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FTIR studies of the cyclosilicate-like structures
Journal of Molecular Structure 596 (2001) 185±189
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FTIR studies of the cyclosilicate-like structures M. Sitarz*, M. Handke, W. Mozgawa Department of Materials Science and Ceramics, University of Mining and Metallurgy (AGH), al. Mickiewicza 30, 30-059 KrakoÂw, Poland Received 11 October 2000; revised 21 February 2001; accepted 21 February 2001
Abstract Ring silicates (cyclosilicates) form a subgroup of oligosilicates. Cyclosilicates include isolated silicooxygen rings of different numbers of members in their structure. Isolated rings occur also in other structures described as cyclosilicate-like ones. In the present paper infrared and Raman study of cyclogermanates, cyclosilicates and cycloaluminosilicates have been carried out. Studies of SrO±SiO2, SrO±GeO2, BaO±GeO2, MgO±Al2O3 ±SiO2 systems allowed to show the in¯uence of different, tetrahedral ring cations (Si 14, Al 13, Ge 14) on the `ring band' positions. Presence of aluminium cations, lighter than silicon cations, causes the shift of ring band to higher wavenumbers. In the case of cyclogermanates, as there is the big mass difference between germanium and silicon, the ring band is observed in signi®cantly lower wavenumber range. The examinations of cyclogermanates with different non-ring cations such as Sr 12 and Ba 12 can show the in¯uence of these cations on the position of the bands. q 2001 Elsevier Science B.V. All rights reserved. Keywords: Vibrational spectroscopy; Cyclosilicate; Ring silicates; Cyclogermante
1. Introduction Group factor analysis carried out for ideal 3, 4 and 6-membered silicooxygen rings (symmetry D3h ; D4h and D6h indicates that one intensive band characteristic of rings structure Ð the so-called `ring band' Ð should appear in the IR spectra of cyclosilicates A 002 in the case of 3-membered rings and A2u in the case of 4and 6-membered rings). In the Raman spectra, except other bands, a single band due to the double degenerated vibration E 00 , characteristic of 3-membered rings, together with two bands, assigned to the vibrations B2g and Eg, also E1g and E2g, connected with the presence of 4-membered and 6-membered rings, respectively, should be observed [1]. Model calculations of the * Corresponding author. Tel.: 148-12-172232; fax: 148-12331593. E-mail address: [email protected] (M. Sitarz).
vibrations (PM3 method) indicate that only 3membered rings showing D3h symmetry. The rings of higher number of members are deformed to some extent, which results in the splitting of the ring band [2,3]. The experimentally obtained spectra are in agreement with the calculations. The ring band appears in the IR spectra of cyclosilicates in the range of 720±700 cm 21 for the 3-membered rings, around 650 cm 21 for the 4-membered rings and around 620 cm 21 for the 6-membered rings [2]. The main goal of this work is to show the in¯uence of various tetrahedral cations on the ring systems vibrations. Thus, the interpretation of spectra is reduced mainly to the range of pseudolattice vibrations. In this range, the ring band characteristic of ring structures is present. Due to the difference in the mass, the position of the band should be different for different ring cations. Changes in the spectra should be much more distinct when the difference in
0022-2860/01/$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S 0022-286 0(01)00714-1
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M. Sitarz et al. / Journal of Molecular Structure 596 (2001) 185±189
Fig. 1. MIR and FIR spectra of (a) Ba3[Ge3O9], (b) Sr3[Ge3O9] and (c) Sr3[Si3O9].
cations mass is greater. The only results of this kind were published by Lazariev et al. [4,5] and they concerned the solid solution of Sr3[Si3O9]± Sr3[Ge3O9]. Based on the structural similarity of Sr3[Si3O9] and Sr3[Ge3O9], the secular equation [6] Lazariev stated that the substitution of Si 14 with Ge 14 should cause a signi®cant decrease of the frequency of the A2 00 vibrations in comparison with E 0 vibrations. 2. Experimental Synthesis of the silicates, germanates and aluminosilicates which were subjected to the studies, was
carried out using solid state reactions (in the case of Sr3[Si3O9], Sr3[Ge3O9], Ba3[Ge3O9]), and by melting of the starting compounds followed by devitri®cation from glassy form (in the case of Mg2Al3[AlSi5O18]). X-ray diffraction was applied to identify the phases present in the samples. Spectroscopic studies in the mid (MIR) and far (FIR) infrared regions were carried out using Fourier Transform Spectrometer Digilab FTS 60V (Bio-Rad). Standard KBr and polyethylene pellet techniques were used, respectively. Raman spectra of Sr3[Si3O9], Sr3[Ge3O9] have been collected using FTS 6000 Bio-Rad Spectrometer with Raman attachment (with Nd:YAG Spectra Physic T10 106 4c laser). Raman spectra of Ba3[Ge3O9] were not measured because of too little amount of this material.
M. Sitarz et al. / Journal of Molecular Structure 596 (2001) 185±189
187
Fig. 2. Raman spectra of (a) Sr3[Ge3O9] and (b) Sr3[Si3O9].
Raman spectrum of cordierite has not been shown because of its poor quality. 3. Results and discussion Fig. 1 shows MIR and FIR spectra of strontium cyclosilicate Ð Sr3[Si3O9] (c), strontium cyclogermanate Ð Sr3[Ge3O9] (b) and barium cyclogermanate Ba3[Ge3O9] (a). All these compounds are isostructural which results in a some resemblance of their spectra. The structures have alternate layers of ternary rings of SiO4 (GeO4) groups and close-packed Sr (Ba) atoms stacked along [001] [7,8,9]. The isolated silicooxygen and germniumoxygen rings, present in their structures, are ¯at and not deformed. It allows to consider their symmetry as D3h : Hence, the interpretation of spectra
of the cyclogermanate can follow the vibrational model of the silicooxygen ring considered as a unit cell built of an appropriate number of tetrahedra [1]. As can be seen in Fig.1, one intensive band (due to ring structure Ð ring band) in the spectra of cyclogermanates Ba3[Ge3O9] (Fig. 1a) and Sr3[Ge3O9] (Fig 1b) appears at 489 and 502 cm 21, respectively. According to the chosen model this band can be assigned to the stretching vibrations of type A 00 2 of Ge±O(Ge). Presence of only one ring band (not splitted) proves high symmetry D3h symmetry) of rings in the cyclogermanates structure. Signi®cant shift of the ring band towards lower wavenumbers (about 200 cm 21) is characteristic of cyclogermante. This band appears at 708 cm 21 in the case of Sr3[Si3O9] (Fig. 1c) and at 502 cm 21 in the case of Sr3[Ge3O9] (Fig. 1b). Such a big difference in positions
188
M. Sitarz et al. / Journal of Molecular Structure 596 (2001) 185±189
Fig. 3. MIR and FIR spectra of cordierite (Mg2Al3[AlSi5O18]).
of the band is due to a big difference in mass of Ge 14 and Si 14. In the Raman spectra (Fig. 2) similar shift of ring band (about 90 cm 21) towards lower wavenumbers can be observed. Only one ring band, occurring at 477 cm 21 in the case of Sr3[Ge3O9] (Fig. 2a) and at 562 cm 21 in the spectrum of Sr3[Si3O9] (Fig. 2b) can be identi®ed. These bands can be assigned to E 00 type vibrations [1]. Hence, the substitution Ge14 ! Si14 (the mass change) in¯uences the locations of the ring band in the Raman spectra to much lower degree (shift of about 90 cm 21) than in the case of IR spectra (shift of about 200 cm 21). It should be stressed, that the ring bands in the IR and Raman spectra are due to the different type vibrations. Comparison of the IR spectra of Sr3[Si3O9], Sr3[Ge3O9] and Ba3[Ge3O9] (Fig. 1) allows to determine the in¯uence of the non-ring cations on the position of the ring band. Presence of Sr 12 and Ba 12 cations connecting the isolated silicooxygen and germaniumoxygen rings does not cause the conjugation of the vibrations of the rings. This is due to the nature of the Ba±O and Sr±O bonds Ð the bonds in the rings are stronger than inter-ring bonds. Neverthe-
less, heavier Ba 12 cation shifts the ring band to lower wavenumbers Ð 489 and 502 cm 21 in the case of Ba3[Ge3O9] (Fig. 1a) and Sr3[Ge3O9] (Fig. 1b), respectively. Occurrence of very sharp bands due to terminal Ge±O 2 vibrations is characteristic of the MIR spectra of cyclogermanates (Fig. 1a,b) (bands at 757 and 743 cm 21 for Sr3[Ge3O9] and Ba3[Ge3O9], respectively). It proves that non-ring cations (Sr 12 and Ba 12) in¯uence on a small rate on the internal ring vibrations. It is con®rmed by the FIR spectra too (Fig. 1). One can observe a sharp boundary between the bands originating from lattice and internal vibrations. The boundary lies at about 400 cm 21 for cyclosilicates; in the FIR spectrum of Sr3[Si3O9] (Fig. 1c) there is a gap (no bands) in the region of 400± 320 cm 21. In the case of cyclogermanate (Sr3[Ge3O9] and Ba3[Ge3O9]) the boundary (because of big difference in masses of Si 14 and Ge 13) shifts to lower wavenumbers and is located at about 280 cm 21(gap in the region of 283±208 cm 21 for Sr3[Ge3O9] (Fig. 1b) and 275±184 cm 21 for Ba3[Ge3O9]) (Fig 1a). Because of the separation of the lattice and internal vibrations we can treat the
M. Sitarz et al. / Journal of Molecular Structure 596 (2001) 185±189 21
position of the ring band at about 502±489 cm as one characteristic of 3-membered cyclogermanates (Fig. 1a,b). Spectroscopic studies of Mg2Al3[AlSi5O18] Ð cordierite were undertaken in the next step of the research. Cordierite can be treated as a 6-membered cycloaluminosilicate or a framework aluminosilicate. This twofold attitude results from the speci®c role of the Al cations [10]. In the structure of cordierite 6memberd silicooxygen rings in which each sixth Si 14 cation is substituted by Al 13 (Coordination Number Ð CN 4) can be distinguished. Cations other than Al 13 CN 6 and magnesium merge the rings into the spatial structure. Several polymorph varieties of cordierite are known. The difference between them depends on the degree of order in the distribution of Si 14 and Al 13 cations in aluminosilicate rings [10,11]. Substitution of one Al 13 cation to the silicooxygen ring has lead to the shift of the ring band to higher wavenumbers. Fig.3 shows MIR and FIR spectra of high-cordierite. An intensive band at 763 cm 21 due to aluminosilicooxygen rings is present in the MIR spectrum. In the case of the ideal symmetry of the ring D6h this band can be assigned to the vibrations of the type A2u [1]. Appearance of this band at such high wavenumbers (about 620 cm 21 in the case of pristine silicooxygen rings [2,3]) results from substitution Al13 CN 4 ! Si14 and the presence of non-ring cations (Al 13 CN 6 and Mg 12) which form stronger bonds with O 22 than alkali and alkaline earth metals cations. The in¯uence of non-ring cation can be observed as the increase of the bandwidth connected with Si±O 2 vibrations Ð band at 950 cm 21 (in comparison with the same bands in the Na4Ca4[Si6O18] and Na6Ca3[Si6O18] spectra [1,2]). FIR spectrum of cordierite (Fig. 3) shows that we have to take into account the in¯uence of the cation sublattice on the intermolecular vibrations of complex anions. This is con®rmed by the presence of the absorption bands in the region of 400±350 cm 21 (Fig. 3). However, the shift of the ring band in this case is smaller than in the case of Ge 14 as the difference of Al 13 and Si 14 mass is smaller in comparison
14
189
14
with Ge and Si . Two small bands at 674 and 626 cm 21 (in the MIR spectrum of cordierite) can be assigned to the deformation of the 6-membered rings. It is known that deformation of the rings leads to the splitting of the ring band [2]. 4. Summary Spectroscopic studies show that in the range of the pseudolattice vibrations the vibrational model of silicooxygen rings can be applied to the interpretation of the spectra of cyclogermanate and cycloaluminosilicate. Substitution of Ge14 ! Si14 causes signi®cant shift of the ring bands towards lower wavenumbers Ð about 200 cm 21 in the case of MIR spectra and about 90 cm 21 in the case of Raman spectra. Similarly, substitution Al13 ! Si14 (in tetrahedral position) shifts the ring band to higher wavenumbers. Position of this band in the spectra depends also on the presence of the non-ring cations. Acknowledgements This work was supported by University of Mining and Metallurgy, contribution no. 10.10.160.237 and grant KBN no. 7T08D 03917. References [1] M. Handke, M. Sitarz, W. Mozgawa, J. Mol. Struct. 450 (1998) 229. [2] M. Sitarz, M. Handke, W. Mozgawa, J. Mol. Struct. 404 (1997) 193. [3] M. Sitarz, M. Handke, W. Mozgawa, Spectrochim. Acta Part A 55 (1999) 2831. [4] A.N. Lazariev, I.S. Ignatiev, Opt. Spektrosk. 5 (1970) 970. [5] I.S. Ignatiev, A.N. Lazariev, Nieorg. Mater. 8 (1972) 268. [6] A.M. Prima, Opt. Spektrosk. 9 (1960) 452. [7] F. Nishi, Acta Cryst. C53 (1997) 534. [8] F. Nishi, Acta Cryst. C53 (1997) 399. [9] O. Yamaguchi, M. Ki, T. Niimi, K. Shimizu, Polyhedron 2 (1983) 1213. [10] G.V. Gibbs, Am. Mineral. 51 (1966) 1068. [11] K. Langer, W. Schreyer, Am. Mineral. 54 (1969) 1442.
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FTIR studies of the cyclosilicate-like structures M. Sitarz*, M. Handke, W. Mozgawa Department of Materials Science and Ceramics, University of Mining and Metallurgy (AGH), al. Mickiewicza 30, 30-059 KrakoÂw, Poland Received 11 October 2000; revised 21 February 2001; accepted 21 February 2001
Abstract Ring silicates (cyclosilicates) form a subgroup of oligosilicates. Cyclosilicates include isolated silicooxygen rings of different numbers of members in their structure. Isolated rings occur also in other structures described as cyclosilicate-like ones. In the present paper infrared and Raman study of cyclogermanates, cyclosilicates and cycloaluminosilicates have been carried out. Studies of SrO±SiO2, SrO±GeO2, BaO±GeO2, MgO±Al2O3 ±SiO2 systems allowed to show the in¯uence of different, tetrahedral ring cations (Si 14, Al 13, Ge 14) on the `ring band' positions. Presence of aluminium cations, lighter than silicon cations, causes the shift of ring band to higher wavenumbers. In the case of cyclogermanates, as there is the big mass difference between germanium and silicon, the ring band is observed in signi®cantly lower wavenumber range. The examinations of cyclogermanates with different non-ring cations such as Sr 12 and Ba 12 can show the in¯uence of these cations on the position of the bands. q 2001 Elsevier Science B.V. All rights reserved. Keywords: Vibrational spectroscopy; Cyclosilicate; Ring silicates; Cyclogermante
1. Introduction Group factor analysis carried out for ideal 3, 4 and 6-membered silicooxygen rings (symmetry D3h ; D4h and D6h indicates that one intensive band characteristic of rings structure Ð the so-called `ring band' Ð should appear in the IR spectra of cyclosilicates A 002 in the case of 3-membered rings and A2u in the case of 4and 6-membered rings). In the Raman spectra, except other bands, a single band due to the double degenerated vibration E 00 , characteristic of 3-membered rings, together with two bands, assigned to the vibrations B2g and Eg, also E1g and E2g, connected with the presence of 4-membered and 6-membered rings, respectively, should be observed [1]. Model calculations of the * Corresponding author. Tel.: 148-12-172232; fax: 148-12331593. E-mail address: [email protected] (M. Sitarz).
vibrations (PM3 method) indicate that only 3membered rings showing D3h symmetry. The rings of higher number of members are deformed to some extent, which results in the splitting of the ring band [2,3]. The experimentally obtained spectra are in agreement with the calculations. The ring band appears in the IR spectra of cyclosilicates in the range of 720±700 cm 21 for the 3-membered rings, around 650 cm 21 for the 4-membered rings and around 620 cm 21 for the 6-membered rings [2]. The main goal of this work is to show the in¯uence of various tetrahedral cations on the ring systems vibrations. Thus, the interpretation of spectra is reduced mainly to the range of pseudolattice vibrations. In this range, the ring band characteristic of ring structures is present. Due to the difference in the mass, the position of the band should be different for different ring cations. Changes in the spectra should be much more distinct when the difference in
0022-2860/01/$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S 0022-286 0(01)00714-1
186
M. Sitarz et al. / Journal of Molecular Structure 596 (2001) 185±189
Fig. 1. MIR and FIR spectra of (a) Ba3[Ge3O9], (b) Sr3[Ge3O9] and (c) Sr3[Si3O9].
cations mass is greater. The only results of this kind were published by Lazariev et al. [4,5] and they concerned the solid solution of Sr3[Si3O9]± Sr3[Ge3O9]. Based on the structural similarity of Sr3[Si3O9] and Sr3[Ge3O9], the secular equation [6] Lazariev stated that the substitution of Si 14 with Ge 14 should cause a signi®cant decrease of the frequency of the A2 00 vibrations in comparison with E 0 vibrations. 2. Experimental Synthesis of the silicates, germanates and aluminosilicates which were subjected to the studies, was
carried out using solid state reactions (in the case of Sr3[Si3O9], Sr3[Ge3O9], Ba3[Ge3O9]), and by melting of the starting compounds followed by devitri®cation from glassy form (in the case of Mg2Al3[AlSi5O18]). X-ray diffraction was applied to identify the phases present in the samples. Spectroscopic studies in the mid (MIR) and far (FIR) infrared regions were carried out using Fourier Transform Spectrometer Digilab FTS 60V (Bio-Rad). Standard KBr and polyethylene pellet techniques were used, respectively. Raman spectra of Sr3[Si3O9], Sr3[Ge3O9] have been collected using FTS 6000 Bio-Rad Spectrometer with Raman attachment (with Nd:YAG Spectra Physic T10 106 4c laser). Raman spectra of Ba3[Ge3O9] were not measured because of too little amount of this material.
M. Sitarz et al. / Journal of Molecular Structure 596 (2001) 185±189
187
Fig. 2. Raman spectra of (a) Sr3[Ge3O9] and (b) Sr3[Si3O9].
Raman spectrum of cordierite has not been shown because of its poor quality. 3. Results and discussion Fig. 1 shows MIR and FIR spectra of strontium cyclosilicate Ð Sr3[Si3O9] (c), strontium cyclogermanate Ð Sr3[Ge3O9] (b) and barium cyclogermanate Ba3[Ge3O9] (a). All these compounds are isostructural which results in a some resemblance of their spectra. The structures have alternate layers of ternary rings of SiO4 (GeO4) groups and close-packed Sr (Ba) atoms stacked along [001] [7,8,9]. The isolated silicooxygen and germniumoxygen rings, present in their structures, are ¯at and not deformed. It allows to consider their symmetry as D3h : Hence, the interpretation of spectra
of the cyclogermanate can follow the vibrational model of the silicooxygen ring considered as a unit cell built of an appropriate number of tetrahedra [1]. As can be seen in Fig.1, one intensive band (due to ring structure Ð ring band) in the spectra of cyclogermanates Ba3[Ge3O9] (Fig. 1a) and Sr3[Ge3O9] (Fig 1b) appears at 489 and 502 cm 21, respectively. According to the chosen model this band can be assigned to the stretching vibrations of type A 00 2 of Ge±O(Ge). Presence of only one ring band (not splitted) proves high symmetry D3h symmetry) of rings in the cyclogermanates structure. Signi®cant shift of the ring band towards lower wavenumbers (about 200 cm 21) is characteristic of cyclogermante. This band appears at 708 cm 21 in the case of Sr3[Si3O9] (Fig. 1c) and at 502 cm 21 in the case of Sr3[Ge3O9] (Fig. 1b). Such a big difference in positions
188
M. Sitarz et al. / Journal of Molecular Structure 596 (2001) 185±189
Fig. 3. MIR and FIR spectra of cordierite (Mg2Al3[AlSi5O18]).
of the band is due to a big difference in mass of Ge 14 and Si 14. In the Raman spectra (Fig. 2) similar shift of ring band (about 90 cm 21) towards lower wavenumbers can be observed. Only one ring band, occurring at 477 cm 21 in the case of Sr3[Ge3O9] (Fig. 2a) and at 562 cm 21 in the spectrum of Sr3[Si3O9] (Fig. 2b) can be identi®ed. These bands can be assigned to E 00 type vibrations [1]. Hence, the substitution Ge14 ! Si14 (the mass change) in¯uences the locations of the ring band in the Raman spectra to much lower degree (shift of about 90 cm 21) than in the case of IR spectra (shift of about 200 cm 21). It should be stressed, that the ring bands in the IR and Raman spectra are due to the different type vibrations. Comparison of the IR spectra of Sr3[Si3O9], Sr3[Ge3O9] and Ba3[Ge3O9] (Fig. 1) allows to determine the in¯uence of the non-ring cations on the position of the ring band. Presence of Sr 12 and Ba 12 cations connecting the isolated silicooxygen and germaniumoxygen rings does not cause the conjugation of the vibrations of the rings. This is due to the nature of the Ba±O and Sr±O bonds Ð the bonds in the rings are stronger than inter-ring bonds. Neverthe-
less, heavier Ba 12 cation shifts the ring band to lower wavenumbers Ð 489 and 502 cm 21 in the case of Ba3[Ge3O9] (Fig. 1a) and Sr3[Ge3O9] (Fig. 1b), respectively. Occurrence of very sharp bands due to terminal Ge±O 2 vibrations is characteristic of the MIR spectra of cyclogermanates (Fig. 1a,b) (bands at 757 and 743 cm 21 for Sr3[Ge3O9] and Ba3[Ge3O9], respectively). It proves that non-ring cations (Sr 12 and Ba 12) in¯uence on a small rate on the internal ring vibrations. It is con®rmed by the FIR spectra too (Fig. 1). One can observe a sharp boundary between the bands originating from lattice and internal vibrations. The boundary lies at about 400 cm 21 for cyclosilicates; in the FIR spectrum of Sr3[Si3O9] (Fig. 1c) there is a gap (no bands) in the region of 400± 320 cm 21. In the case of cyclogermanate (Sr3[Ge3O9] and Ba3[Ge3O9]) the boundary (because of big difference in masses of Si 14 and Ge 13) shifts to lower wavenumbers and is located at about 280 cm 21(gap in the region of 283±208 cm 21 for Sr3[Ge3O9] (Fig. 1b) and 275±184 cm 21 for Ba3[Ge3O9]) (Fig 1a). Because of the separation of the lattice and internal vibrations we can treat the
M. Sitarz et al. / Journal of Molecular Structure 596 (2001) 185±189 21
position of the ring band at about 502±489 cm as one characteristic of 3-membered cyclogermanates (Fig. 1a,b). Spectroscopic studies of Mg2Al3[AlSi5O18] Ð cordierite were undertaken in the next step of the research. Cordierite can be treated as a 6-membered cycloaluminosilicate or a framework aluminosilicate. This twofold attitude results from the speci®c role of the Al cations [10]. In the structure of cordierite 6memberd silicooxygen rings in which each sixth Si 14 cation is substituted by Al 13 (Coordination Number Ð CN 4) can be distinguished. Cations other than Al 13 CN 6 and magnesium merge the rings into the spatial structure. Several polymorph varieties of cordierite are known. The difference between them depends on the degree of order in the distribution of Si 14 and Al 13 cations in aluminosilicate rings [10,11]. Substitution of one Al 13 cation to the silicooxygen ring has lead to the shift of the ring band to higher wavenumbers. Fig.3 shows MIR and FIR spectra of high-cordierite. An intensive band at 763 cm 21 due to aluminosilicooxygen rings is present in the MIR spectrum. In the case of the ideal symmetry of the ring D6h this band can be assigned to the vibrations of the type A2u [1]. Appearance of this band at such high wavenumbers (about 620 cm 21 in the case of pristine silicooxygen rings [2,3]) results from substitution Al13 CN 4 ! Si14 and the presence of non-ring cations (Al 13 CN 6 and Mg 12) which form stronger bonds with O 22 than alkali and alkaline earth metals cations. The in¯uence of non-ring cation can be observed as the increase of the bandwidth connected with Si±O 2 vibrations Ð band at 950 cm 21 (in comparison with the same bands in the Na4Ca4[Si6O18] and Na6Ca3[Si6O18] spectra [1,2]). FIR spectrum of cordierite (Fig. 3) shows that we have to take into account the in¯uence of the cation sublattice on the intermolecular vibrations of complex anions. This is con®rmed by the presence of the absorption bands in the region of 400±350 cm 21 (Fig. 3). However, the shift of the ring band in this case is smaller than in the case of Ge 14 as the difference of Al 13 and Si 14 mass is smaller in comparison
14
189
14
with Ge and Si . Two small bands at 674 and 626 cm 21 (in the MIR spectrum of cordierite) can be assigned to the deformation of the 6-membered rings. It is known that deformation of the rings leads to the splitting of the ring band [2]. 4. Summary Spectroscopic studies show that in the range of the pseudolattice vibrations the vibrational model of silicooxygen rings can be applied to the interpretation of the spectra of cyclogermanate and cycloaluminosilicate. Substitution of Ge14 ! Si14 causes signi®cant shift of the ring bands towards lower wavenumbers Ð about 200 cm 21 in the case of MIR spectra and about 90 cm 21 in the case of Raman spectra. Similarly, substitution Al13 ! Si14 (in tetrahedral position) shifts the ring band to higher wavenumbers. Position of this band in the spectra depends also on the presence of the non-ring cations. Acknowledgements This work was supported by University of Mining and Metallurgy, contribution no. 10.10.160.237 and grant KBN no. 7T08D 03917. References [1] M. Handke, M. Sitarz, W. Mozgawa, J. Mol. Struct. 450 (1998) 229. [2] M. Sitarz, M. Handke, W. Mozgawa, J. Mol. Struct. 404 (1997) 193. [3] M. Sitarz, M. Handke, W. Mozgawa, Spectrochim. Acta Part A 55 (1999) 2831. [4] A.N. Lazariev, I.S. Ignatiev, Opt. Spektrosk. 5 (1970) 970. [5] I.S. Ignatiev, A.N. Lazariev, Nieorg. Mater. 8 (1972) 268. [6] A.M. Prima, Opt. Spektrosk. 9 (1960) 452. [7] F. Nishi, Acta Cryst. C53 (1997) 534. [8] F. Nishi, Acta Cryst. C53 (1997) 399. [9] O. Yamaguchi, M. Ki, T. Niimi, K. Shimizu, Polyhedron 2 (1983) 1213. [10] G.V. Gibbs, Am. Mineral. 51 (1966) 1068. [11] K. Langer, W. Schreyer, Am. Mineral. 54 (1969) 1442.