Strong Improper Ferroelasticity and Weak Canted Ferroelectricity in a Martensitic-Like Phase Transition of Diisobutylammonium Bromide

Article abstract of DOI:10.1002/adma.201501812

Diisobutylammonium bromide is found to be a unique improper ferroelastic in which the elastic degrees of freedom seem to play the essential role, giving rise to a domain pattern resembling that of martensitic phase transitions. A weak canted ferroelectricity turns out switchable by an electric field.

Full text of DOI:10.1002/adma.201501812

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Strong Improper Ferroelasticity and Weak Canted
Ferroelectricity in a Martensitic-Like Phase Transition
of Diisobutylammonium Bromide
Anna Piecha-Bisiorek,* Agata Białon´ska, Ryszard Jakubas, Piotr Zielin´ski,
Martyna Wojciechowska, and Mirosław Gała˛zka
Ferroelectricity of “small-molecule” organic compounds has
been known for almost a century.[1–3] The technological appli-
cability of such materials has not as yet reached the level of
inorganic system-like perovskite-based ceramics, e.g., PZT
(lead zirconate titanate)[4–8] that owe their attractiveness to a
combination of various useful physical properties such as lumi-
nescence, superconductivity, and stable ferroelectricity. The
advantages of organic ferroelectrics are, however, incontestable
so that the synthesis and growth of new such systems are a
challenging research area.
Recently synthesized environment-friendly and small-molec-
ular-weight organic single- and two-components polar mate-
rials[9–21] turned out very promising. An excellent example of
the single-component pure organic ferroelectric with a room-
temperature ferroelectricity is croconic acid whose polar prop-
erties have been studied by Horiuchi et al.[11] The spontaneous
polarization Ps of about 21 µC cm−2 and the coercive field of
order of 14 kV cm−1 allows this material to successfully com-
pete with the inorganic ferroelectric materials. Noteworthy is
the crucial role of the hydrogen bonding and a specific coop-
erative proton tautomerism (the migration of a proton from
the β position to the enol) in the onset of ferroelectricity in
π-conjugated molecular compounds.
Other two-component organic ferroelectrics, diisopropylam-
monium chloride (DIPAC) and diisopropylammonium bro-
mide (DIPAB), belong to the group of molecular-ionic com-
pounds with the spontaneous polarization resulting from
ordering organic cations (order–disorder-type ferroelectric).
A particular behavior of their dielectric parameters (espe-
cially DIPAB) distinguishes them from the formerly reported
organic, hydrogen-bonded ferroelectrics.[19–21] This compound
is characterized by an extremely high value of the spontaneous
polarization (23 µC cm−2), a high Curie temperature (426 K), a
high dielectric constant, small dielectric losses and a low coer-
civity field.[20,21] Additional qualities of DIPAB are: facility of
preparation, low cost, nontoxicity, and good thermal stability.
DIPAB also shows a strong piezoelectric effect and has a well-
defined ferroelectric domain structure.
Numerous newly synthesized materials exhibit a symmetry-
induced coupling of ferroelectricity to a macroscopic strain.[22,23]
The materials fall, thus, into the category of biferroics (ferro-
electric–ferroelastic).[24–26] In most cases, a spontaneous strain
is a secondary order parameter. In this respect, diisobutylam-
+
monium bromide, (DIBAB), [i-(C4H9)2NH2 ][Br−], presented in
this paper, is unique, because the elastic degrees of freedom
seem to play the essential role in the formation of a domain
pattern that resembles that of martensitic phase transitions,[27]
whereas the ferroelectricity appears as a side effect. The lack
of a group–subgroup relation at the phase transition, a very
strong spontaneous strain without cracking of the lattice makes
the system even more intriguing and at the same time closer to
martensitic materials.
DIBAB reveals thermal stability at a wide range of tempera-
tures (up to about 450 K, see Figure S1, Supporting Informa-
tion) and undergoes only one, reversible phase transition
at 285/286 K (cooling/heating) as is presented in Figure 1a.
A well-shaped heat anomaly as well as a thermal hysteresis
(ΔT = 1 K) indicates the discontinuous (first order) character
of this transition. The corresponding value of the transition
entropy, ΔS = 25 J mol−1 K−1, indicates that the room tempera-
ture phase is highly disordered. Structural data reveal an “order-
disorder” underlying mechanism.
At room temperature (293 K), the symmetry of DIBAB is
described by the tetragonal I4/mmm space group. The asym-
metric unit contains a bromide anion and a half of the DIBA+
cation with CH(CH3)2 group disordered over two positions.
The bromide anion occupies a special position 4/mmm whereas
the amine N atom of the DIBA+ cation is located in a vicinity
of another special position 4/mmm. The methylene C atoms
bonded to the amine N atom occupy the special positions
4mm, and the CH(CH3)2 groups bonded to the latter are placed
in general positions. As a result, the amine N atom and the
CH(CH3)2 groups exhibit four and sixteen positioned disorder,
respectively (Figure 1-right). The structure of DIBAB is lay-
ered so that each bromide anion is surrounded by four DIBA+
cations belonging to the same single layer. The isobutyl groups
of the cations are located in the outer parts of the layer while
the amine N atom of the cations and bromide anions lie in its
internal part (see Figure 1-right). At 293 K, bromide anions are
Dr. A. Piecha-Bisiorek, Dr. A. Białon´ska,
Prof. R. Jakubas, M. Wojciechowska
Faculty of Chemistry
University of Wrocław
F. Joliot-Curie 14, 50-383 Wrocław, Poland
E-mail: anna.piecha@chem.uni.wroc.pl
Prof. P. Zielin´ski, Dr. M. Gała˛zka
The H. Niewodniczan´ski Institute
of Nuclear Physics
PAN
ul. Radzikowskiego 152, 31-342 Kraków, Poland
DOI: 10.1002/adma.201501812
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Figure 1. Molecular structure and packing of the DIBAB in the paraelectric phase at 293 K (right) and in the ferroelectric phase at 100 K (left) together
with (a) DSC curves for the DIBAB compound; (b) Temperature dependence of the real part (ε′) of the complex dielectric constant measured in the
cooling mode between 135 Hz and 2 MHz along the c-direction; (c) Ferroelectric properties of DIBAB–temperature dependence of the spontaneous
polarization estimated from the pyroelectric current measurements.
separated from the amine N atom of the disordered cations by
distances of 3.489 and 4.289 Å.
Below 285 K, the symmetry of DIBAB changes to the
orthorhombic Iba2 space group. The orientation of the axes
a and b of the unit cell in the low-temperature (LT) phase
corresponds to the square diagonals of the unit cell of the
high-temperature (HT) phase (see Figure 2). Additionally, in
the LT phase, the lattice constant b is doubled. Every atom in
the LT phase is located in a general position. As at room tem-
perature (HT phase), in the LT phase the ions are assembled
into layers, here parallel to the (001) plane. However, in the LT
phase, the single layer is built up of separated hydrogen bonded
zig-zag chains extended along the [100] direction. In the zig-
zag chains, the amine N atom of DIBA+ cations donates to
two N H+···Br− hydrogen bonds. The neighboring hydrogen
bonded chains along the [010] direction are oriented antipar-
allel (see Figure 3b).
All the N and Br atoms of the chain are located in the same
plane perpendicular to the [001] direction. However, the cat-
ions do not protrude quite perpendicularly to the plane (see
Figure 1-left). The angle (pHB-p+) between the plane of the
···H N H+···Br−··· chain (pHB) and the plane of the cation
(defined by the C2, N1, and C6 atoms) (p+) is equal to 81.9(7)°,
and the angles between the planes (p+) of the neighboring cat-
ions (i) in the hydrogen bonded chain (p+-p+(0.5+x, 1.5−y, z))
and (ii) between the consecutive chains of the layer (p+-p+(1−x,
1−y, z)) are equal to 77.1(8)° and 16.2(17)°, respectively. As
a result, the vectors of the dipole moments are neither par-
allel nor perpendicular to the ···H N H+···Br−··· chain,
but are all directed outwards and slightly inclined towards
the c-axis (see red arrows in Figure 1-left) so that there is a
net dipole moment in the [001] direction (see also Figure S4,
Supporting Information). This gives rise to a weak sponta-
neous polarization in a close analogy to canted magnetism.[28]
Figure 2. Relation between orientation of the unit cell before and after the
phase transition and distribution of symmetry operators in paraelectric
(green) and ferroelectric phases (blue or yellow).
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Figure 3. a) Evolution of the ferroelastic domain pattern (see also films F1 and F2) and its relation with the hydrogen bond geometry; b) packing of
crankshaft-shaped molecular chains in the LT phase.
The polar properties of DIBAB in the LT phase have been
studied by the pyroelectric current measurements (more
details in Supporting Information). As it is seen in Figure 1c
the spontaneous polarization (Ps) was found to be revers-
ible in an external electric dc field ( 6 kV cm−1). The value
of Ps is rather small in comparison to diisopropylammonium
analogues DIPAC and DIPAB, and equals 5 × 10−2 µC cm−2.
This corroborates the structural findings concerning the ori-
entation of molecular permanent dipoles. The behavior of
the spontaneous polarization also resembles that of a canted
ferromagnetism.[28]
The ordering mechanism responsible for the transformation
of the structure is presented in Figure 1-right and -left. Zig-zag
shaped, hydrogen-bonded molecular chains arise and order
approximately in the [001] direction with, however, a certain
slanting angle. The distance of the chains in the [010] direction
diminishes significantly, which explains a 7.5% collapse of the
lattice constant b in the LT phase.
The dynamics of the diisobutylammonium cations in LT
and HT phases is reflected in the dielectric measurements.
As is presented in Figure 1b the real part of the complex
dielectric constant shows an abrupt anomaly, which starts
at 285 K during a cooling cycle. However, the dielectric
response evolves with increasing frequency. In the low-fre-
quency range (between 135 Hz and 3 kHz) the curve ε′ vs.
temperature shows a strong peak (at ≈282 K), which continu-
ously decreases with frequency. It is also noteworthy that
the maxima of the low-frequency peaks appear exactly at the
same temperature (282 K) as the end of dielectric anomalies
for higher frequencies. However, no relaxation process was
detected in the vicinity of the phase transition. Atypical dielec-
tric behavior below room temperature reflects the complicated
mechanism of this transition where the strong spontaneous
deformation is a secondary effect of a molecular ordering
and the canted ferroelectricity is only a residual symptom of
antiferroelectric arrangement of molecular dipoles. In view of
the unit cell quadrupling the ferroelectricity should be clas-
sified as improper (see also Figure S5 and S6, Supporting
Information).[29–31]
Formation and packing of the hydrogen-bonded zig-zag
chains in the LT phase leads to a huge deformation of the
crystal lattice with respect to the HT phase. In the HT phase,
all the bromide anions are located in mutually perpendicular
(110) and (−110) planes, and the nitrogen atoms of the disor-
dered cations are deviated from the planes. The interplanar
distance between (110) or (−110) planes in the HT phase is
equal to 3.868(2) Å. In the LT phase, the nitrogen atoms of
the ordered cations of the neighboring, antiparallel oriented
chains lie approximately in the corresponding planes (perpen-
dicular to the [100] and [010] directions in the LT phase), and
bromide anions lie in the ones of the corresponding planes
(perpendicular to the [010] direction) and are deviated from
the other. In the LT phase, the distances between the anionic
planes perpendicular to the [010] direction determine the
width of the hydrogen-bonded zig-zag chain and the length
separating the neighboring chains in the layer. The distances
are equal to 3.223(5) and 4.011(6) Å, respectively, which are
0.645 Å shorter and 0.143 Å longer than the corresponding
distances in the HT phase (the distance between the (110) or
(−110) planes is equal to 3.868(2) Å). As mentioned above, in
the LT phase, the amine N atoms of cations lie (approximately)
in the planes perpendicular to the [100] direction, and the bro-
mide anions are deviated from the planes. The N Br vectors
are directed outwards on both sides of the planes and the angle
between the N Br vectors directed outwards on opposite sides
of the plane is equal to 19.0(2)°. It is interesting that the angle
19.0(2)° is very close to that observed between certain phase
boundaries when the LT phase grows at the expense of the ini-
tial tetragonal phase (see Figure 3 and Figure S7, Supporting
Information).
The lattice constant b collapses by about 7.5% giving a rise to
a strong macroscopic deformation. Nevertheless, the ferroelas-
ticity is formally improper in this case[32] and the strain is, still
formally, a secondary effect.
An insight into the space groups I4/mmm and Iba2 of HT
and LT phases, respectively, of DIBAB shows that they do not
satisfy a group–subgroup relation in this phase transition (see
Figure 2). Indeed, it is impossible to select, out of the symmetry
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+
elements of the group I4/mmm, the ones forming the space
group Iba2 with the transformation matrix:[33]
monium bromide, [i-(C4H9)2NH2 ][Br−], (DIBAB). The material
exhibits unique and unexpected properties related to ordering
of chains of diisobutylammonium cations. It namely undergoes
a strongly discontinuous phase transition driven by an order–
disorder mechanism strongly coupled to macroscopic defor-
mation. Being essentially antiferroelectric, the crystal shows
a weak canted ferroelectricity. The study is, thus, an evidence
of a canting ferroelectricity recently predicted in DFT calcula-
tions.[36] Despite the deformation and the nonexistence of a
group–subgroup relation, the crystal preserves its integrity and
shows a phase growth and a domain pattern resembling that of
martensitic phase transitions. Undoubtedly, this rare example
of a strong order–disorder induced ferroelasticity giving rise to
a reproducible domain pattern known from martensitic phase
transitions opens interesting prospects for new technological
applications of ferroic crystals.
⎛
⎜
⎜
⎝
⎞
⎟
⎟
⎠
1
2
0
1
−2
0
0
0
1
q1
q2
q3
for whatever translation vector (q1, q2, q3) as follows from the
crystallographic data. Such a symmetry change requires a very
particular rearrangement of atoms. They must, namely, leave
the positions ensuring the “lost” symmetry elements to arrive
at the positions corresponding to the “gained” ones. A rear-
rangement of this type is attained sometimes without breaking
of the lattice despite large differences in geometry and packing.
A condition for this is that a plane in the parent phase to be
structurally close to a plane in the final phase. This plane then
becomes a habit plane, i.e., a coherent interphase boundary.
The behavior is characteristic of martensitic phase transi-
tions[27] (see also films: F1 and F2 recorded during observation
of DIBAB under polarizing microscope during cooling and
heating cycles respectively and Part 6, Supporting Information).
In fact the lattice constants c of DIBAB in both phases are very
close. This also applies to the lattice constant a in LT phase and
a 2 in HT phase (the diagonal of the base of the tetragonal
structure), which are 7.776(4) Å and 7.735(2) Å, respectively.
The planes (110) and (1–10) in the HT phase, parallel to the fer-
roelectric axis in the LT phase seem, therefore, to be good candi-
dates for the habit planes. The evolution of the phase transition
on cooling (Figure 3) corroborates this hypothesis. The new
phase intrudes into the initial one in two perpendicular direc-
tions in the (001) plane and the crystal does not break. Subse-
quent intrusions subtend angles of about 19.08° to 20.08° with
the initial intrusions. As mentioned above, the angle is very
close to the one of some N Br bonds. The resulting pattern is
very similar to that described in Fe(30%)Ni alloy being a result
of a system of accommodation strains.[27] When the phase tran-
sition is completed the system of the domains becomes typical
of a ferroelastic species with perpendicular domain walls. This
can be deduced from the Sapriel’s theory[34] if we consider only
the spontaneous strain accompanying the transition from a
tetragonal to orthorhombic symmetry. The stress-free domain
walls, would then be parallel to the base diagonals of the ele-
mentary cuboid, i.e., [110] and [1–10]. Their angle would be as
different from 90° as 56.513°. In the final state of the phase
transition, the domain walls are quasi perpendicular, however.
This suggests that we have to take into account the deforma-
tion of the elementary square 2a × 2a disregarding the dou-
bling of the unit cell in the b direction. The resulting angle is
86.864°. In contrast to the simple metallic crystals and to typical
martensitic phase transitions the unit cell of DIBAB is four
times larger. Therefore, the existence of antiphase domains
is expected.[35] Such domains are not visible in the polarizing
microscopy but may contribute to the mechanism of the nucle-
ation and growth of the new phase.
Experimental Section
Synthesis: A mixture of diisobuthylamine [(CH3)2CHCH2]2NH, (99%;
Sigma–Aldrich) and concentrated HBr (47%–49%, J.T. Baker) were
dissolved in EtOH/H2O (1:1, v/v) solvent. The solution was slowly
evaporated in 277 K creating thin petals of compounds with a thickness
between 0.2–0.35 mm. Anal. calcd for C8H20NBr: C 45.72, H 9.59, N 6.66;
found: C 45.86, H 9.79, N 6.70. The phase purity was verified by powder
XRD and IR spectra (see Figure S2 and S3, Supporting Information).
Crystal Structures: DIBAB at 293 K: C8H20NBr, Mr = 210.16, Tetragonal,
I4/mmm, a = 5.470(2) Å, b = 5.470(2) Å, c = 19.628(3) Å, V = 587.3(4) Å3,
Z = 2, Dc = 1.188 g cm−3, R1 (I > 2σ(I)) = 0.053, wR2 (all data) = 0.122,
µ = 3.448 mm−1, S = 1.016. DIBAB at 100 K: Orthorombic, Iba2, a =
7.776(4) Å, b = 14.468(15) Å, c = 19.35(3) Å, V = 2177(4) Å3, Z = 8, Dc =
1.282 g cm−3, R1 (I > 2σ(I)) = 0.051, wR2 (all data) = 0.152, µ = 3.720
mm−1, S = 1.092. Details concerning the crystal structure determination
are in Supporting Information.[37,38]
Physical Properties Measurements: Differential scanning calorimetry
(DSC) heating traces were obtained using a PerkinElmer model 8500
differential scanning calorimeter calibrated using n-heptane and indium.
Hermetically sealed Al pans with the polycrystalline material were
prepared in a controlled-atmosphere N2 glovebox. The measurements
were performed between 100 and 310 K. The thermal hysteresis was
estimated from the scans performed at various rates (20, 10, and
5 K min−1) extrapolated to a scanning rate of 0 K min−1. Simultaneous
thermogravimetric analysis (TGA) and differential thermal analysis
(DTA) were performed on Setaram SETSYS 16/18 instrument in the
temperature range 300–650 K with a ramp rate 2 K min−1. The scans
were performed in flowing nitrogen (flow rate: 1 dm3 h−1).
The complex dielectric permittivity, ε* = ε′−i ε″ was measured
between 100 and 310 K by the Agilent 4284A Precision LCR Meter in
the frequency range between 135 Hz and 2 MHz. The overall error
was less than 5%. The single-crystal samples had dimensions ca. 5 ×
3 × 0.3 mm3. Silver electrodes were sticked on the opposite faces.
The dielectric measurements were carried in a controlled atmosphere
(N2) (see also Part 4 in Supporting Information). Pyroelectric current
was measured by Keithley 617 Electrometer (more details in Part 5,
Supporting Information). The ferroelastic domain structure of the DIBAB
crystal was studied by means of an Olympus BX53 optical polarization
microscope. The samples were placed in a LINKAM THM-600 heating/
cooling stage, where the temperature was stabilized to within 0.1 K
(Part 6, Supporting Information).
CCDC 1059278–1059279 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge from
The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif.
In summary, we have successfully synthesized and presented
systematic characterization of an original, environment-friendly
biferroic (ferroelectric and ferroelastic) material diisobutylam-
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Supporting Information
Supporting Information is available from the Wiley Online Library or
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Acknowledgements
This research was supported by the National Science Centre (Poland)
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Adv. Mater. 2015, 27, 5023–5027
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