Analysis of CaOSiO2 and CaOSiO2CaF2 glasses by Raman spectroscopy

Journal of Non-Crystalline Solids 44 (1981) 369-378 North-Holland Publishing Company

369

A N A L Y S I S OF C a O - S i O z A N D C a O - S i O 2 - C a F 2 G L A S S E S BY R A M A N S P E C T R O S C O P Y Yoshiaki T S U N A W A K I * Department of Chemisto', Faculty of General Education, Osaka Industrial University, Naka-Gaito, Daito, Osaka 574, Japan Nobuya IWAMOTO Welding Research Institute, Osaka University, Yamada-Kami, Suita, Osaka 565, Japan Takeshi H A T T O R I a n d Akiyoshi M I T S U I S H I Department of Applied Physics, FaculO' of Engineering, Osaka Universi(v, Yamada-Kami, Suita, Osaka 56.5, Japan Received 10 May 1980 Revised manuscript received 21 January 1981

Raman spectra of CaO-SiO2 and CaO-SiO2-CaF2 glasses were measured to analyze three states of oxygen, i.e. bridging, non-bridging and free oxygen, in a similar way to the previous work on PbO-SiO2 glasses. The stretching vibrational modes of the Si-O bond in CaO-SiO2 and CaO-SiO2-CaF2 glasses corresponded to the bands at 880, 920, 975 and 1050 cm ~. It is suggested from the comparison of the Raman spectra of the glasses with the Raman and infrared absorption spectra of crystalline silicates that these bands would arise from the SiO4 tetrahedron with four, three, two and one non-bridging oxygens, respectively. The fractions of bridging, non-bridging and free oxygens were calculated from the intensities of the four Raman bands and the composition of the glasses. They were in agreement with those obtained from the thermodynamical model for the CaO-SiO2 glasses. When the content of CaF2 was smaller than 15-20 mol.% and the CaO/SiO2 ratio was smaller than unity, CaF2 contributed to the breakage of some Si- O bonds.

1. Introduction The stretching v i b r a t i o n a l m o d e of the S i - O b o n d in silicate glass can be observed i n the frequency range from 800 to 1200 cm-~ i n infrared a b s o r p t i o n a n d R a m a n spectra [1-3]. It is k n o w n that the p o s i t i o n of the b a n d near 1100 c m - 1 due to the v i b r a t i o n of the S i - O b o n d b e t w e e n silicon a n d b r i d g i n g oxygen shifts to the lower frequency or new b a n d s appear in the lower frequency region due to the b r e a k of S i - O b o n d [4,5]. However, the q u a n t i t a tive analysis of the spectrum has so far been incomplete. Previous work o n glasses of the P b O - S i O 2 system [3] indicated that the b r o a d b a n d due to the stretching v i b r a t i o n of the S i - O b o n d in the R a m a n 0022-3093/81/0000-0000/$02.50 © North-Holland

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spectrum was composed of five bands at 890, 920, 960, 1030 and 1150 c m - ~ by a comparison with the infrared dielectric constant obtained from the KramersKroning analysis of the reflectance spectra. From a comparison of these five bands with the Raman and infrared absorption spectra of crystalline silicate, it was suggested that the bands would arise from the SiO4 tetrahedron with four, three, two and one non-bridging oxygens and with four bridging oxygens, respectively. The fractions of bridging oxygen (O°), non-bridging oxygen ( O ) and free oxygen (O 2 ) were calculated from the intensities of these five bands and the composition of the glass. The obtained fractions were in agreement with those calculated from the thermodynamical model [6,7]. In this study, the Raman spectra of C a O - S i O 2 glasses were measured, and the fractions of bridging, non-bridging and free oxygen were obtained by a similar analytical method to that employed in previous work on P b O - S i O 2 glasses [3]. Raman spectra of C a O - S i O 2 glasses containing various amounts of CaF 2 were also measured in order to elucidate the behaviour of CaF 2 in a silicate glass.

2. Experimental C a O - S i O 2 glasses were produced from a mixture consisting of analytical grade reagents of CaCO 3 and SiO 2. Each batch was melted in a platinum Table 1 Compositions of glasses CaO 40 38 36 34 32 3O 28

SiO2 60 57 54 51 48 45 42

42.3 45 45.9 50 40 35 30 52.4

57.7 55 54.1 50 50 50 50 47.6

55.6 52.8 50 48.6

44.4 42.2 40 38.9

56. I

43.9

CaF2 5 10 15 20 25 30

CaO/SiO2 2/3 2/3 2/3 2/3 2/3 2/3 2/3 11/ 15 9/11 17/20 1/I

I0 15 20

5 10 12.5

4/5 7/10 3/5 11/10 5/4 5/4 5/4 5,/4 60/47

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crucible inside an electrical furnace at 1700°C and cooled in air. After being ground to powder with a mortar and pestle, some of the glass was remelted in the same way and cooled in air.The rest was mixed with CaF 2 powder and melted for about 30 min at 1700°C. Most of the melts were allowed to cool in air, while the melts containing a large amount of CaF 2 were rapidly cooled by dipping the platinum crucible into ice water. Compositions of the glasses are shown in table 1 with the mixing ratio of the original powders. Raman spectra were measured using the experimental apparatus previously described in detail [1-3]. Each spectrum was separated into four bands based on a X 2 test by the iterative least-squares procedure with a similar computing program to that employed in the previous work for MOssbauer analysis [8], in order to estimate the fractions of bridging, non-bridging and free oxygen in a glass.

3. Results and discussion The Raman spectra of C a O - S i O 2 glasses are shown in fig. 1. The band near 350 cm-J due to the vibration of the C a - O bond [9] shifts slightly to higher frequencies with the CaO content. The band near 500 c m - ~ due to the bending vibration of the S i - O bond shifts markedly to higher frequencies with an increase of the CaO content. Such shifts have also been observed in alkalisilicate glass [1,4]. Several bands due to the stretching vibration of the S i - O

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,

,

;1500

Y. Tsunawaki et al. / Analysis of glasses

372

bond were observed in the frequency range from 800 to 1200 c m - t. The bands were observed near 880, 975 and 1050 cm-1 for the glass containing less than 50 tool.% CaO. Among these three bands, the lower frequency band increases in relative intensity with CaO content. A new band appears near 920 cm 1 for glass containing more than 55 mol.% CaO. The Raman spectra of C a O - SiO 2 glasses containing various amounts of CaF2 are shown in figs. 2 to 4. The bands corresponding to the stretching vibrational mode of the S i - O bond were also observed near 880, 920, 975 and 1050 c m - L For the C a O / S i O 2 ratio kept at ~ (fig. 2), the relative intensity of the lower frequency band increases with increasing CaF 2 content up to 15-20 mol.% CaF 2. Such a change in relative intensity was not observed for glasses having a C a O / S i O 2 ratio of -~ as shown in fig. 3. The higher frequency band increases in intensity with CaF 2 content for (50-x)CaO50SiO 2 + xmol.%CaF2 glasses as shown in fig. 4. When the Raman bands were analyzed in the frequency region from 800 to 1200 cm -J in the same way as in previous work [3], the bands at 880, 920, 975 and 1050 cm -1 of CaO-SiO 2 and C a O - S i O z - C a F 2 glasses are attributed to the vibration of the SiO4 tetrahedron with four, three, two and one non-bridging oxygens, respectively. The reason for this conclusion will be mentioned again in this paper because of its importance. Raman spectra of crystalline 2PbOSiO 2 obtained by heating the glass are shown in fig. 5. The x-ray diffraction patterns o f these crystals were in good

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Y. Tsunawaki et aL / Analysis o/glasses

373

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agreement with those of GOtz et al. [10]. They suggest that the structure of crystalline 2PbOSiO 2 treated at 425, 500, 620 and 725°C corresponds to T-Pb2SiO4 consisting mainly of Si2 O6- complex anions, M1-Pb2SiO4 consisting of SiO408~- rings, M2-Pb2SiO 4 consisting of Si40~82 rings and (SiO 2- ), chains and H-Pb2SiO4 consisting of (SiO 2- ), chains, respectively. The band at 920 cm-1 is the most intense for the crystal treated at 425°C and reduces in intensity rapidly for the crystals treated at higher temperatures, while the band at 960 cm-~ shows the opposite behavior as shown in fig. 5. If it is assumed

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374

Y. Tsunawaki et al. /Analvsis of glasses

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that the intense band reflects the crystalline state, the band at 920 c m - l is attributed to the vibration of Si2076- . Konijnendijk et al. suggested on the basis of their Raman spectra of alkaline silicate glasses and crystals [11,12] and alkali-alumino-silicate glasses [13] that the bands at 960 cm-~ and 1080 cmcould be attributed to the SiO4 tetrahedra with two and one non-bridging oxygens, respectively. It is, therefore, considered that the band at 960 cm-1 in fig. 5 is due to the vibration of Si4082 - or (SiO 2- ),. On the other hand, Yanagase et al. [5] have indicated the correspondence of the various complex anions to the infrared absorption of silicate glasses with those of minerals. They have suggested that the absorption bands which appeared at 887, 930, 970, 1020, 1068 and 1108 cm -~ correspond to the stretching vibration of the Si-O bond in complex anions of SiO6- , Si2 O 6 - , (SiO 2- ),, (Si2 O 2 - )n, (Si4092- ), and SiO °, respectively. Both bands at 930 and 970 cm -J are consistent with those at 920 and 960 cm-1 for crystalline 2PbOSiO 2. Considering the spectra of the crystalline 2PbOSiO 2 and the suggestions of Konijnendijk et al. [11-13] and Yanagase et al. [5], it is deduced that the Raman bands at 880, 920, 975 and 1050 cm-i of CaO-SiO 2 and CaO-SiO 2 - C a F 2 glasses are due to the vibration of the Si-O bond for the SiO4 tetrahedron with four, three

Y. Tsunawaki et al. / Analysis of glasses

375

two and one non-bridging oxygens, respectively. The fractions of bridging, non-bridging and free oxygens in a glass can be calculated from the composition of the glass and the intensity ratio of each Raman band with the following assumptions: the separated bands caused by the change of symmetry of the SiO 4 tetrahedron are not considered and each Raman spectrum from 800 to 1200 cm t consists of four Gaussian bands at 880, 920, 975 and 1050 cm-~. Fig. 6(a) shows the fractions of SiO 4 tetrahedron with one, two, three and four non-bridging oxygens (denoted by the numbers 1, 2, 3 and 4, respectively) in the C a O - S i O 2 glasses. The fraction of SiO 4 tetrahedra indicated by number 4 increases gradually with CaO content and that indicated by number 3 increases in the glasses having more than 50 mol.% CaO. On the other hand, the fraction of SiO 4 tetrahedra indicated by number 1 shows an abrupt decrease. The fraction indicated by number 2 shows a maximum in the glass containing 53 mol.% CaO. Fig. 6(b) shows the fractions of bridging, nonbridging and free oxygens in the CaO-SiO 2 glasses. It seems that the points obtained in this study lie on the lines extrapolated from the lines obtained on the basis of the thermodynamical calculation by Kapoor et al. [6]. The Raman spectra of CaO-SiO 2 glasses containing various amounts of CaF 2 were analyzed by the same method as mentioned above for the C a O - S i O 2 glasses. The results are shown in figs. 7 - 9 for 2CaO3SiO 2 +xmol.%CaF 2, 5CaO4SiO 2 + xmol.%CaF 2 and (50-x)CaO50SiO 2 + xmol.%CaF 2 glasses, respectively. For all of the glasses, the fraction of non-bridging oxygens is not presented in the figures because it was zero. If fluorine ions of CaF 2 replace oxygens coordinating silicon, breaking S i - O bonds in the way suggested by

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Y. Tsunawaki et al. / Analysis of glasses

376

Kozakevitch [14] and Kumar et al. [15], the S i - F bond can be constructed. It is therefore, necessary to take into account the amount of the S i - F bond in the calculation of ionic fractions of bridging, non-bridging and free oxygen. Such a correction is not considered in this work. When the C a O / S i O 2 ratio is constant at 2, the fractions of SiO 4 tetrahedra indicated by numbers 1 to 3 change and that indicated by number 4 does not show any variation with the CaF 2 content as shown in fig. 7(a). The ionic fractions of bridging and non-bridging oxygens decrease and increase gradually with CaF2 content, respectively, for the glasses containing less than about 20 mol.% CaF 2 as shown in fig. 7(b). When the C a O / S i O 2 ratio is constant at I, all fractions of SiO 4 tetrahedra indicated by numbers 1 and 4 and both bridging and non-bridging oxygens remained unchanged with CaF 2 content as shown in fig. 8. For (50-x)CaO50SiO 2 + xmol.%CaF 2 glasses in which CaO of xmol.% is replaced by CaF 2 of the same amount, the fractions of SiO 4 tetrahedra indicated by numbers 1 to 4 vary with the CaF 2 content in a different manner from other glasses having a constant

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Y. Tsunawaki et al. / Analysis of glasses

377

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C a O / S i O 2 ratio [fig. 9(a)]. The fractions of bridging and non-bridging oxygens increase and decrease with the CaF 2 content, respectively, as shown in fig. 9(b). However, they seem to be constant for a CaF 2 content of more than 15-20 mol.%. If all of the CaF 2 content behave as a network modifier like CaO, the fractions of bridging and non-bridging oxygens will not change with CaF 2 content. Dashed lines show the values obtained from fig. 6(b) for the glasses without CaF 2, i.e. (50-x)CaO50SiO2 glasses. The difference between the solid and dashed lines refers to the amount of bridging and non-bridging diminished and created by the contribution of CaF 2 to the S i - O bond, respectively. The above analysis of Raman spectra of C a O - S i O 2 - C a F 2 glasses indicates that CaF 2 does not contribute to the breakage of the S i - O bond of the SiO 2 network for glasses with a C a O / S i O 2 ratio larger than unity. When the C a O / S i O 2 ratio is smaller than unity and the content of CaF 2 is smaller than 15-20 mol.%, CaF 2 contributes to the breakage of some S i - O bonds. These results probably reflect the explanation of Kumar et al. [15] for the molten C a O - S i O 2 - C a F 2 mixture that fluorine is mostly present as F - ions in the basic mixture, on the other hand fluorine is present both as F - ions and F coordinating silicon in the acidic mixture. The results obtained in this work

378

Y. Tsunawaki et al. / Analysis of glasses

reflect the e x p l a n a t i o n of I w a m o t o et al.[16] for the ( 5 0 - x ) C a O 5 0 S i O 2 x C a F 2 glasses that the C a - F intrerionic distance o b t a i n e d f r o m the m o l a r refractivity of the glasses does not change in the glasses c o n t a i n i n g m o r e than 20 mol.% C a F 2. T h e m e t h o d of o b t a i n i n g the fractions of bridging, n o n - b r i d g i n g a n d free oxygens in a glass could be successfully used in C a O - S i O 2 a n d C a O - S i O 2 C a F 2 glasses. This m e t h o d will be e x a m i n e d in other silicate glasses in further studies. W e wish to t h a n k Prof. S. M i n a m i of O s a k a University for his advice c o n c e r n i n g the analysis of R a m a n spectra a n d Mr. M. M i y a g o of A l p s Electric C o m p a n y for helpful discussions a n d the p r e p a r a t i o n of the glass samples. W e are grateful to Prof. S, S a k k a of M i e U n i v e r s i t y for a review of the manuscript.

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