An above-room-temperature switchable molecular dielectric with a large dielectric change between high and low dielectric states

Article abstract of DOI:10.1007/s11426-012-4820-6

Dielectric anisotropy of anilinium perchlorate is investigated at various temperatures. Crystal structures at different temperatures reveal that significant dielectric change between low and high dielectric states is closely related to the disorder of the anilinium cation and perchlorate anion at high dielectric state; meanwhile, the conductivity after phase transition also contributes a lot to the high dielectric state.

Full text of DOI:10.1007/s11426-012-4820-6

SCIENCE CHINA
Chemistry
July 2013 Vol.56 No.7: 917–922
doi: 10.1007/s11426-012-4820-6
• ARTICLES •
An above-room-temperature switchable molecular dielectric with a
large dielectric change between high and low dielectric states
DU Yang, HAO HuiMin, ZHANG QianChong, ZHAO HaiXia*, LONG LaSheng*,
HUANG RongBin & ZHENG LanSun
State Key Laboratory for Physical Chemistry of Solid Surfaces, Department of Chemistry, College of Chemistry and Chemical Engineering,
Xiamen University, Xiamen 361005, China
Received November 8, 2012; accepted November 28, 2012; published online February 22, 2013
Dielectric anisotropy of anilinium perchlorate is investigated at various temperatures. Crystal structures at different tempera-
tures reveal that significant dielectric change between low and high dielectric states is closely related to the disorder of the ani-
linium cation and perchlorate anion at high dielectric state; meanwhile, the conductivity after phase transition also contributes
a lot to the high dielectric state.
switchable molecular dielectric, ionic conductivity, dielectric anisotropy, phase transition
Anilinium perchlorate, similar to those of pyridinium
1 Introduction
salts, is a molecular-ionic solid. Previous study showed that
a first order phase transition occurred at about 220 K [7]. A
possible mechanism of the phase transition was suggested in
relation to disorder of the hydrogen atoms on nitrogen at-
oms [7]. Owing to the existence of a perchlorate anion in
this compound, it is expected to exhibit an above-room
temperature phase transition connected with the ordering of
the perchlorate anion [3, 8, 9]. On the other hand, the rela-
tively high temperature phase transition would in turn in-
crease the dielectric constant of the compound due to the
Switchable molecular dielectrics, which undergo transitions
between high and low dielectric states, are promising mate-
rials potentially applicable in data communication and sig-
nal processing [1–3]. In principle, switchable molecular
dielectrics could arise from the reorientation of the polar
guests in metal-organic frameworks [4, 5], or order-disorder
molecular-ionic solid, such as pyridinium salts [6]. Some
switchable molecular dielectrics have been successfully
prepared on the basis of these principles. On the one hand,
synthesis of such kind of switchable molecular dielectrics
remains a great challenge due to the difficulty in keeping
the dielectric constant almost unchanged respectively at
high and low dielectric states. On the other hand, the
switchable molecular dielectrics synthesized so far either
suffer from lower phase transition temperature, or relatively
small dielectric change between high and low dielectric
states. As a result, practical applications of those materials
are in fact limited.
−
high temperature favoring the disorder of ClO4 . Here we
report dielectric anisotropy of a molecular-ionic anilinium
perchlorate, namely C6H8NClO4 (APC for short), in the
temperature range from 300 to 460 K. Surprisingly, the
phase transition temperature of APC is up to 442 K, while
its dielectric change between low and high dielectric states
is up to 137 along the a axis.
2 Experimental
2.1 General experimental section
Chemicals were used as purchased from commercial ven-
*Corresponding authors (email: lslong@xmu.edu.cn; hxzhao@xmu.edu.cn)
© Science China Press and Springer-Verlag Berlin Heidelberg 2013
chem.scichina.com www.springerlink.com

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Du Y, et al. Sci China Chem July (2013) Vol.56 No.7
dors without further purification. The C, H, and N microa-
nalyses were carried out with a Vario EL III elemental ana-
lyzer.
500 K. As shown in Figure 1(a), and endothermic peak at
442 K in the heating process and an exothermic peak at 418
K in the cooling process were observed. The DSC result
indicates that the phase transition is reversible. The XRD
patterns were performed at 298 K and 453 K, which are
corresponding to that before and after phase transition tem-
peratures. All the patterns fit very well with the curves sim-
ulated based on X-Ray single crystal data. The XRD pat-
terns at 453 K change a lot comparing to that at 298 K.
Meanwhile, after cooling to 298 K, the XRD patterns fit
very well with the patterns at 298 K (before phase transi-
tion). It further demonstrated that APC undergoes reversible
phase transition (Figure 2). Based on the DSC data, the ∆H
and ∆S (= ∆H/Tc) are estimated to be 3.4 kJ mol1 and 7.65
J mol1K1 respectively. According to ∆S = R ln(N2/N1),
2.2 Synthesis of APC
Compound APC was synthesized by the reaction of per-
chloric acid and aniline in water as described in the previous
literature [7]. Perchloric acid (4 mmol, 2 mol L1) and ani-
line (4 mmol, 0.366 mL) were dissolved into 10 mL deion-
ized water, while stirring at room temperature. The mixture
solution was kept at room temperature in dark place. Col-
ourless sheet crystals were obtained in about 55% yield
(based on aniline) after about one month. IR (KBr, cm1):
3444.1 (m), 2907.8 (s), 2589.3 (m), 1599.4 (w), 1562.3 (m),
1515.4 (w), 1493.7 (s), 1193.7 (w), 1144.8 (s), 1113.6 (s),
1089.3 (s), 1030.3 (w), 743.9 (s), 682.6 (m), 627.4 (s),
525.5 (w), 475.5 (w); Anal. Calcd for 1: C: 37.23, N: 7.24%,
H: 4.16%. Found: C: 37.27%, N: 7.26%, H: 4.11%.
2.3 X-Ray crystal data collection and analysis
Single-crystal X-ray structure determination: Data collec-
tions were performed on an Oxford Gemini S Ultra CCD
area detector using Mo-Kα radiation at 298 K, 453 K for
APC with the same single-crystal. Absorption corrections
were applied by using the multi-scan program. The struc-
tures were solved by direct methods, and non-hydrogen
atoms were refined anisotropically by least-squares on F2
using the SHELXTL-97 program [10]. All the hydrogen
atoms were generated geometrically.
2.4 DSC and TG measurements
DSC (differential scanning calorimetry) measurements were
performed on the NETZSCH DSC 200F3 with sweeping
rate of 10 K/min under nitrogen atmosphere. The TG curve
was measured on Thermal Analyzer SDT Q600 with
sweeping rate of 10 K/min under air atmosphere.
Figure 1 DSC curves (a) and TG curves (b) of APC.
2.5 The impedance and dielectric measurements
Temperature-dependent a.c. impedance plots and dielectric
constants were performed on Wayne Kerr 6500B by using
two-probe a.c. impedance method in frequency range from
1 kHz to 10 MHz. The single crystals were placed into
American Sigma chamber in order to realize variation of the
temperature. The electric contacts were prepared with silver
paste to attach 25-μm gold wires to the single crystal.
3 Results and discussion
3.1 Thermal analysis and dielectric properties
TG analysis clearly demonstrated that APC is stable up to
Figure 2 The XRD patterns at different temperatures exhibiting reversi-
ble phase transition.

Du Y, et al. Sci China Chem July (2013) Vol.56 No.7
919
where R, N1 and N2 are the gas constant and the numbers of
possible orientations of low and high temperature phases,
N2/N1 is about 5/2, indicating that the phase transition is of
typical order-disorder type [11].
Figure 3 showed the dielectric constants of APC along a,
b and c axes in the temperature range from 300 to 460 K. At
f = 10 kHz, the dielectric constant of APC along a axis al-
most keeps a constant (cal. 14) from 300 to 434 K, then it
abruptly increases to 137 at phase transition temperature
and keeps a constant after phase transition. The dielectric
change before phase transition (low dielectric state) and
after phase transition (high dielectric state) in APC is sig-
nificantly larger than that in (Him)2[KFe(CN)6],
[Mn3(HCOO)6](H2O)(CH3OH) and the similar compound
4-methylanilinium perchlorate previously reported [3–5]. Its
phase transition temperature is also significantly higher.
Similarly, the dielectric constant of APC along the b axis
also exhibits temperature-dependent behavior. However, its
dielectric constant significantly decreases with the increase
of temperature after phase transition. In contrast, only a
small dielectric anomaly is observed along the c axis in the
whole temperature range. These results indicate that dielec-
tric constants of APC are crystal-axis dependent. Consist-
ently, in the cooling process the dielectric constants along a,
b and c axes also exhibit crystal-axis dependent behavior
(Figure S3-S4). However, the dielectric peaks along a and b
axes appeared at 415 and 453 K respectively at f = 10 kHz
(Figure S3), significantly different from those in the heating
process. In contrast, the dielectric peak along c axis in the
cooling process appeared at 441 K at f = 10 kHz (Figure S3),
Figure 3 The dielectric constant of APC measured as a function of tem-
peratures along the different axes at 10 kHz.
comparable to that in the heating process. The thermal die-
lectric hysteresis is coincident with the DSC, which shows
that it may be related to the dynamics process in this sys-
tem.
Figure 4 shows dielectric imaginary parts (ε") at different
temperatures along a, b and c axes respectively. The dielec-
tric imaginary parts (ε") exhibits the similar temperature-
and frequency-dependent behavior as that of the dielectric
constant, and dielectric imaginary part (ε") along a axis is
significantly larger than that along b axis, while the later is
significantly larger than that along c axis. Based on the die-
lectric imaginary parts (ε") and its frequency-dependent
behavior, it is concluded that the contribution of long-range
displacement of mobile charge carriers is ineligible [12].
Figure 4 The imaginary parts of the dielectric (ε") along a, b and c axes.

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Du Y, et al. Sci China Chem July (2013) Vol.56 No.7
3.2 Crystal structures
at 453 K (Figure 2), which shows that the solved 453 K
structure is believable.
Although the room temperature crystal structure of APC
is already known, we studied it here to discuss the dielectric
and impedance properties. The crystal structure at 298 K [7,
13] revealed that it crystallized in non-centrosymmetric
space group P212121 (No. 19) with one anilinium cation and
one perchlorate anion in an asymmetric unit [7]. Each per-
chlorate anion is hydrogen-bonded to three adjacent anilin-
ium cations respectively through its three O atoms as proton
acceptors to form a 1D chain structure. Adjacent 1D chain
structures are hydrogen-bonded into a 2D structure through
the fourth O atom of perchlorate anion from one 1D chain
as a proton acceptor and anilinium cation from adjacent
chains as a proton donor. Extension of the 2D structure
through van der Waals Force generates a 3D supramolecular
architecture as shown in Figure 5.
However, a, b, and c axes at 298 K is corresponded to a,
b and c axes at 453 K separately. The c axis is halved at 453
K comparing to that at 298 K. The diffraction spots at 453
K decrease obviously although the exposure time was long-
er than that at 298 K, which proved that the lattice doubly
reduced at high temperature (Figure 7) [11].
The low-high dielectric switchable behavior and ax-
es-dependent anisotropy are mostly caused by two factors
below: (1) Basing on the dielectric loss and its frequen-
cy-dependent behavior, the ionic (or protonic) conductivity
contributes a lot to the high dielectric after phase transition;
(2) the large change of dipole movement (the reorientation

motion of the ClO4 anion and anilinium cation along dif-
ferent axes) within and between layers before and after
phase transition.
To reveal the unique crystal-axis dependence of dielec-
tric constant in APC, crystal structure of APC at 453 K
(namely APC(a)) was measured. As shown in Figure 6, alt-
hough the asymmetric unit in APC(a) also consists of one
anilinium cation and one perchlorate anion, owing to the
phenyl group in the anilinum cation alternately locating
three positions, and the perchlorate anion disordered over
two positions, APC(a) crystallizes in a centrosymmetric
space group Pmmn (No. 59). Each perchlorate anion is hy-
drogen-bonded to two adjacent anilinium cations respec-
tively through its O atoms as proton acceptors to furnish a
1D chain structure (Figure 6(b)). Connections of adjacent
1D chains through hydrogen-bonding interactions between
the perchlorate anion and anilinium cation generate a 2D
structure (Figure 6(c)), which is further extended into a 3D
supramolecular architecture of APC(a) through van der
Waals Force (Figure 6(d)). Although the quality of the
solved crystal structure at 453 K is rather poor, the experi-
ment XRD patterns of APC at 453 K matched very well
with the fitting data simulated based on the solved structure
In order to prove the different mobility of proton existing
in this system at different temperatures, the impedance of
APC at room temperature (300 K) to 460 K was investigat-
ed. Figure 8 shows that the impedance plots of APC exhibit
obvious semi-circle from 446 to 463 K along a axis, which
confirmed that APC at high temperature stage exhibits the
dominant of large conductivity (about 9.3×106 S/cm at 463
Figure 6 (a) One molecular of APC(a); (b) the 1D chain structure in
APC(a) viewed along the b axes; (c) the 2D structure in APC(a) viewed
along the c axes; (d) the 3D structure of APC(a) viewed along the a axis.
Figure 5 (a) The asymmetric unit in APC; (b) the 1D chain structure in
APC viewed along [0,1,1] direction; (c) the 2D structure in APC viewed
along the c axis; (d) the 3D chain structure of APC along the a axis.
Figure 7 The diffraction patterns along c directions for APC at 298 K
and 453 K.

Du Y, et al. Sci China Chem July (2013) Vol.56 No.7
921
tribute to the high-low temperature switchable behavior.
4 Conclusion
In summary, we have reported dielectric anisotropy of a
molecular-ionic anilinium perchlorate in the temperature
range from 300 to 460 K with a single-crystal. Investigation
on the temperature-dependent dielectric behavior of APC
showed that its phase transition temperature is up to 442 K,
while its dielectric change between low and high dielectric
state is up to 137 along the a axis. Study on the crystal
structures of APC at different temperatures revealed that
significant dielectric change between low and high dielec-
tric states is closely related to the disorder of the anilinium
cation and perchlorate anion at high dielectric state. Unam-
biguously, low-high dielectric switchable behavior and ax-
es-dependent anisotropy is the creation of the combining
powers: the ionic (or protonic) conductivity and the reori-
Figure 8 The impedance of APC at different temperatures along a axis.
K). Figure S6 shows that the impedance of APC exhibits a
depressed arc at low temperature stage along a axis, it is
evident that the conductivity is not predominated. The con-
ductivity along b axis exhibits the similar behavior as that of
a axis (Figure S6), meanwhile, the only difference is that
the conductivity is slightly lower (about 5.4×106 S/cm at
463 K). The conductivity along c axis is the lowest (about
5.7×109 S/cm at 463 K), and only up to 463 K, the imped-
ance exhibits a uncompleted semi-circle. It is noticeable that
the defects created during the strong phase transition with
large change of volume may result in additional conductiv-
ity.

entation motion of the ClO4 anion and anilinium cation
along different axes.
This work was supported by the National Natural Science Foundation of
China (21001089, 20825103, 90922031 and 21021061), the 973 project
from MSTC (2012CB821704).
1
2
Pajzderska A, Gonzalez MA, Wasicki J. Quasielastic neutron scat-
tering study of pyridinium cation reorientation in thiourea pyridinium
nitrate inclusion compound. J Chem Phys, 2008, 128(8): 084507(1-9);
Pajzderska A, Fojud Z, Goc R, Wasicki J. Cation dynamics in pyri-
dinium nitrate and bis-thiourea pyridinium nitrate inclusion com-
pound studied by 2H NMR spectroscopy. J Phys: Condens Matter,
2007, 19: 156220(1–12)
Pajzderska A, Czarnecki P, Embs JP, Gonzalez MA, Jurnayi F,
Krawczyk J, Peplińska B, Wasicki J. A study of out-of-plane cation
dynamics in a bis-thiourea pyridinium chloride inclusion compound.
J Phys Chem Chem Phys, 2011, 13: 8908-8914; Pajzderska A, Gon-
zalez MA, Embs JP, Wasicki J. Complex dynamics of pyridinium
cation in ferroelectric bis(thiourea)pyridinium lodide studied by Qua-
After phase transition, the ionic conductivity indeed con-
tributed a lot to the high dielectric state. However, the con-
ductivity along a, b and c axes (Figure 8, Figure S7, S8)
all increases with the increasing temperature at high tem-
perature phase, the dielectric constant keeps almost a con-
stant along a axis, and significantly decreases with the in-
crease of temperature after phase transition along b axis. It
is contrary to the trend of the conductivity-temperature. So,
we concluded that there should be some other factors re-
sulting to the high-dielectric state. Hence, we considered the
large change of dipole movement (the reorientation motion
of the ClO4 anion and anilinium cation along different axes)
within and between layers before and after phase transition.
However, the difference of structure at high and low tem-
perature phase has already confirmed that reorientation mo-
tion of the ClO4 anion and anilinium cation along different
axes indeed occurs at high temperature phase. Before phase
transition, owing to less disorder of the anilinium cation and
perchlorate anion, the dielectric constant of APC is less in-
fluenced by the temperature. After the phase transition, both
anilinium cation and perchlorate anion and significantly
disordered, especially in the (1 1 0) plane, they exhibit the
similar structural characters along a and b axes. The thermal
dielectric hysteresis between the heating and cooling pro-
cesses along different axes further proved that the reorienta-
si-Elastic neutron scattering.
15164–15171
J Phys Chem C, 2011, 115(31):
3
4
5
6
Zhang W, Xu RJ. Distinct phase transitions and dielectric anomalies
in two 4-methylanilinium salts. Sci China Chem, 2012, 55(2):
201–207
Zhang W, Cai Y, Xiong RG, Yoshikawa H, Awaga K. Exceptional
dielectric phase transitons in a perovskite-type cage compoud. Angew
Chem Int Ed, 2010, 49(37): 6608–6610
Cui HB, Takahashi K, Okano Y, Kobayashi H, Wang ZM, Kobayshi
A. Dielectric properties of porous molecular crystals that contain po-
lar molecules. Angew Chem Int Ed Engl, 2005, 44(40): 6508–6512
Pajak Z, Czarnecki P, Wasicki J, Nawrocik W. Ferroelectric properi-
ties of pyridinium periodate.
J Chem Phys, 1998, 109(15):
6420–6423; Maluszyńska H, Czarnecki P, Lewicki S, Wasicki J,
Gdaniec M. Structure and dynamics of ferroelectric pyridinium peri-
odate. J Phys: Condens Matter, 2001, 13: 11053–1065
7
Marchewka MK, Debrus S, Drozd M, Baran J, Barnes AJ, Xue D,
Ratajczak H. Vibrational spectra, phase transition and nonlinear op-
tical properties of anilinium perchlorate molecular-ionic solid.
Polish J Chem, 2003, 77: 1625–1636

tion motion of the ClO4 anion and anilinium cation con-

922
Du Y, et al. Sci China Chem July (2013) Vol.56 No.7
8
Pajak Z, Czarnecki P, Szafrańska B, Maluszyńska H, Fojud Z. Ferro-
electric ordering in imidazolium perchlorate. J Chem Phys, 2006,
124(14): 144502(1–6)
Cai HL, Zhang W, Ge JZ, Zhang Y, Awaga K, Nakamura T, Xiong
RG. 4-(cyanomethyl)anilinium perchlorate: a new displacive-type
molecular ferroelectric. Phys Rev Lett, 2011, 107(14): 147601(1–5);
Zhang Y, Awaga K, Yoshikawa H, Xiong RG. Ferroelastic phase
transition and dielectric anomalies in 2,4,6-trimethylanilinium per-
chlorate. J Mater Chem, 2012, 22: 9841–9845
order-disorder hydrogen bonding transition in metal-organic frame-
works (MOFs) with the perovskite ABX3 architecture. J Am Chem
Soc, 2009, 131(38): 13625-13627; Xu GC, Ma XM, Zhang L, Wang
ZM, Gao S. Order-disorder ferroelectric transition in the metal for-
mate framework of [NH4][Zn(HCOO)3]. J Am Chem Soc, 2010,
132(28): 9588–9590
9
12 Ennaceur N, Jarraya K, Singh A, Ledoux-Rak I, Mhiri T. Phase tran-
sitions, ferroelectric behavior and second-order nonlinear optics of a
new mixed sodium dihydrogen phosphate-arsenate. J Phys Chem
Solids, 2012, 73(3): 418–422
10 SHELXTL 6.10, Bruker Analytical Instrumentation, Madison, WI,
2000
11 Jain P, Ramachandran V, Clark RJ, Zhou HD, Toby BH, Dalal NS,
Kroto HW, Cheetham AK. Multiferroic behavior associated with an
13 Paixao JA, Matos Beja A, Ramos Silva M, Alte da Veiga L, Mar-
+
-
tin-Gil J, Crystal structure of anilinium perchlorate, C6H5NH3 ClO4 .
Z Kristallog -New Crys Struct, 1999, 214: 85–86