Raman and X-ray investigations of ferroelectric phase transition in NH 4HSO4

Article abstract of DOI:10.1021/jp2075868

Temperature-dependent Raman spectroscopy and X-ray diffraction studies have been carried out on NH₄HSO₄ single crystals in the temperature range 77-298 K. Two structural transitions driven by the molecular ordering and change in crys

Full text of DOI:10.1021/jp2075868

ARTICLE
pubs.acs.org/JPCA
Raman and X-ray Investigations of Ferroelectric Phase Transition
in NH HSO
4
4
†
†
†,‡
,†
Diptikanta Swain, Venkata Srinu Bhadram, Papia Chowdhury, and Chandrabhas Narayana*
†
Chemistry and Physics of Materials Unit, Jawaharlal Nehru Centre for Advanced Scientific Research,
Jakkur P.O., Bangalore 560064, India
‡
Also at Department of Physics and Material Science, Jaypee Institute of Information Technology, Noida 201307, India
S Supporting Information
b
ABSTRACT: Temperature-dependent Raman spectroscopy
and X-ray diffraction studies have been carried out on
NH HSO single crystals in the temperature range 77À298 K.
4
4
Two structural transitions driven by the molecular ordering and
change in crystal symmetries are observed below 263 and 143 K.
These phase transitions are marked by the anomalies in the
temperature dependence of wavenumber and fwhm of several
internal vibrational modes. The Raman spectra and X-ray data
enable us to understand the nature of the molecular ordering
resulting in the ferroelectric phase below 263 K, sandwiched
between two nonferroelectric phases. The crystal structure of
the ferroelectric phase is determined correctly as Pc, which has
been earlier solved in Ba symmetry. The temperature dependent Raman and X-ray results suggest that the disorder to order
À
transition leading to lower symmetry below 263 K is driven by the change in HSO ions and that below 143 K is driven by the
change in both HSO4 and NH4 ions.
4
À
+
’
INTRODUCTION
Pepinsky et al. from their dielectric measurements. These phase
transitions in NH HSO were also observed from specific heat
4
4
Ferroelectric materials have been a central theme in both1
21
measurements and modulated differential scanning calorim-
fundamental sciences as well as in electronics in recent times.
22
2
À7
etry. The mechanism of the ferroelectric transition was
Ferroelectricity is observed in numerous materials
including
+
8
À13
explained as only due to the orientation of NH ions below
hydrogen bonded systems. NH HSO belongs to the family
of the hydrogen sulfate compound AHSO , where, A = Cs, NH ,
4
4
4
22
the transition temperature. It was also believed that the
presence of hydrogen bonding in the crystal structure is also
4
4
Rb, K, Na, which are interesting for their fast ion conducting
1
9
1
4À17
responsible for the ferroelectric transition in NH HSO . The
property,
ferroelectricity, and structural phase transitions
4
4
1
8À20
single crystal structures of NH HSO were solved at 293 and 192
with temperature.
These materials are characterized by a
4
4
À
23,24
23
strong OÀH O hydrogen bond between HSO4 ions form-
K.
At 293 K, NH HSO is monoclinic, space group P2 /c,
4 4 1
3
3 3
ing dimer and chain structures. From early dielectric measure-
with eight formula units per unit cell. The asymmetric unit
ments, it was surmised that at normal pressure, NH HSO4
contains two crystallographically independent NH
4
HSO
4
units
4
À
undergoes two phase transitions, one at T = 270 K and the
in which one HSO
4
ion is ordered, while the other is disordered.
c1
20
other at Tc2 = 154 K, respectively. The first transition is
associated with a large anomaly in dielectric constant, whereas
a small anomaly was observed at the second transition tempera-
ture. At T , NH HSO undergoes paraelectric to ferroelectric
The structure has long polymeric OÀH O hydrogen bonds
3 3 3
À
between HSO4 ions. The 192 K structure is also monoclinic,
space group Ba, with sixteen formula units per unit cell and
β ≈ 90° depicting pseudo orthorhombic features as explained
c1
4
4
24
transition and is second order in nature, whereas at T , it goes
by Nelmes. The asymmetric unit contains four independent
NH HSO units. While the structure in ferroelectric phase below
c2
through ferroelectric to nonferroelectric transition and is first
4
4
20
order in nature as explained by Pepinsky et al. The values of
2
43 K was indexed by Pepinsky et al. to be monoclinic, space group
coercive field and spontaneous polarization were about 150 V/cm
Pc, with a = 14.26 Å, b = 4.62 Å, c = 14.80 Å, and β = 121.30°, the
structure below 154 K was also indexed in triclinic system, space
2
20
and 0.4 μC/cm at 260 K, respectively. The maximum value of
2
spontaneous polarization was about 0.8 μC/cm just above 154
K, and it dropped to zero at that temperature, interestingly the
compound showed a piezoelectric property until 77 K. How-
ever, the mechanism of the transitions was not proposed by
Received: August 8, 2011
20
Revised:
December 1, 2011
Published: December 05, 2011
r 2011 American Chemical Society
223
dx.doi.org/10.1021/jp2075868
|J. Phys. Chem. A 2012, 116, 223–230

The Journal of Physical Chemistry A
ARTICLE
Figure 1. (a, b, and c) Packing diagram of NH
hydrogen bonds at 293 K, 200 K, and 125 K. NH
the figure.
4
HSO
4
at 293 K, 200 K, and 125 K (D = disordered, O = ordered). (d, e, and f) Showing OÀH
O
3
3 3
+
4
ions are in the layer formed by the OÀH O hydrogen bonds and are removed for the clarity of
3
3 3
group P1, with a = 14.24 Å, b = 4.56 Å, c = 15.15 Å, α ≈ 90°,
532 nm excitation from a diode pumped frequency doubled
Nd:YAG solid state laser (model GDLM-5015 L, Photop
Suwtech Inc., China) and a custom-built Raman spectrometer
equipped with a SPEX TRIAX 550 monochromator and a liquid
nitrogen cooled CCD (Spectrum One with CCD 3000 con-
20
β = 123.40°,and γ ≈ 90° but remained unsolved.
Raman spectroscopy is an ideal tool for capturing the
dynamics and local structural changes in the viewpoint of
vibrational frequencies whereas X-ray diffraction establishes the
detail estimation of symmetry and periodicity in a crystal during
phase transition. Although the phase transitions in NH HSO
2
5
troller, ISA Jobin Yovn). Laser power at the sample was
∼8 mW, and a typical spectral acquisition time was 3 min. The
4
4
the
2
0À22
À1
were established from various experimental techniques,
spectral resolution chosen was 2 cm . The temperature was
mechanism and dynamics of transitions were not understood
very well. In this article, we have studied the internal vibrations of
molecular units and the crystal structures of NH HSO as a
controlled with an accuracy of ((0.1 K) by using a temperature-
controller (Linkam TMS 94) equipped with a cooling stage unit
(Linkam THMS 600). The spectral profile was fitted using
Lorentzian functions with the appropriate background.
For single crystal X-ray data collections, the crystal was
mounted inside a close-ended Lindemann glass capillary. X-ray
diffraction data were collected on a Bruker AXS SMART APEX
CCD diffractometer. The X-ray generator was operated at 50 kV
4
4
function of temperature by using temperature dependent Raman
spectroscopy and X-ray diffraction to understand the dynamics
of the structural phase transition across the transition tempera-
24
tures. Earlier reported structures suggest strong orthorhombic
pseudo symmetry in ferroelectric phase; however, we report the
crystal structure of NH HSO in ferroelectric phase below the
4
4
and 35 mA using Mo K radiation. For all of the measurements,
α
2
6
first transition temperature to be Pc showing conventional
monoclinic symmetry and the lowest temperature phase to be
P1 showing triclinic symmetry.
6
06 frames per set were collected using SMART in four
different settings of j (0°, 90°, 180°, and 270°) with an ω scan
at À0.3° intervals and with a counting time of 5s, while keeping
the sample to detector distance at 6.054 cm and 2θ value fixed
2
6
’
EXPERIMENTAL SECTION
at À25°. The data were processed using the SAINT-PLUS
protocol, and an empirical absorption correction was applied
using the package SADABS followed by XPREP to determine
Single crystals of NH HSO were grown by slow evapora-
4
4
27
26
tion from aqueous solution containing equimolar quantities of
the space group. The structures were solved and refined by using
(
NH ) SO and H SO . The quality of the crystal used for the
4
2
4
2
4
28
29
experiment was checked under a polarized optical microscope.
the SHELX-97 program present in the WinGX suite. The
The temperature evolution of the Raman spectra of NH HSO4
conventional R factor, the weighted R factor wR, and the goodness
4
2
2
2
was recorded in the 180° backscattering geometry, using a
of fit S are based on F . The threshold expression of F > 2σ(F ) is
2
24
dx.doi.org/10.1021/jp2075868 |J. Phys. Chem. A 2012, 116, 223–230

The Journal of Physical Chemistry A
ARTICLE
Table 1. Crystallographic Data
temp
chemical formula
293 K (disorder)
NH HSO
115.12
200 K (order)
NH HSO
115.12
125 K (order)
NH HSO
115.12
4
4
4
4
4
4
formula weight (mr)
a (Å)
14.393(5)
4.600(2)
14.416(5)
90
14.288(3)
4.571(1)
14.305(3)
90
14.096(5)
4.5953(2)
14.381(3)
89.980(1)
117.149(5)
90.028(2)
triclinic
b (Å)
c (Å)
α (deg)
β (deg)
117.997(5)
90
117.842(3)
90
γ (deg)
crystal system
space group
monoclinic
monoclinic
Pc
P2
1
/c
P1
3
volume (Å )
842.7(5)
8
826.1(3)
8
824.8(5)
8
Z
À3
density (g cm
radiation type
)
1.815
1.851
1.856
X-ray Mo K
α
X-ray Mo K
α
X-ray Mo K
α
crystal form, color
crystal size (mm)
diffractometer
block, colorless
0.50 Â 0.50 Â 0.40
Bruker SMART CCD area detector
θ and j scans
SADABS
block, colorless
0.50 Â 0.50 Â 0.40
Bruker SMART CCD area detector
θ and j scans
SADABS
block, colorless
0.50 Â 0.50 Â 0.40
Bruker SMART CCD area detector
data collection method
absorption correction
θ and j scans
SADABS
0.0366
R
int
0.0367
0.0368
θ
min,max
2
1.65, 26.35
1.66, 26.37
1.59, 26.37
0.0843
2
R[F > 2σ(F )]
goof (S)
0.0509
0.0350
1.108
1.051
1.149
used only for calculation of the R factor and is not relevant to the
choice of reflections for refinements. In situ low-temperature
measurements were carried out with an attached OXFORD cryo-
system, maintaining a cooling rate of 60 K/h by using a temperature
controller with liquid-N flow.
2
’
RESULTS AND DISCUSSION
Crystal Structure. The structure of NH HSO determined
4
4
23
at 293 K confirms the details of the earlier report and is
11,19,30,31
isostructural to RbHSO4
and belongs to the monoclinic
crystal system, space group P2 /c with Z = 8 [see Figure 1a,
1
Supporting Information S1, and Table 1]. All atoms in the
structure are at general crystallographic positions (4e; Wyckoff),
with two crystallographically independent NH HSO units in
4
4
the asymmetric unit, among which one NH HSO unit has
4
4
À
Figure 2. Room temperature Raman spectra of NH HSO in the
disorder in the HSO4 moiety. The sulfate ions have the usual
tetrahedral coordination; however, the angles have small devia-
4
4
À1
frequency range 350À3500 cm
.
2
À
tions from the tetrahedral values for the ordered SO
unit,
4
2
À
analogous to NH HSO , also exhibits a ferroelectric phase transi-
4
4
while that for the disordered SO4 unit, the deviations are large.
11,19,31
tion to space group Pc below 258 K.
The asymmetric unit
There are extensive strong OÀH O hydrogen bond networks
3
3 3
À
contains four independent NH HSO units, with all atoms at
4
4
between HSO ions forming long polymeric chain structures in
4
general crystallographic positions (2a; Wyckoff). The hydrogen
bonding features remain similar to the room temperature
structure, infinite polymeric chains; however, all the NH HSO
4
both the ordered and disordered units [Figure 1d]. In fact, the
hydrogen-bonded chains are formed along the b crystallographic
axis by translational symmetry. In addition to OÀH
O
4
3
3 3
units are ordered in the low temperature phase [Figure 1e]. It is to
hydrogen bonds, there are also NÀH O hydrogen bonds
3
3 3
24
be noted that the structure at 192 K was earlier solved by Nelmes
present in the structure, which help in stabilizing the crystal
packing. Upon cooling to 200 K in situ on the single-crystal X-ray
diffractometer at a ramp rate of 60 K/h, keeping the temperature
steady for 30 min and checking the CCD images at regular
intervals, a structural phase transition to a space group Pc is
observed, with Z = 8 [Figure 1b, Supporting Information S2,
in the monoclinic system, with space group Ba by considering
a = 24.48(4) Å, b = 4.58(2) Å, c = 14.75(3) Å, and β = 90.00°(5)
showing pseudo symmetry; however, we have determined it in
the conventional monoclinic symmetry with a = 14.288(3) Å,
b = 4.571(1) Å, c = 14.305(3) Å, and β = 117.842°(3). With further
cooling to 125 K, we observed another phase transition to a space
and Table 1]. It is of interest to note that RbHSO , which is
4
2
25
dx.doi.org/10.1021/jp2075868 |J. Phys. Chem. A 2012, 116, 223–230

The Journal of Physical Chemistry A
ARTICLE
À1
stretching (850À900 cm ), (iii) symmetric and asymmetric
À1
SÀO stretching (950À1350 cm ), (iv) symmetric and asym-
À1
metric NÀH bending (1350À1800 cm ), and (v) symmetric
À1
and asymmetric NÀH stretching (3000À3200 cm ). Along
with these above fundamental group frequencies, we also ob-
served frequencies that are arising from OÀH O hydrogen
3
3 3
bonds as well as combination modes and overtones of the SÀO
and NÀH vibrations. These frequencies are derived by consider-
2
À
+
ing SO4 and NH4 as distorted tetrahedra. It is established
that, at room temperature, the structure contains two crystal-
lographically independent NH HSO units of which one unit is
4
4
À
disordered at the HSO4 moiety. This phenomenon is well
reflected in the symmetric SÀO stretching region of the Raman
spectra collected at room temperature [Figure 2]. The symmetric
SÀO stretching region has two intense and well-defined Raman
À1
peaks centered at 1013 and 1042 cm and a plateau centered at
1
À1
À1
033 cm at 298 K. The frequencies at 1013 and 1033 cm are
assigned to the symmetric SÀO stretching of the disordered unit
because of its two different possible orientations, whereas the
À1
1
041 cm mode is assigned to the SÀO stretching frequency of
the ordered unit.
Temperature evolutions of the Raman spectra of NH HSO
4
4
for all the regions in the temperature range 77À298 K are shown
in Figures 3À7. Significant changes are observed in the spectral
features around phase transition temperatures (T = 263K and
c1
Tc2 = 143K) and are in agreement with the structural phase
transition observed in the single crystal X-ray diffraction reported
in the present work and also with the earlier dielectric studies by
20
Pepinsky et al. Here, we present the temperature dependence
of the vibrational frequencies and a few selected full width at half
maximum (fwhm) of different molecular subunits of NH HSO
4
4
to explain the dynamics driving the phase transitions. Figure 3
represents the temperature evolution of SÀOÀH and OÀSÀO
À
bending modes of HSO ions in the spectral range 400À
4
À1
8
50 cm . This spectral range consists of six frequencies centered
À1
at 405, 416, 444, 573, 582, and 609 cm at room temperature.
With a decrease in temperature from 298 K approaching the first
transition temperature Tc1 = 263 K, very small changes in the
vibrational frequencies are observed, whereas at T = 143 K, an
c2
À1
abrupt increase in the frequencies at 416, 444, and 609 cm and
À1
an abrupt decrease in the frequency at 582 cm are observed
Figure 3. (a) Temperature evolution of SÀOÀH and OÀSÀO bend-
ing mode frequencies of NH
4
HSO
4
and (b) fwhm of the respective
[Figure 3a]. Similarly, the fwhm of all the six normal modes
exhibit very small changes around the first transition tempera-
ture; however, significant changes are observed around the
second transition temperature [see Figure 3b]. Figure 4 repre-
sents the temperature dependent symmetric SÀOH and SÀO
stretching mode frequencies and their fwhm in the spectral range
bending modes in the temperature range 77À298 K.
group P1, with Z = 8 [Figure 1c and Table 1]. The asymmetric unit
contains eight ordered NH HSO units. The hydrogen bonding
4
4
features remain similar to the room temperature and 200 K
structures [Figure 1f].
Raman Scattering. The sulfate and ammonium ions, with
À1
850À1350 cm . As the temperature is lowered, the symmetric
À1
SÀOH mode centered at 800 cm slowly hardens, and in
À1
tetrahedral (T ) symmetry, have four Raman active normal
addition to that, a new mode appeared at 886 cm around Tc1.
d
modes each, namely, symmetric stretching mode ν , the doubly
Both these modes harden as the temperature is lowered from Tc1
toward T . At T , we observe a slight deviation in the
1
degenerate symmetric bending mode ν , the triply degenerate
2
c2
c2
asymmetric stretching mode ν , and the triply degenerate asym-
metric bending mode ν4. The room temperature Raman
temperature dependence of modes but no discontinuities.
The 970 cm mode could be arising from the combination of
3
32
À1
modes of NH HSO were reported only in the frequency range
the SÀOÀH and OÀSÀO bending modes and is based on the
4
4
À1 37
33
3
50À1300 cm
.
Here, we recorded the Raman spectra of
observation of Moreira et al. The temperature dependence of
À1
NH HSO in the range 350À3600 cm at room temperature,
the wavenumber of this mode presents a small anomaly near Tc1,
4
4
which is shown in Figure 2. All the Raman active modes in
NH HSO at 298 K are assigned based on the information
and above T , we observed an appearance of a new mode at
c2
À1
984 cm . Both the frequencies harden until the temperature
4
4
32À36
available in the literature.
The internal modes in these
approached 77 K. A very interesting behavior is displayed by the
À1
regions are divided into five groups of frequencies: (i) SÀOÀH,
symmetric SÀO stretching modes. The 1033 cm mode, which
À1
OÀSÀO bending (400À850 cm ), (ii) symmetric SÀOH
arises from the disordered SÀO bonds, abruptly decreased and
2
26
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The Journal of Physical Chemistry A
ARTICLE
Figure 4. (a and b) Temperature evolution of symmetric SÀOH and SÀO stretching mode frequencies of NH HSO and (c and d) fwhm of the
4
4
respective stretching modes in the temperature range 77À298 K.
À1
tends toward the 1013 cm mode at the same time the mode at
of a new mode, and beyond T , the three modes are now
c2
À1
1
042 cm shows a discontinuous jump at T . As the tempera-
reduced to two modes with a much lesser frequency difference.
The fwhm of the three modes rapidly decreases from T to T
c2
c1
ture is lowered to T , the mode due to the disordered SÀO unit
c2
c1
softens and gets closer to the counterpart (the other disorder
mode) without merging, while the frequency from the ordered
SÀO unit hardens and continues the trend until 143 K. The
second transition is spread over a temperature range of 20 K and
is observed for all the modes. It is observed that the stretching
modes of the ordered SÀO units and disordered one behave
differently; mainly, the difference in frequency increases as it
approaches the T . Before T itself, we observe the appearance
before they disappear to give rise to two modes.
In the case of the asymmetric stretching modes of sulfate ions,
very small to no change are observed at T at the same time large
c1
changes are observed at T . The temperature dependent Raman
c2
spectra, the vibrational frequencies, and the fwhm of the asym-
metric SÀO stretching modes are shown in the Figure 5. Four
Raman active mode frequencies centered at 1162, 1207, 1246,
À1
and 1279 cm are observed for the asymmetric SÀO stretching
c2
c2
2
27
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The Journal of Physical Chemistry A
ARTICLE
Figure 5. (a and b) Temperature evolution of asymmetric SÀO stretching mode frequencies of NH
modes in the temperature range 77À298 K.
4
HSO
4
and (c) fwhm of the respective stretching
vibration. At T , we also observe the appearance of four new
modes centered at 1152, 1190, 1223, and 1294 cm . The tem-
perature dependent bending modes of ammonium ions are
peaks that could be due to the ν(OH) O mode and the first
3 3 3
c2
À1
+
À1
overtone of ammonium bending mode ν (NH ) at 1444 cm ,
2 4
1
7
which is based on the reports on KHSO
and NaNH4
4
33
shown in Figure 6. In this region, we observed two frequencies
SO 2H O. With the lowering in the temperature, the fre-
quencies gradually decreased until 77 K. Raman modes in the
4
3
2
À1
centered at 1444 and 1675 cm at room temperature. In this
À1
case, we also observe very small anomalies in both the frequen-
region 3000À3600 cm are probably due to the symmetric and
cies at T , but large anomalies are observed at T . Similarly, the
asymmetric ammonium stretching modes as well as the first
c1
c2
+
fwhm also showed a large change around Tc2 [See Figure 6b].
overtone of the ammonium bending mode ν (NH ) at
675 cm , again based on the report on NaNH SO 2H O.
When the temperature is lowered, these regions have interesting
changes. There is evolution of new modes across the transition
temperatures, and the modes are sharp. The modes generally
soften with a decrease in temperature.
4
4
À1
33
Here, too, we observe the appearance of two new modes
1
4 4 2
3
À1
centered at 1461 and 1692 cm at T .
c2
À1
Raman spectra collected in the region 2000À3600 cm are
assigned as NÀH and OÀH group vibrations (Figure 7). The
interpretation of unpolarized Raman spectra in this region is
difficult because it contains the superposition of broad bands due
to the hydrogen bonding and are hard to deconvolute. Vibra-
On the basis of the Raman and single crystal X-ray studies
presented here, we observed three phases of NH HSO evolving
4
4
À1
tional frequencies in the range 2200 to 2600 cm could be due
with decreasing temperature. Above T , the phase is P2 /c,
c1
1
to the coupling of the stretching modes of (SO)ÀH with the
external modes of SO3ÀO(H) groups and are not very sensitive
to the temperature, which is based on the earlier report on
between T and T , it is Pc, and below T , it is P1. In this, we
discuss the temperature dependence of the wavenumber and
fwhm of the vibrational modes showing substantial changes in
c1 c2 c2
3
4
RbHSO4. With a decrease in temperature, the frequencies in
the vicinity of the ferroelectric phase transition at T = 263 K and
c1
this region decreased slightly and split across the transition
nonferroelectric transition at Tc2 = 143 K. The changes in the
temperature behavior of some of the internal modes and their
À1
temperature T . The spectral region 2600À3000 cm contain
c2
2
28
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The Journal of Physical Chemistry A
ARTICLE
Figure 7. Temperature evolution of Raman bands associated with the
NÀH and OÀH region.
transition temperatures can be seen clearly from the fwhm of the
vibratonal modes. From our Raman and X-ray data, it is ruled out
that the ferroelectric phase transition is not due to the orienta-
+
tion of NH4 ions as we did not observe any anomaly in the
temperature dependent NÀH vibrational mode frequencies and
their fwhm [see Figures 6 and 7]. By analyzing the symmetric
SÀO stretching modes [Figure 4b], it is evident that the
disordered sulfate groups are becoming similar in their orienta-
tion, which increases the ordering beyond the first transition
temperature leading to the ferroelectric property. The abrupt
decrease in the fwhm [Figure 4d] signifies the ordering across the
ferroelectric transition.
The second phase transition below T where the material lost
c2
the ferroelectric property could be explained by analyzing the
temperature dependent symmetric SÀO stretching frequency of
À
Figure 6. (a) Temperature evolution of NÀH bending mode frequen-
the HSO4 ion. In the paraelectric phase, the difference between
cies of NH HSO and (b) fwhm of the respective bending modes in the
the symmetric SÀO stretching frequency occurring from two
4
4
À
temperature range 77À298 K.
different crystallographically HSO4 ions is less. As the tem-
perature is lowered below T , the compound attained the
c1
À
+
fwhm associated with the HSO4 and NH4 ions certainly
disclose the existence of structural modifications that occur at
T and T . In the paraelectric phase (P2 /c), the total polariza-
ferroelectric phase, and the difference between the symmetric
SÀO stretching frequency increases gradually until the second
transition temperature, which could be due to strong coupling of
c1
c2
1
tion of the crystal is zero due to the presence of the center of
modes. Below T , the compound lost its ferroelectricity; hence,
c2
inversion. Below T , there is a transition from P2 /c to Pc
leading to a nonzero polarization arising due to the lack of a
center of inversion. An interesting observation is that symmetric
the difference between the symmetric SÀO stretching frequency
reduced drastically, which could be due to less coupling of modes
leading to only the piezoelectric property.
c1
1
stretching modes show a substantial change across T , whereas
the bending and asymmetric stretching modes do not show
changes or show subtle changes at this temperature. Interestingly
We have tried solving and refining the 125 K structure in both
P1 and P1 symmetry. However, the structure could be solved
and refined only with space group P1. The earlier piezoelectric
c1
20
the converse is true across T . This is important as it demon-
study by Pepinsky et al. reported that the NH HSO showed
c2
4 4
strates that T introduces changes in the covalency of the bonds
piezoelectric current until 77 K ;the structure was indexed to the
P1 space group similar to that of our observation. From group
theoretical analysis, it is known that the point group P1 shows
the piezoelectric property, while P1 does not. There are two
crystallographically independent formula units in the asymmetric
c1
because of ordering across the transition. In general, the effect
of covalency on the vibrational parameters of symmetric stretch-
ing modes is significant when compared to the other modes. Tc2
is more due to tilting of the tetrahedra because of further
lowering of the crystal symmetry to P1 and hence would affect
bending and asymmetric stretching and increasing the number of
modes. The ordering in the structure below the first and second
unit of the P2 /c space group; however, when the temperature is
1
reduced to the first transition temperature (T ), the space group
c1
changes to Pc, and the number of independent formula units in
2
29
dx.doi.org/10.1021/jp2075868 |J. Phys. Chem. A 2012, 116, 223–230

The Journal of Physical Chemistry A
ARTICLE
the asymmetric unit are now 4, leading to the appearance of a
new Raman active mode in the SÀOH symmetric stretching
region [see Figure 4a,b]. The compound in the Pc phase has four
different crystallographically independent formula units in the
asymmetric unit similar to the case of P1. We have noticed the
appearance of new Raman active peaks in the SÀO asymmetric
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(
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CONCLUSIONS
The temperature-dependent Raman and X-ray investigation are
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(
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(
2
001, 227, 465.
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À
driven by ordering in the HSO4 ions. It is clear that the internal
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bending vibrational modes in addition to symmetric SÀO stretching
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(
27) Sheldrick, G. M. SADABS; University of G €o ttingen: G €o ttingen,
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(
4
4
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(
(
(
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’
ASSOCIATED CONTENT
(32) Herzberg, G. The Infrared and Raman Spectra of Polyatomic
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S
Supporting Information. P2 /c and Pc hkl plots. This
b
1
material is available free of charge via the Internet at http://pubs.
acs.org.
(
34) Toupry, N.; Poulet, H.; Postollec, M. L. J. Raman Spectrosc.
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Condens. Matter 1999, 11, 889.
1
(
’
AUTHOR INFORMATION
(
(
36) Kwon, S.-B.; Kimt, J.-J. J. Phys.: Condens. Matter 1990, 2, 10607.
37) Jariwala, M.; Crawford, J.; LeCaptain, D.J. Ind. Eng. Chem. Res.
Corresponding Author
*E-mail: cbhas@jncasr.ac.in.
2
007, 46, 4900.
’
ACKNOWLEDGMENT
We would like to acknowledge Professor C.N.R. Rao for useful
discussions during the preparation of this manuscript.
2
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dx.doi.org/10.1021/jp2075868 |J. Phys. Chem. A 2012, 116, 223–230