J . Phys. Chem. 1989, 93, 2147-2151 near the sun so as to limit vaporization of the more volatile ices in the core (primarily H 2 0 ) . One possible scenario is that formaldehyde is condensed as a molecular solid (along with various other materials) while in the interstellar medium. Cosmic rays initiate short-chain polymerization sequences that eventually produce poly(oxymethy1ene) upon warming to 155 K or so (in the early stage of a solar encounter). During the solar encounter, the lower molecular weight poly(oxymethy1ene) chains undergo fragmentation and depolymerization. These fragments therefore
2147
exist in the coma and may be detected via mass s p e ~ t r o m e t r y . ~ Acknowledgment. We are grateful to Professor Cheves Walling for helpful discussions. This research is supported by the Air Force Astronautics Laboratory under Contract No. F04611-87-0023. Partial support was obtained through the Utah Laser Institute and Office of Naval Research under Contract No. N0014-86-K0710. Registry No. CH20,50-00-0.
Raman Spectra of Rapidly Quenched Glasses in the Systems Li,BO,-Li,SiO,-Li,P04 Li,B20,-LisSi207-Li,P207
and
Yoshiyuki Kowada,*it Masahiro Tatsumisago, and Tsutomu Minami Department of Applied Chemistry, University of Osaka Prefecture, Sakai, Osaka 591, Japan (Received: August 5, 1988)
Glasses with large amounts of lithium ions in the systems Li3B03-Li4Si04-Li3P04and Li4B2O5-Li6Si2O7-Li4P2O7were prepared by a rapid-quenching technique. Raman spectra of these glasses deconvoluted into Gaussian peaks assigned to several structural groups. The fractions of a given structural group were determined from the deconvoluted Gaussian peak area. The result suggested that nonbridging oxygens (NBO) were most preferentially formed in the phosphate groups, followed by the borate and the silicate groups. This preference order for NBO formation was consistent with the order of acidity, P205>> B2O3 > Si02, of glass-forming oxides in melts.
Introduction In recent years, rapid-quenching techniques have enabled us to prepare new glasses that could not be formed by usual meltcooling techniques.' For example, in the system Li20-B203 the glass-forming region was extended up to the composition with 70 mol % L i 2 0 by the rapid-quenching technique, although glasses had been prepared in the composition with 0-40 mol % L i 2 0 by the usual cooling. The structure of such glasses has been attractive because they contained extremely large amounts of lithium oxide as a network modifier.24 We previously reported the Raman spectroscopic studies of the rapidly quenched glasses containing as high as 67-70 mol % LizO in the systems Li20-B20,, Li20-Si02, and Li20-P205.5-7 The spectra revealed that such glasses with large amounts of L i 2 0 corresponding to the composition of lithium pyro oxo salts or lithium ortho oxo salts were not constructed by network structure but mainly by monomer and/or dimer anions. In this study, the glasses in the systems Li3B03-Li4Si04Li3P04,which consist of two or three ortho oxo salts, and in the systems Li4B2O5-Li6Si2O7-Li4P20,, which consist of two or three pyro oxo salts, were prepared by rapid quenching. The Raman spectra of these glasses were measured, and the mixing effects of two or three components on the glass structure are discussed in terms of the acidity of the components in melts.
measured intensities by a correction factor proposed by Long9 The peaks observed in measured spectra could be approximated by Gaussian curves for deconvolution.
Experimental Procedures The reagent-grade chemicals Li2C03,B2O3, S O 2 , and NH4H2PO4 were used as the starting materials. The glass samples were prepared by use of a rapid-quenching apparatus combining a thermal-image furnace and a twin roller.8 Raman spectra were obtained at 90° scattering geometries from these glasses with a JASCO N R - 1000 Raman spectrophotometer using the 5 145-A line of an Ar+ laser. The spectroscopic data were read by a microcomputer which was connected with the spectrophotometer. The background corrections were carried out with multiplying
(1)Sarjeant, R.T.; Roy, R. J . A m . Ceram. SOC.1967,50, 500. (2)Kamitsos, E.I.; Karakassides, M. A.; Chryssikos, G. D. J. Phys. Chem. i986,90,452a. (3)Kamitsos, E. I.; Karakassides, M. A,; Chryssikos, G. D. Phys. Chem. Glasses 1987,28,203. (4) Kamitsos, E.I.; Karakassides, M. A.; Chryssikos, G. D. J. Phys. Chem. 1987,91, 1073. ( 5 ) Tatsumisago, M.; Minami, T.; Umesaki, N.; Iwamoto, N. Chem. Lett. 1986, 1371. (6)Tatsumisago, M.; Takahashi, M.; Minami, T.; Tanaka, M.; Umesaki, N.;Iwamoto, N. Yogyo-Kyokai-Shi 1986,94,464. (7) Tatsumisago, M. Kowada, Y.; Minami, T. Phys. Chem. Glasses 1988, 29,63. ( 8 ) Tatsumisago, M.;Minami, T.; Tanaka, M. J. Am. Ceram. Soc. 1981, 64,C97. (9)Long, D.A. Raman Spectrscopy; McGraw-Hill: New York, 1977.
'Present address: Hyougo University of Teacher Education, Yashiro-cho, Kato-gun, Hyogo, 673-14Japan.
0022-365418912093-2147$01.50/0
Results I . Glass-Forming Regions. Figure 1 shows the glass-forming region by rapid quenching for the system Li3B03-Li4Si04-Li3P04 combining three lithium ortho oxo salts. Open and closed circles denote glassy and crystalline samples, respectively. Glasses are widely obtained by the rapid quenching, while no glasses could be formed by the usual melt-cooling technique for any composition in this system. The glass-forming region is spread in the Li3B03-richcompositions. The limit of the region lies near the line tied with two compositions, 80Li4Si04.20Li3P04 and 6OLi3BO3.40Li3PO4. The binary system Li4Si04-Li3P04 shows the narrow glassforming region, which is due to the formation of solid solution in this system; the solid solution formation raises the liquidus temperature and thus causes the difficulty of glass formation. Figure 2 shows the glass-forming region in the system Li,B205-LisSi207-Li4Pz07 combining three lithium pyro oxo salts. Also in this system, glasses could not be prepared by the usual meltcooling technique for any composition. The rapid-quenching
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20
60
40
EO
Kowada et al.
iigPOL
Mol%
Figure 1. Glass-forming region for the system LipBO3-Li4SiO4-Li,PO4.
1200
1000 800 Wavenumber I cm-'
600
Figure 3. Raman spectra of Li3BO3-Li4SiO4-Li3PO4 glasses.
- L ~ i ~ 5
20
60
40
80
L~,,P~O,
MolD/.
Figure 2. Glass-forming region for the system Li4B2O5-Li6Si2O7-Li4-
p207. technique, however, produces glass in almost all the compositions in this system. Glass could not be prepared only for the composition range 30-80 mol % Li6Si207in the binary system Li6Si207-Li4Pz07.This is caused by the fact that the solid solutions are also formed in this system and the liquidus teI'nper2.tUTe is raised. 2. Raman Spectra. 2.1. Glasses with the Mixing of Lithium Ortho Oxo Salts. Figure 3 shows the Raman spectra of rapidly quenched glasses in which two or three lithium ortho oxo salts are mixed. In this figure, the observed spectra are indicated by the solid lines and the deconvoluted Gaussian peaks are indicated by the broken lines. In each spectrum two or three peaks are observed at 850, 930, and 950 cm-'. They are assigned to the stretching vibration of Si-0- bonds in the orthosilicate groups (Si04e),10 that of B-0- bonds in the orthoborate groups (BO3*)," and that of P-0- bonds in the orthophosphate groups (Po43-),12 respectively. For example, in the Raman spectrum of a binary glass 80Li3B03.20Li3P04(mol %), two peaks assigned to orthoborate and orthophosphate groups are respectively observed at 930 and 950 cm-l. In the spectrum of the ternary glass 40Li3BO3.4OLi4SiO4.2OLi3PO4 three peaks assigned to orthoborate, orthosilicate, and orthophosphate groups are respectively observed at 930, 850, and 950 cm-]. Each spectrum exhibits only the Raman peaks due to the groups expected from the chemical compositions. 2.2. Glasses with the Mixing of Lithium Pyro Oxo Salts. Figure 4 shows the Raman spectra of Li4B20S-Li6Si207glasses. The glass LbB2O5, one end component of the binary glasses, shows three peaks at 775, 840, and 930 cm-I. It was previously reported by Konijnendijk that these peaks were characteristic of sixmembered borate rings with BO, unit (B305-), pyroborate groups (BzOs'), and orthoborate groups (B033-), respectively." In the spectrum of the glass Li6Si207,another end component, there are five peaks at 700, 860, 920, 970, and 1040 cm-l. The peak at (10) McMillan, P. Am. Mineral. 1984, 69, 622. ( 1 1) Konijnendijk, W. L. Philips Res. Rep. 1975, Suppl. No. 1. (1 2) Corbridge, D. E. C. In Phosphorus Chemistry; Grayson, M., Griffith, E. J., Eds.; Wiley: New York, 1969; Vol. 6 , p 235.
IO
1000 800 Wavenumber I cm-'
I )O
Figure 4. Raman spectra of Li4B205-Li6Si207 glasses.
700 cm-I is assigned to the Si-0-Si stretching mode. The peak at 860 cm-' is associated with the Si-0- stretching vibration of orthosilicate groups 920 cm-' with that of pyrosilicate groups (Si20T6-), 970 cm-I with that of metasilicate groups (Si032-),and 1040 cm-' with that of disilicate groups (Si2O5-).I0 The Raman spectra of the binary Li4B205-Li6Si207glasses are obtained as a mixing of two spectra of the two end component glasses, Li4B205and Li6Si207. Figure 5 shows the Raman spectra of Li4BzO5-Li4P2O7glasses. In the spectrum of the glass Li4P207,one end component of this binary system, there are four peaks at 760, 950, 1950, and 1120 cm-I. The peak a t 760 cm-I is ascribed to the P-0-P stretching mode. The intense peak at 1050 cm-I is assigned to the P-Ostretching vibration of the pyrophosphate groups (P20,") and the weak peaks at 950 and 1120 cm-' are assigned to the P-0stretching vibration of the orthophosphate and metaphosphate (PO3-) groups, respectively.I2 Since the peak assigned
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I.
1200
1000 800 600 Wavenumber / cm-'
Figure 7. Raman spectra of Li4B205-Li6Si207-Li4P207glasses. 1200
1000 800 Wavenumber I cm-'
600
Figure 5. Raman spectra of Li4B205-Li4P207 glasses.
1200
1000 800 Wavenumber / cm-'
6
Figure 6. Raman spectra of Li6Si207-Li4P207 glasses.
to the pyrophosphate groups is much more intense than the other peaks, this gless is considered to consist mainly of the pyrophosphate groups. The Raman spectra of glasses with the compositions mixing Li4BZ05and Li4Pz07have an intense peak a t 950 cm-' assigned to orthophosphate groups. The relative intensity of this peak against the peak at 1050 cm-I assigned to the pyrophosphate groups is increased as Li4P207is replaced by Li4BZO5. In the spectrum of 20Li4Bz05.80Li4P207 g!ass a weak peak is observed near 1000 cm-I, whose assignment is difficult at present. Since the intensity of this peak is much less than that of other peaks, this peak was neglected in the calculation of the ratio of the peak area in the later discussion. Figure 6 shows the Raman spectra of Li6Siz0,-Li4Pz07 glasses. The spectrum of the glass Li4P207,as previously shown in Figure
5, has the peak assigned to the P-0-P stretching vibration (760 cm-I), the orthophosphate groups (950 cm-'), the pyrophosphate groups (1050 cm-'), and the metaphosphate groups (1 150 cm-').12 In the glass Li6Siz07,as already shown in Figure 4,there are five peaks which originate from the Si-0-Si stretching vibration (700 cm-I), the orthosilicate groups (860 cm-'), the pyrosilicate groups (920 cm-'), the metasilicate groups (970 cm-'), and the disilicate groups (1040 cm-').l0 It is worth noting that the peak due to the orthophosphate groups is also observed in the spectra of Li6Si2Oi-Li4Pz07 glasses, similar to those of Li4BZO5-Li4P2O7glasses. Figure 7 shows Raman spectra of the ternary Li4B205-Li6Siz07-Li4Pz07glasses. The ratio of the Li4BZ05contents to the Li6Si207contents was kept constant to be unity in this system. The peak at 950 cm-' assigned to orthophosphate groups is observed in the spectra and the intensity of this peak increases with an increase in the contents of both of Li4B205and Li6Si207. Discussion 1 . Structure of the Glasses Combining Lithium Qrtho Oxo Salts. The Raman spectra of the Li3B03-Li4Si04-Li3P04 glasses, shown in Figure 3, indicate that these glasses are composed of only ortho oxo groups without network structures. Such an "ionic glass" structure was already reported for the rapidly quenched binary glasses in the system Li3B03-Li4Si04.13 In the Li3B03-Li4Si04 glasses the Raman scattering efficiency of the orthoborate and the orthosilicate groups is almost the same. In the ternary system Li3B03-Li4Si04-Li3P04,however, the intensity of the peak assigned to the orthophosphate groups tends to be larger than that assigned to the orthoborate or the orthosilicate groups. This tendency is caused by the fact that the Raman scattering efficiency of the orthophosphate groups is larger than that of the orthoborate or the orthosilicate groups. The Raman scattering efficiency of the orthophosphate groups was estimated to be about 3 times as large as that of the orthoborate or the orthosilicate groups from the relation of the relative peak intensity and composition of each glass. 2. Structure of the Glasses Combining Lithium Pyro Oxo Salts. The Raman spectra of the Li4B205-L&Si207glasses, shown in Figure 4, seem to be represented as a summation of two spectra of Li4B2O5and Li6Siz07glasses. In fact, all the Raman spectra of these glasses were successfully separated into seven Gaussian peaks assigned to the structural groups present in each end component glass, Li4BZOSand Li6Siz07. Figure 8 shows the proportions of the seven structural groups in the Li4BzO5-Li6SiZO7 (1 3) Tatsumisago, M.; Takahashi, M.; Minami, T.; Umesaki, N.; Iwamoto, N. Phys. Chem. Glasses 1987, 28, 95.
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1 LO1
/
(a)
Figure 8. Fractions of structural units in the Li$205-Li,&07 as a function of Li4B20Scontents.
glasses
(b)
Figure 10. Relative amounts of silicate groups (a) and concentration of nonbridging oxygens in silicate groups (b) for the Li4B205-Li&07 glasses. 100)
=- 0
37
Ll~S205 2o 6o Mol%Lis 9,G,
(a)
1165120,
,
,
,
,
,
,
07
08
09
li l l I([Lil.:Bl)
(b)
Li6Si2G7 ( L '0205 LlSSl20,1,
Figure 9. Relative amounts of borate groups (a) and concentration of
nonbridging oxygens in borate groups (b) for the Li,$205-Li&07 glasses.
Figure 11. Relative amounts of phosphate groups in Li4P207-Li4B205, and Li4P207-(Li4B20S.Li6Si207)1/2 glasses. Li4P207-Li6Si207,
glasses as a function of Li4B205content. The amounts of borate groups increase and those of silicate groups decrease as the L4B20s content increases. However, those increasing and decreasing trends are not monotonous. Figure 9a shows the proportions of each borate group in total borate groups as a function of Li6Si207content. As the Li6Si207 content is increased, the orthoborate group (Bo33-), which has the lowest polymerization degree, tends to increase and the sixmembered borate ring with BO4 unit @,OS-), which has a higher polymerization degree than the pyroborate group (B20se), tends to decrease. The ratio of the nonbridging oxygens (NBO) in borate groups (NB4-/(NB4-+ NBGB))can be calculated on the basis of the amounts of each borate group; the orthoborate group (BOj3-) has three NBO per a boron atom, the pyroborate group (B2OS4-)has two NBO and half of bridging oxygen (BO) per a boron atom, and the six-membered borate ring with BO4 unit NBO and 5 / 3 BO per a boron atom. (B30,-) has In Figure 9b the ratio of NBO in borate groups thus calculated is plotted against the Li6Siz07content for the Li4B207-LiSSi207 glasses. In this figure the composition is indicated by the ratio of the concentration of the lithium atoms against that of the boron atoms in these glasses ([Li]/([Li] + [B])) because it is thought that the addition of L i 2 0 breaks up the network of the glasses in this system and the formation of the nonbridging oxygens in borate groups is associated with the relative amounts of Li20 against BzO3. If the fraction of each structural unit of the borate or the silicate groups in the Li4BzOs-Li6Si2O7glasses is equal to that in Li4B20Sor Li6Si207glasses, the ratio of NBO must be kept constant. The ratio of NBO, however, increases with an increase in the ratio [Li]/([Li] + [B]). This result suggests that NBO is formed more preferentially in borate groups than in silicate groups. In other words the Li20 component added to the glasses tends to be used for NBO formation in borate groups rather than in silicate groups.
Figure 10a shows the proportions of each silicate group in total silicate groups as a function of Li4B205content. The orthosilicate group @io4+), which has the lowest polymerization degree, tends to decrease and the metasilicate group (SO3-), which has a larger polymerization degree than the pyrosilicate group (SiZO7&), tends to increase with an increase in the Li4BZO5content. This result suggests that the average polymerization degree of the whole silicate groups increases with an increase in the Li4BzO5content. Figure 10b shows the ratio of NBO in silicate groups (Nsi4-/ (Nsi+- NsiGa)) as a function of the concentration ratio of the lithium atoms against that of the silicon atoms in these glasses ([Li]/([Li] + [Si])); the orthosilicate group (SO4&) has four NBO per a silicon atom, the pyrosilicate group (Si2Of) has three NBO and half of BO per a silicon atom, the metasilicate group has two NBO and one BO per a silicon atom, and the disilicate group (Si20s-) has one NBO and 3/2 of BO per a silicon atom. The ratio of NBO in silicate groups decreases with an increase in [Li]/([Li] + [Si]). These results suggest that NBO is not formed preferentially in silicate groups in contrast to those in borate groups which is a reasonable agreement with the results shown in Figure 10. It is concluded that NBO in the glasses tends to be formed preferentially in borate groups rather than in silicate groups. In the glasses containing Li4P207,on the other hand, an intense peak (950 cm-') assigned to the PO>-groups has been observed in the Raman spectra (Figures 5-7) in spite of the fact that the composition of glasses were varied. These results suggest that in the Li4Pz07-containingglasses NBO tend to be formed more preferentially in phosphate groups than in borate or silicate groups. Since two peaks (1050 and 950 cm-I) attributed to the pyrophosphate and the orthophosphate groups are mainly observed in the Raman spectra of the glasses containing Li4P207,each
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In other words NBO is much more preferentially formed in the phosphate groups than in the borate or the silicate groups. It is thus concluded that NBO is preferentially formed in the following order: phosphate >> borate > silicate groups. It is of great interest that this order is identical with the order of the acidity, P2O5 >> B203> S O z , of the glass-forming oxides in the melts of binary systems NazO-P2O5, NaZ0-B2O3,and NazOS O z ; the order was decided by the thermodynamic measurem e n t ~ . In ~ ~the melts it has been reported that the bridging oxygen in the glass-forming oxides is an acid and Oz- in the L i 2 0 component is a base.14 As the acidity of the glass-forming oxides increases, the bridging oxygens tend to take the oxygens introduced by LizO with the following equilibria 0 05
0 6 07 08 09 r LI1 I ([ LI I + [ P I )
1
Figure 12. Concentration of the nonbridging oxygens for Li20-P20,, Li4B205-Li4P207,Li6Si207-Li4Pz07,and L4B20S-Li6Si207-Li4P207 glasses as a function of [Li]/([Li] + [PI).
spectrum can be deconvoluted into the two Gaussian peaks. Figure 11 shows the proportions of the two phosphate groups as a function of the composition in Li4PZ07-Li4BZ05,Li4Pz07-Li6Si207,and Li4P207-(Li4Bz05~Li6Siz07)l~z glasses. The ratio of the orthophosphate groups increases and that of pyrophosphate groups decreases as Li4P207is replaced by the other lithium pyro salts. The concentration of NBO present in phosphate groups was calculated from the proportions of the two phosphate groups, and in Figure 12 the ratio of NBO in phosphate groups is plotted against the concentration ratio of the lithium atoms over the phosphorus atoms, [Li]/([Li] [PI), for the mixed pyro oxo salt glasses in the systems Li4B2O5-Li4PZO7,Li6Si207-Li4P207,and Li4BzO5-Li6Si2O7-Li4P2O7. The results for the binary Li20-Pz05 glasses are also plotted for comparison. The ratio of NBO increases linearly with an increase in [Li]/[(Li] [PI) in the binary system Li20-P205 (e),and the plots are located on the extrapolated line of the binary Li20-P205 glasses in the other systems containing one or both of Li4BZ05 and Li6Si207. This result suggests that the L i 2 0 component contained in the systems Li4B205-Li4P207,Li6Si207-Li4Pz07,and Li4B205-Li6Si207-Li4Pz07plays the same role of LizO as in the system LizO-Pz05; that is, the L i 2 0 component added to the glasses is used in order to form NBO in phosphate groups in these systems.
+
+
P-0-P B-0-B Si-0-Si
+ 0 2 F? 2p-o+ 02- 2B-0+ 0” F? 2Si-0-
(1)
(2)
(3)
As the acidity of the glass-forming oxides increases, these equilibria shift to the right-hand side, and therefore the concentration of NBO in these oxides increases.
Conclusion Raman spectra of the rapidly quenched glasses in the systems mixing two or three ortho oxo salts indicate that those glasses are composed of only ortho oxo groups which are expected from the chemical composition and have no network structures. In the systems mixing two or three pyro oxo salts, the rapidly quenched glasses with P2O5 contain the orthophosphate groups as a structural unit. The N B O s in these glasses are formed more preferentially in phosphate groups than in borate or silicate groups. In the glasses without P2O5, N B O s are preferentially formed in borate groups rather than in silicate groups. Acknowledgment. We acknowledge support for the present work by a Grant-in-Aid for Scientific Research on Priority Areas, New Functionality Materials-Design, Preparation and Control, the Ministry of Education, Science and Culture of Japan (No. 63604582). Registry No. LiBO,, 13774-56-6; Li4Si04, 13453-84-4; Li,P04, 10377-52-3; Li4B2OS, 13774-55-5; Li,Sii07, 23304-09-8; LijP207, 13843-41-9. (14) Yokokawa, T.;Kohsaka, S. J . Chem. Eng. Data 1979, 24, 167.
