Phase Transition, Dielectric Properties, and Ionic Transport in the [(CH3)2NH2]PbI3 Organic-Inorganic Hybrid with 2H-Hexagonal Perovskite Structure

Article abstract of DOI:10.1021/acs.inorgchem.6b03095

In this work, we focus on [(CH₃)₂NH₂]PbI₃, a member of the [AmineH]PbI₃ series of hybrid organic-inorganic compounds, reporting a very easy mechanosynthesis route for its preparation at room temperature. We report that this [(CH₃)₂NH₂]PbI₃ compound with 2H-perovskite structure experiences a first-order transition at ≈250 K from hexagonal symmetry P6₃/mmc (HT phase) to monoclinic symmetry P2₁/c (LT phase), which involves two cooperative processes: an off-center shift of the Pb²⁺ cations and an order-disorder process of the N atoms of the DMA cations. Very interestingly, this compound shows a dielectric anomaly associated with the structural phase transition. Additionally, this compound displays very large values of the dielectric constant at room temperature because of the appearance of a certain conductivity and the activation of extrinsic contributions, as demonstrated by impedance spectroscopy. The large optical band gap displayed by this material (E_g = 2.59 eV) rules out the possibility that the observed conductivity can be electronic and points to ionic conductivity, as confirmed by density functional theory calculations that indicate that the lowest activation energy of 0.68 eV corresponds to the iodine anions, and suggests the most favorable diffusion paths for these anions. The obtained results thus indicate that [(CH₃)₂NH₂]PbI₃ is an electronic insulator and an ionic conductor, where the electronic conductivity is disfavored because of the low dimensionality of the [(CH₃)₂NH₂]PbI₃ structure.

Full text of DOI:10.1021/acs.inorgchem.6b03095

Article
pubs.acs.org/IC
Phase Transition, Dielectric Properties, and Ionic Transport in the
(CH ) NH ]PbI Organic−Inorganic Hybrid with 2H-Hexagonal
[
3
2
2
3
Perovskite Structure
†
†
†
‡
§
́ ́
A. García-Fernandez, J. M. Bermudez-García, S. Castro-García, A. L. Llamas-Saiz, R. Artiaga,
§ ∥,⊥
∥,⊥
∥,#
,†
J. Lop
and M. A. Sen
́ ́ ́
ez-Beceiro, S. Hu, W. Ren, A. Stroppa, M. Sanchez-Andujar,*
,
†
̃
arís-Rodríguez*
†
QuiMolMat Group, Department of Fundamental Chemistry, Faculty of Science and CICA, University of A Corun
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
International Centre for Quantum and Molecular Structures, Physics Department, Shanghai University, Shanghai 200444, China
̃
a, Campus A
̃
̃
‡
§
∥
̃
⊥
Materials Genome Institute and Shanghai Key Laboratory of High Temperature Superconductors, Shanghai University, Shanghai
00444, China
2
#
Consiglio Nazionale delle Ricerche, Institute CNR-SPIN, Via Vetoio, 67100 L’Aquila, Italy
S Supporting Information
*
ABSTRACT: In this work, we focus on [(CH ) NH ]PbI , a
3
2
2
3
member of the [AmineH]PbI series of hybrid organic−inorganic
3
compounds, reporting a very easy mechanosynthesis route for its
preparation at room temperature. We report that this [(CH ) NH ]-
3
2
2
PbI compound with 2H-perovskite structure experiences a first-order
3
transition at ≈250 K from hexagonal symmetry P6 /mmc (HT phase)
3
to monoclinic symmetry P2 /c (LT phase), which involves two
1
2+
cooperative processes: an off-center shift of the Pb cations and an
order−disorder process of the N atoms of the DMA cations. Very
interestingly, this compound shows a dielectric anomaly associated
with the structural phase transition. Additionally, this compound
displays very large values of the dielectric constant at room
temperature because of the appearance of a certain conductivity
and the activation of extrinsic contributions, as demonstrated by impedance spectroscopy. The large optical band gap displayed
by this material (E = 2.59 eV) rules out the possibility that the observed conductivity can be electronic and points to ionic
g
conductivity, as confirmed by density functional theory calculations that indicate that the lowest activation energy of 0.68 eV
corresponds to the iodine anions, and suggests the most favorable diffusion paths for these anions. The obtained results thus
indicate that [(CH ) NH ]PbI is an electronic insulator and an ionic conductor, where the electronic conductivity is disfavored
3
2
2
3
because of the low dimensionality of the [(CH ) NH ]PbI structure.
3
2
2
3
7
INTRODUCTION
organic−inorganic hybrids, where the A- and/or X-site
■
inorganic moieties of the conventional perovskites have been
replaced by organic building blocks.
ABX3 compounds with perovskite structure have been
intensively and extensively studied over the past several
decades because of their structural richness and amazing variety
of interesting properties, such as piezoelectricity, ferroelec-
8−12
The multiple
combinations available together with the possibility of
extending well-established concepts (tolerance factor, struc-
ture−property relationship, etc.) and tuning strategies of
inorganic perovskites to this new family of compounds open
up enormous possibilities for finding exciting new (multi)-
functional and (multi)stimulus responsive materials.
This is the case for the families with the general formula
amineH][M(X) ] (amineH = midsized protonated amines, M
1
2
3
tricity, ferromagnetism, magnetoresistance, superconductiv-
4
5
ity, multiferroicity, etc. Traditionally, the studies of perovskite
6
compounds have been focused on ceramic materials, mainly
on transition metal oxides of the general formula ABX (A =
alkali, alkaline earth, or lanthanide cation, B = transition metal
cation, and X = O).
12−15
3
[
3
Interestingly, in the past few years, significant effort has been
devoted to the development of new members of the versatile
families of the so-called perovskite-, chiral-, and niccolite-like
Received: December 22, 2016
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.6b03095
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Inorganic Chemistry
divalent transition metal cations, and X = a polyatomic ligand
Article
=
momentcan give rise to interesting behaviors and properties
−
−
−
such as HCOO , N , or CN ), which have already revealed
as demonstrated in the case of the related metal formates
3
1
7,27−29
very remarkable properties such as noteworthy cooperative
[DMA][M(HCOO) ] with perovskite structure.
For
3
16
magnetic, electric, or elastic order. Moreover, the metal
example, in those formates, the DMA cations experience
thermally induced order−disorder processes, inducing struc-
tural phase transitions, dielectric transitions, and cooperative
2
+
2+
2+
2+
formates [amineH][M(HCOO) ] [M = Mn , Fe , Co , or
3
2+
30
Ni ; amineH = dimethylammonium (DMA)] have been found
to exhibit type I multiferroicity, and methylammonium (MA)
Co-formate, [CH NH ][Co(HCOO) ], coupled between
17
29,30
electric order.
On this basis and the experience we have acquired studying
those metal formates, our goal in this work is to explore
3
3
3
magnetic and electric order, opening the large, flexible,
multifunctional, and designable family of hybrid perovskites
to magnetically induced multiferroic behavior.
Another family of perovskite hybrids that is attracting a great
deal of attention is that of lead halides with the formula
whether DMAPbI also presents such features. This study,
3
18
which will ultimately seek to establish composition−structure−
property relationships of interest, in principle, for dielectric
materials could also provide useful information for improving
our understanding of the origin of the exceptional properties of
[
AmH]PbX (X = Cl, Br, or I) because unprecedented and
3
intriguing photoconductivity was discovered in the methyl-
ammonium lead triiodide perovskite, [CH NH ]PbI
3
the analogue photovoltaic MAPbI compound.
3
To perform this work, we have prepared this compound
3
3
1
9
(
MAPbI3). In this context, the solar energy conversion
using a very easy route, which is different from that described
23
efficiency for a device based on this compound has already
previously, that we present here. We have determined in its
structural characteristics from low temperatures to room
temperature by means of single-crystal X-ray diffraction and
studied its electrical properties by means of dielectric
measurements, impedance spectroscopy, and density functional
theory (DFT) calculations.
2
0
reached 21.6%, a value that surpasses those of dye-sensitized,
quantum dot, organic, and amorphous silicon solar cells, and
other emerging PV technologies, making this compound a very
promising material for low-cost and high-efficiency photovoltaic
applications. Despite the recent substantial progress, perovskite
solar cells based on MAPbI3 have to overcome several
inconveniences to be commercially viable (such as the toxicity
EXPERIMENTAL SECTION
Synthesis. We have developed a new method for synthesizing
■
21
of lead, the bad long-term stability of this compound, its high
sensitivity to humidity, etc.), so that materials beyond MAPbI
3
DMAPbI
3
, which is different and much easier than the one previously
22
23
are currently being addressed.
described for the synthesis of this compound.
In this work, and in the search for new materials and
This method implies, as first step, the synthesis of [(CH ) NH ]I
DMAI). This compound is subsequently reacted with lead iodide by
3 2 2
(
properties in hybrid compounds with ABX stoichiometry, we
3
means of a mechanosynthesis process performed at room temperature,
leading to the production of the desired compound.
have focused on the dimethylammonium lead triiodide
compound, [(CH ) NH ]PbI , DMAPbI . This hybrid, which
3
2
2
3
3
All starting reagents, the (CH ) NH solution (40 wt % in H O),
3
2
2
belongs to the [AmH]PbI series, has been recently prepared
3
PbI (99%), and the HI solution (57 wt % in H O) were of analytical
2
3
2
2
and studied in the search for alternatives to MAPbI3.
This DMA compound has nevertheless revealed important
differences with respect to the MA-iodide.
grade from Sigma-Aldrich and used without further treatment.
First Step: Synthesis of [(CH ) NH ]I (DMAI). The dimethylamine
3
2
2
(CH ) NH solution in a slight excess was reacted with hydriodic acid
3
2
(
i) It does not show a three-dimensional (3D) perovskite
(HI) in an ice bath. The crystallization of dimethylammonium iodide
was achieved using a rotary evaporator. The obtained white
microcrystals were washed with absolute diethyl ether several times
and finally dried in a vacuum line overnight.
22
structure as is the case of MAPbI3. Instead, it crystallizes in a
H-hexagonal polytype consisting of infinite chains of face-
sharing [PbI ] octahedra separated by chains of DMA cations,
the structure that results from an all-hexagonal stacking of the
closely packed AX layers instead of the cubic sequence of the
former.
This difference can be rationalized on the basis of
Goldschmidt’s tolerance factor, which has been recently
2
23
6
Second Step: Synthesis of [(CH ) NH ]PbI (DMAPbI ). The
3
2
2
3
3
DMAPbI compound, as a powder sample, was prepared by means
3
3
of a mechanosynthesis method performed at room temperature. For
2
4
this purpose, equimolar amounts of PbI and DMAI were placed in an
2
agate mortar and ground carefully for 15 min with a pestle until a
visually homogeneous yellow powder was obtained.
25
26
extended to organic−inorganic perovskites by Kieslich et al.,
that in total parallel with the case of inorganic perovskites,
ABO can be used as a guiding tool for hybrid perovskite
Additionally, single crystals of DMAPbI were also prepared. For
3
this purpose, the previously obtained yellow powder of DMAPbI was
3
dissolved in tetrahydrofuran and yellow needle-shaped single crystals
were obtained upon slow evaporation of the solvent at room
temperature.
Scanning Electron Microscopy. The morphology of the as-
obtained samples was studied by means of scanning electron
microscopy (SEM), in a JEOL 6400 microscope.
Thermal Analysis. Thermogravimetric analyses (TGAs) of the
obtained polycrystalline powder were performed in TGA-DTA
Thermal Analysis SDT2960 equipment. For these experiments,
approximately 27 mg of sample was 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 performed
in a TA Instruments MDSC Q-2000 instrument equipped with a liquid
nitrogen cooling system by heating and cooling the sample for several
cycles at a rate of 10 K/min up to 380 K under a nitrogen atmosphere.
Powder X-ray Diffraction. Powder X-ray diffraction (PXRD)
patterns from the obtained polycrystalline powder were recorded in a
3
structures. In this context, tolerance factors (TFs) with values
of 0.9−1 favor almost cubic perovskite structures; meanwhile, a
lower TF of 0.80−0.89 gives rise to distorted perovskite
structures (rhombohedral, tetragonal, etc.). On the other hand,
a TF of >1 leads to hexagonal structures in which layers of face-
sharing octahedra are introduced into the structure.
This is the case for this compound, where the presence of a
larger cation in the A position, DMA instead of the smaller MA,
results in a TF of >1.
(
ii) The DMAPbI compound shows an optical band gap
3
23
(
Eg) of 2.39 eV, which is not suitable for photovoltaic
applications.
In any case, this DMAPbI compound has attracted our
3
attention from a different point of view, as the presence of the
DMA cationswhich are polar and carry an associated dipole
B
DOI: 10.1021/acs.inorgchem.6b03095
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Inorganic Chemistry
Article
Siemens D-5000 diffractometer using Cu Kα radiation at room
temperature. They were also compared with the profiles obtained from
the single-crystal structure at room temperature, which were generated
Kohn−Sham equations were solved using the projector-augmented-
3
5
wave (PAW) method with the PBEsol exchange correlation
3
6
functional using the same procedure as in ref 37. Spin−orbit
coupling (SOC) is not considered in these calculations. A 2 × 2 × 2
supercell (480 atoms) based on the low-temperature structure was
modeled. A plane-wave cutoff energy of 600 eV, a k-point sampling at
the γ point, and the PBEsol exchange correlation functional were
employed. For the initial and final structure relaxation, forces were
31
by Mercury version 3.5.1.
Single-Crystal X-ray Diffraction. Two suitable single crystals
were selected from the sample to collect diffraction data at two
different temperatures, namely, 293 and 100 K, in a Bruker Kappa
diffractometer equipped with an APEX II CCD detector using
monochromatic Mo Kα radiation (λ = 0.71073 Å). Crystals were
mounted on a MiTeGen MicroMount using Paratone (Chevron
Corp.). To perform these experiments, the first crystal was measured
at room temperature and the second one cooled at different rates using
a cold stream of nitrogen gas from an Oxford Cryostream 700 cooler.
Collection, integration, and reduction of data were performed using
the APEX2 V2015.9-0 (Bruker AXS, 2015) suite of software, which
includes the programs reported below. The intensity integration was
performed with SAINT 8.34A and corrected for Lorentz and
polarization effects and also for absorption by multiscan methods on
the basis of symmetry-equivalent data using either SADABS 2008/1 or
TWINABS 2012/1 depending on the presence of twinning. A single
lattice was found for the crystal collected at room temperature (space
−1
converged to <0.01 eV Å .
Migration mediated by ion vacancies was examined using the
34
nudged elastic band and constrained energy minimization methods;
in the latter, the migrating species is propagated along the direction of
migration in a series of small steps with all unconstrained degrees of
freedom relaxed at each step.
Ultraviolet−Visible (UV−vis) Spectroscopy. Optical diffuse-
reflectance measurements of the obtained polycrystalline powder were
performed at room temperature using a Jasco V-730 UV−visible
double-beam spectrophotometer with a single monochromator,
operating from 200 to 900 nm. BaSO was used as a nonabsorbing
4
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
group P6 /mmc). A second crystal was indexed measuring three runs
3
2
of 80 images each at different decreasing temperatures (275, 250, 200,
equation: F(R) = α = (1 − R) /2R, where R is the reflectance and α
150, and 100 K). A complete and highly redundant data set was
values are the absorption coefficients.
collected at 100 K.
The structures were determined by the direct method using the
32
RESULTS AND DISCUSSION
SHELXT2014 program and were refined by the least-squares
■
32
method on SHELXL2014/7. The presence of multiple lattices was
clear from visual inspection of diffraction images collected at low
temperatures. The data set at 100 K was indexed using CELL_NOW
Synthesis and Basic Characterization. We report here a
very easy and quick method for obtaining DMAPbI by means
of a mechanically activated solid state reaction that takes place
at room temperature.
3
2
008/4, producing three orientation matrices that interpret all the
diffraction peaks. The integration of the reflections was performed
taking into account the orientation matrices of the three twin domains
simultaneously.
This method is much simpler than that previously reported
23
in the literature, which requires heating and preparation
under a nitrogen atmosphere.
To refine the structures, anisotropic thermal factors were employed
for the non-H atoms. For 293 K, the hydrogen atoms of the DMA
cation could not be found in the Fourier map because of the
disordered arrangement of this cation.
The obtained polycrystalline powder sample was a single-
phase material as confirmed by comparison between the
experimental PXRD patterns and the profile obtained from the
single-crystal structure at room temperature (Figure S1).
It is worth noting that, different from the related MAPbI3
Meanwhile, for 100 K, the hydrogen atoms of the NH2+ group of
the DMA cations were found in the Fourier map and the rest of the
hydrogen atoms of the DMA were introduced at idealized positions.
All hydrogen atoms were refined using the riding model implemented
in SHELXL2014/7.
compound, the DMAPbI iodide studied here is stable in air for
3
several months at least at room temperature, as confirmed by
PXRD (Figure S2).
Dielectric Properties and Impedance Spectroscopy. The
complex dielectric permittivity (ε = ε′ − iε″ ) of the pelletized
The morphology of the obtained polycrystalline sample and
the single crystals was characterized by SEM. This study
showed that the polycrystalline powders consist of sintered
particles with an average diameter of ∼2 μm (Figure 1a), while
the single crystals have a needlelike morphology with a
diameter of ∼150 μm and lengths of several millimeters
r
r
r
polycrystalline sample was measured as a function of frequency and
temperature with a parallel plate capacitor coupled to a Solar-
tron1260A Impedance/Gain-Phase Analyzer, capable of measuring in
the frequency range from 10 μHz to 32 MHz using an amplitude of 1
V. The capacitor was mounted in a Janis SVT200T cryostat
refrigerated with liquid nitrogen and with a Lakeshore 332 instrument
incorporated to control the temperature from 100 to 350 K. The data
were collected on heating and after having waited 2 min at each
temperature.
(
Figure 1b).
In addition, the TGA results show that this compound is
stable up to ∼623 K (Figure S3). Above that temperature, it
Pelletized samples, made of cold-press polycrystalline powder, with
decomposes in two steps: in the first one (weight loss of
2
−
an area of approximately 133 mm and a thickness of approximately
∼28.27%), it loses the DMA cations and one of the I anions
0.7 mm were prepared to fit into the capacitor, and gold was sputtered
on their surfaces to ensure a good electrical contact with the
electrodes.
All the dielectric measurements were taken in a nitrogen
atmosphere in which several cycles of vacuum and nitrogen gas were
performed to ensure that the sample environment is free of water.
The impedance analysis software SMART (Solartron Analytical)
was used for data acquisition and processing. Impedance complex
plane plots were analyzed using the LEVM program, a particular
33
software for complex nonlinear least-squares fitting.
Density Functional Theory Calculations. To investigate the
minimum energy paths and the migration activation energy of ionic
diffusion, DFT calculations were performed. The calculations were
Figure 1. SEM micrographs of (a) the obtained polycrystalline powder
3
4
performed using the Vienna Ab initio Simulation Package (VASP).
and (b) single crystals of [(CH ) NH ]PbI .
3
2
2
3
C
DOI: 10.1021/acs.inorgchem.6b03095
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
per formula unit); after the second one (weight loss of
Article
(
Table 1. Selected Bond Lengths (angstroms) for DMAPbI3
at 293 and 100 K
∼
71.73%) above 873 K, the compound becomes fully volatized.
Differential Scanning Calorimetry (DSC). According to
2
93 K
100 K
the DSC results, this compound experiences a reversible phase
transition just below room temperature. As shown in Figure 2,
an acute endothermic peak is seen around 257 K upon heating
and the corresponding exothermic peak is observed around 250
K upon cooling.
Pb−I
3.204 (1)
Pb1−I2
Pb1−I3
Pb1−I4
Pb1−I3i
Pb1−I4i
Pb1−I2ii
3.106 (2)
3.190 (2)
3.165 (2)
3.248 (2)
3.272 (2)
3.340 (2)
Figure 2. DSC results as a function of temperature obtained by
heating and cooling the [(CH ) NH ]PbI sample at a rate of 10 K
3
2
2
3
−
1
min .
The observed thermal hysteresis indicates the first-order
character of such a phase transition, which involves an enthalpy
−
1
−1
−1
change (ΔH) of ∼2374 J mol (heating) to 2324 J mol
−1
(
(
cooling) and an entropy change (ΔS) of ∼9.2 J mol
K
−
1
−1
heating) to 9.3 J mol
K
(cooling), as determined from the
area under the heat flow/temperature curve and the peak
temperature, Tmax
Figure 3. Crystal structure of the [(CH ) NH ]PbI HT phase viewed
3 2 2 3
.
along two different orientations, where the N atoms of the DMA
2
+
Taking into account the fact that for an order−disorder
transition ΔS = R ln(N), where R is the gas constant and N is
the ratio of the number of configurations in the disordered and
ordered system, we calculated a value for N of ∼3. This value is
similar to that observed in the hybrid organic−inorganic
cations are disordered in three crystallographic positions and the Pb
cations sit at the center of the octahedra.
long, compared to the size of Pb2+ and I , and the B−X bond
length leading to t > 1 and a hexagonal stacking sequence.
It is also interesting to note that this solid to solid phase
transformation of the HT phase into the LT phase is reversible,
although the crystal becomes twinned when it is cooled below
Tt.
−
29
perovskite [(CH ) NH ][Mn(HCOO) ], which also con-
3
2
2
3
tains the DMA cation in position A, with the nitrogen atoms
evenly distributed in three different positions. In any case, it
should be noted that this model is oversimplified and does not
take into account a number of factors, such as lattice distortions
or intermolecular interactions.
Crystal Structures Determined by Single-Crystal X-ray
Diffraction. The single-crystal X-ray studies reveal that this
compound presents two different crystal structures above and
HT Phase. The structural data for this phase obtained here,
which are summarized in Table 1 and Table S1 and in Figure 3,
are in full agreement with those previously reported in the
23
literature.
In this structure, each Pb2+ cation is connected to its two
Pb2 nearest neighbors through six iodide bridges (Figure 3) in
a regular octahedral environment where the Pb−I distance is
+
below the transition temperature, T . Above that temperature
t
(
HT phase), the sample shows hexagonal symmetry, in space
23
2+
group P6 /mmc as previously reported. Nevertheless, below
∼3.20 Å (Table 1) and the Pb cations sit at the center of the
3
T (LT phase), the crystal structure changes to monoclinic P2 /
octahedra. Most interestingly, the DMA cations are disordered
with nitrogen apparently located in three different possible
positions (Figure 3), as in the case of the [(CH ) NH ][M-
t
1
c down to the lowest measuring temperature, 100 K.
The crystallographic data of these two polymorphs are
summarized in Table 1 and Table S1.
It is interesting to note that both polymorphs display 2H-
hexagonal perovskite crystal structures (Figures 3 and 4), a
polytype that, as indicated above, consists of infinite chains of
3
2
2
2
+
2+
(HCOO) ] compounds mentioned above (with M = Mn ,
3
2+
2+
2+ 17,29
Co , Ni , or Fe ).
LT Phase. In the case of the LT phase, which we report here
for the first time, the Pb2+ cations are no longer in a regular
octahedral environment as six different Pb−I distances are
detected, ranging from ∼3.11 to ∼3.34 Å (with three shorter
Pb−I bond lengths and three longer Pb−I bond lengths)
face-sharing [PbI ] octahedra running along the c-axis of the
6
unit cell separated by DMA cations.
This means that in both temperature intervals the DMA
cation is relatively too large and the A−X bond remains too
2
+
(Table 1 and Figure 4). Very interestingly, the Pb cations are
D
DOI: 10.1021/acs.inorgchem.6b03095
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Inorganic Chemistry
Article
interactions are going to be very weak as the distances are larger
than those normally considered for N−H···I bonds (around 2.7
Å) and also those recently reported for black formamidinium
39
lead halide (N−H···I, 2.75−3.00 Å).
Another interesting point is the ordering of the DMA cations
inside the unit cell, which is cooperative and shows an
antiparallel arrangement of the polar DMA cations.
Therefore, the HT to LT phase transition experienced by this
DMAPbI compound involves two cooperative processes: an
3
2+
off-center shift of the Pb cations and an order−disorder
process of the polar DMA cations. This leads to a LT phase,
which shows an antiferrodistortive arrangement of both the
[
PbI ] chains and the DMA cations.
Dielectric Properties and Impedance Spectroscopy
6
Analysis. Figure 5 shows the temperature dependence of the
Figure 4. Crystal structure of the [(CH ) NH ]PbI LT phase viewed
3
2
2
3
along two different orientations, where the DMA cations are
cooperatively ordered and the Pb2 cations show a cooperative off-
center shift.
+
Figure 5. Temperature dependence of the dielectric constant (ε′ ) of
r
5
[
(CH ) NH ]PbI measured at different frequencies (10−10 Hz).
3
2
2
3
The inset shows the frequency dependence of the dielectric constant
ε′ ) measured at different temperatures.
in fact cooperatively off-center shifted toward one face of such
(
r
octahedra (Figure S4). Such displacement is probably caused
2
by the presence of their 6s electrons (“lone pair”), which are
chemically inactive but can be sterically active, resulting in an
asymmetry of the metal coordination and a distorted crystal
structure as is this case here.
real part of the complex dielectric permittivity (the so-called
dielectric constant, ε′ ) of DMAPbI in the temperature range
r
3
of 100−325 K. As one can see, two different behaviors are
2
+
In this context, it is worth noting that the Pb cations in the
related MAPbI3 compound do not show signs of the
stereochemically active lone pair effect. The manifestation of
this latter can be rationalized through specific orbital
observed as a function of temperature. At <225 K, the dielectric
constant shows a low value (ε′ ≈ 10), which is almost
r
independent of temperature and frequency; meanwhile, at >225
K, ε′ is strongly frequency dependent and increases markedly
r
2
+
interactions between the 6s orbital of the Pb cation and the
with temperature, reaching very large values at room temper-
−
5
p orbitals of the I anion. Typically, the extent of this orbital
ature, especially at low measuring frequencies (for example, ε′
r
mixing is low, and the [PbI ] octahedra are only minimally
≈ 1000 at 325 K for ν = 10 Hz). Another interesting feature is
the small kink that is observed around 260 K, close to the phase
transition temperature, and detected in all the experiments
performed at different measuring frequencies.
6
distorted. Exceptionally, strong orbital mixing can take place
38
and thus activate the lone pair effect.
Therefore, the orbital mixing is stronger in the DMAPbI3
than in the MAPbI compound.
Taking into account the fact that the presence of extrinsic
factors such electronic and/or ionic conductivity could result in
the appearance of artifacts in the dielectric measurements, we
have performed additional studies to explore further the
dielectric behavior displayed by this compound. For this
purpose, we have performed impedance complex plane (Z″ vs
Z′) analysis of the data obtained at different temperatures, as
this is a very powerful tool for unraveling the dielectric
3
As a result of this distortion in the DMA compound, the
[
PbI ] chains become polar in DMAPbI , even if they are
6
3
arranged antiparallel to one other in the unit cell (see below).
Additionally, in this LT phase, the DMA cations are ordered
with the N atoms sitting in a single crystallographic position
(
Figure 4). As for the possibility that the N atoms of the DMA
cations interact with the [PbI ] chains through H-bonds, these
6
40,41
could be of two types in view of the structural data: through
response of materials.
linear and stronger N−H···I interactions (d
∼ 3.11 Å) or
Figure 6 shows typical impedance complex plane plots for
this compound in the temperature interval of 100−225 K
(Figure 6a) and above 225 K (Figure 6b). As one can see, in
N−I
through bifurcated and weaker H-bonds (d
∼ 3.25 Å, and
N−I
dN−I ∼ 3.32 Å) (see Figure S5). In any case, all such
E
DOI: 10.1021/acs.inorgchem.6b03095
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
This means that in this higher-temperature interval the
dielectric response of DMAPbI contains both intrinsic and
3
extrinsic contributions.
Another interesting observation is that while it is in the
interval of 100−225 K this compound behaves as an insulator
(
(
σ < 1 nS/cm) and above 225 K it becomes semiconducting
i.e., σ ∼ 0.183 μS/cm for 300 K).
All these results indicate the activation of interfacial
polarization effects in the DMAPbI compound above 225 K,
3
which would be related to the presence of a space charge layer
at the grain boundaries and/or electrodes, and that would give
rise to the very large dielectric constant values detected at room
temperature.
With this knowledge in hand, another interesting question
refers to the origin of a certain conductivity observed in this
compound above 225 K, which in principle could be of
42
electronic and/or ionic nature.
Nevertheless, the relatively large optical band gap shown by
this compound (E ∼ 2.59 eV) (Figure S6), the value of which
g
is in full agreement with that reported in ref 23, rules out the
possibility that the observed conductivity can have an electronic
origin and points more to the presence of ionic conductivity, an
aspect that we have further investigated by means of DFT
calculations and present in the next section.
Again with respect to the dielectric results, another
interesting feature, briefly mentioned above, concerns the
dielectric anomaly observed at 260 K, close to the structural
phase transition T , which is detected at all the measuring
t
frequencies. We relate it to the cooperative antialignments of
dipolar moments that take place in the LT phase and that
disappear in the HT phase. As we have discussed in the
previous section, the phase transition involves two cooperative
Figure 6. Impedance complex plane plots and equivalent circuits
2
+
obtained for [(CH ) NH ]PbI at (a) 200 and (b) 300 K.
antiferrodistortive processes: an off-center shift of the Pb
3
2
2
3
cations and an order−disorder process of the polar DMA
cations.
In the first case, the displacement of the Pb2+ cation induces
the appearance of electric dipoles whose arrangement along the
the lower-temperature interval (100−225 K), the correspond-
ing impedance complex plane plots show a single large semi-
arc, which can be modeled by an equivalent circuit containing
two elements connected in parallel, a resistance (R) and a
capacitance (C), which is frequency independent. As this large
semi-arc intercepts zero and the order of magnitude of its
capacitance is on the order of picofarads per centimeter, it
seems to be associated with the material bulk response. Taking
into account the fact that this is the only contribution present at
temperatures of ≤225 K, in this temperature range the
observed dielectric response is purely intrinsic.
[PbI ] chains is shown in Figure 7. As one can see, these
6
dipoles are arranged antiparallel along the b-axis even if with a
certain canting, so that a net dipole moment along the c-axis
results. Nevertheless, as the unit cell comprises two chains that
On the other hand above 225 K, the impedance complex
plane plots show two arcs: one larger in the low-frequency
range together with a second smaller one in the high-frequency
range. In this case, the large arc can be modeled by an
equivalent circuit containing three elements connected in
parallel: a resistance (R) and a capacitance (C), which are
frequency independent, and a frequency-dependent distributed
element (DE). The capacitance of the large arc is on the order
of nanofarads per centimeter, typical of extrinsic contributions
such as electrode effects, blocking electrodes, etc.
Meanwhile, the small arc can be modeled as a single RC
connected in series with the circuit that describes the large arc.
As this small arc intercepts zero for at 250 K, and its
capacitance is lower than the values of the large arc, it seems to
be associated with the material bulk response.
Figure 7. Crystal structure of the [(CH ) NH ]PbI LT phase where
3
2
2
3
the blue arrows represent the electric dipoles of the DMA cations and
the dark green arrows represent the electric dipoles induced by the off-
center shift of the Pb2 cation.
+
F
DOI: 10.1021/acs.inorgchem.6b03095
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
are antiparallel to each other, the resulting net polarization
turns out to be zero.
In addition, the order−disorder process of the polar DMA
cations is also involved in the observed dielectric transition. We
have to note that the polar DMA cations display an antiparallel
arrangement in the LT phase (Figure 7), while these are
averaged out in the disordered HT phase.
In this context, the order−disorder transition of the DMA
cations resembles those observed in hybrid organic−inorganic
2
+
2+
2+
perovskites [(CH ) NH ][M(HCOO) ] (M = Mg , Mn ,
Co , Ni , or Fe )
3
2
2
3
2+
2+
2+ 17,29
43
and [(CH ) NH ][Cd(N ) ], even if
3 2 2 3 3
in this latter compound the DMA cations display a ferroelectric
arrangement, instead of the antialignments of dipolar moments
that we are reporting here for the DMAPbI compound.
3
We should also mention that MAPbI shows a similar
3
44
dielectric transition related to a phase transition at 162 K. In
this compound, the low-temperature (<162 K) phase displays a
Figure 8. Energy profile of the iodide ion vacancy migration path in
3D perovskite structure with orthorhombic symmetry (space
[
(CH ) NH ]PbI calculated via DFT with sketches about the location
3 2 2 3
group Pnma), where the MA cations are cooperatively ordered,
of the iodide ions and iodide vacancies (□) in scenarios A−C.
while for the range of 162−330 K, MAPbI displays a 3D
3
perovskite structure with tetragonal symmetry (space group I4/
45
In comparison with that of the MAPbI
), we note that the activation energy for the diffusion of the I
3
compound (Tabl−e
mcm), where the MA cations are disordered. Nevertheless, in
that case, the phase transition is related only to the order−
disorder process of MA cations as no distortion due to off-
2
anions is quite similar in both compounds, while the activation
2+
energy of Pb cations is lower in the DMA than in the MA
2
+
center shifts of the Pb cations has been reported. Another
interesting remark is that in the case of the DMAPbI3
compound studied here such structural and dielectric
transitions take place at 260 K, a rather high temperature
compound. We suggest that this could be related to the
different crystal structures of both compounds, where the face-
sharing [PbI ] octahedra in the DMA compound favor the
6
2+
migration of the Pb cations, while the corner-sharing [PbI6]
(
∼100 K higher) when compared to that of the MA analogue
material.
DFT Calculations. To gain information about the
octahedra in the MA compound hinder cation migration.
Table 2. Calculated Activation Energies for Migration of I−
possibility of ionic conductivity in the DMAPbI , we have
2+
3
and Pb in DMAPbI and MAPbI
3
3
performed DFT calculations to investigate the minimum energy
paths and migration activation energy.
Ea (eV) for I− anion
2+
Ea (eV) for Pb cation
In this context, it is well established that the mobile ionic
species in the solid state is associated with some type of vacancy
or interstitial defects, the concentration of which is controlled
by intrinsic Schottky and Frenkel defect reactions, non-
DMAPbI3
MAPbI
0.68
1.41
0.58 (ref 37)
2.31 (ref 37)
3
4
stoichiometry, or doping. In materials with the ABX3
CONCLUSIONS
We have prepared the DMAPbI3 compound at room
temperature using an easy route and mild conditions
■
perovskite-like structure, vacancy diffusion is the most common
process and interstitial migration has not been observed in
inorganic perovskite oxides or halides because of the lack of
interstitial space in such closely packed structures.
(
mechanosynthesis route). Unlike the analogue MAPbI , the
3
DMAPbI 2H-perovskite is stable under ambient conditions for
3
In the case of DMAPbI , three vacancy diffusion mechanisms
3
several months and starts decomposing above 623 K. From a
structural point of view, DMAPbI experiences a first-order
transition at ≈250 K from space group P6 /mmc (HT phase) to
space group P2 /c (LT phase). This phase transition involves
two cooperative processes: an off-center shift of the Pb
cations and an order−disorder process of the polar DMA
cations.
Very interestingly, this compound shows a dielectric anomaly
associated with the phase transition at 260 K. We attribute it to
an antiparallel arrangement of both the polar [PbI ] chains and
the polar DMA cations in the LT phase, which become apolar
and disordered, respectively, in the HT phase. Additionally, this
compound displays very large values of the dielectric constant
at room temperature and low frequencies due to the
appearance of a certain conductivity and the activation of
extrinsic contributions, as demonstrated by impedance spec-
troscopy.
involving conventional hopping between neighboring position−s
3
were considered (Figure S7). These were (i) migration of I
3
along one-dimensional chains of face-sharing [PbI ] octahedra
6
1
and also interchain migration (3D delocalization), (ii)
2+
migration of Pb2 along one-dimensional chains of face-sharing
+
[
PbI ] octahedra (one-dimensional delocalization), and (iii)
6
+
migration of H into a neighbor DMA cation (3D
delocalization). In this work, we have ruled out migration of
the DMA cation because of its large size, but we suggest that
6
+
the H cation can migrate because it is the smallest and because
of the regular alignment of the DMA cation in the crystal
structure. In this context, high proton conductivity has been
46
reported in the porous coordination polymers and metal−
46
organic framework with aligned imidazolium groups.
The obtained results give the lowest activation energy of 0.68
eV for the diffusion of iodide anions (Figure 8) and the highest
+
activation energy of 2.35 eV for the H of DMA cation. For the
The large optical gap (E ∼ 2.59 eV) displayed by this
material rules out the possibility that the observed conductivity
g
2+
Pb cations, the activation energy of 1.41 eV was obtained.
G
DOI: 10.1021/acs.inorgchem.6b03095
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
can be electronic and points to ionic conductivity, as confirmed
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a
(
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Therefore, the replacement of the methylammonium cation
with the larger dimethylammonium cation provokes important
changes in the crystal, electronic structure, and properties,
(
−
while the ionic conductivity of the I anion is maintained in
(
both the MAPbI and DMAPbI compounds.
3
3
These findings allow us to gain more information about the
(
key ingredients that give rise to the exceptional properties of
& Sons: New York, 1999. (b) Li, W.; Wang, Z.; Deschler, F.; Gao, S.;
Friend, R. H.; Cheetham, A. K. Chemically Diverse and Multifunc-
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the MAPbI compound. For example, a 3D framework based
3
on corner-sharing [PbI ] octahedra is essential to obtaining a
6
1
6099.
8) (a) Zhang, X.; Shao, X.-D.; Li, S.-C.; Cai, Y.; Yao, Y.-F.; Xiong,
semiconductor with optimal band gap and photoconductivity
(
properties. In addition, a framework based on [PbI ] octahedra
6
R.-G.; Zhang, W. Dynamics of a Caged Imidazolium Cation-toward
Understanding the Order-Disorder Phase Transition and the Switch-
able Dielectric Constant. Chem. Commun. 2015, 51, 4568−4571.
seems to favor the ionic mobility of the iodide anions, which is
in turn responsible for the very large dielectric constant values
displayed by the DMAPbI compound at room temperature
3
(b) Xu, W.-J.; Chen, S.-L.; Hu, Z.-T.; Lin, R.-B.; Su, Y.-J.; Zhang, W.-
(
and low frequencies).
X.; Chen, X.-M. The Cation-Dependent Structural Phase Transition
and Dielectric Response in a Family of Cyano-Bridged Perovskite-like
Coordination Polymers. Dalt. Trans. 2016, 45, 4224−4229.
ASSOCIATED CONTENT
Supporting Information
■
*
S
(9) (a) Shang, R.; Xu, G.-C.; Wang, Z.-M.; Gao, S. Phase Transitions,
Prominent Dielectric Anomalies, and Negative Thermal Expansion in
Three High Thermally Stable Ammonium Magnesium-Formate
Frameworks. Chem. - Eur. J. 2014, 20, 1146−1158. (b) Chen, S.;
Shang, R.; Hu, K.-L.; Wang, Z.-M.; Gao, S. [NH NH ][M(HCOO) ]
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.6b03095.
2
3
3
Thermogravimetric analysis, crystal structure details,
UV−vis spectra, a Tauc plot, and vacancy diffusion
mechanisms (PDF)
2+
2+
2+
2+
(
M = Mn , Zn , Co and Mg ): Structural Phase Transitions,
Prominent Dielectric Anomalies and Negative Thermal Expansion,
and Magnetic Ordering. Inorg. Chem. Front. 2014, 1, 83−98.
(10) (a) Zhou, B.; Imai, Y.; Kobayashi, A.; Wang, Z.-M.; Kobayashi,
H. Giant Dielectric Anomaly of a Metal-Organic Perovskite with Four-
Membered Ring Ammonium Cations. Angew. Chem., Int. Ed. 2011, 50,
Crystallographic data for [(CH ) NH ]PbI at room
3
2
2
3
temperature (CIF)
Crystallographic data for [(CH ) NH ]PbI at T = 100 K
3
2
2
3
1
1441−11445. (b) Imai, Y.; Zhou, B.; Ito, Y.; Fijimori, H.; Kobayashi,
(
CIF) (CIF)
A.; Wang, Z.-M.; Kobayashi, H. Freezing of Ring-Puckering Molecular
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Chem. - Asian J. 2012, 7, 2786−2790.
AUTHOR INFORMATION
Corresponding Authors
E-mail: m.andujar@udc.es.
E-mail: m.senaris.rodriguez@udc.es.
■
(11) (a) Mączka, M.; Ciupa, A.; Gągor, A.; Sieradzki, A.; Pikul, A.;
*
Macalik, B.; Drozd, M. Perovskite Metal Formate Framework of
+
[
NH -CH -NH ]Mn(HCOO) ]: Phase Transition, Magnetic, Dielec-
*
2
2
3
tric, and Phonon Properties. Inorg. Chem. 2014, 53, 5260−5268.
ORCID
(b) Mączka, M.; Gągor, A.; Costa, N. L. M.; Paraguassu, W.; Sieradzki,
M. San
Notes
́
chez-Anduj
́
ar: 0000-0002-3441-0994
A.; Pikul, A. Temperature- and Pressure-Induced Phase Transitions in
Niccolite-Type Formate Framework of [H N(CH ) NH ]-
3
3
4
3
The authors declare no competing financial interest.
CCDC 1497286 and 1497287 contain the supplementary
crystallographic data for this paper. The data can be obtained
free of charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/structures.
[Mn
2
(HCOO)
6
]. J. Mater. Chem. C 2016, 4, 3185−3194.
(
́ ́
́ ́
̃
12) (a) Bermudez-García, J. M.; Sanchez-Andujar, M.; Yanez-Vilar,
S.; Castro-García, S.; Artiaga, R.; Lop
́
ez-Beceiro, J.; Botana, L.; Alegría,
́
A.; Sen
̃
arís-Rodríguez, M. A. Role of Temperature and Pressure on the
Multisensitive Multiferroic Dicyanamide Framework [TPrA][Mn-
dca) ] with Perovskite-Like Structure. Inorg. Chem. 2015, 54,
(
3
1
1680−11687. (b) Bermud
́
ez-García, J. M.; San
́
chez-Anduj
́
ar, M.;
ACKNOWLEDGMENTS
■
Yanez-Vilar, S.; Castro-García, S.; Artiaga, R.; Lop
Botana, L.; Alegría, A.; Senarís-Rodríguez, M. A. Multiple Phase and
Dielectric Transitions on a Novel Multi-Sensitive [TPrA][M(dca) ]
́
́
ez-Beceiro, J.;
̃
This work was financially supported by the Ministerio de
̃
́
Economia y Competitividad (MINECO) and EU-FEDER
3
2+
2+
2+
under Project ENE2014-56237-C4-4-R, Xunta de Galicia
under Project GRC2014/042, The National Key Basic
Research Program of China (Grant 2015CB921600), and the
National Natural Science Foundation of China (Grant
(M: Fe , Co and Ni ) Hybrid Inorganic−Organic Perovskite
Family. J. Mater. Chem. C 2016, 4, 4889−4898.
(
13) (a) Kieslich, G.; Forse, A. C.; Sun, S.; Butler, K. T.; Kumagai, S.;
Wu, Y.; Warren, M. R.; Walsh, A.; Grey, C. P.; Cheetham, A. K. Role
of Amine−Cavity Interactions in Determining the Structure and
Mechanical Properties of the Ferroelectric Hybrid Perovskite
1
127422). The Special Program for Applied Research on
Super Computation of the NSFC Guangdong Joint Fund (the
second phase) and the Shanghai Supercomputer Center are
also acknowledged.
[
NH NH ]Zn(HCOO) . Chem. Mater. 2016, 28, 312−317. (b) Kie-
3
2
3
slich, G.; Kumagai, S.; Butler, K. T.; Okamura, T.; Hendon, C. H.; Sun,
S.; Yamashita, M.; Walsh, A.; Cheetham, A. K. Role of Entropic Effects
in Controlling the Polymorphism in Formate ABX Metal-Organic
3
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DOI: 10.1021/acs.inorgchem.6b03095
Inorg. Chem. XXXX, XXX, XXX−XXX