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Effect of high pressures on the structure of anhydrous and hydrated GeO2 glasses
Journal of Non-Crystalline Solids 71 (1985) 429-434 North-Holland, Amsterdam
429
EFFECT OF HIGH PRESSURES ON THE STRUCTURE OF ANHYDROUS AND HYDRATED GeO 2 GLASSES I. KUSHIRO t, Shiv K. SHARMA 2 and Dean W. MATSON 2 I Geological Institute, Universi O, of Tolo'o, Tokyo 113, Japan 2 ttawaii Institute of Geophysics, Universi(v of Hawaii, Honolulu, HI 96822, USA
The effect of high pressures on the structures of anhydrous and hydrated GeO 2 glasses, formed by quenching GeO 2 melts above the liquidus temperature and at P ~< 18 kbar. is investigated using normal and differential Raman spectroscopy. It is shown that at least up to 18 kbar Ge 4+ ions remain fourfold coordinated in GeO 2 melt, and both low and high pressure glasses contain predominantly six-membered rings of GeO 4 tetrahedra. Normal and differential Raman spectra of hydrated GeO 2 glasses clearly show formation of G e - O H bonds in the glass by the appearance of a polarized band at 760 cm 1. It is found that water preferentially attacks the defect sites responsible for the - 520 cm i band. The observed densification of high-pressure quenched GeO 2 glasses is attributed to a decrease in the volume of voids in the melt at high pressures. It is suggested that the observed anomalous decrease in the viscosity of the GeO 2 melt at high pressure results from the increase in the diffusivities of germanium and oxygen ions in the melt at high pressure.
1. Introduction
In recent years advancements in high-pressure technology have made it possible to measure the physical properties of oxide melts at high pressures (~< 40 kbar) and temperatures ( T ~ 1750°C). In addition, optically homogeneous glasses may be prepared by rapidly quenching melts from temperatures above the liquidus and under various pressures. Impetus for the present work was provided by interesting experimental observations which showed that, witfi increasing pressure at constant temperature, the viscosity of GeO2 [1] and several framework silicate and aluminosilicate melts decrease anomalously [2]. The densities (P) and refractive indices (n) of high-pressure quenched glasses of these computations increase with increasing preparation pressure [1-3]. In the geological literature [4] it was speculated that perhaps changes in the coordination of network-forming cations (e.g., A1, Si, Ge) at high pressures might be responsible for the observed changes in the physical properties of the melts at high pressures (for review see ref. [2]). Recent measurements of diffusivities of network-forming cations (Si 4+, AI 3+, Ge 4+ and Ga 3+) in the framework of oxide melts of NaAI(Si, Ge)206 and Na(A1, Ga)Si206 compositions [5], and of 180 tracer in the melt of NaA1Si206 composition [6] have led to the conclusion that diffusivities of network-forming cations and of oxygen ions increase with increasing pressure. These experimental diffusivity data are 0022-3093/85/$03.30 ~Q Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
430
I. Kushiro et al. / Structure of anhydrous and hydrated Ge02 glasses
consistent with the results of molecular dynamics calculations [7,8] and previous Raman spectroscopic measurements [1,9,10] that indicate there is no change in the coordination of network-forming cations at P 4 40 kbar. In the present work we have examined the effect of pressure on the structure of anhydrous and hydrated GeO 2 glasses (quenched melts) using normal and differential Raman spectroscopy.
2. Experimental methods Starting material for the high-pressure runs was an anhydrous GeO 2 glass prepared from high-purity GeO 2 heated at 1350°C for 24 h and subsequently quenched in air. The high-pressure experiments were made on the glass ( - 10 mg) in solid-media high-pr~sure apparatus [11] at 10, 15, and 18 kbar and 1700°C (for details see ref. [1]). The 18 kbar anhydrous GeO 2 glass shattered during quenching and was found to contain quench crystals. Hydrated high-pressure GeO 2 glass samples were prepared with appropriate amounts of distilled water added to the anhydrous GeO 2 glass in sealed platinum capsules. These capsules were pressurized in solid-media high-pressure appartus to 12.5 and 15 kbar, held at 1685°C for 5 min, and subsequently quenched under pressure. The hydrated high-pressure quenched GeO 2 glasses were found to contain some quenched crystals. We isolated small glass fragments from both anhydrous and hydrated GeO 2 samples under a binocular microscope for Raman studies. Raman spectra of both dry and hydrated GeO 2 glasses were studied using a Spex double monochromator (Model 1403) controlled by a Spex Datamate computer. With this system it is possible to make multiple scans of the spectra of small samples to improve signal-to-noise ratio, and to produce differential Raman spectra (for details of this system see ref. [12]). Sample excitation was achieved using the 488.0 nm radiation of an Ar + laser (Spectra Physics Model 165) with a source power of - 600 mW. All spectra were recorded with a 5 cm -~ resolution and a data interval of 2 cm -~. Scattered radiation was collected at 90 ° to the excitation beam. The spectra presented in this report are unpolarized as it was difficult in some cases to get good polarization data from small (~< 0.1 mm diameter) fragments of the high-pressure quenched GeO 2 glass samples.
3. Results and discussion Normal and differential Raman spectra of a dry 18 kbar-quenched GeO 2 glass sample are shown in fig. 1. The differential Raman spectra were produced by normalizing the 856 cm-1 band of the high-pressure GeO 2 glass with the corresponding band of the 1 atm GeO 2 glass, then substracting the digital spectra point by point. The differential Raman spectra are especially useful in
1. Kushiro et aL / Structure of anttvdrous and hydrated GeO 2 glasses
431
IZ
>,
'~
kbar, 1700°C
<
>,. ,~
~
z
!.1.1
..
I-Z
t
m
~
,
,
,
1350°C
/h,,..
~ , 100
a
' . V . ,
D,t[ . . . . . . (xS) 1. _ • ,, .... 1000
,
500
RAMAN
SHIFT [cm I ]
Fig. 1. Normal and differential Raman spectra of anhydrous GeO 2 glasses prepared at ambient and high pressures.
observing subtle changes in the high-pressure GeO 2 glass spectra caused by high pressure only or by the addition of H20 in the melt at high pressures (fig. 2). It is evident from fig. 1 that there are only minor differences in the Raman spectra of melt quenched at 1 atm and at 18 kbar. The strong and polarized band at 416 cm 1 in the spectrum of 1 atm GeO 2 glass corresponds to symmetric stretching of the bridging oxygen, ~ ( G e - O - G e ) , in the network. The slight negative intensity in the differential spectrum (fig. 1) of the 18 kbar glass near the us(Ge-O-Ge) band is probably caused by a slight shift in the position of this band in the spectrum of high-pressure glass toward higher
f--
J
n
~.5 wt. % H 2 0
~
..................
/t
1oo
I
I
~
I
5oo
5
I
I
R A M A N SHIFT
I
t
IOOO
2'.y .
I
I
.
.
.
.
.
I.__
(cm-1)
Fig. 2. Normal and differential Raman spectra of hydrated (1.5 wt% H20) GeO 2 and dry GeO 2 glasses prepared at 15 kbar and 1685°C.
432
1. Kushiro et al. / Structure of anhydrous and hydrated GeO2 glasses
frequency. Similar results were obtained with the spectra of GeO 2 glasses quenched from 10 and 15 kbar and are in agreement with a previous Raman study of high-pressure GeO 2 glasses [1]. The similarity between Raman spectra of GeO 2 melt quenched at 1 atm (1350°C) and high pressures (1700°C) implies that Ge ions remain fourfold coordinated in the high-pressure glasses. The densities of the 18 kbar and 1 atm quenched GeO 2 melts are (3.95 + 0.04) and (3.62 _+ 0.2) g cm ~, respectively, compared to a density of 4.28 g cm 3 for a-quartz form of GeO 2 [1]. Because there is no appreciable difference in the Raman spectra of low- and high-pressure quenched GeO 2 melts, the 9% permanent increase in the density of the 18 kbar glass must result from a decrease in the void space in the GeO 2 melt under high pressure. Similarly, the observed decrease in the viscosity of GeO 2 melt at 1425°C from 6.0 × 10 ~ poise at 1 bar to 1.2 × 103 poise at 9.5 kbar [1] most likely results from an increase in the diffusivities of Ge and oxygen ions in the melt at high pressures as has been found for a number of framework alumino- and gallo-silicate and -germanate glasses [5,6]. Measurements of the Raman spectra of isotope-substituted GeO 2 glasses have considerably improved our knowledge about the origins of various bands in the spectrum of GeO 2 glass [13]. As a result, the assignments of most of the Raman bands of GeO 2 glass are now reasonably well established. The exception is the assignment of the weak and polarized shoulder at - 520 cm 1, still a subject of discussion [14,15]. Recently it has been suggested that the weak and polarized shoulder at - 520 cm ~ in the spectrum of 1 atm GeO 2 glass may originate from threefold planar rings of GeO4 tetrahedra [15], If the 520 cm-~ band indeed originates from the three-membered planar rings, one would expect an increase in the intensity of this band in the spectra of high-pressure quenched glasses as the high pressure would favor denser packing. The intensity of the - 520 cm-1 shoulder does not show an appreciable change in intensity in the spectrum of 18 kbar quenched GeO 2 glass (fig. 1). On the other hand, the intensity of the - 520 cm ~ band is found to increase with neutron irradiation [16], as well as with increasing temperature above Tg, the glass transition temperature [12]. It, therefore, appears that the - 5 2 0 cm-1 glass band most likely originates from thermally induced network defect structures involving partially broken G e - O - G e bonds and that six-membered rings predominate in the GeO 2 glass (for a more detailed discussion see ref. [181). Differences between the Raman spectra (50-1400 cm ~) of dry and hydrated (1.5 wt% H 2 0 ) GeO z glasses prepared at 15 kbar and 1685°C are shown in fig. 2. Fig. 3 shows the Raman spectra of hydrated G e O 2 glasses prepared at 12.5 kbar and 1685°C, and containing 1.34 and 2.46 wt% H20. It can be seen from figs. 2 and 3(a) that a strong polarized band appears at 760 cm-~ and that the weak shoulder at ~ 520 cm-~ decreases in intensity in the spectra of hydrated GeO 2 glasses. In the O H stretching region (fig. 3(b)) the spectra of hydrated G e O 2 glasses containing 1.34 and 2.46 wt% H 2 0 show lines of identical shape that peak at - 3568 cm-~. The new features observed
1. Kushiro et al. / Structure of anh3drous and hydrated GeO 2 glasses
433
in the spectra of high-pressure quenched hydrated GeO 2 glasses are similar to those observed by Galeener and Geils [19] in the spectra of GeO 2 glass samples containing 53 and 870 ppm OH groups. The 760 c m - 1 band originates from G e - ( O H ) stretching in the network. The asymmetry in the shape of the band in the O - H stretching region of the spectra probably indicates that there are at least two types of G e - O H sites similar to those proposed by Walrafen [20] in connection with SiO 2 glass. The observed decrease in the intensity of the - 520 cm- ~ band of dry GeO 2 glass on hydration (figs. 2 and 3a) indicates that water preferentially attacks the defect sites in the GeO 2 melt responsible for the - 520 cm-~ band. The intensity of the 760 cm 1 band in the spectrum of GeO 2 glass prepared with 2.46 wt% H 2 0 at 12.5 kbar (fig. 3a) is about twice the intensity of the
<
2.5 kbar,1685°C 2.46wt.% H20
>E-
12.5 kbar, 1685°C L34wt.% H20
z
LLI I--" Z
] atr'n, 1350°C Dry
I
I
I
1
1
I
I
5O0
100
I
I
I
I
I
I
1000
RAMAN
SHIFT [ c m -1]
%
~5 ''~'~'t2.5 kbar,1685°C
z
UJ FZ
Dry I
3000
3500
4000
RAMAN SHIFT (cm -1]
Fig. 3. Raman spectra of dry GeO 2 glass prepared at 1 atm, 1350°C and hydrated GeO 2 glasses prepared at 12.5 kbar and 1685°C (a) in the low frequency region (50-1400 cm - t ) and (b) in the 3000-4000 cm-1 region. The arrow in (a) marks the 700 cm -1 band of rutile form of GeO 2 quench-crystals in the glass. The sloping background in the high-frequency region of the spectra of hydrated GeO 2 glasses results from sample fluorescence.
434
L Kushiro et al. / Structure of anhydrous and hydrated GeO 2 glasses
corresponding band in the spectrum of GeO 2 glass prepared with 1.34 wt% H20. This observation indicates that most of the water in the high-pressure hydrated GeO 2 glasses exists in the form of G e - O H groups and that as much as 2.46 wt% H 2 0 can be accommodated in the GeO 2 glass network at 12.5 kbar. 4. Conclusions Normal and differential Raman spectra of high-pressure quenched anhydrous and hydrated GeO 2 melts clearly show that, at least up to 18 kbar pressure, Ge remains fourfold coordinated in these melts. The observed increase in the density of the high-pressure GeO 2 glasses is attributed to decrease in the volume of voids in the GeO 2 melt at high pressures. In hydrated GeO 2 glasses most of the water exists in the form of G e - O H groups, and these groups give rise to a sharp, polarized band at 760 cm-% The asymmetry of O - H stretching bands in the spectra of hydrated GeO 2 glass is attributed to the pressure of at least two distinct sites of G e - O H groups in the network. Funding for the Spex Datamate computer was provided by NSF Grant EAR80-26091 (SKS). One of us (SKS) is grateful to Dr Charles E. Helsley, Director, Hawaii Institute of Geophysics, for his encouragement and for providing financial support. Hawaii Institute of Geophysics Contribution No. 1525. References [1] S.K. Sharma, D. Virgo and I. Kushiro, J. Non-Crystalline Solids 33 (1979) 235. [2] I. Kushiro, in: Physics of Magmatic Processes, ed., R.B. Hargraves (Princeton Univ. Press, 1980) p. 93; and refs. therein. [3] P.W. Bridgman and I. Simon, J. Appl. Phys. 24 (1953) 405. [4] H.S. Waft, Geophys. Res. Lett. 2 (1975) 193. [5] I. Kushiro, Geochim. Cosmochim. Acta 47 (1983) 1415. [6] N. Shimizu and I. Kushiro, Geochim. Cosmochim. Acta 48 (1984) 1295. [7] L.V. Woodcock, C.A. Angell and P. Cheesman, J. Chem. Phys. 65 (1976) 1565. [8] C.A. Angell, P.A. Cheesman and S. Tamaddon, Science 218 (1982) 885. [9] S.K. Sharma, D. Virgo and B.O. Mysen, Amer. Mineral. 64 (1979) 779. [10] S.K. Sharma and B. Simons, Amer. Mineral. 66 (1981) 118. [11] F.R. Boyd and J.L. England, J. Geophys. Res. 65 (1960) 741. [12] D.W. Matson, S.K. Sharma and J.A. Philpotts, J. Non-Crystalline Solids 58 (1983) 323. [13] F.L. Galeener, A.E. Geissberger, G.W. Ogan Jr, and R.E. Loeman, Phys. Rev. B28 (1983) 4768. [14] J.C. Phillips, Solid St. Phys. 37 (1982) 93. [15] F.L. Galeener, Solid St. Commun. 44 (1982) 1037. [16] F.L. Galeener, J. Non-Crystalline Solids 40 (1980) 527. [17] F.A. Seifert, B.O. Mysen and D. Virgo, Geochim. Cosmochim. Acta 45 (1981) 1870. [18] S.K. Sharma, D.W. Matson and J.A. Philpotts, J. Non-Crystalline Solids (1984) in press. [19] F.L. Galeener and R.H. Geils, in: The Structure of Non-Crystalline Materials, ed., P.H. Gaskell (Taylor and Francis, London, 1977) p. 223. [20] G.E. Walrafen, J. Chem. Phys. 62 (1975) 297.
429
EFFECT OF HIGH PRESSURES ON THE STRUCTURE OF ANHYDROUS AND HYDRATED GeO 2 GLASSES I. KUSHIRO t, Shiv K. SHARMA 2 and Dean W. MATSON 2 I Geological Institute, Universi O, of Tolo'o, Tokyo 113, Japan 2 ttawaii Institute of Geophysics, Universi(v of Hawaii, Honolulu, HI 96822, USA
The effect of high pressures on the structures of anhydrous and hydrated GeO 2 glasses, formed by quenching GeO 2 melts above the liquidus temperature and at P ~< 18 kbar. is investigated using normal and differential Raman spectroscopy. It is shown that at least up to 18 kbar Ge 4+ ions remain fourfold coordinated in GeO 2 melt, and both low and high pressure glasses contain predominantly six-membered rings of GeO 4 tetrahedra. Normal and differential Raman spectra of hydrated GeO 2 glasses clearly show formation of G e - O H bonds in the glass by the appearance of a polarized band at 760 cm 1. It is found that water preferentially attacks the defect sites responsible for the - 520 cm i band. The observed densification of high-pressure quenched GeO 2 glasses is attributed to a decrease in the volume of voids in the melt at high pressures. It is suggested that the observed anomalous decrease in the viscosity of the GeO 2 melt at high pressure results from the increase in the diffusivities of germanium and oxygen ions in the melt at high pressure.
1. Introduction
In recent years advancements in high-pressure technology have made it possible to measure the physical properties of oxide melts at high pressures (~< 40 kbar) and temperatures ( T ~ 1750°C). In addition, optically homogeneous glasses may be prepared by rapidly quenching melts from temperatures above the liquidus and under various pressures. Impetus for the present work was provided by interesting experimental observations which showed that, witfi increasing pressure at constant temperature, the viscosity of GeO2 [1] and several framework silicate and aluminosilicate melts decrease anomalously [2]. The densities (P) and refractive indices (n) of high-pressure quenched glasses of these computations increase with increasing preparation pressure [1-3]. In the geological literature [4] it was speculated that perhaps changes in the coordination of network-forming cations (e.g., A1, Si, Ge) at high pressures might be responsible for the observed changes in the physical properties of the melts at high pressures (for review see ref. [2]). Recent measurements of diffusivities of network-forming cations (Si 4+, AI 3+, Ge 4+ and Ga 3+) in the framework of oxide melts of NaAI(Si, Ge)206 and Na(A1, Ga)Si206 compositions [5], and of 180 tracer in the melt of NaA1Si206 composition [6] have led to the conclusion that diffusivities of network-forming cations and of oxygen ions increase with increasing pressure. These experimental diffusivity data are 0022-3093/85/$03.30 ~Q Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
430
I. Kushiro et al. / Structure of anhydrous and hydrated Ge02 glasses
consistent with the results of molecular dynamics calculations [7,8] and previous Raman spectroscopic measurements [1,9,10] that indicate there is no change in the coordination of network-forming cations at P 4 40 kbar. In the present work we have examined the effect of pressure on the structure of anhydrous and hydrated GeO 2 glasses (quenched melts) using normal and differential Raman spectroscopy.
2. Experimental methods Starting material for the high-pressure runs was an anhydrous GeO 2 glass prepared from high-purity GeO 2 heated at 1350°C for 24 h and subsequently quenched in air. The high-pressure experiments were made on the glass ( - 10 mg) in solid-media high-pr~sure apparatus [11] at 10, 15, and 18 kbar and 1700°C (for details see ref. [1]). The 18 kbar anhydrous GeO 2 glass shattered during quenching and was found to contain quench crystals. Hydrated high-pressure GeO 2 glass samples were prepared with appropriate amounts of distilled water added to the anhydrous GeO 2 glass in sealed platinum capsules. These capsules were pressurized in solid-media high-pressure appartus to 12.5 and 15 kbar, held at 1685°C for 5 min, and subsequently quenched under pressure. The hydrated high-pressure quenched GeO 2 glasses were found to contain some quenched crystals. We isolated small glass fragments from both anhydrous and hydrated GeO 2 samples under a binocular microscope for Raman studies. Raman spectra of both dry and hydrated GeO 2 glasses were studied using a Spex double monochromator (Model 1403) controlled by a Spex Datamate computer. With this system it is possible to make multiple scans of the spectra of small samples to improve signal-to-noise ratio, and to produce differential Raman spectra (for details of this system see ref. [12]). Sample excitation was achieved using the 488.0 nm radiation of an Ar + laser (Spectra Physics Model 165) with a source power of - 600 mW. All spectra were recorded with a 5 cm -~ resolution and a data interval of 2 cm -~. Scattered radiation was collected at 90 ° to the excitation beam. The spectra presented in this report are unpolarized as it was difficult in some cases to get good polarization data from small (~< 0.1 mm diameter) fragments of the high-pressure quenched GeO 2 glass samples.
3. Results and discussion Normal and differential Raman spectra of a dry 18 kbar-quenched GeO 2 glass sample are shown in fig. 1. The differential Raman spectra were produced by normalizing the 856 cm-1 band of the high-pressure GeO 2 glass with the corresponding band of the 1 atm GeO 2 glass, then substracting the digital spectra point by point. The differential Raman spectra are especially useful in
1. Kushiro et aL / Structure of anttvdrous and hydrated GeO 2 glasses
431
IZ
>,
'~
kbar, 1700°C
<
>,. ,~
~
z
!.1.1
..
I-Z
t
m
~
,
,
,
1350°C
/h,,..
~ , 100
a
' . V . ,
D,t[ . . . . . . (xS) 1. _ • ,, .... 1000
,
500
RAMAN
SHIFT [cm I ]
Fig. 1. Normal and differential Raman spectra of anhydrous GeO 2 glasses prepared at ambient and high pressures.
observing subtle changes in the high-pressure GeO 2 glass spectra caused by high pressure only or by the addition of H20 in the melt at high pressures (fig. 2). It is evident from fig. 1 that there are only minor differences in the Raman spectra of melt quenched at 1 atm and at 18 kbar. The strong and polarized band at 416 cm 1 in the spectrum of 1 atm GeO 2 glass corresponds to symmetric stretching of the bridging oxygen, ~ ( G e - O - G e ) , in the network. The slight negative intensity in the differential spectrum (fig. 1) of the 18 kbar glass near the us(Ge-O-Ge) band is probably caused by a slight shift in the position of this band in the spectrum of high-pressure glass toward higher
f--
J
n
~.5 wt. % H 2 0
~
..................
/t
1oo
I
I
~
I
5oo
5
I
I
R A M A N SHIFT
I
t
IOOO
2'.y .
I
I
.
.
.
.
.
I.__
(cm-1)
Fig. 2. Normal and differential Raman spectra of hydrated (1.5 wt% H20) GeO 2 and dry GeO 2 glasses prepared at 15 kbar and 1685°C.
432
1. Kushiro et al. / Structure of anhydrous and hydrated GeO2 glasses
frequency. Similar results were obtained with the spectra of GeO 2 glasses quenched from 10 and 15 kbar and are in agreement with a previous Raman study of high-pressure GeO 2 glasses [1]. The similarity between Raman spectra of GeO 2 melt quenched at 1 atm (1350°C) and high pressures (1700°C) implies that Ge ions remain fourfold coordinated in the high-pressure glasses. The densities of the 18 kbar and 1 atm quenched GeO 2 melts are (3.95 + 0.04) and (3.62 _+ 0.2) g cm ~, respectively, compared to a density of 4.28 g cm 3 for a-quartz form of GeO 2 [1]. Because there is no appreciable difference in the Raman spectra of low- and high-pressure quenched GeO 2 melts, the 9% permanent increase in the density of the 18 kbar glass must result from a decrease in the void space in the GeO 2 melt under high pressure. Similarly, the observed decrease in the viscosity of GeO 2 melt at 1425°C from 6.0 × 10 ~ poise at 1 bar to 1.2 × 103 poise at 9.5 kbar [1] most likely results from an increase in the diffusivities of Ge and oxygen ions in the melt at high pressures as has been found for a number of framework alumino- and gallo-silicate and -germanate glasses [5,6]. Measurements of the Raman spectra of isotope-substituted GeO 2 glasses have considerably improved our knowledge about the origins of various bands in the spectrum of GeO 2 glass [13]. As a result, the assignments of most of the Raman bands of GeO 2 glass are now reasonably well established. The exception is the assignment of the weak and polarized shoulder at - 520 cm 1, still a subject of discussion [14,15]. Recently it has been suggested that the weak and polarized shoulder at - 520 cm ~ in the spectrum of 1 atm GeO 2 glass may originate from threefold planar rings of GeO4 tetrahedra [15], If the 520 cm-~ band indeed originates from the three-membered planar rings, one would expect an increase in the intensity of this band in the spectra of high-pressure quenched glasses as the high pressure would favor denser packing. The intensity of the - 520 cm-1 shoulder does not show an appreciable change in intensity in the spectrum of 18 kbar quenched GeO 2 glass (fig. 1). On the other hand, the intensity of the - 520 cm ~ band is found to increase with neutron irradiation [16], as well as with increasing temperature above Tg, the glass transition temperature [12]. It, therefore, appears that the - 5 2 0 cm-1 glass band most likely originates from thermally induced network defect structures involving partially broken G e - O - G e bonds and that six-membered rings predominate in the GeO 2 glass (for a more detailed discussion see ref. [181). Differences between the Raman spectra (50-1400 cm ~) of dry and hydrated (1.5 wt% H 2 0 ) GeO z glasses prepared at 15 kbar and 1685°C are shown in fig. 2. Fig. 3 shows the Raman spectra of hydrated G e O 2 glasses prepared at 12.5 kbar and 1685°C, and containing 1.34 and 2.46 wt% H20. It can be seen from figs. 2 and 3(a) that a strong polarized band appears at 760 cm-~ and that the weak shoulder at ~ 520 cm-~ decreases in intensity in the spectra of hydrated GeO 2 glasses. In the O H stretching region (fig. 3(b)) the spectra of hydrated G e O 2 glasses containing 1.34 and 2.46 wt% H 2 0 show lines of identical shape that peak at - 3568 cm-~. The new features observed
1. Kushiro et al. / Structure of anh3drous and hydrated GeO 2 glasses
433
in the spectra of high-pressure quenched hydrated GeO 2 glasses are similar to those observed by Galeener and Geils [19] in the spectra of GeO 2 glass samples containing 53 and 870 ppm OH groups. The 760 c m - 1 band originates from G e - ( O H ) stretching in the network. The asymmetry in the shape of the band in the O - H stretching region of the spectra probably indicates that there are at least two types of G e - O H sites similar to those proposed by Walrafen [20] in connection with SiO 2 glass. The observed decrease in the intensity of the - 520 cm- ~ band of dry GeO 2 glass on hydration (figs. 2 and 3a) indicates that water preferentially attacks the defect sites in the GeO 2 melt responsible for the - 520 cm-~ band. The intensity of the 760 cm 1 band in the spectrum of GeO 2 glass prepared with 2.46 wt% H 2 0 at 12.5 kbar (fig. 3a) is about twice the intensity of the
<
2.5 kbar,1685°C 2.46wt.% H20
>E-
12.5 kbar, 1685°C L34wt.% H20
z
LLI I--" Z
] atr'n, 1350°C Dry
I
I
I
1
1
I
I
5O0
100
I
I
I
I
I
I
1000
RAMAN
SHIFT [ c m -1]
%
~5 ''~'~'t2.5 kbar,1685°C
z
UJ FZ
Dry I
3000
3500
4000
RAMAN SHIFT (cm -1]
Fig. 3. Raman spectra of dry GeO 2 glass prepared at 1 atm, 1350°C and hydrated GeO 2 glasses prepared at 12.5 kbar and 1685°C (a) in the low frequency region (50-1400 cm - t ) and (b) in the 3000-4000 cm-1 region. The arrow in (a) marks the 700 cm -1 band of rutile form of GeO 2 quench-crystals in the glass. The sloping background in the high-frequency region of the spectra of hydrated GeO 2 glasses results from sample fluorescence.
434
L Kushiro et al. / Structure of anhydrous and hydrated GeO 2 glasses
corresponding band in the spectrum of GeO 2 glass prepared with 1.34 wt% H20. This observation indicates that most of the water in the high-pressure hydrated GeO 2 glasses exists in the form of G e - O H groups and that as much as 2.46 wt% H 2 0 can be accommodated in the GeO 2 glass network at 12.5 kbar. 4. Conclusions Normal and differential Raman spectra of high-pressure quenched anhydrous and hydrated GeO 2 melts clearly show that, at least up to 18 kbar pressure, Ge remains fourfold coordinated in these melts. The observed increase in the density of the high-pressure GeO 2 glasses is attributed to decrease in the volume of voids in the GeO 2 melt at high pressures. In hydrated GeO 2 glasses most of the water exists in the form of G e - O H groups, and these groups give rise to a sharp, polarized band at 760 cm-% The asymmetry of O - H stretching bands in the spectra of hydrated GeO 2 glass is attributed to the pressure of at least two distinct sites of G e - O H groups in the network. Funding for the Spex Datamate computer was provided by NSF Grant EAR80-26091 (SKS). One of us (SKS) is grateful to Dr Charles E. Helsley, Director, Hawaii Institute of Geophysics, for his encouragement and for providing financial support. Hawaii Institute of Geophysics Contribution No. 1525. References [1] S.K. Sharma, D. Virgo and I. Kushiro, J. Non-Crystalline Solids 33 (1979) 235. [2] I. Kushiro, in: Physics of Magmatic Processes, ed., R.B. Hargraves (Princeton Univ. Press, 1980) p. 93; and refs. therein. [3] P.W. Bridgman and I. Simon, J. Appl. Phys. 24 (1953) 405. [4] H.S. Waft, Geophys. Res. Lett. 2 (1975) 193. [5] I. Kushiro, Geochim. Cosmochim. Acta 47 (1983) 1415. [6] N. Shimizu and I. Kushiro, Geochim. Cosmochim. Acta 48 (1984) 1295. [7] L.V. Woodcock, C.A. Angell and P. Cheesman, J. Chem. Phys. 65 (1976) 1565. [8] C.A. Angell, P.A. Cheesman and S. Tamaddon, Science 218 (1982) 885. [9] S.K. Sharma, D. Virgo and B.O. Mysen, Amer. Mineral. 64 (1979) 779. [10] S.K. Sharma and B. Simons, Amer. Mineral. 66 (1981) 118. [11] F.R. Boyd and J.L. England, J. Geophys. Res. 65 (1960) 741. [12] D.W. Matson, S.K. Sharma and J.A. Philpotts, J. Non-Crystalline Solids 58 (1983) 323. [13] F.L. Galeener, A.E. Geissberger, G.W. Ogan Jr, and R.E. Loeman, Phys. Rev. B28 (1983) 4768. [14] J.C. Phillips, Solid St. Phys. 37 (1982) 93. [15] F.L. Galeener, Solid St. Commun. 44 (1982) 1037. [16] F.L. Galeener, J. Non-Crystalline Solids 40 (1980) 527. [17] F.A. Seifert, B.O. Mysen and D. Virgo, Geochim. Cosmochim. Acta 45 (1981) 1870. [18] S.K. Sharma, D.W. Matson and J.A. Philpotts, J. Non-Crystalline Solids (1984) in press. [19] F.L. Galeener and R.H. Geils, in: The Structure of Non-Crystalline Materials, ed., P.H. Gaskell (Taylor and Francis, London, 1977) p. 223. [20] G.E. Walrafen, J. Chem. Phys. 62 (1975) 297.