Comparative study of structures, thermal stabilities and dielectric properties for a ferroelectric MOF [Sr(μ-BDC)(DMF)]∞ with its solvent-free framework

Article abstract of DOI:10.1039/c3dt32880e

A ferroelectric MOF with a formula [Sr(μ-BDC)(DMF)]_∞ (1) was transformed into [Sr(μ-BDC)(CH₂Cl₂) _x]_∞ (2) using a solvent exchange approach, where DMF = N,N-dimethylformamide and BDC^2- = benzene-1,4-dicarboxylate. The lattice solvents, CH₂Cl₂ molecules, in 2 were removed by heating to give the solvent-free metal-organic framework [Sr(μ-BDC)] _∞ (3) and the crystal-to-crystal transformation is reversible between 1 and 3. The release of DMF molecules from 1 results in the metal-organic framework of [Sr(μ-BDC)]_∞ expanding a little along the a- and b-axes. The crystal structure optimizations for 1 and 3 disclosed that the lattice expansion is associated with the alternations of the bond distances and angles in the Sr²⁺ ion coordination sphere along the a- and b-axes directions. The metal-organic framework 3 collapses at temperatures of more than 600 °C; such an extremely high thermal stability is related to the closed-shell electronic structure of the Sr²⁺ ion, namely, the coordinate bond between the closed-shell Sr²⁺ ion and the bridged BDC^2- ligands does not have a preferred direction, which is favored for reducing lattice strains and is responsible for the higher thermal stability. The comparative investigations for the dielectric and ferroelectric behaviors of 1 and 3 confirmed that the motion of the polar DMF molecules, but not the [Sr(μ-BDC)]_∞ framework, is responsible for the ferroelectric properties of 1.

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Comparative study of structures, thermal stabilities and
dielectric properties for a ferroelectric MOF
Cite this: Dalton Trans., 2013, 42, 6603
[Sr(μ-BDC)(DMF)]∞ with its solvent-free framework†
Ping-Chun Guo,a,b Zhenyu Chu,a Xiao-Ming Ren,*a,b Wei-Hua Ninga,b and
Wanqin Jin*a
A ferroelectric MOF with a formula [Sr(μ-BDC)(DMF)]∞ (1) was transformed into [Sr(μ-BDC)(CH2Cl2)x]∞ (2)
using a solvent exchange approach, where DMF = N,N-dimethylformamide and BDC2− = benzene-1,4-
dicarboxylate. The lattice solvents, CH2Cl2 molecules, in 2 were removed by heating to give the solvent-
free metal–organic framework [Sr(μ-BDC)]∞ (3) and the crystal-to-crystal transformation is reversible
between 1 and 3. The release of DMF molecules from 1 results in the metal–organic framework of
[Sr(μ-BDC)]∞ expanding a little along the a- and b-axes. The crystal structure optimizations for 1 and 3
disclosed that the lattice expansion is associated with the alternations of the bond distances and angles
in the Sr2+ ion coordination sphere along the a- and b-axes directions. The metal–organic framework 3
collapses at temperatures of more than 600 °C; such an extremely high thermal stability is related to the
closed-shell electronic structure of the Sr2+ ion, namely, the coordinate bond between the closed-shell
Sr2+ ion and the bridged BDC2− ligands does not have a preferred direction, which is favored for reducing
lattice strains and is responsible for the higher thermal stability. The comparative investigations for the
dielectric and ferroelectric behaviors of 1 and 3 confirmed that the motion of the polar DMF molecules,
but not the [Sr(μ-BDC)]∞ framework, is responsible for the ferroelectric properties of 1.
Received 1st December 2012,
Accepted 18th February 2013
DOI: 10.1039/c3dt32880e
www.rsc.org/dalton
remained sparse. At the end of the 1960s, Okada and Sugie
reported the first antiferroelectric MOF, comprised of Cu-
1. Introduction
With the development of the electronic and information (HCOO)2·4H2O, with protonated organic amines and formic
industry, dielectrics (especially ferroelectric materials) have acid as the templates and building blocks, respetively,4 and
been widely used in technologically important fields, such as this MOF has shown an antiferroelectric transition at 227 K.
capacitors, resonators and other key components in microwave Unfortunately, such a fascinating finding has not attracted
communication systems and in information storage devices.1 enough attention in the exploration of dielectric MOFs/CPs
Thus, research on new ferroelectric and the corresponding materials for nearly 40 years. MOFs/CPs have been rapidly
dielectric materials has attracted more and more interest in developed owing to the potential applications in the fields of
recent years. However, the design and oriented synthesis of selective gas separation and storage,5 molecular sensing,6 cata-
these technologically important materials remain
challenge.2,3
a
big lysis7 and luminescence materials8 since the 1990s. Besides
the properties mentioned above, additional functional proper-
Most of the excellent ferroelectric/special dielectric ties of the MOF/CP materials have been explored, which
materials are still traditional inorganic materials, such as cera- include the proton conductivity,9 electronic transport behav-
mics, inorganic salts and their derivatives. To date, studies on ior,10 dielectric property11 and magnetic property12,13. For
metal–organic frameworks (MOFs) or porous coordination instance, Kobayashi and coworkers reported three MOFs,
polymers (CPs) as ferroelectric/special dielectric materials have [Mn3(HCOO)6](H2O)(CH3OH), [Mn3(HCOO)6](C2H5OH)14 and
[La2Cu{NH(CH2COO)2}6](H2O)n,15 where the movable polar
guest or solvent molecules show an intriguing ferroelectricity
and paraelectric-to-ferroelectric transitions that are induced by
the ordering or freezing of the polar guest molecules. A MOF
with a formula of [(CH3)2NH2]Zn(HOOC)3 and Perovskite struc-
ture16 displays antiferroelectric behavior due to the electric
ordering of the organic cations. Besides, the ferroelectric MOF
aState Key Laboratory of Materials-Oriented Chemical Engineering, Nanjing
University of Technology, Nanjing 210009, P. R. China. E-mail: xmren@njut.edu.cn,
wqjin@njut.edu.cn; Fax: +86 25 58139481; Tel: +86 25 58139476
bCollege of Science, Nanjing University of Technology, Nanjing 210009, P. R. China
†Electronic supplementary information (ESI)
available. See DOI:
10.1039/c3dt32880e
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[NH4]Zn(HCOO)3]17 and a series of multiferroic MOFs with 200 mg of the sample, and sealed using a TranSeal. Samples
18
general formula [(CH3)2NH2]M(HCOO)3
or [NH4]M- were degassed at 150 °C for 24 h on a Micromeritics ASAP
19
(HCOO)3 where M = Mn, Fe, Co and Ni, were discovered by 2020 analyzer until the outgas rate was no more than 1 mTorr
different research groups, respectively.
min−1. The degassed sample and sample tube were weighed
In this content, we recently reported that a highly thermal precisely and then transferred back to the analyzer (with the
stable MOF compound, [Sr(μ-BDC)(DMF)]∞ (1) (DMF = N,N- TranSeal preventing exposure of the sample to the air after
dimethylformamide; BDC2−
=
benzene-1,4-dicarboxylate), degassing). The outgas rate was again confirmed to be less
showed ferroelectricity.20 To better understand this issue, a than 1 mTorr min−1. Adsorption isotherms were measured at
comparative investigations of the structure, ferroelectric and 77 K in a liquid nitrogen bath for N2. The isotherm for the
dielectric properties of 1 with the corresponding solvent-free adsorption and desorption of N2 on activated 3 at 77 K and the
compound were completed, and the results are presented in pore size distribution are shown in Fig. S1.†
this paper.
2.4. Details for crystal structure optimization
The crystal structure calculations presented were performed in
the density functional theory (DFT) framework. The starting
crystal structures of 1 and 3 for optimization were taken from
2. Experimental
2.1. Materials and chemicals
the X-ray single crystal structure of 1 at 293 K. The Cambridge
sequential total energy package (CASTEP) module21 was
All commercially available chemicals [Sr(NO3)2 and H2BDC]
and solvents (DMF and CH2Cl2) were of reagent grade and
used as received without further purification.
employed in the calculations. The total plane-wave pseudo-
potential method forms the basis of the CASTEP calculations.
The exchange-correlation effects were treated within the gener-
alized gradient approximation (GGA) with the Perdew–Burke–
2.2. Sample preparation
Ernzerhof functional.22 The plane-wave basis set energy cutoff
was set at 300 eV for 1 and 3. The convergence parameters
were as follows: the SCT tolerance 2 × 10−6 eV per atom, the
total energy tolerance 2 × 10−5 eV per atom, the maximum
force tolerance 0.05 eV Å−1, the maximum stress component
0.1 GPa and the displacement of convergence tolerance
0.002 Å. All above limits can accurately constrain the simulat-
ive process to end at the stable constructions of 1 and 3.
[Sr(μ-BDC)(DMF)]∞ (1) was prepared following the procedure
in a published paper.20 The sample exchanged by dichloro-
methane, [Sr(μ-BDC)(CH2Cl2)x]∞ (2), was obtained by soaking 1
in dichloromethane for 24 h and then refreshing the dichloro-
methane three times. The activated solvent-free compound
[Sr(μ-BDC)]∞ (3) was prepared by heating at 130 °C under
vacuum overnight. The sample of solvent-free 3 was re-
immersed in DMF to give sample 4.
2.3. Physical measurements
Power X-ray diffraction (PXRD) data were collected on a Bruker
D8 diffractometer with Cu Kα radiation (λ = 1.54018 Å). TGA
experiments were performed with a STA449 F3 thermogravi-
metric analyzer in the 30–800 °C range at a warming rate of
10 °C min−1 under a nitrogen atmosphere and the polycrystal-
line samples were placed in an Al2O3 crucible. The powered
samples of 1 and 3 were pressed into a pellet under the
pressure of ca. 10 MPa with a thickness of ca. 0.87 mm,
0.83 mm for 1 and 3, respectively; the area of 1 and 3 is
7.06 mm2. Two opposite surfaces of the pellets were covered
with a gold film via vacuum coating. The measurements of the
temperature and frequency dependent dielectric constant and
loss were carried out by employing a Concept 80 system (Novo-
control, Germany) in the temperature range 0–277 °C
(273–450 K). The powdered pellets of 1 and 3 were sandwiched
by two copper electrodes and the ac frequencies spanned from
1 to 107 Hz. The dielectric hysteresis loop measurements were
carried out on a ferroelectric tester (Radiant Technology) and
the ac electric field frequency ranged from 10 to 1000 Hz, the
pellets covered with a gold film were sandwiched by the
copper electrodes and immersed in insulating oil while
measured.
3. Results and discussion
3.1. Crystal to crystal transformation between 1 and 3
The detailed crystal structure of 1 was reported in our previous
work,20 herein, a simple description is given to assist us for
better understanding the corresponding discussion in the next
sections. The three-dimensional (3-D) framework of 1 is built
by Sr2+ ions and BDC2− bridged ligands and an asymmetric
unit comprised of one Sr2+, one BDC2− together with one DMF
molecule; the coordination sphere of the Sr2+ ion (a distorted
bicapped coordination octahedron) and the packing diagram
projected along the c-axis direction are displayed in Fig. 1a
Fig. 1 (a) Coordination sphere of Si2+ ion where H atoms were omitted for
Low-pressure gas sorption measurements: glass sample
tubes of a known weight were loaded with approximately clarity, (b) packing diagram viewed along the c-axis in the crystal of 1.
6604 | Dalton Trans., 2013, 42, 6603–6610
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and b, respectively. Two types of triangle channels appear in The lattice parameters, as well as the bond distances and
MOF 1, the smaller one is empty and the larger channel is angles in the coordination sphere of the Sr2+ ion, obtained
filled by DMF molecules that are bound directly to the Sr2+ from both the X-ray single crystal structure analyses and the
ions.
crystal structure optimization, are summarized in Tables 1
The DMF molecules, bound to the Sr2+ ions in the channels and 2 for 1 and 3, respectively. The DMF and BDC2− mole-
of crystal 1, can be removed to give the solvent-free compound cule structures and their bond distances in the crystal of 1,
3, by heating at ca. 265 °C or using dichloromethane exchange. obtained from the single crystal structure analysis at 293 K
As shown in Fig. 2, all diffraction peaks are broadened and the and the crystal structure optimization, are shown in
diffraction with the Miller index (h k l) = (1 0 0) is shifted Fig. S2.†
faintly to the lower angle side of the PXRD profile of 3 (with 2θ
The lattice parameters exhibit slight expansion in the opti-
= 9.6288°), compared to that of 1 (with 2θ = 9.3266°), indicat- mized crystal structure compared to the single crystal analysis
ing that DMF molecule removal from the channels of 1 results for 1 (see Table 1). As illustrated in Fig. 3 and Table 2, most of
in reduced crystallinity of the metal–organic framework and the bond distances and angles in the Sr2+ ion coordination
the lattice slightly expands along the a- and b-axes. It is noted sphere are in agreement, and fall well between the single
that 3 can be transformed into 1 by re-immersing 3 in DMF. crystal structure at 293 K and the optimized structure for 1,
This observation demonstrates the crystal-to-crystal transfor- except the bond distances d(Sr1–O1) = 2.679 Å (2.824 Å) and d
mation is reversible between 1 and 3.
(Sr1–O4#3) = 2.767 Å (3.052 Å), and bond angles ∠O1–Sr1–
Generally, lattice shrinking is observed when the lattice O3#3 = 99.8° (103.6°), ∠O2–Sr1–O2#2 = 111.4° (108.0°),
solvents inside the metal–organic framework are removed. In ∠O3#3–Sr1–O4#4 = 147.8° (151.3°) and ∠O1#1–Sr1–O3#3 =
contrast to the common situation, the a- and b-axes slightly 124.2° (119.5°) in the single crystal structure (the optimized
expend with 1 losing solvents in the channels to transform crystal structure), where the hash-marked atoms are generated
into 3. It is unavailable to determine the X-ray single crystal by a symmetric operation with the symmetric codes: #1 = −x +
structure of 3 owing to its bad crystallinity. In order to gain y, 1 − x, −1/3 + z; #2 = 1 − y, 1 + x − y, 1/3 + z; #3 = 1 − x + y,
deeper insight into this anomalous structural change, the 2 − x, −1/3 + z and #4 = −1 + x, y, z. It is worthy of note that
crystal structure optimizations were performed for 1 and 3. the bond lengths in the BDC2− moiety are quite close to each
other between the optimized and the single crystal structures
(ref. Fig. S2†).
Upon comparison of the optimized crystal structures of 1
and 3, it was found that the a- and b-axes expand a little while
the c-axis shrinks slightly after the DMF molecules in the chan-
nels are removed (ref. Table 1), and the lattice parameter alter-
nations stem from the coordination sphere change of the Sr2+
ion along the a- and b-axes. For instance, the significant
changes in the coordination sphere of the Sr2+ ion between 1
and 3 concerns the bond distances and angles below, d(Sr1–
O1) = 2.824 Å (2.694 Å), d(Sr1–O4#3) = 3.052 Å (3.217 Å), ∠O1–
Sr1–O4#4 = 78.9° (86.5°), ∠O1–Sr1–O3#3 = 103.6° (120.7°),
∠O2–Sr1–O3#3
= 81.5° (92.8°), ∠O2–Sr1–O2#2 = 108.0°
(112.1°), ∠O2–Sr1–O4#4 = 117.3° (123.7°), ∠O1#1–Sr1–O3#3 =
119.5° (112.3°), ∠O4#4–Sr1–O4#3 = 161.5° (155.5°), ∠O3#3–
Sr1–O2#2 = 82.3° (92.3°), ∠O3#3–Sr1–O4#4 = 151.3° (143.5°) in
1 (3). Obviously, the significant increase of the bond distance
Sr1–O4#3 and the angle ∠O1–Sr1–O3#3, from 1 to 3, is respon-
sible for the a- and b-axes expansion.
Fig. 2 PXRD profiles where the red, blue and purple lines correspond respect-
ively to the patterns of 1, 3 and 4.
Table 1 Lattice parameters for 1 and 3a
1
3
T = 243 K
T = 293 K
T = 403 K
Optimization
Optimization
a/Å
b/Å
c/Å
10.6123(2)
10.6123(2)
9.8289(2)
958.64(3)
10.62017(15)
10.62017(15)
9.84171(14)
961.31(2)
10.6396(3)
10.6396(3)
9.8731(3)
967.90(5)
10.8133
10.8133
10.0866
1021.39
10.8427
10.8427
10.0379
1021.99
V/Å3
a The lattice parameters of 1 from the X-ray single crystal structure analyses at 243, 293 and 403 K are cited from the literature.20
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Table 2 Bond distances (Å) and angles (°) in the coordination sphere of the Sr2+ ion for 1 and 3a
1
3
T = 243 K
T = 293 K
T = 403 K
Optimization
Optimization
d(Sr1–O1)
d(Sr1–O2)
d(Sr1–O1#1)
d(Sr1–O2#2)
d(Sr1–O3#3)
d(Sr1–O4#3)
d(Sr1–O4#4)
∠O1–Sr1–O2
∠O1–Sr1–O4#4
∠O1–Sr1–O2#2
∠O1–Sr1–O3#3
∠O1–Sr1–O4#3
∠O1–Sr1–O1#1
∠O2–Sr1–O1#1
∠O2–Sr1–O4#3
∠O2–Sr1–O3#3
∠O2–Sr1–O2#2
∠O2–Sr1–O4#4
∠O1#1–Sr1–O4#3
∠O1#1–Sr1–O3#3
∠O1#1–Sr1–O2#2
∠O1#1–Sr1–O4#4
∠O4#3–Sr1–O3#3
∠O4#3–Sr1–O2#2
∠O4#4–Sr1–O4#3
∠O3#3–Sr1–O2#2
∠O3#3–Sr1–O4#4
∠O2#2–Sr1–O4#4
2.679
2.713
2.527
2.557
2.550
2.758
2.500
48.2
77.4
71.7
99.5
109.0
97.1
71.6
69.5
81.3
111.4
116.1
75.6
124.3
156.1
87.8
2.679
2.714
2.534
2.562
2.551
2.767
2.498
48.1
77.7
71.6
99.8
108.8
97.0
71.5
63.4
81.7
111.4
116.2
75.6
124.2
156.0
87.7
2.679
2.723
2.547
2.581
2.550
2.779
2.495
47.8
78.2
71.4
99.7
108.4
96.9
71.2
63.4
81.8
111.1
116.2
75.5
123.9
155.9
87.8
2.824
2.749
2.562
2.615
2.664
3.052
2.516
47.4
78.9
70.1
103.6
107.4
96.8
72.1
62.1
81.5
108.0
117.3
74.0
119.5
157.4
88.1
2.694
2.612
2.564
2.580
2.562
3.217
2.451
49.8
86.5
70.4
120.7
108.1
98.5
71.9
60.8
92.8
112.1
123.7
74.3
112.3
155.1
84.3
48.8
48.7
48.5
45.6
43.6
127.8
162.6
79.1
147.7
69.4
127.8
162.6
79.3
147.8
69.4
127.8
162.6
79.6
148.0
69.4
126.8
161.5
82.3
151.3
71.6
129.9
155.5
92.3
143.5
73.1
a Symmetric codes: #1 = −x + y, 1 − x, −1/3 + z; #2 = 1 − y, 1 + x − y, 1/3 + z; #3 = 1 − x + y, 2 − x, −1/3 + z; #4 = −1 + x, y, z.
3.2. Thermal stabilities of 1 and 3
to one half when the calculation is based on the formula [Sr-
(BDC)(CH2Cl2)]∞. For solvent-free 3, no sizable weight loss was
The TG plots of 1–4 are displayed in Fig. 4. The DMF mole- detected before the metal–organic framework collapsed.
cules bound to the Sr2+ ions in the channels of 1 were comple-
It is worthy of note that the thermal stability of the solvent-
tely released at ca. 265 °C, this temperature is much higher free framework for samples 2–4 is much higher compared to
than the DMF decomposition temperature (151 °C). Interest- the as-synthesized sample 1. This phenomenon is probably
ingly, although the PXRD profiles are almost the same for 1 related to the fact that the blast of large amounts of gas, stem-
and 4, the TG plots show distinctions from each other. The ming from the DMF decomposition, reduces the thermal stabi-
DMF molecules begin to become lost below ∼100 °C in 4, but lity of the framework in 1 with high crystallinity. Another
the temperature to achieve full DMF molecule removal is the interesting issue is that the frameworks in 2–4 can be ther-
same as that for 1. In addition, the thermal stability of the mally stable at more than 600 °C. To date, a large number of
metal–organic framework after the DMF molecules were MOFs have been reported, it was noted that the natures of
removed is higher in 4 than in 1. For 2, the dichloromethane both the metal ions and bridged ligands affect strongly the
molecules inside the channels of the framework are lost thermal stability of a MOF compound, in general, the MOF
between 65 and 110 °C, and this temperature is much lower constructed from the flexible linkers is more thermally stable
than the temperature of DMF release since CH2Cl2 is a weakly than one built from rigid linkers. In addition, MOFs with a
coordinating solvent compared to DMF. The total weight loss main group metal ion or d10 transition metal ion are more
is 12% for the lattice solvents in 2, and this value is only close thermally stable than those with open-shell transition metal
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ions. The relationship between the electronic configuration of polarization (Ps), remnant polarization (Pr) and coercive field
the Sr2+ ion and the thermal stability of the corresponding (Ec) are roughly estimated as 0.025 μC cm−2, 0.018 μC cm−2
,
MOF can be understood, since the coordinate bond between and 7.1 kV cm−1 under an applied field of 14.5 kV cm−1 with
the closed-shell metal ion and ligands does not have a pre- an ac frequency (f) of 16.7 Hz for 1; these values are less than
ferred direction; such a type of bond is favored for reducing the corresponding parameter values obtained from the single
lattice strains, to give the corresponding MOF possessing crystal sample of 1 along the polar c-axis direction.20 This
higher thermal stability.
observation is due to the existence of highly anisotropic polari-
zation in the ferroelectric material.
A ferroelectric hysteresis loop demonstrates that the macro-
scopic polarization in the ferroelectric material is reversed or
switched under an ac applied electric field, and the macro-
scopic polarization reversal originates from the local, micro-
scopic polarization reversal, which includes electronic, atomic
and ionic displacement polarization reversals as well as perma-
nent dipole reversal under an ac electric field. In our previous
study,20 two possible mechanisms for the polarization reversal
were proposed according to the analyses of the variable-temp-
erature single crystal structures, namely, the swing motion of
the polar DMF molecule bound to the Sr2+ ion along the c-axis
direction, as well as the deformation of the Sr2+ coordination
sphere, arising from the DMF displacement along the Sr–O
(DMF) bond direction. From the P–E measurements of 1 and
3.3. Ferroelectricity of 1 and 3
The crystals of 3, obtained from the removal of DMF from the
crystals 1, are much more easily broken, even if they retain the
original shape; as a result, it was not possible to measure the
P–E plot using a single crystal of 3. The field dependence of
polarization was only investigated using the powdered sample
of 3. The P–E plots for the powdered samples of 1 and 3 are
displayed in Fig. 5, and the typically ferroelectric hysteresis
loops appear in the powdered sample of 1, while they were not
clear in the powdered sample of 3. The spontaneous
Fig. 3 Structure for illustrating the changes of both the Sr2+ ion coordination
sphere and the crystallographic a- and b-axes (the hash marked atoms are gen-
erated by a symmetric operation with the symmetric codes: #1 = −x + y, 1 − x,
−1/3 + z; #2 = 1 − y, 1 + x − y, 1/3 + z; #3 = 1 − x + y, 2 − x, −1/3 + z and
#4 = −1 + x, y, z).
Fig. 4 TG plots of 1–4 at a warm rate of 10 °C min−1, respectively.
Fig. 5 Plots of polarization versus electric field of (a) 1 and (b) 3 at ambient temperature.
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Fig. 6 Temperature dependencies of the dielectric constant, ε’, and dielectric loss, tan(δ), of (a, b) 1 and (c, d) 3 in the frequency range 1–107 Hz.
3, it is obvious that the motion of the polar DMF molecules
In the four different dielectric relaxation mechanisms, the
really plays a crucial role in the process of macroscopic polari- one originating from the electronic transitions or the bond
zation reversal and the framework is not responsible for the vibrations within a molecule appears in the high frequency (f)
ferroelectric behavior of 1.
region with f > 1012 Hz, and the dielectric relaxation arising
from the dipole motion or ionic polarization occurs in the fre-
quency regime (f < 1010 Hz). On the one hand, the variable-
temperature single crystal structure analyses indicated that the
coordinated DMF molecule is disordered at higher tempera-
ture; on the other hand, the Sr2+ ion is bound to the O atoms
within the coordination sphere, only through electrostatic
interactions, owing to the closed-shell electronic configuration
of the Sr2+ ion. As a result, it is possible that the dipole orien-
tations of the disordered DMF molecules and the SrO8 coordi-
nation polyhedra are changed under an ac electrical field, and
the dielectric relaxation observed in the 1–104 Hz frequency
range could be attributed to the dipole orientation motions of
the DMF molecules and the SrO8 coordination polyhedra in 1,
while only to the dipole orientation motion of the SrO8 coordi-
nation polyhedra in 3.
3.4. Dielectric properties of 1 and 3
The dielectric behavior of 3 was further investigated and com-
pared with that of 1 in the frequency range 1–107 Hz. The
dielectric constant, ε′, and dielectric loss, tan(δ), as a function
of temperature for 1 and 3 are displayed in Fig. 6, respectively.
The ε′ and tan(δ) values are nearly constant below 325 K for 1,
with ε′ ≈ 5.04 and tan(δ) ≈ 0.002, and at 460 K for 3, with ε′ ≈
4.96 and tan(δ) ≈ 0.041. Both the dielectric constant and loss
rise rapidly with increasing of the temperature above 325 K for
1 and 460 K for 3, especially, under a lower frequency (f <
104 Hz). The frequency-dependent behavior of the ε′ and tan(δ)
values indicated the presence of dielectric relaxation in the
higher temperature region. The distinctions of the dielectric
properties are observed between 1 and 3, for example, (1) ε′ ≈
18 for 1, while ε′ ≈ 4.9 for 3 at 433 K under f = 1 Hz. (2) The
tan(δ) value increases monotonically with temperature for 1,
whereas shoulder is shown for 3. These differences demon-
strated that 1 and 3 possess dissimilar dielectric relaxation
4. Conclusion
behaviors, which are further identified from the different fre- In summary, we investigated comparatively the crystal struc-
quency dependencies of ε′ and tan(δ) for 1 and 3 at various tures, thermal stabilities, ferroelectric and dielectric beha-
temperatures (see Fig. 7).
viors of the ferroelectric MOF [Sr(μ-BDC)(DMF)]∞ (1) and the
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Fig. 7 Frequency dependencies of the dielectric constant, ε’, and dielectric loss, tan(δ), of (a, b) 1 in the temperature range 274–433 K and (c, d) 3 in the tempera-
ture range 293–553 K.
corresponding solvent-free framework [Sr(μ-BDC)]∞ (3).
A
Notes and references
reversible crystal-to-crystal transformation occurs between 1
and 3, and a slight expansion of the lattice a and b para-
meters is observed, which is associated with the bond dis-
tances and angles in the coordination sphere of the Sr2+ ion
increasing along the a- and b-axes. The solvent-free MOF [Sr-
(μ-BDC)]∞ shows an extremely high thermal stability, which
is thought to be related to the closed-shell electronic struc-
ture of the Sr2+ ion. The coordinate bonds between the Sr2+
ion and the O atoms of the BDC2− ligands have no preferred
orientation in space, which is favored for reducing lattice
strains and is responsible for the higher thermal stability.
The ferroelectricity occurs in 1 whereas disappears in 3, indi-
cating that the motion of the DMF molecules in the chan-
nels is responsible for the macroscopic polarization reversal
in 1.
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