Crystal structure, hirshfeld surface analysis and phase transformation behaviour of diisopropylammonium chloride hemihydrate crystals

Article abstract of DOI:10.1016/j.jpcs.2021.110008

The phase stability of the rarely studied diisopropylammonium chloride hemihydrate (DPC-HH) was investigated in a range of temperature from 100 K to 503 K. The single crystals of DPC-HH grown from methanol solution belong to orthorhombic crystal system with space group Ccca at ambient conditions (296 K) and do not show any phase transformation till 100 K upon cooling whereas it is physically less stable when stored at room temperature due to host-guest nature of the diisopropylammonium chloride (DPC) and water in the DPC-HH lattices. While heating above room temperature dehydration of the hydrate molecules occurs which leads to the crystallization of anhydrous DPC in monoclinic crystal system with space group P2₁ at 342 K and further at 439.7 K the DPC undergoes a ferroelectric to paraelectric phase transition through a structural transformation from monoclinic P2₁ to P2₁/m which is reversible. The Hirshfeld surface and fingerprint plots were used to study the intermolecular interactions of DPC-HH. The intermolecular H?H contacts was most significant interactions present in the crystal with over 72% of the total Hirshfeld surface. Powder X-ray diffraction, Raman and FT-IR spectroscopy, TGA and DSC techniques were employed to monitor and confirm the phase stability of DPC-HH crystals and ferro-para transformations occur in the DPC crystal system.

Full text of DOI:10.1016/j.jpcs.2021.110008

Journal of Physics and Chemistry of Solids 153 (2021) 110008
Contents lists available at ScienceDirect
Journal of Physics and Chemistry of Solids
journal homepage: http://www.elsevier.com/locate/jpcs
Crystal structure, hirshfeld surface analysis and phase transformation
behaviour of diisopropylammonium chloride hemihydrate crystals
,b,*
Sampathkumar Pongiappana, Srinivasan Karuppannana
a Crystal Growth Laboratory, Physics Division, DRDO-BU Center for Life Sciences, Bharathiar University Campus, Coimbatore, 641046, Tamil Nadu, India
b Crystal Growth Laboratory, Department of Physics, School of Physical Sciences, Bharathiar University, Coimbatore, 641046, Tamil Nadu, India
A R T I C L E I N F O
A B S T R A C T
Keywords:
The phase stability of the rarely studied diisopropylammonium chloride hemihydrate (DPC-HH) was investigated
in a range of temperature from 100 K to 503 K. The single crystals of DPC-HH grown from methanol solution
belong to orthorhombic crystal system with space group Ccca at ambient conditions (296 K) and do not show any
phase transformation till 100 K upon cooling whereas it is physically less stable when stored at room temperature
due to host-guest nature of the diisopropylammonium chloride (DPC) and water in the DPC-HH lattices. While
heating above room temperature dehydration of the hydrate molecules occurs which leads to the crystallization
of anhydrous DPC in monoclinic crystal system with space group P21 at 342 K and further at 439.7 K the DPC
undergoes a ferroelectric to paraelectric phase transition through a structural transformation from monoclinic
P21 to P21/m which is reversible. The Hirshfeld surface and fingerprint plots were used to study the intermo-
lecular interactions of DPC-HH. The intermolecular H⋯H contacts was most significant interactions present in
the crystal with over 72% of the total Hirshfeld surface. Powder X-ray diffraction, Raman and FT-IR spectros-
copy, TGA and DSC techniques were employed to monitor and confirm the phase stability of DPC-HH crystals
and ferro-para transformations occur in the DPC crystal system.
Ferroelectric materials
Crystal structure
Phase transformation
Hirshfeld surface
Raman spectroscopy
FT-IR spectroscopy
Thermal analysis
1. Introduction
and it crystallizes in the same solution containing the DPC crystals [11].
No clear information on the phase transformation of DPC-HH upon
In recent investigations, researchers reported three new organic
molecules based ferroelectric materials namely diisopropylammonium
chloride (DPC), diisopropylammonium bromide (DPB) and diisopropy-
lammonium perchlorate (DPP) having high Curie temperature as well as
high spontaneous polarization comparable that of the popular ferro-
electric materials such as barium titanate (BaTiO3) [1–8]. Among them,
DPC has Curie temperature at 440 K and shows spontaneous polariza-
cooling below room temperature and heating till the melting point, their
hydration and dehydration behaviours and subsequent structural
changes associated with the above transformations were made available
with best of our knowledge. In this situation, in order to elucidate the
phase transformation behaviours of this material, crystallization ex-
periments from methanol solutions were carried out and the resulted
DPC-HH single crystals were subjected to various studies such as powder
X-ray diffraction (PXRD), single crystal X-ray diffraction (SCXRD) with
structural refinement, Raman and FTIR Spectroscopy, Differential
scanning calorimetry (DSC) and Thermogravimetric analysis (TGA) in a
broad temperature range between 100 K and 503 K and the results are
presented in the present study.
tion of about 8.9
μ
C/cm2 [8]. The DPC was reported to have four
polymorphs including three in the room temperature such as ortho-
rhombic P212121, monoclinic P21, monoclinic P21/c and one high
temperature polymorph of monoclinic P21/m and among them only the
monoclinic P21 form exhibits ferroelectric behaviour with high coercive
field indicating ferroelectric hysteresis loops near the phase transition
temperature [8–10]. Among the few reported, Fu et al., reported the
DPC crystallizes in monoclinic crystal system with space group P21 [8].
G. J. Reib and B. K. Bitschnau reported that the DPC crystallizes with a
hemihydrate namely diisopropylammonium hemihydrate (DPC-HH)
* Corresponding author. Crystal Growth Laboratory, Department of Physics, School of Physical Sciences, Bharathiar University, Coimbatore, Tamil Nadu, 641046,
India.
E-mail addresses: nivas_5@yahoo.com, srini@buc.edu.in (S. Karuppannan).
https://doi.org/10.1016/j.jpcs.2021.110008
Received 30 October 2020; Received in revised form 2 February 2021; Accepted 18 February 2021
Available online 24 February 2021
0022-3697/© 2021 Elsevier Ltd. All rights reserved.

S. Pongiappan and S. Karuppannan
Journal of Physics and Chemistry of Solids 153 (2021) 110008
solution was kept for slow evaporation at room temperature. No
nucleation was observed in the solution over a period of next 15 days
and suddenly many crystal nucleation were appeared on the top surface
of the solution next day owing to the generation of comparatively higher
supersaturation on the surface of the solution and interestingly after few
days of growth, the crystals were diffused into the solution and settled at
the bottom of the vessel due to subsequent gaining of weight. The grown
crystals were having thin elongated plate like morphology, later
confirmed as DPC-HH and were harvested carefully after drying out for
further characterization.
Table 1
Crystal data and structure refinement for DPC-HH at 100 K and 296 K.
Identification code
100 K
296 K
Empirical formula
Formula weight
Temperature/K
Crystal system
Space group
a/Å
C6H17ClNO0.5
146.66
C6H17ClNO0.5
146.66
100
296
orthorhombic
Ccca
orthorhombic
Ccca
10.0761(7)
15.4820(11)
11.4308(8)
90
10.2247(5)
b/Å
15.6439(7)
c/Å
11.5626(6)
◦
α/
90
The grown DPC-HH single crystals were subjected to dehydration
study under the optical microscope attached with the hot stage with
proper temperature control. After recording the micro photograph of the
as grown DPC-HH single crystals kept on the hot stage, the temperature
of the hot stage was increased continuously at successive time intervals
from ambient to till the melting point of DPC-HH (i.e. 303–503 K) and
photographs were recorded successively at various temperatures by
giving sufficient time for temperature stabilization of the crystals.
β/◦
90
90
γ/◦
90
90
Volume/Å3
Z
1783.2(2)
8
1849.49(16)
8
Crystal size/mm3
2Θ range for data
collection/◦
Index ranges
0.6 × 0.3 × 0.2
5.998 to 54.546
0.328 × 0.206 × 0.096
5.208 to 54.27
ꢀ 12 ≤ h ≤ 12, ꢀ 19 ≤ k ≤
19, ꢀ 14 ≤ l ≤ 14
8652
ꢀ 13 ≤ h ≤ 13, ꢀ 20 ≤ k ≤
19, ꢀ 14 ≤ l ≤ 14
18173
Reflections collected
Independent
997 [Rint = 0.0408, Rsigma
= 0.0204]
1025 [Rint = 0.0311, Rsigma
= 0.0115]
2.2. Powder X-ray diffraction
reflections
Goodness-of-fit on F2
Final R indexes [I ≥
1.288
1.121
The purity of the crystalline phase of the grown DPC-HH single
crystals was analysed through powder X-ray diffraction analysis ac-
quired using PANalytical X’Pert Pro X-ray powder diffractometer with
R1 = 0.0480, wR2 = 0.1686
R1 = 0.0496, wR2 = 0.1570
2σ (I)]
Final R indexes [all
data]
R1 = 0.0495, wR2 = 0.1710
R1 = 0.0551, wR2 = 0.1648
Cu Kα (λ = 1.5418 Å) radiation operating at 45 kV/40 mA and samples
were scanned 2θ values from 10 to 40⁰ with the step size of 0.04⁰.
2. Experimental procedure
2.3. Single crystal X-ray diffraction
2.1. Synthesis, crystal growth and dehydration of DPC-HH
Single crystal X-ray diffraction for the grown single crystals of DPC-
HH were collected using Bruker D8 Quest SCXRD instrument with MoK
α
Diisopropylammonium chloride (DPC) was synthesized by dissolving
equimolar ratio of diisopropylamine (>98%, Alfa Aesar) and hydro-
chloric acid (37%, Merck) in methanol (99.9% HPLC Grade, Sigma). The
chemicals obtained were used without any further purification. The
prepared solution was stirred at 500 rpm continuously for about 2 h
using magnetic stirrer at room temperature (303 K). After that the
sealed tube (λ = 0.7107 Å) source having CMOS photon detector and the
low temperature structural data were collected with the help of Oxford
Cryostream 800 Series LN2 low temperature facility. The crystal struc-
ture was solved using Olex2 package [12] with direct methods and
refined using shelxl programme [13,14]. All the non-hydrogen atoms
were refined anisotropically and the hydrogen atoms placed in
Fig. 1. Crystal structure of DPC-HH (a) at 100 K and (b) at 296 K
Fig. 2. ORTEP diagram with 50% ellipsoid probability of DPC-HH (a) at 100 K and (b) at 296 K.
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S. Pongiappan and S. Karuppannan
Journal of Physics and Chemistry of Solids 153 (2021) 110008
with the help of NETSZH D208 DSC Instrument. The samples were
weighed in the aluminium pan and the DSC data was recorded in the
presence of nitrogen gas atmosphere in the temperature range of
305–520 K with the heating rate of 5 K/min.
2.6. Thermogravimetric analysis
A TGA measurement of the grown DPC-HH single crystal was carried
out using TA Instruments Simultaneous Analyser. About 4 mg of sample
was weighed into a ceramic pan and aluminium oxide used as a stan-
dard. TGA was performed in the temperature range of 310–515 K with
the heating rate of 5 K/min.
3. Results and discussion
The crystal structure refined through SCXRD reveals the stoichiom-
etry and the bonding nature of the water molecules with the crystal
lattice. The crystallographic structural data collected for DPC-HH at 296
K and at low temperature (100 K) are summarized in Table 1. Single
crystal X-ray structure revealed that the DPC-HH crystallizes in ortho-
rhombic crystal system with Ccca space group at room temperature and
retains its structure till 100 K. The Z is found to be eight and volume of
the unit cell determined is 1783 Å3 at 100 K and that of 1849 Å3 at 296 K.
Crystallographic asymmetric unit of DPC-HH contains 0.5 occupancy of
diisopropylammonium and a chloride and 0.5 occupancy of water
molecule. In the centrosymmetric space group, the diisopropylammo-
nium cation and the water molecules lie on the two fold axes. The mo-
lecular arrangement of atoms in the diisopropylammonium and the
chloride is similar to that of the anhydrous diisopropylammonium
chloride. Strong hydrogen bond was observed between the chlorine
anion and the nitrogen of the diisopropylammonium cation. A short
hydrogen bond exists between the water molecule and the diisopropy-
lammonium cation. When compared with the previous reported anhy-
drous monoclinic DPC (Z = 2 and V = 429 Å3) this hemihydrate form has
the volume and Z are four times greater than that of the reported
monoclinic polymorph. The refined structure of DPC-HH at 100 K and
296 K are shown in Fig. 1. The displacement ellipsoid plot of DPC-HH
with 50% ellipsoid probability is shown in Fig. 2 and it is clear that
the ellipsoids of the DPC-HH at 100 K are smaller than that of the DPC-
HH at 296 K due to the reduced thermal vibrations at low temperature.
Packing of DPC-HH unit cell along different crystallographic axes is
depicted in Fig. 3. In general, desolvation results in the loss of crystal-
linity due to the removal of the solvent molecules present in the crystal
lattice which results in the structural changes leads to new or existing
crystal structures [17–19]. Fig. 4 shows the packing diagram of the
DPC-HH in which the hydrate molecule is represented as the space ball.
The water molecules shown as space fills and diisopropylammonium
chloride shown as ball and stick model clearly indicate that the water
molecules occupied only in the voids instead of occupying the lattice
positions of the crystal. The water molecules present in the void between
the diisopropylammonium chloride stacks and does not directly
involving the formation of a stable lattice with diisopropylammonium
chloride. Which makes water molecules less stable when compared to
the molecules which occupied in a lattice position and can easily remove
from the crystal.
Fig. 3. Orientation of DPC-HH molecules in different crystallographic axis (a)
along [100] (b) along [010] and (c) along [001].
calculated positions using riding model with isotropic displacement
parameters. The structures were visualized using Mercury 3.1 software
[15] and thermal ellipsoids were envisaged using ORTEP-III [16].
2.4. Raman and FTIR spectroscopy
The presence of hydrate in the grown DPC-HH single crystal was
confirmed by Raman and FTIR spectroscopic techniques. Raman spectra
of the as grown DPC-HH crystals and that of after dehydration were
collected using WITEC ALPHA 300 RA Confocal Raman spectrometer
using DFSS LASER having wavelength 532 nm. Similarly FTIR spectra
for the as grown DPC-HH and after dehydration were recorded using
Hirshfeld surface analysis was carried out to study the nature of
intermolecular interactions and theirs quantitative contributions in the
crystal structure. The Hirshfeld surface [20] and associated
two-dimensional fingerprint plots [21,22] for DPC-HH were calculated
using Crystal-Explorer 17 [23]. With the help of dnorm, it is possible to
differentiate the regions which are participating in the intermolecular
interactions using the colours of the surface. Red coloured regions
indicate the most intense contacts having atoms in close proximity from
the surface, white is generated because of the intermediate atoms and
blue surfaces represent weaker contacts having longer distance from the
Hirshfeld surface. The Hirshfeld surfaces of the DPC-HH are shown in
JASCO-FTIR Spectrophotometer in the range of 4000 to 400 cmꢀ 1
.
2.5. Differential scanning calorimetry
A DSC measurement for the DPC-HH single crystal was performed
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S. Pongiappan and S. Karuppannan
Journal of Physics and Chemistry of Solids 153 (2021) 110008
Fig. 4. Packing diagram of the DPC-HH and the hydrate is represented as the space ball.
Fig. 5. Two different views of Hirshfeld surface mapped over dnorm of DPC-HH at (a, b) 100 K and (c, d) 296 K.
Fig. 5(a–d). In the dnorm map of DPC-HH, the Cl⋯H contacts (labelled as
296 K shown in Fig. 6(a–h). The H⋯H interactions shown most signif-
icant contribution to the total Hirshfeld surface. H⋯H interactions is
about 73 and 72.1% for DPC-HH at 100 K and 296 K, respectively (Fig. 6
c,g). Contribution of Cl⋯H interactions which is associated with
N–H⋯Cl hydrogen bonds found to be only 18.3 and 18.5% for DPC-HH
at 100 K and 296 K, respectively (Fig. 6 b, f) is due to only one Cl atom
1) are visible as deep red spots which are associated to N–H⋯Cl
hydrogen bonds and O⋯H reciprocal contacts (labelled as 3) are asso-
ciated with the O–H⋯O hydrogen bonds. H⋯H contacts (labelled as 2)
are shown deep red spots as well as white over the surface. Fingerprint
plots for the main intermolecular contacts for the structures at 100 K and
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S. Pongiappan and S. Karuppannan
Journal of Physics and Chemistry of Solids 153 (2021) 110008
Fig. 6. Fingerprint plots of DPC-HH at 100 K (a–d) and 296 K (e–h). (a,e) full, (b,f) resolved into Cl⋯H (c,g) H⋯H and (d,h) O⋯H contacts.
involved in this structure. In the fingerprint plot for DPC-HH at 100 K
and 296 K, Cl⋯H reciprocal contacts with pair of spikes observed with
significantly different (de + di) distances, approximately at 2.25 and
2.32 Å, respectively. Contribution of O⋯H interactions is about 8.7 and
9.4% for DPC-HH at 100 K and 296 K, respectively (Fig. 6 d, h). In the
fingerprint plot for DPC-HH at 100 K and 296 K, O⋯H reciprocal con-
tacts with a pair of spikes observed with significantly different (de + di)
distances, approximately at 2.54 and 2.61 Å, respectively. The low
proportion of interactions to the total Hirshfeld surfaces for each
molecule indicates that the hydrogen bonding is the less significant
interaction in the crystal. The difference in the distances is may be due to
the closed packing of the molecules at lower temperatures which was
confirmed from the crystal unit cell volume of 1783 Å3 at 100 K when
compared to 1849 Å3 at 296 K.
sample for longer periods at room temperature environments where the
water molecules are considered to be the guests occupied the spaces of
voids present in the host lattices.
FTIR spectra of the (a) as grown DPC-HH crystals and that of (b)
dehydrated were shown in Fig. 8. It is obvious from the figure that the
presence of absorption peak corresponds to O–H stretching vibration
observed at 3502 cmꢀ 1 in the spectrum (a) confirms the presence of
hydrates in the DPC-HH crystals and is absent in (b) indicates the
possible dehydration. Moreover the presence of peaks correspond to –N-
H and –C-H stretching vibrations observed at 3346 cmꢀ 1 and 2947 cmꢀ 1
respectively in both the cases.
PXRD patterns recorded for the grown DPC-HH crystals (a) as har-
vested, (b) preheated at 323 K (c) preheated at 353 K and (d) exposed to
open environment at room temperature (303 K) for 3 days are given in
Fig. 9. The hydrated and dehydrated forms can clearly be distinguished
from the diffraction peaks corresponding to different hkl planes present
in the respective crystal systems i.e., the transformation of orthorhombic
DPC-HH crystal into monoclinic DPC through dehydration is clearly
revealed. In the diffraction pattern of the (a) as grown DPC-HH crystal
belongs to orthorhombic crystal system with space group Ccca, the
prominent characteristic peak appears at 11.55◦, 2θ corresponds to 020
plane whereas the diffraction pattern of the completely dehydrated
forms such as in (c) and (d) which belong to monoclinic crystal system
with space group P21, the characteristic diffraction peak appears at
12.66◦, 2θ corresponds to 100 planes. In the diffraction pattern of the
intermediate form shown in (b) the prominent characteristic peaks of
both the DPC-HH as well as anhydrous form of DPC are present
The Raman spectra were collected for the grown DPC-HH crystals at
different situations such as (a) as harvested, (b) preheated at 353 K and
(c) exposed to open environment at room temperature are shown in
Fig. 7. Differences in the existence of peaks among the three recorded
Raman spectra clearly explains the presence of –OH groups in (a) and
the absence of the same in (b) and (c). The twin peaks observed around
3350 cmꢀ 1 in (a) corresponds to the –OH groups present in the DPC-HH
lattices which do not exist in either (b) or (c) because of the possible
dehydration of the loosely bounded water molecules with the lattices.
Also it could be observed from the study that the spectra for samples (b)
and (c) are almost similar and have significant differences with (a)
which is the indicative of the dehydration occurred in the DPC-HH lat-
tices either while preheated to elevated temperatures or keeping the
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S. Pongiappan and S. Karuppannan
Journal of Physics and Chemistry of Solids 153 (2021) 110008
Fig. 6. (continued).
Fig. 7. Raman spectra of DPC-HH (a) as harvested (b) preheated at 353 K and
(c) exposed to open environment at room temperature (Inset: Expanded view of
DPC-HH indicating hydrates around 3350 cmꢀ 1).
Fig. 8. FTIR spectra of DPC-HH (a) as harvested and (b) preheated at 353 K.
less stable since it may take up additional moisture in the channels when
they are exposed to the humidity and their crystal lattice may expand or
contract as the hydration or dehydration proceeds effectively changes in
the dimensions of the unit cells [24]. In the case of DPC-HH it was
observed as dehydration and the unit cell transformation was observed
as monoclinic anhydrous DPC which was converted from orthorhombic
indicating the partial dehydration at 323 K.
DPC-HH has non-integer value of hydrate (0.5) associated with one
diisopropylammonium chloride and it falls under the category of non-
stoichiometric hydrates. Hydrates associated with this category are
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S. Pongiappan and S. Karuppannan
Journal of Physics and Chemistry of Solids 153 (2021) 110008
Fig. 11. DSC thermogram for the dehydrated DPC-HH crystal showing the
ferroelectric-paraelectric reversible phase transition.
Fig. 9. Powder X-ray diffraction patterns of DPC-HH crystals (a) as harvested,
(b) preheated at 323 K (c) preheated at 353 K and (d) exposed to open envi-
ronment at room temperature (303 K) for 3 days.
Fig. 12. TGA-DSC thermogram of as grown DPC-HH crystal.
was clearly matches with the PXRD patterns of the preheated samples at
353 K which clearly shows that the effect of humidity induced dehy-
dration effect on the DPC-HH. In both the scenarios of heating to higher
temperatures as well as exposed to open environment we observed that
the DPC-HH was converted into anhydrous DPC. From these results it is
clear that the orthorhombic form of DPC-HH was formed initially as
metastable form or pseudo polymorph of DPC and after dehydration it is
transformed into the stable monoclinic DPC which was confirmed by
comparing with literature [8].
The DSC thermogram recorded with heating rate of 5 K/min in the
temperature range from ambient to 525 K reveals the phase stability and
structural transformation behaviour of (a) as grown DPC-HH crystal and
(b) that of preheated at 353 K as shown in Fig. 10. The recorded ther-
mogram (a) clearly reveals that the as grown DPC-HH crystal exhibits
three endothermic peaks in the above temperature range including the
melting endotherm at 491 K and interestingly, two endotherms before
melting at 342 K and 439.7 K. The endotherm appears at 342 K is
attributed mainly because of two reasons: first one is the induction of
dehydration of water molecules in the lattices and second is the
recrystallization into the stable form i.e. phase transformation from the
room temperature metastable orthorhombic Ccca structure into the
stable monoclinic P21 structure. This endotherm appears as a plateau
instead of a sharp peak because of the onset for the dehydration of water
Fig. 10. DSC thermogram of (a) as grown DPC-HH crystal and (b) pre-heated at
353 K.
DPC-HH. It was confirmed by the PXRD patterns recorded for the sam-
ples exposed to the open environment conditions for 3 days. The changes
in powder X-ray diffraction patterns during 3 days at room temperature
7

S. Pongiappan and S. Karuppannan
Journal of Physics and Chemistry of Solids 153 (2021) 110008
Fig. 13. Optical micrographs of DPC-HH at different temperatures: (a) orthorhombic Ccca, (b–d) mixture of orthorhombic Ccca and monoclinic P21, (e–i) monoclinic
P21 and (j–m) monoclinic P21/m.
molecules which starts at initial heating and slowly increases with
heating. This indicates the poor stability of the DPC-HH crystal lattices
in the temperature range slightly above the room temperature even
though the boiling point of water lies far at 373 K. In general, the dif-
ference between the dehydration onset temperature (Ton) and the
boiling point (Tb) of the solvent (Ton –Tb) determines the stability of the
solvate crystals. This is because the solvent molecules of the solvates are
considered to be guests with weakly bonded to the hosts and frequently
situated in the voids formed in the host structure [25]. The endothermic
peak observed at 439.7 K is attributed mainly for the ferroelectric to
para-electric phase transition of the monoclinic P21 to monoclinic
P21/m. This coincides with the previous report where the anhydrous
DPC crystallized in monoclinic P21 space group exhibited the ferro-
electric to para-electric phase transition at 440 K [8]. The final melting
8

S. Pongiappan and S. Karuppannan
Journal of Physics and Chemistry of Solids 153 (2021) 110008
endotherm of the dehydrated DPC-HH peaked at 491 K is also in-line
with the literature value [8]. Whereas in the case of thermogram (b)
recorded for the pre-heated sample at 353 K for 30 min, there is no
endothermic peak observed around 342 K because of the known reasons
but only an endothermic peak that appears at 439.7 K which corre-
sponds to the ferroelectric to paraelectric phase transition from mono-
clinic P21 to monoclinic P21/m as in the case of DPC-HH. Moreover, the
orthorhombic to monoclinic phase transformation in the DPC-HH
observed at 342 K is monotropic in nature as reported for diisopropy-
lammonium bromide (DPB) [4] the analogy material of DPC [8].
Whereas the ferroelectric to paraelectric phase transition from mono-
clinic P21 to monoclinic P21/m is reversible; this is obvious from the
heating as well as cooling thermogram recorded for the dehydrated
DPC-HH in this temperature range shown in Fig. 11 in which the
endothermic peak while heating appears at 439.7 K and the corre-
sponding peak appears at 436.4 K while cooling. Relatively high thermal
hysteresis of 3.3 K suggests the phase transition is first order in nature
[8].
chloride (DPC) and water molecules in the DPC-HH lattices. The dehy-
dration study carried out in the temperature range 303–503 K shows
that the grown DPC-HH crystals undergo a loss of transparency and
becomes opaque as a result of dehydration with increase in temperature.
Raman and FTIR spectral analysis confirm the dehydration of water
molecules upon heating and the PXRD study reveals the monotropic
structural transformation of the orthorhombic DPC-HH crystal with
space group Ccca to the stable monoclinic DPC crystal with space group
P21 upon heating at 342 K as a result of dehydration. The DSC thermal
analysis also confirms that the DPC further undergoes a first order
ferroelectric to paraelectric phase transition at 439.7 K with structural
transformation from monoclinic P21 to monoclinic P21/m and is also
reversible upon cooling.
Accession codes
CCDC 1947703 and 1947704 contains the crystallographic data for
this paper. These data can be obtained free of charge via www.ccdc.cam.
ac.uk/data_request/cif, or by emailing data_request@ccdc.cam.ac.uk, or
by contacting The Cambridge Crystallographic Data Centre, 12 Union
Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
The TGA-DSC thermogram recorded to study the weight loss with
temperature shown in Fig. 12. The thermogravimetric profile shows a
weight loss of 5.5% begins almost as soon as heating process starts and
ends sharply at 342 K. From stoichiometric calculation it is estimated
that the weight loss corresponds to the presence of one half of the
molecule of water per diisopropylammonium chloride molecule (0.5:1).
The weight loss value determined by TGA is little lower than the value of
theoretical weight loss (6.1%) and is because of dehydration due to
humidity present in the room temperature storage environment between
the time interval of harvesting and the sample preparation.
Declaration of competing interest
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.
At room temperature (303 K) as harvested DPC-HH single crystals
appear as clear crystals and with heating of them to higher temperatures
leads to lose of transparency. At 318 K, the edges of the DPC-HH start to
become opaque and at 343–353 K, the entire DPC-HH crystal boule
becomes opaque. At 473 K, the DPC-HH starts to melt and at 503 K it
becomes completely melted. The series of optical micrographs recorded
during the course of the transformation of the DPC-HH crystals from
303 K to 503 K is shown in Fig. 13. The reason behind the loss of
transparency upon heating at high temperatures is due to the loss of
hydrate molecules attached to the primary DPC-HH molecules to form
the dehydrated stable DPC. The loss of transparency is mainly because of
the structural transformation of the orthorhombic DPC-HH crystal with
space group Ccca to the stable monoclinic DPC crystal with space group
P21 as a result of dehydration. In which each single orthorhombic Ccca
unit cell with Z = 8 of the DPC-HH crystal transforms into four mono-
clinic P21 unit cell with Z = 2 of the DPC crystal. The low stability of the
DPC-HH crystal lattices at higher temperatures is mainly because of the
loosely bound water molecules in the host lattices which were also
confirmed by structural and thermal studies.
Acknowledgements
Authors would like to acknowledge the Defence Research and
Development Organization (DRDO), New Delhi, Government of India
for providing the financial support (Grant No. DLS/86/50071/DRDO-
BU Centre/Phase-II dated February 20, 2014) to carry out the research
through DRDO-BU CLS, Phase II Programme.
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