The Soft Molecular Polycrystalline Ferroelectric Realized by the Fluorination Effect

Article abstract of DOI:10.1021/jacs.0c05372

For a century ferroelectricity has attracted widespread interest from science and industry. Inorganic ferroelectric ceramics have dominated multibillion dollar industries of electronic ceramics, ranging from nonvolatile memories to piezoelectric sonar or ultrasonic transducers, whose polarization can be reoriented in multiple directions so that they can be used in the ceramic and thin-film forms. However, the realization of macroscopic ferroelectricity in the polycrystalline form is challenging for molecular ferroelectrics. In pursuit of low-cost, biocompatible, and mechanically flexible alternatives, the development of multiaxial molecular ferroelectrics is imminent. Here, from quinuclidinium perrhenate, we applied fluorine substitution to successfully design a multiaxial molecular ferroelectric, 3-fluoroquinuclidinium perrhenate ([3-F-Q]ReO4), whose macroscopic ferroelectricity can be realized in both powder compaction and thin-film forms. The fluorination effect not only increases the intrinsic polarization but also reduces the coercive field strength. More importantly, it is also, as far as we know, the softest of all known molecular ferroelectrics, whose low Vickers hardness of 10.5 HV is comparable with that in poly(vinylidene difluoride) (PVDF) but almost 2 orders of magnitude lower than that in BaTiO3. These attributes make it an ideal candidate for flexible and wearable devices and biomechanical applications.

Full text of DOI:10.1021/jacs.0c05372

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Article
The Soft Molecular Polycrystalline
Ferroelectric Realized by Fluorination Effect
Yongfa Xie, Yong Ai, Yu-Ling Zeng, Wen-Hui He, Xue-Qin Huang,
Da-Wei Fu, Ji-Xing Gao, Xiao-Gang Chen, and Yuan-Yuan Tang
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.0c05372 • Publication Date (Web): 22 Jun 2020
Downloaded from pubs.acs.org on June 22, 2020
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The Soft Molecular Polycrystalline Ferroelectric Realized by Fluori-
nation Effect
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Yongfa Xie,† Yong Ai,† Yu-Ling Zeng,† Wen-Hui He,† Xue-Qin Huang,† Da-Wei Fu,*,‡ Ji-Xing
Gao,‡ Xiao-Gang Chen,‡ and Yuan-Yuan Tang*,†
†Ordered Matter Science Research Center, Nanchang University, Nanchang 330031, People’s Republic of China
‡Institute for Science and Applications of Molecular Ferroelectrics, Key Laboratory of the Ministry of Education for
Advanced Catalysis Materials, Zhejiang Normal University, Jinhua 321004, People’s Republic of China
ABSTRACT: For a century ferroelectricity has attracted widespread interest from science and industry. Inorganic ferroelec-
tric ceramics have dominated multibillion dollar industries of electronic ceramics, ranging from nonvolatile memories to
piezoelectric sonar or ultrasonic transducers, whose polarization can be reoriented in multiple directions so that they can
be used in the ceramic and thin film forms. However, the realization of macroscopic ferroelectricity in the polycrystalline
form is challenging for molecular ferroelectrics. In pursuit of low-cost, biocompatible and mechanical flexible alternatives,
the development of multiaxial molecular ferroelectrics is imminent. Here, from quinuclidinium perrhenate, we applied
fluorine substitution to successfully design a multiaxial molecular ferroelectric, 3-fluoro-quinuclidinium perrhenate ([3-F-
Q]ReO4), whose macroscopic ferroelectricity can be realized in both powder compaction and thin-film forms. The fluori-
nation effect not only increases the intrinsic polarization, but also reduces the coercive field strength. More importantly, it
is also, as far as we know, the softest of all known molecular ferroelectrics, whose low Vickers hardness of 10.5 HV is com-
parable with that in polyvinylidene difluoride (PVDF) but almost two orders of magnitude lower than that in BaTiO3. These
attributes make it an ideal candidate for flexible and wearable devices, and biomechanical applications.
growth, worsening the microstructure and thus the
INTRODUCTION
performance for ferroelectric ceramics.4
Ceramics, whose history can be traced back to 27000
years ago, are inorganic, non-metallic materials shaped
and subjected to heat. In addition to the importance as
conventional domestic, building and art products, their
unique electrical, magnetic, and optical properties also
bring forth the booming of advanced ceramics in many key
technologies.1 Particularly, inorganic ferroelectric ceramics
that were born in the early 1940s have been the heart and
soul of several multibillion dollar industries of electronic
ceramics.2 For the usage of such inorganic materials,
however, the main limitations primarily include the
intrinsic brittleness and the toxicity brought by heavy
metal-containing. Their processing also poses some
noticeable challenges. The fabrication of inorganic
ferroelectric ceramics traditionally starts with formulated
oxide powders, which are then hot-pressed or sintered to
the required sizes and shapes following the common
sequence of mixing, drying, calcining, milling, forming,
firing, and assembly.2c The process flow is complicated,
time consuming, energy intensive, as well as necessitates
high cost.3 More than that, the high-temperature sintering
or hot-pressing (even above 1000 C for periods up to ten
hours) will cause not only the loss of volatile components
like Pb, Bi or Li, making it difficult to maintain the
stoichiometric compositions, but also the abnormal grain
Molecular ferroelectrics, with the merits of mechanical
flexibility, lightweight, structural tunability, ease of
processing, biocompatibility, and low acoustical
impedance, are known as promising complementary
materials for the inorganic ferroelectric ceramics.5 The past
decade has witnessed the renew of research interest on
new molecular ferroelectrics.6 Some of them were found to
catch up with the commercial inorganic ones such as
BaTiO3 in terms of the spontaneous polarization (Ps), Curie
temperature (Tc), piezoelectric coefficient (d33) or the
number of equivalent polarization directions. A major
superiority of inorganic ferroelectrics is that they can be
used in polycrystalline form after poling. When a strong
external electric field (10–100 kV cm−1), often in
combination with elevated temperature, is applied to
orient the randomly distributed dipoles or domains, these
polycrystalline ferroelectric materials will be able to
possess both ferroelectric and piezoelectric properties, and
act very similar to a single crystal. The practical use of
inorganic ceramic ferroelectrics has been close to 80 years.
Nevertheless, with respect to molecular ferroelectrics, the
macroscopic
ferroelectricity
in
cold-pressing
polycrystalline form is still in the initial stage of
exploration.7
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Figure 1. Molecular design strategy and crystal packing diagrams in different phases for [3-F-Q]ReO4. (a) Molecular Design
steps for [3-F-Q]ReO4. (b) The crystal structure in the ferroelectric phase of Pmn21 at 93 K. The F atom of the 3-F-
quinuclinium cation is disordered over the six possible positions, giving the S- or R-configuration, respectively. Only one
distribution is illustrated for clarity. (c) The crystal structure in the ferroelectric phase of Amm2 at 298 K. The 3-F-
quinuclinium cation is disordered over two possible positions. The F atom is disordered as the same as in the phase at 93
K. (d) The crystal structure in the ferroelectric phase of R3m at 343 K. The 3-F-quinuclinium cation is disordered around
the 3-fold axis. The molecular 3-fold axis is not superimposed with the crystallographic 3-fold axis. The F atom is disordered,
and not modeled. The [ReO4]- anion is also disordered around the 3-fold axis, and the molecular 3-fold axis coincides with
̅
the crystallographic 3-fold axis. (e) The crystal structure in the paraelectric phase of Pm3m at 393 K. The organic cation is
totally disordered, and modeled as a ball. The [ReO4]- anion is also totally disordered, and modeled as an octahedron.
In contrast to the rigid inorganic systems in which the
basic properties are always modified and improved by
using dopants or other special chemical constituents,
molecular ferroelectrics leave ample room for tuning
functionalities through molecular design. We introduced
an F atom to the organic cation in quinuclidinium
perrhenate ([Q]ReO4) by chemical synthesis strategy
systematically (see Figures 1, S1-4 and experimental part for
details), and then the single crystals of 3-fluoro-
quinuclidinium perrhenate ([3-F-Q]ReO4) were obtained
by slow evaporation of room-temperature solution.7a
Owing to the close van der Waal’s radii and the similar
steric parameters between H and F atoms, this new
compound maintains the polar crystal structures and
ferroelectric property. And, such incorporation of the most
electronegative species also raised the strength of
molecular dipole moment and the dynamic structural
disorder might make the polarization switching easier.
Moreover, [3-F-Q]ReO4 also has low Vickers hardness of
10.5 HV comparable to that in PVDF, allowing that the
powder of [3-F-Q]ReO4 could be easily compressed into
dense compaction with moderate pressure. The all room-
temperature fabrication process not only reduces the cost,
but also avoids any fracture during high-temperature
annealing and broadens the application potential. Since
there is no risk of mechanic crack-down during cooling
process, customized shape would be possible using die-
cast technique. As a result, it is particularly exciting to note
that, single-crystal-level ferroelectric performance was
successfully recorded on a fine-grained compaction pellet
of the microcrystalline powders made by cold-pressing at
room temperature. This discovery points out the bright
future of molecular ferroelectrics in the polycrystalline
form, while the simple room-temperature processing
without sintering or hot-pressing will be a deciding factor
in the design of flexible and wearable electronic devices.
RESULTS AND DISCUSSION
The synthetic strategy was shown in Figure 1a, which
started with quinuclidin-3-one. All three fluorine-
containing products react with perrhenic acid to verify
their potential ferroelectricity. The first obtained fluorine-
containing product is 3,3-difluoroquinuclidine. Its
perrhenate salt crystallizes in centrosymmetric point
group 2/m. Under the influence of fluorination, the
symmetry of [3,3-difluoro-quinuclidinium]ReO4 is lowered
compared with [Q]ReO4 crystal, but the anti-parallel 3,3-
difluoro-quinuclidinium
cations
lead
to
the
centrosymmetric packing of crystal (Figure S1).7a Thus no
ferroelectricity can be expected. The second fluorine-
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Figure 2. Ferroelectric properties of [3-F-Q]ReO4. (a) SEM image of the powder compaction. (b) Temperature dependence of
dielectric constant at different frequencies. The insert shows the PE hysteresis loops obtained on the powder compaction sample
at different temperatures. (c) Linear PE relationship of some typical molecular ferroelectrics with the powder compaction form
under the same electric field. (d) Optical microscope photograph of thin film. (e) PE hysteresis loops measured on the thin film
sample under different temperatures at 1 kHz. (f) Fatigue characteristics of the ferroelectric thin film at room temperature, and
the inset shows the PE relationship after different switching cycles.
̅
containing product is 3-fluoro-1-azabicyclo[2.2.2]oct-2-ene.
The carbon atom at the 3 position is changed from sp3
hybridization to sp2 hybridization. The fluorination effect
also lowers the symmetry of crystal, and it crystallizes in a
polar point group 2. The potential ferroelectricity is
examined by PFM domain mapping and switching
spectroscopy (Figures S2-4), which shows the switchable
polarization with relative large coercive field (about 5
times larger than that of [3-F-Q]ReO4). The large coercive
field may be due to the ordered structure stabilized by
hydrogen bonding effect. Finally, 3-fluoroquinuclidine was
obtained by catalytic hydrogenation from 3-fluoro-1-
azabicyclo[2.2.2]oct-2-ene. Compared with [Q]ReO4
crystal, only one H atom is substituted by one F atom. Such
subtle fluorination effect can introduce a chiral center,
leading to a variety of possible molecular design results. In
this case, the stoichiometric R- and S-3-fluoroquinuclidine
crystallize together with perrhenic acid. The symmetry of
the resultant crystal maintains mm2 point group, which is
the same of [Q]ReO4 crystal. Therefore, the advantages of
fluorination can be summarized as follows: (1) The
inherent CF dipole could enhance the ferroelectric
polarization essentially; (2) It is possible to regulate the
polarization direction or equivalent number of
polarization directions since the crystal symmetry can be
maintained or lowered due to fluorination effect; (3) Chiral
centers could be introduced by fluorination, which can be
used to targeted design of chiral ferroelectrics.
temperature phase (HTP) Pm3m (Figures 1 and S5). Also,
temperature-dependent second harmonic generation
measurement shows the structural symmetry variation
from centrosymmetric paraelectric phase to non-
centrosymmetric ferroelectric phases (Figure S6). The
crystal structure is similar to that of non-fluorine-
substituted specimen,7a which adopts the simple cubic
structure in the paraelectric phase, where the [ReO4]-
anions occupy the vertexes of cubic cell, and the 3-F-
quinuclinium cation occupies the center (Figure 1e). The
structural differences for different phases are mainly in the
orientation states of the isolated cations and anions. The
substitution of an F atom for an H atom does not affect the
structures significantly, and accordingly the crystal
symmetry for three of the four phases is the same as that
for non-fluorine-substituted specimen, whereas a new
phase of Amm2 is observed. The polarization direction of
Pmn21 and Amm2 phases is confined to [110]-direction with
̅
regard to the high-temperature cubic Pm3m phase, which
has 12 equivalent polarization directions. While the
polarization direction of the R3m phase is along the [111]-
direction, which has 8 equivalent directions. For three
crystals, the packing structures are very similar, while the
anti-parallel 3,3-difluoro-quinuclidinium cations lead to
the centrosymmetric structure and the parallel 3-fluoro-1-
azabicyclo[2.2.2]oct-2-ene and 3-F-quinuclidinium cations
result in the polar structure with the ferroelectric
spontaneous polarization (Figure S7).
The [3-F-Q]ReO4 crystal undergoes three phase
transitions at around 285, 319 and 370 K, from the low-
temperature phase (LTP) Pmn21, to the intermediate-
temperature phase (ITP1) Amm2, to the intermediate-
temperature phase (ITP2) R3m, and to the high-
The macroscopic ferroelectricity of [3-F-Q]ReO4 was
first examined by the powder compression form. [3-F-
Q]ReO4 can exhibit impressive mechanical properties,
where the Vickers hardness of 10.5 HV was obtained on
single crystalline sample. We measured the Vickers
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hardness of several representative ferroelectrics with the
electronic applications. Moreover, the remanent
polarization of thin film can be maintained till 10^6 cycles
without any deterioration (Figure 2f), shows excellent
fatigue properties.
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same nanoindenter and compared their values as listed in
Table S1. [3-F-Q]ReO4 possesses the smallest Vickers
hardness among molecular ferroelectrics, which is
comparable with ferroelectric polymer PVDF. Hence, the
powder could be easily compressed into dense compaction
disc with moderate pressure at room temperature. In
Figure 2a, scanning electron microscope (SEM) image
shows that a dense pore-less powder compaction, which is
similar to the ceramic-form of inorganic ferroelectrics.
Then, the polarization vs. electric field (PE) hysteresis
loops of such powder compaction are measured. As shown
in Figure 2b, by applying a 50 Hz AC voltage, well-defined
rectangular PE hysteresis loops can be obtained at
different temperatures, which undoubtedly confirm the
ferroelectricity in the powder compaction form. The
remanent polarization (Pr) is extracted with zero electric
field of Pr = 6.0 μC cm-2 at room temperature. The fact that
the Pr is smaller than the theoretical value is possibly due
to the randomly oriented polycrystalline grains in the
powder sample.8 For the powder compaction form, as
increasing temperature from 298 to 313 K, the Pr decreases
slightly, which is most likely caused by the increase of
thermal fluctuation that disrupts the long-range order of
polarization upon heating. From 313 K to 333 K, the drop of
remanent polarization is more prominent, which can be
attributed to the orthorhombic-to-trigonal phase
transition at 319 K. Moreover, the coercive field (Ec) shows
a decrease with increasing temperature, which should be
attributed to the weakening of the coupling stabilizing
effect between the domains.9 For comparison, the
ferroelectricity of some typical molecular ferroelectrics
(MDABCO-NH4I3,6a DIPAB,6b DIPAC,10 TMCM-CdCl3,6c
[Q]ReO4,7a croconic acid,11 and PVDF) with powder
compaction form are examined under the same condition
at room temperature (Figure 2c). Under the maximum
electric field 65 kV/cm, only linear P−E relationship are
observed, indicating that the dipoles are hard to be
switched in the polycrystalline form. Therein, the relative
large coercive field and mono-axis character might be two
main factors for the unswitchable ferroelectricity in the
powder compaction form.
Using piezoresponse force microscopy (PFM), we
observed the stable and switchable polarization in the [3-
F-Q]ReO4 film, which is the most intrinsic property of
ferroelectrics (Figure 3).12 Because of the presence of stress
imposed by the matrix, the as-grown film surface exhibits
various domain patterns at room temperature in the ITP1
(Figure S8), including the intriguing herringbone
multidomain structure (Figure 3a). As shown in Figure S8b,
the striking 180° contrasts both in the out-of-plane and in-
plane phase images indicate that the polarizations are
antiparallel in two adjacent domains. Meanwhile, in the
amplitude images, the signal is necessarily nulling at
domain walls and nearly equal in adjacent domains,
suggesting they are 180° domain walls. In contrast, Figure
S8c exhibits inconsistent phase contrasts and various
amplitude signals in adjacent domains, where these
domain walls should be non-180° ones. Through
comprehensive PFM studies, we clearly identified the
coexistence of 180° and non-180° ferroelectric domains,
confirming the multiaxial nature in [3-F-Q]ReO4.
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Along with the rapid development of integrated
electronic application, the predominance of molecular
ferroelectrics that can be easily fabricated by low-
temperature and cost-saving solution method thin films
becomes more and more obvious. As seen from Figure 2d,
the as-prepared thin-film of [3-F-Q]ReO4 shows large area
with high coverage. The ferroelectric performance of the
thin film was tested with AC frequency of 1 kHz, in the
temperature range of 293 to 353 K. The obtained PE
hysteresis loops always retain good rectangularity (Figure
2e), which discloses the robust ferroelectricity of the [3-F-
Q]ReO4 thin film in a wide temperature range and thus the
suitability for high-temperature working environment.
The Pr values remain 8 to 5 μC cm-2, close to that of the
theoretical value. Notably, the polarization switching
speed (1 kHz) is fast among molecular ferroelectrics, which
provides [3-F-Q]ReO4 more practical potential in
Figure 3. Microscopic ferroelectric properties of [3-F-Q]ReO4
at room temperature in the ITP1. (a) Out-of-plane PFM phase
image in the film, showing the beautiful herringbone ferroe-
lectric domain structure. (b) Hysteretic dependence of out-of-
plane PFM phase and amplitude with applied DC bias for the
film. (c) Out-of-plane phase image for 0.6 μm thick film with
written box-in-box patterns under reversed DC bias ±12 V. (d)
Out-of-plane phase image of domains written by +15 V bias
pulses of various durations 0.3 s, 0.6 s, 0.9 s and 1.2 s in a 0.6
µm thick film. Scale bars: 2 µm.
To characterize the switching behavior, we performed
local PFM-based hysteresis loop measurements and local
polarization manipulation experiments in the [3-F-Q]ReO4
films with different thicknesses (Figures 3 and S9-11). The
characteristic hysteresis and butterfly loops constitute a
proof of the switching of ferroelectric domains. By
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averaging the minima of the amplitude hysteresis loops, we strategy to enhance the ferroelectric polarization
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can estimate that local coercive voltages (Vc) of 1.5, 1 and
0.6 μm thick films are 15.5, 9 and 6.6 V, respectively
(Figures S9a-c). Such low operating voltages will be
desirable for memory devices. Figures S9d-f demonstrate
the out-of-plane phase images of 1.5, 1 and 0.6 μm thick
films after writing box-in-box patterns with reversed DC
bias in the centre. The large phase contrast (~180°) can be
observed, suggesting that opposite polarizations have been
written in the [3-F-Q]ReO4 film. The switching voltage in a
film with thickness about 1.5 µm is only 20 V, much smaller
than 100 V in the [3-F-Q]ReO4 film with the same
thickness.13 Figures 3d and S11 present the poling map
acquired by applying a constant DC bias of +15 V to the tip
with a pulse duration ranging from 0.3 to 1.2 s for a 0.6 µm
thick film, where the written domains vary markedly with
the change of time. All the above evidence establishes the
presence of ferroelectricity in the [3-F-Q]ReO4 film.
performance.
On the other hand, the F-quinuclidine molecules have
experienced different dynamic states with increasing
temperature. Intermolecular interactions between F-
quinuclidine molecules and surrounding environments in
this process were quantified using Hirshfeld surfaces
(Figures 4b, 4d, and S12).14 The color coding in Hirshfeld
surfaces (dnorm mode) shows the normalized contact
distance based upon the location and magnitudes of
intermolecular interactions. Where atoms make
intermolecular contacts closer than the sum of their van
der Waals radii, these contacts will be highlighted in red
on the dnorm surface. Longer contacts are blue, and contacts
around the sum of van der Waals radii are white. At 223 K,
large red areas in the Hirshfeld surface are observed from
the CF peripheries, indicating strong intermolecular
interactions. Below 223 K, there is no distinct variation of
the F-quinuclidine dynamic behavior, so the Hirshfeld
surfaces and corresponding fingerprint plots only change
slightly (Figure S12). On the other side, the F-quinuclidine
structure is disordered along 2-fold rotation axis at 298 K.
Consequently, the red areas in the Hirshfeld surface
transferred to the side parts (Figure 4d), which means the
2-fold disorder dominates the inter-molecular interactions
instead of elongate CF group. At the same time, the
decoded two-dimensional (2D) fingerprint plots shifting to
the longer contact direction also indicates the relaxed
inter-molecular interactions and then probably lower the
coercive field for the ferroelectric switching (Figures 4c
and 4e). Similarly, the short contact red areas shrink and
move to the end of the body diagonal ends since the F-
quinuclidine displays 3-fold disorder at 434 K (Figures S12i-
j), and finally disappear at the high-degree disordered
cubic states (Figures S12k-l). In the process of changing
with temperature, the globularity (is a measure of the
degree to which the surface area differs from the value for
a sphere of the same volume) of the F-quinuclidine
increases monotonically, also reflects its dynamic behavior
(Figure S13). We also compared the intermolecular
contacts of [3-F-Q]ReO4 with those of [3,3-difluoro-
quinuclidinium]ReO4 and [3-fluoro-1-azabicyclo[2.2.2]oct-
2-en-1-ium]ReO4 by the 2D fingerprint plots of cations. As
shown in Figures S12 and S14, the short contacts between
cation and its surroundings with minimum (di, de) of (0.703,
1.060) in [3,3-difluoro-quinuclidinium]ReO4 are much
stronger than those of (0.895, 1.182) in [3-fluoro-1-
azabicyclo[2.2.2]oct-2-en-1-ium]ReO4 and (0.899, 1.158) in
[3-F-Q]ReO4. This may be one reason why its phase
transition temperature (388 K) is relatively higher than
theirs (366 K and 370 K).
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Figure 4. Ferroelectric polarization characteristics of [3-F-
Q]ReO4. (a) Schematic drawing of polarization along crystal-
lographic c-axis originating from ionic displacement (Pion) and
CF dipoles (magenta arrows). Hirshfeld surfaces (b, d) and
2D fingerprint plots (c, e) of F-quinuclidine molecule at differ-
ent temperatures.
The ferroelectric polarization in [3-F-Q]ReO4 crystal is
firstly evaluated by point charge model analysis (Tables S2
and S3). The obvious displacement between positive
charged [3-F-Q]+ and negative [ReO4]- gives a major source
of polarization about 8.30 μC/cm2 and 8.52 μC/cm2 along c
axis at 223 K and 298 K, respectively, which are obviously
larger than [Q]ReO4 crystal (7.58 μC/cm2).13 Secondly, to
investigate the CF dipole on the ferroelectric polarization
in [3-F-Q]ReO4 crystal, DFT calculations on the dipole
moment are carried out. When the 3-H atom was
substituted by F atom, the large dipole moment of CF
bond (2.092.34 D) would have a huge impact on the
polarization value. Considering the direction of the CF
bond in [3-F-Q]ReO4 crystal (Figure 4a), the effective
contribution from polar CF bond to the ferroelectric
polarization along crystallographic c axis is about 1.0
μC/cm2. Therefore, through F atom substitution, both the
ionic displacement and CF dipole can enhance the total
polarization of [3-F-Q]ReO4 crystal, which can be
estimated to be 7.6 μC/cm2. Compared with [Q]ReO4
crystal, the polarization value increases 30% (1.72 μC/cm2)
due to the F-substitution effect. From this point of view,
the well-designed F atom substitution is an effective
CONCLUSION
Through elaborate fluorine atom substitution, we have
found excellent ferroelectricity in a molecular ferroelectric
of 3-fluoro-quinuclidinium perrhenate, which displays
macroscopic ferroelectricity in both powder compaction
and thin-film forms. On the one hand, the fluorination
effect not only enhance the intrinsic ferroelectric
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Deng, J.; Yu, R.; Scott, J. F.; Xing, X. Giant polarization in super-
tetragonal thin films through interphase strain. Science 2018, 361,
494.
(3) Ramadan, K. S.; Sameoto, D.; Evoy, S. A review of piezoelec-
tric polymers as functional materials for electromechanical trans-
ducers. Smart Mater. Struct. 2014, 23, 33001.
(4) Kong, L. B.; Zhang, T. S.; Ma, J.; Boey, F. Progress in synthe-
sis of ferroelectric ceramic materials via high-energy mechano-
chemical technique. Prog. Mater Sci. 2008, 53, 207.
polarization without changing the symmetry of the point
group of the crystal, but also lower the coercive field. On
the other hand, unlike BaTiO3 and other ferroelectric
ceramics which need long-time high-temperature
sintering, the powder compaction of [3-F-Q]ReO4 is
fabricated by simple cold-pressing method at room
temperature due to the smallest Vickers hardness among
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molecular
ferroelectrics.
The
room-temperature
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(5) (a) Li, W.; Wang, Z.; Deschler, F.; Gao, S.; Friend, R. H.;
Cheetham, A. K. Chemically diverse and multifunctional hybrid
organic–inorganic perovskites. Nat. Rev. Mater. 2017, 2, 16099. (b)
Xu, W.-J.; Li, P.-F.; Tang, Y.-Y.; Zhang, W.-X.; Xiong, R.-G.; Chen,
X.-M. A Molecular Perovskite with Switchable Coordination
Bonds for High-Temperature Multiaxial Ferroelectrics. J. Am.
Chem. Soc. 2017, 139, 6369. (c) Zhang, Z.; Li, P.-F.; Tang, Y.-Y.; Wil-
son, A. J.; Willets, K.; Wuttig, M.; Xiong, R.-G.; Ren, S. Tunable
electroresistance and electro-optic effects of transparent molecu-
lar ferroelectrics. Sci. Adv. 2017, 3, e1701008. (d) Gao, W.; Zhang,
Z.; Li, P. F.; Tang, Y. Y.; Xiong, R. G.; Yuan, G.; Ren, S. Chiral Mo-
lecular Ferroelectrics with Polarized Optical Effect and Electrore-
sistive Switching. ACS Nano 2017, 11, 11739. (e) Li, D.; Zhao, X.-M.;
Zhao, H.-X.; Dong, X.-W.; Long, L.-S.; Zheng, L.-S. Construction
of Magnetoelectric Composites with a Large Room-Temperature
Magnetoelectric Response through Molecular–Ionic Ferroelec-
trics. Adv. Mater. 2018, 30, 1803716. (f) Zhang, H.-Y.; Song, X.-J.;
Chen, X.-G.; Zhang, Z.-X.; You, Y.-M.; Tang, Y.-Y.; Xiong, R.-G.
Observation of Vortex Domains in a Two-Dimensional Lead Io-
dide Perovskite Ferroelectric. J. Am. Chem. Soc. 2020, 142, 4925.
(6) (a) Ye, H. Y.; Tang, Y. Y.; Li, P. F.; Liao, W. Q.; Gao, J. X.;
Hua, X. N.; Cai, H.; Shi, P. P.; You, Y. M.; Xiong, R. G. Metal-free
three-dimensional perovskite ferroelectrics. Science 2018, 361, 151.
(b) Fu, D. W.; Cai, H. L.; Liu, Y. M.; Ye, Q.; Zhang, W.; Zhang, Y.;
Chen, X. Y.; Giovannetti, G.; Capone, M.; Li, J. Y.; Xiong, R. G.
Diisopropylammonium Bromide Is a High-Temperature Molecu-
lar Ferroelectric Crystal. Science 2013, 339, 425. (c) You, Y. M.; Liao,
W. Q.; Zhao, D. W.; Ye, H. Y.; Zhang, Y.; Zhou, Q. H.; Niu, X. H.;
Wang, J. L.; Li, P. F.; Fu, D. W.; Wang, Z. M.; Gao, S.; Yang, K. L.;
Liu, J. M.; Li, J. Y.; Yan, Y. F.; Xiong, R. G. An organic-inorganic
perovskite ferroelectric with large piezoelectric response. Science
2017, 357, 306. (d) Liao, W. Q.; Zhao, D. W.; Tang, Y. Y.; Zhang, Y.;
Li, P. F.; Shi, P. P.; Chen, X. G.; You, Y. M.; Xiong, R. G. A molecular
perovskite solid solution with piezoelectricity stronger than lead
zirconate titanate. Science 2019, 363, 1206. (e) Anetai, H.; Takeda,
T.; Hoshino, N.; Kobayashi, H.; Saito, N.; Shigeno, M.; Yamaguchi,
M.; Akutagawa, T. Ferroelectric Alkylamide-Substituted Helicene
Derivative with Two-Dimensional Hydrogen-Bonding Lamellar
Phase. J. Am. Chem. Soc. 2019, 141, 2391. (f) Li, P.-F.; Liao, W.-Q.;
Tang, Y.-Y.; Qiao, W.; Zhao, D.; Ai, Y.; Yao, Y.-F.; Xiong, R.-G. Or-
ganic enantiomeric high-Tc ferroelectrics. Proc. Natl. Acad. Sci. U.
S. A. 2019, 116, 5878. (g) Han, S.; Liu, X.; Liu, Y.; Xu, Z.; Li, Y.; Hong,
M.; Luo, J.; Sun, Z. High-Temperature Antiferroelectric of Lead
Iodide Hybrid Perovskites. J. Am. Chem. Soc. 2019, 141, 12470. (h)
Li, L.; Liu, X.; Li, Y.; Xu, Z.; Wu, Z.; Han, S.; Tao, K.; Hong, M.; Luo,
J.; Sun, Z. Two-Dimensional Hybrid Perovskite-Type Ferroelectric
for Highly Polarization-Sensitive Shortwave Photodetection. J.
Am. Chem. Soc. 2019, 141, 2623. (i) Tang, Y.-Y.; Ai, Y.; Liao, W.-Q.;
Li, P.-F.; Wang, Z.-X.; Xiong, R.-G. H/F-Substitution-Induced Ho-
mochirality for Designing High-Tc Molecular Perovskite Ferroe-
lectrics. Adv. Mater. 2019, 31, 1902163. (j) Yao, Z. S.; Yamamoto, K.;
Cai, H. L.; Takahashi, K.; Sato, O. Above Room Temperature Or-
ganic Ferroelectrics: Diprotonated 1,4-Diazabicyclo[2.2.2]octane
Shifts between Two 2-Chlorobenzoates. J. Am. Chem. Soc. 2016,
138, 12005. (k) Chen, L.; Liao, W.-Q.; Ai, Y.; Li, J.; Deng, S.; Hou, Y.;
Tang, Y.-Y. Precise Molecular Design Toward Organic–Inorganic
Zinc Chloride ABX3 Ferroelectrics. J. Am. Chem. Soc. 2020, 142,
6236.
fabrication not only reduces cost, but also avoids any
fracture during high-temperature annealing and broadens
the application potential. Molecular ferroelectrics can be
used in the cold-pressing polycrystalline form like
inorganic ferroelectrics, which opens up a new path for the
application of ‘molecular polycrystalline ferroelectrics’.
Furthermore, the thin-film of [3-F-Q]ReO4 can be simply
prepared by aqueous method, which has potential
application in soft piezoelectrics, thin-film memory,
flexible electronics, etc.
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ASSOCIATED CONTENT
Supporting Information.
Sample preparation, X-ray crystallographic, differential scan-
ning calorimetry, dielectric, SHG, PFM characterizations,
computational modelling, Figures S1–S14, and Tables S1–S3.
This material is available free of charge via the Internet at
http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*dawei@zjnu.edu.cn; tangyuanyuan@ncu.edu.cn
Notes
The authors declare no competing financial interests.
ACKNOWLEDGMENT
This work was financially supported by the National Natural
Science Foundation of China (21991141, 21975114, and 11904151),
the Natural Science Foundation of Zhejiang Province
(LZ20B010001), and starting grants from Zhejiang Normal
University.
REFERENCES
(1) (a) Saito, Y.; Takao, H.; Tani, T.; Nonoyama, T.; Takatori, K.;
Homma, T.; Nagaya, T.; Nakamura, M. Lead-free piezoceramics.
Nature 2004, 432, 84. (b) Pullar, R. C. Hexagonal ferrites: A review
of the synthesis, properties and applications of hexaferrite ceram-
ics. Prog. Mater. Sci. 2012, 57, 1191. (c) Minh, N. Q. Ceramic Fuel
Cells. J. Am. Ceram. Soc. 1993, 76, 563.
(2) (a) Scott, J. F. Applications of modern ferroelectrics. Science
2007, 315, 954. (b) Scott, J. F.; Araujo, C. P. d. Ferroelectric mem-
ories. Science 1989, 246, 1400. (c) Haertling, G. H. Ferroelectric ce-
ramics: history and technology. J. Am. Ceram. Soc. 1999, 82, 797.
(d) Li, F.; Cabral, M. J.; Xu, B.; Cheng, Z.; Dickey, E. C.; LeBeau, J.
M.; Wang, J.; Luo, J.; Taylor, S.; Hackenberger, W.; Bellaiche, L.;
Xu, Z.; Chen, L.-Q.; Shrout, T. R.; Zhang, S. Giant piezoelectricity
of Sm-doped Pb(Mg1/3Nb2/3)O3-PbTiO3 single crystals. Science
2019, 364, 264. (e) Qiu, C.; Wang, B.; Zhang, N.; Zhang, S.; Liu, J.;
Walker, D.; Wang, Y.; Tian, H.; Shrout, T. R.; Xu, Z. Transparent
ferroelectric crystals with ultrahigh piezoelectricity. Nature 2020,
577, 350. (f) Zhang, L.; Chen, J.; Fan, L.; Diéguez, O.; Cao, J.; Pan,
Z.; Wang, Y.; Wang, J.; Kim, M.; Deng, S.; Wang, J.; Wang, H.;
ACS Paragon Plus Environment

Page 7 of 8
Journal of the American Chemical Society
(7) (a) Harada, J.; Shimojo, T.; Oyamaguchi, H.; Hasegawa, H.;
ferroelectricity in a single-component molecular crystal. Nature
2010, 463, 789.
Takahashi, Y.; Satomi, K.; Suzuki, Y.; Kawamata, J.; Inabe, T. Di-
rectionally tunable and mechanically deformable ferroelectric
crystals from rotating polar globular ionic molecules. Nat. Chem.
2016, 8, 946. (b) Harada, J.; Yoneyama, N.; Yokokura, S.;
Takahashi, Y.; Miura, A.; Kitamura, N.; Inabe, T. Ferroelectricity
and Piezoelectricity in Free-Standing Polycrystalline Films of
Plastic Crystals. J. Am. Chem. Soc. 2018, 140, 346. (c) Tang, Y. Y.;
Li, P. F.; Liao, W. Q.; Shi, P. P.; You, Y. M.; Xiong, R. G. Multiaxial
Molecular Ferroelectric Thin Films Bring Light to Practical Appli-
cations. J. Am. Chem. Soc. 2018, 140, 8051.
(8) Damjanovic, D. Ferroelectric, dielectric and piezoelectric
properties of ferroelectric thin films and ceramics. Rep. Prog. Phys.
1998, 61, 1267.
(9) Jin, L.; Li, F.; Zhang, S.; Green, D. J. Decoding the Finger-
print of Ferroelectric Loops: Comprehension of the Material Prop-
erties and Structures. J. Am. Ceram. Soc. 2014, 97, 1.
1
2
3
4
5
6
7
8
(12) (a) Garcia, V.; Fusil, S.; Bouzehouane, K.; Enouz-Vedrenne,
S.; Mathur, N. D.; Barthelemy, A.; Bibes, M. Giant tunnel electro-
resistance for non-destructive readout of ferroelectric states. Na-
ture 2009, 460, 81. (b) Lee, D.; Lu, H.; Gu, Y.; Choi, S.-Y.; Li, S.-D.;
Ryu, S.; Paudel, T. R.; Song, K.; Mikheev, E.; Lee, S.; Stemmer, S.;
Tenne, D. A.; Oh, S. H.; Tsymbal, E. Y.; Wu, X.; Chen, L.-Q.;
Gruverman, A.; Eom, C. B. Emergence of room-temperature fer-
roelectricity at reduced dimensions. Science 2015, 349, 1314.
(13) Tang, Y.-Y.; Li, P.-F.; Shi, P.-P.; Zhang, W.-Y.; Wang, Z.-X.;
You, Y.-M.; Ye, H.-Y.; Nakamura, T.; Xiong, R.-G. Visualization of
Room-Temperature Ferroelectricity and Polarization Rotation in
the Thin Film of Quinuclidinium Perrhenate. Phys. Rev. Lett. 2017,
119, 207602.
(14) (a) McKinnon, J. J.; Jayatilaka, D.; Spackman, M. A. To-
wards quantitative analysis of intermolecular interactions with
Hirshfeld surfaces. Chem. Commun. 2007, 37, 3814. (b) McKinnon,
J. J.; Mitchell, A. S.; Spackman, M. A. Hirshfeld surfaces: A new
tool for visualising and exploring molecular crystals. Chem. - Eur.
J. 1998, 4, 2136.
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
(10) Fu, D. W.; Zhang, W.; Cai, H. L.; Ge, J. Z.; Zhang, Y.; Xiong,
R. G. Diisopropylammonium Chloride: A Ferroelectric Organic
Salt with a High Phase Transition Temperature and Practical Uti-
lization Level of Spontaneous Polarization. Adv. Mater. 2011, 23,
5658.
(11) Horiuchi, S.; Tokunaga, Y.; Giovannetti, G.; Picozzi, S.; Itoh,
H.; Shimano, R.; Kumai, R.; Tokura, Y. Above-room-temperature
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