Unveiling Solvent-Related Effect on Phase Transformations in CsBr-PbBr2 System: Coordination and Ratio of Precursors

Article abstract of DOI:10.1021/acs.chemmater.8b00537

All-inorganic cesium lead halide perovskite nanocrystals have emerged as attractive optoelectronic nanomaterials owing to their stabilities and highly efficient photoluminescence. However, the inorganic perovskites of CsPbBr₃ synthesized by the

Full text of DOI:10.1021/acs.chemmater.8b00537

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Article
Unveiling Solvent-related effect on Phase Transformations in
CsBr-PbBr2 System: Coordination and Ratio of Precursors
Mei Liu, Jiangtao Zhao, Zhenlin Luo, Zhihu Sun, Nan Pan, Huaiyi Ding, and Xiaoping Wang
Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b00537 • Publication Date (Web): 16 Aug 2018
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Unveiling Solvent-related effect on Phase Transformations in
CsBr-PbBr System: Coordination and Ratio of Precursors
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1*
Mei Liu , Jiangtao Zhao , Zhenlin Luo , Zhihu Sun , Nan Pan , Huaiyi Ding ,
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Xiaoping Wang
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Hefei National Laboratory for Physical Sciences at the Microscale, University of
Science and Technology of China, Hefei, Anhui 230026, P. R. China
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National Synchrotron Radiation Laboratory and CAS Key Laboratory of Materials
for Energy Conversion, University of Science and Technology of China, Hefei, Anhui,
2
*
30026, China.
Correspondence and requests for materials should be addressed to Huaiyi Ding,
Xiaoping Wang (email: dinghy@ustc.edu.cn, xpwang@ustc.edu.cn,)
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Abstract
Allꢀinorganic cesium leadꢀhalide perovskite nanocrystals have emerged as attractive
optoelectronic nanomaterials owing to their stabilities and highly efficient
photoluminescence. However, the inorganic perovskites of CsPbBr
3
synthesized by
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the solution method are often suffered from byꢀproducts such as Cs PbBr and
4
6
CsPb
2 5
Br . Herein, we have investigated thoroughly the solventꢀrelated effect on the
phase formation in CsBrꢀPbBr
2
system through single crystal Xꢀray diffraction
measurement. It is found that the prepared product is dominantly determined by the
coordination number (CN) of Pb (II) and the ratio of precursors. Using
2+
dimethylsulfoxide (DMSO) or dimethylformamide (DMF) as the solvent, Pb is
found to be surrounded by sixꢀcoordination sites, and the products can be tuned from
CsPbBr to Cs PbBr by increasing the precursor ratio of CsBr to PbBr . On the
3
4
6
2
2+
contrary, in the solvent of water, only Pb eightꢀcoordinated crystal of CsPb
2 5
Br can
be produced, regardless of the ratio of CsBr to PbBr . More importantly, with the
2
investigation on the extended Xꢀray absorption fine structure (EXAFS) for Pb LꢀII
edge in precursor solutions, we identify that the CN of Pb (II) of resultants are the
same as those of the corresponding plumbite oligomers in precursor solutions. In
addition, the phase transitions from Cs
CsPb Br triggered by water vapor have also been observed clearly. This work not
only enriches our understanding of the phase formation of CsBrꢀPbBr system but also
4 6 3
PbBr to CsPbBr , amorphous state and
2
5
2
provides the knowledge of the degradation of halide perovskites in the environment of
humidity.
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The CsBrꢀPbBr
2
system possesses a complicated phase diagram containing three
1
compounds, CsPbBr , CsBrꢀrich Cs PbBr , and PbBr ꢀrich CsPb Br (CPBs).
3
4
6
2
2
5
Recently, the narrower bandꢀgap semiconductor material CsPbBr
3
with perovskite
structure attracts a great deal of research attention in the field of optoelectronic
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2
ꢀ5
devices
by the virtue of high photoluminescence (PL) quantum yield, large
6
absorption crossꢀsections and facile emission color tunability. Whereas the studies of
CsPb
2 5 4 6
Br and Cs PbBr are very rare, resulting in greatly divergences on their
photophysical behavior. Actually, both of them were regarded as materials with strong
7
ꢀ9
greenꢀemission and greenꢀlasing properties in some reports initially,
but the
followꢀup studies suggested that they are wideꢀgap semiconductors with bandꢀgap
1
0ꢀ12
2
.979 eV and 3.95 eV respectively and have no visible PL emission.
conflicting results may stem from a few CsPbBr nanocrystals embeddedinCs PbBr
6
These
3
4
2 5
or CsPb Br , owing to the fact that CPBs are easily interconvertible to form mixed
11, 13ꢀ14
phasese.
3
On the other hand, the composites of the CsPbBr containing
Cs PbBr or CsPb Br might have a better performance in optoelectronic devices than
4
6
2
5
15ꢀ19
intrinsic CsPbBr
CsPbBr
frequencyꢀupconverted optical gain. Lack of ligands in CsPbBr
Cs PbBr matrix ensure its thermal stability and high temperature operation.
3
.
For instance, Wang et al. synthesized embedded ligandsꢀfree
3
nanocrystals in a wider bandꢀgap Cs
4
PbBr matrix as temperatureꢀinsensitive
6
3
and protection from
1
9
4
6
The single crystals of CPBs can be readily synthesized in solutionꢀprocess through
inverse temperature and antiꢀsolvent vaporꢀassisted crystallization method, which has
2
0ꢀ22
greater yield and lowꢀcost.
dimethylformamide (DMF) and H
Generally, dimethyl sulfoxide (DMSO) or
O are frequently used as effective solvents for
2
dissolving lead halides and CsBr. Interestingly, the resultants are deeply affected by
the solvents. CsPbBr and Cs PbBr bulk crystals have been synthesized in DMSO,
3
4
6
1
3ꢀ14, 23
and they are often cocrystallized.
2 5
Pure powders of CsPb Br were obtained in
2
2
water by M. Rodová et al. Meanwhile，it was reported that the lead halide in solvent
2
n
ꢀn
forms a serious of PbX
(n≥2, X=Cl, Br, I) complexes, which play a key role in
2
4ꢀ27
building PbBr
n
polyhedron framework of CPBs crystals.
However, the evolution
2ꢀn
process from PbX
n
to the CPBs in different solvents is still ambiguous.
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In this paper we focused on the roles of the precursors ratio and solvent properties
in the formation of CPBs. Based on the extended Xꢀray absorption fine structure
(EXAFS) of Pb LꢀII edge in precursor solutions and single crystal Xꢀray diffraction
(
XRD) measurement on the resultants, we reveal that the the coordination number
CN) of the Pb (II) of resultants are controlled crucially by the intermediates mediated
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(
by the solvents in the precursor solutions, while the stoichiometry of CPBs can be
regulated by the precursors ratio of CsBr to PbBr . Furthermore, by utilizing the
coordination engineering and solubility of precursors, we achieved the phase
transitions from Cs PbBr to CsPbBr and CsPb Br in humid environment.
2
4
6
3
2
5
The ratio of precursors. We began our investigation on solution synthesis with the
2
1
solvent of DMSO. In a typical antiꢀsolvent vaporꢀassisted crystallization, 10 mL of
a DMSO solution containing CsBr (0.2 M) and PbBr with various concentrations
2
(Pb:Cs) 9:1, 4:1, 1:1, 1:4 and 1:9 acted as the precursors, and dichloromethane (DCM)
as antiꢀsolvent. XRD patterns of the resultants are shown in Figure 1a. We found that
only PbBr •2DMSO is produced when the ratio of PbBr to CsBr is 9. However, a
2
2
precursor with a ratio of 4 yields pure CsPbBr
3
(orthorhombic, space group: Pbnm; a
o
22
= 8.207 Å, b = 8.255 Å, c = 11.759 Å, α = β = γ = 90 ) precipitate. As decreasing
the ratio of PbBr
2
4 6
to CsBr to 1, Cs PbBr (trigonal, space group: Rꢀ3c; a = b =13.722
o
o
28
Å, c = 17.299 Å; α = β = 90 , γ = 120 ) appears gradually and dominates the
products at the ratio down to 1/4. Further decreasing the PbBr
disappears and Cs PbBr ꢀCsBr is crystallized from the solution. The results suggest
that the variation tendency of stoichiometry of the products is consistent well with the
ingredients of the precursors, i.e., with PbBr ratio decreasing in the precursor solution,
the products are gradually changed from PbBr •2DMSO to CsPbBr and then to
2 3
proportion, CsPbBr
4
6
2
2
3
Cs PbBr and finally to CsBr. Similarly, when using DMF as the solvent, PbBr •DMF,
4
6
2
CsPbBr
3
, Cs
4
PbBr
6
and CsBr crystals are formed as the products, which results are
shown in Figure S1 and Table S1.
We next change the solvent to H
2
O with cooling crystallization method through
22
cooling down the precursor solutions. XRD results of the products with various
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concentration ratios (Pb: Cs) of 9:1, 4:1, 1:1, 1:4 and 1:9 are shown in Figure 1b. As
seen, with the ratio decreasing, the products are gradually changed from 3PbBr
2 2
•2H O
to CsPb Br (tetragonal, space group: I4/mcm; a = b = 8.45 Å and c = 15.07 Å, α = β
2
5
o
29
=
γ = 90 ). However, further decreasing the ratio of PbBr
2
: CsBr to as low as 1:9,
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only CsPb Br is obtained, no CsPbBr and Cs PbBr signal can be detected in XRD
2
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3
4
6
patterns. Therefore, we suggest that, in addition to the ratio of precursors, the solvent
also plays an important role in the phases of final products.
Figure 1. XRD patterns of the resultants produced by varying the precursor ratios of
2 2
PbBr to CsBr in different solvents. (a) DMSO and (b) H O.
It should be mentioned that, besides the solvent, both of the antiꢀsolvent and the
crystallization technique also play roles in the final phase formation. This is because
that the antiꢀsolvent with higher dielectric constant can screen the interaction between
the solvated salts and the solvent, and hence govern the final products, as reported by
23
Rakita et al. At the same time, in the solution preparation of crystal, the route of
crystallization is dominantly controlled by either thermodynamic or kinetic process,
which distinctly depends on the crystallization technique. In this work, we adopted
cooling crystallization method and DCM as antiꢀsolvent, which has less impact on the
final products due to its small relatively dielectric constant (ε≈9.1) as compared to
the solvents of DMSO (ε≈42) and DMF (ε≈38) solvent.
The coordination of Pb (II). To get more insight into the solvent effect on CPBs
formation, we focus on the CN of Pb (II) of products. It is wellꢀknown that the
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structures of CPBs can be divided into two categories by their building blocks of
4
−
CPBs: one is CsPbBr
3
or Cs
4
PbBr
6
with [PbBr
6
]
octahedral units (sixꢀcoordinate
6−
sites), and the other is CsPb Br with [PbBr ] hendecahedral units (eightꢀcoordinate
2
5
8
sites), whereas there are rarely reports on the products of PbBr
and 3PbBr •2H O. To this end, the single crystals of PbBr •2DMSO, PbBr
PbBr •2H O are firstly separated respectively from PbBr solutions in DMSO, DMF
2 2
•2DMSO, PbBr •DMF
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9
0
2
2
2
2
•DMF and
3
2
2
2
2
and H O solvents to determine their corresponding Pb (II) CN. Figure 2 and Figure S2
show their crystal structures deduced from single crystal Xꢀray crystallographic
analysis. As seen, each lead ion is coordinated by four bromide ions and two oxygen
30
atoms of DMSO ligands for PbBr •2DMSO (Figure 2a), but by five bromide ions
2
and one oxygen atom of DMF ligands for PbBr
II) ions in both of these crystals have sixꢀCN. However, as shown in Figure 2b, each
Pb (II) ion has eightꢀCN in 3PbBr •2H O crystal, where Pb (II) binds either eight
2
•DMF (FigureS2). In other words, Pb
(
2
2
bromide ions or seven bromides ions and one oxygen atom of H
2
O ligands. The
results imply that the coordination number of Pb (II) resultants is six in the crystals
2
produced from the solvent of DMSO or DMF, but becomes eight for H O solvent.
30
Figure 2. The crystal structures of (a) PbBr •2DMSO and (b) 3PbBr •2H O
2
2
2
characterized and deduced by single crystal Xꢀray crystallographic analysis.
The whole synthesis process of lead perovskite consists of the dissolution of
reagent salts in solvent and the crystallization from the supersaturation solution.
–
During the process, the solvent molecules or Br was coordinated to Pb (II) to form
2
ꢀn
−
2−
PbBr
n
anion, such as PbBr
2
, PbBr
3
and PbBr
4
species when the lead halide
31ꢀ32
precursor dissolved in aqueous, DMF or other polar solutions.
In this regards, it is
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ꢀn
reasonable to speculate that the structure of intermediate PbBr
n
in solution plays a
key role on the formation of CPBs resultants. We want to know whether the
coordination number of intermediate is six in the solvent of DMSO or DMF, and
become eight in H
out to probe the CN of the Pb (II) in precursor solutions with different solvents of
DMSO, DMF and H O. It is well known that EXAFS is resulted from the oscillatory
2
O? To this end, EXAFS measurement at Pb LꢀII edge was carried
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region of an Xꢀray absorption spectrum above an absorption edge. The fine oscillatory
behavior is attributed to the backscattering of photoelectrons from various shells of
neighboring atoms, and therefore it can be used to identify the coordination
environment of the atoms. The details of the experiment can be found in the
supplementary information.
Figure 3 shows the experimental results (open squares) of Fourier transform (FT)
2
curves of the [k χ(k)] in the R space for the intermediates in different solvents, where
k is the wave vector of photoelectron and χ(k) is the oscillation function. The FT
curves exhibit two coordination peaks, corresponding to Pb−O and Pb−Br bonds,
respectively, while the oscillation positions and intensities vary with the solvents.
Note that the chemical bands of PbꢀN and PbꢀO in DMF solvent can not be
distinguished directly from the EXAFS. However, with the knowledge DMF
2+
coordinated with Pb ions in solid state (PbBr •DMF) from our single crystal Xꢀray
2
2
+
crystallographic analysis, we speculate that Pb ion tends to be bound to O not N of
DMF.In order to know the exact CN of Pb (II), the experiment data are fitted by the
ATHENA (version 0.8.056) and ARTEMIS (version 0.8.011) modules implemented
33ꢀ34
in the IFEFFIT software packages.
The fitting results are shown in the Figure 3
with solid lines, and the structural parameters extracted from the fitting are
summarized in Table 1. As seen, the total coordination number of Pb (II), is around
six in DMSO or DMF, but becomes around eight in H
2
O. We also notice that, due to
the different sizes of DMSO, DMF and H O molecules, PbꢀO and PbꢀBr have
2
different bond lengths and CN under different solvents (Table S2). However, the
results of the single crystal Xꢀray crystallographic analysis imply that the difference
does not change the configuration of Pb coordinations but only distorts the structure
of octahedron (or hendecahedron). Consequently, we can conclude that the CN of Pb
(
II) of resultants are the same as those of the corresponding plumbite oligomers in the
precursor solutions, i.e., the structure of intermediate in solution plays a decisive role
on the formation of CPBs product, and PbꢀO bond is replaced by PbꢀBr gradually and
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Figure 3. Experimental (open squares) and fitting (solid lines) results of the Fourier
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transformed [k χ(k)] in the R space for the intermediates in DMSO, DMF and H
2
O
solvents.
Table 1. Fitting parameters for real space EXAFS.
2
ꢀ3
2
Sample
DMF
Bond
N
Total N
0
R (Å) σ (10 Å ) ꢀE (eV)
PbꢀO
PbꢀBr
PbꢀO
PbꢀBr
PbꢀO
PbꢀBr
2.0±0.3
4.1±0.6
4.1±0.6
3.7±0.6
1.9±0.3
3.6±0.5
6.1±0.9
2.51
2.72
2.59
2.69
2.63
2.76
6.3
11.6
7.2
ꢀ4.0
ꢀ9.9
7.8±1.2
5.5±0.8
1.7
2
H O
13.8
5.7
ꢀ17.2
3.2
DMSO
12.0
ꢀ11.3
E0, edge
2
N, coordination number; R, bond distance;
σ
, DebyeꢀWaller factor;
ꢀ
energy shift. EXAFS CN values have 10ꢀ15% fitting errors.
The reason why the coordination number of lead halide can be affected by the
solvent is that DMSO, DMF and H O can be as the ligands to participate the Pb(II)
2
coordination. Owing to the hindrance role of sterics, the larger ligands favor lower
coordination number, while the small ones incline to form complicated high
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coordination number. In this regard, the ligands of DMSO or DMF can form lower
2
coordination to Pb (II) as compared to that of H O. Based on the experimental
observation and the above discussion, here we propose a plausible mechanism of
growing CPBs in different solvents as schematically shown in Figure 4. As illustration
in the top of Figure 4a,the coordination number of Pb (II) changes firstly from seven
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in PbBr
CsBr to PbBr
from [PbBr (DMSO)
whereas the coordination number of Pb (II) maintains six. Addition of antiꢀsolvent can
further drive PbBr –DMSO ions assembled into bulk crystal. As a consequence, the
resultants change from PbBr •2DMSO to CsPbBr and finally to Cs PbBr
successively. Similarly, as shown in Figure 4b, the coordination number of Pb (II)
changes to eight when dissolves into H O, and the resultants can be tuned accordingly
from 3PbBr •2H O to CsPb Br with increasing the ratio of CsBr to PbBr
2
crystal to six when adding the solvent of DMSO. With increasing the ratio of
2
, as shown in the bottom of Figure 4a, the intermediate can be tuned
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4ꢀ
2
4
] to higherꢀorder plumbate complexes,
6
such as [PbBr ] ,
n
m
2
3
4
6
2
2
2
2
5
2
.
Figure 4. Schematic of the CPBs formation in the solvent of (a) DMSO and (b)
O. PbBr dissolves in solvents forming lead polyhalide framework with different
CN, six in DMSO and eight in H O. Increasing the ratio of CsBr to PbBr induces the
intermediate to higherꢀorder plumbate complexes and different resultants.
H
2
2
2
2
In situ XRD observation of phase transformation. Preventing the CPBs from
degradation in humid environment is critical for their application. More importantly,
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the transformation process of CPBs in moisture can supply more insight on the role of
ratio and coordination of precursors discussed above. In the following, inꢀsitu
synchrotron XRD experiment was used to investigate the humid effect of environment
on the material stability and phase change. The schematic of the experiment setup is
shown in Figure S3. Briefly, the Cs PbBr powder was placed in a homemade sample
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cell at the diffractometer center. The relative humidity (RH) in the cell was controlled
by tuning the flow rate of nitrogen gas which carried water vapor. The RH value was
realꢀtime monitored by a commercial hygrometer and regulated to be about 80%
during the whole experiment. The cell was sealed with kapton film to confine the
35
moisture but not to block the incident and diffractive xꢀray beam passing through.
Figure 5a is in situ XRD result for the successive phase transformations from
Cs PbBr to CsPbBr and CsPb Br with time, and Figure 5b shows the typical XRD
4
6
3
2
5
patterns at 1 min, 25 min, 33 min, and 45 min in the phase transformation process,
respectively. As shown in Figure 5a and b, XRD patterns confirm that the initial CPBs
is Cs PbBr . With time increasing, the intensities of the diffraction peaks of Cs PbBr
4
6
4
6
gradually decrease due to its degradation. At the same time, the orthorhombic
CsPbBr diffraction peaks emerge and the related intensities become enhanced
3
gradually. As time going, all the diffraction peaks decrease and disappear abruptly,
and only a weak and broad diffraction peak left as shown in Figure 5b with blue line,
indicating the emergence of anamorphous phase of CPBs. This amorphous state is not
2 5
stable and can be reꢀcrystallized to the CsPb Br in the end. More XRD patterns
corresponding to the phase transformation process can be found in Figure S4.
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Figure 5(a) In situ XRD observation for the phase transformations from Cs PbBr
to CsPbBr
and 45 min in the phase transformation process, respectively. (c) Schematic of the
possible degradation mechanism for the Cs PbBr crystal.
3 2 5
and CsPb Br . (b) Representative XRD patterns at 1 min, 25 min, 33 min,
4
6
On the basis of the results discussed above, we suggest that the transformation
process is probably controlled by both of the solubility of CsBr and coordination
number of Pb (II) in water. A plausible schematic of the crystal structure evolution is
displayed in Figure 5c, and the related processes can be described as follows: When
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Cs PbBr stored in humid environment, H O molecular from the moisture can initially
permeate into it and strip the CsBr component out from the Cs PbBr crystal. This will
4
6
induce the shrink of the crystal lattice and transform the Cs
4
PbBr
6
3
phase into CsPbBr ,
while the framework of PbBr
6
octahedron is retained. With time increasing, more
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water vapor can interact with CsPbBr intensely, resulting in not only striping more
3
CsBr but also changing the coordination number of Pb (II) from six to eight. During
the process, the distinct change of PbꢀBr coordination environment leads CsPbBr
3
crystal to be decomposed to an amorphous state, which can transform to CsPb
crystal finally due to the instability of intermediate amorphous phase.
Br
2 5
Absorption and Raman properties of CPBs. The absorption spectra of as grown
pure CPBs crystals are shown in Figure 6a. The bandgaps value (E ) are obtained on
the basis of the Tauc plot as the intercept value of the plot of ( hν) against light
is the absorption coefficient. It is found
and Cs PbBr , indicating the feature of
g
1
/m
α
energy hν, as shown in Figure S5, where
that m is equal to 1/2 for both CsPbBr
α
3
4
6
directꢀbandꢀgap semiconductor. On the contrary, m is equal to 2 for CsPb Br ,
2
5
suggesting its indirectꢀbandꢀgap. The band gaps are estimated to be about 3.67 eV,
2.26 eV and 3.23 eV for Cs
consistent with previously reports.
CsPbBr , Cs PbBr
common behavior in the synthesis of Cs
the weak peak appeared at 17.2° in the XRD pattern of Cs
with red # mark). However this interpretation is doubted by some reports that the
4
6 3 2 5
PbBr , CsPbBr and CsPb Br , respectively, which are
10ꢀ13
Note that, due to the encapsulated nanoscale
3
4
6
has a little absorption in the region of 340ꢀ530 nm, which is a
11
4
6
PbBr crystals. This can also be verified by
4
6
PbBr in Figure 5b (labeled
2
6
3, 28, 36ꢀ38
origin of this emission to the shallow and deep energy defects of Cs
4
PbBr
.
Raman spectra of these crystals provide the vibrational modes of the metalꢀhalide
lattice (Figure 6b). When the perovskite crystal was excited by 633 nm laser at room
temperature, sharp and wellꢀresolved Raman signals were collected. Strong peak at 70
−1
−1
3
cm and broad peak at 125 cm of CsPbBr are assigned to the vibrational mode of
4−
+
[
PbBr
6
] octahedron and the motion of Cs cations, respectively. For Cs
4
PbBr
6
crystal,
−1
−1
−1
strong peaks at 81.8 cm and 123 cm and a shoulder peak at 67.3 cm are
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14,
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4 6
consistent to previous reports for meltꢀgrown and solutionꢀgrown Cs PbBr crystals.
3
9
−1
In the Raman spectrum of CsPb
2 5
Br , the strong peaks at 75.8 cm and 131.5 cm
are observed.
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Figure 6.(a) Absorption and (b) Raman scattering spectra of asꢀprepared CPBs.
Inserts in (a) are their photographs.
Conclusions. In this work, CPBs bulk single crystals with different phases have been
synthesized using roomꢀtemperature solution method by changing the solvents and
ratio of precursors. Using DMSO or DMF as the solvent, various CPBs with Pb (II)
sixꢀCN can be produced. On the contrary, in the solvent of water, only Pb (II)
2 5 2 2
eightꢀcoordinated crystals, such as CsPb Br and 3PbBr •2H O, can be formed. Based
on the EXAFS of Pb LꢀII edge in precursor solutions and single crystal Xꢀray
diffraction measurement on the resultants, we identify that the CN of a plumbite
oligomers in precursor solution are the same as those of the corresponding resultants.
The finding suggests that the coordination number of the product is controlled
dominantly by the solvents, while the stoichiometry of the resultants can be regulated
2
by the precursors ratio of CsBr to PbBr . Additionally, the phase change process of
CPBs under the humid environment is further verified by the inꢀsitu synchrotron
radiation XRD. Knowledge of how the solvent affects the phase formation by
coordination geometry of Pb (II) is very helpful in synthesis of a pure phase in
CsBrꢀPbBr
2
system and is also beneficial to understand the degradation of halide
perovskites in humid environment.
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ASSOCIATED CONTENT
Supporting Information.
Details of all of the experimental methods and synthesis; absorption spectra ,
extended Xꢀray absorption fine structure and single crystal Xꢀray diffraction
measurements (PDF)
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Crystallographic data for PbBr
2
•DMF in CIF format (CIF)
•2H O in CIF format (CIF)
Crystallographic data for 3PbBr
2
2
ACKNOWLEDGMENT
We acknowledge the financial supports from MOST of China (2016YFA0200602),
National Natural Science Foundation of China (21421063, 11474260, 11374274,
11504364, 21633007, 11675179), the Chinese Academy of Sciences (XDB01020200),
the Fundamental Research Funds for the Central Universities (WK2030020027,
WK2060190027, WK3510000004), and Anhui Natural Science Foundation
(1608085QA17).
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