Journal of Non-Crystalline Soiids 205-207 (1996) 225-230
Raman spectroscopic study of alkali silicate glasses and melts Norimasa Umesaki ap* ) Masanari Takahashi b, Masahiro Tatsumisago ‘, Tsutomu Minami ’ a Osaka National Research Institute (ONRI), AIST, I-8-31, Midorigaoka, Ikeda, Osaka 563, Japan ’ Osaka Municipal Technical Research Institute, Morinomiya, Joto-ku, Osaka 563, Japan ’ Deparhnent of Applied Materials Science, Universiry of Osaka Prefecture, Mozu-Umemachi, Sakai, Osaka 591, Japan
Abstract Raman scattering measurements have been performed on M,O .4SiO, (M: K and Rb) glasses at room temperature and in the corresponding melts to study the structural relationship between the glasses and the melt. The fractions of SiOz- chain, Si,Oz- sheet and SiOi three-dimensional networkunitsin the glasses and melts were determined from the Raman results. It is found that above the glass transitiontemperatureT’ the neutral SiOi species are gradually destroyedand the highly charged
Si,O,‘- sheetscreatedas the temperatureincreased.
1. Introduction In metallurgical and welding processes, silicate mixtures applied as slags or flux are always used in the molten state. It is therefore important to understand the structures which control the various physical properties such as density, viscosity, diffusion, electrical conductance, etc., of molten silicates. Silicates made up of tetrahedral SiO, units readily form glassesupon solidification from the melt. For this reason, as a zeroth-order approximation, the melt structures have been estimated from the corresponding glass structures to avoid the experimental difficulties of high temperature measurements.Most data on the structure of silicate melts have been obtained through measurementson quenched samplesat room
* Corresponding author. Present address: Osaka National Research Institute (ONRI), AIST, l-8-31, Midoriga-oak, Ikeda, Osaka 563, Japan. Tel.: +Sl-727 9536; fax: + 81-727 519 631; e-mail: [email protected]
0022-3093/96/$1.5.00 Copyright Pii SOO22-3093(96)00439-S
0 1996 Elsevier
temperature, and only very limited direct information on the structure of melts at high temperature is available [ 11.It is still controversial whether the glass structures are in fact similar to the corresponding melt structures. Therefore, in order to relate the structural information of silicate glassesto the structural features of corresponding melts, it is necessary to accumulate direct experimental evidence of structural similarities between the two phases. It is generally accepted that the introduction of network modifiers such as alkali oxide, M,O, to SiO, glass and melt breaks the Si-0-Si bridging bonds and forms units with 0, 1, 2, 3 and 4 NBO/Si (non-bridging oxygen per Si). According to the modified random network (MRN) model , the addition of one M,O modifier unit causesone bridging oxygen (BO) ion between two connected SiO, tetrahedra to be replaced by two non-bridging oxygen (NBO) ions. The number of NBO/Si in silicate glassesand melts increaseswith increasing modifier oxide content [3,4], and also with increasing temperature [S].
All rights reserved.
et al. / Journnl
Raman spectroscopy is a powerful method for identifying distinct SiO, units in silicate crystals and glasses.The stretching vibrations of Si-0 bonds in silicate crystals and glassescan be easily observed in the frequency region from 800 to 1200 cm-’ by Raman spectroscopy.Many earlier investigations on silicate glasseswere reported [6-121. The 800-1200 cm -I frequency region of the measured Raman spectra can generally be assigned to the Si-0 stretching motions of five SiO, units with O-4 NBO/Si: SiOg three-dimensionalnetwork (NBO/Si = O), Si,Oz- sheet (NBO/Si = l), SiOt- chain (NBO/Si = 2), S&O;- dimer (NBO/Si = 3) and SiOimonomer (NBO/Si = 4). Iwamoto and Tsunawaki [9,10] determined the fractions of bridging oxygen, non-bridging oxygen, and free or fullyactive oxygen PbO-SiO,, CaO-SiO, and CaOCaF,-SiO, glassesfrom the relative intensity of the Raman stretching bands. Recently, Umesaki et al. 1131found that the relative Raman intensities of the four SiO, units with 1, 2, 3 and 4 NBO/Si in rapidly quenched Li,O-SiO, glassesare equivalent to the abundanceof the correspondingSiO, units. It is the purpose of this study to determine the structures of M,O .4SiO, (M: K and Rb) melts by high temperature Raman spectroscopy, as one of a seriesof studies of molten alkali silicates. The temperature dependenceof the structuresin theseglasses and melts is also discussed.
A furnace for high-temperature Raman spectroscopy was fabricated as follows. The heating elements consist of a Pt main heater and nichrome sub-heater in order to maintain high temperature easily. The melted sample could be held on the Pt ring wire by its surface tension. The furnace can easily generate a temperature of 15OO”C, but the Ran-ran spectroscopic measurementsin this study were limited to 1200°C due to the disturbance of thermal radiation from the furnace and samples.The sample temperature was controlled within a maximum error of &5”C! throughout the measurement. The uncertainty in the sampletemperaturecausedby local heating by the laser beam can be ignored. With the exception of the glasses,the sampleswere held at temperature for times exceeding the relaxation time so that structural equilibrium, though metastable, was attained. Each Raman spectrum obtained was deconvoluted into several Gaussianpeaks  after temperature correction .
2. Experimental procedures K,O .4SiO, and Rb,O .4SiO, binary silicate glasseswere prepared from analytical reagent grade powders of SiO,, K,CO, and Rb,CO,. About S-10 g batches were mixed and then melted at temperatures about 100°C above their melting points in a Pt crucible inside an electrical furnace for about 2 h. The melts were cooled to room temperatureto obtain the glasssamples. Unpolarized Raman spectra were measuredon a JASCO R-800 double-grating spectrometerat a scan$ng angle of 90”. The excitakion source was a 5145 A (19435.6 cm-‘) or 4880 A (19435.6 cm-‘) line NEC GLG-3300 argon laser at power levels from 300 to 800 mW.
Fig. 1. Raman spectra of K,O.4SiO, temperatures.
glass and melt at different
et al. /Journal
The bands appearing in the Raman spectra can be grouped conveniently into two frequency regions, 500-800 cm-’ and 800-1200 cm-‘. The 800-1200 cm- ’ frequency region is attributed to the Si-0 stretching modes of three SiO, units with O-2 NBO/Si, that is, SiOi three-dimensional network, Si,O:sheet, and SiOi- chain. Fig. 1 and Fig. 2 show the Raman spectra from the K,O .4SiO, Rb,O * 4Si0, glasses and melts, respectively. As the temperature increases there is a systematic intensity decrease in the peak shoulder around 1150 cm-‘. It can bee also seen in these figures that the weak band in the 940-970 cm-’ range is considerably broader above the glass transition temperature Tg (T,/T, = 2/3 ; Tg = 516°C in K,O .4SiO,). There are no literature data available for comparison for the range from room temperature through the glass transition up to the melting temperature.
The bulk NBO/Si can be easily calculated from the expression [13,14,17,18]
,, 1100 frequency
Fig. 2. Raman temperatures.
spectra of Rb,O-4Si0,
glass and melt at different
C X,ni = NBO/Si,
where Xi is the mole fraction of SiO, units with IZ~ non-bridging oxygen per silicon. The fraction Xi is related to the corresponding relative Raman intensity, Ai, by Xi = cqA;,
(2) where ai is the normalized Raman cross-section of SiO, units, and Ai in Eq. (1) corresponds to the ratio of the area of Gaussian peak i to the total area, that is, the relative intensity. The c+ factors were determined to be 1.04, 1.02, 1.15 and 0.90 for S&O:sheet, SiO:chain, S&O:dimer and SiOimonomer in the L&O-SiO, glasses . This result demonstrates that the scattering efficiencies of the four SiO, units with 1, 2, 3 and 4 NBO/Si are almost equivalent in alkali-silicate glasses. Fig. 3 and Fig. 4 show the temperature dependence of the proportions of SiO, units with O-2 NBO/Si and the fractions of bridging oxygen (0’) and non-bridging oxygen (0-j in the M,O * 4Si0, (M = K and Rb) glasses and melts, respectively. As shown in these figures, the Raman results indicate that the proportions of SiO, units in the M,O .4SiO, glasses and melts demonstrates no clear change between room temperature and Tg, and then gradually changes above Ts. It may be attributed to depolymerization of SiO, units by rupturing Si-0-Si bonds in the network with increasing temperature so that the concentration of Si,Ozsheet gradually increases while that of SiO: three-dimensional network decreases. The peak positions of the chain, sheet and three-dimensional network species were observed to be only weakly temperature dependent. This temperature effect may be as follows. Shiraishi and Granasy 1201 deduced the breakage of Si-0 bridging bonds with increasing of temperature from the viscosity behavior of silicate melts. The M,O . 4Si0, (M = K and Rb) glasses and melts consist mostly of Si,O:sheet and SiOi three-dimensional network, with small amounts of SiOz- chain units. The variation of SiO, units between the glasses and
et al. / Journal
Fig. 3. Temperature dependence of the fractions of (a) SiO, units with 0, 1 and 2 NBO/Si, and (b) 0’ and O- in K,0.4SiO, glass and melt. Included in this figure for comparison are the data for M,O,4SiO, (M = Na and K) glasses obtained by XPS .
400 600 Temperature
Fig. 4. Temperature dependence of the fractions of (a) SiO, units with 0, 1 and 2 NBO/Si, and (b) 0’ and O- in Rb,O.4SiO, glass and melt. Included in this figure for comparison are the data for M,O.4SiO, (M = Na and K) glasses obtained by XPS .
Fig. 5. Composition dependence of the coordination numbers (dotted line) of nearest-neighbor Si-Si, 0-Si, and O-O pairs in molten M,O ’ 4Si0, (M = Na and K) and Li,O .2SiO, at 1130°C IS] obkined from the Raman spectra. The solid lines represent the coordination numbers est:mated from the compositional dependence of silicate units. Included in this figure for comparison are the data for molten Na,O-SiO, obtained by X-ray diffraction  (dashed line).
et al. / Journal
melts suggests that these SiO, species should arise from the following reaction:
Raman spectra of K,O .4SiO, and Rb,O .4SiO, glasses and melts at different temperatures were measured by using high-temperature Raman spectroscopy. Raman spectra observed in the frequency region from 800 to 1200 cm-’ consist of four Raman bands near 960-980 cm-‘, 1020-1060 cm-‘, 1090-1110 cm-’ and 1140-1160 cm-‘. These bands are attributed to the Si-0 stretching vibrations for SiOz- chain, Si,Oisheet and SiOg three-dimensional network units with two, one and zero non-bridging oxygen per silicon atom, respectively, and can be approximated by Gaussian curves. As a result, above the glass transition temperature T,, SiOi species are gradually destroyed and the highly sheet created with increasing temcharged Si,O:perature. The Raman results suggests that the reaction (3) is shifted to the left with increasing temperature. It is believed that the depolymerization of SiO, units with increasing temperature corresponds to the observed viscosity behavior of silicate melts. However, there is only slight difference between the Raman spectra of M,O . 4Si0, glasses and melts, so that the glass structure is roughly similar to the corresponding melt as a zeroth-order approximation. The coordination numbers of the nearest-neighbor correlations Si-Si, 0-Si and O-O pairs in K,O . 4Si0, and Rb,O . 4Si0, glasses and melts are estimated from the fractions of the three SiO, species, and found to be in good agreement with the values in K,O .4SiO, and Rb,O .4SiO, crystals.
* SiOi + 2SiOi-
(3) By using the proportion of SiO, units present in the glasses and melts, the average coordination numbers can be calculated for the nearest-neighbor atomic pairs Si-Si, 0-Si and O-O, i.e., Nsi,si, No,,, and N oio. These coordination numbers can be estimated using the following method. The average coordination numbers of the nearestneighbor correlations Si-Si, 0-Si and O-O for the SiO, units with O-4 NBO/Si, i.e., nsi,si, no,,, and no/O, are well understood. Therefore, the relationship between coordination numbers and proportions of SiO, units can be given as follows: NSi/Si
= 5.25f,,, + 4.50s,,,,
(6) where Ai, (i = 1-2) is the proportion of SiO, units with NBO/Si = i. The results calculated from this method are indicated in Fig. 5. The estimated values Nsi/si, No/s, and No/o drop reasonably with increasing M,O content due to the depolymerization of SiO, units. As shown in Fig. 5, the trends in coordination numbers obtained from the Raman results confirm the prediction from simple considerations of alkali silicate melt structures. The values of coordination numbers for molten Na,O-SiO, obtained from X-ray diffraction measurements by Waseda  are included in Fig. 5 for comparison. As shown in Fig. 5, Waseda’s results indicate that the addition of Na,O up to 60 mol% has no effect on Nsi,si and No,, in molten Na,O-SiO, , which is not in agreement with our Raman results. When M,O (where M is an alkali metal) is added to an SiO, melt, it is widely accepted that physical properties such as viscosity drastically change , owing to the rupture of three-dimensional network structure. In the MRN model , the addition of one network modifier unit, M,O, causes one BO between two connected SiO, tetrahedra to be replaced by two NBO atoms. The negative charge on the singly charged NBOs is balanced by the positively charged Mf ions. Therefore, the present authors consider that there are grounds for the X-ray results of Waseda  to be re-measured.
References [I] H. Ohno, N. Igarashi, N. Umesaki and K. Furukawa, X-ray Diffraction Analysis of Ionic Liquids, Molten Salt Forum, Vol. 3 (TransTech, Aedermannsdorf, Switzerland, 1994).  B.E. Warren and J. Biscoe, J. Am. Ceram. 21 (1938) 259.  R. Dupree, D. Halland, P.W. McMillan and R.F. Pettifer, J. Non-Cry&. Soiids 68 (1984) 399,  CM. Schramm, B.H.W.S. de Jong and V.E. Parziale, J. Am. Chem. Sot. 106 (1984) 4396.  N. Iwamoto, N. Umesaki and K. Dohi, Trans. Jpn. Inst. Met. 47 (1983) 382 (in Japanese).  S.A. Brawer and W.B. White, J. Chem. Phys. 63 (1975) 2421.  B.O. Mysen, D. Virgo and CM. Scare, Am. Mineral. 65 (1980) 690.
[81 D. Virgo, B.O. Mysen and T. Kushiro, Science 208 (1980) 1371.  N. Iwamoto, Y. Tsunawaki and S. Miyago, Trans. Jpn. Inst. Met. 43 (1979) 1138 (in Japanese). [IO] Y. Tsunawaki, N. Iwamoto, T. Hatori and A. Mitsuishi, J. Non-Cryst. Solids 44 (1981) 369. Ill] T. Furukawa, K.E. Fox and W.B. White, J. Chem. Phys. 75 (1980) 3226. 1121 P. McMillan, Am. Mineral. 69 (1984) 622. 1131 N. Umesaki, M. Takahashi, M. Tatsumisago and T. Minami, J. Mater. Sci. 28 (1993) 3473. 1141 B.O. Mysen, L.W. Finger, D. Virgo and F.A. Seifert, Am. Mineral. 67 (1982) 686. 1151 D.A. Long, Raman Spectroscopy (McGraw-Hill, New York, 1977) p. 82.
I161 H. Scholze, Glass-Nature, Structure and Properties, translated by M.J. Lakin (Springer, New York, 1990) p. 73.  B.O. Mysen, D. Virgo and F.A. Seifert, Am. MineraI. 70 (1985) 88. [IS] E.I. Kamitos, J.A. Kapoutsis, H. Jain and C.H. Hsein, J. Non-Cryst. Solids 171 (1994) 31.  B.M. Smets and T.P.A. Lommen, Phys. Chem. Glasses 22 (1981) 158; J. Non-Cryst. Solids 46 (1981) 21.  Y. Shiraishi and L. Granasy, Bulletin on the Research Institute of Mineral Dressing and Metallurgy Tohoku University 42 (1986) 42 (in Japanese). [21j Y. Waseda, Progr. Mater. Sci. 26 (1981) 85.