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Vibrational study of the protonic superionic conductor Cs5H3 (SO4)4·H2O
Journal of
MOLECULAR STRUCTURE
ELSEVIER
Journal of Molecular Structure 326 (1994) 93-98
Vibrational study of the protonic superionic conductor A.M. Fajdiga-Bulat”, bLaboratoire
F. Romainb, M.H. Limageb, A. LautiCb9*
“J. Stefan Institute, Jamova 39, 6111 I Ljubljana, Slovhnia de Spectrochimie Infrarouge et Raman, C.N.R.S., 2 rue Henri Dunant, 94320 Thiais, France
Received 30 March 1994
Abstract
Infrared and Raman spectra of CsZH3(S0&.H20 have been investigated in the 4000-lOcm-’ region between 450 and 90 K. An assignment of bands in terms of OH group, distorted sulphate ion and crystalline water group frequencies is proposed. One phase transition is observed at 415 K on heating and the spectral changes which occur between 150 and 240 K have been explained by progressive freezing out of the dynamically disordered acid hydrogen bonds and rotationally disordered water molecules on cooling. The spectroscopic results are compared with dielectric relaxation and NMR measurements on CsSH3(S0& -Hz0 as well as with crystallographic data on the hexagonal high temperature phase of isomorphic CsSH3(Se0&.H20.
1. Introduction
CS~H~(SO~)~- Hz0 belongs to a new family of protonic, superionic conductors with the general formula MSX3 (AO,), . H20; M=K, Rb, Cs; X=H, D; A=S, Se. Only crystals with M=Cs have been investigated so far [l-3]. At room temperature, CS~H~(SO~)~- Hz0 as well as isomorphous CsgH3(Se04)4 - Hz0 in high temperature phase, belong to the hexagonal space group D& = P6&nmc, Z = 2 [l, 21. The basic structural pattern is similar to the one in the well known M5X(A04)2 [4] crystals, with layers of SO4 tetrahedra linked by hydrogen bonds and layers of Cs atoms. Hz0 molecules are located between the layers. Crystallographic data [3] indicate that there are two different types of SO, tetrahedra
* Corresponding author. 0022-2860/94/$07.00 0 1994 SSDI 0022-2860(94)08344-H
and the acid hydrogen bonds linking them are of different lengths. While CsgH3 (SeO& - Hz0 undergoes on cooling an improper ferroelastic phase transition at T = 345 K, it has been suggested that CsSH3 (SO4)4 - Hz0 instead undergoes a glass-like freezing of the two-dimensional dynamically disordered O-H . . . 0 bonded network below room temperature [l]. With the aim of determining precise proton dynamics and the occurrence of phase transitions, we have investigated the infrared and Raman spectra of CSSH~(SO~)~. Hz0 in a broad temperature range (400-90 K) and report here the main results concerning hydrogen bonding and phase transitions in this substance. 2. Experimental
Infrared spectra of powdered CS~HJ(SO~)~ - Hz0 Elsevier Science B.V. All rights reserved
A.M. Fajdiga-Bulat et al/J. Mol. Struct. 326 (1994) 93-98
I
)
480
,
780
I
I
I
iO80
1350
2675
I
4000
Wavenua&er/cD-1
Fig. 1. Temperature dependence of infrared spectra, as suspensions of powdered CsSH3(S04)4. Hz0 in Fluorolube (4000-1345cn-‘) and Nujol (1345%18Ocm-I), between 433 K and liquid nitrogen temperature. The intensities of the Fluorolube and Nujol regions are not correlated. *Nujol band.
as a suspension in Fluorolube and Nujol were recorded on a Perkin Elmer 983 spectrophotometer. A Perkin Elmer high temperature cell and a conventional liquid nitrogen cooled cell were used to obtain spectra at different temperatures. Raman spectra of monocrystalline samples were obtained on a RTI 30 DILOR triple monochromator with the 514.5 nm Spectra Physics Ar+ laser exciting line. Conventional furnace and cryostat were used.
3. Results and discussion The temperature dependence of the infrared and Raman spectra of CsSH3(S0& - Hz0 in the ~-180~-’ region is shown in Figs. 1 and 2. The low-frequency Raman spectra (3 lo- 10 cm-‘)
are shown in Fig. 3. The waven~~rs, relative intensities and the proposed assignment of the spectra measured at 95, 300 and 433 IS are given in Table 1. The bands due to internal vibrations are assigned in terms of OH and Hz0 group frequencies and frequencies of distorted tetrahedra of SO;- ions designated by vl, v2, y and v4. There seem to be two kinds of OH - . - 0 hydrogen bonds of different lengths, henceforward designated as short and long (though the “long” one is still very short), in CS~H~(SO~)~H~O. The latter are evidenced by three very broad bands which can be distinguished between 3200 and 17OOcm-’ in the infrared spectrum of Fig. 1 at all investigated temperatures. These A, B, C bands [5], with maxima near 2860, 2440 and 1700cm-’ at room temperature, generally originate from a Fermi resonance between the v(OH) stretching and the overtones of the in-plane &OH) and out-of-plane r(OH) bending modes; they are a frequent feature of crystals with strong hydrogen bonds with R(O.. -0) lengths between 2.6 and 2.5& like in (CH3)2A~OOH 161.The band near lOOOcm_’ at room temperature can thus be assigned to the +OH) mode and the band at 139Ocm-‘, that appears at low temperatures, to the &OH) mode of the long 0-H.. ~0 bond. There may be another, still stronger hydrogen bond giving rise to the very broad background Y(OH) mode centered near 1100 cm-‘. From this wavenumber, its corresponding R(0 . - ~0) length is estimated as being between 2.5 and 2.4A. With this assumption, the corresponding y(OH) mode probably participates in the broad band near 12OOcn-‘ . However, in such strong OH- . -0 bonds, we have shown that only by inelastic neutron scattering is it possible to determine the OH bending modes unambiguously [7]. the corresponding absorptions are Indeed, observed with difficulty on infrared spectra. The other bands in the 1200-4OOcm-’ region can be assigned to the internal vibrations of the sulfate entities. The v2, y and u4 modes are split into several components due to the distortions of the SO4 tetrahedra. According to crystallographic data at 300K, there are two slightly different kinds of SO4 tetrahedra in CS~H~(SO~)~- H20 both with C3, site symmet~. For the factor group Lf,,, this brings us to expect only 2v3, 2v4
95
A.M. Fajdiga-Bulat et al./J. Mol. Struct. 326 (1994) 93-98 Table 1 Infrared and Raman wavenumbers and assignments of CssHs(SO& Raman
Infrared 9SK
300K
3520 s 3475 s
3560 m 3480 m
2810 s,b 2450 s,b 1740 s,b 1050 vb 164Om
2860 s,b 2440 s,b 1700 s,b 1lOOvb 164Om
1390 m 1320 vw 1200 s,b 117Osh 113ovw 1090 s 1080 sh 1052 m 1042 sh 1030 sh 1000 s 986 sh 880 m 870 s 854 m 840s 770 sh 636 s 615 s 590 vw 580 vs 470 m 455 m 442s 432 s
- Hz0 at 93,300 and 430 K
433 K
95K
Assignment 300 K
433 K dH2O) ~0520)
A B ~@H)I,,, C1 v(OH) short @W)
2920 m,b 2400 m,b 1700 m,b 1100 vb
WWI0Il* 1206 s,b 1120 w 1096 m
1207 s,b 116Osh 1120sh 1050 s
1240w 1193m 1091 vw 1083 w
1225 w 1193m 1118 VW 1085 VW
ly(OWsh*?l 1204w, b Y(SO4)
1169~~
1052 w 1044W
1010 s
1015 w
1025 vw
1059 vs 984 s 966 vw
1055 vs 980 s 968 vw
888 s 860 w,b
882 s 850 w,b
I
970 sh
984 sh
860 s
r@H)
long
v,
1045 vs 975 s
HSOi VIW- )
838 mb, V(SOs -OH)
835 s 780 vw 635 m 625 vw 610 m 584 vs 468 m 440m 430 sh
840 s,b 780 sh 610 sh 580 vs
455 vw 430 s
1 637 w 625 w 611 w 592 s 584 vw
638 VW 622vw 608 VW 590s
476 w 443vw 435 w 423 s
464w
s: strong; m: medium; w: weak; b: broad; v: very; sh: shoulder.
u4(SO4)
1 dSO4)
435 w 423 s
237 vw 91 w,b 55 m,b 34 w,b 20 m
610 VW 584 s
430 w 419 s v(O...O)
89 w,b 49 m,b 28 w,b 17m
Lattice modes
96
A.M. Fajdiga-Bulat et al.lJ. Mol. Struct. 326 (1994) 93-98
T/K
300
600
900
1200
l!
WavenuriberIcm-1 Fig. 2. Raman spectra of CsgH3(S0& - Hz0 between 1500 and 3OOcm-’ at T = 433,300 and 95 K.
and 2~~ infrared active vibrations (El, symmetry). However, from the chemical point of view, also two different sulfate entities are expected, namely SOiand HSOZ ions, and Cs, is only a statistically averaged site symmetry. The true local symmetry is certainly lower because of the disorder present in superionic conductor crystals; so more components can be expected for each sulfate and hydrogen sulfate modes. Finally, the bands at 3560, 3480 and 1640 cm-’ are assigned to the v,(HzO), vs(H20) and 6(H20) vibrations of crystalline water, respectively. The wavenumbers of valence modes are high and characteristic of quasi-free water molecules linked to the sulfate framework by very weak OH. . .O hydrogen bonds with R(0 ‘. .O) distances of about 2.9-3.0& as in numerous hydrates metallic salts [6]. This interpretation is consistent with the structural data [4]. The infrared spectrum of CS~H~(SO~)~- Hz0 at liquid nitrogen temperature, while not very different from the room temperature one, does show, besides the S(OH) mode at 139Ocm-‘, a splitting of the v(S-OH) mode at 87Ocm-’ into four components, an additional splitting of the v2, y modes, as well as intensity changes in some of these modes and in the va(H20) and v~(H~O) modes. On heating from liquid nitrogen temperature back to room temperature, these additional splittings gradually
diminish in the tem~rature region between 150 and 24OK, but there are no abrupt sharp changes in the spectrum. The v(S-OH) splitting starts to change already at around 150 K, and at 200 K this vibration shows only two discernible components. Between 200 and 260K, this splitting vanishes as well as the &OH) mode and the additional y and v4 vibrations (Fig. 1). The wavenumber shifts between 90 and 300 K are weak, indicating only a light reinforcement of strong and weak hydrogen bonds. On heating from room temperature up to 450 K, we observe a phase transition near 415K. The infrared spectrum at 433 K lacks the crystalline H20 modes, the splitting of the sulphate vibrations almost vanishes and the v2, v3 and u4 modes become mostly broad, single bands. This transition is reversible and, in particular, the Hz0 modes reappear below transition temperature, showing that water molecules are always present in the crystal, even in high temperature phase. The disappearance of Hz0 valence modes therefore reflects an important dynamic disordering of these molecules in the crystalline network, probably of reorientational type. The Raman spectrum of CS~H~(SO~)~- Hz0 (Fig. 2) at room temperature shows bands assignable to the internal sulfate vibrations, but as expected [8] no equivalents of the OH stretching or crystalline water modes. The strong v1 band is a triplet (1055, 980 and 882cn-t) and we also observe an additional, very weak vr band at 968cm-’ (from crystallographic data, at most two strong vr bands would be expected, originating from the two different SO4 tetrahedra). This splitting may be considered as a spectroscopic indication of asymmetric hydrogen bonds where, by analogy to the spectra of MHS04 [9] and M3H(S04), [8] compounds, SOi- [ll] and HSOT entities can be distinguished. The bands at 1055 and 882 cm-’ correspond to yl(S03) and Y(S-OH) of HSO;. The lowest wavenumber can be correlated with S-OH distances. So, in the case of KHS04 [9], the lowest skeletal stetching frequencies are observed in the 850~890cm-’ range, the S-OH distance being 1.57A [lo] and this correlates well with the observed frequency of the line at 882cm-‘.
A.M. Fajdiga-Bulat
et a1.l.I. Mol. Struct. 326 (1994) 93-98
T/K
233
__-
< ce
193
10
1
I
I
I
I
I
60
110
160
210
260
310
WavenumerIcn-1 Fig. 3. Temperature dependence of low frequency Raman spectra (310-lOcm-‘) ofCsSH3(S04)4 - Hz0 between 433 and 83 K. *Plasma lines.
The Raman spectrum at liquid nitrogen temperature is not significantly different from the room temperature one, although some additional splittings corresponding to those in the infrared spectrum do occur. Thus, it seems that the SO4 framework is practically not modified at all during the temperature variations between 90 and 450 K. The important broadening of the lines above 415 K is consistent with an increasing disorder in the high temperature phase. The same is true for the lattice bands, in the region below 300 cm-r (Fig. 3); that is the general pattern of the lattice bands, which are usually very sensitive to structural modifications, do not change in the temperature range between 400 and 77K and are relatively broad even below liquid nitrogen temperature, indicating a persisting, strong, structural disorder in the crystal.
91
There is a drastic change in the low frequency range Raman spectrum at high temperatures. The spectrum at 433 K becomes “liquid-like”, with no vibrations distinguishable from a broad background, characteristic for an orientationally and translationally disordered phase. The bandwidth of sulphate vibrations also increases more than twice on heating from 410 to 433K and their frequencies are shifted to lower values, e.g. the line of the Y(S-OH) mode moves from 875 to 838 cm-’ and its width at half height increases from roughly 24 to 60cm-‘. The changes observed in the Raman spectra of CS~H~(SO~)~- Hz0 in the lattice and SO4 vibrations region at the phase transition to the high temperature phase are very similar to changes seen in Raman spectra of CsHS04 and CsDS04 at the ferroelastic to super-ionic phase transition [12, 131. The broadening of the v(SOJ modes can be explained by an increase of the anharmonicity of the sulfate vibrations due to the fast reorientations of SO4 tetrahedra and the disordering of the protons in the O-H bonds that occurs in the superionic phase. We may conclude that, like in CsHS04 and CS~H(SO~)~, a dynamic 0-H.. .O hydrogen bonded network, formed by reorienting HS04 groups, is also responsible for the high protonic conductivity in CS~H~(SO~)~- H20. The infrared spectra of CsSH3(S04)4 - Hz0 show bands characteristic of compounds with strong hydrogen bonds. Two spectroscopically different kinds of hydrogen bonds can be distinguished: a longer bond with a R(0 . . . 0) length between 2.6 and 2.5 A, and a shorter bond with its OH stretching frequency lower than its OH bending frequency, indicating that the R(O.. .O) length lies between 2.5 and 2.4A. These observations can be compared with crystallographic data on the hexagonal phase of isomorphic CsSH3(Se04&, - Hz0 [3], where a crystallographically symmetric and an asymmetric hydrogen bond with R(0 . . . 0) lengths of 2.59 and 2.81 A are reported. However, in our case, the hydrogen bonds are shorter, even in the high temperature phase. Besides, the 2.8 1 A R(0 . . . 0) distance seems too long because no v(OH) band appears around 3300cm-r [6] in this compound family. Probably,
98
A.M. Fajdiga-Bulat et al./J. Mol. Struct. 326 (1994) 93-98
the crystallographic length corresponds to an averaged position of oxygen atoms, due to an orientational disordering between different potential wells, and the real R(0 . . . 0) distance is certainly about 2.6-2.7 A. Comparing our results with previously published dielectric relaxation measurements [l] and NMR experiments [14], we observe that our results are in agreement with the conclusions reached there. While no phase transition occurs in CS~H~(SO~)~- HZ0 on cooling down from room temperature to 77K, the observed gradual changes in the infrared and Raman spectra are compatible with a glassy freezing out of the dynamically disordered acid hydrogen bonds and rotationally disordered Hz0 molecules. Strong, static, structural disorder still persists in the crystal at the lowest investigated temperature. The phase transition that occurs on heating near 413 K leads to a super-ionic phase as shown by the “liquid-like” Raman spectrum in the lattice vibration region. This agrees with the ‘H NMR self-diffusion coefficient measurements. There are two possible explanations for the disappearance of the Hz0 vibration modes in the superionic phase: either water molecules are free to move through the crystal lattice or water molecules get out of the crystal. From spectroscopic measurements it is not possible to distinguish between these two possibilities, but the reversibility of the phenomenon is in favor of the first mechanism. We believe that a somewhat enlarged crystal lattice unit cell with groups of SO4 tetrahedra linked together by hydrogen bonds and
layers of Cs atoms still exists in this high temperature phase.
References [I] AI.
Baranov, O.A. Kabalov, B.V. Merinov, L.A. Shuvalov and V.V. Dolbinina, Ferroelectrics, 127 (1992) 257. [2] B.V. Merinov, AI. Baranova, L.A. Shuvalov and I.M. Shchagina, Kristallografya, 36 (1991) 584. [3] B.V. Merinov, A.I. Baranov, L.A. Shuvalov, J. Schneider and H. Schulz, submitted for publication. [4] AI. Baranov, V.P. Khiznichenko, V.A. Sandler and L.A. Shuvalov, Ferroelectrics, 81 (1988) 183. [S] D. Hadzi and N. Kobilarov, J. Chem. Sot. A, (1966) 439. [6] A. Novak, Struct. Bonding (Berlin), 18 (1974) 177. [7] F. Fillaux, A. Lautit, J. Tomkinson and G.J. Kearley, Chem. Phys., 154 (1991) 135. [8] M. Damak, M. Kamoun, A. Daoud, F. Romain, A. Lautie and A. Novak, J. Mol. Struct., 130 (1985) 245. [9] A. Goypiron, J. de Villepin and A. Novak, J. Raman Spectrosc., 9 (1980) 297. [lo] F. Payan and R. Haser, Acta Crystalogr., Sect. B, 32 (1976) 1875. [l l] K. Nakamoto, Infrared and Raman Spectra of Inorganic and Coordination Compounds, 3rd edn, Wiley, New York, 1978, p. 142. [12] V.P. Dmitriev, V.V. Loshkarev, L.M. Rabkin and L.A. Shuvalov, Ferroelectr. Lett., 9 (1988) 7. [13] V.P. Dmitriev, V.V. Loshkarev, L.M. Rabkin, L.A. Shuvalov and I.M. Shchagina, Kristallografiya, 33 (1988) 151. [14] A.M. Fajdiga-Bulat, R. Blinc, B. Loiar, J. Slak, G. Lahajnar, I. ZupanEiE and L.A. Shuvalov, submitted for publication.
MOLECULAR STRUCTURE
ELSEVIER
Journal of Molecular Structure 326 (1994) 93-98
Vibrational study of the protonic superionic conductor A.M. Fajdiga-Bulat”, bLaboratoire
F. Romainb, M.H. Limageb, A. LautiCb9*
“J. Stefan Institute, Jamova 39, 6111 I Ljubljana, Slovhnia de Spectrochimie Infrarouge et Raman, C.N.R.S., 2 rue Henri Dunant, 94320 Thiais, France
Received 30 March 1994
Abstract
Infrared and Raman spectra of CsZH3(S0&.H20 have been investigated in the 4000-lOcm-’ region between 450 and 90 K. An assignment of bands in terms of OH group, distorted sulphate ion and crystalline water group frequencies is proposed. One phase transition is observed at 415 K on heating and the spectral changes which occur between 150 and 240 K have been explained by progressive freezing out of the dynamically disordered acid hydrogen bonds and rotationally disordered water molecules on cooling. The spectroscopic results are compared with dielectric relaxation and NMR measurements on CsSH3(S0& -Hz0 as well as with crystallographic data on the hexagonal high temperature phase of isomorphic CsSH3(Se0&.H20.
1. Introduction
CS~H~(SO~)~- Hz0 belongs to a new family of protonic, superionic conductors with the general formula MSX3 (AO,), . H20; M=K, Rb, Cs; X=H, D; A=S, Se. Only crystals with M=Cs have been investigated so far [l-3]. At room temperature, CS~H~(SO~)~- Hz0 as well as isomorphous CsgH3(Se04)4 - Hz0 in high temperature phase, belong to the hexagonal space group D& = P6&nmc, Z = 2 [l, 21. The basic structural pattern is similar to the one in the well known M5X(A04)2 [4] crystals, with layers of SO4 tetrahedra linked by hydrogen bonds and layers of Cs atoms. Hz0 molecules are located between the layers. Crystallographic data [3] indicate that there are two different types of SO, tetrahedra
* Corresponding author. 0022-2860/94/$07.00 0 1994 SSDI 0022-2860(94)08344-H
and the acid hydrogen bonds linking them are of different lengths. While CsgH3 (SeO& - Hz0 undergoes on cooling an improper ferroelastic phase transition at T = 345 K, it has been suggested that CsSH3 (SO4)4 - Hz0 instead undergoes a glass-like freezing of the two-dimensional dynamically disordered O-H . . . 0 bonded network below room temperature [l]. With the aim of determining precise proton dynamics and the occurrence of phase transitions, we have investigated the infrared and Raman spectra of CSSH~(SO~)~. Hz0 in a broad temperature range (400-90 K) and report here the main results concerning hydrogen bonding and phase transitions in this substance. 2. Experimental
Infrared spectra of powdered CS~HJ(SO~)~ - Hz0 Elsevier Science B.V. All rights reserved
A.M. Fajdiga-Bulat et al/J. Mol. Struct. 326 (1994) 93-98
I
)
480
,
780
I
I
I
iO80
1350
2675
I
4000
Wavenua&er/cD-1
Fig. 1. Temperature dependence of infrared spectra, as suspensions of powdered CsSH3(S04)4. Hz0 in Fluorolube (4000-1345cn-‘) and Nujol (1345%18Ocm-I), between 433 K and liquid nitrogen temperature. The intensities of the Fluorolube and Nujol regions are not correlated. *Nujol band.
as a suspension in Fluorolube and Nujol were recorded on a Perkin Elmer 983 spectrophotometer. A Perkin Elmer high temperature cell and a conventional liquid nitrogen cooled cell were used to obtain spectra at different temperatures. Raman spectra of monocrystalline samples were obtained on a RTI 30 DILOR triple monochromator with the 514.5 nm Spectra Physics Ar+ laser exciting line. Conventional furnace and cryostat were used.
3. Results and discussion The temperature dependence of the infrared and Raman spectra of CsSH3(S0& - Hz0 in the ~-180~-’ region is shown in Figs. 1 and 2. The low-frequency Raman spectra (3 lo- 10 cm-‘)
are shown in Fig. 3. The waven~~rs, relative intensities and the proposed assignment of the spectra measured at 95, 300 and 433 IS are given in Table 1. The bands due to internal vibrations are assigned in terms of OH and Hz0 group frequencies and frequencies of distorted tetrahedra of SO;- ions designated by vl, v2, y and v4. There seem to be two kinds of OH - . - 0 hydrogen bonds of different lengths, henceforward designated as short and long (though the “long” one is still very short), in CS~H~(SO~)~H~O. The latter are evidenced by three very broad bands which can be distinguished between 3200 and 17OOcm-’ in the infrared spectrum of Fig. 1 at all investigated temperatures. These A, B, C bands [5], with maxima near 2860, 2440 and 1700cm-’ at room temperature, generally originate from a Fermi resonance between the v(OH) stretching and the overtones of the in-plane &OH) and out-of-plane r(OH) bending modes; they are a frequent feature of crystals with strong hydrogen bonds with R(O.. -0) lengths between 2.6 and 2.5& like in (CH3)2A~OOH 161.The band near lOOOcm_’ at room temperature can thus be assigned to the +OH) mode and the band at 139Ocm-‘, that appears at low temperatures, to the &OH) mode of the long 0-H.. ~0 bond. There may be another, still stronger hydrogen bond giving rise to the very broad background Y(OH) mode centered near 1100 cm-‘. From this wavenumber, its corresponding R(0 . - ~0) length is estimated as being between 2.5 and 2.4A. With this assumption, the corresponding y(OH) mode probably participates in the broad band near 12OOcn-‘ . However, in such strong OH- . -0 bonds, we have shown that only by inelastic neutron scattering is it possible to determine the OH bending modes unambiguously [7]. the corresponding absorptions are Indeed, observed with difficulty on infrared spectra. The other bands in the 1200-4OOcm-’ region can be assigned to the internal vibrations of the sulfate entities. The v2, y and u4 modes are split into several components due to the distortions of the SO4 tetrahedra. According to crystallographic data at 300K, there are two slightly different kinds of SO4 tetrahedra in CS~H~(SO~)~- H20 both with C3, site symmet~. For the factor group Lf,,, this brings us to expect only 2v3, 2v4
95
A.M. Fajdiga-Bulat et al./J. Mol. Struct. 326 (1994) 93-98 Table 1 Infrared and Raman wavenumbers and assignments of CssHs(SO& Raman
Infrared 9SK
300K
3520 s 3475 s
3560 m 3480 m
2810 s,b 2450 s,b 1740 s,b 1050 vb 164Om
2860 s,b 2440 s,b 1700 s,b 1lOOvb 164Om
1390 m 1320 vw 1200 s,b 117Osh 113ovw 1090 s 1080 sh 1052 m 1042 sh 1030 sh 1000 s 986 sh 880 m 870 s 854 m 840s 770 sh 636 s 615 s 590 vw 580 vs 470 m 455 m 442s 432 s
- Hz0 at 93,300 and 430 K
433 K
95K
Assignment 300 K
433 K dH2O) ~0520)
A B ~@H)I,,, C1 v(OH) short @W)
2920 m,b 2400 m,b 1700 m,b 1100 vb
WWI0Il* 1206 s,b 1120 w 1096 m
1207 s,b 116Osh 1120sh 1050 s
1240w 1193m 1091 vw 1083 w
1225 w 1193m 1118 VW 1085 VW
ly(OWsh*?l 1204w, b Y(SO4)
1169~~
1052 w 1044W
1010 s
1015 w
1025 vw
1059 vs 984 s 966 vw
1055 vs 980 s 968 vw
888 s 860 w,b
882 s 850 w,b
I
970 sh
984 sh
860 s
r@H)
long
v,
1045 vs 975 s
HSOi VIW- )
838 mb, V(SOs -OH)
835 s 780 vw 635 m 625 vw 610 m 584 vs 468 m 440m 430 sh
840 s,b 780 sh 610 sh 580 vs
455 vw 430 s
1 637 w 625 w 611 w 592 s 584 vw
638 VW 622vw 608 VW 590s
476 w 443vw 435 w 423 s
464w
s: strong; m: medium; w: weak; b: broad; v: very; sh: shoulder.
u4(SO4)
1 dSO4)
435 w 423 s
237 vw 91 w,b 55 m,b 34 w,b 20 m
610 VW 584 s
430 w 419 s v(O...O)
89 w,b 49 m,b 28 w,b 17m
Lattice modes
96
A.M. Fajdiga-Bulat et al.lJ. Mol. Struct. 326 (1994) 93-98
T/K
300
600
900
1200
l!
WavenuriberIcm-1 Fig. 2. Raman spectra of CsgH3(S0& - Hz0 between 1500 and 3OOcm-’ at T = 433,300 and 95 K.
and 2~~ infrared active vibrations (El, symmetry). However, from the chemical point of view, also two different sulfate entities are expected, namely SOiand HSOZ ions, and Cs, is only a statistically averaged site symmetry. The true local symmetry is certainly lower because of the disorder present in superionic conductor crystals; so more components can be expected for each sulfate and hydrogen sulfate modes. Finally, the bands at 3560, 3480 and 1640 cm-’ are assigned to the v,(HzO), vs(H20) and 6(H20) vibrations of crystalline water, respectively. The wavenumbers of valence modes are high and characteristic of quasi-free water molecules linked to the sulfate framework by very weak OH. . .O hydrogen bonds with R(0 ‘. .O) distances of about 2.9-3.0& as in numerous hydrates metallic salts [6]. This interpretation is consistent with the structural data [4]. The infrared spectrum of CS~H~(SO~)~- Hz0 at liquid nitrogen temperature, while not very different from the room temperature one, does show, besides the S(OH) mode at 139Ocm-‘, a splitting of the v(S-OH) mode at 87Ocm-’ into four components, an additional splitting of the v2, y modes, as well as intensity changes in some of these modes and in the va(H20) and v~(H~O) modes. On heating from liquid nitrogen temperature back to room temperature, these additional splittings gradually
diminish in the tem~rature region between 150 and 24OK, but there are no abrupt sharp changes in the spectrum. The v(S-OH) splitting starts to change already at around 150 K, and at 200 K this vibration shows only two discernible components. Between 200 and 260K, this splitting vanishes as well as the &OH) mode and the additional y and v4 vibrations (Fig. 1). The wavenumber shifts between 90 and 300 K are weak, indicating only a light reinforcement of strong and weak hydrogen bonds. On heating from room temperature up to 450 K, we observe a phase transition near 415K. The infrared spectrum at 433 K lacks the crystalline H20 modes, the splitting of the sulphate vibrations almost vanishes and the v2, v3 and u4 modes become mostly broad, single bands. This transition is reversible and, in particular, the Hz0 modes reappear below transition temperature, showing that water molecules are always present in the crystal, even in high temperature phase. The disappearance of Hz0 valence modes therefore reflects an important dynamic disordering of these molecules in the crystalline network, probably of reorientational type. The Raman spectrum of CS~H~(SO~)~- Hz0 (Fig. 2) at room temperature shows bands assignable to the internal sulfate vibrations, but as expected [8] no equivalents of the OH stretching or crystalline water modes. The strong v1 band is a triplet (1055, 980 and 882cn-t) and we also observe an additional, very weak vr band at 968cm-’ (from crystallographic data, at most two strong vr bands would be expected, originating from the two different SO4 tetrahedra). This splitting may be considered as a spectroscopic indication of asymmetric hydrogen bonds where, by analogy to the spectra of MHS04 [9] and M3H(S04), [8] compounds, SOi- [ll] and HSOT entities can be distinguished. The bands at 1055 and 882 cm-’ correspond to yl(S03) and Y(S-OH) of HSO;. The lowest wavenumber can be correlated with S-OH distances. So, in the case of KHS04 [9], the lowest skeletal stetching frequencies are observed in the 850~890cm-’ range, the S-OH distance being 1.57A [lo] and this correlates well with the observed frequency of the line at 882cm-‘.
A.M. Fajdiga-Bulat
et a1.l.I. Mol. Struct. 326 (1994) 93-98
T/K
233
__-
< ce
193
10
1
I
I
I
I
I
60
110
160
210
260
310
WavenumerIcn-1 Fig. 3. Temperature dependence of low frequency Raman spectra (310-lOcm-‘) ofCsSH3(S04)4 - Hz0 between 433 and 83 K. *Plasma lines.
The Raman spectrum at liquid nitrogen temperature is not significantly different from the room temperature one, although some additional splittings corresponding to those in the infrared spectrum do occur. Thus, it seems that the SO4 framework is practically not modified at all during the temperature variations between 90 and 450 K. The important broadening of the lines above 415 K is consistent with an increasing disorder in the high temperature phase. The same is true for the lattice bands, in the region below 300 cm-r (Fig. 3); that is the general pattern of the lattice bands, which are usually very sensitive to structural modifications, do not change in the temperature range between 400 and 77K and are relatively broad even below liquid nitrogen temperature, indicating a persisting, strong, structural disorder in the crystal.
91
There is a drastic change in the low frequency range Raman spectrum at high temperatures. The spectrum at 433 K becomes “liquid-like”, with no vibrations distinguishable from a broad background, characteristic for an orientationally and translationally disordered phase. The bandwidth of sulphate vibrations also increases more than twice on heating from 410 to 433K and their frequencies are shifted to lower values, e.g. the line of the Y(S-OH) mode moves from 875 to 838 cm-’ and its width at half height increases from roughly 24 to 60cm-‘. The changes observed in the Raman spectra of CS~H~(SO~)~- Hz0 in the lattice and SO4 vibrations region at the phase transition to the high temperature phase are very similar to changes seen in Raman spectra of CsHS04 and CsDS04 at the ferroelastic to super-ionic phase transition [12, 131. The broadening of the v(SOJ modes can be explained by an increase of the anharmonicity of the sulfate vibrations due to the fast reorientations of SO4 tetrahedra and the disordering of the protons in the O-H bonds that occurs in the superionic phase. We may conclude that, like in CsHS04 and CS~H(SO~)~, a dynamic 0-H.. .O hydrogen bonded network, formed by reorienting HS04 groups, is also responsible for the high protonic conductivity in CS~H~(SO~)~- H20. The infrared spectra of CsSH3(S04)4 - Hz0 show bands characteristic of compounds with strong hydrogen bonds. Two spectroscopically different kinds of hydrogen bonds can be distinguished: a longer bond with a R(0 . . . 0) length between 2.6 and 2.5 A, and a shorter bond with its OH stretching frequency lower than its OH bending frequency, indicating that the R(O.. .O) length lies between 2.5 and 2.4A. These observations can be compared with crystallographic data on the hexagonal phase of isomorphic CsSH3(Se04&, - Hz0 [3], where a crystallographically symmetric and an asymmetric hydrogen bond with R(0 . . . 0) lengths of 2.59 and 2.81 A are reported. However, in our case, the hydrogen bonds are shorter, even in the high temperature phase. Besides, the 2.8 1 A R(0 . . . 0) distance seems too long because no v(OH) band appears around 3300cm-r [6] in this compound family. Probably,
98
A.M. Fajdiga-Bulat et al./J. Mol. Struct. 326 (1994) 93-98
the crystallographic length corresponds to an averaged position of oxygen atoms, due to an orientational disordering between different potential wells, and the real R(0 . . . 0) distance is certainly about 2.6-2.7 A. Comparing our results with previously published dielectric relaxation measurements [l] and NMR experiments [14], we observe that our results are in agreement with the conclusions reached there. While no phase transition occurs in CS~H~(SO~)~- HZ0 on cooling down from room temperature to 77K, the observed gradual changes in the infrared and Raman spectra are compatible with a glassy freezing out of the dynamically disordered acid hydrogen bonds and rotationally disordered Hz0 molecules. Strong, static, structural disorder still persists in the crystal at the lowest investigated temperature. The phase transition that occurs on heating near 413 K leads to a super-ionic phase as shown by the “liquid-like” Raman spectrum in the lattice vibration region. This agrees with the ‘H NMR self-diffusion coefficient measurements. There are two possible explanations for the disappearance of the Hz0 vibration modes in the superionic phase: either water molecules are free to move through the crystal lattice or water molecules get out of the crystal. From spectroscopic measurements it is not possible to distinguish between these two possibilities, but the reversibility of the phenomenon is in favor of the first mechanism. We believe that a somewhat enlarged crystal lattice unit cell with groups of SO4 tetrahedra linked together by hydrogen bonds and
layers of Cs atoms still exists in this high temperature phase.
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Baranov, O.A. Kabalov, B.V. Merinov, L.A. Shuvalov and V.V. Dolbinina, Ferroelectrics, 127 (1992) 257. [2] B.V. Merinov, AI. Baranova, L.A. Shuvalov and I.M. Shchagina, Kristallografya, 36 (1991) 584. [3] B.V. Merinov, A.I. Baranov, L.A. Shuvalov, J. Schneider and H. Schulz, submitted for publication. [4] AI. Baranov, V.P. Khiznichenko, V.A. Sandler and L.A. Shuvalov, Ferroelectrics, 81 (1988) 183. [S] D. Hadzi and N. Kobilarov, J. Chem. Sot. A, (1966) 439. [6] A. Novak, Struct. Bonding (Berlin), 18 (1974) 177. [7] F. Fillaux, A. Lautit, J. Tomkinson and G.J. Kearley, Chem. Phys., 154 (1991) 135. [8] M. Damak, M. Kamoun, A. Daoud, F. Romain, A. Lautie and A. Novak, J. Mol. Struct., 130 (1985) 245. [9] A. Goypiron, J. de Villepin and A. Novak, J. Raman Spectrosc., 9 (1980) 297. [lo] F. Payan and R. Haser, Acta Crystalogr., Sect. B, 32 (1976) 1875. [l l] K. Nakamoto, Infrared and Raman Spectra of Inorganic and Coordination Compounds, 3rd edn, Wiley, New York, 1978, p. 142. [12] V.P. Dmitriev, V.V. Loshkarev, L.M. Rabkin and L.A. Shuvalov, Ferroelectr. Lett., 9 (1988) 7. [13] V.P. Dmitriev, V.V. Loshkarev, L.M. Rabkin, L.A. Shuvalov and I.M. Shchagina, Kristallografiya, 33 (1988) 151. [14] A.M. Fajdiga-Bulat, R. Blinc, B. Loiar, J. Slak, G. Lahajnar, I. ZupanEiE and L.A. Shuvalov, submitted for publication.