Room-temperature ferroelectricity in diisopropylammonium bromide

Article abstract of DOI:10.1039/c2ce26580j

A room-temperature ferroelectric, diisopropylammonium bromide (DPB), with dielectric constant ε ≈ 12000 and a clear hysteresis loop at T _c = 425 K is reported. At 417 K DPB undergoes the irreversible phase transition from nonpolar orthorhombic P2₁2₁2₁ to the ferroelectric monoclinic phase (P2₁) and subsequently, at 425 K, to the paraelectric prototype phase (P2₁/m). The molecular mechanism of the paraelectric-ferroelectric transition is ascribed to the 'order-disorder' behaviour of the diisopropylammonium cations.

Full text of DOI:10.1039/c2ce26580j

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Room-temperature ferroelectricity in
diisopropylammonium bromide3
Cite this: CrystEngComm, 2013, 15,
40
9
a
b
a
a
Anna Piecha,* Anna G a˛ gor, Ryszard Jakubas and Przemysław Szklarz
A room-temperature ferroelectric, diisopropylammonium bromide (DPB), with dielectric constant e #
1
2 000 and a clear hysteresis loop at T
transition from nonpolar orthorhombic P2
subsequently, at 425 K, to the paraelectric prototype phase (P2
c
= 425 K is reported. At 417 K DPB undergoes the irreversible phase
to the ferroelectric monoclinic phase (P2 ) and
/m). The molecular mechanism of the
Received 28th September 2012,
Accepted 13th November 2012
2 2
1 1 1
1
1
DOI: 10.1039/c2ce26580j
paraelectric–ferroelectric transition is ascribed to the ‘order–disorder’ behaviour of the diisopropylammo-
nium cations.
www.rsc.org/crystengcomm
It is widely known that ferroelectric crystals are important
basic materials for technological applications in capacitors
and in piezoelectric, pyroelectric, and electrooptical devices.
complicated. Two polymorphs of DPB are known at room
6
7
1 1 1 1
temperature: monoclinic P2 and orthorhombic P2 2 2 . The
1
crystal structure of the latter polymorph (abbreviated in this
Their nonlinear characteristics were found to be very useful,
for example, in optical second-harmonic generators and other
nonlinear optical devices. Much of the attention in this field
has been focused on developing ferroelectric inorganic
paper as O) was presented as a private communications by G. J.
Reiss. The monoclinic polymorph was obtained accidentally
7
during the reaction of BrRe(CO)
mide. It undergoes an irreversible PT below room tempera-
ture to the monoclinic phase (P2 /n). In turn, O presented in
1
and lithium diisopropyla-
5
6
2
8
compounds such as KH PO (KDP), perovskite-type com-
2
4
3
this paper was obtained during the reaction of HBr and
diisopropylamine and did not disclose any PTs below room
temperature. Our present studies has revealed that O under-
goes a polymorphic irreversible structural PT to a polar
monoclinic phase (M(II)) at 417 K that is associated with the
substantial rearrangement of the crystal packing. This new
phase is stable from 90 to 425 K and at 425 K transforms to
pounds (BaTiO
3
) and LiNbO
3
. Over the past two decades one
can observe a great deal of progress in the synthesis of acentric
inorganic–organic hybrids which may be characterized by
4
polar/ferroelectric packing arrangements. Recently, the mole-
cule-based ferroelectric, simple alkylammonium organic salt
diisopropylammonium chloride (abbreviated as DPC) has
5
been synthesized and characterized. It undergoes a reversible
1
paraelectric phase P2 /m.
phase transition (PT) from a high temperature paraelectric
It should be emphasized that the high Curie temperature
c
(T ) exceeding room temperature (RT) in ferroelectric materials
is a very important parameter for ongoing design of all-organic
electronics.
The fact that ionic organic compound DPB exhibits strong
ferroelectricity suggests that other simple organic salts may
have comparable ferroelectric properties to those encountered
in inorganic ferroelectrics. Hence, this discovery is surely
expected to encourage the development of ferroelectric organic
materials.
phase (2/m) to a low temperature ferroelectric phase (2) at ca.
4
40 K. DPC exhibits one of the largest values of spontaneous
2
2
s
polarization, P = 8.9 mC cm , within the molecule-based
ferroelectrics, thus this group of compounds may be very
promising from the point of view of practical applications.
Initially, the closely related bromine analog, diisopropylam-
monium bromide (DPB), had seemed to be a good candidate
to exhibit polar properties in the room temperature phase,
however, further research indicated that the phase situation
and a rich polymorphism of DPB appeared to be quite
The main purpose of the present study is to characterize the
ferroelectric properties in DPB based on X-ray diffraction and
dielectric measurements over a wide temperature range.
Colorless crystals of O were prepared by reaction of
diisopropylamine with 48% aqueous HBr (molar ratio 1 : 1).
After a few days, transparent solids were formed by a slow
evaporation from the colorless solution. The obtained poly-
crystalline material was twice recrystallized in a mixture of
methanol and ethanol (1 : 1) at decreased temperature (about
263 K). The single crystals of DPB were grown at the constant
a
Faculty of Chemistry, University of Wrocław, F. Joliot-Curie 14, 50-383 Wrocław,
Poland. E-mail: anna.piecha@chem.uni.wroc.pl; Fax: +48 713757270;
Tel: +48 713757288
b
W. Trzebiatowski Institute of Low Temperature and Structure Research PAS, P.O.
Box 1410, 50-950 Wrocław, Poland
3
(
Electronic supplementary information (ESI) available: Details of the thermal
DSC and TGA), dielectric and X-ray diffraction measurements as well as tables
with selected bond lengths and angles. CCDC 893209–893211. For ESI and
crystallographic data in CIF or other electronic format see DOI: 10.1039/
c2ce26580j
9
40 | CrystEngComm, 2013, 15, 940–944
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temperature of 263 K. Chemical analysis gave the following
mass percentages (values in brackets are theoretical): C, 39.32
ture. In O the cations are arranged in such a way that dihedral
angle between two neighbouring diisopropylammonium
cations is equal to 85.7u, whereas after the transition similar
cations are oriented parallel to each other.
The new arrangement is induced by reorientation of all the
counterions along the internal long axis by approx. 90u and
rotation along the [100] orthorhombic direction. Fig. 2(a) and
(b) show the H-bonded chains in polymorph O and M(II),
respectively. Phase transition changes the arrangement of
H-bonded patterns.
¡
0.1 (39.57); N, 7.75 ¡ 0.1 (7.69); H 8.61 ¡ 0.05 (8.86). The
calorimetric measurements (DSC) of O revealed a heat
anomaly at ca. 417 K indicating an irreversible PT (O A
M(II)) occurring in this system (scan 1 in Fig. 1). Further
heating of the sample disclosed sequential reversible PT at 425
K which is observable during successive heating and cooling
scans (3, 4 in Fig. 1).
On the basis of the thermal characteristic (DSC) it is
difficult to state unequivocally the character of the high-
temperature PT (M(II) « M(I)). The heat anomaly visible as a
well-shaped peak may suggest a first order PT. Nevertheless,
neither the temperature hysteresis nor the phase front close to
Reorientation of counterions changes the packing of the
chains and induces deformation of the anionic substructure.
After the transition, the chains elongate and the distance
2
between two Br ions in a chain increases from 5.42 Å to
T
c
was seen during the observations under a polarizing
5
.52 Å. At the same time there is a contraction of the crystal in
2
microscope, thus the PT at 425 K seems to be close to the
second order type. The compound remained stable up to 470 K
the direction perpendicular to the chains and mutual Br
distances decrease from 7.97 Å to 7.85 Å (see Fig. 3).
Consequently, the crystal volume changes ca. y0.5%, however
3
(Fig. S1, ESI ) and above this temperature a continuous
decomposition took place. Furthermore, the calorimetric
measurements excluded the PT below room temperature (90–
2
the total displacement of Br ions from initial positions that is
given by the square root of the sum of the square of all atomic
displacements is significant (0.17 Å). It is worth noting that
both structures are described at the same room temperature,
so the thermal expansion does not affect the distances. This
rather complex rearrangement of the crystal structure has an
insignificant impact on the hydrogen bonds which have
similar strength and geometry in both polymorphs (see
Table 1). It means that mainly steric effects are responsible
for the transformation.
8
300 K) which is mentioned by Haberecht et al. in the
monoclinic polymorph P2
1
.
The crystal structure of as-grown DPB{ is isomorphic to
9
2
DPC. The counterions and the Br anion interact via two
hydrogen bonds and form one-dimensional chains propagat-
ing in the a-axis.
Above 417 K the crystal structure transforms to monoclinic
M(II) during a non-reversible phase transition. The new polar
phase is stabilized down to 90 K after cooling. The second
polymorph is isomorphic to the crystal of DPB obtained as
reported in ref. 6. The key difference between polymorph O
and M(II) is the mutual orientation of the neighboring
hydrogen bonded chains. The transformation between the
polymorphs requires reconstruction of the cationic substruc-
1
0
1
0
Polymorph M(II) experiences phase transition to the
10
structurally
disordered
M(I)
phase
at
425
K.
Diisopropylammonium cations adopt two equivalent, oppo-
sitely directed positions. Such an arrangement introduces the
symmetry centre and the polar properties vanish. A similar
microscopic model of the high temperature transformation
5
has been recently proposed for isomorphic DPC crystals,
although the interpretation of the diffraction data has been
intuitive rather than based on the results of calculations.
Fig. 1 (a) DSC curves for the DPB compound upon cooling and heating runs (2 K
2
1
min , m = 11.6 mg); (b) phase identification based on X-ray diffraction
Fig. 2 H-bonded chains in (a) O and (b) M(II), T = 295 K. Phase transition
changes the orientation of H-bonded patterns whereas the geometry as well as
the strength of H-bonds is similar in both forms. The dihedral angle between
two neighbouring C–C–N–C–C planes in O is marked. In M(II) the same angle is
equal to 0u. Hydrogen atoms not involved in H-bonds are omitted for clarity.
measurements.
{
Crystal data, experimental details and structure refinement results for DPB are
in the ESI.
3
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Fig. 4 (a) Model for the disorder of the diisopropylammonium counterions in
1
the paraelectric P2 /m phase. The site occupation factors equal 0.5 for both
available positions. The mirror plane introduces two equivalent positions, (b)
position of the counterions in the non-polar M(I) and polar M(II) phases.
The transition to centrosymmetric M(I) phase has a low
2
impact on positions of Br anions. Distortion amplitude for
2
Br is equal to 0.055(1) Å when comparing atomic positions at
2
2
95 and 435 K. Additionally displacement of Br ions in the
Fig. 3 Crystal packing of (a) O and (b) M(II) as seen along the H-bonded chains,
T = 295 K. The planes on which the hydrogen bonds are located are parallel in
M(II), whereas in O they are rotated about 22u and 24u to each other. The unit
cell of polymorph O may be transformed to M(II) using (0 0.5 20.5, 21 0 0, 0
polar b direction equals only 0.006(1) Å.
The temperature dependence of the real part of the
complex dielectric permittivity of the DPB was measured upon
heating and cooling at frequencies ranging between 20 Hz and
0
.5 0.5) rotation matrix and (0.25 0 0.25) translation vector.
2
MHz (Fig. 5(a)). The polymorphic irreversible PT at 417 K is
accompanied by a significant increase in the dielectric
constant (see inset of Fig. 5(a)), whereas the PT at 425 K
manifests itself as a characteristically strong peak in the
dielectric permittivity curve. The maximum value of the
permittivity, eb(max), reaches y12 000 at the Tc.
The behavior of the electric permittivity as a function of the
temperature suggests that this PT has a strong ferroelectric
character. Our preliminary dielectric dispersion studies
between 20 Hz and 1 MHz reveal that the fundamental
relaxation process over the M(I) phase takes place in the
Model of disorder in DPB appears to be more complex than
those proposed for the chlorine analog, but at least it does not
change the conformation of the counterion and gives good
reliability factors. Fig. 4(a) shows two equivalent orientations
of diisopropylammonium counterions in paraelectric, M(I),
phase. Both are 50% occupied and related to the mirror plane.
It seems that a maximum of two carbon atoms lie on the
mirror plane and the whole cation may rotate along its long
axis.
Compared to the low temperature phase, the coordinates of
the counterion change only slightly; the displacement of N
atom from its initial position is very weak and equals to 0.02 Å
with dy equal to 20.002 Å. In both phases the dihedral angle
formed between the (010) plane and the plane on which C–C–
N–C–C atoms are located equals to 44.5u. The symmetry centre
that appears in the parent phase allows for the antiparallel
arrangement of dipoles and the resultant dipole moment
vanishes to zero. Fig. 4(b) presents the unit cells in both
monoclinic phases.
kilohertz frequency region. The Curie–Weiss law (e = e + C+/2/
0
(T 2 T ) is well obeyed close to the T (see Fig. S5, ESI ). C was
o
c
3
+
3
estimated to be 5 6 10 K, which is comparable with the
values encountered in typical ‘order–disorder’ ferroelectrics
3
(e.g., for TGS C = 3.2 6 10 K) and the ratio C /C = 6 is
+
+
2
characteristic of first-order ferroelectric transitions.
To check the existence of the spontaneous polarization,
which could be reversible in an external electric field, we
performed hysteresis loop measurements. The positive result
of this measurement is a crucial evidence of the ferroelectric
character of the M(II) phase (Fig. 5(b)).
a
Table 1 Hydrogen bonds in polymorphs O, M(II) and M(I)
…
…
…
…
Polymorph, T (K)
D–H
A
D–H (Å)
H
A (Å)
D
A (Å)
D–H A (u)
…
O, 295
O
M(II), 295
M(II)
M(I), 435
M(I)
N1–H1A Br1
0.90(3)
0.85(2)
0.85(3)
0.91(4)
0.89
2.46(3)
2.49(3)
2.51(4)
2.43(4)
2.47
3.353(2)
3.334(2)
3.346(3)
3.335(3)
3.360(3)
3.393(3)
178(2)
172(2)
168(3)
173(3)
177.2
…
i
N1–H1B Br1
…
N1–H1A Br1
…
ii
N1–1B Br1
…
N1–H1A Br1
…
ii
N1–1B Br1
0.89
2.51
178.1
a
i
ii
Symmetry code(s): 2x + 1, 2y + 1, 2z + 1; 2x + 1, y + 1/2, 2z + 1.
9
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polarization (P
s
), thus the arrangement of two diisopropylam-
monium cations in the unit cell is crucial in the explanation of
the ferroelectric properties and especially the P value, which is
s
relatively large in this type of crystal.
In summary, the present work has successfully demon-
strated that very simple (1 : 1) alkylammonium organic salt,
diisopropylamine bromide (DPB), exhibits ferroelectric proper-
ties at room temperature and seems to be a very promising
candidate for applications.
Acknowledgements
This work was supported by the Polish State Committee for
Scientific Research (project register No. N N204 147837).
Fig. 5 (a) Temperature dependence of the real part of the complex electric
permittivity measured along the b-direction (on cooling). The inset in (a)
presents the results during first heating (irreversible PT at 417 K); (b) hysteresis
c
loops observed at various temperatures close to T (sample thickness = 0.6 mm, f
=
20 Hz).
References
1
2
J. F. Scott, Science, 2007, 315, 954.
G. Busch and P. Scherrer, Naturwissenschaften, 1935, 23,
737.
(a) M. E. Lines and A. M. Glass, Principles and Applications
of Ferroelectrics and Related Materials, Clarendon Press,
Oxford, 1977; (b) R. E. Newnham and L. E. Cross, MRS Bull.,
The obtained value of the spontaneous polarization was
2
3
22
found to be ca. 1.5 6 10
C m
c
at 424 (1 K below T ).
3
Because of the strong increase of the coercive field (E . 10 kV
c
2
1
cm ) we were not able to observe the electric hysteresis loop
for (T 2 T) . 1.5 K. The value of P at 424 K is far from the
expected saturated value of P , which in the case of the
chlorine analog is close to 9 6 10 mC cm (ca. 20 K below
). Taking into account the fact that dielectric constant in
c
s
2005, 30, 845; (c) C. D. Chandler, C. Roger and M.
s
J. Hampden-Smith, Chem. Rev., 1993, 93, 1205; (d)
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Mitsui, Landolt–B o¨ rnstein New Series, Springer-Verlag,
Berlin, 2006, vol. III/36.
2
2
22
T
c
DPB is distinctly larger than that in DPC the saturated
spontaneous polarization should be comparable in both
crystals.
In general, the molecular mechanism of paraelectric–
ferroelectric transition seems to be common for both
diisopropylammonium analogs, DPC and DPB. The dynamics
of the organic cations contribute mainly to the ‘order–
disorder’ mechanism of this transition. Nevertheless, a
substantial difference in the disorder of the organic cations
of both analogs should be emphasized. In the case of DPC, in
the paraelectric phase four carbon atoms are placed on the
mirror plane, whereas in DPB only two C atoms occupy this
plane. The molecular mechanism of PT proposed in this paper
seems to justify the fact that reorientational motion of
diisopropylammonium cations should take place around its
center of gravity. The thermodynamic properties of the
ferroelectric transition in DPC and DPB only partially reflect
the isomorphism of these crystals. The entropy transitions are
comparable, being of the order of 1.14 (DPC) and 1.43 (DPB) J
4
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5
6
7
8
9
2
1
21
mol
K
, however they are significantly smaller than they
10 (a) Crystal data for DPB-O: T = 298 K, C
82.11, orthorhombic P2 , a = 8.0233(1) Å, b = 8.3080(2)
calc
Å, c = 13.5868(3) Å, Z = 4, V = 905.66(3) Å , D = 1.336 g
H16BrN, M =
6 r
1
1 1 1
2 2
are predicted theoretically for a simple two-site positions
3
2
1
21
model (DS = Rln2 (= 5.76 J mol
K
)), which suggests that a
2
3
21
cm , m = 0.644 mm , 8913 reflections measured, 1711
‘displacive’ contribution to the molecular mechanism of PT
unique (R = 0.0206), 85 parameters, 2 restraints. The
int
should be also considered. Nevertheless, the preliminary
dielectric dispersion analysis, especially the presence of the
relaxation process in the radio-frequency region, supports the
2
2
2
final R(F . 2s(F )), wR(F ), S values were 0.0187, 0.0446
and 1.06, respectively. Asymmetric unit contains one
2
diisopropylammonium counterion and Br ion. All atoms
‘order–disorder’ type ferroelectric. Because the ‘order disorder’
adopt general positions with C site symmetry; (b) Crystal
1
contribution seems to be the main origin of the spontaneous
6 r
data for DPB-M(II): T = 298 K, C H16BrN, M = 182.11,
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monoclinic P2
CrystEngComm
3
23
1
, a = 7.8586(5) Å, b = 8.0818(5) Å, c =
116.379(11), Z = 2, V = 463.05(14) Å , D
calc
= 1.306 g cm , m
3
21
7
1
1
.8918(5) Å, b = 116.290(4), Z = 2, V = 449.38(5) Å , Dcalc
=
= 2.634 mm , 4620 reflections measured, 944 unique (R
= 0.0429), 59 parameters, 5 restraints. The final R(F
int
2
3
21
2
.346 g cm , m = 4.944 mm , 4536 reflections measured,
533 unique (Rint = 0.0275), 84 parameters, 3 restraints.
.
2
2
2s(F )), wR(F ), S values were 0.0444, 0.1459 and 1.124,
respectively. Asymmetric unit contains one diisopropylam-
monium counterion and Br ion. Bromine atoms as well as
two carbons from the counterion lie on the mirror plane
2
2
2
The final R(F . 2s(F )), wR(F ), S values were 0.0230,
.0453 and 0.898, respectively. Asymmetric unit contains
2
0
2
one diisopropylammonium counterion and Br ion. All
atoms adopt general positions; (c) Crystal data for DPB-
M(I): T = 435 K, C
(C
s
site symmetry), whereas remaining atoms adopt general
6 r 1
H16BrN, M = 182.11, monoclinic P2 /m, a
positions.
=
7.9465(14) Å, b = 8.1567(14) Å, c = 7.9743(14) Å, b =
9
44 | CrystEngComm, 2013, 15, 940–944
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