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Raman scattering in silica glass in the permanent densification region
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Journal of Non-Crystalline Solids 144 (1992) 151-158 North-Holland
NON-CRYSTALLINE SOLIDS
Raman scattering in silica glass in the permanent densification region H. S u g i u r a a n d T. Y a m a d a y a Department of Physics, Yokohama City University, 22-2 Seto, Kanazawaku, Yokohama 236, Japan Received 6 June 1991 Revised manuscript received 10 February 1992
Raman spectra of silica glasses were measured at pressures up to about 20 GPa. The bandwidth of a broad band in the region 300-500 cm 1 reduced and approached D 1 observed at 495 cm 1 with increasing pressure. The change was reversible with compression up to about 10 GPa. Above 10 GPa, the D 2 line was enhanced and the change in the 300-500 cm 1 band became irreversible. The resultant spectrum, which was that of densified silica glass, had fundamentally the same structure as that of normal silica glass, although each band was different from that in normal glass in bandwidth, relative intensity, and peak position. D 1 and D2, which were the mode associated with intercluster layer or cluster surface, were dominant in Raman scattering in high density silica glass.
1. Introduction T w o n a r r o w lines o b s e r v e d n e a r 490 a n d 603 c m - 1 a r e r e m a r k a b l e f e a t u r e s o f t h e R a m a n scatt e r i n g [1,2] in silica glass. T h e s e lines a r e d e n o t e d as D 1 a n d D2, r e s p e c t i v e l y [3]. O t h e r b a n d s a r e f o u n d n e a r R a m a n p e a k s o f crystalline silica, l o w - c r i s t o b a l i t e [4] or l o w - q u a r t z [5]. A l t h o u g h a b r o a d b a n d c e n t e r e d at a b o u t 430 c m - 1 is f o u n d to b e q u i t e d i f f e r e n t f r o m t h e case o f crystalline silica, t h e c e n t e r p o s i t i o n o f this b a n d is very close to p o s i t i o n s o f p r o n o u n c e d p e a k s in cryst a l l i n e silica. This b r o a d b a n d is c a l l e d the m a i n band. T h e o b s e r v e d R a m a n b a n d s e x c e p t for two n a r r o w lines c a n b e a s s o c i a t e d with v i b r a t i o n s in t h e c o n t i n u u m r a n d o m n e t w o r k ( C R N ) [6] o r clusters a n a l o g o u s to crystalline silica [3]. T h e two i n t e r p r e t a t i o n s a r e d i f f e r e n t in style, b u t a c o m m o n s t r u c t u r a l f e a t u r e is s e e n which is c a l l e d t h e bulk part of the cluster model. Correspondence to: Dr H. Sugiura, Department of Physics, Yokohama City University, 22-2 Set, Kanazawa-ku, Yokohama 236, Japan. Tel: +81-45 781 1311.
V i b r a t i o n s a s s o c i a t e d with t h e D 1 a n d D 2 have b e e n in controversy. T h e C R N m o d e l was n o t initially a b l e to r e p r o d u c e t h e s e n a r r o w lines. G a l e e n e r et al. [7] p r o p o s e d a m o d e l in which t h e D 1 a n d D 2 a r e r e l a t e d to f o u r - m e m b e r e d rings a n d t h r e e - m e m b e r e d rings, respectively. T h e m o d e s of such rings d e c o u p l e d v i b r a t i o n a l l y f r o m t h e C R N . T h e C R N with p e r i o d i c b o u n d a r y conditions r e c e n t l y r e p r o d u c e d t h e s e lines w i t h o u t c h a r a c t e r i s t i c f e a t u r e s [8]. H o w e v e r t h e D t o n t h e c a l c u l a t e d s p e c t r u m is n o t as c l e a r as o b s e r v e d a n d is difficult to d i s t i n g u i s h f r o m t h e m a i n b a n d . I n t h e c l u s t e r m o d e l , D 1 a n d D 2 a r e r e l a t e d to v i b r a t i o n s on t h e c l u s t e r s u r f a c e o r in t h e interc l u s t e r layer. A s t r u c t u r a l m o d e l for t h e interclust e r l a y e r has n o t b e e n w e l l - e s t a b l i s h e d . F o u r m e m b e r e d rings a r e t h e p r i n c i p a l units in t h e i n t e r c l u s t e r l a y e r a c c o r d i n g to o n e i n t e r p r e t a t i o n [3]. I n t h a t layer, s o m e o f t h e O a t o m s a r e d o u b l y b o n d e d to Si a t o m s w i t h o u t l i n k a g e to any rings. I n t h e o t h e r case, - S i - S i - a n d - O - O - b o n d s a r e p r i n c i p a l [9]. T h e o t h e r i n t e r e s t i n g p h e n o m e n o n in silica glass is a p e r m a n e n t i n c r e a s e in d e n s i t y a f t e r c o m p r e s s i o n w i t h h e a t t r e a t m e n t [10,11] o r a f t e r
0022-3093/92/$05.00 © 1992 - Elsevier Science Publishers B.V. All rights reserved
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H. Sugiura, T. Yamadaya / Raman scattering in silica glass
neutron irradiation [2,12]. The irreversible change found in compression up to 17 G P a [13] or 30 GPa [14] without heating should also be a result of the densification process. There is evidence of a similarity [14] between the R a m a n spectrum of quenched silica glass and that of glass densified by neutron irradiation. The pronounced feature in the spectrum of quenched silica glass is a set of two broad bands around 500-600 cm -1. These two bands are likely the enhanced and broadened D~ and D 2. The densification appears to be related to the origin of the D 1 and D 2 lines. The relationship between pronounced peaks of quenched silica glass and the D1 and D 2 lines is not clear. A series of R a m a n spectra of silica glass under pressures was measured by Hemley et al. [14]. The pronounced p e a k in the spectra measured at high pressures was assigned to the main band in which the bandwidth reduced by narrowing of S i - O - S i angle distribution. The D~ line seems to vanish from the series of spectra. Although the spectrum of quenched silica glass s e e m s to be analogous to those measured at high pressures, two types of spectrum have been said to differ from each other [14]. The density of silica glass is close to that of low cristobalite, while the bulk modulus is close to that of low quartz. The bulk 'modulus of silica glass decreases with compression up to about 2.3 G P a [15]. If the structure of silica glass in interpreted as a skeleton [16], and the two narrow lines are related to such a structure, the initial change in these two lines with compression in low pressure range is of interest. The change of the R a m a n spectrum of silica glass has not been reported in the low pressure region. R a m a n spectra of silica glass were measured from low pressures to the range for densification. The aim of these measurements w a s to check the effects of pressure on the main band and two lines, D 1 and D 2.
Methanol was used as pressure transmitting medium instead of methanol and ethanol because the R a m a n lines at about 400 c m - 1 from ethanol interfered with the silica glass spectra. Differences in hydrostaticity of methanol from the usual mixture were not serious, because the width of ruby fluorescence line was broadened little and the compression of silica glass was reversible up to about 10 GPa. R a m a n spectra were measured in D A C using a 1 m double monochrometer, Ramanol U1000 by Jobin Yvon with conventional photon counting. The light source was the 488 nm line Ar ion laser, GLG-3200 by N E C at 200 or 400 mW. Neither polarizer nor analyzer was used. Silica glass samples were T1000 by Toshiba Ceramics shaped as thin plate about 0.04 m m thick. R a m a n spectra of silica glass in the range from 400 to 700 cm -1 were measured under pressures with small increases of pressure in the low pressure range. Several runs were carried out for different pieces. In the first run, a sample was pressurized to 12.6 GPa, and returned to atmospheric pressure. In the process of pressurizing, the sample was heated to 100°C at about 10.2 GPa. In the next run, another sample was pressurized to 20.4 GPa, and also returned to atmospheric pressure. In addition to these two runs, several series of measurements below 8 G P a were carried out on different pieces in order to observe changes in the R a m a n bands at low pressure. Several samples were densified using a pistoncylinder high pressure apparatus at pressures and temperatures up to 3.5 GPa and 900°C. The densities and the peak positions of the D 1 and D 2 were measured on the recovered samples. A mixture of diiodomethane and dichloromethane was used for the density measurement.
3. Results 2. Experimental A gasketted diamond-anvil cell (DAC) apparatus was used for pressurizing the sample. The pressure was estimated by the ruby scale [17].
Figure 1 shows the continuous changes in the R a m a n spectra of silica glass under pressures to about 8 GPa. Although the intensity of a R a m a n band was difficult to compare in different spectra, the results of several runs were reproducible
153
H. Sugiura, T. Yamadaya / Raman scattering in silica glass
a n d a r e s u m m a r i z e d in fig. 1. B e l o w 3.35 G P a , t h e intensity o f t h e m a i n b a n d i n c r e a s e s with p r e s s u r e in t h e c e n t e r o f this b a n d . Since t h e D 1 line displays no sign o f d i s a p p e a r i n g , t h e D 1 i n t e n s i t y i n c r e a s e s as well as t h e i n t e n s i t y in t h e m a i n b a n d . A b o v e 3.35 G P a , t h e intensity o f t h e m a i n b a n d b e g i n s to d e c r e a s e at low frequency. T h e m a i n b a n d s e e m s to b e c o m b i n e d with t h e D1 line a n d is i n d i s t i n g u i s h a b l e f r o m it. T h e D 2 b e c o m e s w e a k with i n c r e a s i n g p r e s s u r e as c o m p a r e d with t h e D v F i g u r e 2 shows t h e p r e s s u r e shifts of p e a k p o s i t i o n s in t h e r a n g e f r o m 400 to 700 cm - t . Open squares are the positions obtained during p r e s s u r i z a t i o n o f n o r m a l silica glasses. T h e c e n t e r of t h e m a i n b a n d shifts r a p i d l y t o w a r d high freq u e n c y with p r e s s u r e i n c r e a s e s u p to a b o u t 8 G P a , b u t g r a d u a l l y a p p r o a c h e s t h e D 1 line at h i g h e r p r e s s u r e . I n t h e s p e c t r a at l o w e r p r e s -
700
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PRESSURE EGP~] Fig. 2. Pressure shifts of the peak positions observed in the range from 400 to 700 cm- 1. In lower segment, the main band is shifted approximately from 440 to 500 cm-1 up to 7 GPa. A segment at the middle shows the positions of the original D l line and that combined with the main band. The D 2 lines are shown in upper segment fiom 600 to 680 cm-1 Open squares are the observed positions in the process of increasing pressure. Open and closed circles are the positions in the reducing process. Maximum pressures are 12.6 and 20.4 GPa for open circles and closed circles, respectively.
>--
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400
500
WFIVE N U M B E R
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Fig. 1. Continuous feature of the changes of the main band, D 1 and D 2 lines in the Raman spectrum of silica glass with pressure up to about 8 GPa.
sures, t h e D 1 was s e p a r a t e d f r o m t h e o v e r l a p o f t h e D 1 a n d t h e m a i n b a n d , a s s u m i n g t h a t the D 1 was a s i m p l e L o r e n t z i a n p e a k a n d t h e rest was t h e m a i n b a n d . A b o v e 8 G P a , t h e two p e a k s c o m b i n e d into a s y m m e t r i c p e a k w h i c h was insepa r a b l e . T h e c o m b i n e d p e a k shifts with p r e s s u r e a l o n g t h e line w h i c h r e p r e s e n t s t h e e x t e n s i o n of t h e shift o f D 1 at l o w e r p r e s s u r e s . T h e p r e s s u r e shift o f D 2 is small to a few G P a , b u t it m o n o t o n ically i n c r e a s e s at h i g h e r p r e s s u r e . T h e c h a n g e in t h e s p e c t r u m o f silica glass with c o m p r e s s i o n was, in g e n e r a l , n o t d r a s t i c b u t g r a d ual a n d c o n t i n u o u s . A d r a s t i c c h a n g e was ob-
H. Sugiura, T. Yamadaya / Raman scattering in silica glass
154
served after heat treatment under high pressure. Figure 3 shows the R a m a n spectrum of silica glass before and after heat treatment at 10.2 GPa. The pressure was reduced to 8.73 G P a in this treatment. The spectrum measured at 8.42 G P a before the treatment is also shown in fig. 3 for comparison. The important difference in these spectra is an enhanced band around 630 c m - 1 in the spectrum at 8.73 G P a after heat treatment. Two peaks are seen in the spectrum of silica glass above 8.73 G P a after that treatment. Figure 4 shows continuous features of these two peaks during pressure reduction after compression to 12.6 GPa. The positions of both peaks were also checked during pressure reduction as shown in fig. 2. The peak around 550 c m - 1 shifts along the line for the shift of the D 1 and returns to the vicinity of the initial position of D 1. On the low
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Fig. 4. Continuous feature of the change of the R a m a n spectrum of silica glass in releasing process. In this run, the sample was heated to 100°C at 10.2 G P a and pressurized up to 12.6 GPa. >--
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400
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Fig. 3. Change of the R a m a n spectrum of silica glass with heat-treatment. 10.2 G P a and 8.73 G P a are pressures before and after heat treatment, respectively. 8.42 GPa is a step in pressurizing process before treatment.
frequency side of this peak, the main band grows with pressure reduction as shown in fig. 4. The intensity after compression is not as high as that under pressure for the entire spectrum. The other peak around 630 cm -1 returns almost to the initial position of D 2. The final spectrum is similar to that before compression. Figure 5 shows continuous features of the spectra measured in the second run in the pressure range from about 10 to 20 G P a and in the reducing process. Although the p e a k corresponding to D 2 is seen only as a shoulder in high frequency side of the combined peak, it is clearly found that the shoulder grows with pressure increases above 12 GPa, and the shoulder can be separated from the combined peak during pres-
155
H. Sugiura, T. Yamadaya / Raman scattering in silica glass
sure reduction. The peak positions during pressure reduction plotted in fig. 2 deviate from those obtained during pressurization. There is an increase on the low frequency side of the combined peak in the spectrum in fig. 5 measured as pressure fell to 0.02 GPa. Although it seems to be a weak feature, a change in curvature of the low frequency tail is clear. Such an increase is a trace of the main band and similar to the restoration of the main band shown in the first run. The intensities of the two peaks remain high after compression. The density of silica glasses recovered from the piston-cylinder apparatus depended on applied pressure, temperature and treatment time.
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Fig. 6. D I and D2 positions of pressurized silica glass in DAC (close squares) and recovered high density silica glasses (open squares) as functions of density. >co z
The maximum density was 2.45 M g / m 3. In several runs, low quartz crystallized in densified glass. In such cases, the center positions of the main bands were very close to the position of the low quartz peak. Figure 6 shows the peak positions of the D 1 and D 2 as functions of density. Open squares are densified and quenched glasses, while closed squares are results of in situ measurements in D A C up to 3.19 GPa in the reversible compression range. The densities of glass in D A C were estimated from the equation of state of silica glass after Kondo et al. [15].
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Fig. 5. Continuous feature of the change of the Raman spectrum of silica glass in compression from about 10 to 20 GPa and releasing process. The spectra are placed from bottom to top in order of measurements. The maximum pressure was 20.4 GPa.
4. Discussion The R a m a n spectra observed were rugged as compared with those after Hemley et al. [14]. Our
156
H. Sugiura, T. Yamadaya / Raman scattering in silica glass
conventional system for optical m e a s u r e m e n t was less favorable to D A C experiments than their system. Despite disadvantages in the measurement system, the continuous changes in the spectrum were much more intelligible especially in the low pressure region, maybe because of small pressurizing steps. Each spectrum in a common pressure range showed an almost equal feature. The main band is combined with the D 1 up to 8 GPa. Although the D 2 becomes weak up to 8 GPa, a corresponding band is enhanced with heat treatment (first run) or compression above 12 G P a (second run). Therefore, there are two remarkable bands in the R a m a n spectrum of silica glass in high pressure region. Such a feature is also found in fig. 2 which shows the shifts of p e a k positions. The main band moves close to D 1, and disappears from this figure, but the other two bands remain. All bands shown in fig. 2 except the main band can be classified into two segments. It is clear that these are respectively the D 1 and D2, although the D 1 is not pure D 1 but likely a combination with the main band. It is noted that the main band does not disappear, but becomes weak and indistinguishable from D 1. In the low pressure range, the spread of the main band shrinks, and the center position of this band shifts to higher frequency with increasing pressure. These changes in the main band are rapid compared with those in other peaks up to about 8 GPa, but are considered to slow down above 8 GPa. On the other hand, the shift of the D 1 with pressure is almost linear up to 20 GPa. The frequency of the main band cannot exceed that of Da; it is as if the main band is restricted by D 1. This suggests that these two bands associated with the same mode on similar bond units in different configurations. Although this result partly supports the ring model [7], the ring associated with D 1 does not decouple from the network but seems to constitute a type of network-like skeletal surface, which is a word used in ref. [16]. Such a feature resembles the model of domains glued together by interfacial material [18]. The D 2 e n h a n c e m e n t closely resembles that of neutron irradiation [12], which gives rise to densification. In the case of compression, the degree of densification depends on pressure and temper-
ature [10,11]. The compression of silica glass is reversible up to 8 G P a in argon [14], and 9.5 G P a in a methanol and ethanol mixture [13]. The limit of reversible and elastic compression must therefore be lower than 10 GPa, and pressure above 10 G P a is high enough to irreversibly densify, even if another substance is used as the pressure medium. Since heat treatment at 10.2 G P a accelerates densification, a result of which is the enhancement of the D2, it is clear that D 2 relates to the densification process. The other characteristic of D 2 is a weak dependence of p e a k position on pressure up to about 3 GPa. The structural unit related to D 2 is considered to be a part difficult to deform or a dangling part on the skeletal surface. If the former is, for example, a SiO 4 tetrahedron, it will be too difficult to fit the case. The part associated with the D 2 is a type of 'defect', which increases in the densification process. The feature in the intercluster layer is important for the structural change in the densification process. The increase of the 'defect' is, however, not considered to cause the increase in density. It is expected that the intercluster layer grows in the densification process. If the density of the intercluster layer is higher than that of cluster inside, the growth of the layer can be related to a permanent increase in density. A structural unit in that layer corresponds to D 1. Such a unit is not a new form at all, but existed in normal silica glass. The cluster in normal silica glass is said to be analogous to cristobalite, one of the silica polymorphs. Another form is expected to exist in the intercluster layer, that is a morphology analogous to coesite. The density of coesite is much higher than cristobalite. Although there is no evidence, it is interesting that R a m a n spectra of glass obtained from coesite are similar to that of densified silica glass [19]. If a form, analogous to coesite, grows in the intercluster layer, the unit corresponding to D 1 is likely to be the f o u r - m e m b e r e d ring, which is characteristic of coesite. The intensity of the main band is weak in densified silica glass. The main band is found as a weak shoulder on the combined peak in the spectrum at 0.02 G P a after compression to about 20
H. Sugiura, T. Yamadaya / Raman scattering in silica glass
GPa. The combined peak is still strong at 0.02 GPa. D1 is considered as the main constituent of this peak. Although the area of the cluster is reduced by the growth of the intercluster layer, the weakening of the main band and enhancement of the D 1 are not interpreted only by the change in size of each portion. The reason is that these phenomena are also observed in reversible compression. The growth of the layer gives a type of restriction to the cluster such as elastic compression and brings about deformation in both regions. Weakening of the main band and enhancement of D~ are related to deformations in the cluster and intercluster layer, where some types of selection rules possibly appear. The difference between compressed silica glass and densified silica glass is in cluster size and in the intercluster layer. Since there is expected to be a common situation for both glasses that the cluster and the intercluster layer are mutually restricted, two glasses are compared by a scale of density. The maximum density, 2.43 M g / m 3, of densified glass corresponds to compressed glass up to about 3.1. GPa. The main band almost remains in both cases. The shifts of D 1 and D 2 with density are approximately equal in both cases as shown in fig. 6. It can be said that the degree of restriction depends on density. The peak position of D 2 is almost independent of density in this range. This is the same as the effect of pressure on D 2 mentioned above. Quantitative agreements in slopes for both lines in fig. 6 were unexpected, but the important thing is the similarity of each tendency. There are large deviations in peak positions (see fig. 2) between pressurization and depressurization in the second run of D A C experiment up to about 20 GPa. Such deviations are understood to be the result of the difference in density brought about by the densification. If the density can be estimated during depressurization and the peak positions are plotted as a function of density, the result will coincide with that of pressurization. The extent of deviation is related to the degree of densification. Large deviations correspond to irreversible changes observed in the R a m a n spectrum.
157
5. Conclusions Three R a m a n bands, the main band, the D 1 and the D2, were observed in the range from 400 to 700 cm-1. Although each band changed shape and position with compression, and the main band appeared to hide in the D 1 band, all remained under high pressure. No new bands were observed under high pressure. These are natural results for reversible compression, and are common to densification. Changes in the Raman bands brought about by change in density were almost independent of whether the density change was irreversible or reversible. Present results fit the structure model with cluster and intercluster layer or domain and inteffacial material. The clusters are surrounded by the intercluster layer. Densification is understood to be a result of growth of the intercluster layer, the density of which is higher than that of the cluster inside. There is a structural unit related to the D 1 mode in the intercluster layer. Such a unit is considered to play a role as skeleton in the glass. On the other hand, the unit related to the D 2 is likely to be a part dangling on the skeleton, because of insensitivity to compression at low pressure.
References [1] P. Flubacher, A.J. Leadbetter, J.A. Morrison and B.P. Stoicheff, J. Phys. Chem. Solids 12 (1959) 53. [2] R.H. Stolen, J.T. Krause and C.R. Kurkjian, Discuss. Faraday Soc. 50 (1970) 103. [3] J.C. Phillips, J. Non-Cryst. Solids 63 (1984) 347. [4] J.B. Bates, J. Chem. Phys. 57 (1972) 4042. [5] J.F. Scott and S.P.S. Porto, Phys. Rev. 161 (1967) 903. [6] R.J. Bell, N.F. Bird and P. Dean, J. Phys. C1 (1968) 299. [7] F.L. Galeener, R.A. Barrio, E. Martinez and R.J. Elliot, Phys. Rev. Lett. 53 (1984) 2429. [8] R.A. Murray and W.Y. Ching, Phys. Rev. B39 (1989) 1320. [9] J.C. Phillips, Phys. Rev. B35 (1987) 6409. [10] J.D. Mackenzie, J. Am. Ceram. Soc. 46 (1963) 461. [11] J. Arndt and D. Stoffier, Phys. Chem. Glasses 10 (1969) 117. [12] J.B. Bates, R.W. Hendricks and L.B. Shaffer, J. Chem. Phys. 61 (1974) 4163. [13] M. Grimsditch, Phys. Rev. Lett. 52 (1984) 2379.
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[14] R.J. Hemley, H.K. Mao, P.M. Bell and B.O. Mysen, Phys. Rev. Lett. 57 (1986) 747. [15] K. Kondo, S. Iio and A. Sawaoka, J. Appl. Phys. 52 (1981) 2826. [16] J.C. Phillips, Phys. Rev. B33 (1986) 4443. [17] G.J. Piermarini, S. Block, J.D. Barnett and R.A. Forman, J. Appl. Phys. 46 (1975) 2774.
[18] C:S. Marians and J.K. Burdett, J. Non-Cryst. Solids 124 (1990) 1. [19] R.J. Hemley, in: High-Pressure Research in Mineral Physics, ed. M.H. Manghnani and Y. Shono (Terra Scientific, Tokyo, 1987) p. 347.
Journal of Non-Crystalline Solids 144 (1992) 151-158 North-Holland
NON-CRYSTALLINE SOLIDS
Raman scattering in silica glass in the permanent densification region H. S u g i u r a a n d T. Y a m a d a y a Department of Physics, Yokohama City University, 22-2 Seto, Kanazawaku, Yokohama 236, Japan Received 6 June 1991 Revised manuscript received 10 February 1992
Raman spectra of silica glasses were measured at pressures up to about 20 GPa. The bandwidth of a broad band in the region 300-500 cm 1 reduced and approached D 1 observed at 495 cm 1 with increasing pressure. The change was reversible with compression up to about 10 GPa. Above 10 GPa, the D 2 line was enhanced and the change in the 300-500 cm 1 band became irreversible. The resultant spectrum, which was that of densified silica glass, had fundamentally the same structure as that of normal silica glass, although each band was different from that in normal glass in bandwidth, relative intensity, and peak position. D 1 and D2, which were the mode associated with intercluster layer or cluster surface, were dominant in Raman scattering in high density silica glass.
1. Introduction T w o n a r r o w lines o b s e r v e d n e a r 490 a n d 603 c m - 1 a r e r e m a r k a b l e f e a t u r e s o f t h e R a m a n scatt e r i n g [1,2] in silica glass. T h e s e lines a r e d e n o t e d as D 1 a n d D2, r e s p e c t i v e l y [3]. O t h e r b a n d s a r e f o u n d n e a r R a m a n p e a k s o f crystalline silica, l o w - c r i s t o b a l i t e [4] or l o w - q u a r t z [5]. A l t h o u g h a b r o a d b a n d c e n t e r e d at a b o u t 430 c m - 1 is f o u n d to b e q u i t e d i f f e r e n t f r o m t h e case o f crystalline silica, t h e c e n t e r p o s i t i o n o f this b a n d is very close to p o s i t i o n s o f p r o n o u n c e d p e a k s in cryst a l l i n e silica. This b r o a d b a n d is c a l l e d the m a i n band. T h e o b s e r v e d R a m a n b a n d s e x c e p t for two n a r r o w lines c a n b e a s s o c i a t e d with v i b r a t i o n s in t h e c o n t i n u u m r a n d o m n e t w o r k ( C R N ) [6] o r clusters a n a l o g o u s to crystalline silica [3]. T h e two i n t e r p r e t a t i o n s a r e d i f f e r e n t in style, b u t a c o m m o n s t r u c t u r a l f e a t u r e is s e e n which is c a l l e d t h e bulk part of the cluster model. Correspondence to: Dr H. Sugiura, Department of Physics, Yokohama City University, 22-2 Set, Kanazawa-ku, Yokohama 236, Japan. Tel: +81-45 781 1311.
V i b r a t i o n s a s s o c i a t e d with t h e D 1 a n d D 2 have b e e n in controversy. T h e C R N m o d e l was n o t initially a b l e to r e p r o d u c e t h e s e n a r r o w lines. G a l e e n e r et al. [7] p r o p o s e d a m o d e l in which t h e D 1 a n d D 2 a r e r e l a t e d to f o u r - m e m b e r e d rings a n d t h r e e - m e m b e r e d rings, respectively. T h e m o d e s of such rings d e c o u p l e d v i b r a t i o n a l l y f r o m t h e C R N . T h e C R N with p e r i o d i c b o u n d a r y conditions r e c e n t l y r e p r o d u c e d t h e s e lines w i t h o u t c h a r a c t e r i s t i c f e a t u r e s [8]. H o w e v e r t h e D t o n t h e c a l c u l a t e d s p e c t r u m is n o t as c l e a r as o b s e r v e d a n d is difficult to d i s t i n g u i s h f r o m t h e m a i n b a n d . I n t h e c l u s t e r m o d e l , D 1 a n d D 2 a r e r e l a t e d to v i b r a t i o n s on t h e c l u s t e r s u r f a c e o r in t h e interc l u s t e r layer. A s t r u c t u r a l m o d e l for t h e interclust e r l a y e r has n o t b e e n w e l l - e s t a b l i s h e d . F o u r m e m b e r e d rings a r e t h e p r i n c i p a l units in t h e i n t e r c l u s t e r l a y e r a c c o r d i n g to o n e i n t e r p r e t a t i o n [3]. I n t h a t layer, s o m e o f t h e O a t o m s a r e d o u b l y b o n d e d to Si a t o m s w i t h o u t l i n k a g e to any rings. I n t h e o t h e r case, - S i - S i - a n d - O - O - b o n d s a r e p r i n c i p a l [9]. T h e o t h e r i n t e r e s t i n g p h e n o m e n o n in silica glass is a p e r m a n e n t i n c r e a s e in d e n s i t y a f t e r c o m p r e s s i o n w i t h h e a t t r e a t m e n t [10,11] o r a f t e r
0022-3093/92/$05.00 © 1992 - Elsevier Science Publishers B.V. All rights reserved
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neutron irradiation [2,12]. The irreversible change found in compression up to 17 G P a [13] or 30 GPa [14] without heating should also be a result of the densification process. There is evidence of a similarity [14] between the R a m a n spectrum of quenched silica glass and that of glass densified by neutron irradiation. The pronounced feature in the spectrum of quenched silica glass is a set of two broad bands around 500-600 cm -1. These two bands are likely the enhanced and broadened D~ and D 2. The densification appears to be related to the origin of the D 1 and D 2 lines. The relationship between pronounced peaks of quenched silica glass and the D1 and D 2 lines is not clear. A series of R a m a n spectra of silica glass under pressures was measured by Hemley et al. [14]. The pronounced p e a k in the spectra measured at high pressures was assigned to the main band in which the bandwidth reduced by narrowing of S i - O - S i angle distribution. The D~ line seems to vanish from the series of spectra. Although the spectrum of quenched silica glass s e e m s to be analogous to those measured at high pressures, two types of spectrum have been said to differ from each other [14]. The density of silica glass is close to that of low cristobalite, while the bulk modulus is close to that of low quartz. The bulk 'modulus of silica glass decreases with compression up to about 2.3 G P a [15]. If the structure of silica glass in interpreted as a skeleton [16], and the two narrow lines are related to such a structure, the initial change in these two lines with compression in low pressure range is of interest. The change of the R a m a n spectrum of silica glass has not been reported in the low pressure region. R a m a n spectra of silica glass were measured from low pressures to the range for densification. The aim of these measurements w a s to check the effects of pressure on the main band and two lines, D 1 and D 2.
Methanol was used as pressure transmitting medium instead of methanol and ethanol because the R a m a n lines at about 400 c m - 1 from ethanol interfered with the silica glass spectra. Differences in hydrostaticity of methanol from the usual mixture were not serious, because the width of ruby fluorescence line was broadened little and the compression of silica glass was reversible up to about 10 GPa. R a m a n spectra were measured in D A C using a 1 m double monochrometer, Ramanol U1000 by Jobin Yvon with conventional photon counting. The light source was the 488 nm line Ar ion laser, GLG-3200 by N E C at 200 or 400 mW. Neither polarizer nor analyzer was used. Silica glass samples were T1000 by Toshiba Ceramics shaped as thin plate about 0.04 m m thick. R a m a n spectra of silica glass in the range from 400 to 700 cm -1 were measured under pressures with small increases of pressure in the low pressure range. Several runs were carried out for different pieces. In the first run, a sample was pressurized to 12.6 GPa, and returned to atmospheric pressure. In the process of pressurizing, the sample was heated to 100°C at about 10.2 GPa. In the next run, another sample was pressurized to 20.4 GPa, and also returned to atmospheric pressure. In addition to these two runs, several series of measurements below 8 G P a were carried out on different pieces in order to observe changes in the R a m a n bands at low pressure. Several samples were densified using a pistoncylinder high pressure apparatus at pressures and temperatures up to 3.5 GPa and 900°C. The densities and the peak positions of the D 1 and D 2 were measured on the recovered samples. A mixture of diiodomethane and dichloromethane was used for the density measurement.
3. Results 2. Experimental A gasketted diamond-anvil cell (DAC) apparatus was used for pressurizing the sample. The pressure was estimated by the ruby scale [17].
Figure 1 shows the continuous changes in the R a m a n spectra of silica glass under pressures to about 8 GPa. Although the intensity of a R a m a n band was difficult to compare in different spectra, the results of several runs were reproducible
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a n d a r e s u m m a r i z e d in fig. 1. B e l o w 3.35 G P a , t h e intensity o f t h e m a i n b a n d i n c r e a s e s with p r e s s u r e in t h e c e n t e r o f this b a n d . Since t h e D 1 line displays no sign o f d i s a p p e a r i n g , t h e D 1 i n t e n s i t y i n c r e a s e s as well as t h e i n t e n s i t y in t h e m a i n b a n d . A b o v e 3.35 G P a , t h e intensity o f t h e m a i n b a n d b e g i n s to d e c r e a s e at low frequency. T h e m a i n b a n d s e e m s to b e c o m b i n e d with t h e D1 line a n d is i n d i s t i n g u i s h a b l e f r o m it. T h e D 2 b e c o m e s w e a k with i n c r e a s i n g p r e s s u r e as c o m p a r e d with t h e D v F i g u r e 2 shows t h e p r e s s u r e shifts of p e a k p o s i t i o n s in t h e r a n g e f r o m 400 to 700 cm - t . Open squares are the positions obtained during p r e s s u r i z a t i o n o f n o r m a l silica glasses. T h e c e n t e r of t h e m a i n b a n d shifts r a p i d l y t o w a r d high freq u e n c y with p r e s s u r e i n c r e a s e s u p to a b o u t 8 G P a , b u t g r a d u a l l y a p p r o a c h e s t h e D 1 line at h i g h e r p r e s s u r e . I n t h e s p e c t r a at l o w e r p r e s -
700
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Fig. 1. Continuous feature of the changes of the main band, D 1 and D 2 lines in the Raman spectrum of silica glass with pressure up to about 8 GPa.
sures, t h e D 1 was s e p a r a t e d f r o m t h e o v e r l a p o f t h e D 1 a n d t h e m a i n b a n d , a s s u m i n g t h a t the D 1 was a s i m p l e L o r e n t z i a n p e a k a n d t h e rest was t h e m a i n b a n d . A b o v e 8 G P a , t h e two p e a k s c o m b i n e d into a s y m m e t r i c p e a k w h i c h was insepa r a b l e . T h e c o m b i n e d p e a k shifts with p r e s s u r e a l o n g t h e line w h i c h r e p r e s e n t s t h e e x t e n s i o n of t h e shift o f D 1 at l o w e r p r e s s u r e s . T h e p r e s s u r e shift o f D 2 is small to a few G P a , b u t it m o n o t o n ically i n c r e a s e s at h i g h e r p r e s s u r e . T h e c h a n g e in t h e s p e c t r u m o f silica glass with c o m p r e s s i o n was, in g e n e r a l , n o t d r a s t i c b u t g r a d ual a n d c o n t i n u o u s . A d r a s t i c c h a n g e was ob-
H. Sugiura, T. Yamadaya / Raman scattering in silica glass
154
served after heat treatment under high pressure. Figure 3 shows the R a m a n spectrum of silica glass before and after heat treatment at 10.2 GPa. The pressure was reduced to 8.73 G P a in this treatment. The spectrum measured at 8.42 G P a before the treatment is also shown in fig. 3 for comparison. The important difference in these spectra is an enhanced band around 630 c m - 1 in the spectrum at 8.73 G P a after heat treatment. Two peaks are seen in the spectrum of silica glass above 8.73 G P a after that treatment. Figure 4 shows continuous features of these two peaks during pressure reduction after compression to 12.6 GPa. The positions of both peaks were also checked during pressure reduction as shown in fig. 2. The peak around 550 c m - 1 shifts along the line for the shift of the D 1 and returns to the vicinity of the initial position of D 1. On the low
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Fig. 3. Change of the R a m a n spectrum of silica glass with heat-treatment. 10.2 G P a and 8.73 G P a are pressures before and after heat treatment, respectively. 8.42 GPa is a step in pressurizing process before treatment.
frequency side of this peak, the main band grows with pressure reduction as shown in fig. 4. The intensity after compression is not as high as that under pressure for the entire spectrum. The other peak around 630 cm -1 returns almost to the initial position of D 2. The final spectrum is similar to that before compression. Figure 5 shows continuous features of the spectra measured in the second run in the pressure range from about 10 to 20 G P a and in the reducing process. Although the p e a k corresponding to D 2 is seen only as a shoulder in high frequency side of the combined peak, it is clearly found that the shoulder grows with pressure increases above 12 GPa, and the shoulder can be separated from the combined peak during pres-
155
H. Sugiura, T. Yamadaya / Raman scattering in silica glass
sure reduction. The peak positions during pressure reduction plotted in fig. 2 deviate from those obtained during pressurization. There is an increase on the low frequency side of the combined peak in the spectrum in fig. 5 measured as pressure fell to 0.02 GPa. Although it seems to be a weak feature, a change in curvature of the low frequency tail is clear. Such an increase is a trace of the main band and similar to the restoration of the main band shown in the first run. The intensities of the two peaks remain high after compression. The density of silica glasses recovered from the piston-cylinder apparatus depended on applied pressure, temperature and treatment time.
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Fig. 6. D I and D2 positions of pressurized silica glass in DAC (close squares) and recovered high density silica glasses (open squares) as functions of density. >co z
The maximum density was 2.45 M g / m 3. In several runs, low quartz crystallized in densified glass. In such cases, the center positions of the main bands were very close to the position of the low quartz peak. Figure 6 shows the peak positions of the D 1 and D 2 as functions of density. Open squares are densified and quenched glasses, while closed squares are results of in situ measurements in D A C up to 3.19 GPa in the reversible compression range. The densities of glass in D A C were estimated from the equation of state of silica glass after Kondo et al. [15].
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Fig. 5. Continuous feature of the change of the Raman spectrum of silica glass in compression from about 10 to 20 GPa and releasing process. The spectra are placed from bottom to top in order of measurements. The maximum pressure was 20.4 GPa.
4. Discussion The R a m a n spectra observed were rugged as compared with those after Hemley et al. [14]. Our
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conventional system for optical m e a s u r e m e n t was less favorable to D A C experiments than their system. Despite disadvantages in the measurement system, the continuous changes in the spectrum were much more intelligible especially in the low pressure region, maybe because of small pressurizing steps. Each spectrum in a common pressure range showed an almost equal feature. The main band is combined with the D 1 up to 8 GPa. Although the D 2 becomes weak up to 8 GPa, a corresponding band is enhanced with heat treatment (first run) or compression above 12 G P a (second run). Therefore, there are two remarkable bands in the R a m a n spectrum of silica glass in high pressure region. Such a feature is also found in fig. 2 which shows the shifts of p e a k positions. The main band moves close to D 1, and disappears from this figure, but the other two bands remain. All bands shown in fig. 2 except the main band can be classified into two segments. It is clear that these are respectively the D 1 and D2, although the D 1 is not pure D 1 but likely a combination with the main band. It is noted that the main band does not disappear, but becomes weak and indistinguishable from D 1. In the low pressure range, the spread of the main band shrinks, and the center position of this band shifts to higher frequency with increasing pressure. These changes in the main band are rapid compared with those in other peaks up to about 8 GPa, but are considered to slow down above 8 GPa. On the other hand, the shift of the D 1 with pressure is almost linear up to 20 GPa. The frequency of the main band cannot exceed that of Da; it is as if the main band is restricted by D 1. This suggests that these two bands associated with the same mode on similar bond units in different configurations. Although this result partly supports the ring model [7], the ring associated with D 1 does not decouple from the network but seems to constitute a type of network-like skeletal surface, which is a word used in ref. [16]. Such a feature resembles the model of domains glued together by interfacial material [18]. The D 2 e n h a n c e m e n t closely resembles that of neutron irradiation [12], which gives rise to densification. In the case of compression, the degree of densification depends on pressure and temper-
ature [10,11]. The compression of silica glass is reversible up to 8 G P a in argon [14], and 9.5 G P a in a methanol and ethanol mixture [13]. The limit of reversible and elastic compression must therefore be lower than 10 GPa, and pressure above 10 G P a is high enough to irreversibly densify, even if another substance is used as the pressure medium. Since heat treatment at 10.2 G P a accelerates densification, a result of which is the enhancement of the D2, it is clear that D 2 relates to the densification process. The other characteristic of D 2 is a weak dependence of p e a k position on pressure up to about 3 GPa. The structural unit related to D 2 is considered to be a part difficult to deform or a dangling part on the skeletal surface. If the former is, for example, a SiO 4 tetrahedron, it will be too difficult to fit the case. The part associated with the D 2 is a type of 'defect', which increases in the densification process. The feature in the intercluster layer is important for the structural change in the densification process. The increase of the 'defect' is, however, not considered to cause the increase in density. It is expected that the intercluster layer grows in the densification process. If the density of the intercluster layer is higher than that of cluster inside, the growth of the layer can be related to a permanent increase in density. A structural unit in that layer corresponds to D 1. Such a unit is not a new form at all, but existed in normal silica glass. The cluster in normal silica glass is said to be analogous to cristobalite, one of the silica polymorphs. Another form is expected to exist in the intercluster layer, that is a morphology analogous to coesite. The density of coesite is much higher than cristobalite. Although there is no evidence, it is interesting that R a m a n spectra of glass obtained from coesite are similar to that of densified silica glass [19]. If a form, analogous to coesite, grows in the intercluster layer, the unit corresponding to D 1 is likely to be the f o u r - m e m b e r e d ring, which is characteristic of coesite. The intensity of the main band is weak in densified silica glass. The main band is found as a weak shoulder on the combined peak in the spectrum at 0.02 G P a after compression to about 20
H. Sugiura, T. Yamadaya / Raman scattering in silica glass
GPa. The combined peak is still strong at 0.02 GPa. D1 is considered as the main constituent of this peak. Although the area of the cluster is reduced by the growth of the intercluster layer, the weakening of the main band and enhancement of the D 1 are not interpreted only by the change in size of each portion. The reason is that these phenomena are also observed in reversible compression. The growth of the layer gives a type of restriction to the cluster such as elastic compression and brings about deformation in both regions. Weakening of the main band and enhancement of D~ are related to deformations in the cluster and intercluster layer, where some types of selection rules possibly appear. The difference between compressed silica glass and densified silica glass is in cluster size and in the intercluster layer. Since there is expected to be a common situation for both glasses that the cluster and the intercluster layer are mutually restricted, two glasses are compared by a scale of density. The maximum density, 2.43 M g / m 3, of densified glass corresponds to compressed glass up to about 3.1. GPa. The main band almost remains in both cases. The shifts of D 1 and D 2 with density are approximately equal in both cases as shown in fig. 6. It can be said that the degree of restriction depends on density. The peak position of D 2 is almost independent of density in this range. This is the same as the effect of pressure on D 2 mentioned above. Quantitative agreements in slopes for both lines in fig. 6 were unexpected, but the important thing is the similarity of each tendency. There are large deviations in peak positions (see fig. 2) between pressurization and depressurization in the second run of D A C experiment up to about 20 GPa. Such deviations are understood to be the result of the difference in density brought about by the densification. If the density can be estimated during depressurization and the peak positions are plotted as a function of density, the result will coincide with that of pressurization. The extent of deviation is related to the degree of densification. Large deviations correspond to irreversible changes observed in the R a m a n spectrum.
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5. Conclusions Three R a m a n bands, the main band, the D 1 and the D2, were observed in the range from 400 to 700 cm-1. Although each band changed shape and position with compression, and the main band appeared to hide in the D 1 band, all remained under high pressure. No new bands were observed under high pressure. These are natural results for reversible compression, and are common to densification. Changes in the Raman bands brought about by change in density were almost independent of whether the density change was irreversible or reversible. Present results fit the structure model with cluster and intercluster layer or domain and inteffacial material. The clusters are surrounded by the intercluster layer. Densification is understood to be a result of growth of the intercluster layer, the density of which is higher than that of the cluster inside. There is a structural unit related to the D 1 mode in the intercluster layer. Such a unit is considered to play a role as skeleton in the glass. On the other hand, the unit related to the D 2 is likely to be a part dangling on the skeleton, because of insensitivity to compression at low pressure.
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[14] R.J. Hemley, H.K. Mao, P.M. Bell and B.O. Mysen, Phys. Rev. Lett. 57 (1986) 747. [15] K. Kondo, S. Iio and A. Sawaoka, J. Appl. Phys. 52 (1981) 2826. [16] J.C. Phillips, Phys. Rev. B33 (1986) 4443. [17] G.J. Piermarini, S. Block, J.D. Barnett and R.A. Forman, J. Appl. Phys. 46 (1975) 2774.
[18] C:S. Marians and J.K. Burdett, J. Non-Cryst. Solids 124 (1990) 1. [19] R.J. Hemley, in: High-Pressure Research in Mineral Physics, ed. M.H. Manghnani and Y. Shono (Terra Scientific, Tokyo, 1987) p. 347.