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Raman and infrared studies of lithium and cesium carbonates
SpectrochimicaActa, Vol. 4SA, No. 7, pp. 99’hlOOS. 1992 Printed in Great Britain
osw-g539/92 t5.oo+o.rm @ 1992 Pergamcm Press Ltd
Raman and infrared studies of lithium and cesium carbonates MURRAY H. BROOKER*
and JIANFANG WANG
Department of Chemistry, Memorial University of Newfoundland, St. John’s Newfoundland, Canada AlB 3X7 (Received 18 May 1991; accepted 20 December 1991) Abstract-Raman spectra have been obtained for solid lithium and cesium carbonate over a wide range of temperatures. The results are consistent with the crystal structures, C2k for lithium carbonate, and P2,/c for cesium carbonate. No phase transitions were detected for the solids over the temperature range from 77 K to the melting points. The difference in the crystal structures results in significantly different correlation-fieldsplitting effects in the out-of-plane bending mode, vr of the carbonate ion. The stacking of the planar carbonate ions in lithium carbonate results in very large correlation field splitting in the vr and 2vr regions. An anomalously small shift of the peak position of the vr band of ‘%ZO$-relative to the main band of %@- for LizCOphasalsobeen explained. The ‘3CO:- ion in natural abundance can be treated as a partially decoupled guest impurity in the lattice of the strongly coupled ‘%Z@- host.
INTRODUCTION ALKALI
metal carbonates and their mixtures provide useful molten media for use as a flux in the benefication of minerals and as non-aqueous electrolytes for medium temperature batteries and fuel cells. Although the alkali metal carbonates have been studied by spectroscopic [l-11] and thermal methods [12-161 and the phase diagrams for the mixtures of some of the salts are known [17-191 there are still wide gaps in the knowledge of the physical and chemical properties of these materials. There is considerable uncertainty as to the existence of phase transitions and the thermal stability limits. These problems are exacerbated by the possibility of water, hydroxide and oxide impurities. REISMAN [13] reported a phase transition at 410°C for L&CO3 but subsequent X-ray studies by PAPIN [ 151 and Raman studies by BATES et al. [4] did not reveal a phase change. REISMAN did not detect any thermal anomalies up to the melting point of Cs3C03. Phase transitions are known and well documented for the sodium, potassium and rubidium salts [16]. As part of a program to study the phase equilibria and compound formation for binary mixtures of alkali metal carbonates the vibrational spectra of L&CO3 were re-measured. Although the previously reported IR and Raman spectra of L&CO3 were consistent with the known crystal structure, not all of the predicted bands were detected [l]. The anomalously small separation between the main IR active component of the v2 band and the 13CO]- component prompted a more detailed study of Li2C03. Similar anomalous isotope shifts have been explained on the basis of strong intermolecular coupling of the vibrational modes of the host lattice in which the uncoupled isotopomer may be treated as an impurity [20-221. The Raman spectrum of Cs3CO3 at room temperature has been reported by HASE [9] but not all of the expected bands were detected. The Raman spectra of Na2C03, K&O3 and Rb2C03 have been studied in detail [l-4]. In the present study new Raman bands were detected for Li2C03 and C&O3 that were consistent with the unit cell group analysis. The IR spectrum for C&CO3 was measured for the first time. For Li2C03 the detection of new bands in the Raman spectrum at 893, 1409, 1718 and 1785 cm-’ was in excellent agreement with predictions based on the reported crystal structure [l, 231. The crystal structure of Cs3C03 has not been reported but the vibrational spectrum indicated that the cesium salt was isomorphous with the rubidium and potassium salts [2,9]. The fact that the crystal structure of K&O3 has been determined [24] permited a partial vibrational analysis of CS~CO~.A comparison of the vibrational spectra for the Li2C03 and Cs3C03 provided an excellent example to illustrate *Author to whom correspondence should bc addressed.
loo0
MURRAYH.
BROOKER and JIANFANG WANG
the effect of different packing arrangements on the magnitude splittings for the out-of-plane bending mode of carbonate.
of correlation
field
EXPERIMENTAL A number of different commercial and synthetic samples of L&CO3 were studied in an attempt to obtain Raman spectra with superior signal to noise. Commercial L&CO3 samples tended to give high background scattering and fluorescence. Attempts were made to grow single crystals from the melts but these were unsuccessful. As a matter of routine, reagent grade L&CO3 and C&O3 were dissolved in double distilled water, treated with activated carbon, filtered through a fine sintered glass frit and recrystallized. The solids were carefully dried in nickel boats under vacuum by slowly raising the tempeature to 250°C over two days. The samples were transferred under nitrogen in sealed quartz tubes, further heated to 250°C and sealed under vacuum. One commercial sample of Li2C03 gave a weak peak at 952 cm-’ due to SO:-. A sample of Li2C03 was precipitated by stoichiometric addition of aqueous NazCOs to aqueous LiCl and the dried sample gave excellent spectra. The Li2C03 was not very sensitive to exposure to room air but the CS~CO~ is known to form hydrates and even short exposures result in partial hydration. A weak peak was detected at 1044 cm-’ for partly hydrated samples of Cs2C03. Although it has still not been possible to perform measurements with single crystals, with the use of purified samples, higher laser power, careful sample alignment and appropriate instrument settings it was possible to obtain Raman spectra with more than a factor of five improvement in signal to noise over the previous study [l]. Raman spectra were measured with a Coderg PHO spectrophotometer. Sample excitation was achieved with the 488 nm line of a Laser Ionics Argon ion laser. The usual laser power was about 0.5 W at the sample but occasionally as much as 1.2 W was used without apparent sample damage. The instrument slit settings were normally 1.0 cm-’ but were as high as 4.0 cm-’ for studies of Li&03. Attempts to obtain Raman spectra of L&CO3 with better signal to noise by the use of even larger slits were frustrated by the high background which swamped the detector. Raman spectra were measured for samples at room temperature and up to just below the melting points in a temperature controlled furnace. The system has been described in detail elsewhere [21]. Raman spectra were measured at 77 K for samples sealed under vacuum in 1 mm id thin-walled glass tubes and mounted on the cold-tip of an evaporating liquid nitrogen cryostat as described previously [ 11. The bands in the Raman spectrum of Cs2C03 were much more clearly resolved at 77 K than at room temperature. On the other hand the Raman bands for L&CO3 were not significantly altered when the sample was cooled to 77 K and the best spectra were obtained for samples at room temperature. The sample preparation for the IR spectrum of anhydrous C&CO3 was to rub a thin film of the dry powder onto an AgBr plate in a dry box under nitrogen. A second AgBr plate was sealed on as a cover and the sample transferred to the cold tip of an Air Products cryostat. The IR transmission spectrum was obtained immediately with a Perkin-Elmer 225 spectrometer with nominal slit widths of 0.5 cm-* for the sample at room temperature and approximately 10 K. The Cs2C03 sample proved too hygroscopic to permit measurement of the reflection spectrum.
RESULTS AND DISCUSSION
The observed peak frequencies and assignments for &JO3 and Li2C03 are collected in Tables 1 and 2 along with literature values to complete the assignments. In general the present results are in good agreement with previous measurements and consistent with the predictions of the unit-cell group analyses [l-4,9] based on the known crystal structures [24,23]. Raman spectra measured at selected temperatures from 77 K to just below the melting points for each compound retained the basic patterns of the room temperature phases. Although the bands broadened with increased temperature there were no abrupt frequency or halfwidth changes. These results suggest that each salt has only one stable phase over this temperature interval. The Raman and IR spectra of Cs&OJ were found to be very similar to those reported for K2C03 and Rb2COS [2,9], a fact which indicated that the three salts have isomorphic room temperature structures. Lithium and sodium carbonate have different, unique crystal structures and distinctive IR and Raman spectra [l, 111. The new bands detected in the ~~(1409 cm-‘),
Raman and IR studies of lithium and cesium carbonates
1001
Table 1. Raman and IR peak frequencies* (cm-‘) for anhydrous C&COs Raman 875 K
Raman 298 K
Raman 77K
IR 10K
Assignments
30
120
56 72 114 150
673.3
43 54 63 77 125 143 160w 170 672.3
External modes
667 673
673
675.7 679.8 680.7
1037
1370
1021 1030 1041.3 1363 1372 1380 1393
675.3 680.9 681.8 1021 1030 1041.7 1363 1375 1383 1397
V4
677 851.0 877.5 (878)t 1041 vw -1380
(1450)t 1718 1754
1756.0 1757.5
1757.2 1759.5
“co:Au + WTO) Au+ B&O)
C’“O’W c”ow-
vz VI
Au+&
4 v3
LO v, + v.@Ml + 667 cm-’ = 1718 cm-‘) 2v,(2 x 878 = 1756 cm-t) 2v,(2 x 879 = 1758 cm-‘)
* Frequencies are accurate to about 1 cm -’ but sharp peaks’ frequencies are quoted to 0.1 cm-’ for comparison within a region. t The LO mode frequency has been estimated from the band width and by comparison to results for Rb,CO, (Ref. [2]).
~~(893cm-‘) and 2~3(1718 and 1785 cm-‘) regions for L&CO3 provide further evidence to support the reported structure. The unusual modulated structure [ll, 161 of the Na3CO3 incommensurate room temperature phase appears to have no counterpart. The vibrational spectra of the alkali metal carbonate solids can be interpreted on the basis of the vibrational modes of the carbonate ion modified by crystal packing and intermolecular coupling among neighboring carbonate ions. There are six internal modes of vibration of the unperturbed ion with D3,,point group symmetry: y1 (A;) at about 1050 cm-‘, Raman active only; y2 (A;‘) at about 870 cm-‘, IR active only; and q and v,, (E’) at about 1400 and 700 cm-‘, respectively, both IR and Raman active. There are also external modes of vibration in the crystals which are primarily due to rotatory and translatory motions of the anisotropic carbonate group [l, 21. The complication of transverse optic-longitudinal optic (TO-LO) splitting is not a serious problem. Since the crystals are centrosymmetric there are no TO-LO splittings for the Raman active modes. Only the v3 and v2 IR modes have sufficiently large transition dipole values to give measurable TO-LO splitting [l-3]. CS3CO3 Assignments (Table 1) for Q&O3 are based on the unit-cell group analysis for I&CO3 (space group P2i/c, Z= 4) and comparison with the IR and Raman spectra for K&O3
MURRAYH. BROOKER and JIANFANG WANG
1002
and Rb2C03 [2,9]. Peak frequencies discussed below refer to values for the sample at 77 K (Raman) or 10 K (IR). At room temperature the’individual components could still be resolved in the v3, v4 and 215 regions but the components gradually merged into broad bands as the temperature was increased (Figs l-4, Table 1). However, there were no abrupt changes in peak maxima which might have indicated a phase change. Two Raman bands (Ag+ BE) and two infrared bands (A,+ B,) are predicted in both the vi and v2 regions. One sharp intense Raman band was observed at 1041.7 cm-’ (Fig. 1) due to the symmetric stretching mode of the carbonate ion and assigned to the A, component. The B, was not detected and may be coincident with the A, mode or too weak to be detected. The very weak IR peak (A, or B,) at 1041 cm-’ was essentially coincident with the Raman peak. Weak Raman bands at 1030 and 1021 cm-’ have been assigned to the v1 modes of C170160$- and C’80160~- based on frequency shifts and relative intensities (Fig. 1). These results indicate that the correlation field coupling between carbonate groups is very weak in the v1 region. Only one peak was observed in the v2 region of the IR spectrum at 877.5 cm-’ (Fig. 2) and it is assigned to the A,, and/or B, mode. No shoulder was detected as was the case for the potassium and rubidium salts. The TO and LO components appeared to be almost Table 2. Raman and IR peak frequencies(cm-‘) for anhydrousLirCOr Raman %SK 90
Raman 298 K
711 728 -
- (133,134) - (140,141) 156(157,160) 192(192,196) 219(220,220)
1420
-
CO]- reorientation modes (external)
- (254,254) 272(275,277) 380(380,408) 375(375,402) 410(412,442) 431(433,459) (486,518) 510(513,547) 712 747
-
893 1081
Assignments
95(97, wt 126(128,130)
-
140 171
IR’ 298 K
1091 1409
440(434,465) 500(496,523)
713 740 847 859 867 1089
-
1420
1459 -1480 1600 1718 1785 -
1805 1837
Li’-O modes (external)
-
A,or Bt A, or B, A, or B, v4 A, or Bg wo:B&W vz BdW A,6 B,) AdTO) VI A8 A, or Bb A, or B.(TO) A, or Bb v3 A, or B, A.+B. (LO)
2v2(2 x 893 = 1786 cm-‘) 2v,(2 x 859 = 1718 cm’) vi + ~~(713+ 1091= 1804 cm-‘) v, + v,(748 + 1091= 1839 cm-‘)
* Infrared peak positionsfrom Ref. [l]. t Values in parentheses are IR and Raman values for (‘LirCOs, 6Li2C03) reported by HASEand YOSHIDA (Refs [7,8]).
Raman and IR studies of lithium and cesium carbonates
1003
L-
298K
I, I
.
.
I
_77K .
1040
1050
.
.
.
I
1030
cm-‘. Fig. 1. Raman spectra of the q region of Cs#ZOSmeasured at 298 K (top) and 77 K (bottom). The instrument slit setting was 0.5 cm -‘. The insert shows the bands due to naturally abundant C17016@2-and f?*O%- and 1030 and 1021 cm-r. The instrument slit setting was l.Ocm-‘.
coincident. The value of the LO component has been estimated as 878 cm-’ by analogy to Rb.$O,. No peaks were observed in the Raman spectrum in this region. The weak sharp peak at 851 cm-’ in the IR spectrum (Fig. 2) has been assigned to the v2 mode of the uncoupled 13CO:- ion. The fact that the 27 cm -’ frequency shift for the peak due to the 13CO:- is identical to the calculated value from the Teller-Redlich product rule and normal coordinate methods suggests that the correlation field coupling in the v2 region for Cs2C03 is small. Further evidence to substantiate the weak intermolecular coupling can be obtained from the 2v2 region which is Raman active. Two closely spaced lines
%T
800
700
cm-’ Fig. 2. Infrared spectrum of C&O3 at about 10 K. l The high frequency intensity maxima are the result of anomalous transmission (Christiansen filter effect). The spike results in the region of an absorption band when the refractive index of the sample approaches the value of refractive index of the transmission plate.
MURRAY
1004
t
I
.,
H.
BROOKER and JIANFANGWANG
77K
I
1770
I
1760
I
1
1750
I
1400
cm-’
I
I I360
I
I I360
I
cm-’
Fig. 3. Raman spectra of the 2v, and V~regions of C&CO3 at 298 K (top) and 77 K (bottom).
were detected.at 1757.2 and 1759.5 cm-’ (Fig. 3) due to the overtones of the correlation field components of the fundamental. It can be deduced that the correlation field components of the v2 must be of the order of only 1 cm -l. It is probable that the Raman active A, or B, component would occur about 879 cm-‘. The small correlation field splitting for this mode is not unexpected since there are Cs+ ions between the CO:- in the stacks of CO:- ions with the result that the carbonate ions are too far apart to permit significant coupling of the out-of-plane motions. The correlation field splitting pattern (Fig. 5) for the v2 region of Cs,CO, is in marked contrast to that for Li2C03 discussed below. Infrared and Raman spectra for the v3 (Fig. 3) and v4 (Fig. 4) regions have identical splitting and intensity patterns to those observed for &CO3 and R&CO3 [2]. All four predicted Raman bands were detected in both the v3 and vq regions. Three of the predicted four IR bands were detected in the v4 region but only one broad intense IR band was detected in the v3 region. Anomalous dispersion and the presence of LO components often complicate the band shape of very intense IR bands. The estimated value of 1450cm-’ for the major LO component of the v3 (Table 1) is based on a comparison to the measured values for Rb2C03 [2]. The absence of evidence for Raman active LO components is consistent with the centric crystal structure. The Raman spectra (Figs 3 and 4) for the v3 and v4 regions were sufficiently similar at 77 K and 298 K to suggest that there was no phase transition over this temperature range. The external mode region (Fig. 6) for C&CO3 at 77 K was found to give rather weak Raman scattering with relatively sharp bands at 43, 54 and 63 cm-l and broad features centered around 125 and 16Ocm- ‘. It would appear that further cooling below 77 K would result in better resolution as the band structure is just resolvable at 77 K. Thermal motions of the carbonate ion must still be significant at 77 K. The bands are all much broader for the sample at room temperature and the two broad peaks at 114 and 150 cm-’ are the dominant features. Above room temperature these two bands merge
I..
1..
*
680
*
I. 670
cm-’ Fig. 4. Raman spectra of the V~region for C&OS at 298 K (top) and 77 K (bottom).
Raman and IR studies of lithium and cesium carbonates
1005
cs, co,
I2m*676 cm-’ lo,
”
(879)
(23* = 1756 em-@)
(618) 677.5 cm-’
(24, = 1756 cm-‘)
co,z-
051 cm-’ do
Li2CO3
893 cm4
%
@$,=I786
cm“)
1pco,2-
674 cm-l Y)u
-‘c__________6” (LO) 667 em-’ 6” (To) 659 an-’
(23,=178 cm-‘)
%o,*847alr’
too
0
047 cm-’
Fig. 5. Correlation diagrams for the v, components of Cs,CO, and LizC03. ‘Ihe static field wavenumber values UJ,,for the host ‘*CO:- ions were calculated by adding the calculated 27 cm-’ isotope shift to the static field values of the uncoupled “CO$- ions present as natural impurities.
into a single band about 110 cm-‘. These results are very similar to those for the same region for the Raman spectra of the KzC03 and Rb2C03 salts. Raman intensity in this region is primarily associated with the rotatory motion of the polarizability of the anisotropic carbonate group [l ,2]. The lack of well defined structure in the most intense bands at room temperature and above suggests considerable thermal broadening and large thermal amplitudes of vibration. In contrast to these results for C&O3 the spectral features of the external lattice modes for L&CO3 remained relatively sharp and well defined up to temperatures near the melting point.
I
150
.
.
.
.
1
KXI
.
.
.
.
I..
c
50
cm-’ Fig. 6. Raman spectra of the external mode region of Cs$03 at 298 K (top) and 77 K(bottom). #(A) 4sl7-s
1006
MURRAYH. BROOKER and JIANFANG WANG
c
I800
1750
1700
cm-’ Fig. 7. Raman spectra of selected regions of L&CO3measured at 298 K. The instrument slit setting was set at 4.0 cm-‘. (a) I+ and vd regions; (b) v3 region; and (c) 2v, region.
L&CO3 Remeasured IR and Raman spectra for L&CO3 were essentially identical to those previously reported [l, 7,8] except for the observation of new Raman peaks at 893,1409, 1718 and 1785 cm-’ (Table 2, Fig. 7). Only the new results will be discussed in detail. The spectra were not very sensitive to the sample temperature and the peak frequencies below refer to room temperature values because better spectra were obtained for room temperature samples. Peak frequencies decreased and bands broadened with increased sample temperature (Table 2) but there were no abrupt changes which might indicate a phase change. In both the vi and vz regions the unit-cell group analysis predicts one IR and one Raman active band [l]. In the v1 region the Raman active A, component and the IR active A,(TO) components have been reported at 1091 and 1089 cm-’ in agreement with predictions [l]. The A,(LO) component was not detected and should be coincident with the TO component for such a weak IR band. The correlation field effects were small as expected. The v2 region is much more complicated. The infrared active B,(TO) mode has been detected at 859 cm-’ in the transmission spectrum but the Raman active B, component at 893 cm-’ was not previously detected [l]. The B,(LO) component has been found from reflection studies to be near 867 cm-‘. The peak due to the v2 mode of the decoupled 13CO:- ion has been detected at 847 cm- ‘. Since the calculated isotope shift for 13CO:- is 27 cm-’ the measured isotope shift seemed anomalously small from either the TO or LO component. In the present study the Raman active B, mode was detected at 893 cm-’ (Fig. 7a). The presence of the B, band in this region was confirmed by measurements of the 2v2 region where a Raman peak was detected very close to 2 x 893 cm-‘. The full assignment of the v2 region is now possible and it is apparent that the correlation field splitting and TO-LO splitting are significant. The magnitude of the correlation field splitting can be inferred from the 2v2 region (Fig. 7~). Raman intensity extends from 1710 to 1790 cm-’ with maxima at 1718 and 1785 cm-’ corresponding to two times the IR fundamental at 859 cm-’ and two times the Raman fundamental at 893 cm-‘. The intensity between 1710 and 1790 cm-’ can be assigned to combination modes. It should be noted that for overtones and combinations the wavevector selection
Raman and IR studies of lithium and cesium carbonates
1007
for the wavevector k becomes (ki+ kj)+O and combinations with component non-zero wavevectors are possible [25]. The correlation field splitting pattern and assignments for the v2 region are given (Fig. 5). The apparently small frequency shift for the v2 mode of the uncoupled 13CO]- impurity ion from the main band due to the host 12CO$- can be explained by the large correlation field and TO-LO splittings [20-221. The isotope shift measurement should be referred to a theoretical static field value, o. (874 cm-‘) which represents a mean of the correlation and TO-LO components. The value for o. of 874 cm-’ can be estimated from the 847 cm-’ value for the uncoupled mode of the 13CO:- ion by adding the 27 cm-’ calculated isotope shift. In the crystal structure of Li2C03 the carbonate ions occur in stacks with no intervening cations. This structure places the carbonate planes very close and enhances the intermolecular coupling of the out-of-plane bending modes of neighboring carbonate ions. The magnitude of the correlation field splitting is similar to the aragonite form of calcium carbonate and potassium nitrate which have similar packing of the planar anions [25-281. A previously unreported Raman band has been detected in the v3 region at 1410 cm-’ (Fig. 7b). The two Raman (Ag, Bg) and two IR (A,, B,) active bands predicted for each of the v3 and V~regions by the unit-cell group analysis have now been observed. In the low frequency region it has been possible to confirm the presence of weak bands in the 300 to 500 cm-’ region reported by HASE and YOSHIDA[7,8]. These bands appear to be due to Li+ motions against the oxygens of carbonate because HASE and YOSHIDA have measured the lithium isotope dependence through the use of 6Li+. TARTE [6] has also used lithium isotopes to assign bands in the 400 to 500 cm-’ region of several lithium salts to motions of Li+ in an octahedral hole created by the oxygen atoms of polyatomic anions. The stronger, sharper bands at 95, 126, 156, 192, 219 and 272cm-’ have been assigned to rotatory and translatory motions of the carbonate ion because these bands were found to be insensitive to lithium isotope substitution [8]. The fact that the external modes of CO:- are very sharp indicates that there is little thermal disorder in Li2C03 at room temperature. When the sample temperature was increased all of the low frequency bands broadened and decreased in frequency. Most of the weaker bands merged into the Rayleigh wing but the main bands at 90, 140 and 171 cm-’ could still be identified up to temperatures just below the melting temperature. It is interesting to compare the halfwidths of the symmetric stretching vibrations of Cs2C03 and Li2C03. For Cs3CO3 at room temperature the halfwidth is only 2.4 cm-’ and this decreases to about 1.5 cm-’ at 77 K. The values for Li2C03 are considerably larger at 5.6cm-’ at room temperature and 3.2cm-’ at 77 K. Halfwidths are related to the vibrational relaxation processes which are primarily due to environmental dephasing (T2), and population relaxation through energy relaxation of the excited state (T1) mechanism [29]. Since the v1 mode is isolated from nearby energy states the contribution to the bandwidth from the energy relaxation process would be expected to be small and pure dephasing should be the predominant relaxation process. The relatively small halfwidth value for Cs3C03 is indicative of a long lifetime for the excited vibrational state and suggests that randomization of the phase of vibration by collisions with the cation sub-lattice is not as important for the C&O3 salt as for the Li2C03. The large thermal amplitude of the Li+ in the lattice gives rise to a wide range of environments for the carbonate ion. In order to reconcile these results with the information obtained from the low frequency external modes it is necessary to propose that the cation-anion (and anion-anion) interactions are greater for the lithium salt since the rotatory modes of Li2C03 are very sharp even at room temperature. It would appear that the broad diffuse scattering in the low frequency region of &CO3 can be attributed to relatively unhindered rotatory motions of the carbonate ion in a rigid cesium ion lattice. rule
CONCLUSIONS The vibrational spectra of Cs3CO3 and Li2C03 were found to be consistent with unit-cell-group analyses based on the proposed crystal structures. The different crystal
1008
MURRAYH. BROOKERand JIANFANGWANG
structures for the two salts result in different intermolecular coupling of the internal modes of the carbonate ions. A most interesting difference was observed in the y2 region of the out-of-plane bending mode. In Li2C03 the planar carbonate ions are stacked directly above one another with the result that the out-of-plane modes of the carbonate ions are strongly coupled and lead to a relatively large value for correlation field splitting (34 cm-‘). In CszC03 the cations separate the carbonate ions of a stack with the result that there is only weak intermolecular coupling of the out-of-plane modes and a small correlation field splitting (1 cm-‘). The interpretation of the correlation field splitting was aided by the detection of the v2 mode of the uncoupled ‘3CO$- present in natural abundance. Acknowledgemenr-This Council of Canada.
work was supported
in part by the Natural Sciences and Engineering
Research
REFERENCES [l] M. H. Brooker and J. B. Bates, J. Chem. Phys. 54,4788 (1971). (21 M. H. Brooker and J. B. Bates, Spectrochim. Actu 3OA,2211 (1974). [3] J. B. Bates and M. H. Brooker, Chem. Phys. Lett. 21, 149 (1973). [4] J. B. Bates, M. H. Brooker, A. S. Quist and G. E. Boyd, 1. Whys. Chem. 76, 1565 (1972). [5] K. Buijs and C. J. H. Schutte, Spectrochim. Actu 17, 927 (l%l). [6] P. Tarte, Spectrochim. Acta 21, 313 (1965). [7] Y. Hase and I. V. P. Yoshida, Spectrochim. Acta 35A, 377 (1979). [8] Y. Hase and I. V. P. Yoshida, Spectrochim. Acta 35A, 379 (1979). [9] Y. Hase, An. Acad. Brusil. Cienc. 52,521 (1980). (lo] H. Meekes, Th. Rasing, P. Wyder, A. Janner and T. Janssen, Phys. Rev. B34,4240 (1986). [ll] A. Maciel, J. F. Ryan and P. J. Walker, J. Phys. C: Solid State Phys. 14, 1611 (1981). [12] A. Reisman, Anal. Chem. 32, 1566 (1960). [13] A. Reisman, J. Am. Chem. Sot. 80,3558 (1958). [14] M. RoIin and J.-M. Recapet, Bull. Sot. Chim. Fr. 2504 (1964). [15] G. Papin, Compt. Rendu. Acad. SC. Paris, Series C 277,691 (1973). [16] P. M. de Wolff and F. Tuinstra, in Incommensurate Phases in Dielectrics 2. Materials (Edited by R. Blinc and A. P. Levanyuk), p. 253; Modern Problems in Condensed Matter Sciences Series, Vol. 14.2 (Edited by V. M. Agranovich and A. A. Maradudin). Elsevier North Holland (1986). [17] G. J. Janz and M. R. Lorenz, J. Chem. Eng. Data. 9, 1419 (1963). [18] M. Christmann and G. Papin, Rev. Chim. Mineral. 16, 485 (1979). [19] M. Christmann, N. Sadeghi and G. Papin, Rev. Chim. Mineral. 15, 312 (1978). [20] M. V. Belousov, D. E. Pogarev and A. A. Shultin, Soviet Physics-Solid State 11, 2185 (1970). [21] M. H. Brooker, J. G. Shapter and K. Drover, J. Phys.: Condens. Matter 2,2259 (1990). [22] M. H. Brooker, J. Chen, Spectrochim. Acta 47A, 315 (1991). [23] H. Effenberger and J. Zemann, Z. Krist. 150, 133 (1979). [24] B. M. Gatehouse and D. J. Lloyd, J. Chem. Sot. Dalton Truns. 70 (1973). [25] J. C. Decius and R. M. Hexter, Molecular Vibrations in Crystals. McGraw Hill, Toronto (1977). [26] J. C. Decius, J. Chem. Phys. 23, 1290 (1955). [27] W. Sterzel, Z. anorg. allg. Chem. 368, 308 (1%9). [28] M. H. Brooker, Can. /. Chem. 55, 1242 (1977). [29] L. Angeloni and R. Righini, Chem. Phys. Lett. 154, 115 (1989).
osw-g539/92 t5.oo+o.rm @ 1992 Pergamcm Press Ltd
Raman and infrared studies of lithium and cesium carbonates MURRAY H. BROOKER*
and JIANFANG WANG
Department of Chemistry, Memorial University of Newfoundland, St. John’s Newfoundland, Canada AlB 3X7 (Received 18 May 1991; accepted 20 December 1991) Abstract-Raman spectra have been obtained for solid lithium and cesium carbonate over a wide range of temperatures. The results are consistent with the crystal structures, C2k for lithium carbonate, and P2,/c for cesium carbonate. No phase transitions were detected for the solids over the temperature range from 77 K to the melting points. The difference in the crystal structures results in significantly different correlation-fieldsplitting effects in the out-of-plane bending mode, vr of the carbonate ion. The stacking of the planar carbonate ions in lithium carbonate results in very large correlation field splitting in the vr and 2vr regions. An anomalously small shift of the peak position of the vr band of ‘%ZO$-relative to the main band of %@- for LizCOphasalsobeen explained. The ‘3CO:- ion in natural abundance can be treated as a partially decoupled guest impurity in the lattice of the strongly coupled ‘%Z@- host.
INTRODUCTION ALKALI
metal carbonates and their mixtures provide useful molten media for use as a flux in the benefication of minerals and as non-aqueous electrolytes for medium temperature batteries and fuel cells. Although the alkali metal carbonates have been studied by spectroscopic [l-11] and thermal methods [12-161 and the phase diagrams for the mixtures of some of the salts are known [17-191 there are still wide gaps in the knowledge of the physical and chemical properties of these materials. There is considerable uncertainty as to the existence of phase transitions and the thermal stability limits. These problems are exacerbated by the possibility of water, hydroxide and oxide impurities. REISMAN [13] reported a phase transition at 410°C for L&CO3 but subsequent X-ray studies by PAPIN [ 151 and Raman studies by BATES et al. [4] did not reveal a phase change. REISMAN did not detect any thermal anomalies up to the melting point of Cs3C03. Phase transitions are known and well documented for the sodium, potassium and rubidium salts [16]. As part of a program to study the phase equilibria and compound formation for binary mixtures of alkali metal carbonates the vibrational spectra of L&CO3 were re-measured. Although the previously reported IR and Raman spectra of L&CO3 were consistent with the known crystal structure, not all of the predicted bands were detected [l]. The anomalously small separation between the main IR active component of the v2 band and the 13CO]- component prompted a more detailed study of Li2C03. Similar anomalous isotope shifts have been explained on the basis of strong intermolecular coupling of the vibrational modes of the host lattice in which the uncoupled isotopomer may be treated as an impurity [20-221. The Raman spectrum of Cs3CO3 at room temperature has been reported by HASE [9] but not all of the expected bands were detected. The Raman spectra of Na2C03, K&O3 and Rb2C03 have been studied in detail [l-4]. In the present study new Raman bands were detected for Li2C03 and C&O3 that were consistent with the unit cell group analysis. The IR spectrum for C&CO3 was measured for the first time. For Li2C03 the detection of new bands in the Raman spectrum at 893, 1409, 1718 and 1785 cm-’ was in excellent agreement with predictions based on the reported crystal structure [l, 231. The crystal structure of Cs3C03 has not been reported but the vibrational spectrum indicated that the cesium salt was isomorphous with the rubidium and potassium salts [2,9]. The fact that the crystal structure of K&O3 has been determined [24] permited a partial vibrational analysis of CS~CO~.A comparison of the vibrational spectra for the Li2C03 and Cs3C03 provided an excellent example to illustrate *Author to whom correspondence should bc addressed.
loo0
MURRAYH.
BROOKER and JIANFANG WANG
the effect of different packing arrangements on the magnitude splittings for the out-of-plane bending mode of carbonate.
of correlation
field
EXPERIMENTAL A number of different commercial and synthetic samples of L&CO3 were studied in an attempt to obtain Raman spectra with superior signal to noise. Commercial L&CO3 samples tended to give high background scattering and fluorescence. Attempts were made to grow single crystals from the melts but these were unsuccessful. As a matter of routine, reagent grade L&CO3 and C&O3 were dissolved in double distilled water, treated with activated carbon, filtered through a fine sintered glass frit and recrystallized. The solids were carefully dried in nickel boats under vacuum by slowly raising the tempeature to 250°C over two days. The samples were transferred under nitrogen in sealed quartz tubes, further heated to 250°C and sealed under vacuum. One commercial sample of Li2C03 gave a weak peak at 952 cm-’ due to SO:-. A sample of Li2C03 was precipitated by stoichiometric addition of aqueous NazCOs to aqueous LiCl and the dried sample gave excellent spectra. The Li2C03 was not very sensitive to exposure to room air but the CS~CO~ is known to form hydrates and even short exposures result in partial hydration. A weak peak was detected at 1044 cm-’ for partly hydrated samples of Cs2C03. Although it has still not been possible to perform measurements with single crystals, with the use of purified samples, higher laser power, careful sample alignment and appropriate instrument settings it was possible to obtain Raman spectra with more than a factor of five improvement in signal to noise over the previous study [l]. Raman spectra were measured with a Coderg PHO spectrophotometer. Sample excitation was achieved with the 488 nm line of a Laser Ionics Argon ion laser. The usual laser power was about 0.5 W at the sample but occasionally as much as 1.2 W was used without apparent sample damage. The instrument slit settings were normally 1.0 cm-’ but were as high as 4.0 cm-’ for studies of Li&03. Attempts to obtain Raman spectra of L&CO3 with better signal to noise by the use of even larger slits were frustrated by the high background which swamped the detector. Raman spectra were measured for samples at room temperature and up to just below the melting points in a temperature controlled furnace. The system has been described in detail elsewhere [21]. Raman spectra were measured at 77 K for samples sealed under vacuum in 1 mm id thin-walled glass tubes and mounted on the cold-tip of an evaporating liquid nitrogen cryostat as described previously [ 11. The bands in the Raman spectrum of Cs2C03 were much more clearly resolved at 77 K than at room temperature. On the other hand the Raman bands for L&CO3 were not significantly altered when the sample was cooled to 77 K and the best spectra were obtained for samples at room temperature. The sample preparation for the IR spectrum of anhydrous C&CO3 was to rub a thin film of the dry powder onto an AgBr plate in a dry box under nitrogen. A second AgBr plate was sealed on as a cover and the sample transferred to the cold tip of an Air Products cryostat. The IR transmission spectrum was obtained immediately with a Perkin-Elmer 225 spectrometer with nominal slit widths of 0.5 cm-* for the sample at room temperature and approximately 10 K. The Cs2C03 sample proved too hygroscopic to permit measurement of the reflection spectrum.
RESULTS AND DISCUSSION
The observed peak frequencies and assignments for &JO3 and Li2C03 are collected in Tables 1 and 2 along with literature values to complete the assignments. In general the present results are in good agreement with previous measurements and consistent with the predictions of the unit-cell group analyses [l-4,9] based on the known crystal structures [24,23]. Raman spectra measured at selected temperatures from 77 K to just below the melting points for each compound retained the basic patterns of the room temperature phases. Although the bands broadened with increased temperature there were no abrupt frequency or halfwidth changes. These results suggest that each salt has only one stable phase over this temperature interval. The Raman and IR spectra of Cs&OJ were found to be very similar to those reported for K2C03 and Rb2COS [2,9], a fact which indicated that the three salts have isomorphic room temperature structures. Lithium and sodium carbonate have different, unique crystal structures and distinctive IR and Raman spectra [l, 111. The new bands detected in the ~~(1409 cm-‘),
Raman and IR studies of lithium and cesium carbonates
1001
Table 1. Raman and IR peak frequencies* (cm-‘) for anhydrous C&COs Raman 875 K
Raman 298 K
Raman 77K
IR 10K
Assignments
30
120
56 72 114 150
673.3
43 54 63 77 125 143 160w 170 672.3
External modes
667 673
673
675.7 679.8 680.7
1037
1370
1021 1030 1041.3 1363 1372 1380 1393
675.3 680.9 681.8 1021 1030 1041.7 1363 1375 1383 1397
V4
677 851.0 877.5 (878)t 1041 vw -1380
(1450)t 1718 1754
1756.0 1757.5
1757.2 1759.5
“co:Au + WTO) Au+ B&O)
C’“O’W c”ow-
vz VI
Au+&
4 v3
LO v, + v.@Ml + 667 cm-’ = 1718 cm-‘) 2v,(2 x 878 = 1756 cm-t) 2v,(2 x 879 = 1758 cm-‘)
* Frequencies are accurate to about 1 cm -’ but sharp peaks’ frequencies are quoted to 0.1 cm-’ for comparison within a region. t The LO mode frequency has been estimated from the band width and by comparison to results for Rb,CO, (Ref. [2]).
~~(893cm-‘) and 2~3(1718 and 1785 cm-‘) regions for L&CO3 provide further evidence to support the reported structure. The unusual modulated structure [ll, 161 of the Na3CO3 incommensurate room temperature phase appears to have no counterpart. The vibrational spectra of the alkali metal carbonate solids can be interpreted on the basis of the vibrational modes of the carbonate ion modified by crystal packing and intermolecular coupling among neighboring carbonate ions. There are six internal modes of vibration of the unperturbed ion with D3,,point group symmetry: y1 (A;) at about 1050 cm-‘, Raman active only; y2 (A;‘) at about 870 cm-‘, IR active only; and q and v,, (E’) at about 1400 and 700 cm-‘, respectively, both IR and Raman active. There are also external modes of vibration in the crystals which are primarily due to rotatory and translatory motions of the anisotropic carbonate group [l, 21. The complication of transverse optic-longitudinal optic (TO-LO) splitting is not a serious problem. Since the crystals are centrosymmetric there are no TO-LO splittings for the Raman active modes. Only the v3 and v2 IR modes have sufficiently large transition dipole values to give measurable TO-LO splitting [l-3]. CS3CO3 Assignments (Table 1) for Q&O3 are based on the unit-cell group analysis for I&CO3 (space group P2i/c, Z= 4) and comparison with the IR and Raman spectra for K&O3
MURRAYH. BROOKER and JIANFANG WANG
1002
and Rb2C03 [2,9]. Peak frequencies discussed below refer to values for the sample at 77 K (Raman) or 10 K (IR). At room temperature the’individual components could still be resolved in the v3, v4 and 215 regions but the components gradually merged into broad bands as the temperature was increased (Figs l-4, Table 1). However, there were no abrupt changes in peak maxima which might have indicated a phase change. Two Raman bands (Ag+ BE) and two infrared bands (A,+ B,) are predicted in both the vi and v2 regions. One sharp intense Raman band was observed at 1041.7 cm-’ (Fig. 1) due to the symmetric stretching mode of the carbonate ion and assigned to the A, component. The B, was not detected and may be coincident with the A, mode or too weak to be detected. The very weak IR peak (A, or B,) at 1041 cm-’ was essentially coincident with the Raman peak. Weak Raman bands at 1030 and 1021 cm-’ have been assigned to the v1 modes of C170160$- and C’80160~- based on frequency shifts and relative intensities (Fig. 1). These results indicate that the correlation field coupling between carbonate groups is very weak in the v1 region. Only one peak was observed in the v2 region of the IR spectrum at 877.5 cm-’ (Fig. 2) and it is assigned to the A,, and/or B, mode. No shoulder was detected as was the case for the potassium and rubidium salts. The TO and LO components appeared to be almost Table 2. Raman and IR peak frequencies(cm-‘) for anhydrousLirCOr Raman %SK 90
Raman 298 K
711 728 -
- (133,134) - (140,141) 156(157,160) 192(192,196) 219(220,220)
1420
-
CO]- reorientation modes (external)
- (254,254) 272(275,277) 380(380,408) 375(375,402) 410(412,442) 431(433,459) (486,518) 510(513,547) 712 747
-
893 1081
Assignments
95(97, wt 126(128,130)
-
140 171
IR’ 298 K
1091 1409
440(434,465) 500(496,523)
713 740 847 859 867 1089
-
1420
1459 -1480 1600 1718 1785 -
1805 1837
Li’-O modes (external)
-
A,or Bt A, or B, A, or B, v4 A, or Bg wo:B&W vz BdW A,6 B,) AdTO) VI A8 A, or Bb A, or B.(TO) A, or Bb v3 A, or B, A.+B. (LO)
2v2(2 x 893 = 1786 cm-‘) 2v,(2 x 859 = 1718 cm’) vi + ~~(713+ 1091= 1804 cm-‘) v, + v,(748 + 1091= 1839 cm-‘)
* Infrared peak positionsfrom Ref. [l]. t Values in parentheses are IR and Raman values for (‘LirCOs, 6Li2C03) reported by HASEand YOSHIDA (Refs [7,8]).
Raman and IR studies of lithium and cesium carbonates
1003
L-
298K
I, I
.
.
I
_77K .
1040
1050
.
.
.
I
1030
cm-‘. Fig. 1. Raman spectra of the q region of Cs#ZOSmeasured at 298 K (top) and 77 K (bottom). The instrument slit setting was 0.5 cm -‘. The insert shows the bands due to naturally abundant C17016@2-and f?*O%- and 1030 and 1021 cm-r. The instrument slit setting was l.Ocm-‘.
coincident. The value of the LO component has been estimated as 878 cm-’ by analogy to Rb.$O,. No peaks were observed in the Raman spectrum in this region. The weak sharp peak at 851 cm-’ in the IR spectrum (Fig. 2) has been assigned to the v2 mode of the uncoupled 13CO:- ion. The fact that the 27 cm -’ frequency shift for the peak due to the 13CO:- is identical to the calculated value from the Teller-Redlich product rule and normal coordinate methods suggests that the correlation field coupling in the v2 region for Cs2C03 is small. Further evidence to substantiate the weak intermolecular coupling can be obtained from the 2v2 region which is Raman active. Two closely spaced lines
%T
800
700
cm-’ Fig. 2. Infrared spectrum of C&O3 at about 10 K. l The high frequency intensity maxima are the result of anomalous transmission (Christiansen filter effect). The spike results in the region of an absorption band when the refractive index of the sample approaches the value of refractive index of the transmission plate.
MURRAY
1004
t
I
.,
H.
BROOKER and JIANFANGWANG
77K
I
1770
I
1760
I
1
1750
I
1400
cm-’
I
I I360
I
I I360
I
cm-’
Fig. 3. Raman spectra of the 2v, and V~regions of C&CO3 at 298 K (top) and 77 K (bottom).
were detected.at 1757.2 and 1759.5 cm-’ (Fig. 3) due to the overtones of the correlation field components of the fundamental. It can be deduced that the correlation field components of the v2 must be of the order of only 1 cm -l. It is probable that the Raman active A, or B, component would occur about 879 cm-‘. The small correlation field splitting for this mode is not unexpected since there are Cs+ ions between the CO:- in the stacks of CO:- ions with the result that the carbonate ions are too far apart to permit significant coupling of the out-of-plane motions. The correlation field splitting pattern (Fig. 5) for the v2 region of Cs,CO, is in marked contrast to that for Li2C03 discussed below. Infrared and Raman spectra for the v3 (Fig. 3) and v4 (Fig. 4) regions have identical splitting and intensity patterns to those observed for &CO3 and R&CO3 [2]. All four predicted Raman bands were detected in both the v3 and vq regions. Three of the predicted four IR bands were detected in the v4 region but only one broad intense IR band was detected in the v3 region. Anomalous dispersion and the presence of LO components often complicate the band shape of very intense IR bands. The estimated value of 1450cm-’ for the major LO component of the v3 (Table 1) is based on a comparison to the measured values for Rb2C03 [2]. The absence of evidence for Raman active LO components is consistent with the centric crystal structure. The Raman spectra (Figs 3 and 4) for the v3 and v4 regions were sufficiently similar at 77 K and 298 K to suggest that there was no phase transition over this temperature range. The external mode region (Fig. 6) for C&CO3 at 77 K was found to give rather weak Raman scattering with relatively sharp bands at 43, 54 and 63 cm-l and broad features centered around 125 and 16Ocm- ‘. It would appear that further cooling below 77 K would result in better resolution as the band structure is just resolvable at 77 K. Thermal motions of the carbonate ion must still be significant at 77 K. The bands are all much broader for the sample at room temperature and the two broad peaks at 114 and 150 cm-’ are the dominant features. Above room temperature these two bands merge
I..
1..
*
680
*
I. 670
cm-’ Fig. 4. Raman spectra of the V~region for C&OS at 298 K (top) and 77 K (bottom).
Raman and IR studies of lithium and cesium carbonates
1005
cs, co,
I2m*676 cm-’ lo,
”
(879)
(23* = 1756 em-@)
(618) 677.5 cm-’
(24, = 1756 cm-‘)
co,z-
051 cm-’ do
Li2CO3
893 cm4
%
@$,=I786
cm“)
1pco,2-
674 cm-l Y)u
-‘c__________6” (LO) 667 em-’ 6” (To) 659 an-’
(23,=178 cm-‘)
%o,*847alr’
too
0
047 cm-’
Fig. 5. Correlation diagrams for the v, components of Cs,CO, and LizC03. ‘Ihe static field wavenumber values UJ,,for the host ‘*CO:- ions were calculated by adding the calculated 27 cm-’ isotope shift to the static field values of the uncoupled “CO$- ions present as natural impurities.
into a single band about 110 cm-‘. These results are very similar to those for the same region for the Raman spectra of the KzC03 and Rb2C03 salts. Raman intensity in this region is primarily associated with the rotatory motion of the polarizability of the anisotropic carbonate group [l ,2]. The lack of well defined structure in the most intense bands at room temperature and above suggests considerable thermal broadening and large thermal amplitudes of vibration. In contrast to these results for C&O3 the spectral features of the external lattice modes for L&CO3 remained relatively sharp and well defined up to temperatures near the melting point.
I
150
.
.
.
.
1
KXI
.
.
.
.
I..
c
50
cm-’ Fig. 6. Raman spectra of the external mode region of Cs$03 at 298 K (top) and 77 K(bottom). #(A) 4sl7-s
1006
MURRAYH. BROOKER and JIANFANG WANG
c
I800
1750
1700
cm-’ Fig. 7. Raman spectra of selected regions of L&CO3measured at 298 K. The instrument slit setting was set at 4.0 cm-‘. (a) I+ and vd regions; (b) v3 region; and (c) 2v, region.
L&CO3 Remeasured IR and Raman spectra for L&CO3 were essentially identical to those previously reported [l, 7,8] except for the observation of new Raman peaks at 893,1409, 1718 and 1785 cm-’ (Table 2, Fig. 7). Only the new results will be discussed in detail. The spectra were not very sensitive to the sample temperature and the peak frequencies below refer to room temperature values because better spectra were obtained for room temperature samples. Peak frequencies decreased and bands broadened with increased sample temperature (Table 2) but there were no abrupt changes which might indicate a phase change. In both the vi and vz regions the unit-cell group analysis predicts one IR and one Raman active band [l]. In the v1 region the Raman active A, component and the IR active A,(TO) components have been reported at 1091 and 1089 cm-’ in agreement with predictions [l]. The A,(LO) component was not detected and should be coincident with the TO component for such a weak IR band. The correlation field effects were small as expected. The v2 region is much more complicated. The infrared active B,(TO) mode has been detected at 859 cm-’ in the transmission spectrum but the Raman active B, component at 893 cm-’ was not previously detected [l]. The B,(LO) component has been found from reflection studies to be near 867 cm-‘. The peak due to the v2 mode of the decoupled 13CO:- ion has been detected at 847 cm- ‘. Since the calculated isotope shift for 13CO:- is 27 cm-’ the measured isotope shift seemed anomalously small from either the TO or LO component. In the present study the Raman active B, mode was detected at 893 cm-’ (Fig. 7a). The presence of the B, band in this region was confirmed by measurements of the 2v2 region where a Raman peak was detected very close to 2 x 893 cm-‘. The full assignment of the v2 region is now possible and it is apparent that the correlation field splitting and TO-LO splitting are significant. The magnitude of the correlation field splitting can be inferred from the 2v2 region (Fig. 7~). Raman intensity extends from 1710 to 1790 cm-’ with maxima at 1718 and 1785 cm-’ corresponding to two times the IR fundamental at 859 cm-’ and two times the Raman fundamental at 893 cm-‘. The intensity between 1710 and 1790 cm-’ can be assigned to combination modes. It should be noted that for overtones and combinations the wavevector selection
Raman and IR studies of lithium and cesium carbonates
1007
for the wavevector k becomes (ki+ kj)+O and combinations with component non-zero wavevectors are possible [25]. The correlation field splitting pattern and assignments for the v2 region are given (Fig. 5). The apparently small frequency shift for the v2 mode of the uncoupled 13CO]- impurity ion from the main band due to the host 12CO$- can be explained by the large correlation field and TO-LO splittings [20-221. The isotope shift measurement should be referred to a theoretical static field value, o. (874 cm-‘) which represents a mean of the correlation and TO-LO components. The value for o. of 874 cm-’ can be estimated from the 847 cm-’ value for the uncoupled mode of the 13CO:- ion by adding the 27 cm-’ calculated isotope shift. In the crystal structure of Li2C03 the carbonate ions occur in stacks with no intervening cations. This structure places the carbonate planes very close and enhances the intermolecular coupling of the out-of-plane bending modes of neighboring carbonate ions. The magnitude of the correlation field splitting is similar to the aragonite form of calcium carbonate and potassium nitrate which have similar packing of the planar anions [25-281. A previously unreported Raman band has been detected in the v3 region at 1410 cm-’ (Fig. 7b). The two Raman (Ag, Bg) and two IR (A,, B,) active bands predicted for each of the v3 and V~regions by the unit-cell group analysis have now been observed. In the low frequency region it has been possible to confirm the presence of weak bands in the 300 to 500 cm-’ region reported by HASE and YOSHIDA[7,8]. These bands appear to be due to Li+ motions against the oxygens of carbonate because HASE and YOSHIDA have measured the lithium isotope dependence through the use of 6Li+. TARTE [6] has also used lithium isotopes to assign bands in the 400 to 500 cm-’ region of several lithium salts to motions of Li+ in an octahedral hole created by the oxygen atoms of polyatomic anions. The stronger, sharper bands at 95, 126, 156, 192, 219 and 272cm-’ have been assigned to rotatory and translatory motions of the carbonate ion because these bands were found to be insensitive to lithium isotope substitution [8]. The fact that the external modes of CO:- are very sharp indicates that there is little thermal disorder in Li2C03 at room temperature. When the sample temperature was increased all of the low frequency bands broadened and decreased in frequency. Most of the weaker bands merged into the Rayleigh wing but the main bands at 90, 140 and 171 cm-’ could still be identified up to temperatures just below the melting temperature. It is interesting to compare the halfwidths of the symmetric stretching vibrations of Cs2C03 and Li2C03. For Cs3CO3 at room temperature the halfwidth is only 2.4 cm-’ and this decreases to about 1.5 cm-’ at 77 K. The values for Li2C03 are considerably larger at 5.6cm-’ at room temperature and 3.2cm-’ at 77 K. Halfwidths are related to the vibrational relaxation processes which are primarily due to environmental dephasing (T2), and population relaxation through energy relaxation of the excited state (T1) mechanism [29]. Since the v1 mode is isolated from nearby energy states the contribution to the bandwidth from the energy relaxation process would be expected to be small and pure dephasing should be the predominant relaxation process. The relatively small halfwidth value for Cs3C03 is indicative of a long lifetime for the excited vibrational state and suggests that randomization of the phase of vibration by collisions with the cation sub-lattice is not as important for the C&O3 salt as for the Li2C03. The large thermal amplitude of the Li+ in the lattice gives rise to a wide range of environments for the carbonate ion. In order to reconcile these results with the information obtained from the low frequency external modes it is necessary to propose that the cation-anion (and anion-anion) interactions are greater for the lithium salt since the rotatory modes of Li2C03 are very sharp even at room temperature. It would appear that the broad diffuse scattering in the low frequency region of &CO3 can be attributed to relatively unhindered rotatory motions of the carbonate ion in a rigid cesium ion lattice. rule
CONCLUSIONS The vibrational spectra of Cs3CO3 and Li2C03 were found to be consistent with unit-cell-group analyses based on the proposed crystal structures. The different crystal
1008
MURRAYH. BROOKERand JIANFANGWANG
structures for the two salts result in different intermolecular coupling of the internal modes of the carbonate ions. A most interesting difference was observed in the y2 region of the out-of-plane bending mode. In Li2C03 the planar carbonate ions are stacked directly above one another with the result that the out-of-plane modes of the carbonate ions are strongly coupled and lead to a relatively large value for correlation field splitting (34 cm-‘). In CszC03 the cations separate the carbonate ions of a stack with the result that there is only weak intermolecular coupling of the out-of-plane modes and a small correlation field splitting (1 cm-‘). The interpretation of the correlation field splitting was aided by the detection of the v2 mode of the uncoupled ‘3CO$- present in natural abundance. Acknowledgemenr-This Council of Canada.
work was supported
in part by the Natural Sciences and Engineering
Research
REFERENCES [l] M. H. Brooker and J. B. Bates, J. Chem. Phys. 54,4788 (1971). (21 M. H. Brooker and J. B. Bates, Spectrochim. Actu 3OA,2211 (1974). [3] J. B. Bates and M. H. Brooker, Chem. Phys. Lett. 21, 149 (1973). [4] J. B. Bates, M. H. Brooker, A. S. Quist and G. E. Boyd, 1. Whys. Chem. 76, 1565 (1972). [5] K. Buijs and C. J. H. Schutte, Spectrochim. Actu 17, 927 (l%l). [6] P. Tarte, Spectrochim. Acta 21, 313 (1965). [7] Y. Hase and I. V. P. Yoshida, Spectrochim. Acta 35A, 377 (1979). [8] Y. Hase and I. V. P. Yoshida, Spectrochim. Acta 35A, 379 (1979). [9] Y. Hase, An. Acad. Brusil. Cienc. 52,521 (1980). (lo] H. Meekes, Th. Rasing, P. Wyder, A. Janner and T. Janssen, Phys. Rev. B34,4240 (1986). [ll] A. Maciel, J. F. Ryan and P. J. Walker, J. Phys. C: Solid State Phys. 14, 1611 (1981). [12] A. Reisman, Anal. Chem. 32, 1566 (1960). [13] A. Reisman, J. Am. Chem. Sot. 80,3558 (1958). [14] M. RoIin and J.-M. Recapet, Bull. Sot. Chim. Fr. 2504 (1964). [15] G. Papin, Compt. Rendu. Acad. SC. Paris, Series C 277,691 (1973). [16] P. M. de Wolff and F. Tuinstra, in Incommensurate Phases in Dielectrics 2. Materials (Edited by R. Blinc and A. P. Levanyuk), p. 253; Modern Problems in Condensed Matter Sciences Series, Vol. 14.2 (Edited by V. M. Agranovich and A. A. Maradudin). Elsevier North Holland (1986). [17] G. J. Janz and M. R. Lorenz, J. Chem. Eng. Data. 9, 1419 (1963). [18] M. Christmann and G. Papin, Rev. Chim. Mineral. 16, 485 (1979). [19] M. Christmann, N. Sadeghi and G. Papin, Rev. Chim. Mineral. 15, 312 (1978). [20] M. V. Belousov, D. E. Pogarev and A. A. Shultin, Soviet Physics-Solid State 11, 2185 (1970). [21] M. H. Brooker, J. G. Shapter and K. Drover, J. Phys.: Condens. Matter 2,2259 (1990). [22] M. H. Brooker, J. Chen, Spectrochim. Acta 47A, 315 (1991). [23] H. Effenberger and J. Zemann, Z. Krist. 150, 133 (1979). [24] B. M. Gatehouse and D. J. Lloyd, J. Chem. Sot. Dalton Truns. 70 (1973). [25] J. C. Decius and R. M. Hexter, Molecular Vibrations in Crystals. McGraw Hill, Toronto (1977). [26] J. C. Decius, J. Chem. Phys. 23, 1290 (1955). [27] W. Sterzel, Z. anorg. allg. Chem. 368, 308 (1%9). [28] M. H. Brooker, Can. /. Chem. 55, 1242 (1977). [29] L. Angeloni and R. Righini, Chem. Phys. Lett. 154, 115 (1989).