Reentrant Structural and Optical Properties and Large Positive Thermal Expansion in Perovskite Formamidinium Lead Iodide

Article abstract of DOI:10.1002/anie.201609538

The structure of the hybrid perovskite HC(NH₂)₂PbI₃(formamidinium lead iodide) reflects competing interactions associated with molecular motion, hydrogen bonding tendencies, thermally activated soft octahedral rotations, and the propensity for the Pb²⁺lone pair to express its stereochemistry. High-resolution synchrotron X-ray powder diffraction reveals a continuous transition from the cubic α-phase (Pm3m, #221) to a tetragonal β-phase (P4/mbm, #127) at around 285 K, followed by a first-order transition to a tetragonal γ-phase (retaining P4/mbm, #127) at 140 K. An unusual reentrant pseudosymmetry in the β-to-γ phase transition is seen that is also reflected in the photoluminescence. Around room temperature, the coefficient of volumetric thermal expansion is among the largest for any extended crystalline solid.

Full text of DOI:10.1002/anie.201609538

Angewandte
Communications
Chemie
International Edition: DOI: 10.1002/anie.201609538
German Edition: DOI: 10.1002/ange.201609538
Perovskite Phases
Reentrant Structural and Optical Properties and Large Positive
Thermal Expansion in Perovskite Formamidinium Lead Iodide
Douglas H. Fabini, Constantinos C. Stoumpos, Geneva Laurita, Andreas Kaltzoglou,
Athanassios G. Kontos, Polycarpos Falaras, Mercouri G. Kanatzidis, and Ram Seshadri*
Dedicated to Professor Anthony K. Cheetham, FRS on the occasion of his 70th birthday
Abstract: The structure of the hybrid perovskite HC-
(NH2)2PbI3 (formamidinium lead iodide) reflects competing
interactions associated with molecular motion, hydrogen
bonding tendencies, thermally activated soft octahedral rota-
tions, and the propensity for the Pb2+ lone pair to express its
stereochemistry. High-resolution synchrotron X-ray powder
diffraction reveals a continuous transition from the cubic a-
symmetry for the a- and b-phases on the basis of laboratory
single-crystal X-ray diffraction,[3] while a subsequent report
assigned the structure of the cubic perovskite aristotype for
the a-phase from neutron powder diffraction.[4] The structure
of the g-phase has not been reported.
The disordered molecular cation, challenges associated
with twinning in single crystals, and issues of pseudosymmetry
led us to employ high-resolution synchrotron X-ray powder
diffraction to follow the structure evolution of HC(NH2)2PbI3
between 90 K and 490 K. The temperature-dependent scat-
tering intensity around instructive low-angle Bragg peaks is
given in Figure 1. The continuous a–b phase transition around
285 K is evident in the emergence of the 211 tetragonal peak
and the splitting of the 200 cubic peak into the 002 and 220
tetragonal peaks on cooling. The first-order b–g transition can
be seen in the abrupt change in intensities and peak positions
at 140 K (transition temperature from reported scanning
¯
phase (Pm3m, #221) to a tetragonal b-phase (P4/mbm, #127)
at around 285 K, followed by a first-order transition to
a tetragonal g-phase (retaining P4/mbm, #127) at 140 K. An
unusual reentrant pseudosymmetry in the b-to-g phase tran-
sition is seen that is also reflected in the photoluminescence.
Around room temperature, the coefficient of volumetric
thermal expansion is among the largest for any extended
crystalline solid.
P
hotovoltaic absorbers based on the hybrid perovskite
HC(NH2)2PbI3 (formamidinium lead iodide) and its alloys
exhibit impressive performance,[1] but the description of the
crystal structure of this material is incomplete. In addition to
this technological motivation,[2] dense hybrid materials with
3D inorganic connectivity and isolated organic molecular ions
combine features of traditional inorganic solids and open-
framework materials, and their composition–structure rela-
tions are of fundamental interest.
The initial report of the preparation and characterization
of HC(NH2)2PbI3 proposed perovskite structures of trigonal
[*] D. H. Fabini, Dr. G. Laurita, Prof. Dr. R. Seshadri
Materials Research Laboratory, University of California
Santa Barbara, CA 93106 (USA)
E-mail: seshadri@mrl.ucsb.edu
D. H. Fabini, Prof. Dr. R. Seshadri
Materials Department, University of California
Santa Barbara, CA 93106 (USA)
Dr. C. C. Stoumpos, Prof. Dr. M. G. Kanatzidis
Chemistry Department, Northwestern University
Evanston, IL 60208 (USA)
Figure 1. X-ray scattering intensity from HC(NH2)2PbI3 around selected
low-angle Bragg peaks between 90 K and 490 K, normalized to
maximum peak intensity. a) The 211t tetragonal Bragg peak emerges
upon cooling through a continuous phase transition around 285 K
from the cubic a-phase to the tetragonal b-phase. b) The 200c cubic
Bragg peak splits continuously on cooling due to the emergent
tetragonality. A first-order transition to the pseudo-cubic g-phase with
tetragonal symmetry occurs at 140 K.[5] The 002t and 220t tetragonal
peaks “fuse” across the b–g transition while the 211t tetragonal peak
remains.
Dr. A. Kaltzoglou, Dr. A. G. Kontos, Dr. P. Falaras
Institute of Nanoscience and Nanotechnology, NCSR Demokritos
Athens 15310 (Greece)
Prof. Dr. R. Seshadri
Department of Chemistry and Biochemistry, University of California
Santa Barbara, CA 93106 (USA)
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under http://dx.doi.org/10.
1002/anie.201609538.
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calorimetry, which confirms the order of the transitions).[5]
Remarkably, the 002 and 220 tetragonal peaks “fuse” in the g-
phase, while the 211 tetragonal peak remains, suggesting the
possibility of a tetragonal phase with cubic pseudosymmetry.
Le Bail and Rietveld analyses of diffraction data confirm
this hypothesis. In agreement with the results of Weller
preferential orientations for the molecular cation due to the
tilting of the octahedra.
Remarkably, the structure of the g-phase is also that of
a0a0c+ tilts with P4/mbm symmetry, as shown in Figure 2c,
though there are discontinuous changes in many structure
parameters (see below) as required for such a first-order
transition. Alternative structures exhibiting simple tilts that
maintain 3D octahedral connectivity[9] and that allow ferroic
Pb2+ displacements[10] were screened and rejected based on
unindexed peaks, absences of expected peaks, or both (details
given in the Supporting Information).
Given this highly unusual phase evolution, we examined
the temperature dependence of structure parameters from
Rietveld refinement of diffraction data across the temper-
ature range of 90 k to 490 K, as well as temperature-
dependent optical and dielectric properties. The tetragonal
lattice parameter ratio is given in Figure 3a, and reflects the
continuous emergence of tetragonality on cooling into the b-
phase, as well as the reentrant cubic pseudosymmetry in the g-
phase.
et al.,[4] the a-phase is well described by the cubic Pm3m
¯
aristotype (a = 6.35788 ꢀ at 299 K), as shown in Figure 2a.
For all phases, the A-site [HC(NH2)2]+ cation has been
modeled as a pseudo-atom with equivalent scattering power
due to the dynamic disorder (see below) and limited
sensitivity in the X-ray experiment.
The structure of the b-phase is that of P4/mbm symmetry,
with a0a0c+ tilts in Glazer notation[6] (temperature-dependent
structure parameters for all phases are provided in the
Supporting Information). This in-phase uniaxial tilt pattern
(Figure 2b) is in line with computational predictions of the
preferred tilts for tetragonal perovskite iodides[7] and con-
trasts with the out-of-phase tilts observed in the tetragonal
[8]
phase of CH3NH3PbI3 and many oxides. The anisotropy of
the A-site atomic displacement parameters (ADPs) suggests
The photoluminescence (PL) emission peak energy, EPL,
is given in Figure 3b. EPL blue-shifts with temperature in the
a- and b-phases, consistent with the effects of thermal
expansion on the unusual electronic structure as well as
possible dynamic Pb2+ displacements.[11] EPL plateaus above
370 K, a range which was not accessed in previous studies. On
Figure 3. Tetragonal lattice parameter ratio, photoluminescence peak
energy, and static dielectric permittivity of HC(NH2)2PbI3. a) The
tetragonal lattice parameter ratio reflects the continuous a–b transi-
tion on cooling, as well as the reentrant cubic pseudosymmetry in the
g-phase. Error bars are smaller than the markers and are omitted.
b) The photoluminescence (PL) emission peak energy, EPL, red-shifts
on warming through the g–b transition. In the b- and a-phases, EPL
blue-shifts with temperature, consistent with the effects of lattice
expansion[11] on the unusual electronic structure of the main-group
halide perovskites, and subsequently plateaus above 370 K. c) Static
dielectric permittivity, er’, extracted from capacitance-loss measure-
ments at 20 kHz. Despite a slight kink at the b–g transition, er’
declines substantially with temperature in both phases, suggesting
reorientation of the dipolar axis of the A-site cation is minimally
inhibited even at these temperatures.
Figure 2. Rietveld refinement of X-ray powder diffraction and resulting
crystal structures of the phases of HC(NH2)2PbI3. Recorded scattering
intensities are indicated by black dots, modelled intensities by gray
lines, and difference curves are offset below. The A-site [HC(NH2)2]+
cation is modelled as a pseudo-atom with the same form factor due to
disorder and limited sensitivity in the X-ray experiment, and atomic
displacement parameter (ADP) ellipsoids are visualized at the 50%
¯
level. a) Cubic a-phase (Pm3m, #221) at 299 K, Rw =12.4%. The
formamidinium cation, [HC(NH2)2]+ is illustrated. b) Tetragonal b-
phase (P4/mbm, #127) at 151 K, Rw =15.1%, viewed down the c-axis.
c) Tetragonal g-phase (P4/mbm, #127) at 91 K, Rw =16.1%, viewed
down the c-axis. Dashed lines in (b,c) indicate the unique axis of the
A-site ADP ellipsoid.
2
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Table 1: Volumetric thermal expansion coefficients, aV, of HC(NH2)2PbI3
and selected other compounds near room temperature.
cooling through the b–g transition, EPL blue-shifts substan-
tially, in agreement with existing reports.[12,13] This is similar to
the observed behavior at the tetragonal–orthorhombic tran-
sition in CH3NH3PbI3,[13] where the blue-shift arises from
reduced Pb–I orbital overlap due to the lowered symmetry,
though in HC(NH2)2PbI3 the blue-shift despite reduced tilting
suggests crystallographically hidden disorder of the Pb–I
network, though no disorder-induced emission broadening is
apparent.
Composition
Phase
T [K]
aV [10ꢀ6 Kꢀ1
]
Reference
HC(NH2)2PbI3
HC(NH2)2PbI3
CH3NH3PbI3
CsSnI3
Ag3[Co(CN)6]
Hg
b (solid)
a (solid)
b (solid)
g (solid)
solid
200–299
348–489
280
298
276
206–202
100–98.6
132
126
121
This work
This work
[19,20]
[21]
[22]
[21]
liquid
293
182
The static dielectric permittivity, er’, indirectly probes the
rotational freedom of the polar axis of the molecular
cation.[14] For the well-studied CH3NH3PbI3, er’ rises on
cooling in the tetragonal phase and drops abruptly through
the tetragonal–orthorhombic phase transition,[15] as the severe
tilting of the octahedra constrains the reorientation of the C–
N axis. In contrast, er’ for HC(NH2)2PbI3 has no discontinuity
and continues to rise on cooling in the g-phase (Figure 3c),
suggesting minimally inhibited reorientation of the polar axis.
The smaller value and reduced temperature dependence of er’
compared to that for CH3NH3PbI3 are consistent with the
smaller dipole moment for [HC(NH2)2]+. The persistence of
considerable cation motion in the g-phase is in line with our
previous observation of a glassy slowing of dynamics below
100 K in CH3NH3PbI3 and HC(NH2)2PbI3, with HC-
(NH2)2PbI3 exhibiting greater glass fragility.[5]
coefficient, aV, with those of other main-group halides and
selected framework materials and liquids is given in Table 1.
Fit over a temperature range of 200 K to 299 K, aV is greater
than 200 ꢁ 10ꢀ6 Kꢀ1 (R2 = 99.8%). Thus, aV for the b-phase is
larger than that for liquid mercury[21] and appears to be nearly
the largest for any extended crystalline solid near ambient
temperature. It was recently reported that aV = 220 ꢁ 10ꢀ6 Kꢀ1
for the mixed A-site [CH3NH3]0.5[HC(NH2)2]0.5PbI3, though
the precise temperature and fitting range are unspecified.[23]
In the b-phase, linear thermal expansion is ca. 60% greater in
the ab-plane (aa = 7.7 ꢁ 10ꢀ5 Kꢀ1) than in the c-direction (ac =
4.9 ꢁ 10ꢀ5 Kꢀ1), suggesting anisotropic mechanical properties.
While still large, aV is substantially reduced in the cubic a-
phase, consistent with a negative contribution from large
amplitude dynamic octahedral tilts as in A-site vacant
ReO3[16,17] and ScF3.[18]
The unit cell volume per formula unit, V, is given in
Figure 4a. A comparison of the volumetric thermal expansion
The isotropic ADP for Pb, Uiso, is given in Figure 4b, and
increases monotonically with temperature. While the value is
significantly elevated relative to typical values in oxides
(partially a consequence of 6s2 lone pair-induced anharmo-
nicity),[11] there are no discontinuities that would suggest
crystallographic Pb2+ displacements that are not captured in
our structure models for the low temperature phases.
The Pb-I-Pb octahedral tilt angle is given in Figure 4c, and
reflects the continuous a–b transition. In the g-phase, the tilt
angle jumps closer to linear, underscoring the unusual nature
of this transition.
ꢀ
Iodine ADPs in the directions orthogonal to the Pb I
bonds, U?, are given in Figure 4d. Consistent with thermally-
activated dynamic octahedral tilts, the values of U? decrease
monotonically on cooling in the a- and b-phases. However,
they increase significantly in the g-phase, again consistent
with disorder that is not captured by this crystallographic
model. The discrepancy between ADPs for the two distinct I
sites in the g-phase reflects parameter correlations that arise
from Bragg peak overlap.
Together, the photoluminescence, iodine ADPs, and
persistence of molecular motion in the g-phase suggest
complex (possibly modulated) disorder in this regime. This
is supported by the observation of weak (intensity < 0.5%
that of the strongest Bragg peak) diffuse scattering features
between Q = 1.55 ꢀꢀ1 and Q = 2.00 ꢀꢀ1 that are not indexed
by any plausible space group symmetry.[9,10] Nonetheless, the
pseudo-cubic unit cell is unambiguous, and underscores the
Figure 4. Selected crystallographic parameters for HC(NH2)2PbI3. Error
bars for (a)–(c) are smaller than the markers and are omitted. a) The
unit cell volume per formula unit, V, reveals unusually large volumetric
thermal expansion coefficients (aV =203ꢀ10ꢀ6 Kꢀ1 at 274 K). aV is
reduced in the cubic phase, consistent with a negative contribution
from large amplitude dynamic octahedral tilts.[16–18] b) The isotropic
ADP for Pb, Uiso, is large compared to typical values in oxides but
reveals no discontinuities associated with masked displacements in
the tetragonal phases. c) The Pb-I-Pb octahedral tilt angle reflects the
continuous a–b transition, and jumps back toward linear through the
unusual b–g transition. d) The ADPs for I in the directions orthogonal
ꢀ
to the Pb I bonds, U?, decline monotonically with cooling in the a-
ꢀ
different N H···I interactions between CH3NH3PbI3 and
and b-phases, consistent with reduced dynamic octahedral tilting. The
increased values of the U? ADPs on cooling to the g-phase suggest
disorder that is not fully captured by this crystallographic model.
HC(NH2)2PbI3, where the diamine may hydrogen-bond to
halogens on both sides of the cage.
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In summary, we find that this class of materials, already so
Acknowledgements
fascinating on account of their outstanding functionality in
photovoltaic devices, display fundamental properties that are
surprising and even counterintuitive. The implications for
technological applications are profound. For example, the
very large volumetric thermal expansion creates substantial
challenges in device applications. The atomistic origins of the
reentrant structural and optical behavior deserve further
scrutiny.
This work was supported by the U.S. Department of Energy,
Office of Science, Basic Energy Sciences under award number
DE-SC-0012541. We thank Professor Anthony K. Cheetham
FRS for illuminating discussions. This research used resources
of the Advanced Photon Source, a U.S. Department of
Energy (DOE) Office of Science User Facility operated for
the DOE Office of Science by Argonne National Laboratory
under Contract No. DE-AC02-06CH11357. D.H.F. thanks the
National Science Foundation Graduate Research Fellowship
Program for support under Grant DGE 1144085. The MRL
Shared Experimental Facilities are supported by the MRSEC
Program of the NSF under Award No. DMR 1121053;
a member of the NSF-funded Materials Research Facilities
Network (http://www.mrfn.org). P.F. acknowledges financial
support from FP7 European Union (Marie Curie ITN
DESTINY/316494).
Experimental Section
HC(NH2)2PbI3 was prepared following a modification of the
previously reported procedure.[3] 4.46 g (20 mmol) of PbO was
dissolved in 15 mL of concentrated aqueous HI (57% w/w) and the
solution temperature was raised and held to boiling (ca. 1308C). In
a separate beaker, 2.08 g (20 mmol) of (HC(NH2)2)(O2CCH3) or
1.61 g (20 mmol) of freshly prepared HC(NH2)2Cl (prepared from
(HC(NH2)2)(O2CCH3) and concentrated HCl (37% w/w), followed
by copious washing with toluene to remove excess acetic acid)[4] were
dissolved in 5 mL of concentrated aqueous HI (57% w/w). The latter
step was found to be necessary since commercially available HC-
(NH2)2Cl is extremely hygroscopic and contains hydrolysis products.
Addition of the HC(NH2)2I solution to the PbI2 solution resulted
immediately in a fine black precipitate. The reaction was stopped
after 5 min and the solution was filtered hot under vacuum to avoid
co-precipitation of hydrated phases obtained for the cooled solution.
During filtration, the black solid turns to yellow, completely
converting over a period of 5 to 10 min. The yellow solid was
transferred in a vacuum oven and heated at 1208 C overnight, yielding
a black solid that is stable for a period of 20 to 30 days before
converting back to the yellow phase. Note that if the latter step is
performed in an evacuated ampule under static vacuum, the black
phase converts to the yellow one within 5 to 10 min. The obtained
black material consists of the crystallographically pure a-phase of
HC(NH2)2PbI3. Yield: 9 to 10 g (70% to 80%).
Keywords: perovskite phases · phase transitions ·
solid-state structures · structure elucidation · thermal expansion
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Received: September 28, 2016
Revised: October 21, 2016
Published online: && &&, &&&&
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Communications
Perovskite Phases
Extraordinary phase evolution: The
hybrid perovskite formamidinium lead
iodide undergoes two structural transi-
tions in the 90 K to 490 K range. Deci-
phering these transitions points to
unusually large thermal expansion in the
tetragonal phases, and strong coupling
between the reentrant lower temperature
structural transition and the optical band
gap.
D. H. Fabini, C. C. Stoumpos, G. Laurita,
A. Kaltzoglou, A. G. Kontos, P. Falaras,
M. G. Kanatzidis,
R. Seshadri*
&&&& — &&&&
Reentrant Structural and Optical
Properties and Large Positive Thermal
Expansion in Perovskite Formamidinium
Lead Iodide
6
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