Possible manifestation of proton disorder in δ-KIO 3·HIO3 crystal in its IR spectra

Article abstract of DOI:10.1016/j.molstruc.2004.02.005

For getting more information about the behavior of hydrogen atoms in partially occupied sites in δ-KIO₃·HIO₃, we have measured temperature-dependent infrared spectra of this crystal at 14-300 K in the spectral range of OH stretching

Full text of DOI:10.1016/j.molstruc.2004.02.005

Journal of Molecular Structure 692 (2004) 237–241
www.elsevier.com/locate/molstruc
Possible manifestation of proton disorder
in d-KIO3·HIO3 crystal in its IR spectra
a
a,
a
b
b
c
T. Gavrilko , G. Puchkovska , I. Sekirin , B. Engelen , M. Panthofer , J. Baran , H. Ratajzcak
c
*
¨
aDepartment of Photoactivity, Institute of Physics, National Academy of Sciences of Ukraine, 46 Nauky Prosp., 03028 Kyiv, Ukraine
bDepartment of Inorganic Chemistry, University of Siegen, 57068 Siegen, Germany
cInstitute of Low Temperature and Structure Research, Polish Academy of Sciences, 50-950 Wroclaw, Poland
Received 16 January 2004; revised 16 January 2004; accepted 2 February 2004
Abstract
For getting more information about the behavior of hydrogen atoms in partially occupied sites in d-KIO3·HIO3, we have measured
temperature-dependent infrared spectra of this crystal at 14–300 K in the spectral range of OH stretching and bending vibrations. It was
found that n(OH) band (centered at ,2800 cm21) has complicated profile which, at temperatures below 220 K, includes weak low-frequency
satellites at its low-frequency slope. At 14 K, we have main peak n(OH) at 2766, satellites at 2606, 2485, and 2387 cm21, and strong 2d(OH)
overtone peak at 2274 cm21 enhanced due to Fermi resonance. At the same time, the low-frequency slope of d(OH) band (centered at
1189 cm21) shows equidistant satellites separated with period of approximately 25 cm21. The observed features suggest anharmonic
coupling between OH high-frequency vibrations and various low-frequency motions of the crystal lattice.
q 2004 Elsevier B.V. All rights reserved.
Keywords: d-KIO3·HIO3; H-bond; Phase transitions; FTIR spectra; Proton dynamics
1. Introduction
atoms; however, the crystal structure does not contain the
sites of (8a) type suitable for occupation by hydrogen atoms,
but it contains two such sites in general position (16b)
which, at full occupation, would give 32 hydrogen atoms;
therefore, at least one of these sites should be only partially
occupied. One of these sites corresponds to short hydrogen
bond (O· · ·O distance equal to 267.1 pm) and seems to be
fully occupied, while the other corresponds to greater O· · ·O
distance (290.7 pm) and could be partially occupied; for
getting 24 hydrogen atoms, it should be one-half occupied.
The resulting structure contains O23–H1· · ·O13· · ·H2–O33
fragments (Fig. 1), where one-half of H2 atoms is absent.
In Ref. [5], the coordinates of H2 were reported as 0.023;
0.20; and 0.390; after correction, they are as follows: 0.020;
Polymorphism of potassium biiodate, KIO3·HIO3, has
been known for decades (see Ref. [1] and the literature cited
therein). The stable delta polymorph of the title crystal,
d-KIO3·HIO3, was found to exhibit low temperature
phase transition [1], non-linear dielectric and optical
properties [1,2]; a dynamic jump type proton disorder was
supposed for this polymorph [2,3]. The heavy atom part of
the d-KIO3·HIO3 crystal structure (K, I, O) was determined
several times at room temperature from X-ray diffraction
data [2–4] and refined in our previous work [5]. According
to these works, the space group of the title crystal is Fdd2;
with Z ¼ 24: The lattice unit contains: three crystal-
lographically non-equivalent pyramidal IO3 groups, each
in general position (16b), which gives 48 IO3 groups; one K
atom in general position (16b), and one K atom in a special
position (8a) at C2 axis, which gives 24 K atoms. From
chemical structure, the unit cell should contain 24 hydrogen
0.131; and 0.407. The structure can be described as
3/22
consisting of [I3O9H3/2
]
ions possibly originating from
occupationally disordered [I3O9H2]2 and [I3O9H]22 anions
(with short I–O contacts of IO3 groups). These ions are
connected via hydrogen bonds to form plane grids parallel
to (100) with the Kþ ions placed between them. This agrees
with the known structures of the oxosalts, oxoacids, and acid
oxosalts of iodine(V), i.e. compounds of the types MIIO3,
Hn(IO3)n, and MIm(IO3)m·Hn(IO3)n, respectively, which
*
Corresponding author. Tel.: þ380-44-265-1552; fax: þ380-44-265-
1589.
E-mail address: puchkov@iop.kiev.ua (G. Puchkovska).
0022-2860/$ - see front matter q 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2004.02.005

238
T. Gavrilko et al. / Journal of Molecular Structure 692 (2004) 237–241
of the IR spectra in the 14–300 K temperature range were
measured using the ADP cryogenics closed-cycle helium
refrigerator system, which allows one to maintain the
temperature of the sample within 1 K. The measurements
were performed with 10–30 K steps and a 20 min
stabilization time for each temperature.
3. Results and discussion
The fragments of the temperature-dependent FTIR
spectra of d-KIO3·HIO3 crystal recorded in the frequency
range of OH stretching and bending vibrations (1800–3400
and 900–1300 cm21) are shown in Figs. 2 and 3,
respectively. The lengths of the two types of the OH· · ·O
bonds in this crystal, as reported in Ref. [5], are 290.7 and
267.1 pm and, therefore, according to well-known Novak’s
classification [6,7], one can ascribe them to weak and
medium strength hydrogen bonds. It is known [6,7] that for
weak and medium strength hydrogen bonds one can
distinguish three IR active modes corresponding to
vibrations of OH· · ·O fragments: (1) stretching modes
n(OH) with 3500 $ nOH $ 2800 cm21; (2) in-plane bend-
ing modes d(OH) with 1300 $ dOH $ 1150 cm21; and (3)
out-of-plane bending modes g(OH) with gOH < 550 cm21
[7]. The frequencies vary with the strength of the hydrogen
bond and, within the series of similar compounds,
they correlate with each other and with the strength
Fig. 1. Hydrogen bond system of d-KIO3·HIO3 with the octahedral
surrounding of the 8a sites at 0 0 1/2. Dashed lines represent hydrogen
bonds [5].
contain trigonal pyramidal IO23 ions or IO2OH molecules,
which tend to form dimeric or oligomeric species with
intermolecular I· · ·O contacts. Our previous results of
infrared (IR) and Raman studies of d-KIO3·HIO3 [5] show
that this species undergoes a second-order phase transition
(without changes in its crystal structure) near 220 K, which
manifests itself as a change in the slope of the temperature
dependence of some spectral parameters and can be related
to the proton sublattice. However, until now the mechanism
of this phase transition is not clear due to the lack
of experimental data in the OH stretching range. It is
known [6,7] that in hydrogen bonded systems the profile and
frequency shift of OH stretching band is correlated with
both the intermolecular distance O· · ·O, and the intramole-
cular bond length, and the temperature dependence of the
bands related to the vibrations of O–H bonds can give
valuable information concerning the dynamics of the proton
subsystem. In the present work, we perform a comprehen-
sive analysis of the temperature-dependent FTIR spectra of
the d-KIO3·HIO3 crystal in the region of OH stretching and
bending vibrations. The aim of this analysis is to
characterize in more detail the lattice dynamics of d-
KIO3·HIO3 crystal and to get insight into the mechanism of
the phase behavior of this crystal.
2. Experimental
Specimens of the d-KIO3·HIO3 crystal were obtained
according to Ref. [4] by slowly cooling down the
concentrated solutions of KIO3 and HIO3 from 413 to
353 K during 3 months. The temperature-dependent IR
spectra of the powdered crystal suspended in Nujol were
recorded on the Bruker FTIR spectrometer IFS-88 model in
400–4000 cm21 spectral range. Temperature dependencies
Fig. 2. Temperature evolution of FTIR spectra of polycrystalline
d-KIO3·HIO3 (suspension in Nujol) in the region of n(OH) stretching
mode. Asterisks denote the Nujol absorption bands.

T. Gavrilko et al. / Journal of Molecular Structure 692 (2004) 237–241
239
usually assigned to in-plane bending modes d(OH) of
I–O–H· · ·O–I fragments. Temperature evolution of the
peak position and half-width of the d(OH) band were
measured in our previous work [5], and respective
dependencies showed sharp changes in their slopes near
220 K, which was presumably attributed to a phase
transition in the proton sublattice. In the present paper, we
focused on the temperature evolution of the overall profile
of the respective bands. First of all, one could see
pronounced ‘modulation’ in the low-frequency slopes of
the n(OH) and d(OH) bands, which clearly develops at
lower temperatures. The frequencies of the d(OH) band
satellites at 99 K are 1146, 1119, 1093, 1070, 1039, 1012,
982, 956, and 932 cm21. The differences between the
neighboring peaks in this progression are almost constant
and equal to 27 ^ 4 cm21. The n(OH) band satellites in the
spectra, which are more clearly seen on the low-frequency
slope at the temperatures below 220 K, are located (14 K) at
2766 (main maximum), 2606, 2485, and 2387 cm21. The
respective difference between the maxima are 160, 120, and
98 cm21, which correlates well with lattice phonons
reported for the d-KIO3·HIO3 crystal in Ref. [5]. When
passing from 220 K to lower temperatures, all the satellite
bands become sharper; however, they retain their relative
positions and intensities. It is worth mentioning that the
number of satellite bands observed in the OH bending
mode region is three times greater than the number of
corresponding satellites in the OH stretching region. Similar
Fig. 3. Temperature evolution of FTIR spectra of polycrystalline d-
KIO3·HIO3 in NaCl pellet in the region of the d(OH) in-plane bending
mode.
and the RO· · ·O distance of the respective hydrogen bond.
According to this classification, the OH stretching band of
weaker hydrogen bond with O–H· · ·O distance of 290.7 pm
should be located at higher frequencies of about 3500 cm21
.
It was observed experimentally in our FTIR spectra at about
3585 cm21. It is also known that for medium strength
H-bonds Fermi resonance often affects the n(OH) band
shape [8]. As it was mentioned above, a characteristic
feature of d-KIO3·HIO3 crystal structure is the presence of
K3/2[I3O9H3/2] moieties. The interatomic distances and
hydrogen bond lengths in d-KIO3·HIO3 are similar to
those reported for a-HIO3 crystal [9] where HIO3 molecules
are linked by chain-like H-bonds (see Table 1), and their
low-frequency phonon spectra are similar, too [5,9]. This
can be the reason for similarity of the spectral distribution
observed in the region of n(OH) stretching band in the
spectra of these two crystals. Following [9], we assign a
very strong broad band centered at 2800 cm21 (at room
temperature) and less intensive more narrow band centered
at 2274 cm21 to Fermi resonance doublet formed by n(OH)
stretching vibration and 2d(OH) overtone. In the
1000–1300 cm21 region, there is a band of middle intensity
with peak position 1189 cm21 at room temperature. Bands
of similar frequencies were also observed in the IR spectra
of the other H-bonded iodates [9,10], and they are
Table 1
I–O bond distances (pm) of a-HIO3 and d-KIO3·HIO3 to oxygen atoms
acting as hydroxyl groups (I–OH), H-bond acceptors (I–O· · ·H) or
bridging atoms to neighbouring I atoms (O–I· · ·O) with short (#265 pm)
I· · ·O contacts, and O–H· · ·O distances
I–OH I–O· · ·H O–I· · ·O I· · ·O O–H· · ·O Ref.
a-HIO3 190
d-KIO3·HIO3 193
188
181
181
181
181
182
181
250
235
256
268
[10]
[5]
Fig. 4. Temperature-dependent IR spectra of single crystal a-HIO3
(yz plane; EjjZ thickness of plate ,5 mm in the region of n(OH) stretching
mode.
267.1
290.7
[5]

240
T. Gavrilko et al. / Journal of Molecular Structure 692 (2004) 237–241
Table 2
protons demonstrate a peculiar lattice dynamics. Of special
importance are the recent studies performed by Fillaux and
co-authors [17–19]. Using the incoherent inelastic neutron
scattering technique, they have investigated the vibrational
dynamics for protons in various solids and revealed that
proton dynamics is almost totally decoupled from the
surrounding heavy atoms. In addition, Fillaux notes that
the proton subsystem demonstrates collective dynamics.
By studying electronic absorption spectra in the benzoic
acids crystal, Rambaud and Trommsdorff [20] have observed
the cooperative proton transfer and tunneling (motion offour
protons). Similar four-position protonic moieties can be
selected in the d-KIO3·HIO3 crystal (Fig. 1). For a-HIO3
crystal, which at room temperature belongs to the orthor-
hombic shape group P212121 with four formula per unit cell
[9], there are also evidences [21] that the protons are
randomly distributed in general positions between the
oxygen atoms belonging to different iodate groups. So one
can expect collective vibrations of protons in certain
structures formed by statically disordered hydrogen atoms,
whose vibrations are only weakly coupled with the vibrations
of the neighboring hydrogen atoms (and which would be
absent for ideally ordered proton subsystem). As it follows
from our data [5], the OH bending vibration band is more
sensitive to proton disorder than the OH stretching band.
Therefore, the satellite bands observed on the low-frequency
slope of d(OH) band can be assigned to oscillations of
disordered OH groups forming a periodic structure in the
crystal. The resulting spectral distribution in the OH bending
and stretching vibration regions is determined by the
interplay between the adiabatic coupling between the high-
frequency OH stretching/bending and the low-frequency
lattice phonons and resonance interaction between the
respective OH fundamental and various combination modes.
Frequencies of n(OH) and d(OH) bands of HIO3 and d-KIO3·HIO3 at 300
and 14 K
a-HIO3
d-KIO3·HIO3
Assignment
300 K
14 K
300 K
14 K
2274
2304
2378
2511
2656
2893
3062
77 K
2263
2274
2407
2490
2606
2774
2d(OH)
Two-phonon excitations
2604
2952
2606
2858
n (OH)
d (OH)
200 K
1155
293 K
99 K
1165
1183
1146
1119
1093
1070
–
1189
1146
1119
1093
1070
–
1060
1063
2g (OH)
1039
1012
982
1039
1012
982
956
956
(but less regular) band progressions were also observed for
n(OH) band in the spectra of a-HIO3 [9] (see Fig. 4) and
a-KIO3·HIO3 [11] crystals, as well as for other systems
[12–14]. The frequencies of n(OH) and d(OH) bands are
presented in Table 2. The observed extra bands can be
ascribed to strong anharmonic coupling between the high-
frequency vibrational modes of hydrogen bonds and low-
frequency lattice phonons as described in Ref. [15].
According to the theoretical model proposed in Ref. [15],
for solids containing strong hydrogen bonds one can expect
the appearance of a multiband substructure developed at the
low-frequency slope of the n(OH) absorption band in the IR
spectra. To understand the origin of the fine structure of
n(OH) band which emerges when cooling the crystal
(Fig. 4), it is worth mentioning that for a-HIO3 crystal the
submaxima observed in the 2300–2700 cm21 region
correspond to the gaps of the polariton branch in the
Raman spectra on polaritons [16], and therefore the
submaxima were attributed to dipole active two-particle
states. This suggests that the observed structure is caused by
resonant or quasiresonant interaction between the two-
particle excitation zones and the one-particle state n(OH)
which leads to formation of coupled or quasicoupled pairs
of phonons. The observed fine structure can be also
connected with distribution of the density of states, as
well as with excitation of the type 2dðOHÞ þ vs; where vs
corresponds to low-frequency lattice vibrations.
4. Conclusions
The detailed temperature variable FTIR spectroscopic
study of the d-KIO3·HIO3 crystal has been performed in the
spectral range of OH stretching and bending vibrations at
temperatures down to 14 K. Powder FTIR spectra of the title
crystal show a series of submaxima, which are developed at
the temperatures below 220 K and are superimposed on the
carrying OH stretching or bending absorption band.
We associate the presence of this extra bands in the IR
spectra with anharmonic coupling between OH high-
frequency vibrations and collective vibrations of disordered
hydrogen atoms in proton sublattice of the crystal, with
possible participation of lattice phonons.
Moreover, with account for rather complicated crystal
structure of the d-KIO3·HIO3 crystal ðZ ¼ 24Þ and presumed
proton disordering, one should consider contribution of
proton sublattice into the lattice dynamics of the studied
crystal. It is well known that solids with hydrogen atoms or
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