Experimental and theoretical demonstration of ferroelectric anisotropy in a one-dimensional copper(ii)-based coordination polymer

Article abstract of DOI:10.1039/b820500k

The anisotropy of polarization in a 1D copper(ii)-based coordination polymer was investigated experimentally and theoretically for the first time, revealing that the origin of the ferroelectricity and its anisotropic nature are closely related to the coor

Full text of DOI:10.1039/b820500k

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Experimental and theoretical demonstration of ferroelectric anisotropy
in a one-dimensional copper(II)-based coordination polymerw
Hai-Xia Zhao,a Gui-Lin Zhuang,a Shu-Ting Wu,a La-Sheng Long,*a Hai-Yan Guo,b
Zuo-Guang Ye,*ab Rong-Bin Huang*a and Lan-Sun Zhenga
Received (in Cambridge, UK) 17th November 2008, Accepted 13th January 2009
First published as an Advance Article on the web 4th February 2009
DOI: 10.1039/b820500k
The anisotropy of polarization in a 1D copper(II)-based
coordination polymer was investigated experimentally and
theoretically for the first time, revealing that the origin of the
ferroelectricity and its anisotropic nature are closely related to
the coordination geometry of the metal ion and the packing
mode of the coordination polymer.
Two similar yet distinct Cu(II) centers are identified in the
asymmetric unit, each featuring the coordination of two
imidazole and two o-phthalate. Each of the neutral imidazole
provides one of its two N atoms, while each o-phthalate offers
two carboxylate O atoms for coordination. Thus, the Cu(II)
ion formally is situated in an octahedral coordination sphere.
However, two of the Cu–O distances, 1.943 and 1.993 A, are
significantly shorter than the other two (2.727 and 3.081 A).
Therefore, the o-phthalate is effectively monodentate and
the coordination of Cu(II) can be described as a severely
elongated octahedron, which typically arises from the
Jahn–Teller effect of Cu(II). Each crystallographically
independent copper(II) center coordinated by two imidazoles,
and adjacent two crystallographically independent copper(II)
centers bridged by o-phthalate in chelating mode, generate a
1D chain structure similar to those of (C28H24Cu2N8O8)nꢀ
3nH2O, (C16H16CoN4O4)n and (C32H32N8O8Zn2)n.5 The 1D
chains are zipped to each other via hydrogen-bonding
interaction between the imidazole as a proton donor and
the carboxylate group as proton acceptor (N(2)ꢀ ꢀ ꢀO(3) =
2.718(6) A, N(2)–Hꢀ ꢀ ꢀO(3) = 1.87 A, +N(2)–Hꢀ ꢀ ꢀO(3) =
167.41, N(4)ꢀ ꢀ ꢀO(5) = 2.872(5) A, N(4)–Hꢀ ꢀ ꢀO(5) = 2.04 A,
+N(2)–Hꢀ ꢀ ꢀO(5) = 161.41) to give a 2D structure that is
further extended into a 3D supramolecular architecture
through van der Waals interactions.
Ferroelectric materials are characterized by spontaneous
electric polarization. When subject to an appropriate external
electric field, this spontaneous polarization can be switched.
Because of such unique properties, ferroelectrics have found
broad applications in microelectronics (e.g. tunable capacitors),
computing (non-volatile memory devices) and transducers,1
while more extensive applications are being envisioned.
Development of new ferroelectric materials and realization
of novel applications depend critically on our understanding
of the structure–property relationship at the molecular level.
So far the research on ferroelectric materials is focused on
inorganic salts and organic polymers.2 Recently, ferroelectricity
in supramolecular assemblies of organic molecules and in
metal–organic frameworks have been reported.3,4 Because
the characterization of the ferroelectric properties of these
materials was mainly carried out on polycrystalline pellets or
powders,3 the question of how the local chemical environment
induces the macroscopic ferroelectricity remains unclear,
which hinders the rational design of ferroelectric materials.
Here we report the synthesis, structural determination, and
characterization of anisotropic ferroelectricity of single crystals
of a coordination polymer formed by copper–(o-phthalate)
with terminal imidazole.
Since 1 crystallizes in the non-centrosymmetric space group
Pna21, one of the ten polar point groups necessary for
ferroelectricity to occur, the dielectrical hysteresis loops along
different crystal axes of the single crystals were measured.
Fig. 2 shows the polarization vs. electric field (P–E) curves of 1
at various ac electric fields of triangular waveform applied
along the three main crystallographic axes. The hysteretic P–E
loops were clearly displayed along the polar c-axis, indicating
the presence of ferroelectricity. A saturation polarization (Psa)
of ca. 0.48 mC cmꢁ2 and a remanent polarization (Pr) of ca.
0.17 mC cmꢁ2 are obtained with a coercive field (Ec) of ca.
2.0 kV cmꢁ1. The Psa value for 1 is significantly higher than
that of 0.25 mC cmꢁ2 reported for the typical ferroelectric
compound NaKC4H4O6ꢀ4H2O and comparable to that of
Single crystals of [Cu(C8H4O4)(C3H4N2)2]n (1) of a few
millimeters dimensions were obtained by the hydrothermal
reaction of copper acetate, imidazole and o-phthalic acid.z
X-Ray structural analysisy reveals that 1 crystallizes in the
non-centrosymmetric and polar space group Pna21 (no. 33).
The metal coordination environments are shown in Fig. 1.
a State Key Laboratory for Physical Chemistry of Solid Surfaces,
Department of Chemistry, College of Chemistry and Chemical
Engineering, Xiamen University, Xiamen, 361005, PR China.
E-mail: lslong@xmu.edu.cn. E-mail: rbhuang@xmu.edu.cn;
Fax: 0592-2183047; Tel: 0592-2182093
b Department of Chemistry and 4D LABS, Simon Fraser University,
Burnaby, Canada BC V5A 1S6. E-mail: zye@sfu.ca
w Electronic supplementary information (ESI) available: Spontaneous
polarization of 1 along the c axis calculated using different DFT
methods. Crystallographic CIF file CCDC 704347 for 1. For ESI and
crystallographic data in CIF or other electronic format see DOI:
10.1039/b820500k
Fig. 1 The 1D chain structure in 1.
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Fig. 3 The polarization vs. electric field (P–E) curves of 1 along the c
axis at higher electric field.
Fig. 2 The polarization vs. electric field (P–E) curves of 1 at
various ac electric fields applied along a (top, left), b (top, right) and
c (bottom) axes.
0.50 mC cmꢁ2 for C13H14ClN5O2Cd.4 Interestingly, while 1
exhibits clearly typical ferroelectric feature along the c axis,
no P–E hysteresis loops are displayed along the a and
b axes, indicating strong anisotropy in the ferroelectricity,
which is consistent with the characteristics of the polar point
group mm2.
Fig. 4 The real part of dielectric permittivity (left) and the dielectric
dissipation factor (right) for 1 measured as a functions of temperature
at various frequencies (note that the values at the Td temperature and
above arise from the decomposed material).
The spontaneous polarization of 1 was also calculated
theoretically based on the structural model. Given that the
unit cell contains a n glide plane along a and b axes, and a 21
screw along the c axis, the local dipoles along a and b axes are
cancelled out by symmetry,6 and the total macroscopic
polarization lies parallel to the c axis, which constitutes the
unique polar axis. Thus, the total polarization of 1 along the c
axis in a unit cell (Z = 4) is calculated on the basis of density
functional theory (DFT)7 using the GAUSSIAN 03 package.8
between 300 and 350, and the loss tangent is almost
temperature independent and low (o0.02) in the temperature
range from ꢁ60 to 180 1C. Above 180 1C, both e0 and tan d at
all measuring frequencies increase very sharply, and then reach
the peak values at 195 1C. The temperature dependence of
the dielectric properties clearly indicates that no solid–solid
structural phase transition takes place in crystal 1 up to the
decomposition point Td, which is consistent with the thermal
analysis of 1 (Fig. 5). It is therefore reasonable to deduce that
the ferroelectric Curie temperature TC would be higher than
Td. In other words, the ferroelectric state must be very
stable at room temperature, which makes it difficult to switch
the polarization fully. This could account for the observation
that the polarization Psa measured is smaller than the
calculated value.
The spontaneous polarization of
1 along the c axis
calculated using different DFT methods9 ranges from 1.138
to 1.270 mC cmꢁ2 (ESI,w Table S1).
The above analyses confirm the structural origin of the
spontaneous polarization and the observed anisotropy. However,
the total polarization calculated on the basis of structural data
is larger than that of 0.48 mC cmꢁ2 measured. The possible
reason for such a discrepancy is that the P–E loops do not
saturate, i.e. the polarization switching was not complete.
Such situation mainly originates from the following reasons:
(1) the leakage of charges arising from the conduction of the
crystal; (2) the Curie temperature is much higher than room
temperature, thus requiring a higher field to fully switch the
polarization.
Consistently, when a higher electric filed was applied along
the c axis, the leakage of charges was observed (Fig. 3). To find
the Curie temperature (TC), the variations of the real part of
dielectric permittivity (e0, i.e. dielectric constant) and the
dissipation factor (i.e. loss tangent, tan d) were measured as
a function of temperature at various frequencies. As shown in
Fig. 4, the dielectric constant remains relatively flat with values
Fig. 5 Thermal analysis for 1 (dotted line represents TG curve, solid
line is heat flow).
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In summary, we have demonstrated a large anisotropy of
polarization in a 1D copper(II)-based coordination polymer
which crystallizes in the non-centrosymmetric polar space
group Pna21. Ferroelectric coercivity was present only along
the polar c axis of the crystals. Theoretical investigation
reveals that the polar anisotropy and ferroelectricity are
closely related to the coordination geometry of the metal ion
and the packing mode of the coordination polymer. Thus, the
present work would be useful for our rational design of
ferroelectric coordination polymers.
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This work was supported by the NNSFC (Grant Nos.
20825103 and 20721001), the 973 Project from MSTC (Grant
2007CB815304), the Natural Science Foundation of Fujian
Province of China (Grant No. 2008J0010), and the Natural
Science and Engineering Research Council of Canada
(NSERC).
Notes and references
6 S. Horiuchi, F. Ishii, R. Kumai, Y. Okimoto, H. Tachibana,
N. Na-gaosa and Y. Tokura, Nat. Mater., 2005, 4, 163.
7 R. G. Parr and W. Yang, in Density Functional Theory of Atoms and
Molecules, Oxford University Press, Oxford, 1989.
z Preparation of 1: o-Phthalic acid (0.166 g, 1.0 mmol), imidazole
(0.140 g, 2.0 mmol) and cupric acetate (0.200 g, 1.0 mmol) were added
into the mixture of 4 mL distilled water and 12 mL ethanol, while
stirring at room temperature. When the pH value of the mixture was
adjusted to about 6.2 with ammonia solution (B12%, v/v), the cloudy
solution was put into a 25 mL Teflon-lined Parr and heated to 100 1C
8 M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria,
M. A. Robb, J. R. Cheeseman, J. A. Montgomery, Jr., T. Vreven,
K. N. Kudin, J. C. Burant, J. M. Millam, S. S. Iyengar, J. Tomasi,
V. Barone, B. Mennucci, M. Cossi, G. Scalmani, N. Rega,
G. A. Petersson, H. Nakatsuji, M. Hada, M. Ehara, K. Toyota,
R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda,
O. Kitao, H. Nakai, M. Klene, X. Li, J. E. Knox,
H. P. Hratchian, J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo,
R. Gomperts, R. E. Stratmann, O. Yazyev, A. J. Austin, R. Cammi,
C. Pomelli, J. Ochterski, P. Y. Ayala, K. Morokuma, G. A. Voth,
P. Salvador, J. J. Dannenberg, V. G. Zakrzewski, S. Dapprich,
A. D. Daniels, M. C. Strain, O. Farkas, D. K. Malick,
A. D. Rabuck, K. Raghavachari, J. B. Foresman, J. V. Ortiz,
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G. Liu, A. Liashenko, P. Piskorz, I. Komaromi, R. L. Martin,
D. J. Fox, T. Keith, M. A. Al-Laham, C. Y. Peng, A. Nanayakkara,
M. Challacombe, P. M. W. Gill, B. G. Johnson, W. Chen,
M. W. Wong, C. Gonzalez and J. A. Pople, GAUSSIAN 03
(Revision D.01), Gaussian, Inc., Wallingford, CT, 2004.
for 1000 min, and then cooled to room temperature at a rate of 2 1C hꢁ1
.
Dark-blue block crystals of 1 were obtained in 65% yield (based on
cupric acetate). IR (KBr): 3154(s), 3076(m), 2871(m), 2851(m),
1472(w), 2964(w), 1602(s), 1091(s), 1457(s), 1546(s), 1485(m),
1447(m), 1076(s), 963(w), 864(w), 831(m), 768(m), 745(m), 713(w),
704(w), 655(m), 625(w), 466(w) cmꢁ1; Anal. Calc. for 1: C 46.22, H
3.32, N 15.40. Found: C 45.20, H 3.14, N 14.81%.
y Crystal data for 1 (Cu(C8H4O4)(C3H4N2)2): orthorhombic, space group
Pna21, Flack parameter = 0.051(2), T = 298(2) K, a = 16.578(3),
b = 15.421(3), c = 11.988(2) A, V = 3064.9(10) A3, Z = 4, Dc
=
1.577 g cmꢁ3, M = 727.63, m(Mo-Ka) = 1.450 mmꢁ1, number of
reflections measured/number of independent reflections = 23055/5997,
Rint = 0.0514, R1(obs.) = 0.0454, wR2 (all data) = 0.1113.
1 M. E. Lines and A. M. Glass, in Principles and Applications of
Ferroelectrics and Related Materials, Oxford Univ. Press,
New York, 1977; H. N. Lee, D. Hesse, N. Zakharov and
U. Gosele, Science, 2002, 296, 2006; A. V. Bune, V. M. Fridkin,
9 M. Tokunaga, J. Phys. Soc. Jpn., 1988, 57, 4275.
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1646 | Chem. Commun., 2009, 1644–1646