Composition related structural transition between mechanosynthesized CsPbBr3 and CsPb2Br5 perovskites and their optical properties

Article abstract of DOI:10.1016/j.jallcom.2019.152612

A facile solvent-free mechanochemical synthesis mechanism of perovskites of Cs–Pb–Br system has been demonstrated in the present study. Inherent structural and microstructural characteristics of CsPbBr₃ and CsPb₂Br₅ perovskites are investigated by analyzing powder XRD patterns using Rietveld refinement analysis and formation of these perovskites has been ascertained. The compositional phase transition between orthorhombic CsPbBr₃ and tetragonal CsPb₂Br₅ perovskites has been explained from the structural point of view. The optically active orthorhombic CsPbBr₃ perovskite transforms to optically inactive tetragonal CsPb₂Br₅ perovskite under flowing water vapour within a short duration and the reverse transformation is noticed when the tetragonal perovskite is heat-treated at elevated temperature. Optical properties of these perovskites corroborate well with structural phase transitions of these compounds. This new approach of structural conversion not only provides a controlled synthesis of the ternary Cs–Pb–Br system but also demonstrates a clear underlying mechanism for the structural instability and associated optical properties.

Full text of DOI:10.1016/j.jallcom.2019.152612

Journal Pre-proof
Composition related structural transition between mechanosynthesized CsPbBr and
3
CsPb Br perovskites and their optical properties
2
5
G. Maity, S.K. Pradhan
PII:
S0925-8388(19)33858-7
DOI:
https://doi.org/10.1016/j.jallcom.2019.152612
JALCOM 152612
Reference:
To appear in: Journal of Alloys and Compounds
Received Date: 29 July 2019
Revised Date: 2 October 2019
Accepted Date: 8 October 2019
Please cite this article as: G. Maity, S.K. Pradhan, Composition related structural transition between
mechanosynthesized CsPbBr and CsPb Br perovskites and their optical properties, Journal of Alloys
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and Compounds (2019), doi: https://doi.org/10.1016/j.jallcom.2019.152612.
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Composition Related Structural Transition Between Mechanosynthesized
CsPbBr and CsPb Br Perovskites and Their Optical Properties
3
2
5
*
G. Maity and S. K. Pradhan
Materials Science Division Department of Physics, The University of Burdwan, Golapbag,
Burdwan-713104, West Bengal, India
Abstract
A facile solvent-free mechanochemical synthesis mechanism of perovskites of Cs-Pb-Br system
has been demonstrated in the present study. Inherent structural and microstructural
characteristics of CsPbBr and CsPb Br perovskites are investigated by analyzing powder XRD
3
2
5
patterns using Rietveld refinement analysis and formation of these perovskites has been
ascertained. The compositional phase transition between orthorhombic CsPbBr and tetragonal
3
CsPb Br perovskites has been explained from the structural point of view. The optically active
2
5
orthorhombic CsPbBr perovskite transforms to optically inactive tetragonal CsPb Br perovskite
3
2
5
under flowing water vapour within a short duration and the reverse transformation is noticed
when the tetragonal perovskite is heat-treated at elevated temperature. Optical properties of these
perovskites corroborate well with structural phase transitions of these compounds. This new
approach of structural conversion not only provides a controlled synthesis of the ternary Cs-Pb-
Br system but also demonstrates a clear underlying mechanism for the structural instability and
associated optical properties.
*
Corresponding author; e-mail: skpradhan@phys.buruniv.ac.in; skp_bu@yahoo.com,
Phone: +91 342 2657800, Fax: +91 342 2530452
Keywords: Cs-Pb-Br perovskites, mechanosynthesis, phase-transition, Rietveld, optical property

1
. Introduction:
All inorganic cesium lead halide perovskites (CsPbX ) have attracted great attention due
3
to their incredible photophysical properties, like high photoluminescence quantum yields, widely
tunable band gaps and narrow emission bandwidth [1, 2]. Halide perovskites are new generation
promising materials for solar cells [3, 4], lasers [5, 6], light-emitting diodes (LEDs) [7] and other
optoelectronic applications [8, 10]. Nanocrystalline with different morphologies and
compositions of Cs-Pb-Br system had already been designed by different methods, such as the
solvothermal method [11], colloidal synthesis [12, 13], hot injection method [12] and chemical
3 2 5 4 6
vapour deposition method [14]. These CsPbBr , CsPb Br and Cs PbBr compounds are different
by their layer stacking while Pb and Br ions assemble into PbBr octahedra in their crystal
6
structures. It is well known that CsPbBr may crystallize in orthorhombic, tetragonal and cubic
3
structures with the preference of temperature and total system energy. However, the CsPb Br
2
5
perovskite-related system belongs to the tetragonal structure. Some fundamental properties and
structural stability of halide perovskites depend on the synthesis procedure. Kulbak et al. showed
3
that CsPbBr compound has better efficiency, thermal stability and can be operated for a longer
duration without significant degradation in comparison to other halide perovskites [3]. Recently,
there are few reports on the synthesis, stability and optical property of CsPb Br compound.
2
5
Some of the research groups reported on the transformation of 3D CsPbBr to highly
3
luminescence 2D CsPb Br system [15-17]. Yu et al. demonstrated the synthesis of CsPb Br and
2
5
2
5
its enhancement of photoluminescence (PL) in the visible region (512 nm) with a high quantum
yield of 87% [16]. Sun et al. reported the enhancement of luminescence of CsPbBr is due to the
3
formation of a by-product CsPb
2
Br
5
at the synthesis stage of CsPbBr
3
[17]. However, Zhang et
, which emerged
al. claimed that the visible emission of CsPb
2
Br was explicitly due to CsPbBr
5
3

2 5 3
as a by-product due to thermally induced phase alteration between CsPb Br and CsPbBr [18].
On the other hand, like other inorganic halide perovskites (IHPs) CsPbBr can also be
3
decomposed by a polar solvent, especially in water, also that decomposition start around 600 K
[19, 20]. There are many reasons for photo-degradation, including oxidations, photo-assisted
ionization and aggregation, but to date, the phase transformation of this perovskite at the
presence of polar solvents is under debate. Therefore, further investigation on the inter-
3 2 5
conversion between the CsPbBr and CsPb Br compounds is somewhat important to justify their
significant difference in optical properties.
Recently, it is reported that the possible inter-conversion of ternary Cs-Pb-Br compounds
by physical and chemical treatments was associated with the ligands on their surface and they
were realized with mediation by a ligand [15, 21, 22]. In some earlier cases, interconversion
between perovskite halides by adding either CsBr or PbBr precursor through some mechanical
2
synthetic approach was studied by Pal et al. [23]. But, without ligand mediations, investigations
on the interconversion in the Cs-Pb-Br systems were rarely reported, which is essential to reveal
3
the instability mechanism of CsPbBr perovskite. However, the preparation strategy also plays a
crucial role in physical and chemical properties like crystallinity, morphology, grain size and
other structural parameters. It is difficult to synthesize environmental friendly halide perovskites
by a chemical route at low temperature and minimization of secondary reaction products.
Solvent-free mechanosynthesis technique gives several advantages over the traditional chemical
route. Recently, very few reports on the mechanosynthesis technique are devolved for IHPs
formation. Zhang et al. used the same method but added oleylamine as a non-polar solvent to
form perovskite during wet milling [2]. Therefore, the growing interest in Cs-Pb-Br perovskites

motivated us to develop a facile synthesis method and to study their phase conversion
mechanism with desired optical properties.
The main objectives of this present work are to (i) a new strategy to synthesize solvent-free
CsPbBr and CsPb Br perovskites by one step mechanical alloying in ambient condition, (ii)
3
2
5
study the crystal structure, microstructure and surface morphology of prepared samples by
Rietveld’s analysis of XRD patterns and FESEM images, (iii) confirm and explain the
3 2 5
compositional related phase transformation in between CsPbBr and CsPb Br perovskites by
water treatment or heat treatment and (iv) study the optical properties of all compounds and
resolve the controversy about the sources of enhanced luminescence.
2
. Experimental:
2
.1 Synthesis of CsPbBr and CsPb Br perovskites by mechanical alloying
3 2 5
CsPbBr and CsPb Br nanocrystalline (NCs) compounds are synthesized by a facile
3
2
5
mechanosynthesis technique. Pure CsBr (Sigma-Aldrich, purity 99.99%) and PbBr (Sigma-
2
Aldrich, purity 99.99%) powders were taken without further purification as raw materials with
two different molar ratios, 1:1 and 1:2. For both compositions, mechanical alloying was carried
out for 4 h (with 15 min pause after every 15 min of milling) under argon atmosphere in an 80 ml
chrome steel vial with 30 chrome steel balls of 10 mm diameter using a planetary ball mill
(
model-P5, M/S Fritsch, GmbH, Germany).
.2 Forward conversion from CsPbBr to CsPb Br
5
2
3
2
A water vapour treatment process was applied to the as-synthesized orthorhombic
CsPbBr compound for transforming it into CsPb Br tetragonal phase. Purified steam was flown
3
2
5
into CsPbBr
3
sample through a two-ended open tube for about 10 min until the CsPbBr
3
perovskite (yellow colour) converts completely to CsPb
2
Br perovskite (white colour). After full
5

conversion, the collected sample was dried in an open-air to obtain the sample in the form of fine
powder. Therefore, the proposed chemical reaction occurs during the phase transformation can
be expressed as, 2CsPbBr ꢀ+ H O↓ → CsPb Br + CsBr +H O↑
3
2
2
5
2
2
.3 Reverse conversion from CsPb
The conversion from CsPb
annealing of milled CsPb Br
decomposition process can be proposed as, CsPb Br → CsPbBr + PbBr
2
Br
5
to CsPbBr
3
2
Br
5
to CsPbBr
3
perovskite was accomplished by thermal
2
5
powder for 3 h at 673 K in a programmable furnace. This
2
5
3
2
2
.4 Materials characterization
X-ray powder diffraction (XRD) profiles of ball-milled powders were recorded using Ni-filtered
Cu Kα radiation (λ = 1.5418 Å) from an automated X-ray powder diffractometer (Bruker AXS,
D8 Advance, Da Vinci) operated at 40 kV and 40 mA. The surface morphologies of sheet-like
perovskite are first revealed from FESEM images. For UV–Vis and PL spectroscopy
measurements, a small amount of powder was fairly dispersed in toluene using vigorous
sonication and then transferred to the quartz cuvette of the spectrophotometer. UV-Vis
absorption spectra (SHIMADZU, UV-1800, JAPAN) were recorded to ensure the wavelength
range for the highest absorption region of CsPbBr and CsPb Br NCs. The emission PL spectra
3
2
5
are recorded in the wavelength range of 420–700 nm at the excitation wavelength 400 nm in a
PL spectrophotometer (Perkin Elmer, 1555). The direct observation of microstructure and
selected area electron diffraction patterns of ball-milled synthesized sample were revealed by
high-resolution transmission electron microscopy (HRTEM) operated at 120kV, equipped with a
CCD camera. A pinch of synthesized powder was dispersed ultrasonically in toluene and a drop
of colloidal solution was placed on a carbon-coated Cu-grid.

2
.5 Method of analysis
The Rietveld refinement method [24-27] is considered as one of the best analytical methods to
determine different structure and microstructure parameters such as lattice parameters,
crystallites size, r.m.s. strain and phase volume fraction etc. In the present work, we have
adopted Maud software [28] to obtain structure and microstructure parameters of all prepared
samples by analyzing respective XRD patterns, considering all appeared reflections of all
pertinent phases. FESEM images were analyzed by ImageJ software. Atomic-level modellings
are prepared by ATOM software using Rietveld refinement outputs.
3
. Results and discussion:
3
.1 Structural characterization
From the Rietveld refinement of powder XRD (PXRD) patterns (Fig. 1) of 4 h milled
CsBr and PbBr mixtures, it is shown that both milled compounds are composed of well-
2
crystalline phases with sharp reflections. It is evident that all reflections in Fig. 1(a) belong to
3
the orthorhombic perovskite CsPbBr phase (ICSD # 1533062) only and there is no reflection
from any of the ingredient phases, taken in 1:1 molar ratio. Some of the strongest reflections are
indexed and detailed structural parameters are tabulated in Table 1.

Fig.1. Partially indexed PXRD patterns of (a) orthorhombic CsPbBr
perovskites synthesized by mechanical alloying.
3 2 5
and (b) tetragonal CsPb Br
As shown in Fig. 1(b), the PXRD pattern of compound matched perfectly well with the
tetragonal CsPb Br (JCPDS # 25-0211) perovskite phase and in this case, also, there is no
2
5
impurity reflection either from the precursors (taken in 1:2 molar ratio) or from the milling
media. The post synthesized treatment of orthorhombic perovskite CsPbBr powder (yellow)
3
under flowing water vapour for a short duration results in a partial phase transition to another
perovskite CsPb Br powder (white) (94 mol %) and CsBr (6 mol%) (ICSD # 05-0588)
2
5
compounds. The post-synthesis treatment of tetragonal CsPb Br phase at 673K results in partial
2
5
transformation to orthorhombic CsPbBr powder (64 mol %) and PbBr (4 mol %) (ICSD # 64-
3
2
8
528) compounds. Fig. 2 depicts the typical Rietveld refined PXRD patterns of pure and post
synthesized compounds. It may be noted that all these PXRD patterns are fitted with a pseudo-
Voigt profile fitting function to account for the individual contribution of small crystallite size
and lattice strain to the peak broadening. The relative phase abundances of different phases after
post-synthesis treatment have been estimated from the Rietveld refinement as mentioned above.
All structural and micro/nano-structural parameters of these compounds are revealed by
analyzing respective PXRD patterns employing Rietveld refinement analysis and results are
summarized in Table 1.

Fig.2. Typical Rietveld analysis outputs of (a) mechanosynthesized orthorhombic CsPbBr , (b)
3
mechanosynthesized tetragonal CsPb Br , (c) post synthesized CsPb Br prepared from CsPbBr , (d) post
2
5
2
5
3
synthesized CsPbBr prepared from CsPb Br . Experimental (I ) data are shown as (o), simulated patterns
3
2
5
O
(
I
C O C
) are shown by solid lines, (I –I ) represents the corresponding difference for each of the pattern.

Table.1. Structural and microstructural parameters of cesium lead perovskites obtained from the Rietveld
refinement.
Cell parameters (Å)
Crystall R.m.s. Atomic
ite size micro- Species
Atomic coordinates
a
b
c
x
y
z
(nm)
strain
−
5
(x10 )
Cs
Pb
Br1
Br2
0.016 0.525 0.25
0.0 0.0 0.50
0.202 0.296 0.476
0.026 0.999 0.75
8
.2079 8.2537 11.7579 205.41
2.745
Cs
Pb
Br1
Br2
0.5
0.336 0.830
0.5 0.5
0.5
0.25
0
0
8
8
8
.4931
.4926
15.1884 85.21
15.1974 319.56
3.622
0.169
0.155 0.656 0.132
Cs
Pb
Br1
Br2
0.5
0.329 0.829
0.5 0.5
0.5
0.25
0
0
0.152 0.653 0.131
Cs
Pb
0.0200 0.521 0.25
.2091 8.2551 11.7598 176.15
0.5694
0.0
0.0
0.5
Br1
Br2
0.2011 0.298 0.469
0.0261 0.997 0.75
3
.2 Inter-conversion and stability
In this modified synthesis method, the compositions and structural transformation of
CsPbBr orthorhombic phase to CsPb Br tetragonal phase by water vapour treatment process are
3
2
5
confirmed by XRD pattern analysis using Rietveld refinement method as well as optical
absorption measurements. The synthesis mechanism as well as inter-conversion is represented by
a schematic diagram in Fig. 3. It is clearly demonstrated that the CsPbBr perovskite could be
3
fully converted to the CsPb Br perovskite with the extraction of CsBr compound by water
2
5

vapour. The water vapour treatment to orthorhombic CsPbBr compound results in intercalation
3
of two water molecules of resultant diameter ~3.4 Å preferably along c-axis and the compound
undergoes through a phase transition to tetragonal CsPb Br phase with an elongated c-axis of ~
2
5
1
5.19 Å, followed by the transformation: ~11.76 Å (length of c-axis of CsPbBr ) + ~ 3.4 Å
3
(
2H O added to c-axis of CsPbBr ) = ~ 15.16 Å (length of c-axis of CsPb Br ) (Table 1) [29-31].
2 3 2 5
Due to the intercalation of a layer containing a couple of adjacent water molecules into the
2
CsPbBr
3
unit cell, almost square like an orthorhombic basal plane (~8.21x8.25 Å ) becomes
2
perfectly square basal plane (~8.49 Å ) of tetragonal CsPb
2
Br
5
lattice. It is also evident from the
tabulated data that all atoms in the unit cell are displaced from their respective positions due to
the inclusion of water molecules. When the tetragonal CsPb Br compound is heat-treated at
2
5
elevated temperature, these water molecules are evaporated from the compound and
consequently, it transforms to the orthorhombic CsPbBr phase with a small distortion in the
3
basal plane. It may be noted that the fractional atomic coordinates of transformed lattices are
very close to those of the pure lattices as obtained directly by mechanical alloying the powder
mixtures.
+
The water-induced phase transformation accelerates the removal of Cs ions from CsPbBr
3
. This
is because of the high solubility of CsBr in water and strong ionic nature of CsPbBr compound.
3
As a result, the anisotropic growth of 2D layered structured CsPb Br compound takes place.
2
5

Fig. 3. The atomic-level structures of orthorhombic CsPbBr
proper polyhedral arrangements.
3 2 5
and tetragonal CsPb Br perovskites with
The maximum transformation level into CsPb
treatment through breaking of PbBr octahedra in orthorhombic CsPbBr
arranging chemically bonded alternating Cs and (Pb
related structure. On the other hand, the direct transformation from CsPb Br to CsPbBr was
2
Br
5
was achieved within 5 min by water vapour
perovskite lattice and
) sandwich-like layered perovskite-
6
3
+
-
2
Br
5
2
5
3
achieved by heat treatment in the open air. In this case, at high temperature, some Br atoms are
displaced and formed an octahedron with Pb atom at the centre by segregation of PbBr2
molecules. Schematic atomic level diagrams of CsPbBr and CsPb Br compounds are generated
3
2
5
based on the refined structural parameters revealed from the Rietveld analysis of respective XRD
patterns and shown in Fig. 4. The tetragonal phase of CsPb Br exhibits an interesting sandwich
] layers with intercalated Cs cations present in a 2D structure. In
2
5
-
+
2 5
structure consisting of [Pb Br
-
(
Pb Br ) layer, there are two types of Br atoms, one being four-coordinated with Pb atom in the
2
5
same plane and the other located by the two sides of Pb forming a distorted octahedron. One Pb
atom and eight Br atoms make a hendecahedron, which is distinct from the crystal structure of
CsPbBr with a three-dimensional connected structure.
3

Fig. 4. Illustrated polyhedral arrangement of CsPbBr and CsPb Br structure is shown by atomic
3
2
5
modelling.
Distorted 3D perovskite (orthorhombic) crystal structure is based on the corner-shared PbBr
6
+
octahedra and Cs ions occupy the cuboctahedral voids.

Morphology and microstructure of compounds revealed from FESEM and HRTEM
images:
Fig. 5. High and low magnification FESEM images of the mechanically synthesized (a, b) CsPbBr and
3
(
c, d) CsPb
2 5 .
Br perovskites
Inorganic halide perovskites have been synthesized with various morphology and size by
varying different conditions including temperature, molar ratio, nature of capping agents and
time of preparation by using different methods [32-34]. Fig. 5 represents FESEM images of
CsPbBr and CsPb Br compounds with high and low magnifications. It is evident that after 4 h
3
2
5
of milling the granular layered structure of CsPbBr is formed and these layers are oriented in all
3
possible directions but they are not entangled to each other (Fig. 5a). A magnified image of this

layered shape morphology of CsPbBr reveals that the compound is mesoporous in nature and
3
size of an isolated particle is ~220 nm (Fig. 5b). In the case of CsPb Br compound almost
2
5
similar layered structure morphology is evident with quite a large particle size of ~580 nm,
without any interlayer porosity (Figs. 5(c,d)). The particle size values of these compounds
corroborate well with results of respective crystallite sizes obtained from Rietveld refinement
analysis. It is evident that the morphology, shape and size of particles mostly depend on the
compositions of the Cs-Pb-Br system.
3 2 5
Fig. 6. HRTEM and SAED images of CsPbBr (a,b) and CsPb Br (c,d) perovskites.
HRTEM images of CsPbBr compound reveal that it is composed of several thin 2D
3
flake-like tiny single crystals stacked paralleled over each other and their average size is ~192

nm (Fig. 6a) which is closed to the crystallite size obtained from the Rietveld refinement (~205
nm). The SAED pattern shown in Fig (6b) clearly reveals the single-crystalline nature of the
compound as all diffraction spots are aligned in different layer lines. The phase formation of
orthorhombic CsPbBr compound is confirmed by indexing some of the intense spots as (110),
3
(112), (002) reflections and their multi-order reflections after identifying the orientation of such
single crystals along the zone axis [1-10]. Intensities as well as the interplanar spacings of these
spots are in good agreement with the respective XRD pattern (Fig. 2) and in accordance with the
ICSD file (File No. 1533062).
3 2 5
In contrast to the CsPbBr compound, the tetragonal CsPb Br compound is composed of
agglomerated spherical nanocrystals with average size ~76 nm (Fig. 6c), which is quite
comparable with the crystallite size obtained from Rietveld refinement (85 nm). Single
crystalline nature is also noticed in the SAED pattern of the compound and these nanocrystals
are found to oriented along [001] zone axis. Based on the crystal orientation, some of the intense
spots in the layered SAED pattern have been identified and indexed accordingly as (200), (202)
and (110) reflecting crystal planes with a tetragonal structure of the compound (JCPDS file. 25-
0
211) (Fig. 6d). It is to be noted that the intensities and interplanar spacings of these planes are
in good agreement with the respective XRD pattern of the tetragonal CsPb Br compound (Fig.
2
5
2
).

3
3
.3 Optical property analysis:
.3.1 UV-Vis spectra analysis
Optical absorption and emission properties of these perovskites are studied to verify their
potential application as optoelectronic materials. The optical absorption spectra of all as-prepared
and post-synthesis samples are obtained in the wavelength range 200–1000 nm and shown in
Fig. 6. The absorption coefficients of these samples are expected to depend on several factors,
such as dispersion effect, surface roughness, etc. Band structure calculations for the 3D
3
orthorhombic CsPbBr compound at ambient and high-temperatures confirmed the direct nature
of its band gap [35-38].

Fig. 7. UV–Visible absorbance spectra of (a) mechanosynthesized CsPbBr
CsPb Br , (c) transformed CsPb Br from CsPbBr and (d) transformed CsPbBr
Tauc plot and image of powder sample under visible light of respective compound)
3
, (b) mechansynthesized
2
5
2
5
3
3
from CsPb Br .(Inset:
2
5
It is evident from Fig. 7(a) that the absorption peak at 505 nm is attributed to CsPbBr3
compound and the peak at 402 nm (Fig. 7b) arises only due to CsPb Br compound, both
2
5
synthesized directly by mechanical alloying. The optical band gap energy (E ) of these
g
compounds are ~ 2.45 eV and ~ 3.08 eV respectively as obtained from their respective
3
absorption peaks. It may be noted that the yellowish orthorhombic CsPbBr compound obtained
by mechanical alloying is transformed primarily to whitish CsPb Br compound after water
2
5
vapour treatment with a small amount of impurity of CsBr phase. From Fig. 7(c) it is evident that
the absorption spectrum is almost identical to Fig. 7(b), except an additional absorption peak at ~
6
39 nm, which may be attributed to the CsBr impurity phase. Heat treatment of
mechanosynthesized whitish tetragonal CsPb Br compound results in the partial transformation
2
5
of the compound to yellowish orthorhombic CsPbBr compound and a substantial amount of
3
PbBr phase. Fig. 7(d) represents the absorption spectrum of the heat-treated compound, which is
2
not identical with Fig. 7(a). Also, the colour of mechanosynthesized CsPbBr
exact to the transformed compound after heat-treatment due to the presence of impurity of
CsPb Br phase and PbBr phase.
3
compound is not
2
5
2

Fig. 8. UV–Visible absorbance spectra of transformed CsPbBr compound and mechanosynthesized
3
CsPbBr and CsPb Br compounds.
3
2
5.
It is evident from Fig. 8 that the absorption peak of transformed CsPbBr compound is the
3
superposition of absorption of mechanosynthesized CsPbBr and CsPb Br compounds. The
3
2
5
consistency between the Rietveld refinement results and absorption results demonstrates that the
absorption peaks at 505 nm and 402 nm are indeed induced by CsPbBr and CsPb Br
5
3
2
compounds, which is further confirmed through the structural changes between two compounds.
3
.3.2 PL spectrum analysis
The study of optical emission spectra of nanomaterials is a general technique for
estimating different structural defects and optical properties by photogenerated carriers through
confirming different recombination and transition levels [39-44]. As per the previous reports, the

PL emission of pristine CsPbBr NCs could be within an energy interval comprised between 1.88
3
eV and 3.03 eV [45-48]. In the present study, PL emission spectra of mechanosynthesized
CsPbBr and post synthesized CsPbBr compounds are recorded in the wavelength range 450-
3
3
7
9
00 nm at the excitation wavelength λexc ~ 400 nm, as shown in Fig. 9. It is evident from Fig.
(a), only a sharp and distinct emission peak appears at 530 nm and 522 nm in
mechanosynthesized and post synthesized CsPbBr
3
compounds respectively, which is due to the
presence of the interconnected PbBr octahedra in this compound with 3D crystal structure. The
6
PL intensity of post synthesized compound is quite low and also blue-shifted from the
mechanosynthesized compound. The decrease in PL intensity can be attributed to the presence of
the substantial amount of PL inactive CsPb Br compound produced due to heat treatment and
2
5
the blue shift may be reasoned due to (i) transformed compound formed with smaller particles
~176nm) compared to mechanosynthesized compound (205nm) ( Table 1), (ii) dielectric PL
inactive CsPb Br environment around the CsPbBr compound and (iii) change in photon
(
2
5
3
propagation in the presence of impurity phases [49].
3 2 5
Fig. 9. PL spectra of (a) CsPbBr and (b) CsPb Br perovskites.

It is evident from Figs. 9(a) and 9(b) that the PL performances of orthorhombic CsPbBr
3
and
tetragonal CsPb Br compounds are quite different. Therefore, the PL spectra of the post
2
5
synthesized compounds are the combination of both materials and the orthorhombic CsPbBr3
compound dominates the whole PL signals [15, 50]. As shown in Fig. 9(b) the PL measurement
exhibits a weak performance for both mechanosynthesized and post synthesized tetragonal
CsPb Br phase respectively. The optical properties of CsPb Br compound signify a topic of
2
5
2
5
2 5
conflicting interpretations. Jiang and co-workers reported that tetragonal CsPb Br nanosheets
with large indirect bandgap showed a PL inactive behaviour [50]. Dursun et. al also concluded
from DFT calculation the inactive intrinsic luminescence from indirect bandgap ternary halogen-
plumbate single crystal [51]. On the other side, Tang et. al demonstrated that CsPb Br
2
5
microplates exhibit low-threshold lasing under both one- and two-photon excitation [52]. In our
observation, the obvious optical absorption edge at 380 nm could be noted which is different
from the previously reported value. In these PL spectra, low emission peaks at 401 nm, 420 nm,
4
43 nm and 470 nm are observed but no intrinsic luminescence peak in the green region is
detected. Thus, the emission peak around 530 nm is due to only the CsPbBr phase and the
impurity phase PbBr does not show any visible emission at room temperature [53]. To confirm
these speculations, CsPb Br compound is heat-treated at 673K and transition to CsPbBr is
3
2
2
5
3
noticed in the XRD pattern. Associated absorption and PL properties also corroborated well
with the changes in XRD patterns with dominating PL intensity of CsPbBr phase. In this case,
3
the absorption edge at 380 nm was not distinguishable any more due to being besieged by the
absorption of CsPbBr , which strength explains the absence of the absorption edge at 380 nm as
3
observed in previous reports. Turedi et. al reported that introducing water on the 3D film of
3
CsPbBr compound led to direct transformation of this perovskite to a highly emissive and

2 5
thermally stable 2D CsPb Br perovskite-related material (PRM) films [21]. They also explained
the reason of high PL intensity and quantum yield (QY) by none of aggregation-induced PL
quenching in the solid-state CsPb Br perovskite and loss of ligand by thermal annealing or
2
5
oxidation with water. From the electronic structure of orthorhombic CsPbBr compound, it is
3
known that the 4p-orbitals of Br predominantly contribute to the upper valence band edge and
4
p-orbitals of Pb make the most contribution to the conduction band [54-56]. It seems that Cs
has an insignificant effect on the electronic structure of the band edge. So, the excitation and
recombination of the electron are confined within the PbBr octahedral. Actually, the ionic
6
character of halide perovskite makes them sensitive to polar solvents. In the present study, it is
noticed that CsPbBr compound degrades by polar solvent and transforms to indirect band gap
3
CsPb Br compound which emits radiation in the UV region.
2
5
4
. Conclusion:
Nanocrystalline CsPbBr3 and CsPb Br5 halide perovskite compounds have been
2
successfully synthesized by one step one-pot mechanical alloying method by ball milling the
stoichiometric powder mixtures of CsBr and PbBr within a very short duration of 4 h. Crystal
2
structures of these compounds are precisely investigated employing Rietveld refinement analysis
using XRD data. Lattice parameters of tetragonal CsPbBr and orthorhombic CsPb Br
3
2
5
compounds, crystallite sizes, r.m.s. microstrains and atomic coordinates of mechanically
synthesized and post synthesized compounds are calculated with Rietveld's analysis and
microstructure/morphology are revealed from FESEM images. These ionic perovskites crystals
usually decompose easily in a high-humid environment and their optoelectronic as well as
optical performances degrade extensively. Structural phase transitions between these two
compounds occurred due to water vapour and heat treatments have been verified both by

structural analyses and photoluminescence measurements. The role of water molecules in the
structural phase transition from luminescent orthorhombic 3D CsPbBr phase to PL inactive 2D
3
layered tetragonal CsPb Br phase has been proposed with structural models for the first time.
2
5
UV-Vis absorption spectra of these compounds reveal that yellowish CsPbBr compound with a
3
direct bandgap of 2.45 eV transformed to whitish CsPb Br compound with an indirect band gap
2
5
of 3.08 eV. The visible PL emissions around at 530 nm are noticed to emerge from the
orthorhombic CsPbBr compound and the tetragonal CsPb Br compound is found to be PL
inactive in the UV-Vis regions. In the air, the water vapour can extract CsBr from CsPbBr phase
and results in conversion to tetragonal CsPb Br phase, which subsequently triggers in the
3
2
5
3
2
5
degradation of luminescence. Especially easy reaction procedure with some reaction conditions
makes the mechano-chemical reaction highly reproducible and has been adopted in the present
study. The novel finding of role of water (or any polar solvent) molecules resolves the mystery
of PL and QY degradation of the orthorhombic CsPbBr compound and any preventing measure
3
(thin film coating or keeping it under vacuum) to arrest the water molecules in the compound
will result in preventing the degradation of the compound and its optical properties can be
retained for a longer time. The compound can also be charged further with eliminating water
molecules by heat treatment and the optical properties can be revived completely. These
compounds may also be used for optical switching devices.

Conflicts of interest:
There are no conflicts to declare
Acknowledgements
Authors (G. Maity, S. K. Pradhan) wish to acknowledge Department of Science and
Technology (DST) for providing XRD facility under the FIST programme and University Grant
Commission (UGC) India for granting the “Center of Advanced Study” programme under the
thrust area “Condensed Matter Physics including Laser Applications” to The Department of
Physics, Burdwan University under the financial assistance of which the present work has been
carried out.
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