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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 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 undergoes 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 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 zerodimensional dimers (see Figure 1). The structural phase transitions 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 HSO4which is involved in polymeric hydrogen bond chain is disordered at the O and H crystallographic sites. 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.
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 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. 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 (βS-OH) and O-S-O deformation (δOSO) (400-650 cm-1), (ii) S-OH stretching (νS-OH) (855, 872 cm-1), and (iii) symmetric and asymmetric S-O stretching
10.1021/jp103005g 2010 American Chemical Society Published on Web 08/30/2010
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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, corresponding to the chain and dimer units. The Raman modes correspond to these units have been distinguished by Dey et al.29 by 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 (βS-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-halfmaximum (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
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. Raman spectra collected in the region 2000-3000 cm-1 are assigned as OH group vibrations (see Figure 8). The interpretation 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
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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.
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Figure 6. Temperature evolution of νS-OS(C) and νS-Oa(D) modes; the inset shows corresponding line widths (D ) dimer, C ) chain).
Figure 5. Temperature evolution of βSOH 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. 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 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 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 temperaturedependent 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
Figure 7. Temperature dependence of external lattice modes.
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. 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 associated with chain HSO4- units weakens the hydrogen bonding between two chain units and facilitates the proton to
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Swain et al. been revised and sentence 2 in the Results and Discussion has been revised. The correct version posted on September 16, 2010. References and Notes
Figure 8. Temperature-dependent evolution of Raman bands associated with the O-H region.
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. Note Added after ASAP Publication. This article posted ASAP on August 30, 2010. Sentence 12 in the Introduction has
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