Journal of Molecular Structure 596 (2001) 139±143
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Structural inhomogeneity in glasses from the system Li2O3 ±Al2O3 ±SiO2 revealed by IR spectroscopy Marek Nocun a,b,*, Mirosl¤aw Handke c b
a Surface Spectroscopy Laboratory, University of Mining and Metallurgy, KrakoÂw, al. Reymonta 23, Poland Surface Spectroscopy Laboratory, Joint Centre for Chemical Analysis and Structural Research, Jagiellonian University, KrakoÂw, Poland c Department of Materials Science and Ceramic, University of Mining and Metallurgy, KrakoÂw, al. Mickiewicza 30, Poland
Received 7 November 2000; revised 15 February 2001; accepted 15 February 2001
Abstract MIR spectra of lithium silicates and aluminosilicates were measured and compared with the simulated spectra. Simulation was carried out by mathematical summation of appropriate glass spectra taken in the right proportion. It was shown that the spectra obtained in this way are very similar to the measured spectra. It con®rms phase inhomogeneity of studied glasses with short or even medium range of order. Structures of lithium silicate and aluminosilicate glasses consist of structural elements resulting from its composition and position in the ®eld of primary crystallisation. The structure of these elements is very similar to their crystalline analogues with the same chemical composition, but they are much more disordered and do not have translation symmetry. q 2001 Elsevier Science B.V. All rights reserved. Keywords: Lithium silicates; Aluminosilicates; Infrared spectroscopy; Structure
1. Introduction Discussion on the structure of glasses started in 1921 when Lebiediev published his microcrystalline hypothesis [1]. Lebiediev's hypothesis postulates that the glasses are built of small crystals. A completely different theory, continuous random network, was proposed by Zachariasen in 1932 [2]. According to Zachariasen theory, the glass structure is continuous and completely disordered with no sign of order. Since then many hypotheses and theories were published which were based mainly on one of the * Corresponding author. Address: Surface Spectroscopy Laboratory, University of Mining and Metallurgy, al. Reymonta 23, 30-059 KrakoÂw, Poland. E-mail addresses:
[email protected] (M. NocunÂ),
[email protected] (M. Handke).
above-mentioned concepts [3±5]. Development of analytical tools and new non-oxide glasses give new evidences supporting one of these theories. At present, most of the researches are of the idea that different glasses can have completely different structures and there is no universal theory which can describe the structure of all kinds of glasses [6,7]. Even a single component glass can have different structures depending on, for example, melting conditions as in the case of phosphate glasses or method of glass preparation. Babcock was probably the ®rst to postulate that the MIR spectra of multiphase polycrystalline compounds could be obtained by mathematical summation of MIR spectra of single phases taken in appropriate proportion [8,9] in accordance with the triangle composition rule. Babcock's suggestion was fully con®rmed in the case of crystalline lithium silicates
0022-2860/01/$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S 0022-286 0(01)00701-3
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2. Experimental procedure
Fig. 1. Phase diagram of the Li2O3 ±Al2O3 ±SiO2 system with marked compositions.
and aluminosilicates [10]. The main goal of this work was to show that this procedure also works in the case of glasses spectra. Such a simulation can have important structural implications Ð can con®rm phase heterogeneity of studied glasses and give information concerning possible structure of these heterogeneous regions.
Glasses were synthesised by the conventional hightemperature melting technique. The baths were prepared with pure reagents of Li2CO3, Al2O3, and SiO2. Chemical compositions of studied glasses are shown in Fig. 1. Glasses were melted in a platinum crucible at the appropriate temperature for 1 h. The melt was then poured into the container ®lled with liquid nitrogen. Such a quenching method gives high cooling rate and prevents crystallisation as well as chemical degradation by water vapour. By using this method even lithium silicate glass was obtained with no trace of crystallisation. The degree of crystallisation was checked by X-ray analysis. IR absorption spectra were measured using the standard KBr-pellet technique with a Bio-Rad Fourier transform spectrometer model FTS-6000. Computer simulations were carried out on the IBM personal computer with the Spectra-Calc programme (Galctic Industries Corporation). All mathematical operations on the FTIR spectra were performed taking into account the remarks of Mysen et al. [11], and their validation in
Fig. 2. IR spectra for the sample with the ªA1º composition. LS 1 E represents the computer-simulated spectra; A1 denotes the measured spectra.
M. NocunÂ, M. Handke / Journal of Molecular Structure 596 (2001) 139±143
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Fig. 3. MIR spectra of the ªA2º sample. S 1 LS 1 E is the result from the summation of metasilicate, eucriptite and spodumene spectra; A2 represents the measured spectra for glass with composition ªA2º.
Fig. 4. Results obtained for glass with the ªlithium orthoclaseº composition Ð ªA3º. P is the petalite spectrum; S is the spodumene spectrum; R denotes the measured spectra for the ªA3º sample; P 1 S denotes the simulated spectra.
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the case of the studied system has been con®rmed [12]. 3. Results We based our study on the system Li2O±Al2O3 ± SiO2 mainly because it is fairly well known and glasses from this system crystallise relatively easily. More details concerning this system and crystalline compounds can be found in literature [13±17]. The ®rst samples we examined had a composition on the conode line Ð A1 composition (Fig. 1). Glass with such a composition should crystallise to lithium metasilicate (Li2SiO3) and b-eucriptite (LiAlSiO4). The result of mathematics summation of metasilicate and eucriptite spectra, taken in appropriate proportions, in comparison with measured spectra is shown in Fig. 2. Both these spectra are almost identical. Some differences are observed in the intensity of particular bands Ð 940, ,700 cm 21. These differences are due to the different natures of measured and simulated spectra. In Fig. 3, we compared the measured and simulated spectra for glass A2 with composition in the ®eld of triangle composition of lithium silicate, b-eucriptite and b-spodumene (LiAlSi2O6). The simulated spectrum was obtained by adding IR spectra of glasses with the composition of lithium silicate, b-eucriptite and b-spodumene. These spectra are also very similar. The difference is visible at about 400 cm 21 but in this range a greater error is expected because of the absorption edge. In Fig. 4, the spectra taken for glass with a ªlithium orthoclase (LiAlSi3O8)º composition-A3 is compared with the simulated spectra resulting from summation of petalite (LiAlSi4O10) and spodumene glass spectra. Some difference in intensity of 1022 and 730 cm 21 are visible. It is obvious that the IR spectra of glass samples with a multiphase composition (with chemical composition different to that of a single compound) must contain bands due to vibration of atoms at interface regions. These regions are much more disordered than in the glass with a single compound composition because of the different symmetry of the structural units. Symmetry disorder leads to broadening of absorption bands. Such bands cannot be simulated by simple summation methods
and it is for this reason that the simulated spectra have much sharper bands than measured ones. 4. Discussion IR absorption spectra of all studied glasses have their own characteristic unique shapes. It was shown in our previous work [18], that the maxima observed in IR spectra are directly connected with the bands present in the polycrystalline samples with the same chemical composition. It was proved that the structure of glass contains domains with the structure of a crystalline analogue. The lack of sharp bands in the spectra of glassy substances results from the high distribution of bond length and the angles between bonds. The simulations carried out lead us to the conclusion that the structural units with which the structure of the analysed glasses are built have a chemical composition and symmetry of appropriate crystals. The centres of these units consist of a few unit cells with higher symmetry, but there is no translation symmetry. The chemical composition of such units changes with distance from the centre and as the symmetry is reduced. Such units are connected between each other and give a continuous glass structure. ªContinuousº does not mean ªhomogeneousº in the chemical sense of high symmetry of small structural units Ð changes in chemical composition, and structural Ð but a lack of translation symmetry. The placement of connections is highly disordered with structural defects such as broken bonds or double bonds being present. Also, voids can be formed as a result of structural unit mismatch. In the case of glasses with a composition within the ®eld of the triangle composition, the structure consists of structural units that are characteristic of the crystalline phase at the top of the triangle. Further, it was shown that the relationship between these units is in agreement with the lever-arm principle. We would like to stress however that the relationships described above are only valid in case of studied glasses and not necessarily be observed for other type of glasses. References [1] A.A. Lebiediev, Turdy Gos. Opt. Inst. 2 (1921) 1.
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W.H. Zachariasen, J. Am. Chem. Soc. 54 (1932) 3841. C.H.L. Goodman, Nature 257 (1975) 370. E.A. Porai-Koshits, J. Glass Phys. Chem. 18 (1992) 408. P.N. Sen, M.F. Thorpe, Phys.Rev. 1315 (1977) 4030. L. Stoch, Polish Ceramic Bulletin 5, Ceramics 42 (1993) 21. P.H. Gaskell. Proc. Glass Congresse, National Committee Netherlands Glass Industry (NCNG) Amsterdam, 15-17 May, 2000 C.L. Babcock, J. Am. Ceram. Soc. 51 (1968) 163. C.L. Babcock, J. Am. Ceram. Soc. 52 (1969) 151. M. NocunÂ. Doctor thesis, KrakoÂw, 1994 B.O. Mysen, L.W. Finger, D. Virgo, F.A. Seifert, Am. Mineral. 67 (1982) 686.
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[12] M. Handke, W. Mozgawa, M. NocunÂ, J. Mol. Struct. 325 (1994) 129. [13] R. Roy, D. Roy, E.F. Osborn, J. Am. Ceram. Soc. 33 (1950) 152. [14] D. Tranqui, R.D. Shannon, H.Y. Chen, Acta Crystallogr. B35 (1979) 2479. [15] G. Donnay, J.D.H. Donnay, Am. Mineral. 38 (1953) 163. [16] F. Liebau, Acta Crystallogr. 14 (1961) 389. [17] B.J. Skinner, H.T. Evans, Am. J. Sci. 258A (1960) 312. [18] M. Handke, M. NocunÂ, Polish Ceramic Bulletin 5, Ceramics 43 (1993) 213.
