of 300-500 K, comparable to that of the
Bi
2
Te
3
-based materials [23–29].
This review focuses on these n-type Mg
3
Sb
2-x
Bi
x
alloys
with promising thermoelectric performance. We first sum-
marize the effects of alloying Mg
3
Sb
2
with Mg
3
Bi
2
on the
band structure (e.g., bandgap, effective mass, and carrier
mobility). The defect-controlled electronic transport in
Mg
3
Sb
2-x
Bi
x
thermoelectric materials will then be dis-
cussed, including defect-chemistry-inspired dopant explo-
ration and the defect-induced near-room-temperature
shift in the carrier-scattering mechanism. Furthermore,
promising applications in power generation and cooling
are also discussed. The strategies mentioned here are
believed to be equally applicable to many other thermo-
electric materials. Some ideas for possible further improve-
ment of thermoelectric performance in n-type Mg
3
Sb
2-x
Bi
x
materials are also presented.
2. Electronic Structure
Alloying of Mg
3
Sb
2
with Mg
3
Bi
2
has a significant impact on
the thermoelectric transport properties and band structures
of the alloys. Zhang et al. [30] calculated the band alignments
of Mg
3
Sb
2-x
Bi
x
alloys and found that Mg
3
Bi
2
alloying results
in a moderate increase in the energy separation between the
conduction band minima K and CB
1
, decreasing the
0.0 0.5 1.0 1.5 2.0
0.4
0.8
1.2
1.6
Bi content of Mg3Sb2–xBix
md
⁎ (me)
(c)
0.0 0.5 1.0 1.5 2.0
0.0
0.2
0.4
0.6
Shu et al.
Bandgap (eV)
Bi content of Mg3Sb2–xBix
eoretical prediction
Mao et al.
(b)
(a)
Mg(1)
Mg(2)
Sb
ab
c
Figure 1: (a) Crystal structure of Mg
3
Sb
2
. Reproduced with permission from Ref. [17]. Copyright 2016 John Wiley and Sons. (b) Bandgap
energy of Mg
3
Sb
2-x
Bi
x
as a function of composition [23, 32]. (c) Density of state effective mass (md
∗) for n-type Mg
3
Sb
2-x
Bi
x
as a function
of composition [23, 28, 34, 35].
2 Research

contribution of the secondary band minimum K to the elec-
trical transport. Since Mg
3
Bi
2
is a semimetal [31] and Mg
3
Sb
2
is a semiconductor, the bandgap of Mg
3
Sb
2-x
Bi
x
will be
reduced with increasing Mg
3
Bi
2
content (Figure 1(b)), lead-
ing to an enhanced bipolar contribution for the Bi-rich com-
positions [23, 32]. Thus, such compositions are not suitable
for applications at higher temperatures. Considering the
empirical trend of bandgap dependence on the application
temperature range, the room temperature thermoelectric
materials exhibit similar bandgaps, so the bandgap of
Bi
2
Te
3-x
Se
x
provides a hint for choosing Mg
3
Sb
2-x
Bi
x
compo-
sitions with the proper Bi/Sb ratios [32].
In addition, the effective mass will be reduced with
increasing Mg
3
Bi
2
concentration [31]. Theoretically, with
increasing Bi content in Mg
3
Sb
2-x
Bi
x
, the density of states
effective mass (md
∗) is reduced from ~1.53 m
0
(Mg
3
Sb
2
)to
~1.23 m
0
(Mg
3
SbBi) to ~0.87 m
0
(Mg
3
Bi
2
) based on the sim-
ulation from the BoltzTraP software package with spin orbit
coupling (SOC) (300 K, carrier concen tration: ~4×10
19 c
m−3), leading to a smaller Seebeck coefficient and higher car-
rier mobility [31]. Such a trend has been verified experimen-
tally although the values seem to be lower than the theoretical
calculation, as shown in Figure 1(c). It is clear that Bi alloying
significantly reduces the density of states effective mass,
indicating that it is an effective strategy to enhance the carrier
mobility of Mg
3
Sb
2-x
Bi
x
alloys. Therefore, the alloying
concentration of Mg
3
Bi
2
is crucial for balancing the carrier
mobility and the Seebeck coefficient, as well as the bipolar
effect. Pan et al. [33] showed the band evolution from
Mg
3
Bi
2
to Mg
3
Sb
2
through angle-resolved photoemission
spectroscopy (ARPES) combined with density functional
theory (DFT) calculations, which also indicated the effec-
tiveness of adjusting the Bi/Sb ratio in improving thermo-
electric performance.
3. Chemical Doping
Defect chemistry has been widely investigated in thermoelec-
tric Zintl compounds in order to understand their intrinsic
defects and to explore effective extrinsic dopants that can
optimize their electronic transport properties [36–38]. In
Mg
3
Sb
2-x
Bi
x
alloys, native Mg vacancies caused by the low
defect formation energy and high vapor pressure of Mg result
in p-type conduction and abnormal electronic transport
behavior near room temperature. Recent studies have shown
that adding excess Mg could suppress the formation of such
vacancies, leading to a reduction in hole concentration and
further resulting in n-type conduction behavior [22]. How-
ever, due to the intrinsic doping limit, the electron concentra-
tion achieved is only ~10
18
cm
-3
, which is significantly lower
than the optimal carrier concentration (~10
19
cm
-3
) needed
to maximize the zT. Thus, further optimization of the elec-
tron concentration via extrinsic doping at the Mg or Sb/Bi
sites is especially necessary in this case.
Gorai et al. [39, 40] used first principle defect calculations
to study n-type doping strategies for Mg
3
Sb
2-x
Bi
x
alloys,
including (i) Sb substitution by mono- (Br, I) or divalent
(Se, Te) anions, (ii) Mg substitution by trivalent or higher
valence cations (La, Y, Sc, Nb), and (iii) insertion of cation
interstitials (Li, Zn, Cu, Be), which are represented by black
spheres and denoted by i(1), i(2), and i(3) in Figure 2(a).
The chemical trends of various dopants have been revealed
in terms of their solubility and maximum achievable electron
concentration, and the discussion here mainly focuses on Sb
and Mg substitution. For the Sb substitution strategy, the
defect formation energy around the conductive band mini-
mum in Te
Sb
is lower than that in Se
Sb
under the Mg-rich
condition (Figure 2(b)), indicating that Te may have a higher
doping limit and greater efficiency, both of which have been
confirmed experimentally [20, 35, 41]. On the other hand,
substitution by La, Y, and Sc at the cation site has been also
explored. It has been found that the defect formation energy
values of La
Mg(1)
,Y
Mg(1)
, and Sc
Mg(1)
are each lower than that
of Te
Sb
, indicating that Mg substitution is even more effective
than Sb substitution by Se or Te. The predicted carrier con-
centration in (La, Y, Sc)-doped Mg
3
Sb
2
could exceed
~10
20
cm
-3
. The relationship between the dopant concentra-
tion and the measured electron concentration of Mg
3
Sb
2-x
Bi
x
for different dopants, i.e., La [42], Y [43], Sc [34], Se [35, 44],
and Te [45], is illustrated in Figure 2(c). For each dopant, the
carrier concentration gradually saturates at a given value with
increasing doping level, which is slightly different from the
theoretical predictions (dashed lines). This may be closely
related to the limited solubility of dopants in Mg
3
Sb
2-x
Bi
x
alloys. Additionally, the optimized carrier concentration for
power generation is in the range of ~3−5×10
19 cm−3, and
it is slightly lower for cooling, and such carrier concentra-
tions can be achieved by doping with Te, Y, Sc, and La. Actu-
ally, most studies reported thus far have focused on how to
improve the zT value, ignoring the structural origin: e.g.,
how the electronic and atomic structures of the alloys,
including the chemical bonding and the chemical state,
evolve after introducing the dopant; how the band structures
vary due to doping; and whether a chemical reaction occurs
at high temperature. Such lack of structural understanding
limits further improvement in the thermoelectric perfor-
mance of the Mg
3
Sb
2-x
Bi
x
alloys.
Additionally, it should be noted that dopants may affect
the thermal stability of the n-type Mg
3
Sb
2-x
Bi
x
alloys, with
studies suggesting that degradation in performance would
occur with their long-term operation at high temperatures
(≥673 K) and that cation-site doping (Y, La, Yb, etc.) via
replacing excess Mg may improve their thermal stability
and delay such decline in the thermoelectric properties
[42, 46, 47]. This can be explained by the changing defect
energetics and the fewer Mg deficiencies. Considering the
differences in vapor pressure between Mg and Bi/Sb, the
decreasing thermal stability has been attributed to the
significant Mg loss (defects) at high temperature [48].
Cation-site doping can effectively eliminate Mg deficiencies
and improve the thermal stability. On the other hand, by
applying coating (such as boron nitride, etc.) on the sur-
faces of the Mg
3
Sb
2-x
Bi
x
alloys, their thermal stability can
be also effectively improved since such coating prevents
Mg loss. Thus, both cation-site doping and coating technol-
ogy are beneficial for improving thermal stability and pro-
moting practical applications, especially power generation
at elevated temperatures.
3Research

4. Manipulating the Carrier-
Scattering Mechanism
In addition to tuning the carrier concentration, suppres-
sion of Mg vacancies in n-type Mg
3
Sb
2-x
Bi
x
could also be
employed to manipulate the carrier-scattering mechanism,
thereby enhancing carrier mobility and improving the zT,
which is particularly significant near room temperature. By
exploring the Hall carrier mobility (μ
H
) temperature (T)
relation, ionized impurity scattering was found to domi-
nate the electron transport around room temperature,
resulting in low carrier mobility [45]. In order to reduce
Mg vacancies and suppress ionized impurity scattering in
Mg
3.2
Sb
1.5
Bi
0.49
Te
0.01
, Mao et al. [25] introduced transition-
metal elements (Fe, Co, Hf, Ta) into the material matrix,
eventually increasing the room-temperature carrier mobility
from ~16 cm
2
V
−1
s
−2
to ~81 cm
2
V
−1
s
−2
(Figure 3(a)). Simi-
larly, other transition-metal elements, such as Nb [24] and
Mn [5, 32, 44], have also been shown to have a dominant effect
in shifting the scattering mechanism from ionized impurity
scattering to a mixture of ionized impurity scattering and
acoustic phonon scattering around room temperature. Addi-
tionally, since defects are highly sensitive to preparation
conditions, Mao et al. [50] reported that manipulating the
hot-pressing temperature could also tune the carrier-
scattering mechanism and thereby substantially enhance the
carrier mobility of Mg
3.2
Sb
1.5
Bi
0.49
Te
0.01
.
On the other hand, grain boundary scattering has also
attracted increasing attention as a carrier-scattering mecha-
nism other than ionized impurity scattering because samples
with large grain size have been shown to demonstrate higher
carrier mobility, which is particularly noticeable around
(i) Sb substitution (ii) Mg substitution (iii) Interstitial
Te, Se, Br, I
i(1)
i(2)
i(3)
Mg(2)
Mg(1)
Sb
La, Y, Sc, Nb Li, Zn, Cu, Be
(a)
0.0 0.1 0.2 0.3 0.4 0.5 0.6
0.0
0.4
0.8
1.2
1.6
ScMg(2)
ScMg(1)
YMg(1)
YMg(2)
LaMg(2)
LaMg(1)
SeSb
TeSb
VMg(2)
𝛥HD,q (eV)
EF (eV)
VMg(1)
(b)
0.1 1
1
10
Y
Te
La
Se
Te
La
Y
Sc
nH (1019 cm–3)
Doping level (%)
Dashed lines: theoretical predictions
Sc
(c)
Figure 2: (a) Mg
3
Sb
2
contains two unique Mg Wyckoffpositions denoted by Mg(1) and Mg(2)andoneuniqueSbWyckoffposition. Reproduced
with permission from Ref. [40]. Copyright 2018 Royal Society of Chemistry. (b) Defect formation energy (ΔHD,q) of various dopants as a function of
the Fermi energy (EF) under the Mg-rich condition [39, 40]. (c) Doping efficiencyofsomedopants(Te,Se,La,Y,Sc)inMg
3
Sb
2-x
Bi
x
at 300 K, with a
comparison to ideal doping (dashed lines) assuming that each donor releases one electron [27, 34, 42, 45, 49].
4 Research

room temperature [51, 52]. The Mg
3.2
Sb
1.5
Bi
0.49
Te
0.01
samples prepared at a higher sintering temperature show
noticeably enlarged grain size as well as higher electrical con-
ductivity (Figure 3(b)). For example, the room-temperature
electrical conductivity is ~4×10
4Sm
−1for the sample with
an average grain size of ~7.8 μm, and it is ~1×10
4Sm
−1for
the sample with an average grain size of ~1.0 μm [53].
Similarly, the grain size of Mg
3
Sb
2-x
Bi
x
alloys was increased
by annealing [54] or hot deforming [27, 34, 55], and
improvement in mobility was also observed. It should be
noted that the defects would be also reduced, in addition
to the increasing grain size, by increasing the sintering
temperature or by annealing. Thus, in these cases, the ion-
ized impurity scattering was also reduced, eventually lead-
ing to the increased electrical conductivity. Kuo et al.
explored the defect compositions near the grain boundary
of Mg
3.05
Sb
1.99
Te
0.01
(nominal composition) using 3D
atom-probe tomography (APT) (Figure 3(c)), from which
the planar defect is clearly noticeable (as marked by the
arrow), and it is a maximum 5 at. % Mg deficiency [56].
As discussed above, a Mg deficiency could easily induce
a high Mg vacancy (V
Mg
2-
) concentration in the vicinity
of the boundary and result in the depletion of free n-
type carriers since V
Mg
2-
serves as an effective electron-
killing defect (Figure 3(d)). Single-crystal n-type Mg
3
Sb
2
was thus grown and used to investigate the underlying
charge-scattering mechanism [33, 57, 58]. As indicated in
Figure 3(e), acoustic phonon scattering dominates the charge
transport in the single-crystal sample that lacks grain bound-
ary electrical resistance, resulting in the sample’ssignificantly
increased weighted mobility near room temperature. This
may support the proposition that grain boundary scattering
dominates the carrier transport of n-type Mg
3
Sb
2-x
Bi
x
alloys in the near-room-temperature range but does not
exclude the ionized impurity scattering existing in the samples
that do have lots of defects. Actually, in comparison to poly-
crystal Mg
3
Sb
2-x
Bi
x
, not only grain boundaries but also defects
are reduced in the single-crystal sample. Thus, additional
300 450 600 750 900
20
40
60
80
100
Only Te
Fe
Co
Hf
Ta
T–2.05
T–2.02
T–2.06
T–2.05
T–1.73
T0.12
T0.55
T0.59
T0.64
T1.94
300 400 500 600 700
1
2
3
4
5
SPS 1123 K
SPS 873 K
(a) (b)
Te
Sb
Mg
at. %
[VMg
2–]
VMg
2– conc.
(c) (d) (e)
250
–30
–30
–20
–10
0
10
20
30
–20 –10 0 10 20 30 300 350 400 450 500 550 600
0
50
100
150
200
250
300
Single crystal
𝜇W (cm2 V–1 S–1)
𝜇H (cm2 V–1 S–1)
T (K)
T (K)
T (K)
XD (mm)
YD (mm)
Polycrystal
Grain boundary eect
5 𝜇m5 𝜇m
Figure 3: Electronic properties and structures of Mg
3
Sb
2-x
Bi
x
. (a) Temperature-dependent Hall mobility [25]. (b) Temperature-dependent
electrical conductivity and electron backscatter diffraction (EBSD) crystal orientation maps. Reproduced with permission from Ref. [53].
Copyright 2018 American Institute of Physics. (c) Projected atomic density map from 3D APT measurement showing the planar Mg-
deficiency defect (arrow); (d) such Mg deficiency in the grain boundary region (gray area, top panel) induces a higher Mg vacancy (V
Mg
2-
)
concentration (bottom panel). Reproduced with permission from Ref. [56]. Copyright 2019 John Wiley and Sons. (e) Temperature-
dependent weighted mobility [57]. Reproduced with permission from Ref. 50. Copyright 2020 John Wiley and Sons.
5Research

details are needed to clarify the carrier-scattering mechanism,
which is also crucial for further improving the thermoelec-
tric performance of n-type Mg
3
Sb
2-x
Bi
x
.
5. Power Generation and Cooling Applications
Mg
3
Sb
2-x
Bi
x
alloys have shown promise for applications in
power generation and cooling due to their high performance.
Generally, the Sb-rich compositions (Mg
3
Sb
2
-based alloys)
are promising for power generation at medium temperature
although they may lack good stability due to Mg loss at high
temperature (≥673 K). For example, Zhu et al. [59] reported
that the conversion efficiency of Mg
3.1
Co
0.1
Sb
1.5
Bi
0.49
Te
0.01
could be up to ~10.6% at a temperature difference of 400 K
in the range from 300 K to 700 K, suggesting good potential
for midtemperature heat conversion.
The Bi-rich compositions (Mg
3
Bi
2
-based materials), on
the other hand, show more potential for cooling applications.
In this case, concerns regarding thermal stability can be
ignored due to the low temperature range. Mao et al. [23]
reported that optimized Mg
3.2
Sb
0.5
Bi
1.498
Te
0.02
exhibits a
room temperature zT of more than 0.7 and that the uni-
couple of Mg
3.2
Sb
0.5
Bi
1.498
Te
0.02
and Bi
0.5
Sb
1.5
Te
3
exhibits
a large temperature difference of ~91 K at the hot-side tem-
perature of 350 K, comparable to that of commercial
coolers based on the Bi
2
Te
3
alloys. Imasato et al. [26] also
fabricated n-type Mg
3
Sb
0.6
Bi
1.4
with a zT of 1.0-1.2 at
400-500 K, which surpasses that of the n-type Bi
2
Te
3
. Fur-
thermore, Mg
3
Sb
2-x
Bi
x
alloys are inexpensive compared to
Bi
2
Te
3
-based materials because they minimize the need
for expensive elemental Te, largely reducing the material
cost. In addition, unlike the nanostructured n-type Bi
2
Te
3
-
based materials that suffer from high contact resistance
between the thermoelectric legs and the electrodes, such
contact resistance can be greatly reduced for Mg
3
Sb
2-x
Bi
x
by forming a sandwiched structure of Fe/Mg
3
Sb
2-x
Bi
x
/Fe.
All of these examples show the great potential that the
Mg
3
Sb
2-x
Bi
x
alloys have for becoming good candidates to
replace the traditional Bi
2
Te
3
, promoting their application
in thermoelectric technology. In particular, the high cooling
performance of Mg
3
Bi
2
-based alloys inspires researchers to
explore these semimetals as potential thermoelectric mate-
rials for cooling.
6. Conclusions
In summary, strategies like alloying, as well as defect-
controlled carrier-concentration optimization and manipu-
lation of the carrier-scattering mechanism, have been
successfully used to improve the thermoelectric performance
of Mg
3
Sb
2-x
Bi
x
alloys. Further research efforts are warranted
to explore other effective and inexpensive dopants for wider
temperature application such as in power generation and
solid-state cooling, including the structural variation induced
by these dopants, and effective strategies to improve thermal
stability. In addition, the carrier-scattering mechanism
needs to be clarified (whether ionized impurity scattering
or grain boundary scattering can better explain the dra-
matic increase in mobility around room temperature) in
the near future in order to further enhance the zT. Even
so, Mg
3
Sb
2-x
Bi
x
alloys show great potential for power gen-
eration and cooling applications.
Conflicts of Interest
The authors declare no conflicts of interest.
Authors’Contributions
Hongjing Shang and Zhongxin Liang contributed equally to
this work.
Acknowledgments
Part of this work was supported by the National Natural
Science Foundation of China (Grant No. U1832131 and
Grant No. 51721005) and the Beijing Natural Science
Foundation (Grant No. 3202034).
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8 Research