Effect of counter anions on ferroelectric properties of diisopropylammonium-cation based molecular crystals

Article abstract of DOI:10.1002/pssa.201700029

Diisopropylammonium cation based single crystals with different counter anions (F^?, Cl^?, Br^?, I^?, NO^-₃, and CIO^-₄) were prepared. Their crystal structures and ferroele

Full text of DOI:10.1002/pssa.201700029

Phys. Status Solidi A, 1700029 (2017) / DOI 10.1002/pssa.201700029
a
www.pss-a.com
applications and materials science
Effect of counter anions on
ferroelectric properties of
diisopropylammonium-cation
based molecular crystals
,1
,1
,1
,1,2
Chunli Jiang*** , Wen-Yi Tong*** , Hechun Lin* , Chunhua Luo1, Hui Peng** , and Chun-Gang Duan1,2
1 Department of Electrical Engineering, Key Laboratory of Polar Materials and Devices, East China Normal University, Ministry of
Education, Shanghai 200241, China
2 Collaborative Innovation Center of Extreme Optics, Shanxi University, Taiyuan, Shanxi 030006, China
Received 20 January 2017, revised 12 February 2017, accepted 28 February 2017
Published online 24 March 2017
Keywords density-functional theory calculation, diisopropylamine, molecular ferroelectrics, polarization
* Corresponding author: e-mail hclin@ee.ecnu.edu.cn, Phone: þ0086 215434 5114, Fax: þ86 21 5434 5119
** e-mail hpeng@ee.ecnu.edu.cn, Phone: þ0086 215434 2726, Fax: þ86 21 5434 5119
*** These authors contributed equally
Diisopropylammoꢀnium cationꢀbased single crystals with different of diisopropylammonium-based single crystals are closely related
counter anions(F ,Clꢀ,Brꢀ,I ,NO3ꢀ,andClOꢀ4 ) were prepared. to the electronegativity and structure of the counter anions. For
Their crystal structures and ferroelectric properties were monoatomic anions, higher electronegativity results in higher
investigated by single-crystal X-ray diffraction spectroscopy, polarizationandphase-transitiontemperatureofthecorresponding
differential scanning calorimetry, and dielectric measurements. molecular–ionic ferroelectrics. The results of density-functional
The experimental results illustrate that the ferroelectric properties theory (DFT) calculations also support the experimental results.
ß 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
1 Introduction Ferroelectrics, which have spontane- also developed by a self-assembly of charge acceptor and
ous electric polarization that can be reversed by external different donors [11–14]. In these crystalline lattices,
electric field, are essential to many practical applications, dipoles have originated from collective transfer of electrons
including electronics, electro-optics, and electrome- from donor to acceptor molecules [12]. Although great
chanics [1, 2]. Since the discovery of the first ferroelectric efforts have been made in this research area, the discovered
crystal, Rochelle salt [3], research on ferroelectrics was small molecular ferroelectrics with a Curie point (Tc) above
mainly focused on inorganics and polymers, such as lead room temperature are rare. The molecular design and
zirconate titanate (PZT) [4, 5], BaTiO3-based ceramics growth of new small-molecule ferroelectrics are still
[6, 7], and polyvinylidene fluoride [8, 9]. Recently, small- challenging.
molecule ferroelectrics, including single- and two-compo-
In the process of searching for new small-molecule
nent organic compounds, have been receiving immense ferroelectrics, nitrogenous organic compounds showed
academic and practical attention due to the advantages of extraordinary talents as dielectric, piezoelectric, and
lightweight, flexibility and ease of synthesis and integration, ferroelectric materials. Thiourea was reported to be the
which make them good candidates for the development of first low molecular mass organic ferroelectric [15], glycine
all-organic electronics. For example, croconic acid crystal molecule exhibited room-temperature ferroelectricity in its
shows a room-temperature ferroelectricity with a spontane- salts [16], 2,2,6,6-tetramethyl-1-piperidinyloxy free radical
ous polarization (Ps) of about 21 mC cmꢀ2 and a coercive presented the first para-ferroelectric transition [17], and
field of 14 kV cmꢀ1, which are comparable to those of imidazolium-based molecular ferroelectric possessed high
inorganic ferroelectric materials. [10] A series of three- Curie point (Tc) of 373 K and low coercive field [18].
dimensional ferroelectric supramolecular networks were Among these excellent achievements, the discovery of
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C. Jiang et al.: Diisopropylammonium-cation based molecular crystals
diisopropylammonium bromide (DIPAB), which has prop- program) and refined by full-matrix least-squares on F2
erties comparable with perovskite ceramics [19, 20], is a (SHELXL-97 program) using Olex2014 software. Differ-
milestone in this research area [21]. DIPAB shows a Tc ential scanning calorimetry (DSC) measurements were
value of 426 K and a spontaneous polarization value of performed on a DSC-60 (Shimadzu Corporation) instrument
23 mC cmꢀ2, as well as good thermal and mechanical under a nitrogen atmosphere. The heating and cooling rates
stability. The origin of ferroelectricity in DIPAB was were both 6 K minꢀ1 and pure aluminum was used for the
ascribed to the order–disorder transition of the diisopropy- calibration of temperature and enthalpy. For dielectric and
lammonium cations [21, 22]. Upon replacing Brꢀ ion with ferroelectric measurements, the electrodes were prepared on
Clꢀ, the spontaneous polarization of diisopropylammonium crystals by using silver paste. Ferroelectric hysteresis loops
chloride (DIPAC) dramatically decreases to 8 mC cmꢀ2 and were recorded on a Precision LC ferroelectric tester
Tc increases to 440 K [23]. The ferroelectricity of DIPAC (Radiant Technology) and dielectric permittivities were
was also attributed to the order–disorder transitions of the measured on an Agilent E4980A.
diisopropylammonium cations [23]. Therefore, the big
difference in the disorder of the polarization values of these
two crystals cannot be fully understood.
3 Results and discussion
3.1 Synthesis and crystal structures The single
In our previous work, the scaling behavior of dynamic crystals of diisopropylammonium (DIPA)-based salts were
hysteresis of the DIPAB crystal indicated the coexistence of grown in methanol by slow evaporation. The obtained single
the order–disorder and displacive transitions in this small- crystals were all colorless as shown in Fig. 1. The crystal
molecule ferroelectric [24]. Then a question arises: how have structures were examined by single-crystal XRD and the
the counter anions affected the ferroelectric properties of results are presented in Fig. 2 and Table 1. DIPA-X single
diisopropylammonium cation-based molecular ferroelec- crystals were formed by protonated diisopropylamine and
trics? Motivated by this question, we embarked on the anions through electronic interaction and hydrogen bonding
exploration of the relationship between the ferroelectric between the hydrogen atoms of NH2 group and the anions.
propertiesandthecounteranionsofthismolecular–ionic-type The lengths of the hydrogen bonds varied with anions, as
ferroelectric. A series of diisopropylammonium-anion singꢀle seen in Fig. 2. The single crystals prepared with halogen
crystals with different anions (DIPA-X, X ¼ HF2ꢀ, Clꢀ, Br , acids adopted the same space group of P212121 at 296 K. As
Iꢀ, NOꢀ3 , ClO4ꢀ) were prepared. The crystal structures, phase- reported in previous works [21, 24], P212121 phase can be
transition temperatures and ferroelectric properties of these converted to ferroelectrically active P21 phase by heat
crystals were investigated. At the same time, the spontaneous treatment at 430 K. It is worth noting that DIPA-HF2 instead
polarizations were calculated based on the density-functional of DIPA-F was obtained from hydrofluoric acid due to the
theory (DFT). The obtained results were compared with the strong hydrogen bonding between Fꢀ and HF. DIPA-NO3
experimental ones.
and DIPA-ClO4 were in P21/n and P1 space groups,
respectively.
2 Experimental
2.1 Materials Diisopropylamine (99%) was pur-
3.2 Differential scanning calorimetry The phase-
chased from J&K Scientific Ltd., (Beijing, China). Hydro- transition temperature (Tc) is one of the most concerned
fluoric acid (40%), hydrochloric acid (40%), hydrobromic characters of ferroelectric materials. Above Tc, ferroelectric
acid (48%), hydroiodic acid (55%), and absolute methanol materials turn into the paraelectric state, thus a high Tc is
(99.8%) were obtained from Aladdin Chemistry Co., Ltd. desirable in terms of practical applications. Differential
(Shanghai, China).
scanning calorimetry (DSC) measurements were performed
to detect the phase transitions of the six crystals. The
2.2 Synthesis By utilizing diisopropylammonium obtained DSC curves of polycrystalline samples are
(DIPA) as a cation, six single crystals with different presented in Fig. 3 and the transition temperatures are
monovalent counter anions were synthesized, namely summarized in Table 2. It is clear that all crystals except
DIPA-HF2, DIPA-Cl, DIPA-Br, DIPA-I, DIPA-NO3, and
DIPA-ClO4. In a general synthesis process, equal molar
amounts of diisopropylamine and the corresponding concen-
trated acid were dissolved in water. After slow evaporation at
room temperature, the produced colorless crystals were
collected. The obtained crystals were recrystallized in
methanol for the growth of single crystals. In the case of
DIPA-HF2, plastic beakers were used to prevent the formation
of (DIPA)2 [SiF6] [25].
2.3 Characterization Single-crystal XRD data of
the crystals were recorded with a Bruker D8 Venture. All the
structures were solved by direct methods (SHELXS-9713 Figure 1 Photographs of DIPA-X crystals.
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Original
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Phys. Status Solidi A (2017)
(3 of 6) 1700029
Fig. 2, an increasꢀing trend can be found in the order of Fꢀ,
NOꢀ3 , ClOꢀ4 , Cl , Brꢀ, and Iꢀ, which is also in good
agreement with the decrease of their phase-transition
temperatures, except for ClOꢀ4 . These results indicate that
the phase-transition temperatures of the DIPAX crystals are
closely related to the electronegativity of the counter
anions. Higher electronegativity leads to higher phase-
transition temperature.
3.3 Dielectric properties As an important evidence
for the ferroelectric nature of the phase transitions,
temperature-dependent dielectric constants of the crystals
were investigated during a heating process. The real parts
(e0) of the complex dielectric constants at a frequency of
Figure 2 Unit structures of DIPA-X: (a) ꢀHFꢀ2 ; (b) ꢀClꢀ; (c)
ꢀBrꢀ; (d) ꢀIꢀ; (e) ꢀNO3ꢀ, and (f) ꢀClOꢀ4 .
DIPA-HF2 showed reversible endothermic and exothermic 1 MHz in the temperature range of 290–450 K are shown in
peaks, clearly illustrating the occurrence of reversible phase Fig. 4. Prominent dielectric anomalies with the change of
transitions in these crystals. The phase-transition temper- temperature were observed for DIPA-Cl, DIPA-Br, DIPA-I,
atures are in a sequence of DIPA-NO3> DIPA-Cl > DIPA- DIPA-NO3, and DIPA-ClO4, which occurred at around 439,
Br > DIPA-I > DIPA-ClO4, as shown in Table 2. For 422, 375, 440, and 337 K, respectively. The transition
DIPA-HF2, no peak appeared until the sample melted above processes were very rapid within a temperature range of 2 K,
450 K, which may indicate that the phase-transition which benefits the applications of switchable dielectrics.
temperature of DIPA-HF2 was higher than 450 K. As the The temperatures of the dielectric phase transition were well
mechanism of the paraelectric–ferroelectric transition of consistent with the results of DSC measurements. For
DIPA-based molecular ferroelectrics was ascribed to the DIPA-HF2, no dielectric anomaly was observed until the
“order–disorder” behavior of the N atom [21, 22], the single crystal gradually collapsed and melted at about 460 K
energy that hinders the “order–disorder” transformation (data not given).
thus played an important role in the phase-transition
temperature. Higher energy levels would result in a higher
3.4 Ferroelectric properties To verify and compare
phase-transition temperature. In the DIPA-X systems, the ferroelectric properties of the DIPA-X crystals,
attractive forces between DIPA and X anions in the polarization–electric field (P–E) loops were measured. As
direction of hydrogen bonds of N–HꢁꢁꢁX mainly depended presented in Fig. 5, ferroelectric hysteresis loops of DIPA-
on the electronegativity of X andꢀinterꢀatomic distances [26]. HF2, DIPA-Cl, DIPA-Br, DIPA-I, and DIPA-ClO4 were
The electronegativities of Fꢀ, Cl , Br , Iꢀ, NO3ꢀ, and ClOꢀ4 obtained at a frequency of 100 Hz, with spontaneous
are 1.59, 1.08, 0.84, 0.42, 4.06, and 4.39, respectively [27], polarizations of 10.67, 10.48, 9.94, 5.17, and 0.16 mC cmꢀ2
,
which are in good accordance with the trend of transition respectively. During the attempt of P–E measurement for
temperatures of the crystals measured by DSC except for DIPA-NO3, only linear curves were obtained until the
that of DIPA-ClO4. If we look at the interatomic distances breakdown of the crystal by a high applied electric field,
between the DIPA cations and X anions, which were indicating paraelectric characteristics of DIPA-NO3 under
represented as the hydrogen bond lengths, as shown in the test conditions. The Ps values for DIPA-HF2, DIPA-Cl,
Table 1 Brief crystal and structure refinement data of DIPA-X.
anion
HFꢀ2
Clꢀ
Brꢀ
Iꢀ
NOꢀ3
ClOꢀ4
triclinic
crystal-system orthorhombic orthorhombic monoclinic orthorhombic monoclinic orthorhombic monoclinic
space-group
P212121
141.21
893.4 (6)
4
7.726 (2)
8.057 (3)
14.352 (6)
90
90
90
0.0298
0.0709
P212121
137.65
857.84 (10)
4
7.831 (5)
8.255 (5)
13.270 (9)
90
90
90
0.0287
0.0722
P21
137.65
429.25 (18) 904.3 (18)
P212121
182.11
P21
182.10
450.91 (4)
2
7.8639 (4)
8.0996 (4)
7.8961 (4)
90
116.292 (1) 90
90
0.0300
0.0793
P212121
227.08
972.9 (5)
4
8.306 (3)
8.306
14.101 (6)
90
P21/n
164.21
978.1 (11)
4
8.186 (5)
8.358 (6)
14.394 (10) 8.77 (3)
90
96.640
90
0.0499
0.1610
P1
201.65
543 (4)
2
8.15 (3)
8.52 (4)
Mr [g molꢀ1
]
3
̊
volume/A
Z
a/A
b/A
c/A
2
4
̊
̊
7.674 (15)
7.945 (16)
7.766 (16)
90
8.019 (8)
8.303 (12)
13.583 (12)
90
̊
a/8
b/8
g/8
R1
82.77 (19)
65.14 (13)
79.9 (2)
0.1098
114.95 (3)
90
90
90
90
0.0164
0.0417
0.0347
0.0945
0.0857
0.2737
wR2
0.3450
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C. Jiang et al.: Diisopropylammonium-cation based molecular crystals
300
ClO4
350
400
450
300
350
400
450
125
120
115
110
105
1.7
ClO4
0.0
-1.7
-3.4
133
126
119
112
NO3
5
0
-5
NO3
I
135
126
117
108
I
1.0
0.5
0.0
-0.5
Br
1.4
0.0
-1.4
451
440
429
418
407
Br
Cl
Cl
0.8
0.4
0.0
Cooling
300
290
280
270
Heating
400
300
350
450
Temperature (K)
300
350
400
450
Temperature (K)
Figure 3 DSC curves of the DIPA-X crystals.
Figure 4 Temperature-dependent dielectric constants at 1 MHz of
the DIPA-X crystals.
DIPA-Br and DIPA-I have generally decreased with
increasing atomic number, and DIPA-ClO4 has the lowest
Ps, showing a similar variation trend to that of the phase-
transition temperatures.
dynamic disorder, with four O atoms randomly occupying
the six vortices of an octahedron [29, 31]. While, in the
ferroelectric phase, the ClOꢀ4 anions are well ordered in
spite of a strong cationic disorder [29, 31]. The small off-
center displacement together with a slight distortion of the
ClOꢀ4 anion contributed to the Ps of these materials with a
magnitude of 0.1–1.0 mC cmꢀ2. The Curie temperatures of
the related ferroelectrics were relatively low, ranging from
184 to 373 K [30–32]. In our case, the phase-transition
temperature of DIPA-ClO4 was 100 K lower than the
analogs of DIPA-Br and DIPA-Cl, although ClOꢀ4 has a
much higher electronegativity and similar interatomic
distances to the DIPA cation. Thus, the phase transition
of DIPA-ClO4 may come from the order–disorder transition
of the ClOꢀ4 rather than DIPA, which reasonably explains
the observed small polarization of this crystal.
As discussed above, both the phase-transition tempera-
ture and the ferroelectric polarizations of DIPA-ClO4 were
the exceptions when the phase transition and ferroelectricity
of this type of ferroelectrics was considered to only originate
from the arrangement transition of DIPA cation. In the
crystal lattice of DIPA-ClO4, the ClOꢀ4 anion is well
ordered, adopting a tetrahedral geometry with four O atoms
occupying four vortices of a tetrahedron. The Cl–O bond
̊
distances are 1.181–1.293 A and the O–Cl–O bond angles
are 103.17–119.5428. Although stable, the ClOꢀ4 anion can
readily change its position by varying the temperature
because of its moderate size and highly symmetric
shape [28]. In fact, several ferroelectric organic ammonium
perchlorates, like imidazolium perchlorate, pyridinium
perchlorate, and 4-(cyanomethyl)anilinium perchlorate,
have been developed and the driving force of phase
transition was attributed to the order–disorder transition of
the ClOꢀ4 anions [29–32]. In the paraelectric phase of these
ferroelectrics, the ClOꢀ4 anions were found to show a
3.5 Theoretical calculations To verify our mea-
surement of spontaneous polarizations, we performed
density-functional theory (DFT) calculations using the
accurate full-potential projector augmented wave (PAW)
Table 2 Summary of phase-transition temperatures (K) from DSC and dielectric measurements.
crystals
DIPA-HF2
DIPA-Cl
DIPA-Br
DIPA-I
DIPA-NO3
DIPA-ClO4
phase-transition temperatures (K) from DSC peaks
heating
cooling
melted
439
436
424
420
378
358
441
438
338
316
dielectric anomalies (K) from dielectric data at 1 MHz
heating melted 439
422
375
440
337
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Original
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Phys. Status Solidi A (2017)
(5 of 6) 1700029
12
9
representative DIPA-Br, and then performed anion
substitution, followed by sufficient structural relaxation
and optimization. The convergence criterion for the
electronic energy is 10ꢀ6 eV and the structures are relaxed
until the Hellmann–Feynman forces on each atom are less
12
9
6
6
3
3
0
0
-3
-6
-9
-12
-3
-6
than 1 meV Aꢀ1. The Berry-phase method [35] is employed
̊
DIPA-Cl
10 20
DIPA-HF2
-9
for the evaluation of the crystalline polarization. Note that
as the ionic contribution to the dipole moment largely
depends on the choice of cell basis [36], there might be
overestimation of polarization values. In our calculations,
the potential modulo eR Vꢀ1 has already been considered
and eliminated. In order to confirm the reliability of our
first-principles calculations, the model predication of
spontaneous polarizations upon the simple assumption
that the positive and negative charge are located on
the protonated nitrogen atom and halogen atom is also
carried out.
-12
-20 -10
0
-20 -10
0
10 20
E (kV/cm)
E (kV/cm)
6
12
9
4
2
6
3
0
0
-3
-6
-9
-12
-2
-4
-6
DIPA-Br
10 20
DIPA-I
-20 -10
0
-20 -10
0
10 20
E (kV/cm)
E (kV/cm)
As listed in Table 3, when the anions for DIPA are
halogen elements, all of the relaxed ferroelectric structures
are in polar P21 symmetry. It can be seen that the calculated
structure data are in good agreement with experimental ones
in the cases of DIPA-Cl and DIPA-Br. With the
enhancement of atomic radius, the volume not surprisingly
increases. The difference of averaged coordinates between
nitrogen and halogen atoms along the b-axis, however,
gradually reduces, due to the smaller inductive effect. Both
the decreasing trend of spontaneous polarization from
DIPA-F to DIPA-I and the polarization values themselves
are in good agreement with ferroelectric hysteresis loop
measurements. The consistency between first-principles
calculations and model predictions guarantees the accuracy
of our theoretical results. It is interesting to point out that the
DFT values are generally greater than the model ones,
since only the ionic dipole moment is included in the model
prediction, the electronic contribution is neglected for
simplicity. In addition, similar calculations are also
applied to the ferroelectric phase of DIPA-ClO4. The tiny
calculated dipole moment reproduces the experimental
result, indicating that the choice of anions will largely affect
the spontaneous polarization for DIPA.
0.20
0.15
0.10
0.05
0.00
-0.05
-0.10
-0.15
-0.20
DIPA-ClO4
-6 -4 -2
0
2
4
6
E (kV/cm)
Figure 5 Polarization–electric field hysteresis loops of the DIPA-
X crystals.
method, as implemented in the Vienna ab initio Simulation
Package (VASP) [33]. The exchange-correlation potential
is treated in the Perdew–Burke–Ernzerhof (PBE) form [34]
of the generalized gradient approximation (GGA) with a
kinetic-energy cutoff of 500 eV. A 4 ꢂ 4 ꢂ 4 and 6 ꢂ 6 ꢂ 6
Monkhorst–Pack k-point mesh centered at the G point are,
respectively, adopted in the geometry optimization and
self-consistent calculations. For DIPA-Cl, DIPA-Br, and
DIPA-ClO4, structural relaxations start from experimental
data. For other cases, we referenced the structure of
Table 3 Summary of structural and polarization information for ferroelectric phase of DIPA-F, DIPA-Cl, DIPA-Br, DIPA-I, and DIPA-
ClO4.
anions
Fꢀ
Clꢀ
Brꢀ
Iꢀ
ClOꢀ4
3
̊
volume/A
Z
385.30
2
7.64
7.48
7.49
90
115.98
90
10.65
9.54
10.67
412.44
2
7.63
7.84
7.67
90
115.94
90
7.06
6.51
10.48
427.04
2
7.73
7.96
7.75
90
116.51
90
6.32
5.81
9.94
451.71
2
7.91
8.12
7.91
90
117.32
90
5.57
5.09
5.17
460.32
2
̊
a/A
7.59
8.07
8.31
81.80
66.73
81.25
0.26
0.34
0.16
̊
b/A
̊
c/A
a/8
b/8
g/8
DFT-Ps (mC cmꢀ2
)
model-Ps (mC cmꢀ2
)
measured Ps (mC cmꢀ2
)
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C. Jiang et al.: Diisopropylammonium-cation based molecular crystals
4 Conclusions In summary, six diisopropylammo- [12] A. S. Tayi, A. K. Shveyd, A. C.-H. Sue, J. M. Szarko, B. S.
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nium (DIPA) cation-based single crystals were synthesized,
five of which displayed above-room-temperature dielectric
transitions and ferroelectric polarizations. A comparative
study revealed that the associated anions (Fꢀ, Clꢀ, Brꢀ, Iꢀ,
NOꢀ3 , and ClOꢀ4 ) played an important role in the
ferroelectric properties of the DIPA-X compounds, even
when the ferroelectricity originated from an order–disorder
transition of the DIPA cation. The monoatomic anions
(halogen anions) of the coordinated crystals exhibited more
prominent ferroelectric properties than the polyatomic ones,
which was attributed to different mechanisms for the
origination of ferroelectricity, namely, the former was from
the transformation of diisopropylammonium cations while
the latter was from the configuration transformation of
anions. On the other hand, the ferroelectric phase-transition
temperature and polarization of the halogen anions
coordinated crystals decreased with the increase of the
atomic number from Fꢀ to Iꢀ, which were ascribed to the
change of the electronegativity of the anions. From this
study, monoatomic anions of high electronegativity were
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Acknowledgements This work was supported by the 973
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