Superionic phase transition in KHSO4: A temperature-dependent raman investigation

Article abstract of DOI:10.1021/jp103005g

Temperature-dependent Raman spectroscopic studies have been carried out on KHSO₄ single crystals in the temperature range 298-493 K. A structural phase transition driven by the lattice and molecular disorder is observed at 473 K. The spectral data enable an understanding of the nature of the lattice disorder across the phase transition leading to the superionic phase. The disorder in the HSO₄^- polymeric hydrogen-bonded chain leading to a higher symmetry in the high temperature phase is clearly captured from our Raman results. The internal S-OH and S-O stretching modes and, to a limited extent, the external modes throw light on the disorder mechanism and the enhancement of conductivity after transition.

Full text of DOI:10.1021/jp103005g

10040
J. Phys. Chem. A 2010, 114, 10040–10044
Superionic Phase Transition in KHSO4: A Temperature-Dependent Raman Investigation
Diptikanta Swain,† Venkata Srinu Bhadram,† Gopal K. Pradhan,† S. Venkataprasad Bhat,†
Chandrabhas Narayana,*,† and C. N. R. Rao†,‡
Chemistry and Physics of Materials Unit and International Center for Materials Science, Jawaharlal Nehru
Centre for AdVanced Scientific Research, Jakkur P.O., Bangalore 560064, India
ReceiVed: April 3, 2010; ReVised Manuscript ReceiVed: June 29, 2010
Temperature-dependent Raman spectroscopic studies have been carried out on KHSO4 single crystals in the
temperature range 298-493 K. A structural phase transition driven by the lattice and molecular disorder is
observed at 473 K. The spectral data enable an understanding of the nature of the lattice disorder across the
-
phase transition leading to the superionic phase. The disorder in the HSO4 polymeric hydrogen-bonded
chain leading to a higher symmetry in the high temperature phase is clearly captured from our Raman results.
The internal S-OH and S-O stretching modes and, to a limited extent, the external modes throw light on
the disorder mechanism and the enhancement of conductivity after transition.
Introduction
is, therefore, of interest to study the internal as well as external
vibrations of molecular units as a function of temperature to
understand the mechanism of fast ion conduction and the
dynamics of the structural phase transition resulting in the
superionic phase. The room-temperature vibrational modes of
KHSO4 have already been reported.29 In the present study, we
Ion-conducting solids are of great importance because of their
potential applications in fuel cells, steam electrolysis, and
sensors.1-5 Ion conduction occurs in numerous types of
materials,6-11 including hydrogen-bonded systems.12-20 Among
the ion-conducting solids, superionic solids exhibit anomalously
high conductivity in the high temperature phase.12-20 KHSO4
belongs to the family of alkali sulfates AHSO4 (A ) Cs, Rb,
NH4), which are known for their ferroelectric property.21,22
KHSO4 does not, however, exhibit ferroelectricity23 but under-
goes a superionic phase transition at high temperature.24,25 The
high conductivity in the superionic phase in AHSO4 could be
due to disorder in crystallographic sites, rotational motion of
-
have investigated the dynamics of HSO4 units using variable
temperature Raman spectroscopy to understand the changes in
structure, conductivity, and the nature of the disorder associated
with KHSO4 during the phase transition.
Experimental Section
Single crystals of KHSO4 were grown by slow evaporation
from aqueous solution containing equimolar quantities of the
K2SO4 and H2SO4. The quality of the crystal used for the
experiment was checked under a polarized optical microscope.
The temperature evolution of the Raman spectra of KHSO4 was
recorded in the 180° backscattering geometry, using a 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 charge-coupled device (CCD; Spectrum One with CCD
3000 controller, ISA Jobin Yovn). Laser power at the sample
was ∼8 mW, and a typical spectral acquisition time was 1 min.
The spectral resolution chosen was 2 cm-1. The temperature
was controlled with an accuracy of ((0.1 K) by using a
temperature-controller (Linkam TMS 94) equipped with a
heating stage unit (Linkam THMS 600). The spectral profile
was fitted using Lorentzian functions with the appropriate
background.
-
HSO4 , or the dynamical motion of the crystal lattice. The room
temperature crystal structure of KHSO4 is orthorhombic (space
group Pbca) with 16 formula units per unit cell. The structure
contains two crystallographically independent KHSO4 units. The
HSO4- ion is usually found in a distorted tetrahedral symmetry.
Furthermore, it is connected with short O-H · · · O hydrogen
bonds forming one-dimensional long polymeric chains and zero-
dimensional dimers (see Figure 1). The structural phase transi-
tions in KHSO4 were studied by several authors using thermal
analysis,24 NMR spectroscopy, and conductivity measurements.25
On the basis of their conductivity measurements, Sharon and
Kalia26 predicted that the superionic phase transition in KHSO4
is driven by the conversion of dimer units into chains and
increase in the rotational degree of freedom of the chain units.
A more detailed and accurate analysis of the phase transition
was given by Swain and Guru Row27 using in situ single crystal
X-ray diffraction. They reported that the structure still remains
orthorhombic after the phase transition, but the space group
symmetry increases from Pbca to Cmca, keeping 16 formula
-
units per unit cell intact. In the Cmca structure, one of the HSO4
which is involved in polymeric hydrogen bond chain is
disordered at the O and H crystallographic sites.
Results and Discussion
Figure 2 shows the unpolarized Raman spectra of KHSO4
recorded at room temperature in the range of 50-1350 cm-1
arising from the external and internal vibrational modes. The
internal modes in the region 300-1350 cm-1 are divided into
three groups: (i) S-OH bending (ꢀS-OH) and O-S-O deforma-
tion (δOSO) (400-650 cm-1), (ii) S-OH stretching (νS-OH) (855,
872 cm-1), and (iii) symmetric and asymmetric S-O stretching
Raman spectroscopy is an ideal tool for capturing the
dynamics and local structural changes in crystal systems.28 It
* To whom correspondence should be addressed. E-mail: cbhas@
jncasr.ac.in.
† Chemistry and Physics of Materials Unit.
‡ International Center for Materials Science.
10.1021/jp103005g  2010 American Chemical Society
Published on Web 08/30/2010

Superionic Phase Transition in KHSO4
J. Phys. Chem. A, Vol. 114, No. 37, 2010 10041
Figure 1. Packing diagram showing hydrogen bonding schemes (dimers and chains) at 293 K. Potassium atoms are placed between the layer
formed by dimers and chains and are removed for the clarity of the diagram.
(νS-O) (1002, 1027, 1165, 1241 cm-1) by considering SO42- as
a distorted tetrahedra in C3V symmetry.29 The structure contains
two crystallographically independent HSO4- units, correspond-
ing to the chain and dimer units. The Raman modes correspond
to these units have been distinguished by Dey et al.29 by
frequencies plotted in Figure 5 also show abrupt changes with
some of the modes disappearing above TC. We observe an
anomalous increase in the fwhm of the νS-O mode across the
TC, as can be seen from Figure 4b. The line-width of the νS-O
mode of the chain increases abruptly from 26 to 34 cm-1 at
473 K and increases steeply to 46 cm-1 at 493 K. A similar
behavior can be seen in the case of the νS-O mode of the chain
at 1161 cm-1. It abruptly increases to a higher frequency at TC
as depicted in Figure 6. The anomalous broadening of the νS-O
mode can also be seen clearly in the inset of Figure 6. The
reason behind this anomalous behavior will be discussed later.
As in the case of the internal modes, the external lattice modes
of KHSO4 shown in Figure 7 also exhibit significant changes
around TC. Most of the external modes decrease in intensity
with temperature and collapse into a broad wing after the phase
transition.
-
considering the interaction among the HSO4 units. The
interactions are strong in the case of the dimer rather than the
chain. Hence, the frequencies of vibrational modes observed in
Raman and IR spectra occur at higher frequencies in the case
of the dimer. In contrast, the vibrational modes associated with
S-OH stretching and deformation in the dimer should appear
at lower frequencies than in the chain (see Figure 2a). This is
due to the strong hydrogen bonding in the dimer causing the
hydrogen atom getting to be pulled away from the oxygen in
S-OH. The lattice modes at 84, 90, 99, and 108 cm-1 are
assigned to the librational modes and those at 125, 139, 165,
179, and 192 cm-1 to the translational modes (see Figure 2b).
Temperature evolution of the Raman spectra of KHSO4 in
the temperature range 298-493 K is shown in Figure 3. We
observe major changes in the spectral features around the phase
transition temperature (TC ) 473 K) and in agreement with the
structural phase transition reported by Swain and Guru Row.27
The important changes in the Raman spectra are the following:
the disappearance of splitting of the symmetric S-O stretching
modes (νS-O); the sudden increase in frequency of the S-OH
stretching mode (νS-OH); the disappearance of the Raman modes
at 452 cm-1 and 571 cm-1 belonging to S-OH bending (ꢀS-OH);
and O-S-O deformation (δO-S-O) at TC. These changes are
shown in Figures 4 and 5, where we have plotted the temperature
evolution of phonon frequencies and the full-width-at-half-
maximum (fwhm) of some of the modes. With the increase in
temperature, νS-OH decreases, whereas νS-O increases for both
the dimer and the chain. At TC, there is an abrupt change in the
frequency of both νS-O and νS-OH modes. The bending mode
Raman spectra collected in the region 2000-3000 cm-1 are
assigned as OH group vibrations (see Figure 8). The interpreta-
tion of unpolarized Raman spectra in the OH stretching region
is difficult, because it contains the superposition of broad bands,
which are hard to deconvolute. Vibrational frequencies from
2200 to 2500 cm-1 may be due to coupling of the stretching
modes of (S)O-H with the external modes of SO3-O(H) groups
and are not very sensitive to the temperature, based on the earlier
report on RbHSO4.30 The spectral region 2600-3000 cm-1
contains one broad peak close to 2800 cm-1 with a shoulder
around 2650 cm-1. The former is the dimer ν(OH)· · ·O mode,
and the latter is assigned to chain mode, based on the reports
on KHSO4.29 With an increase in the temperature, the frequency
corresponding to chain mode increases and merges with the
dimer mode. The features become very broad after the transition.
We shall now examine the implications of the changes
observed in the vibrational modes of KHSO4 across the
transition. The anomalous changes in νS-O and νS-OH vibrational

10042 J. Phys. Chem. A, Vol. 114, No. 37, 2010
Swain et al.
Figure 2. Room temperature Raman spectra of KHSO4 in the
frequency range (a) 300-1350 cm-1 and (b) 50-250 cm-1 (D ) dimer,
C ) chain).
Figure 4. (a) Temperature evolution of νS-O and νS-OH mode
frequencies, (b) fwhm of νS-OH modes in the temperature range
298-493 K (D ) dimer, C ) chain).
mode frequencies across the phase transition are mainly due to
the variations in the bond lengths leading to structural phase
transition at TC. The disappearance of some of the modes such
as the bending modes at TC implies a higher symmetry structure
expected at high temperatures. The sudden increase in the line
width of the chain νS-O and νS-OH modes shown in Figure 4b
and in the inset of Figure 6 is interesting. Due to the lattice
disorder associated with O and H positions in the high-
Figure 3. Temperature-dependent Raman spectra of KHSO4 in the spectral range (a) 300-700 cm-1 and (b) 800-1300 cm-1
.

Superionic Phase Transition in KHSO4
J. Phys. Chem. A, Vol. 114, No. 37, 2010 10043
a
Figure 6. Temperature evolution of νS-OS(C) and νS-O (D) modes;
the inset shows corresponding line widths (D ) dimer, C ) chain).
Figure 5. Temperature evolution of ꢀSOH and δOSO deformation
frequencies in the temperature range 298-493 K (D ) dimer, C )
chain).
temperature phase, there is a large variation in bond strength
associated in these molecular subunits. This results in the
broadening of these modes. Since the lattice modes are much
weaker than the internal modes, this is more evident in their
behavior as a function of temperature. The increase in the width
leads to the loss of the fine features in the external mode at
high temperatures. The onset of disorder in the system due to
the variation in the position of oxygen and hydrogen atoms in
the system, initiating the transition, is clearly evidenced in the
behavior of both the internal and the external modes.
Figure 7. Temperature dependence of external lattice modes.
Sharon and Kalia26 proposed that the high conductivity of
the high temperature phase of KHSO4 is due to the conversion
of dimer units into chain and the subsequent rotation of the
time a decrease in hydrogen bonding does the reverse. In the
case of the OH stretching of the chain mode, we observe an
increase in the mode frequency with temperature, and it merges
with the broad band associated with the dimer mode after
transition. Since the OH stretching of the dimer is unaffected
by the transition, it supports the earlier observation that there
is no dimer to chain conversion across the transition.
-
HSO4 tetrahedra. However, our experimental results suggest
that there is no such dimer to chain conversion as all of the
dimer vibrational modes are still present after the phase
transition (see Figure 4a, for example). In fact the excessive
-
broadening suggests that HSO4 units are disordered in the
Our Raman results demonstrate that the transition is a result
of lattice disorder, which sets in much before the transition. It
has been suggested in case of chain hydrogen-bonded systems
that ion conduction is mainly due to the proton transfer from
one end of the chain to other.31 From our Raman results, we
strongly believe that, in our system, the lattice disorder
polymeric hydrogen-bonded chains leading to a higher symmetry
in the high temperature phase. In the region of 2600-3000 cm-1
associated with OH stretching region, we observe the weakening
of the chain hydrogen bonding suggested from the temperature-
dependent chain ν(OH)· · ·O mode frequency around 2650 cm-1
(see Figure 8). It is common to observe that strong hydrogen
bonding brings down the OH stretching frequency; at the same
-
associated with chain HSO4 units weakens the hydrogen
bonding between two chain units and facilitates the proton to

10044 J. Phys. Chem. A, Vol. 114, No. 37, 2010
Swain et al.
been revised and sentence 2 in the Results and Discussion has
been revised. The correct version posted on September 16, 2010.
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hop from one unit to another, resulting in the high conductivity
reported in KHSO4.25
Conclusions
The present temperature-dependent Raman investigation is
able to explain the dynamics of the structural phase transition
in KHSO4. The disappearance and discontinuities of phonon
frequencies at TC are attributed to the structural changes in the
crystal during the phase transition. On the basis of our
experimental results, the conversion of dimer units into chain
during phase transition can be ruled out. The superionic phase
transition in KHSO4 is mainly driven by disorder in chain
-
HSO4 ions due to the disorder in the oxygen and hydrogen
atom positions. It is clear that the internal S-OH and S-O
stretching modes and, to a limited extent, the external modes
throw light on the disorder mechanism and the enhancement of
conductivity after transition.
(31) Aruldhas, G. Bull. Mater. Sci. 1992, 15 (3), 229.
Note Added after ASAP Publication. This article posted
ASAP on August 30, 2010. Sentence 12 in the Introduction has
JP103005G