Precise Molecular Design Toward Organic-Inorganic Zinc Chloride ABX3 Ferroelectrics

Article abstract of DOI:10.1021/jacs.0c00315

Organic-inorganic ABX₃ (A, B = cations, X = anion) hybrids with perovskite structure have recently attracted tremendous interest due to their structural tunability and rich functional properties, such as ferroelectricity. However, ABX₃ hybrid ferroelectrics with other structures have rarely been reported. Here, we successfully designed an ABX₃ hybrid ferroelectric [(CH₃)₃NCH₂F]ZnCl₃ with a spontaneous polarization of 4.8 μC/cm² by the molecular modification of [(CH₃)₄N]ZnCl₃ through hydrogen/halogen substitution. It is the first zinc halide ABX₃ ferroelectric, which contains one-dimensional [ZnCl₃]^-_n chains of corner-sharing ZnCl₄ tetrahedra, distinct from the anionic framework of corner-sharing or face-sharing BX₆ octahedra in the ABX₃ perovskites. From zero dimension to one dimension, the high symmetry of ZnCl₄ tetrahedra is broken, and all of them align along one direction to form a polar [ZnCl₃]^-_n chain, beneficial to the generation of ferroelectricity. This finding provides an efficient polar anionic framework for enriching the family of hybrid ferroelectrics by assembling with various cations and should inspire further exploration of new classes of organic-inorganic ABX₃ ferroelectrics.

Full text of DOI:10.1021/jacs.0c00315

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Article
Precise Molecular Design Toward Organic
3
-Inorganic Zinc Chloride ABX Ferroelectrics
Lizhuang Chen, Wei-Qiang Liao, Yong Ai, Junyi Li, Siyu Deng, Yunlong Hou, and Yuan-Yuan Tang
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.0c00315 • Publication Date (Web): 11 Mar 2020
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Precise Molecular Design Toward Organic−Inorganic Zinc Chloride
ABX3 Ferroelectrics
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Lizhuang Chen,† Wei-Qiang Liao,‡ Yong Ai,‡ Junyi Li,† Siyu Deng,† Yunlong Hou,† and Yuan-
Yuan Tang*,‡
†School of Environmental and Chemical Engineering, Jiangsu University of Science and Technology, Zhenjiang
212003, People’s Republic of China
‡Ordered Matter Science Research Center, Nanchang University, Nanchang 330031, People’s Republic of China
Supporting Information
ABSTRACT: Organic−inorganic ABX3 (A, B = cations, X = anion) hybrids with perovskite structure have recently attracted
tremendous interest due to their structural tunability and rich functional properties, such as ferroelectricity. However,
ABX3 hybrid ferroelectrics with other structures have rarely been reported. Here, we successfully designed an ABX3 hybrid
ferroelectric [(CH3)3NCH2F]ZnCl3 with a spontaneous polarization of 4.8 μC/cm2, by the molecular modification of
[(CH3)4N]ZnCl3 through hydrogen/halogen substitution. It is the first zinc halide ABX3 ferroelectric, which contains one-
dimensional [ZnCl3]-n chains of corner-sharing ZnCl4 tetrahedra, distinct from the anionic framework of corner-sharing or
face-sharing BX6 octahedra in the ABX3 perovskites. From zero dimension to one dimension, the high symmetry of ZnCl4
tetrahedra is broken and all of them align along one direction to form a polar [ZnCl3]-n chain, beneficial to the generation
of ferroelectricity. This finding provides an efficient polar anionic framework for enriching the family of hybrid ferroelec-
trics by assembling with various cations, and should inspire further exploration of new classes of organic−inorganic ABX3
ferroelectrics.
tetrahedral. However, their ferroelectricity was rarely ex-
plored, and it is still a great challenge to precisely design
INTRODUCTION
Organic−inorganic ABX3 (A, B = cations, X = anion) hy-
brids with perovskite structure have recently been exten-
sively investigated for their fascinating properties and
functions such as ferroelectricity, piezoelectricity, photo-
an organic−inorganic ABX3 ferroelectric because of the
lack of deep knowledge on the structure-property rela-
tionship.
Exploring the relationship between structure and prop-
erties is essential to the design and optimization of func-
tional materials. Based on the Neumann principle, ferroe-
lectricity exists only in one of ten polar point groups: 1
(C1), 2 (C2), m (C1h), mm2 (C2v), 4 (C4), 4mm (C4v), 3 (C3),
3m (C3v), 6 (C6) and 6mm (C6v).6 Introducing polar mole-
cule or framework into the crystal to induce polar struc-
ture may be one of the most effective ways to construct
ferroelectrics. During our systematic search for organ-
ic−inorganic ABX3 hybrids with the distinct polar anionic
luminescence,
magnetism,
and
photovoltaic
performance.1 Among them, the ferroelectricity, as an
important functional property, plays essential roles in
non-volatile memory, capacitors and sensors, etc.2 Struc-
turally, hybrid ABX3 perovskites typically adopt the same
structures as the inorganic ABX3 perovskites CaTiO3 and
BaNiO3, containing the three-dimensional BX3 framework
of corner-sharing octahedra and one-dimensional BX3
framework of face-sharing BX6 octahedra, respectively.3
Owing to the unique structure, hybrid materials possess
both the benefits of organic molecules and advantageous
characteristics of inorganic framework. Numerous mem-
bers in these hybrid ABX3 perovskites have been discov-
ered as ferroelectrics, and some of them show large pie-
zoelectricity and excellent ferroelectricity of high Curie
temperature and large spontaneous polarization (Ps).4 In
addition to perovskite structures, ABX3 inorganic com-
pounds also crystallize in other structure types, such as
pyroxene (mainly inosilicate), hexagonal manganites, co-
rundum, bixbyite structures.5 Correspondingly, there are
also some less common classes of organic−inorganic ABX3
hybrids, constructed by BX3 framework of pentahedra or
framework, we found that
a zinc chloride one,
[(CH3)4N]ZnCl3, adopts a polar crystal structure contain-
ing one-dimensional [ZnCl3]-n polar chains of corner-
sharing ZnCl4 tetrahedra.7 All the ZnCl4 tetrahedra align
approximately along one direction, which results in a po-
lar [ZnCl3]-n chain, different from both one-dimensional
perovskite and inosilicate structures, where the dipole
moments tend to cancel each other out (Scheme 1).1c,8
Unfortunately, although the polar symmetry satisfies the
requirement for ferroelectrics, systematic characteriza-
tions (see below) indicate that [(CH3)4N]ZnCl3 shows no
phase transition and ferroelectricity, probably because of
the large coercive field. Subtle changes in the molecular
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structure will directly affect the crystal structure and
[(CH3)3NCH2I]ZnCl3, revealing that they undergoes no
phase transitions. However, the DSC curves of
[(CH3)3NCH2F]ZnCl3 and [(CH3)3NCH2Cl]ZnCl3 clearly
exhibit a pair of endothermic/exothermic peaks at around
232.4/215.2 K and 255.3/235.5 K (Figures 1b and S2), re-
spectively, which indicates that the reversible phase tran-
sition occurs at 232.4 K for [(CH3)3NCH2F]ZnCl3 and 255.3
K for [(CH3)3NCH2Cl]ZnCl3. From the DSC results, the
introduction of F or Cl atoms on TMHM cations success-
fully induces the occurrence of obvious phase transition.
The phase transition of [(CH3)3NCH2F]ZnCl3 was also veri-
fied by the dielectric anomaly near transition temperature
in the temperature-dependent real part (εʹ) of complex
dielectric permittivity (Figure S3).
properties.9 For example, Xiong et al. used the molecular
design strategy of fluorine substitution to successfully
increase the Curie temperature from 295 K in (pyrroli-
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dinium)MnCl3 to 333
K
in (R)- and (S)-3-
(fluoropyrrolidinium)MnCl3.10
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Scheme 1. ABX3 compounds with perovskite, inosilicate and
distinct [ZnCl3]-n chain structures.
In this work, we replaced one hydrogen atom of
spherical [(CH3)4N]+ cation with more electronegative
species like halogen atoms to construct polar quasi-
spherical cations. Under precise molecular modifications
or tailoring, we obtained four new ABX3 zinc chloride
polar crystals, [(CH3)3NCH2X]ZnCl3 (X = F, Cl, Br, I),
isostructural to [(CH3)4N]ZnCl3. Interestingly, different
from [(CH3)4N]ZnCl3, [(CH3)3NCH2F]ZnCl3 successfully
exhibits both the phase transition and remarkable
ferrolectric behaviours. For [(CH3)3NCH2X]ZnCl3 (X = Cl,
Br, I), there is no valid evidence to show their ferroelectric
properties. The stronger electronegative F atoms in
[(CH3)3NCH2F]+ cation affect the Cl atoms in [ZnCl3]-n
chains to form a relatively strong F···Cl interaction, a type
of halogen bonding interaction, which acts as a bridge
between the cations and anionic chains. In this way, when
the cations switch, it can drive the anion chains to switch
together, which can reduce the energy required for
polarization switching and cause the coercive field to
become smaller. Since the electronegativity of Cl, Br and I
atoms are relatively small, the ability to attract electrons
is not sufficient to induce significant ferroelectric
behavior. This work opens an effective route to design
novel and excellent organic−inorganic ABX3 ferroelectrics
through introducing a polar anionic framework, of which
the structural adjustability and easy processing will
provide new opportunities for the modern intelligent
electronics field.
Figure 1. (a) DSC curves of [(CH3)3NCH2X]ZnCl3 (X = H, F, Cl,
Br, I) upon heating, respectively. (b) DSC curves of
[(CH3)3NCH2F]ZnCl3 upon heating and cooling respectively.
Crystal
structural determinations
reveal
that
[(CH3)4N]ZnCl3 crystallizes in the orthorhombic polar
space group Pmc21 (point group mm2) at 293 K (Table S1).
After hydrogen/halogen substitution, the polar mm2
point group maintains in [(CH3)3NCH2X]ZnCl3 (X = F, Cl,
Br, I), and [(CH3)3NCH2X]ZnCl3 (X = Cl, Br, I) adopt the
same Pmc21 space group while [(CH3)3NCH2F]ZnCl3 has
the Pmn21 one (Table S1). The crystal structures of
[(CH3)3NCH2X]ZnCl3 (X = H, F, Cl, Br, I) are isostructural,
consisting of one-dimensional [ZnCl3]-n anionic chains of
corner-sharing ZnCl4 tetrahedra and [(CH3)3NCH2X]+ cat-
ions (Figures 2 and S4). The anionic and cationic layers
RESULTS AND DISCUSSION
Differential scanning calorimetry (DSC) measurements
were firstly performed to examine the existence of phase
transitions in [(CH3)3NCH2X]ZnCl3 (X = H, F, Cl, Br, I). As
shown in Figure 1a, DSC results show the melting points
of [(CH3)3NCH2X]ZnCl3 (X = H, F, Cl, Br, I) are up to 545.3,
511.2, 448.2, 463.6 and 487.3 K, respectively. No thermal
anomalies were detected below the melting point in the
DSC curves of [(CH3)4N]ZnCl3, [(CH3)3NCH2Br]ZnCl3 and
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Figure 2. Crystal structures of (a) [(CH3)4N]ZnCl3 and (b) [(CH3)3NCH2X]ZnCl3 at 293 K, respectively. The light blue dashed
bonding in (b) is the selected F−Cl bonding. (c) Chemical structures of [ZnCl3]-n chains and intercalated TMXM (X = H, F, C, B, I)
cations in [(CH3)3NCH2X]ZnCl3 (X = H, F, Cl, Br, I).
show an alternating arrangement, similar to that found in
the one-dimensional ABX3 perovskite structure. However,
the anionic framework of [ZnCl3]-n chains of corner-
sharing ZnCl4 tetrahedra in [(CH3)3NCH2X]ZnCl3 (X = H, F,
Cl, Br, I) is distinct from the [BX3]-n chains of face-sharing
BX6 octahedra in one-dimensional ABX3 perovskites such
as (pyrrolidinium)MnCl3.11 In the [ZnCl3]-n chain, there are
two independent distorted ZnCl4 tetrahedra (Table S2), in
which the positive Zn2+ and negative Cl- charge centers do
not coincide. All the ZnCl4 tetrahedra align approximately
along one direction, leading to a polar [ZnCl3]-n chain,7
which may contribute to the polar crystal structures of
[(CH3)3NCH2X]ZnCl3 (X = H, F, Cl, Br, I). The Cl–Zn bond
lengths and Cl–Zn–Cl bond angles in [(CH3)3NCH2X]ZnCl3
(X = H, F, Cl, Br, I) are comparable to each other (Table
S2), indicating that the configuration of the [ZnCl3]-n
chain show no obvious change after the hydro-
gen/halogen substitution.
Nevertheless, the state of [(CH3)3NCH2X]+ cations in
[(CH3)3NCH2X]ZnCl3 (X = F, Cl, Br, I) is significantly dif-
ferent from that of [(CH3)4N]+ cations in [(CH3)4N]ZnCl3.
[(CH3)4N]ZnCl3 contains two independent [(CH3)4N]+ cat-
ions, both of which display a ordered state (Figure 2a). By
replacing a hydrogen atom on the [(CH3)4N]+ cation with
a larger F atom, the quasi-spherical [(CH3)3NCH2F]+ cati-
ons in [(CH3)3NCH2F]ZnCl3 become severely disordered,
with the atomic coordinates of F atoms of two independ-
ent [(CH3)3NCH2F]+ cations occupying two and three pos-
[(CH3)3NCH2X]+ cations in [(CH3)3NCH2X]ZnCl3 (X = Br, I),
one is ordered and the other is slightly disordered with
the Br and I atomic coordinates splitting into two parts
(Figure S4). In the organic-inorganic system, the phase
transition is generally governed by the dynamics of organ-
ic cations.6a In the case of [(CH3)3NCH2F]ZnCl3 and
[(CH3)3NCH2Cl]ZnCl3, the phase transitions should be
related to the dynamics of severely disordered
[(CH3)3NCH2F]+ and [(CH3)3NCH2Cl]+ cations, respectively.
We thus determine the crystal structures of
[(CH3)3NCH2F]ZnCl3 and [(CH3)3NCH2Cl]ZnCl3 at 93 K
below the phase transition temperature. At 93 K,
[(CH3)3NCH2F]ZnCl3 adopts the same space group as that
at 293 K (Table S3), indicating that the phase transition in
[(CH3)3NCH2F]ZnCl3 is an isostructural one. Although the
disordered models of [(CH3)3NCH2F]+ are similar to those
at 293 K, the atomic anisotropic displacement parameters
become remarkably lower, which reveals that the severely
disordered state of [(CH3)3NCH2F]+ cations at 293 K
changes to a less disordered one below the phase transi-
tion temperature (Figure S5). For [(CH3)3NCH2Cl]ZnCl3,
the Pmn21 space group at 293 K changes to the Pna21 one
at 93 K (Table S3), and the [(CH3)3NCH2Cl]+ cations be-
come totally ordered (Figure S6).
Another striking structural difference after hydro-
gen/halogen substitution is the intermolecular interac-
tions between the cations and anionic [ZnCl3]-n chains. In
[(CH3)3NCH2X]ZnCl3 (X = F, Cl, Br, I), the cations are
linked to the anionic [ZnCl3]-n chains via halogen-halogen
interactions rather than hydrogen bonding interactions in
[(CH3)4N]ZnCl3 (Figures 2 and S4). Halogen bonding ef-
fect that involves the utilization of intermolecular
sible
positions,
respectively
(Figure
2b).
In
[(CH3)3NCH2Cl]ZnCl3, one kind of [(CH3)3NCH2Cl]+ cation
is also severely disordered with two possible positions for
the Cl atomic coordinate while the other one shows a
ordered state (Figure S4). For two independent
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Figure 3. The Hirshfeld dnorm surfaces (top) and 2D fingerprint plots (bottom) of (a) [(CH3)4N]+ and (b) [(CH3)3NCH2F]+ cations
at 293 K, respectively. The intensity of molecular interaction is mapped onto the Hirshfeld surface by using a red-blue-white
color scheme: where the white regions exactly correspond to the distance of Van der Waals contact, the blue regions correspond
to longer contacts, and the red regions represent closer contacts. In 2D fingerprint plots, each point represents an individual pair
(di, de), reflecting the distances to the nearest atom inside (di) and outside (de) of the Hirshfeld surface, and the frequency of
occurrence for these points corresponds to the colors from blue (low), through green, to red (highest).
interactions to design a variety of crystals with desired
physical properties, is an indispensable tool in the devel-
opment of crystallographic engineering, particularly for
the intermolecular interactions, like a rope that connects
the cations and anionic chains. Because of the low rota-
tional energy barrier, the cations are easier to rotate, and
meanwhile the rope would pull on the anionic chains so
that they rotate with the cations. This synergy can reduce
the energy required for polarization switching and allow
the coercive field to become smaller, which may be re-
sponsible for the remarkable ferroelectric behavior in
[(CH3)3NCH2F]ZnCl3. Compared to F atom, the electro-
negativity of Cl, Br and I atoms are relatively small, result-
ing in the weaker intermolecular interactions. Moreover,
their larger steric resistance also increases the rotational
energy barrier of cations, which leads to the larger coer-
cive field that makes polarization switching difficult to
realize.
organic-inorganic
hybrid
crystal
systems.12
In
[(CH3)3NCH2X]ZnCl3 (X = F, Cl, Br, I), the Cl atom in the
[ZnCl3]-n chain is nucleophilic while halogen atoms of
cations are electrophilic, and thus halogen-halogen bond-
ing effect, C−X···Cl−Zn bonds occur (Figures 2 and S4).
The average bond lengths of C−X bonds are 0.970 Å for
[(CH3)4N]+, 1.225 Å for [(CH3)3NCH2F]+, 1.609 Å for
[(CH3)3NCH2Cl]+, 1.9 Å for [(CH3)3NCH2Br]+ and 2.114 Å for
[(CH3)3NCH2I]+ (Figure 2c), which gives that the crystal
cell volume for [(CH3)3NCH2X]ZnCl3 increases gradually
with the increasing atomic radius (Table S1). The shortest
halogen-halogen distances are 3.222, 3.541, 3.486 and 3.575
Å at 293 K for [(CH3)3NCH2X]ZnCl3 (X = F, Cl, Br, I), re-
spectively.
We further confirm the non-centrosymmetric nature of
crystal structures of [(CH3)3NCH2X]ZnCl3 (X = H, F, Cl, Br,
I) by second harmonic generation (SHG) measurements.
As shown in Figure 4a, [(CH3)3NCH2X]ZnCl3 (X = H, F, Cl,
Br, I) polycrystalline samples exhibit obvious SHG re-
sponses with non-zero intensities at room temperature,
which reveals that their crystal symmetries are non-
centrosymmetric, consistent with the polar mm2 point
group. When increasing the temperature, the SHG inten-
sities of [(CH3)3NCH2X]ZnCl3 (X = H, F, Cl, Br, I) keep
stable below the melting point. This indicates that the
polar symmetries of [(CH3)3NCH2X]ZnCl3 (X = H, F, Cl, Br,
I) are able to maintain below the melting point, which
can be attributed to the existence of polar one-
dimensional [ZnCl3]-n chains of corner-sharing ZnCl4 tet-
rahedral in the crystal structure. Because I atom has the
highest atomic number and smallest electronegativity, its
electron cloud is most easily polarized, which might lead
to the strongest SHG intensity in [(CH3)3NCH2I]ZnCl3.
To further analyze the intermolecular interactions, the
Hirshfeld surface and 2D-fingerprint plots of
[(CH3)3NCH2X]+ (X = H, F, Cl, Br, and I) cations are pre-
sented (Figures 3 and S7). The intensity of molecular in-
teraction is mapped onto the Hirshfeld surface by using a
red-blue-white color scheme: where the white regions
exactly correspond to the distance of Van der Waals con-
tact, the blue ones correspond to longer contacts, and the
red ones represent closer contacts. Obviously, the propor-
tion of significant contacts (represented by the large deep
red circular depressions on the surfaces) for
[(CH3)3NCH2X]+ (X = F, Cl, Br, and I) cations are larger
than that for [(CH3)4N]+ cation. Among them, the
[(CH3)3NCH2F]+ cations have the greatest significant con-
tacts, which can be attributed to the larger electronegativ-
ity of F atoms and the strong electron-withdrawing ability,
resulting in the relatively strong interaction between cati-
on and anion. The introduction of F atoms forms the rela-
tively strong F···Cl halogen bonding interaction to change
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polarization was also evaluated by point charge model
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(see Supporting Information for details). The estimated Ps
value of [(CH3)3NCH2F]ZnCl3 (4.47 μC/cm2) is close to the
test value, larger than those of [(CH3)4N]ZnCl3 and small-
er than those of [(CH3)3NCH2X]ZnCl3 (X = Cl, Br, I).
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Figure 5. Domains observed in the as-grown
[(CH3)3NCH2F]ZnCl3 film. Topography (a), lateral PFM am-
plitude (b) and phase (c) images in the same region.
To
further
evidence
the
ferroelectricity
of
[(CH3)3NCH2F]ZnCl3, we performed piezoresponse force
microscopy (PFM) measurements in its thin films. PFM
has become a well-established characterization technique
that is used to map and manipulate the domains by using
the conductive probe.17 The typical PFM imaging consists
of the information about local magnitude of the piezore-
sponse, the so-called PFM amplitude; and the phase shift
between the applied oscillating voltage and sample de-
formation, the so-called PFM phase which indicates the
domain orientation. Figure 5 displays the lateral PFM im-
ages of domains observed in the as-grown
[(CH3)3NCH2F]ZnCl3 thin film, as well as the correspond-
ing topographic image. We note that the domains assem-
ble into a stripe arrangement. The domain walls can be
clearly seen in the amplitude image (Figure 5b), which
separate the adjacent domains. In the phase image (Fig-
ure 5c), the purple and yellow color tones demonstrate
the existence of spontaneous polarization with different
orientations in the (TMFM)ZnCl3 thin films.
Figure 4. (a) Temperature dependent SHG intensity of
[(CH3)3NCH2X]ZnCl3 (X = H, F, Cl, Br, I), respectively. Potas-
sium dihydrogen phosphate (KDP) was used as a reference.
(b) P–E hysteresis loops of [(CH3)3NCH2F]ZnCl3 measured at
323 K by using the double-wave method.
The polar crystal symmetry of [(CH3)3NCH2X]ZnCl3 (X =
H, F, Cl, Br, I) meets the symmetry requirement for ferro-
electrics. We thus carried out the polarization–electric
Another criterion for the verification of ferroelectricity
is that the spontaneous polarization can be switched by
external electric field. Additional DC voltages can be ap-
plied on the PFM tip to manipulate the domains, visualiz-
ing the ferroelectric switching directly. We firstly scanned
an area in the [(CH3)3NCH2F]ZnCl3 thin film, where the
PFM amplitude and phase responses are uniform, show-
ing a single domain state (Figure 6a). Then, we applied
voltage of +80 V on the central zone marked by black
dashed box. Induced by the electric field, the polarization
in the central region was switched, generating a new pur-
ple domain with the phase contrast of approximately 180°
(Figure 6b). The clear domain wall can be discerned in
the corresponding amplitude image, which separate the
new domain from the parent one. Hence, the phase con-
trast arises from the polarization switching, instead of
charge injection after poling. We further switched back
the purple domain by applying an opposite voltage of -70
V over the zone marked as white dashed box. As a result,
partial purple domain switched from purple to yellow
(Figure 6c). Therefore, the polarization in
field
(P–E)
hysteresis
loops
experiments
on
[(CH3)3NCH2X]ZnCl3 (X = H, F, Cl, Br, I) thin films to in-
vestigate their ferroelectricity. By using the double-wave
method, we obtained a standard P–E hysteresis loop for
[(CH3)3NCH2F]ZnCl3 at 323 K (Figure 4b), directly verify-
ing the ferroelectric property of [(CH3)3NCH2F]ZnCl3. Un-
fortunately, although we tried many times, the ferroelec-
tric P–E hysteresis loops of [(CH3)3NCH2X]ZnCl3 (X = H,
Cl, Br, I) were not obtained. Therefore, there are no
enough
effective
evidences
to
prove
that
[(CH3)3NCH2X]ZnCl3 (X = H, Cl, Br, I) are ferroelectrics,
which will be further confirmed by PFM measurements
(see below). The Ps value of [(CH3)3NCH2F]ZnCl3 obtained
from the P–E hysteresis loop is about 4.8 μC/cm2. This
value is a relatively large one among molecular ferroelec-
trics and is greater than those of some ABX3 perovskite
ones, such as the one-dimensional [N(CH3)4]CdBr3 (0.12
μC/cm2)13,
[C4H10N]CdCl3 (3.6 μC/cm2)4c, and the three-dimensional
(3-ammoniopyrrolidinium)RbBr3 (2.3
μC/cm2)15,
[NH2NH3][Mn(HCOO)3] (3.58 and
μC/cm2)16
[NH2NH3][Zn(HCOO)3] (3.48 μC/cm2)16. The ferroelectric
[C5H9NH3]CdCl3
(1.7
μC/cm2)14
and
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Figure 6. Domain switching of [(CH3)3NCH2F]ZnCl3 thin film. Topography (top row), PFM amplitude (middle row) and PFM
phase (bottom row) images for initial state (a), after electric poling with voltage of +80 V (b), and back switching operation with
voltage of -70 V (c).
[(CH3)3NCH2F]ZnCl3 is switchable by external electric field,
providing robust evidence for the existence of ferroelec-
tricity. We performed PFM measurements to image the
domains and investigate the domain swtiching for
[(CH3)3NCH2X]ZnCl3 (X = H, Cl, Br, I) as well. As shown in
Figures S8-S11, the as-grown domains can be found in the
thin film of [(CH3)3NCH2X]ZnCl3 (X = H, Cl, Br, I).
However, we can not detect the domain switching
behavior in these films. Therefore, there are no robust
evidences to demonstrate their ferroelectric properties.
dral in the classical organic-inorganic ABX3 perovskite
ferroelectrics. Considering the tunable compositions and
structures in organic−inorganic hybrids, one can expect
more such kind of hybrid ABX3 ferroelectrics to be dis-
covered with excellent ferroelectricity. This work enriches
the family of organic-inorganic ABX3 ferroelectrics and
provides a fine molecular design strategy to design them.
MATERIALS AND METHODS
Synthesis.
The
crystals
of
[(CH3)4N]ZnCl3,
[(CH3)3NCH2F]ZnCl3,
[(CH3)3NCH2Cl]ZnCl3,
CONCLUSION
[(CH3)3NCH2Br]ZnCl3 and [(CH3)3NCH2I]ZnCl3 were easily
obtained by slow evaporation of the deionized water solu-
tions containing ZnCl2 and corresponding organic chlo-
ride salts in a 1:1 molar ratio. The grown crystals were
about 1  1  5 mm3 in size. The detailed procedures for
preparing these organic chloride salts are described in the
Supporting Information. Thermogravimetric (TG) analy-
sis reveals the thermal stability of these as-grown crystals
(Figure S1).
In summary, by substituting a hydrogen atom of
[(CH3)3N]+ in [(CH3)4N]ZnCl3 with a more electronegative
halogen atom, we have successfully obtained an organic-
inorganic zinc chloride ABX3 type ferroelectric,
[(CH3)3NCH2F]ZnCl3. Compared to [(CH3)4N]ZnCl3,
[(CH3)3NCH2F]ZnCl3 undergoes a structural phase transi-
tion
and
shows
remarkable
ferroelectricity.
[(CH3)3NCH2F]ZnCl3 is a new kind of organic-inorganic
ABX3 type ferroelectric, in which the anionic framework
of [ZnCl3]-n polar chains of corner-sharing ZnCl4 tetrahe-
dral is significantly distinct from the [BX3]-n anionic
framework of corner-sharing or face-sharing BX6 octahe-
Methods. Methods for DSC, single crystal X-ray dif-
fraction, dielectric, SHG, P–E hysteresis loops, and PFM
measurements are described in the Supporting Infor-
mation. For P–E hysteresis loops and PFM measurements,
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solid solution with piezoelectricity stronger than lead zirconate
[(CH3)3NCH2X]ZnCl3 (X = H, F, Cl, Br, I) thin film samples
were prepared by spreading a drop (20 μL) of the precur-
sor solution containing 10 mg as-grown crystals of
[(CH3)3NCH2X]ZnCl3 in 1 mL deionized water on a clean
ITO-coated glass substrate. Slow evaporation of the sol-
vent results in the formation of [(CH3)3NCH2X]ZnCl3 (X =
H, F, Cl, Br, I) thin films.
titanate. Science 2019, 363, 1206. (c) Xu, W. J.; He, C. T.; Ji, C. M.;
Chen, S. L.; Huang, R. K.; Lin, R. B.; Xue, W.; Luo, J. H.; Zhang,
W. X.; Chen, X. M. Molecular Dynamics of Flexible Polar Cations
in a Variable Confined Space: Toward Exceptional Two-Step
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ASSOCIATED CONTENT
Supporting Information.
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Sample preparation, methods, point charge model analyses,
Figures S1–S11 and Tables S1–S3. This material is available free
of charge via the Internet at http://pubs.acs.org. The struc-
tures have been deposited at the Cambridge Crystallographic
Data Centre (deposition numbers: CCDC 1976710-1976716),
and can be obtained free of charge from the CCDC via
www.ccdc.cam.ac.uk/getstructures.
AUTHOR INFORMATION
Corresponding Author
*tangyuanyuan@ncu.edu.cn
Notes
(6) (a) Shi, P.-P.; Tang, Y.-Y.; Li, P.-F.; Liao, W.-Q.; Wang, Z.-X.;
Ye, Q.; Xiong, R.-G. Symmetry breaking in molecular
ferroelectrics. Chem. Soc. Rev. 2016, 45, 3811. (b) Pan, L.; Liu, G.;
Li, H.; Meng, S.; Han, L.; Shang, J.; Chen, B.; Platero-Prats, A. E.;
The authors declare no competing financial interests.
ACKNOWLEDGMENT
This work was supported by the National Natural Science
Foundation of China (21671084, 21975114 and 11904151).
Lu, W.; Zou, X.; Li, R.-W.
A Resistance-Switchable and
Ferroelectric Metal–Organic Framework. J. Am. Chem. Soc. 2014,
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Sato, O. Above Room Temperature Organic Ferroelectrics:
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Wu, Z.; Han, S.; Tao, K.; Hong, M.; Luo, J.; Sun, Z. Two-
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