Rational Design of Ceramic-Like Molecular Ferroelectric by Quasi-Spherical Theory

Article abstract of DOI:10.1021/jacs.9b11665

Molecular ferroelectrics are attracting tremendous interest because of their easy and environmentally friendly processing, light weight, low acoustical impedance, and mechanical flexibility, which are viable alternatives or supplements to conventional ceramic ferroelectrics. However, reports of ceramic-like molecular ferroelectrics that can be applied in the polycrystalline form have been scarce. Here, according to the "quasi-spherical theory", we successfully synthesized a ceramic-like molecular ferroelectric with an m3mFmm2 type phase transition at 357 K, 1,5-diazabicyclo[3.2.1]octonium tetrafluoroborate ([3.2.1-dabco]BF₄), which can show excellent ferroelectric performance in the polycrystalline thin-film form at room temperature. On the basis of the reported molecular ferroelectric [2.2.2-dabco]BF₄ (2.2.2-dabco = 1,4-diazabicyclo[2.2.2]octonium) with an Aizu notation of 4/mmmFmm2 and two polar axes, we changed the [2.2.2-dabco]⁺ cation to the [3.2.1-dabco]⁺ cation to reduce the molecular symmetry and keep the quasi-spherical shape simultaneously, making the number of polar axes up to six. Moreover, the spontaneous polarization P_s gets successfully increased from 4.9 μC cm^-2 in [2.2.2-dabco]BF₄ to 5.5 μC cm^-2 in [3.2.1-dabco]BF₄. This precise molecular design strategy offers an efficient pathway to design ceramic-like molecular ferroelectrics.

Full text of DOI:10.1021/jacs.9b11665

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Article
Rational Design of Ceramic-Like Molecular
Ferroelectric by Quasi-Spherical Theory
Zhen-hong Wei, Zhen-Tao Jiang, Xiu-Xiu Zhang, Ming-Li Li, Yuan-
Yuan Tang, Xiao-Gang Chen, Hu Cai, and Ren-Gen Xiong
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.9b11665 • Publication Date (Web): 08 Jan 2020
Downloaded from pubs.acs.org on January 8, 2020
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Rational Design of Ceramic-Like Molecular Ferroelectric by Quasi-
Spherical Theory
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Zhen-Hong Wei,† Zhen-Tao Jiang,† Xiu-Xiu Zhang,† Ming-Li Li,† Yuan-Yuan Tang,† Xiao-Gang
,‡
Chen,‡ Hu Cai,*,† and Ren-Gen Xiong*,†
†Ordered Matter Science Research Center, Nanchang University, Nanchang 330031, People’s Republic of China
‡Jiangsu Key Laboratory for Science and Applications of Molecular Ferroelectrics, Southeast University, Nanjing
211189, People’s Republic of China
Supporting Information
ABSTRACT: Molecular ferroelectrics are attracting tremendous interest because of their easy and environmentally friend-
ly processing, light weight, low acoustical impedance and mechanical flexibility, which are viable alternatives or supple-
ments to conventional ceramic ferroelectrics. However, reports of ceramic-like molecular ferroelectrics that can be ap-
plied in the polycrystalline form have been scarce. Here, according to the “quasi-spherical theory”, we successfully synthe-
̅
sized
a ceramic-like molecular ferroelectric with an m 3 mFmm2 type phase transition at 357 K, 1, 5-
diazabicyclo[3.2.1]octonium tetrafluoroborate ([3.2.1-dabco]BF4), which can show excellent ferroelectric performance in
the polycrystalline thin-film form at room temperature. Based on the reported molecular ferroelectric [2.2.2-dabco]BF4
(2.2.2-dabco = 1,4-diazabicyclo[2.2.2]octonium) with an Aizu notation of 4/mmmFmm2 and two polar axes, we changed
the [2.2.2-dabco]+ cation to the [3.2.1-dabco]+ cation for reducing the molecular symmetry and keeping the quasi-spherical
shape simultaneously, making the polar axes up to six. Moreover, the spontaneous polarization Ps gets successfully in-
creased from 4.9 μC·cm-2 in [2.2.2-dabco]BF4 to 5.5 μC·cm-2 in [3.2.1-dabco]BF4. This precise molecular design strategy of-
fers an efficient pathway to design the ceramic-like molecular ferroelectrics.
lecular ferroelectrics gradually become one of the focus of
scientists’ research.3 In the recent decade, molecular fer-
roelectrics have achieved many striking advances, such as
large Ps up to 22 μC/cm2 in Mdabco-NH4I3 (Mdabco is N-
methyl-N'-diazabicyclo[2.2.2]octonium) and diisoprop-
ylammonium bromide, high Tc up to 499.6 K in [2.2.2-
INTRODUCTION
Ferroelectric materials, as a special kind of polar mate-
rials, the spontaneous polarization of which can be
switched by an applied electric field, have a wide range of
industrial and commercial applications such as ferroelec-
tric random access memory (FeRAM), piezoelectric sonar
or ultrasonic transducers, sensors, and optoelectronic
devices.1 During almost 100-years development of ferroe-
lectric, a large number of diverse ferroelectric materials
have emerged, including inorganic oxides, molecular crys-
tals, liquid crystals, and polymers.2 Most of the state-of-
the-art ferroelectric and piezoelectric materials are based
on inorganic ceramics and polymers, such as BaTiO3,
Pb(Zr,Ti)O3 (PZT), Pb(Mg1/3Nb2/3)O3-PbTiO3 (PMN-PT)
and polyvinylidene fluoride (PVDF).2 Ferroelectric ceram-
ics have been widely applied because of their superior
performance, i.e., high spontaneous polarization (Ps),
high Curie temperature (Tc) and multiple polar axes,
while the high sintering temperature, high cost, toxicity
and mechanical rigidity have hindered more widespread
applications. For ferroelectric polymers, they are soft and
flexible that can be easily tailored to meet the require-
ments for next-generation flexible and wearable devices,
whereas the low crystallinity, large coercive field, and
mono polar axis severely limit large-scale integration.
Because of low weight, low cost, environmental friendli-
ness, low acoustical impedance, and easy processing, mo-
dabco]ReO4
(2.2.2-dabco
=
1,4-
diazabicyclo[2.2.2]octonium), low coercive voltage (Vc)
down to ∼4.2 V in [2.2.2-dabco]BF4.4 If molecular ferroe-
lectrics can be applied in the polycrystalline ceramic-like
form, they become promising and viable alternatives or
supplements to inorganic and polymer ones. However,
ceramic-like molecular ferroelectrics have rarely been
reported.
Here, according to the “quasi-spherical theory”, a phe-
nomenological theory based on the Curie symmetry prin-
ciple, we successfully designed a ceramic-like molecular
ferroelectric, 1,5-diazabicyclo[3.2.1]octonium tetrafluorob-
orate ([3.2.1-dabco]BF4), which exhibits excellent ferroe-
lectric performance in the polycrystalline thin-film form
with small Vc and large Ps.5 Based on the reported mo-
lecular ferroelectric [2.2.2-dabco]BF4, which undergoes a
ferroelectric phase transition with an Aizu notation of
4/mmmFmm2 and two polar axes, we carefully modulated
the [2.2.2-dabco]+ cation to its structural isomer, [3.2.1-
dabco]+ cation, for reducing the molecular symmetry.4d As
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Scheme 1. (a) Design strategy of thin-film-form ceramic-like molecular ferroelectric, [3.2.1-dabco]BF4. (b) Schematic illustration
of [3.2.1-dabco]BF4 in the plastic crystal phase (HTP) with cubic symmetry structure.
embodying symmetry elements such as a 3-fold rotation
a highly symmetric diamine with a globular shape, 1,4-
axis and three σv mirror planes. For the isomer [3.2.1-
diazabicyclo[2.2.2]octane is desirable for constructing
ferroelectrics through order-disorder conformational
transformations. However, its high molecular symmetry
means that the paraelectric phase with centrosymmetry is
dabco]+, it can be clearly found that the breaking of three
σv mirror planes and 3-fold rotation axis result in the low-
er symmetry of [3.2.1-dabco]+ cation with a quasi-
spherical geometry, which induces the formation of wavy
easy to achieve for the corresponding crystal. When re-
NHN hydrogen bonds and bears a dipole moment
along the direction of NHN hydrogen bonds, i.e., [001]
direction in the ferroelectric phase unit cell. Simultane-
ously, it was noteworthy that NHN hydrogen bond
ducing the molecular symmetry and keeping the quasi-
spherical shape, the ferroelectric phase transition would
also exist and the symmetry of paraelectric phase would
increase. As expected, [3.2.1-dabco]BF4 undergoes an
̅
interactions of [3.2.1-dabco]BF4 (1.932(4) Å and 2.049(4) Å )
are relatively weak compared with those of [2.2.2-
dabco]BF4 (1.864(3) Å ) (Table S2).
m3mFmm2 type ferroelectric phase transition at 357 K,
which has a multiaxial characteristic with six equivalent
polar axes. Moreover, the Ps value gets successfully in-
creased from 4.9 μC·cm-2 in [2.2.2-dabco]BF4 to 5.5 μC·cm-
2
in [3.2.1-dabco]BF4, conducive to the data storage in the
ceramic-like form.4d This finding opens up a new strategy
for designing the ceramic-like molecular ferroelectrics,
which will promote the practical applications in wearable
devices, flexible materials, biomachines, and so on.
RESULTS AND DISCUSSION
[3.2.1-dabco]BF4 was found to show a phase transition
at 355 K by the differential scanning calorimetry and die-
lectric analyses (see below), similar to its isomer [2.2.2-
dabco]BF4.6 Thus, crystal structures of [3.2.1-dabco]BF4
below (low-temperature phase, LTP) and above (high-
temperature phase, HTP) Tc were determined by single-
crystal X-ray diffraction to investigate the phase transi-
tion mechanism. The crystal data for [3.2.1-dabco]BF4 at
173 K, 223 K, 293 K and 373 K are summarized in Table S1.
At 173 K (LTP), [3.2.1-dabco]BF4 crystallizes in the ortho-
rhombic crystal system (a = 13.1375(9) Å , b = 7.6331(5) Å
and c = 17.7668(11) Å ) with the non-centrosymmetric po-
lar space group Pca21. As shown in Figure 1a, the crystal
structure of [3.2.1-dabco]BF4 consists of monopronated
[3.2.1-dabco]+ cations and BF4 anions. Different from the
-
Figure 1. (a) Crystal structure in the LTP at 173 K of [3.2.1-
dabco]BF4 (outlined in blue solid lines representing the fer-
roelectric crystal cell and outlined in red solid lines repre-
senting the paraelectric crystal cell). (b, c) The twisted cage
in the ferroelectric phase (red arrow, [001]-direction repre-
senting the polarization direction of [3.2.1-dabco]BF4 in the
ferroelectric crystal cell) and the cubic cage in the paraelec-
ideal linear NHN hydrogen-bonded chains observed in
[2.2.2-dabco]BF4 and other [2.2.2-dabco] salts, the [3.2.1-
dabco]+ cations form a one-dimensional wavy chain along
c-axis by the head-to-tail hydrogen-bonding interactions
(Figures 1a and S1).6-7 As shown in Figure S1, [2.2.2-dabco]+
is a high-symmetric molecule with spherical geometry,
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tric phase (arrows in the cage representing the twelve possi-
crystal-to-plastic transition, which holds a great potential
for refrigeration technologies based on solid-state caloric
effects (Figure S4).9 Based on the formula ΔS = RlnN, (R
and N, the gas constant and the ratio of the numbers of
respective geometrically distinguishable orientations,
respectively), for [3.2.1-dabco]BF4, one can obtain the fit-
ted N = 28.29, which presents a direct evidence for the
ordered-disordered phase transition.10 Meanwhile, the
temperature-dependent 휀′ (the real part of the dielectric
permittivity) at 1 MHz of [3.2.1-dabco]BF4 shows an obvi-
ously step-like dielectric anomaly, further manifesting the
first-order phase transition at around 357 K (Figure 2b).
Besides, the peak values of the maximum 휀' of the nota-
ble anomaly are above 4 times larger than those in the
peak stable states, indicating a proper ferroelectric-to-
paraelectric transition.
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ble equivalent polarization directions of [3.2.1-dabco]BF4 in
the paraelectric crystal cell). Hydrogen atoms bonded to C
atoms were omitted for clarity.
In order to explore the ferroelectric origin, we have
tried to determine the crystal structure of paraelectric
phase (HTP). Unfortunately, their single-crystal X-ray
diffractions show few or even indistinguishable peaks
with relatively weak intensity, especially for those in the
relatively high-angle region. From these poor diffractions
in the HTP, only the cubic unit cell parameters (a = 6.16(8)
Å , V = 234.3(5) Å 3) were obtained, while the detailed
structure could not be known (Figure S2). Therefore, var-
ious-temperature powder X-ray diffraction (PXRD) meas-
urements for [3.2.1-dabco]BF4 were performed at 293 K
(LTP) and 373 K (HTP), respectively. The pattern of PXRD
at 293 K matches well with the simulated pattern from
single-crystal structure, revealing the good crystallinity
and purity of the phase (Figure S3a). The pattern in the
HTP shows obvious changes in the number of diffraction
peaks. The diffraction characteristic reminds us the plas-
tic phase, which is very similar to that in recently discov-
ered plastic phase transition materials with pseudo-
spherical structure, such as [quinuclidinium][X] (X =
ReO4, ClO4 and PF6), and [2.2.2-dabco]ReO4.3a,4c Im-
portantly, the indexing of PXRD patterns in the HTP re-
veals cubic lattice with a = 6.16 Å , which coincides with
crystal cell parameters predicted by single-crystal X-ray
diffractions. The Pawley refinements speculate a few pos-
sible cubic space groups, among which, the most possible
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̅
space group is Pm3m. Therefore, one can assume that the
paraelectric phase of [3.2.1-dabco]BF4 possesses the simi-
lar cubic structure like [3-quinuclidinone]ClO4.8 Then, the
paraelectric phase structure of [3.2.1-dabco]BF4 can be
deduced from the ferroelectric phase. Firstly, one can
easily find a twisted cage from the ferroelectric phase,
Figure 2. Ferroelectricity and related properties of [3.2.1-
dabco]BF4. (a) DSC curves in a heating-cooling run, showing
a high Tc of 357 K. (b) Temperature dependent 휀' (the real
part of complex dielectric permittivity) at 1 MHz. (c) The
temperature dependence of the SHG signal. (d) P–E hystere-
sis loop performed using double-wave method at 293 K.
-
where BF4 anions occupy the vertexes, while [3.2.1-
dabco]+ cations occupy the cavities surrounded by eight
anions, respectively (Figure 1b). Then, it is not difficult to
imagine that the distorted cage will be transformed into a
cubic cage during the ferroelectric-paraelectric phase
transition, and both the cations and anions are tumbling
and thus show a totally spherical geometry (Figure 1c).
According to the symmetry changes between paraelectric
phase and ferroelectric phase, the ferroelectric species
Second harmonic generation (SHG) is usually utilized
to detect and characterize the symmetry change of crystal
structures during the transition process. As depicted in
Figure 2c, the signal of SHG is active at room temperature,
verifying the non-centrosymmetric crystal structure cor-
responding to the Pca21 space group in the LTP. With the
temperature increasing, the SHG intensity suddenly de-
creases to zero at around Tc, which demonstrates a cen-
trosymmetry crystal structure in the HTP, corresponding
̅
with Aizu notion m3mFmm2 of [3.2.1-dabco]BF4 have 6
polar axes, corresponding to 12 crystallographically equiv-
alent polarization directions, revealing a multiaxial fer-
roelectric characteristic.
̅
to the centrosymmetric Pm3m space group.
We then directly measured the polarization-electric
field (P–E) hysteresis loops to characterize the intrinsic
ferroelectric feature of [3.2.1-dabco]BF4 by using double-
wave method. The typical ferroelectric J–E (current densi-
ty-electric filed) curve is shown in Figure 2d with two op-
posite peaks at 293 K. According to the J–E curve, we ob-
tained a perfect P–E hysteresis loop by counting the cur-
rent. Under the influence of applied voltage, the Ps in-
creases quickly and reaches its maximum value of about
5.5 μC·cm-2, which is close to the calculated one (5.65
Next, we utilized differential scanning calorimetry (DSC)
and dielectric anomalies to verify the structural phase
transition of [3.2.1-dabco]BF4. As illustrated in Figure 2a,
DSC measurement presents a pair of abnormal peaks dur-
ing the heating (357 K) and cooling (287 K) runs with a
hysteresis of approximately 70 K and the entropy changes
(ΔS) of about 27.79 J·mol-1·K -1, revealing a reversible first-
order phase transition. It is worth noting that the ΔS dur-
ing the phase transition process is even larger than that
during the melting process, indicating a characteristic of
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μC·cm-2) depending on the point charge model (Table S3).
switching of ferroelectric domains. And the local coercive
voltage (Vc) value estimated by averaging the minima of
the amplitude hysteresis loops is ~56 V.
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4
This value is larger than other quasi-spherical molecular
ferroelectrics which are found recently, such as
(hmtaH2)(NH4)Br3 (hmta = hexamethylenetetramine, 1.08
nC·cm-2)
and
[AH]ClO4
(AH
=
1-
azabicyclo[2.2.1]heptanium, 4.1 μC·cm-2).11
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Figure 4. (a) Phase and amplitude signals as functions of the
tip voltage for a selected point, showing local PFM hysteresis
loops. Panels in each row are arranged as the following se-
quence: vertical PFM phase (top) and amplitude (bottom)
images of the film surface for [3.2.1-dabco]BF4: (b) Initial
state. (c) After the first switching produced by poling with
the tip bias of +70 V for 2 s. (d) After the succeeding back-
switching produced by poling with the tip bias of -60 V for
0.5 s.
To get a more intuitive observation of domain switch-
ing process, local polarization manipulation experiments
were carried out on the thin film of [3.2.1-dabco]BF4 for
the as-grown state. Initially, a single domain area was
selected as the initial state for switching (Figure 4b), and
then we applied an electric field between the conductive
tip and ITO substrate with a DC tip bias of +70 V for 2 s in
the center. In Figure 4c, the polarization direction in the
central region was switched with a rectangle shape, which
is denoted by a deep red box. Meanwhile, the appearance
of domain walls shown in the amplitude images further
manifests the polarization switching of [3.2.1-dabco]BF4.
Afterwards, subsequent poling with an opposite bias of -
60 V for 0.5 s was applied, the polarization direction of
central region (Figure 4d) could also be reversed back,
suggesting that the polarization in these domains can be
reversibly switched forth and back. Given the above, the
efficient switching of polarization direction on the thin
film of [3.2.1-dabco]BF4 with an external electric field will
further motivate the practical utilizations of multiaxial
molecular ferroelectrics.
Figure 3. The PFM phase, topography and amplitude images
for the thin film of [3.2.1-dabco]BF4 at one region with 180
domain (a, b) and another region with non-180 domain (c, d)
in the lateral (a, c) and vertical (b, d) components, respec-
tively.
The ferroelectricity and multiaxial characteristic of
[3.2.1-dabco]BF4 were investigated on the thin film using
piezoresponse force microscopy (PFM), which can yield
nondestructive visualization of ferroelectric domains with
ultrahigh spatial resolution.12 A PFM image, containing
both vertical (out-of-plane) and lateral (in-plane) compo-
nents, has the amplitude and phase data providing direct
evidence of polarization intensity and orientation, respec-
tively. Evidently, with the PFM tests conducted on the
film surface, the coexistence of 180 and non-180 domain
structure was detected, which firmly confirms the multi-
axial characteristic of [3.2.1-dabco]BF4. Figures 3a and 3b
show the lateral and vertical components of 180 domain
structure, where amplitude signals of neighbor domains
are the same, while phase ones are totally different with a
clear contrast, indicating the presence of two antiparallel
domains. While in Figures 3c and 3d, the non-180 do-
main structure can be explicitly signified in the lateral
and vertical components with different phase and ampli-
tude signals, agreeing with the multiaxial characteristic of
[3.2.1-dabco]BF4 deduced from the above structure anal-
yses.
CONCLUSION
In summary, we have synthesized a ceramic-like multi-
axial molecular ferroelectric [3.2.1-dabco]BF4, with the
high phase-transition temperature Tc = 357 K. Multiaxial
ferroelectric characteristic was attributed to the transfor-
mation between the highly reorientation of the monopro-
nated [3.2.1-dabco]+ cations and BF4- anions with spherical
geometry in the HTP and the freezing-state ions with
quasi-spherical geometry in the LTP. Due to the uniform
distribution of six polar axes in the 3D space, the switch-
ing of ferroelectric domains and the well-defined rectan-
gular P−E hysteresis are successfully achieved in the poly-
crystalline thin-film form of [3.2.1-dabco]BF4. This finding
demonstrates that the quasi-spherical theory has im-
portant guiding significance for achieving superior mo-
lecular ferroelectrics.
Electrically switchable spontaneous polarization, the
most intrinsic characteristic for ferroelectric, reveals the
switching process among several spontaneously polarized
states under sufficiently large electrical load. From this,
local PFM-based hysteresis loop measurement was per-
formed to verify the ferroelectric characteristic of [3.2.1-
dabco]BF4 by applying an AC electric field superimposed
on a DC triangular square-stepping wave. As such, under
the resonance-enhanced PFM mode, the typical rectangu-
lar phase hysteresis loop and butterfly amplitude loop
were developed (Figure 4a), which strongly prove the
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6369; (e) 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 applications. J. Am. Chem. Soc. 2018, 140, 8051; (f) Tang,
Y.-Y.; Ai, Y.; Liao, W.-Q.; Li, P.-F.; Wang, Z.-X.; Xiong, R.-G. H/F-
substitution-induced homochirality for designing high-Tc molecular
perovskite ferroelectrics. Adv. Mater. 2019, 31, 1902163; (g) Zhang, Z.;
Li, P.-F.; Tang, Y.-Y.; Wilson, A. J.; Willets, K.; Wuttig, M.; Xiong, R.-
G.; Ren, S. Tunable electroresistance and electro-optic effects of
transparent molecular ferroelectrics. Sci. Adv. 2017, 3, e1701008; (h) 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
ferroelectrics. Adv. Mater. 2018, 30, 1803716.
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Synthesis. 1,5-Diazabicyclo[3.2.1]octane was synthesized
according to the reported literature by reactions of homopi-
perazine with formaldehyde aqueous solution in tolu-
ene/H2O systems.13 The title compound [3.2.1-dabco]BF4 was
obtained through slow evaporation of a solution of metha-
nol/H2O (1:1) containing 1,5-diazabicyclo[3.2.1]octane (1.12 g,
10 mmol), and tetrafluoroboric acid (40% in water, 10 mmol)
at 323 K. The white sheet crystals were formed in six hours.
Powder X-ray diffraction (PXRD) (Figure S2) and infrared (IR)
spectroscopy (Figure S5) results confirmed the phase purity
of the bulk crystals.
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Measurements. The methods of single-crystal X-ray dif-
fraction, DSC, dielectric, SHG, P−E hysteresis loop, PFM,
PXRD, and IR experiments were described in detail previous-
ly.14 The thin film sample with thickness of ∼3 μm of [3.2.1-
dabco]BF4 was used for the P−E hysteresis loop and PFM
measurements. The precursor solution was prepared by dis-
solving 20 mg [3.2.1-dabco]BF4 in 1 mL methanol. Spreading
20 µL of the precursor solution on a clean indium-doped tin
oxide (ITO) glass substrate. The high-quality thin film crys-
tals were formed after the slow solvent evaporation at 300 K.
For the P−E hysteresis loop measurement, we dropped the
liquid GaIn eutectic on the thin film sample as the top elec-
trode to form an ITO/[3.2.1-dabco]BF4 thin film/GaIn capaci-
tor architecture.
ASSOCIATED CONTENT
Supporting Information.
Figures S1–S5, Tables S1–S3, and discussion. This material is
available free of charge via the Internet at http://pubs.acs.org.
The structures have been deposited at the Cambridge Crys-
tallographic Data Centre (deposition numbers: CCDC
1945432-1945434), and can be obtained free of charge from
the CCDC via www.ccdc.cam.ac.uk/getstructures.
(4) (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.; Ye, Q.; Zhang, W.; Zhang, Y.; Chen, X. Y.;
Giovannetti, G.; Capone, M.; Li, J.; Xiong, R. G.
Diisopropylammonium bromide is a high-temperature molecular
ferroelectric crystal. Science 2013, 339, 425; (c) Tang, Y.-Y.; Li, P.-F.;
Zhang, W.-Y.; Ye, H.-Y.; You, Y.-M.; Xiong, R.-G. A multiaxial
molecular ferroelectric with highest curie temperature and fastest
polarization switching. J. Am. Chem. Soc. 2017, 139, 13903; (d) Shi, P.-
P.; Tang, Y.-Y.; Li, P.-F.; Ye, H.-Y.; Xiong, R.-G. De novo discovery of
[Hdabco]BF4 molecular ferroelectric thin film for nonvolatile low-
voltage memories. J. Am. Chem. Soc. 2017, 139, 1319.
AUTHOR INFORMATION
Corresponding Author
*caihu@ncu.edu.cn; xiongrg@seu.edu.cn
Notes
The authors declare no competing financial interests.
ACKNOWLEDGMENT
(5) Zhang, H.-Y.; Tang, Y.-Y.; Shi, P.-P.; Xiong, R.-G. Toward the
targeted design of molecular ferroelectrics: modifying molecular
symmetries and homochirality. Acc. Chem. Res. 2019, 52, 1928.
(6) Katrusiak, A.; Szafrański, M. Ferroelectricity in NH⋯ N
hydrogen bonded crystals. Phys. Rev. Lett. 1999, 82, 576.
(7) Szafrański, M.; Katrusiak, A.; McIntyre, G. Ferroelectric order
of parallel bistable hydrogen bonds. Phys. Rev. Lett. 2002, 89, 215507.
(8) Yang, C. K.; Chen, W. N.; Ding, Y. T.; Wang, J.; Rao, Y.; Liao, W.
Q.; Xie, Y.; Zou, W.; Xiong, R. G. Directional intermolecular
interactions for precise molecular design of a high-Tc multiaxial
molecular ferroelectric. J. Am. Chem. Soc. 2019, 141, 1781.
(9) Li, B.; Kawakita, Y.; Ohira-Kawamura, S.; Sugahara, T.; Wang,
H.; Wang, J.; Chen, Y.; Kawaguchi, S. I.; Kawaguchi, S.; Ohara, K.; Li,
K.; Yu, D.; Mole, R.; Hattori, T.; Kikuchi, T.; Yano, S.-i.; Zhang, Z.;
Zhang, Z.; Ren, W.; Lin, S.; Sakata, O.; Nakajima, K.; Zhang, Z.
Colossal barocaloric effects in plastic crystals. Nature 2019, 567, 506.
(10) Li, P.-F.; Liao, W.-Q.; Tang, Y.-Y.; Qiao, W.; Zhao, D.; Ai, Y.;
Yao, Y.-F.; Xiong, R.-G. Organic enantiomeric high-Tc ferroelectrics.
Proc. Natl. Acad. Sci. U. S. A. 2019, 116, 5878.
This work was supported by the National Natural Science
Foundation of China (21991142, 21831004, 21975114, 11904151,
21571094, 21661021 and 21865015).
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