7Pb4X15 (X = Cl- and Br-), 2D-Perovskite Related Hybrids with Dielectric Transitions and Broadband Photoluminiscent Emission

Article abstract of DOI:10.1021/acs.inorgchem.7b03217

We have prepared two new lead halides with the novel general formula of DMA₇Pb₄X₁₅ (DMA = [(CH₃)₂NH₂]⁺ and X = Cl^- or Br^-) by using an easy route under mild conditions at room temperature. These compounds exhibit an unprecedented crystal structure, are formed by layers of distorted [PbX₆] octahedra, which share corners and faces, and contain intercalated DMA cations. Very interestingly, they display dielectric transitions, which are related to a partial order-disorder process of the DMA cations between 160 and 260 K. Additionally, these new layered hybrids exhibit a broadband photoluminiscent emission, which is related to the structural distortions of the [PbX₆] octahedra. These findings not only open up large possibilities for future optoelectronic applications of these materials, but they also offer a novel playground for an easy modulation of electrical and optical properties of hybrid organic-inorganic materials. We anticipate that this novel A₇Pb₄X₁₅ formula can be adequate to tune the family of the hybrid lead halides using other alkylammonium cations, such as methylammonium, formamidinium, or ethylammonium, to improve their photoelectronic properties.

Full text of DOI:10.1021/acs.inorgchem.7b03217

Article
pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
[(CH3)2NH2]7Pb4X15 (X = Cl− and Br−), 2D-Perovskite Related Hybrids
with Dielectric Transitions and Broadband Photoluminiscent
Emission
Alberto García-Fernandez,*,† Juan Manuel Bermudez-García,† Socorro Castro-García,†
́
́
Antonio Luis Llamas-Saiz,‡ Ramon Artiaga,§ Jorge Jose Lopez-Beceiro,§ Manuel Sanchez-Andujar,*,†
́
́
́
́
́
and María Antonia Senarís-Rodríguez†
̃
†QuiMolMat Group, Department of Chemistry, Faculty of Science and CICA, University of A Coruna, Campus A Coruna, 15071 A
̃
̃
Coruna, Spain
̃
‡RIAIDT X-ray Unit, University of Santiago de Compostela, 15782 Santiago de Compostela, Spain
§Department of Industrial Engineering II, University of A Coruna, Campus Ferrol, 15403 Ferrol, Spain
̃
S
* Supporting Information
ABSTRACT: We have prepared two new lead halides with the
novel general formula of DMA7Pb4X15 (DMA = [(CH3)2NH2]+
and X = Cl− or Br−) by using an easy route under mild conditions
at room temperature. These compounds exhibit an unprecedented
crystal structure, are formed by layers of distorted [PbX6]
octahedra, which share corners and faces, and contain intercalated
DMA cations. Very interestingly, they display dielectric
transitions, which are related to a partial order−disorder process
of the DMA cations between 160 and 260 K. Additionally, these
new layered hybrids exhibit a broadband photoluminiscent
emission, which is related to the structural distortions of the
[PbX6] octahedra. These findings not only open up large
possibilities for future optoelectronic applications of these
materials, but they also offer a novel playground for an easy
modulation of electrical and optical properties of hybrid organic−inorganic materials. We anticipate that this novel A7Pb4X15
formula can be adequate to tune the family of the hybrid lead halides using other alkylammonium cations, such as
methylammonium, formamidinium, or ethylammonium, to improve their photoelectronic properties.
hydrophobic, which avoids one of the main drawbacks of the
perovskite lead halides, their instability toward moisture.17,18
Hence, 2D lead halides usually show better chemical stability
than the 3D perovskite materials.19,20
INTRODUCTION
■
In the past years, lead halide materials with hybrid organic−
inorganic components have received much attention due to the
discovery of the unprecedented photovoltaic and optoelec-
tronic properties in APbX3 compounds with a perovskite
structure,1−6 where A= Cs+, [CH3NH3]+ (methylammonium,
MA) and/or [HC(NH2)2]+ (formamidinium, FA) and X = Cl−,
Br−, and/or I−.7,8
Owing to the flexibility and ease of manipulation of such a
perovskite structure, many substitutions have been explored in
the search for optimized materials.9,10 In addressing materials
beyond hybrid halide APbX3 perovskite, the A cation (MA and
FA) has been replaced by larger organic cations.11,12 This
replacement often makes the 3D perovskite structure no longer
favorable, while the lower-dimensional structures become more
stable. This decrease of the dimensionality from 3D to 2D is in
general an interesting strategy to tune the electronic and optical
properties of materials.13,14 In addition, very versatile organic
cations can be incorporated to obtain new crystal struc-
tures.15,16 Moreover, longer organic cations are typically
It is worth highlighting that the organic cations and the
anionic inorganic framework have a templating influence on
each other, allowing a certain degree of control over the final
structures and properties.21,22 The choice of the organic
cation(s) and the stoichiometry of the reaction are among
the most influential parameters on the orientation and
deformation of the resultant inorganic frameworks.23 In that
context, in most of the known low-dimensional lead halide
perovskites, the organic cation A must contain terminal
functional groups that can ionically interact with the anionic
inorganic framework, such as mono- or di-ammonium cations
of general formula (RNH3)+ or (NH3RNH3)2+,24 where R
represents an organic functional group. The framework is based
Received: December 27, 2017
© XXXX American Chemical Society
A
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Article
((CH3)2NH3Br) was achieved using a rotary evaporator; a white
colored powder was formed indicating successful crystallization. The
white microcrystals of (CH3)2NH3Br were washed with absolute
diethyl ether several times and finally dried under vacuum overnight.
Synthesis of [(CH3)2NH2]7Pb4X15 (X= Cl− and Br−). Single-
crystals of both DMA7Pb4X15 have been obtained by crystallization
from a N,N-dimethylformamide (DMF) solution. For this purpose,
stoichiometric amounts of PbX2 (X = Cl or Br) (4 mmol) and of
DMAX (X = Cl or Br) (7 mmol) were dissolved by adding 5 mL of
DMF. The obtained solution was filtered through a 0.45 μm PTFE
filter to minimize nucleation sites. Colorless, needle-shaped single-
crystals of DMA7Pb4Cl15 and colorless, cubic single-crystals of
DMA7Pb4Br15 were obtained upon slow evaporation of the solvent
at room temperature after several days.
on one layer or several layers of corner-sharing [PbX6]
octahedra. The most common low-dimensional perovskite
derivatives display the following general formula:
2 5
2 6
(RNH3 )2 An − 1 Pbn X3 n + 1
,
A′2 Am Pbm X3 m + 2
,
and
11
A′2Aq−1PbqX3q+3
.
A property recently discovered in these 2D-hybrid perov-
skites is their broadband light emission;27−30 this makes them
interesting as broadband phosphors, for applications as artificial
illumination or the manufacturing of displays, among others.
While most broadband inorganic phosphors must contain
emissive extrinsic dopants or surface defect sites, the broad
photoluminescence (PL) of those 2D-hybrid perovskites is
supposed to be due to self-trapped excitons, STEs (i.e., excited
electron-hole pairs stabilized though coupling to a deformable
lattice).31,32 Therefore, the emission in those layered perov-
skites could be amenable with a fine synthetic tuning.15 In this
context, it has been recently reported that a distortion of the
[PbX6] octahedra has an important role on the PL emission of
different 2D-lead halides.13,23,33−36
Additionally, powder of [(CH3)2NH2]7Pb4X15 (X= Cl− and Br−)
compounds were easily obtained by a solid state reaction at room
temperature. For that purpose, 4 mmol of PbX2 (X = Cl or Br) and 7
mmol of DMAX (X = Cl or Br) were placed in an agate mortar and
ground carefully with a pestle until a visually homogeneous white
powder was obtained.
Thermal Analysis. Thermogravimetric analysis (TGA) were
carried out using TGA-DTA Thermal Analysis SDT2960 equipment.
For these experiments, approximately 27 mg of grounded single
crystals were heated at a rate of 5 K/min from 300 to 1200 K using
corundum crucibles under a flow of dry nitrogen.
Differential scanning calorimetric (DSC) analyses were carried out
in TA Instruments MDSC Q-2000 equipped with a liquid nitrogen
cooling system by heating and cooling the samples during several
cycles at 10 K/min from 160 up to 300 K under a nitrogen
atmosphere.
On the other hand, the 2D-hybrid perovskites of general
formula A2MX4 (A= organic cation, M metal ions, and X =
halide) constitute a large family of organic−inorganic hybrids
with well-known interesting ferroelectric (FE) properties, this
made them interesting for applications as a capacitor and FE
random-access memory (RAM), among others.37 In those
systems, the organic cations, A, located in the anionic cavities
are believed to be responsible for the ferroelectric ordering in
most cases. Also, it has been shown that the dimethylammo-
nium [(CH3)2NH2]+ (DMA) cations can experience thermally
induced order−disorder processes in several hybrid perovskites,
giving rise to structural phase transitions with associated
interesting properties.38−41 Very recently, we reported a first-
order phase transition and a dielectric transition at around 250
K in a [(CH3)2NH2]PbI3 2H-hexagonal perovskite.42
Powder X-ray Diffraction. Powder X-ray diffraction (PXRD)
patterns were collected in a Siemens D-5000 diffractometer using Cu
Kα radiation at room temperature. The PXRD patterns were analyzed
by the Le Bail method using GSAS+EXPGUI software.43,44
Single Crystal X-ray Diffraction. Single-crystal data sets were
collected at different temperatures (275 and 100 K) in a Bruker Kappa
X-ray diffractometer equipped with an Apex II CCD detector and
using monochromatic Mo Kα radiation (λ = 0.71073 Å). A suitable
crystal for each compound was chosen and mounted on a glass fiber
using a two components epoxy glue (Araldit 7070, Ceys) for the Cl
compound, and on a MiTeGen MicroMountTM using Paratone N
(Hampton Research) for the Br compound. To carry out these
experiments, the crystals were cooled at different rates using a nitrogen
gas cold stream from an Oxford Cryosystems cooler (700 Series). The
presence of multiple lattices was clear from visual inspection of
diffraction images collected at 275 K for both crystals. Both data sets,
at 275 K, were indexed using CELL_NOW 2008/4, and obtained two
solutions that interpreted all the diffraction peaks, with I/σ(I) > 8
found in the 1337 and 2855 images measured for the Cl and Br
compounds, respectively. The data integration, reduction, and
correction for Lorentz and polarization effects were performed using
SAINT V8.37A included in the APEX2 V2015.9-0 (Bruker AXS, 2015)
suite of software. The integration of the reflections for all data sets
were performed taking into account the orientation matrices of the
two twin domains simultaneously. The reflection intensities were
corrected for absorption effects by semiempirical methods on the basis
of symmetry-equivalent data using TWINABS 2012/1 included in the
Bruker software suite.
All of these results suggest that the substitution of the halide
in the DMAPbI3 compound, together with changes in its
stoichiometry, can offer new possibilities to achieve novel
hybrid lead halide perovskites with interesting dielectric and
photophysical properties.
In this context, in this work, we report the synthesis of two
new hybrid lead halides designed by combining the possibilities
offered by a framework based on more or less distorted [PbX6]
octahedra with a peculiar connectivity and DMA guests prone
to experience order−disorder transitions. As we will show, the
obtained compounds, which display a new and original
structure and formula, (DMA)7Pb4X15, where X= Cl− and
Br−, have been characterized in depth by single-crystal-X-ray
diffraction (SCXRD), thermogravimetric analysis (TGA), and
differential scanning calorimetry (DSC). Their dielectric and
optical (optical diffuse−reflectance and emission spectra)
characterizations reveal that these materials display dielectric
transitions below room temperature, and a broadband photo-
luminiscent emission at room temperature.
The structures were solved by direct methods using the
SHELXT2014/545 program and were refined by least-squares methods
on SHELXL2014/7. To refine the structures, anisotropic thermal
factors were employed for the non-H atoms. The hydrogen atoms of
EXPERIMENTAL SECTION
■
Materials. All starting materials were of analytical grade from
commercial sources, with the exception of the (CH3)2NH2Br.
(CH3)2NH2Cl (99%) (DMACl), (CH3)2NH solution (40 wt % in
H2O), PbCl2 (99%), PbBr2 (99%), HCl solution (37 wt % in H2O),
and HBr solution (48 wt % in H2O) were purchased from Sigma-
Aldrich and used without further treatment.
+
the NH2 group of the DMA cations could be found in the Fourier
maps, and the rest of the hydrogen atoms of the DMA were
introduced at idealized positions. All hydrogen atoms were restrained
using the riding model implemented in SHELXL2014/7.
Dielectric Properties. The complex dielectric permittivity (εr = ε′r
− iε″r) of the cold-press pelletized samples was measured as a function
of frequency and temperature with a parallel-plate capacitor coupled to
a Solartron 1260A impedance/gain-phase analyzer, capable of
Synthesis of (CH3)2NH2Br (DMABr). Dimethylamine (CH3)2NH
solution in a slight excess was reacted with hydrobromic acid (HBr) in
an ice bath. The crystallization of dimethylammonium bromide
B
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measuring in the frequency range from 10 μHz up to 32 MHz using an
amplitude of 2 V. The capacitor was mounted in a Janis SVT200T
cryostat refrigerated with liquid nitrogen, and with a Lakeshore 332
incorporated to control the temperature from 100 up to 350 K. The
data were collected on heating, and before carrying out the
measurements, the pellets were kept for 2 min at each temperature
so as to reach the thermal equilibrium.
Differential Scanning Calorimetry (DSC). The differ-
ential scanning calorimetry analysis of grounded single crystals
(Figure. 1) shows that both compounds exhibit endothermic
Pelletized samples, made out of cold-press grounded single-crystals,
with an area of approximately 530 mm2 and a thickness of
approximately 0.7 mm were prepared to fit into the capacitor. Gold
was sputtered on the surfaces of the pelletized samples to ensure a
good electrical contact.
All the dielectric measurements were carried out in a nitrogen
atmosphere where several cycles of vacuum and nitrogen gas were
performed to ensure that the sample environment was free of water.
Ultraviolet−Visible (UV−vis) Spectroscopy. Optical diffuse−
reflectance measurements of grounded single-crystals were performed
at room temperature using a Jasco V-730 UV−visible double-beam
spectrophotometer with single monochromator, operating from 200 to
900 nm. BaSO4 was used as a nonabsorbing reflectance reference. The
generated reflectance-versus-wavelength data were used to estimate
the band gap of the material by converting reflectance to absorbance
data according to the Kubelka−Munk equation:46 F(R)= α = (1 −
R)2/2R, where R is the reflectance data and α is the absorption
coefficient.
Emission spectra were measured on a Horiba FluoroMax Plus-P
spectrofluorometer equipped with a 150 W ozone-free xenon arc lamp
and a R928P photon counting emission detector, as well as a
photodiode reference detector for monitoring lamp output. Samples
were excited using a 150 W xenon arc lamp at 320 nm, and then the
emission was measured from 400 to 700 nm.
The chromaticity coordinates were calculated using the Color-
Calculator by OSRAM Sylvania, Inc.47 and plotted using the 1931
color space chromaticity diagram.
RESULTS AND DISCUSSION
■
Figure 1. DSC results as a function of temperature obtained by
heating and cooling the samples (a) DMA7Pb4Cl15 and (b)
DMA7Pb4Br15 at a rate of 10 K min−1.
Synthesis and Basic Characterization. For the first time,
we have prepared two new compounds with the novel
composition of DMA7Pb4X15 (X = Cl or Br) by using an
easy route under mild conditions at room temperature.
These compounds were obtained not only as single crystals
but also as powdered samples by using a mechanosynthesis
method. In this context, it is worth noting that we have tried to
prepare the iodide compound, but we did not succeed and we
have always obtained the DMAPbI3 hexagonal perovskite phase
instead of the desired compound.
We have confirmed the purity of the so-prepared samples by
Le Bail refinements of their PXRD data collected at room
temperature, which show good agreement between the
experimental data and the model. The new DMA7Pb4Cl15
structure displays orthorhombic symmetry, with cell parameters
of a = 18.8778(4) Å, b = 15.9548(3) Å, and c = 32.0809(7) Å
and space group Pbca (Figure S1). Meanwhile the structure of
the DMA7Pb4Br15 compound exhibits monoclinic symmetry,
with a = 17.0859(3) Å, b = 19.6358(3) Å, c = 16.4307(3) Å,
and β = 105.7190(10) and space group P21/c (Figure S2). As
we will show below, the so-obtained cell parameters and space
groups are in agreement with those obtained by single-crystal
X-ray diffraction (see below).
(on heating) and exothermic (on cooling) peaks, in the
temperature range of 160−300 K, probably due to one or
several phase transitions. Nevertheless, both compounds
display slightly different behaviors, while DMA7Pb4Cl15
presents a single acute endothermic peak on heating at around
262 K (and the corresponding exothermic peak on cooling at
around 256 K), DMA7Pb4Br15 exhibits three weak endothermic
peaks, two broad peaks at ∼182 and ∼202 K and an acute peak
at ∼252 K (with the corresponding exothermic peaks at ∼178,
∼ 198, and ∼248 K).
The changes in the enthalpy ΔH (J/mol) and the entropy
ΔS (J/mol K) were determined from the area under the peaks
of the heat flow curves. The results yield ΔH ∼ 2984 J/mol
(endo) − 3536 J/mol (exo) and ΔS ∼ 11.38 J/mol (endo) −
13.48 J/mol (exo) K for the Cl compound, and ΔH ∼ 173.08
J/mol (endo) − 269.31 J/mol (exo) and a ΔS ∼ 0.68 J/(mol
K) (endo) − 1.07 J/(mol K) (exo) for Br compound (only
considering the high temperature).
This DSC behavior is reminiscent of that shown by other
perovskite systems containing DMA cations, such as DMAMn-
(HCOO)3 and DMAPbI3,42 which exhibit structural tran-
48
Interestingly, we have observed that both compounds are
stable in air at room temperature for several months, and their
X-ray diffraction patterns remain unchanged after this period.
As for their thermal stability, according to our TGA results
(Figure S3), these compounds are stable up to ∼450 K (Cl)
and up to ∼470 K (Br).
sitions associated with order−disorder processes of DMA
cations. Using these studies as a reference, we have evaluated a
possible order−disorder process in our systems by using the
calculated entropy. In that context, for an order−disorder
transition, entropy is related with N (the ratio of the number of
C
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configurations in the disordered and ordered systems) by using
the expression ΔS = R ln(N), where R is the gas constant. Such
an estimation leads to quite different N-values for the here
studied Cl and Br compounds, 4 and 1, respectively. Therefore,
even if the DSC results point to an order−disorder mechanism
of the DMA cations, the DMA7Pb4Br15 compound shows some
particularities compared to the Cl analogous and to other
reported hybrids, such as lower values of ΔH and ΔS and
multiphase transitions.49 These differences can be due to the
extent of the order−disorder process, which takes place in steps
and along a wider temperature range in the case of the Br
compound.
Crystal Structure by Single-Crystal X-ray Diffraction.
Single-crystal X-ray diffraction experiments were carried out at
temperatures well above and below the observed DSC
transitions, namely at 275 and 100 K. The space groups
found corroborate the PXRD results reported above, giving
Pbca for DMA7Pb4Cl15 and P21/c for DMA7Pb4Br15. The
crystallographic data of these two compounds are compiled in
Table S1 and S2 of the SI The unit cell parameters of both
compounds were analyzed and the following relationships were
observed, a (Br) ∼ c/2 (Cl), b (Br) ∼ a (Cl), and c (Br) ∼ b
(Cl). Also, it was observed that the cell parameters of the Cl
compound are slightly lower than these of the Br compound,
which is related with the smaller size of the chloride anions.
It is worth commenting that differences with the case of
other hybrids,38−42 such space groups, do not change as a
function of temperature in the temperature range studied.
Another aspect that is worthwhile to highlighting is that,
despite the fact that both compounds crystallize under different
space groups, they display similar crystal structures that, very
interestingly, are novel and different from any other crystal
structure reported so far for lead halides.
Figure 2. (a) View of the crystal structure of the DMA7Pb4Cl15
compound at T = 100 K along the b-axis. (b) View of the crystal
structure of the DMA7Pb4Br15 compound at T = 100 K along the c-
axis. As it can be seen, the DMA cations are cooperatively ordered in
both structures.
Both structures consist on single-layered lead halide
frameworks with intercalated DMA cations (Figure 2). Their
asymmetric unit contains four independent Pb2+ cations, 15 X−
anions bounded to the metal cations, and seven DMA cations.
The Pb2+ cations are in octahedral coordination with six halide
ligands. The four [PbX6]4− octahedra of the asymmetric unit
are forming a linear [Pb4X15]7− chain, where two central
[PbX6]4− octahedra are sharing one corner, and they are linked
to the terminal [PbX6]4− octahedra by face-sharing. Addition-
ally, each [Pb4X15]7− unit is sharing four corners with
neighboring [Pb4X15]7− moieties (Figure 3), forming a layered
structure. Six DMA cations per asymmetric unit are located
between the layers, while one DMA cation occupies a cavity
within the layer. The DMA cations are linked by H-bonds to
the halide anions of the framework (Figure S4).
It is worthwhile to highlight another two interesting features
of the structure:
(1) The fact that all [PbX6]4− octahedra of the framework
display a high distortion, with Pb-X bond lengths
ranging, at 100 K, from 2.633(7) to 3.297(7) Å for the
chloride compound, and from 2.779(3) to 3.290(3) Å for
the bromide compound, with Pb-Xterminal shorter than
Pb-Xbridging (Tables S1, S2, and S3).
(2) Another important feature concerns the DMA cations of
the structure, which at T = 100 K are ordered, with all
the C- and N-atoms sitting in a single crystallographic
position (Figure 2). Nevertheless, at 275 K a certain
disorder is observed (Figure S5) on the DMA cations
located inside and close to the cavities of the framework,
Figure 3. Details of the crystal structure of DMA7Pb4X15 compounds,
where the 2D arrangement of the framework is shown. The linear
[Pb4X15]7− chain is highlighted inside a dot line.
with all atoms located in two different positions. This
disorder is also higher in the case of the Cl-compound
with three of the DMA cations disordered per formula,
while the Br-compound displays only two disordered
DMA cations per formula. Therefore, we confirm by
SCXRD that both compounds exhibit a partial order−
disorder process of the DMA cations, which takes place
to a larger extent in the Cl-compound, with the DMA
D
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cations fully ordered at T = 100 K, and a coexistence of
Article
ordered and disordered DMA cations at T= 275 K.
Dielectric Properties. The dielectric properties of the
DMA7Pb4X15 samples were studied in the range of temper-
atures where the structural phase transitions were observed by
DSC. Figure 4 shows the temperature dependence of the real
Figure 5. Optical absorption spectra of DMA7Pb4X15 (X = Cl (red)
and Br (blue)) at room temperature.
Additionally, the Br-compound has clear excitonic features near
the absorption edge, as indicated by the shoulder found around
400 nm in the absorbance spectrum, and comparable to the
feature exhibited by other 2D semiconductors.18,36 The optical
band gaps were calculated using the Kubelka−Munk equation,
and values of 3.5 and 2.8 eV were obtained for the Cl- and the
Br-compound, respectively. These values are also similar to
those reported for other related 2D lead halides.23 Thus, the
obtained band gap of the chloride hybrid is ∼0.7 eV larger than
that of the bromide one, always in accordance with the values
found for comparable hybrid lead halide compounds.50,51
Moreover, the solid steady-state photoluminescence (PL)
spectra were measured at room temperature (see Figure 6a).
Both compounds display PL emission in the blue-light region
(see below the chromaticity coordinates).
The results obtained here are in agreement with the
commented literature,13,23,33−36 which predicts the broadband
photoluminiscent emission at room temperature for those 2D
lead halides, related with the [PbX6] octahedra distortions.
Surprisingly, we have found that the chloride compound
exhibits a notable broadband PL emission (peak centered at
407 nm, fwhm ∼195 nm) in comparison with the bromide
compound (peak centered at 438 nm, fwhm ∼70 nm). To
rationalize this observed broadband PL emission of the chloride
hybrid, we have analyzed, in detail, the structural distortion of
[PbX6] octahedra in both structures. As discussed above, both
compounds display a large distortion of the [PbX6] octahedra,
but it is different for Cl and Br hybrids. To quantify the
octahedral distortion Δd, on the basis of the Pb-X bond
lengths, we have used the expression52,53
Figure 4. Temperature dependence of the dielectric constant (ε′r)
measured at different frequencies (10−100 kHz) for (a) DMA7Pb4Cl15
and (b) DMA7Pb4Br15.
part of their complex dielectric permittivity (also known as
dielectric constant, ε′r) at different frequencies. The first
general observation is that the values of ε′r of both compounds
are strongly temperature dependent, while they are independ-
ent of frequency. This behavior is similar to that previously
reported for the 1D lead iodide DMAPbI3 2H-hexagonal
polytype42 and ascribed to the above-mentioned thermally
activated order−disorder processes of the DMA cations.
Nevertheless, there are significant differences in the dielectric
behavior of both compounds. As it can be seen in Figure 4a,
while in the Cl hybrid, the initial increase in ε′r is followed by a
sharp kink at about 270 K; in the case of the Br compound
(Figure 4b), ε′r shows a broad dielectric anomaly over a larger
temperature range, which takes place already at lower
temperatures, between 160 and 200 K.
UV−Visible Spectroscopy. In order to check the
optoelectronic properties of the new compounds, we have
evaluated their optical absorption. The optical spectra were
measured in diffuse reflection mode by UV−visible spectro-
photometry, and the corresponding results are shown in Figure
5. Absorption cutoff wavelengths of ∼425 and ∼350 nm were
observed for the bromide and chloride hybrids, respectively.
2
⎤
⎡
⎛ ⎞
dn − d
1
⎝ ⎠
6
⎜ ⎟
Δd =
⎥
⎦
∑
⎢
⎣
d
where d is the mean Pb−X distance and dn is the six individual
Pb−X distances. The obtained values show that the chloride
compound has a very high distortion (Δd ranging from 35.82 ×
10−4 to 22.37 × 10−4) at room temperature. Nevertheless, the
bromide one is less distorted (Δd ranging from 24.97 × 10−4 to
17.87 × 10−4). Therefore, a higher distortion and higher
difference between the less and the high distorted octahedra
seem to be responsible of the unexpected broadband emission
of the chloride compound.
E
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acute and takes place at a higher temperature in the case of the
Cl-one, as also expected from the DSC results.
Very interestingly, the chloride compound displays a
broadband PL emission (peak centered at 407 nm, fwhm
∼195 nm) in comparison with the bromide compound (peak
centered at 438 nm, fwhm ∼70 nm). This peculiar broadband is
explained on the basis of the [PbX6] octahedral distortions,
which are clearly larger in the chloride hybrid than in the
bromide one.
Finally, it is worthwhile to highlight that as the use of the
midsize DMA cations allow to prepare a novel family of
A7Pb4X15 lead halides, we anticipate that this novel formula can
be adequate to prepare a larger family of compounds with other
alkylammonium cations typically used for other hybrid lead
halides, such as methylammonium, formamidinium, or ethyl-
ammonium. In addition, we suggest that the Pb2+ cation can be
substituted by another divalent cation (such as Sn2+, Ge2+,
Mn2+, etc.). These materials open a novel playground for an
easy modulation of electrical and optical properties of new
organic−inorganic hybrid materials.
ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.7b03217.
Le Bail refinement of the PXRD data obtained at room
temperature, thermogravimetric curves, additional crys-
tallographic figures and tables, and crystallographic files
for DMA7Pb4X15 (X= Cl, Br), CCDC: 1812870;
DMA7Pb4Cl15 (100 K), CCDC: 1812867; DMA7Pb4Cl15
(275 K), CCDC: 1812868; DMA7Pb4Br15 (100 K),
CCDC: 1812869; and DMA7Pb4Br15 (275 K) (PDF)
Figure 6. (a) PL spectra of DMA7Pb4X15 (X = Cl (red) and Br (blue))
at room temperature. (b) Chromaticity coordinates of DMA7Pb4X15
(X = Cl (red) and Br (blue)) in a 1931 color space chromaticity
diagram.
It is worthwhile to note that the PL spectra also shows some
anomalies from 500 to 700 nm, which can be due to sub-band
gap states.
Accession Codes
CCDC 1812867−1812870 contain the supplementary crystal-
lographic data for this paper. These data can be obtained free of
charge via www.ccdc.cam.ac.uk/data_request/cif, or by email-
ing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
Finally, the chromaticity coordinates of the obtained halides
are plotted in Figure 6b, using the 1931 color space
chromaticity diagram. DMA7Pb4Cl15 has chromaticity coor-
dinates of (0.22, 0.22) whereas DMA7Pb4Br15 coordinates are
(0.21, 0.19), clearly showing that both materials display a
similar blue fluorescence.
CONCLUSIONS
AUTHOR INFORMATION
■
■
We have prepared two new lead halides with a general formula
of DMA7Pb4X15 (X = Cl or Br), that are based on the APbX3
photovoltaic hybrid perovskite, and have novel compositions
and crystal structures. These new halides were prepared with an
easy route and under mild conditions at ambient temperature.
Their crystal structures contain single-layered lead halide
frameworks with intercalated DMA cations. Such frameworks
display a peculiar arrangement, where linear [Pb4X15]7− chains
are linked to each other and are forming layers. Those
[Pb4X15]7− chains are formed by [PbX6] distorted octahedra,
which are linked to each other through corners and/or faces.
Six DMA cations of the asymmetric unit are located between
the layers, while the seventh occupies a cavity of the [Pb4X15]7−
framework layer. The DMA cations display a thermally induced
order−disorder process, with all of them ordered in the low
temperature structure (at 100 K), while several of them are
disordered at higher temperatures (at around 275 K). This
order−disorder process gives rise to the dielectric transitions
observed in both Cl- and Br-hybrids. Such a transition is more
Corresponding Authors
*E-mail: alberto.garcia.fernandez.
*E-mail: msanchez@udc.es.
ORCID
Alberto García-Fernandez: 0000-0003-1671-9979
́
Juan Manuel Bermudez-García: 0000-0001-7381-4409
́
Manuel Sanchez-Andujar: 0000-0002-3441-0994
́
́
María Antonia Senarís-Rodríguez: 0000-0002-0117-6855
̃
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was financially supported by the Ministerio de
́
Economia y Competitividad (MINECO) and EU-FEDER
under the project ENE2014-56237-C4-4-R and by Xunta de
Galicia under the project GRC2014/042. J.M.B.-G. acknowl-
edges Xunta de Galicia for a Postdoctoral Fellowship.
F
DOI: 10.1021/acs.inorgchem.7b03217
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
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Dimension Engineering on Cesium Lead Iodide for Efficient and
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