Crystal structure of the high-temperature polymorph of C(NH2)3PbI3 and its thermal decomposition

Article abstract of DOI:10.1016/j.jallcom.2020.158104

The synthesis of guanidinium lead iodide, C(NH₂)₃PbI₃ (GUAPbI₃), was conducted by slow evaporation of the mixture obtained by dissolving PbI₂ and C(NH₂)₃I in acetonitrile. When the evaporation is done at 40 oC, a yellow needle-like crystals are being formed. The sample was characterized by elemental analysis, density measurements, scanning electron microscopy, thermal analyses, high-temperature X-ray powder diffraction and infrared spectroscopy measurements. The elemental analysis of the obtained crystals confirmed the proposed stoichiometry. The performed thermal analyses showed an endothermic peak associated with structural transition around 160 oC. On the other hand, the endothermic temperature effects above 300 oC are accompanied with mass loss and were interpreted as compound degradation. The crystal structure of high temperature polymorph between 160 oC and 300 oC was determined using high-temperature powder diffraction data measurements at 280 oC using simulated annealing technique in order to obtain initial structural model. The structure was refined using the Rietveld method. At temperatures higher than 160 oC, C(NH₂)₃PbI₃ crystallizes in hexagonal space group P6₃mc with unit cell parameter a increasing from 9.269 ? to 9.337 ? between 160 oC and 300 oC and c parameter increasing from 15.211 ? to 15.287 ? in the same temperature range. The structure consists of PbI₆ octahedra couples sharing a common face, linked with corners. Guanidinium cations are situated in the channels between Pb₂I₉ couples in a manner that the plane of the molecule is perpendicular to the c-axis.

Full text of DOI:10.1016/j.jallcom.2020.158104

Journal of Alloys and Compounds 864 (2021) 158104
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jalcom
Crystal structure of the high-temperature polymorph of C(NH2)3PbI3 and
its thermal decomposition
Sandra Dimitrovska-Lazovaa,⁎, Miha Bukleskia, Peter Tzvetkovb, Slobotka Aleksovskaa,
Daniela Kovachevab
a University “Ss Cyril and Methodius”, Faculty of Natural Sciences and Mathematics, Institute of Chemistry, Arhimedova 5, 1000 Skopje, Macedonia
b Institute of General and Inorganic Chemistry, Bulgarian Academy of Sciences, Acad. G. Bonchev Str, Bd.11, 1113 Sofia, Bulgaria
a r t i c l e i n f o
a b s t r a c t
Article history:
The synthesis of guanidinium lead iodide, C(NH2)3PbI3 (GUAPbI3), was conducted by slow evaporation of
the mixture obtained by dissolving PbI2 and C(NH2)3I in acetonitrile. When the evaporation is done at 40 ºC,
a yellow needle-like crystals are being formed. The sample was characterized by elemental analysis, density
measurements, scanning electron microscopy, thermal analyses, high-temperature X-ray powder diffraction
and infrared spectroscopy measurements. The elemental analysis of the obtained crystals confirmed the
proposed stoichiometry. The performed thermal analyses showed an endothermic peak associated with
structural transition around 160 ºC. On the other hand, the endothermic temperature effects above 300 ºC
are accompanied with mass loss and were interpreted as compound degradation. The crystal structure of
high temperature polymorph between 160 ºC and 300 ºC was determined using high-temperature powder
diffraction data measurements at 280 ºC using simulated annealing technique in order to obtain initial
structural model. The structure was refined using the Rietveld method. At temperatures higher than 160 ºC,
C(NH2)3PbI3 crystallizes in hexagonal space group P63mc with unit cell parameter a increasing from 9.269 Å
to 9.337 Å between 160 ºC and 300 ºC and c parameter increasing from 15.211 Å to 15.287 Å in the same
temperature range. The structure consists of PbI6 octahedra couples sharing a common face, linked with
corners. Guanidinium cations are situated in the channels between Pb2I9 couples in a manner that the plane
of the molecule is perpendicular to the c-axis.
Received 14 September 2020
Received in revised form 20 November 2020
Accepted 23 November 2020
Available online 26 November 2020
Keywords:
GUAPbI3
High-temperature polymorph structure
Simulated annealing
Rietveld structure refinement
© 2020 Elsevier B.V. All rights reserved.
1. Introduction
development of perovskite solar cells has been focused on in-
creasing both PCE and their stability [13].
Over the past decade, organic-inorganic perovskites (hybrid
perovskites) emerged as one of the most promising materials for
solar cell fabrication combining both high efficiency and low cost
[1,2]. Starting with the work of Kojima et al. [3] on CH3NH3PbI3
(MAPbI3) perovskite solar cell, which showed 3.8% power con-
version efficiency (PCE), the intense work on this field resulted in
increasing PCE up to 22% [4,5] in just a few years. Hybrid per-
ovskites have shown unique properties suitable for photovoltaic
applications, such as high light absorption coefficients [3,6], long
charge carrier (electron-hole) diffusion lengths [7], tunable band
gaps [8,9] etc. However, organic-inorganic perovskites for solar
cell applications suffer from instability under external conditions
such as humidity, heat, light etc. [10–12]. Therefore, the further
Recent investigation on organic-inorganic perovskites, has shown
that the organic cation in the structure has a great influence on both
the stability and performances [14], so a number of attempts were
made on exchanging and modification of the monovalent organic ca-
tion [9,14,15]. A promising candidate among the organic cations, is
guanidinium cation [(NH2)3C]+ (GUA) [16,17]. Unfortunately, GUA has a
significantly larger ionic radius (278 pm), and is assumed as not sui-
table to form a 3D perovskite structure as the sole A-cation [14,16].
However, Giorgi et al. [18] have theoretically characterized the guani-
dinium lead iodide system, GUAPbI3 using density functional theory
calculations (DFT). The results of their calculations pointed out to better
thermodynamic stability, low formation energy and low band gap
compared to MAPbI3 and FAPbI3. The authors suggest that this is due to
a high symmetry of guanidinium cation and zero dipole moment as a
result of its D3h symmetry. In addition, the three NH2 groups are ex-
pected to frustrate the bulkier GUA cation rotation in the semi-
conductor cavity of the proposed perovskite structure [18].
⁎
Corresponding author.
E-mail address: sandra@pmf.ukim.mk (S. Dimitrovska-Lazova).
https://doi.org/10.1016/j.jallcom.2020.158104
0925-8388/© 2020 Elsevier B.V. All rights reserved.

S. Dimitrovska-Lazova, M. Bukleski, P. Tzvetkov et al.
Journal of Alloys and Compounds 864 (2021) 158104
Stoumpos et al. [19] synthesized and determined the single
crystal structure of GUAGeI3 at room temperature, which appeared
to be monoclinic (P21/c) consisting of a 1D chains, and a band gap of
2.7 eV. In addition, Stoumpos et al. [20] synthesized GUASnI3 and
found it to be hexagonal (P63/m) at room temperature, with 1D
connectivity that is a combination of face-sharing and corner-
sharing octahedra.
thermal behavior. Considering the growing interest for GUA-based
perovskite-like compounds, in this work we present a detailed study
of the high-temperature polymorph in respect to its crystal struc-
ture, thermal behavior within the range from room temperature to
700 °C, morphology and IR spectra.
2. Experimental
The lead containing guanidinium halides were also investigated.
Years ago, Szafrański synthesized and investigated the instability of
GUA2PbCl4, GUA2PbBr4, GUA2PbI4 and of GUAPbI3, by DTA and XRD
[21]. He reported that all studied compounds undergo phase tran-
sitions at elevated temperature. As to the GUAPbI3 he registered two
first-order phase transitions at 255 K (–8.5 ºC) and 432 K (158.85 ºC)
respectively. The crystal structure of the phase stable between 255
and 432 K (phase II in Szafrański notation) was reported to be or-
thorhombic of the KCdCl3-type, space group Pnma with unit cell
parameters close to that of CsPbI3. He suggested an order-disorder
mechanism for the phase transitions in this compound. According to
Jodlowski et al. [22] the GUAPbI3 has 1D octahedral connectivity,
whereas for the GUA2PbI4 it is 2D.
Mixed cation perovskites involving guanidinium ions have also
been synthesized [23–28]. Jodlowski et al. [24] prepared
MA1–xGUAxPbI3, a highly stable 3D crystalline structure that exhibits
superior photovoltaic performance and material stability compared
with MAPbI3, due to the increased number of H-bonds within the
inorganic framework. Improved interfacial and bulk electronic in
(GUA)0.10(MA)0.90PbI3, was confirmed by Prochowicz et al. [25]. For a
GUA content of 14%, good optoelectronic properties were obtained
and a high PCE surpassing 20%. Kubicki et al. [26] provided atomic-
level evidence that GUA is directly incorporated into the MAPbI3 and
FAPbI3 lattices, forming pure GUAxMA1–xPbI3 or GUAxFA1-xPbI3
phases. Nazarenko et al. [27] presented a new layered hybrid lead
halide, namely [CH(NH2)2][C(NH2)3]PbI4, wherein puckered lead-
iodide layers are separated by two stable organic cations: for-
mamidinium, CH(NH2)2+, and guanidinium, C(NH2)3+. Recently, Soe
2.1. Sample preparation
As starting materials for the synthesis of C(NH2)3PbI3, PbI2 and
C(NH2)3I were used. The PbI2 was obtained by adding concentrated
HI (57%, w/w, Carlo Erba Reagents) to PbCO3 (Merck Millipore).
Guanidinium iodide was synthesized by dropwise addition of HI to
the water solution of NH2C(=NH)NH2·½H2CO3. The product was
obtained by slow evaporation at 40 ºC of the stoichiometric mixture
of previously prepared solutions of PbI2 and C(NH2)3I in acetonitrile.
The use of PbCO3 in the synthesis of PbI2, instead of Pb2+ water
soluble salts has several advantages. The main one is that PbCO3 is
water insoluble and hence environmental more friendly. Also, PbI2
obtained as described, has higher purity compared to the product
obtained by simple precipitation from Pb2+ water solutions. Due to
its low solubility, PbI2 is not listed as a substance that is threat to the
environment. Last, the lead perovskites, according to the conducted
research, have not yet been proven to be hazardous to the human or
the environment [30,31].
2.2. Thermal analysis
The differential thermal analyses and the thermogravimetric in-
vestigations (DTA/TG) were carried out on LabsysEvo 1600 apparatus
equipped with Pfeiffer OmniStar GSD 301 mass spectrometer. The
measurements were made with a temperature heating rate of 10°/
min under a flowing argon atmosphere and Al2O3 crucible as re-
ference material.
et al. [28] presented
a series of low dimensional perovskites
(C(NH2)3)(CH3NH3)nPbnI3n+1 (n=1, 2, and 3) using guanidinium and
formamidinium as interspacer.
2.3. Powder X-ray diffraction
The growing interest in adding guanidinium cations to lead io-
dide systems, in an attempt to modify their optical properties and
film stability, has become obvious. Thus, Deng et al. [29] examined
(GUA)xPbIx+2 (x=1, 2, and 3) by single crystal XRD at room and low
temperature. The yellow needle-like GUAPbI3, at room temperature
crystallizes in the orthorhombic space group Pnma. The crystal
adopts a 1D double chain structure, which is in agreement with
previously reported study [22]. The authors found that on cooling, at
120 K (–53 ºC), a phase transition takes place resulting in a mono-
clinic cell with inorganic framework that retains the same double
edge-sharing connectivity as the room temperature phase. The or-
ange crystals of (GUA)2PbI4 at room temperature crystallize in the
space group P21/n, which is consistent with the previous work of
Szafransky [21]. The structure consists of double layers of corner-
sharing octahedra with 2D connectivity and GUA+ filling the cavities
between two polyhedra within a layer and between layers. The third
compound, (GUA)3PbI5, which has not been reported previously, is
yellow at room temperature and crystallizes in the monoclinic space
group C2/c. The structure is characterized by a PbI6 network of
corner-sharing octahedra forming 1D chains extending along the c-
axis. The authors have also calculated band structures of the iodo-
plumbates and confirm that their band gaps increase with reducing
dimensionality. It is also shown that octahedral connectivity affects
the bandgap, with face-sharing octahedra resulting in compounds
with the largest bandgap.
Powder X-ray diffraction (XRD) patterns for characterization of
phase transition temperatures of the sample were collected from
20 ºC to 300 ºC in air with measuring interval of 10 ºC, on a Bruker D8
Advance diffractometer using CuKα radiation and LynxEye PSD de-
tector. Heating of the sample was performed by a custom-made
vertical quartz tube furnace with aluminum foil mounted at the top
of the tube. The sample was powdered on the foil and gently flat-
tened with a glass plate. The temperature was adjusted with an
automatic on/off controller and thermocouple mounted next to the
bottom of the sample. The furnace was calibrated with RbNO3 for
temperature gradient between the bottom and surface of the
sample. The measurement range was from 5 º to 90 º 2θ with a step
of 0.02 º 2θ and 0.1 s/strip counting time of the PSD detector (total
17.5 s/step) were used for phase analysis, calculation of the unit cell
parameters and determination of the phase transition temperatures.
The patterns for structure refinement were collected at room tem-
perature and 280 ºC within 5–120 º 2θ range with a step of 0.02º 2θ
and 10 s/strip (total 175 s/step) counting time. Powder pattern
Table 1
Elemental composition of the C(NH2)3PbI3 sample.
Element/ (w/%)
C
H
N
Pb
I
Theoretical
Experimental
EDX
1.85
1.87(7)
0.92
1.04 (9)
6.48
6.56 (5)
31.94
31.86 (8)
34(4)
58.79
61(3)
Taking into consideration the literature survey, it is evident that
the structure of the high-temperature phase, reported by Szafransky
[21], is not yet structurally characterized, as well as its detailed
2

S. Dimitrovska-Lazova, M. Bukleski, P. Tzvetkov et al.
Journal of Alloys and Compounds 864 (2021) 158104
Fig. 1. SEM micrographs of the RT-phase of C(NH2)3PbI3 with magnifications of (a) ×500 and scale bar of 50 µm, and (b) × 5000 and scale bar of 5 µm.
Powdered sample of C(NH2)3PbI3 was measured six consecutive
times with five iterations during each measurement. The calculated
average value of 3.98(1) g/cm3 was used for further structural con-
siderations.
2.7. Infrared spectroscopy
The IR absorption spectra were recorded on a Perkin-Elmer
System 2000 FTIR Spectrometer using a heating cell, accessory
purchased by the same brand. For the absorption measurements, the
KBr pellet technique was used. The absorption spectra were recorded
with a resolution of 2 cm–1, using 32 scans (both for the background
and the sample spectra) at 25 and 200 ºC on the same pellet. The
CO2/H2O software compensation was used to reduce the bands in-
tensity originating from the atmospheric moisture and the CO2.
3. Results and discussion
Fig. 2. Rietveld plot of room temperature C(NH2)3PbI3 phase.
The results from the combined elemental analysis are presented
in Table 1. As can be seen, the experimental elemental composition
is very close to the theoretical one and confirms the proposed stoi-
chiometry.
The SEM micrographs of the as obtained phase are presented in
Table 2
Crystal structure data for RT-phase of C(NH2)3PbI3 at 20 ºC.
Atom
Wyck.
x
y
z
Biso
Fig. 1. The micrographs reveal the presence of two fractions of
needle-like shape crystals of the obtained C(NH2)3PbI3. The large
needles are about 2 µm thick and more than 100 µm long, while the
little crystals show about 10 times smaller size. Very different
morphology was reported for C(NH2)3PbI3 synthesized by mechan-
ochemical activation of C(NH2)3I and PbI2 with subsequent an-
nealing at 160 ºC [22]. The reported SEM micrographs represent
dense masses of plate-like to isometric crystals ranging in size from
2 to 4 µm.
Pb1
I1
I2
4a
4a
4a
4a
4a
4a
4a
4a
0.6546(2)
0.7706(2)
0.4711(2)
0.8169(2)
0.595(3)
0.542(2)
0.539(3)
0.684(2)
0.5523(1)
0.6842(2)
0.5959(2)
0.4931(2)
0.821(3)
0.764(2)
0.869(2)
0.830(2)
0.653(2)
0.772(5)
0.189(7)
0.183(7)
0.25(6)
0.25(4)
0.17(7)
2.51(6)
2.51(6)
2.51(6)
2.51(6)
2.51(6)
2.51(6)
2.51(6)
2.51(6)
I3
C1
N1
N2
N3
0.43(2)
SG. (33) Pna21, a=11.9694(2) Å, b=20.8583(3) Å, c=4.4715(3) Å, Rexp=2.52%, Rwp=7.40%,
Rb=3.67%, χ2=8.58
The powder diffraction pattern of the RT-phase of C(NH2)3PbI3
was indexed in the orthorhombic space group, Pna21. The crystal
structure of RT-phase was refined on the basis of the model pro-
posed by Jodlowski et al. [22]. Rietveld plot for the RT-phase of
GUAPbI3 is shown on Fig. 2. The refined structural parameters are
presented in Table 2. For the sample obtained in this study, they are
close to those reported by Jodlowski et al. [22]. On the other hand,
the observed diffraction pattern reflects the strong difference be-
tween the morphology of the sample obtained in this study and the
one obtained by Jodlowski et al. [22]. A significant preferred or-
ientation due to needle-like morphology of our sample is observed.
The crystal structure of the RT-phase of GUAPbI3 along c and a
axes is shown on Fig. 3, respectively. In this structure the lead ca-
tions are octahedrally coordinated with six iodine anions and the
edge sharing PbI6 octahedra are arranged in double chains along the
c-axis of the unit cell. The planar guanidinium cations are situated
between the chains. The plane of the guanidinium group is almost
perpendicular to the c-axis. Guanidinium groups possess high
indexing, ab initio crystal structure determination and Rietveld re-
finement were performed by using Bruker Topas v.4.2 program.
2.4. Scanning electron microscopy
The morphology of the samples was studied by scanning electron
microscopy (SEM) on JEOL 6390 instrument.
2.5. Elemental analysis
The determination of the content of C, H and N in the obtained
compound was performed on Carlo Erba 1106 equipment.
2.6. Density measurement
The density measurements were performed on Mikromeritics
AccuPyc 1330 gas pycnometer under nitrogen atmosphere.
3

S. Dimitrovska-Lazova, M. Bukleski, P. Tzvetkov et al.
Journal of Alloys and Compounds 864 (2021) 158104
Fig. 3. The crystal structure of RT phase of C(NH2)3PbI3 along c and a axis.
released in this temperature interval are NH3 and H2O. It should
be mentioned that the temperatures at which ammonia is being
released are in good agreement with the data published by Ma
et al. [32] for the decomposition of MAPbI3 and FAPbI3. The second
region is between 380 ºC and 500 ºC. The endothermic peak at
396 ºC is not associated with mass loss and may be due to struc-
tural rearrangement of the residual part of the structure after the
ammonia was produced. The total mass loss in this region is about
20%. It is associated with the formation of molecules with 44 mass
– either N2O or a fragment C2H6N+. Species of HNCO are observed
at 467 °C. The release of N2 starts at 450 °C. The mass loss (30%)
within the third temperature interval above 500 ºC is supposed to
be due to the release (through sublimation) of iodine containing
species. The presence of oxygen containing species among the
released gases suggests the strong interaction of the sample
with air.
On the other hand, in the case of decomposition in the
Ar atmosphere (Fig. 4) the three regions of decomposition appear at
different temperatures. The endothermic peaks in the first region are
shifted to higher temperatures. The generation of NH3 is observed at
around 350 ºC. In this case the formation of water vapor and HNCO
during decomposition are not detected. The observed release of the
species with 44 mass, in this case, appears at three temperatures
350 ºC, 443 ºC and 500 ºC. For that reason, it is most probable that
these peaks belong to the fragment C2H6N+. What is somehow
strange, is that in the case of decomposition in the Ar atmosphere
the release of N2 is observed within the entire temperature interval
with a maximum at around 430 ºC. Based on the data obtained as
well as from general considerations, the following decomposition
equation can be proposed:
Table 3
Calculated interatomic distances and bond-valence sums for RT- C(NH2)3PbI3.
Bond length distances
PbI6 polyhedra angles
Pb1−I1/Å
Pb1−I3/Å
Pb1−I3/Å
Pb1−I2/Å
Pb1–I2/ Å
Pb1–I2/ Å
Pb1–Pb1/Å
C1–N1/Å
C1–N2/Å
C1–N3/Å
Pb1 BVS
Pb1I6 distortion
3.128(6)
3.12(3)
3.30(3)
3.16(3)
3.38(3)
3.441(5)
4.844 (4)
1.35(7)
I1–Pb1–I2/º
I3–Pb1–I3/º
I2–Pb1–I2/º
I3–Pb1–I2/º
167.5(7)
88.2(1)
86.3(1)
88.5(6)
1.26(14)
1.35(18)
2.02(4)
15.262 × 10–4
symmetry and ability to form hydrogen bonds. These factors are
important when considering the structure and the properties of
guanidinium containing compounds. The distances within PbI6 oc-
tahedra in the RT-phase are between 3.12 Å and 3.44 Å (Table 3). The
effects related to Pb2+ 6s2 lone pair electrons were not observed in
the RT-phase structure.
The results from DTA/TG analyses performed in air and argon
atmosphere are presented in Fig. 4. In both cases a small en-
dothermic peak and the absence of mass loss is observed at 159 ºC. It
is worth mentioning that both the position and the intensity of this
peak are independent on whether the experiment is conducted
under nitrogen or air. This peak is associated with structural tran-
sition from RT-phase to HT-phase. This observation is in good
agreement with the data, published earlier. Szafranski [21] reported
for the transition temperature of C(NH2)3PbI3 between 142 °C and
159 °C depending on the heating rate, while Jodlovski [22] gives
values of 134 ºC and 165 ºC depending on the thermal treatment
history of the material. The study of the behavior of C(NH2)3PbI3 at
high temperatures was complemented by investigation of the
thermal degradation products. When the temperature increases
above 300 ºC the endothermic effects related to phase decomposi-
tion and mass losses vary depending on the different atmosphere
used. The gas products of C(NH2)3PbI3 decomposition in air and
argon atmosphere are also presented in Fig. 4. Due to spectrometer
limitations only masses below 50 were detected.
C(NH2)3PbI3(s)→PbI2(s)+CH3I(g)+NH3(g)+N2(g)
(1)
This equation is expected to describe the decomposition of the
initial compound under argon atmosphere, while heating in air
should lead to products that may include oxygen.
To elucidate the structural transformation, powder diffraction
patterns were collected within the temperature interval from RT
to 300 ºC. The summarized XRD plot is presented on Fig. 5. It can
be seen that within the range of 20–140 ºC the pattern corre-
sponds to the orthorhombic phase of C(NH2)3PbI3 crystallizing in
space group Pna21. Starting from 140 ºC peaks of a new phase (HT-
phase) appear together with the peaks of the RT-phase. The
transition completes at about 170 ºC. These observations are in
In the case of decomposition in air, three main regions of de-
composition should be outlined (Fig. 4). The first one being be-
tween 300 ºC and 380 ºC includes three endothermic peaks. They
are associated with the mass loss of about 10%. The detected gases
4

S. Dimitrovska-Lazova, M. Bukleski, P. Tzvetkov et al.
Journal of Alloys and Compounds 864 (2021) 158104
Fig. 4. DTA/TG plot within the range 25–650 ºC for thermal decomposition of C(NH2)3PbI3 together with the results for the masses of gases released in air atmosphere (left
column) and in Ar-atmosphere (right column).
good agreement with the results from DTA/TG thermal analyses.
The appearance of a new set of peaks at 290 ºC is an indication for
a thermal decomposition. One may consider that the range of
stability of the high-temperature polymorph of C(NH2)3PbI3 is
between 140 ºC and 290 ºC.
crystalline state (with minimal energy). In the present case at the
beginning all heavy atoms (lead and iodine) were placed in the cell
and the program determined their positions in the cell successfully.
The couples of Pb2I9 octahedra were revealed. The positions of
guanidine groups were found further by introducing a “dummy”
atom with atomic scattering factor equal to the sum of scattering
factors of the atoms in guanidine group. After finding the principal
positions of the guanidine groups their true atomic configuration
was built in these places. Structural models were tested within all
possible space groups (SGNo 159, 163, 186, 190, 194). Best fit with the
experimental powder diffraction pattern were obtained for the
models built in P31c and P63mc groups. It was found that no sub-
stantial difference in the crystal structure is obtained within these
two groups. That is why further Rietveld refinement of the crystal
structure, on data collected at 280 ºC was performed in the group
with higher symmetry P63mc. Rietveld plot for HT-phase of
C(NH2)3PbI3 is presented in Fig. 6. Table 4 contains the crystal
structure data for HT-phase of C(NH2)3PbI3. In order to fix the origin
of the unit cell, the z fractional parameter of Pb1 atom was con-
strained to zero. The positions and thermal displacement parameters
The structure of HT-polymorph of C(NH2)3PbI3 was determined
on the basis of high-temperature powder diffraction data measure-
ments at 280 ºC. Solving crystal structure from powder diffraction
pattern consists usually of several steps. The first one is indexing the
powder diffraction pattern. The second step is determining the
possible space groups (usually several with the same extinction
conditions). The third step is to extract the integral intensities of the
peaks. From density measurements, formula units per unit cell (Z) is
obtained. In the present case, the indexing of the peaks in the dif-
fraction pattern resulted in a hexagonal (or trigonal) unit cell with
parameters a ≈ 9.3 Å and c ≈ 15.2 Å. The systematic extinctions re-
vealed five possible space groups. The initial structure models for the
HT-phase were obtained by simulated annealing technique. Atoms
are introduced to the unit cell in stoichiometric quantities in arbi-
trary positions. After that the algorithm implemented in the pro-
gram tries to mimic the annealing of the sample to reach the good
5

S. Dimitrovska-Lazova, M. Bukleski, P. Tzvetkov et al.
Journal of Alloys and Compounds 864 (2021) 158104
Fig. 5. Powder XRD patterns of GUAPbI3 at different temperatures indicative for the phase transition.
Table 4
Crystal structure data for HT-phase of C(NH2)3PbI3 at 280 ºC.
Atom
Wyck.
x
y
z
Biso
Pb1
Pb2
I1
2a
2a
6c
6c
2a
2b
6c
6c
1/3
1/3
0.827(1)
0.487(1)
0
1/3
-0.080(5)
0.250(4)
2/3
2/3
-0.827(1)
-0.487(1)
0
2/3
0.080(5)
-0.250(4)
0.000
3.9(2)
3.9(2)
3.9(2)
3.9(2)
3.9(2)
3.9(2)
3.9(2)
3.9(2)
0.263(1)
0.624(1)
0.861(1)
0.332(4)
0.602(5)
0.332(4)
0.602(5)
I2
C1
C2
N1
N2
SG. (186) P63mc, a=9.3262(4) Å, c=15.2681(7) Å, Rexp=2.95%, Rwp=7.71%, Rb=7.39%,
χ2=6.86
Table 5
Calculated interatomic distances and bond-valence sums for HT-C(NH2)3PbI3.
Bond length distances
PbI6 polyhedra angles
Pb1–I1/Å
Pb1–I2/Å
Pb2–I1/Å
Pb2–I2/Å
Pb1–Pb2/Å
C1–N1/Å
C2–N2/Å
Pb1 BVS
Pb1I6 distortion
Pb2 BVS
3.21(1)×3
3.27(1)×3
3.35(2)×3
3.26(1)×3
4.02 (1)
I1–Pb1–I2/º
I2–Pb1–I2/º
I1–Pb1–I1/º
I2–Pb1–I1/º
I1–Pb2–I2/º
I2–Pb2–I2/º
I1–Pb2–I1/º
I2–Pb2–I1/º
175.7(5)
82.3(4)
88.7(5)
94.4(4)
167.8(6)
100.8(5)
84.1(5)
86.9(3)
Fig. 6. Rietveld plot of high temperature C(NH2)3PbI3 phase at 280 ºC.
of the hydrogen atoms were not refined. Selected interatomic dis-
tances and bond-valence sums for HT-phase are presented in Table 5.
Polyhedral representation of the structure of HT modification of
C(NH2)3PbI3 is given on Fig. 7.
1.292(1)×3
1.346(1)×3
1.99(3)
0.834 × 10–4
1.67(3)
The structure of the HT-phase consists of couples of PbI6 octa-
hedra sharing a common face, linked to other couples with corners.
The close values of the Pb-I distances around Pb1 (Table 5), indicate
that the octahedron around Pb1 shows low distortion, while that
around Pb2 is highly distorted. These distances lead to the bond
valence sums close to 2 for Pb1 and slightly lower for Pb2. The ob-
tained values are in good agreement with typical Pb-I distances,
Pb2I6 distortion
1.885 × 10–4
observed for example in PbI2 (3.1–3.3 Å) [33] and with the distances
calculated for the RT-phase [22]. The BVS(Pb1)=1.99 indicates that
the distances around Pb(1) are very close to the usual distances
found for Pb2+ ion. In the case of Pb(2), the lower BVS value is due to
6

S. Dimitrovska-Lazova, M. Bukleski, P. Tzvetkov et al.
Journal of Alloys and Compounds 864 (2021) 158104
Fig. 7. Polyhedral representation of high temperature phase of C(NH2)3PbI3 along (010) and (001) directions.
low stability of this phase. The fact that HT-phase transforms to a RT-
modification after cooling to room temperature advocates that HT-
phase is metastable. Some related face-sharing Pb-I species have
also been reported for Pb4I15 ions [34]. A recent theoretical work on
organic lead iodides [35] predicted the configuration found in this
study and
a similar structure was recently found for θ-acet-
amidinium tin iodide CH3C(NH2)2SnI3 [20]. The present structure is
the first representative of the 4H-polytype with (hchc) anion se-
quence found for guanidinium cation containing lead iodides. The
tricapped trigonal-prismatic cavities between Pb2I9 polyions form
hexagonal channels that run along the c-axis. In the studied struc-
ture guanidinium groups are situated in the channels between Pb2I9
couples in a manner that the plane of the molecule is perpendicular
to the c axis of the unit cell. It should be noted that at increased
temperatures the thermal displacement parameters of the atoms in
the guanidinium group became very high. This indicates that the
rotation of some GUA+ groups leads to symmetry lowering of the
structure.
As it was mentioned from other authors [21], the transition be-
tween the RT and HT phases was assumed to be of order-disorder
type. The results presented in this study contradict this assumption.
It was found that the transformation is connected with the dis-
placement and rearrangement of the iodine sublattice leading to the
breaking of the double chains from the edge-sharing PbI6 octahedra
and to the formation of face-sharing Pb2I9 polyions linked with
corners.
Fig. 8. Dependence of the unit cell volume of the high temperature phase on tem-
perature.
the asymmetric elongation of the octahedron along the 3-fold axis.
Namely, three of the distances are longer (3.35 Å), which reflects on
the higher polyhedral distortion for Pb2 octahedron. It should be
mentioned that the observed values are not unusual. They suggest
that the strength of the corresponding bonds is lower, leading to the
Fig. 9. IR spectra of C(NH2)3PbI3 recorded at 25 ºC (lower, black spectra) and at 200 ºC (upper, red spectra) in the regions where the most significant changes are observed: (a)
stretching NH2 vibration, (b) bending NH2 vibrations, (c) bending CN3 and rocking NH2 vibrations.
7

S. Dimitrovska-Lazova, M. Bukleski, P. Tzvetkov et al.
Journal of Alloys and Compounds 864 (2021) 158104
The obtained data suggest that at temperatures higher than
150 ºC, C(NH2)3PbI3 crystallizes in hexagonal space group P63mc
with unit cell parameter a increasing from 9.269 to 9.337 Å between
160 and 300 ºC and c parameter increasing from 15.211 to 15.287 Å in
the same temperature range. Shortly after the temperature of phase
transition the unit cell parameters at 170 °C are a = 9.290(1) Å and
c = 15.193(2) Å. The change of the unit cell volume with temperature
is shown in Fig. 8. A good linear trend of the cell volume is observed
with the increase of the temperature.
spectra of the HT phase show that the guanidinium ion is sig-
nificantly more symmetrical compared to the one in the RT-phase.
Also, the results based on the IR spectra suggest stronger hydrogen
bonding of the NH2 groups in the HT phase and confirm the dis-
tortion of the Pb octahedra determined by the means of XRD. The
thermal analyses under inert atmosphere, supported by mass
spectrometry detection of the degradation products, revealed three
regions of GUAPbI3 decomposition connected to releasing of CH3I,
NH3 and N2 and PbI2 as a final solid residue.
The changes in the structure that corresponds to RT→HT phase
transition can also be observed in the mid-IR spectra of the
C(NH2)3PbI3. They point to a reorientation of the NH2 groups around
the carbon atom. This conclusion is made based on the band shift
CRediT authorship contribution statement
Sandra Dimitrovska-Lazova: Investigation, Conceptualization,
Writing - review & editing. Miha Bukleski: Investigation, Writing -
3426
→
3438 cm−1 (Fig. 9a) associated with the asymmetric
stretching NH2 vibrations, νas(NH2). The same effect is visible for the
νs(NH2), as well. The observed result suggests stronger hydrogen
bonding in the high temperature phase. This may be linked to the
review
& editing, Visualization. Peter Tzvetkov: Investigation,
Methodology, Writing - original draft, Writing - review & editing,
Visualization. Slobotka Aleksovska: Conceptualization, Writing - ori-
ginal draft, review & editing, Supervision, Project administration,
Funding acquisition. Daniela Kovacheva: Validation, Writing - review
& editing, Supervision, Project administration, Funding acquisition.
4–
changes in the arrangement of PbI6 octahedra that influences the
position of the NH2 groups. As it can be seen by the illustration in
Fig. 9 for the RT phase, the GUA+ are not completely perpendicular in
respect to c-axis, while the structure for the HT-phase suggests that
the GUA+ ions fully lie in the ab plane. Namely, in RT-phase, octa-
hedra are sharing an edge while in HT-phase they share a face, for-
cing NH2 groups to move.
Declaration of Competing Interest
The authors declare that they have no known competing fi-
nancial interests or personal relationships that could have appeared
to influence the work reported in this paper.
The most significant differences between the IR spectra of RT and
HT-phase are evident in the region around the δ(NH2) band (Fig. 9b).
The position of this band confirms the conclusion regarding the
changes in the NH2 groups. In this case, the band at 1660 cm–1
Acknowledgement
vanishes and at the same time, a new band appears at 1655 cm–1
.
This result is more straightforward when the temperature depen-
dent IR spectra are studied. Further details about the temperature
dependent structural changes related to the IR spectra in this region,
will be considered elsewhere.
This work was performed with financial support of a collabora-
tive project between Bulgarian Academy of Sciences and
Macedonian Academy of Sciences and Arts “Structural character-
ization and investigation of the electrical and catalytic properties of
newly synthesized inorganic and organic-inorganic perovskites”.
If the band at 516 cm–1 is taken into account (Fig. 9c), then it can
be concluded that in the high temperature phase, the guanidinium
ion is significantly more symmetrical compared to the one in the RT-
phase structure. Again, the given structures on Figs. 3 and 7 can
confirm this conclusion for the RT and HT-phase, respectively. The
band at 516 cm–1 present in the RT phase is a result of the CN3
bending vibrations. When all NH2 groups are placed almost per-
pendicular to the c-axis, then the change in the dipole momentum of
References
[1] H.J. Snaith, Perovskites: the emergence of a new era for low-cost, high-efficiency
solar cells, J. Phys. Chem. Lett. 4 (2013) 3623–3630, https://doi.org/10.1021/
jz4020162.
[2] N.-G. Park, Perovskite solar cells: an emerging photovoltaic technology, Mater.
Today 18 (2015) 65–72, https://doi.org/10.1016/j.mattod.2014.07.007.
[3] A. Kojima, K. Teshima, Y. Shirai, T. Miyasaka, Organometal halide perovskites as
visible-light sensitizers for photovoltaic cells, J. Am. Chem. Soc. 131 (2009)
6050–6051, https://doi.org/10.1021/ja809598r.
[4] S. Luo, W.A. Daoud, Recent progress in organic-inorganic halide perovskite solar
cells: mechanisms and material design, J. Mater. Chem. A 3 (2015) 8992–9010,
https://doi.org/10.1039/C4TA04953E.
[5] M. Saliba, T. Matsui, K. Domanski, J.-Y. Seo, A. Ummadisingu,
S.M. Zakeeruddin, J.-P. Correa-Baena, W.R. Tress, A. Abate, A. Hagfeldt,
M. Gratzel, Incorporation of rubidium cations into perovskite solar cells
improves photovoltaic performance, Science 354 (2016) 206–209, https://
doi.org/10.1126/science.aah5557.
+
the C(NH2)3 for the δ(CN3) vibration is zero and the band vanishes,
as seen in the HT phase of C(NH2)3PbI3. This is a reflection of the
surrounding of the NH2 groups. In order not to have band from the
δ(CN3) vibrations it is necessary to have symmetrical arrangement of
iodides around the NH2 groups. That leads to a conclusion that the
octahedra around Pb needs to be distorted in order to achieve the
desired symmetry of GUA+ so the δ(CN3) does not appear. This result
is in line with the obtained structural data from the XRD pattern.
[6] W.-J. Yin, T. Shi, Y. Yan, Unique properties of halide perovskites as possible ori-
gins of the superior solar cell performance, Adv. Mater. 26 (2014) 4653–4658,
https://doi.org/10.1002/adma.201306281.
4. Conclusions
[7] S.D. Stranks, G.E. Eperon, G. Grancini, C. Menelaou, M.J.P. Alcocer, T. Leijtens,
L.M. Herz, A. Petrozza, H.J. Snaith, Electron-hole diffusion lengths exceeding 1
micrometer in an organometal trihalide perovskite absorber, Science 342 (2013)
341–344, https://doi.org/10.1126/science.1243982.
[8] J.H. Noh, S.H. Im, J.H. Heo, T.N. Mandal, S. Il, Seok, chemical management for
colorful, efficient, and stable inorganic-organic hybrid nanostructured solar cells,
Nano Lett. 13 (2013) 1764–1769, https://doi.org/10.1021/nl400349b.
[9] G.E. Eperon, S.D. Stranks, C. Menelaou, M.B. Johnston, L.M. Herz, H.J. Snaith,
Formamidinium lead trihalide: a broadly tunable perovskite for efficient planar
heterojunction solar cells, Energy Environ. Sci. 7 (2014) 982, https://doi.org/10.
1039/c3ee43822h.
[10] T. Leijtens, G.E. Eperon, N.K. Noel, S.N. Habisreutinger, A. Petrozza, H.J. Snaith,
Stability of metal halide perovskite solar cells, Adv. Energy Mater. 5 (2015)
1500963, , https://doi.org/10.1002/aenm.201500963.
[11] E.J. Juarez-Perez, Z. Hawash, S.R. Raga, L.K. Ono, Y. Qi, Thermal degradation of
CH3NH3PbI3 perovskite into NH3 and CH3I gases observed by coupled thermo-
Taking into consideration the obtained results in this investiga-
tion, several conclusions can be proposed. The investigated GUAPbI3,
that is orthorhombic at room temperature, undergoes phase tran-
sition around 160 ºC leading to the formation of hexagonal structure
P63mc. The present structure is the first representative of the 4 H-
polytype found for guanidinium cation containing perovskites. The
phase transition is not due to order-disorder transition of the gua-
nidinium cation, but rather to the displacement and rearrangement
of the iodine sublattice. Thus, instead of double chains of edge-
sharing PbI6 octahedra in RT structure, in HT structure face-sharing
Pb2I9 polyions linked with the corners are observed. The open in-
organic framework of the compound is possibly the reason why it is
the most air-sensitive among the studied compounds because the
PbI6 octahedra are more exposed to attack by oxygen. The FTIR
gravimetry-mass spectrometry analysis, Energy Environ. Sci.
3406–3410, https://doi.org/10.1039/C6EE02016J.
9
(2016)
8

S. Dimitrovska-Lazova, M. Bukleski, P. Tzvetkov et al.
Journal of Alloys and Compounds 864 (2021) 158104
[12] E.J. Juarez-Perez, L.K. Ono, M. Maeda, Y. Jiang, Z. Hawash, Y. Qi,
Photodecomposition and thermal decomposition in methylammonium halide
lead perovskites and inferred design principles to increase photovoltaic device
guanidinium cation mixed with methylammonium in lead iodide perovskites for
19% efficient solar cells, Nat. Energy 2 (2017) 972–979, https://doi.org/10.1038/
s41560-017-0054-3.
stability, J. Mater. Chem.
C8TA03501F.
[13] P.P. Boix, K. Nonomura, N. Mathews, S.G. Mhaisalkar, Current progress and future
perspectives for organic/inorganic perovskite solar cells, Mater. Today 17 (2014)
16–23, https://doi.org/10.1016/j.mattod.2013.12.002.
A
6
(2018) 9604–9612, https://doi.org/10.1039/
[25] D. Prochowicz, M.M. Tavakoli, A.Q. Alanazi, S. Trivedi, H. Tavakoli Dastjerdi,
S.M. Zakeeruddin, M. Grätzel, P. Yadav, Charge accumulation, recombination, and
their associated time scale in efficient (GUA)x(MA)1– xPbI3 -based perovskite
solar cells, ACS Omega 4 (2019) 16840–16846, https://doi.org/10.1021/acsomega.
9b01701.
[14] Z. Wang, Z. Shi, T. Li, Y. Chen, W. Huang, Stability of perovskite solar cells: a
prospective on the substitution of the a cation and X anion, Angew. Chem. Int.
Ed. 56 (2017) 1190–1212, https://doi.org/10.1002/anie.201603694.
[15] N. Pellet, P. Gao, G. Gregori, T.-Y. Yang, M.K. Nazeeruddin, J. Maier, M. Grätzel,
Mixed-organic-cation perovskite photovoltaics for enhanced solar-light har-
vesting, Angew. Chem. 126 (2014) 3215–3221, https://doi.org/10.1002/ange.
201309361.
[16] G. Kieslich, S. Sun, A.K. Cheetham, Solid-state principles applied to organic–i-
norganic perovskites: new tricks for an old dog, Chem. Sci. 5 (2014) 4712–4715,
https://doi.org/10.1039/C4SC02211D.
[17] G. Kieslich, S. Sun, A.K. Cheetham, An extended tolerance factor approach for
organic-inorganic perovskites, Chem. Sci. 6 (2015) 3430–3433, https://doi.org/
10.1039/C5SC00961H.
[18] G. Giorgi, J.-I. Fujisawa, H. Segawa, K. Yamashita, Organic-inorganic hybrid lead
iodide perovskite featuring zero dipole moment guanidinium cations: a theo-
retical analysis, J. Phys. Chem. C 119 (2015) 4694–4701, https://doi.org/10.1021/
acs.jpcc.5b00051.
[19] C.C. Stoumpos, L. Frazer, D.J. Clark, Y.S. Kim, S.H. Rhim, A.J. Freeman,
J.B. Ketterson, J.I. Jang, M.G. Kanatzidis, Hybrid germanium iodide perovskite
semiconductors: active lone pairs, structural distortions, direct and indirect
energy gaps, and strong nonlinear optical properties, J. Am. Chem. Soc. 137
(2015) 6804–6819, https://doi.org/10.1021/jacs.5b01025.
[20] C.C. Stoumpos, L. Mao, C.D. Malliakas, M.G. Kanatzidis, Structure-band gap re-
lationships in hexagonal polytypes and low-dimensional structures of hybrid tin
iodide perovskites, Inorg. Chem. 56 (2017) 56–73, https://doi.org/10.1021/acs.
inorgchem.6b02764.
[21] M. Szafrański, Investigation of phase instabilities in guanidinium halogen-
oplumbates(II), Thermochim. Acta 307 (1997) 177–183, https://doi.org/10.1016/
S0040-6031(97)00408-5.
[22] A.D. Jodlowski, A. Yépez, R. Luque, L. Camacho, G. de Miguel, Benign-by-design
solventless mechanochemical synthesis of three-, two-, and one-dimensional
hybrid perovskites, Angew. Chem. Int. Ed. 55 (2016) 14972–14977, https://doi.
org/10.1002/anie.201607397.
[26] D.J. Kubicki, D. Prochowicz, A. Hofstetter, M. Saski, P. Yadav, D. Bi, N. Pellet,
J. Lewiński, S.M. Zakeeruddin, M. Grätzel, L. Emsley, Formation of stable mixed
guanidinium-methylammonium phases with exceptionally long carrier lifetimes
for high-efficiency lead iodide-based perovskite photovoltaics, J. Am. Chem. Soc.
140 (2018) 3345–3351, https://doi.org/10.1021/jacs.7b12860.
[27] O. Nazarenko, M.R. Kotyrba, S. Yakunin, M. Aebli, G. Rainò, B.M. Benin, M. Wörle,
M.V. Kovalenko, Guanidinium-formamidinium lead iodide: a layered perovskite-
related compound with red luminescence at room temperature, J. Am. Chem.
Soc. 140 (2018) 3850–3853, https://doi.org/10.1021/jacs.8b00194.
[28] C.M.M. Soe, C.C. Stoumpos, M. Kepenekian, B. Traoré, H. Tsai, W. Nie, B. Wang,
C. Katan, R. Seshadri, A.D. Mohite, J. Even, T.J. Marks, M.G. Kanatzidis, New type
of 2D perovskites with alternating cations in the interlayer space, (C(NH2)3)
(CH3NH3)nPbn I3n+1: structure, properties, and photovoltaic performance, J. Am.
Chem. Soc. 139 (2017) 16297–16309, https://doi.org/10.1021/jacs.7b09096.
[29] Z. Deng, G. Kieslich, P.D. Bristowe, A.K. Cheetham, S. Sun, Octahedral con-
nectivity and its role in determining the phase stabilities and electronic struc-
tures of low-dimensional, perovskite-related iodoplumbates, APL Mater.
(2018) 114202, , https://doi.org/10.1063/1.5046404.
6
[30] A. Babayigit, A. Ethirajan, M. Muller, B. Conings, Toxicity of organometal halide
perovskite solar cells, Nat. Mater. 15 (2016) 247–251, https://doi.org/10.1038/
nmat4572.
[31] J. Li, H.-L. Cao, W.-B. Jiao, Q. Wang, M. Wei, I. Cantone, J. Lü, A. Abate, Biological impact
of lead from halide perovskites reveals the risk of introducing a safe threshold, Nat.
Commun. 11 (2020) 310, https://doi.org/10.1038/s41467-019-13910-y.
[32] L. Ma, D. Guo, M. Li, C. Wang, Z. Zhou, X. Zhao, F. Zhang, Z. Ao, Z. Nie,
Temperature-dependent thermal decomposition pathway of organic-inorganic
halide perovskite materials, Chem. Mater. 31 (2019) 8515–8522, https://doi.org/
10.1021/acs.chemmater.9b03190.
[33] R.W.G. Wyckoff, Note: cadmium iodide structure, Crystal Structure, 2nd ed.,
Interscience Publishers, New York, 1963, pp. 239–444.
[34] E.V. Campbell, B. Dick, A.L. Rheingold, C. Zhang, X. Liu, Z.V. Vardeny, J.S. Miller,
Structures of a complex hydrazinium lead iodide, (N2H5)15Pb3I21, possessing
(Pb2I9)5−, (PbI6)4−, and I− Ions and α- and β-(N2H5)PbI3, Chem. - A Eur. J. 24 (2018)
222–229, https://doi.org/10.1002/chem.201704356.
[23] N. De Marco, H. Zhou, Q. Chen, P. Sun, Z. Liu, L. Meng, E.-P. Yao, Y. Liu, A. Schiffer,
Y. Yang, Guanidinium: a route to enhanced carrier lifetime and open-circuit
voltage in hybrid perovskite solar cells, Nano Lett. 16 (2016) 1009–1016, https://
doi.org/10.1021/acs.nanolett.5b04060.
[35] A.S. Thind, X. Huang, J. Sun, R. Mishra, First-principles prediction of a stable
hexagonal phase of CH3NH3PbI3, Chem. Mater. 29 (2017) 6003–6011, https://doi.
org/10.1021/acs.chemmater.7b01781.
[24] A.D. Jodlowski, C. Roldán-Carmona, G. Grancini, M. Salado, M. Ralaiarisoa,
S. Ahmad, N. Koch, L. Camacho, G.D. Miguel, M.K. Nazeeruddin, Large
9