Abstract The densification processes in SiO2 glass in- duced by shock-wave compression up to 43.4 GPa are in- vestigated by Raman spectroscopy. At first ...
Springer-Verlag 1999
Phys Chem Minerals (1999) 26:304±311
ORIGINAL PAPER
M. Okuno ´ B. Reynard ´ Y. Shimada ´ Y. Syono C. Willaime
A Raman spectroscopic study of shock-wave densification of vitreous silica
Received: March 30, 1998 / Revised, accepted: August 21, 1998
Abstract The densification processes in SiO2 glass induced by shock-wave compression up to 43.4 GPa are investigated by Raman spectroscopy. At first, densification increases with increasing shock pressure. A maximum densification of 11% is obtained for a shock pressure of 26.3 GPa. This densification is attributed to the reduction of the average SiOSi angle, which occurs first by the collapse of the largest ring cavities, then by further reduction of the average ring size. For higher shock pressures, a different structural modification is observed, resulting in decreasing densification with increasing shock pressure. Indeed, the recovered densification becomes very small, with values of 1.8 and 0.5% at 32 and 43.4 GPa, respectively. This is attributed to partial annealing of the samples due to high after shock residual temperatures. The study of the annealing process of the most densified glass by in situ high temperature Raman spectroscopy confirms that relaxation of the SiOSi angle starts at a lower temperature (about 800 K) than that of the siloxane rings (about 1000 K), thus explaining the high intensity of the siloxane defect bands in the samples schocked at compressions of 32 and 43.4 GPa. The large intensity of the siloxane bands in the nearly undensified samples shocked by compressions above 30 GPa may be explained by the relaxation during decompression of five- and six-fold coordinated silicon species formed at high pressure and high temperature during the shock event.
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M. Okuno ( ) ´ Y. Shimada Department of Earth Sciences, Faculty of Science, Kanazawa University, Kanazawa, 920-1192, Japan B. Reynard1 ´ C. Willaime GØosciences Rennes CNRS UPR 4661, UniversitØ de Rennes 1, F-35042 Rennes Cedex, France Y. Syono Institute for Material Research, Tohoku University, Sendai, 980-8577, Japan Present address: Laboratoire de Sciences de la Terre, CNRS UMR 5570, Ecole Normale SupØrieure, 46 AllØe d'Italie, F-69364 Lyon cedex 07, France
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Key words SiO2 glass ´ Shock compression ´ Densified glass ´ Raman spectroscopy
Introduction The structure and physical properties of silicate melts and glasses under high pressure and high temperature conditions are of interest for various fields such as earth sciences and material sciences. SiO2 glass can be permanently densified under high pressure conditions (Bridgeman and Simon 1959; Roy and Cohen 1961; Mackenzie 1963) and by neutron irradiation (Bates et al. 1974). The structure of densified SiO2 glasses and the densification processes have been inferred from X-ray diffraction measurements (Couty 1977; Yamana 1980), Raman spectroscopy (Walrafen and Krishnan 1981; McMillan et al. 1984; Williams et al. 1993) and XANES spectroscopy (Davoli et al. 1992). These studies have provided evidence for a decrease in the mean SiOSi angle in the permanently densified SiO2 glass, which are further substantiated by molecular dynamics calculations (Kubicki and Lasaga 1988) and LCAO calculations (Murray and Ching 1989). However, the densification varies with pressure, temperature and technical conditions (Mackenzie 1963). Recently, in situ high temperature and high pressure measurement techniques have been developed for structural studies of amorphous solids. Under static compression, Hemley et al. (1986) reported irreversible changes in the Raman spectrum between 8 and 30 GPa and a breakdown in the medium-range order structure under higher pressure (>30 GPa). Meade et al. (1992) measured the X-ray diffraction intensities of SiO2 glass up to 42 GPa in the DAC and reported that the coordination of Si increase from four at 8 GPa to close to six at 42 GPa. The study of the structural change of SiO2 glass induced by shock-wave compression is important for understanding of the formation of impact craters on the earth, and provides a simple model for more complex terrestrial materials. In particular, this information can aid in elucidating the shock pressure and temperature and residual
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temperature during impacts. Therefore, a detailed scheme of the processes involved in SiO2 glass densification and relaxation is required in order to fully understand its behaviour under dynamic compression. However, only a few structural studies on shock-densified glasses have been carried out so far (Arndt et al. 1971; Mashimo et al. 1980). In this study, we investigate the densification processes and their high-temperature annealing in experimentally shock-subjected SiO2 glass by Raman spectroscopy and show that the recovered structures and properties can be simply rationalized in terms of the PT paths followed by the sample during the shock.
Experimental Shock-wave experiments A commercially available synthetic transparent SiO2 glass rod was cut in thin plates (10 mm in diameter, 2 mm in thickness), and was used as starting material for shock experiments. These samples were encased in a stainless steel container. Shock-wave experiments were performed by using a single stage propellant gun (25 mm bore and 4 m length; Goto and Syono 1984). The specimen container was hit by a stainless steel flyer, which was accelerated to a velocity of up to about 2.0 km/s. Five shock-wave experiments were performed with shock pressures of 17.8, 21.8, 26.3, 32.0 and 43.4 GPa. These pressures were estimated from measured projectile velocities, using the impedance matching method. The samples were recovered from peak pressures of 17.8 and 21.8 GPa as powder like aggregates, from 26.3 GPa as white non-transparent block, and from 32.0 and 43.4 GPa as semitransparent blocks. Refractive index and density measurements The refractive indices (n) of shock-wave densified SiO2 glasses (densified glass) and non densified SiO2 glass (normal glass) were measured by the immersion method and by observation of Becke line with a microscope. This method gives index values with an accuracy of about 1104. Densities (r) were determined by the suspension method in mixtures of CHI2 and acetone. Measured refractive indices and densities are listed in Table 1. Raman spectroscopy Raman spectra were recorded using a Dilor XY double subtractive spectrograph equipped with a premonochromator (1200 g/mm holographic gratings) and ORTEC CCD nitrogen-cooled detector. The 514.5 nm line (green) of a Spectra Physics Ar+ laser was used to excite Raman scattering. The Ar+ laser light was focused onto the sample through a Leitz UTK40 objective with a working distance of 14 mm and numerical aperture of 0.32 of a modified Olympus BH-2 Table 1 Refractive indices (n) and densities (r) of unshocked and shock-densified SiO2 glasses
petrographic microscope. Raman light was collected in the back scattering geometry through the same objective and focused in the spectrometer through a 100 m slit defining a spectral resolution of about 10 m, giving a band pass of about 2.5 cm1. Operating laser power was 200 mW and counting time was 120 s. Raman spectra in the range 115±1200 and 400±1400 cm1 were measured for each specimen. Raman spectra were obtained at high temperatures using a Leitz 1350 heating stage. Temperature was monitored with a PtPt/ Rh10% thermocouple calibrated against known melting point compounds. The accuracy of measured temperature is estimated as 5 K. The spectra of SiO2 sample subjected to shock at 26.3 GPa were recorded in the range 115±1100 cm1 at up to 1177 K. At each temperature step, the spectrum was registered after 10±20 min of keeping the temperature. Raman spectrum was measured at each temperature. Because most of the shock-subjected samples present a strong fluorescent background due to the existence of broken SiO bond, the fluorescence was corrected by subtracting a smooth polynomial function fitted on high frequency regions where no Raman signal exists. All the observed Raman spectra were then corrected for the temperature and frequency dependence of the first order (Stokes) Raman scattering (Long 1977; Piriou and Alain 1979; Mysen et al. 1982; McMillan et al. 1994), using the following correction relation: Icorr Iobs w
w0 ÿ wÿ4 1 ÿ exp
ÿhw=kB T
1
where wo is the wave number of the incident laser light (19435 cm1 for the green Ar+ laser line), w the Raman shift, kB is Bortzmann's constant, and T the absolute temperature (K).
Results Refractive index and density The variations of refractive index and density with shock pressure are depicted in Fig. 1. These two variations are very similar, as expected from the close relationship between density and refractive index. Densification of SiO2 glass increases rapidly with shock pressures above about 15 GPa and reaches a maximum (about 11%) at about 25 GPa before decreasing rapidly at higher shock pressures. For 43.4 GPa, the density has dropped back to almost the same density as normal glass. The maximum density increment (Dr/ro =11.0% for 26.3 GPa) is very similar to the value (10.3%) obtained by Arndt et al. (1971) on SiO2 glass densified by shock compression. It is intermediate between of the values of about 20% obtained by static compression at 530 C and 8 GPa (Couty and Sabatier 1978) or at room temperature and 15 GPa (Cohen and Roy 1965), and the density increase of 4% (Simon 1957) and 2.38% (Bates et al. 1974) observed on SiO2 glass densified by fast neutron irradiation.
Pressure (GPa)
n
Dn/n0 (%)
r
Dr/r0 (%)
Room temperature Raman spectra
0.0001 17.8 21.8 26.3 32.0 43.4
1.4588 (no ) 1.4781 1.5030 1.5059 1.4672 1.4610
0 1.32 3.02 3.23 0.58 0.15
2.18 (r0 ) 2.26 2.39 2.42 2.22 2.19
0 3.7 9.6 11.0 1.8 0.5
The correct spectra of glasses subjected to shock and those not are shown in Fig. 2 (hereafter shocked and unshocked glasses). The Raman spectrum of the starting material is consistent with the spectra previously published (e.g. Seifert et al. 1982; McMillan et al. 1994). Typical broad bands of amorphous materials are observed at
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Fig. 1 a Refractive index and b density of SiO2 glass as a function of shock pressure. Solid circle: present work; open circle: Arndt et al. (1971)
around 455, 800, 1060 and 1200 cm1, onto which are superimposed two sharp peaks at 490 and 602 cm1, the ªdefectº bands denoted D1 and D2 bands, respectively. With increasing shock pressure, the Raman bands display significant variations in both their frequencies, widths and relative intensities. The frequency of the broad and intense band (at 455 cm1 in unshocked SiO2 glass) reaches a maximum value of about 500 cm1 for 26.3 GPa (Fig. 3a) before decreasing to 470 cm1 for 43.4 GPa, its position in the intermediate sample shocked at 32 GPa being ill-defined due to overlapping with the intense D1 band. The peak width of this asymmetric band is inversely correlated with its frequency (Fig. 3b). The frequency of the D1 band at 490 cm1 shows, with increasing shock pressure, a positive shift of 12 cm1 (Fig. 3c). This is much smaller than that of the broad 455 cm1 band (about 45 cm1). The frequency of 602 cm1 D2 band also increases (Fig. 3d), with an even smaller frequency shift (9 cm1). The most important variation for these two bands is their strong increase in relative intensity for shock pressures up to 32 GPa (Fig. 3e). This is followed by a slight intensity decrease in the sample shocked at 43.4 GPa, but the intensity remains higher than in the starting material.
Fig. 2a, b Raman spectra of shock densified SiO2 glass. a 115± 1000 cm1 frequency region; b high frequency region
The frequencies of the highest energy bands decrease with increasing shock pressure. The shift is small for the band near 800 cm1. It is larger for the bands at 1060 and 1200 cm1 and is accompanied by a broadening and decrease in intensity. However, the detailed evolution of these two bands is difficult to follow due to their weak intensity. High temperature Raman spectra High temperature Raman spectra of the sample shocked at 26.3 GPa are shown in Fig. 4a. The most important changes with increasing temperature are observed in the region below 700 cm1, as evidenced from the difference spectra, obtained by subtracting the Raman spectra at room temperature to those at high temperatures (Fig. 4b). In particular, the broad band near 500 cm1 shifts down to about 470 cm1 at 1100 K and the intensities of the D1 and D2 bands decrease. The spectrum of the sample quenched from 1177 K is indistinguishable from that of the starting material, indicating that most of the structural changes induced by shock compression have been annealed during heating. The temperature-induced changes are not uniform and become large above 800
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Fig. 3a±e Variations of Raman frequency, peak width and intensity as a function of shock pressure. a Frequency and b peak width of the broad 455 cm1 band; c frequency of the D1 band; d frequency and e intensity of the D2 band
K. The temperature variations of frequency of the broad 500 cm1 band and intensity of the 606 cm1 band are shown in Fig. 5. The frequency of the broad band shows almost no change up to 770 K and rapidly decreases above this temperature, while the intensity of the D2 band decreases rapidly above about 900 K.
Discussion Pressure dependences of refractive index and density The variation of refractive index with shock pressure (Fig. 1a) is similar in shape to that reported by Arndt et al.
(1971). However, the shock pressure which gives a maximum index value in this work is higher (26.3 GPa) than that of 13.5 GPa reported by Arndt et al. (1971). This discrepancy was probably caused by the difference in the sample assemblies. We used samples of 10 mm in diameter and 2 mm thick while their samples are 20 mm higher and 10 mm in diameter. Arndt et al. (1971) have shown that the increase in refractive index was not homogeneous in their samples due to inhomogeneous schock wave propagation in their thick samples, while we find homogeneous material after shock in our runs. This can well explain the discrepancy, as Mashimo et al. (1980) have reported similar effects in a-quartz. In addition to measuring the variations of refractive index with shock pressure,
308 Fig. 4 a High temperature Raman spectra of SiO2 glass densified at 26.3 GPa. b Difference spectra obtained by subtracting the ambient temperature spectrum before heating from the high temperature Raman spectra. (25 C* shows spectrum after heating)
Arndt et al. (1971) have also reported the annealing effects at room pressure on a sample densified by about 10%. They have reported a rapid decrease (within about one minute) of refractive index at 773 K before it reaches a stable value after about 5 to 10 min, but it is only for annealing temperatures of 1173 K that the refractive index of the sample goes back to that of the starting material. The high temperature and relatively large times (about 1 min) required to recover the original refractive index account for the densification versus shock pressure curve. A first regime is that where increasing densities are recovered with increasing shock pressure due to shock compression and relatively low residual temperatures. A second regime is that where decreasing densities are recovered with higher shock pressures, due to high residual temperatures that allow the recovery of the sample at amient pressure after the shock. From the qualitative agreement between our results and those of Arndt et al. (1971), we can assume that similar mechanisms can account for the observed density versus shock pressure evolution in spite of the pressure measurement discrepancies discussed. This explanation is further substantiated by the structural mechanisms inferred from our Raman spectroscopic results discussed later. Structural evolution in shock-densified SiO2 glasses
Fig. 5a, b Variations of Raman frequency and intensity as a function of annealing temperature. a Frequency of the broad band; b intensity of D2 band (ini: initial state, fin: after heating state, non: non shocked state)
We first summarize the current understanding of the room temperature Raman spectrum of SiO2 glass, which has been extensively studied (see also McMillan et al. 1994; Mysen 1988). From isotopic substitution experiments (Galeener and Mikkelsen 1981; Galeener and Geissberger 1983), the major broad Raman band near 450 cm1 is inferred to involve mainly oxygen displacement and generally assigned to a symmetric oxygen vibration of the bent SiOSi linkages, with oxygen motion perpendicular to the Si ´´´ Si line (Galeener and Geissberger 1983; Sharma et al. 1984; McMillan and Hess 1990). It is also generally accepted that the sharp bands at 490 cm1 (D1 ) and 602
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cm1 (D2 ) correspond to the symmetric oxygen breathing vibration of four- and three-membered siloxane rings of SiO4 tetrahedra, respectively, embedded as ªdefectsº within the glass structure (Sharma et al. 1981; Galeener 1982a, b; O'Keeffe and Gibbs 1984; Warren et al. 1991; Kubicki and Sykes 1993; Sykes and Kubicki 1996). The band at 800 cm1 is assigned to the SiO stretching vibration with dominant Si motion (Mysen 1988). In the higher frequency region (n>900 cm1), weak Raman band at 1060 cm1 and about 1200 cm1 are assigned to asymmetric SiO stretching vibration. The broad band near 455 cm1 shows the most important variations in frequency with shock pressure. As mentioned, this band is due to oxygen breathing vibration of SiOSi linkage within large rings of SiO4 tetrahedra in the glass framework. There is generally an inverse correlation between the frequency of this band and the SiOSi angle in condensed silicate (Galeener and Sen 1978; McMillan 1984), and its width is considered to reflect the breadth of the SiOSi angle distribution. Therefore, the pressure variations of frequency and peak width of this band indicate a decrease of the mean SiOSi angle and its distribution width with shock pressure up to 26.3 GPa. Assuming a relation of approximately 1 change in SiOSi angle for 10 cm1 shift (Lazarev 1972; Sen and Thorpez 1977; Galeener and Sen 1978), this implies that the average SiOSi angle changed from 144 in the starting material to 140 in the sample shocked at 26.3 GPa. This is smaller than the change from 144 to 136 determined by Shimada et al. (1996) on the same samples by X-ray diffraction. The latter angular variation is similar to that determined by Couty (1977) on a glass densified by 15.1% at 6.25 GPa and 530 C. He found that SiO and OO distance remained constant but SiSi distance decreased from 3.07 to 3.02 on densification, with resulting decrease in average SiOSi angle from 144 to 137 . Because the variations in frequency of the broad band near 450 cm1 follow roughly those of the refractive index and density, the variation of the SiOSi angle is the main mechanism for glass densification in our glasses. Our results suggest, however, that the frequency change associated with the SiOSi angle decrease is smaller than formerly proposed (about 5.5 cm1 per degree instead of 10 cm1). Given the associated density increase of 11% for a shock pressure of 26.3 GPa (Table 1), the relative density (r) change with band frequency (n, in cm1) is obtained as ¶lnr/¶n»2.81´103 cm. This can be compared with the value of 2.2´103 cm obtained using the angle change observed by Couty (1977) for a densification of 15% and the frequency-angle dependence of 10 cm1 per degree commonly used (see earlier). Using the observed positive frequency shift with increasing temperature of the 450 cm1 band in SiO2 glass and supercooled liquid (McMillan et al. 1994), a negative thermal expansivity is expected of about 3 to 4´ 105 K1 in the glass (300±1500 K range) and 1 to 1.5´104 K1 in the supercooled liquid (1500±1950 K range). Such a negative thermal expansivity will counterbalance the positive one arising from SiO bond length
increase and from configurational changes (in the supercooled liquid), and can quantitatively account for the resulting low thermal expansivity of amorphous SiO2 in that temperature range and for the density maximum near 1500 C (e.g. Brückner 1970). As mentioned, the D1 band near 490 cm1 was assigned to four-membered siloxane rings by Sharma et al. (1981) and Galeener (1982a). The SiOSi angle for a planar four-membered ring lies near 160 , this value may be reduced by deformation of ring. However, the average SiOSi angle in such rings is more constrained than those of larger rings. This is consistent with the small frequency shift of this band, which indicates a slight decrease of the SiOSi angle of fourmembered rings with increasing shock pressure. The intensity of this band increases with shock pressure, although this effect is difficult to quantify with accuracy because of overlapping with the broad band which reaches similar frequencies at the highest density. The D2 band was assigned to three-membered siloxane rings. The SiOSi angles within these rings are constrained to have values near 130 by the almost constant SiO bond length and tetrahedral OSiO angle (Galeener 1982a). This is consistent with the very small frequency shift of this band. The relative intensity of this peak increases strongly with increasing shock pressures from 21.8 to 32.0 GPa (Fig. 3d). This indicates a strong increase of the number (or concentration) of three-membered siloxane rings with increasing shock pressure, which reaches a maximum for a shock pressure of 32.0 GPa before slightly decreasing for a shock pressure of 43.4 GPa. Actually, the behaviour of the D1 and D2 bands with increasing shock pressure is very similar, suggesting that they are formed by a similar mechanism. Finally, the highest frequency bands in the region of 1000±1300 cm1 display a behaviour which is consistent with the decrease in SiOSi angle inferred for the 455 cm1 band shift (Hemley et al. 1986). High-temperature relaxation mechanisms in densified SiO2 glass The spectra obtained at high-temperature on the sample shocked at 26.3 GPa allow to discuss the microscope mechanisms that are responsible for the relaxation of the densified glasses. Although we did not measure the density or the refractive index of the sample after heating, we are confident that this has evolved back to the value close to the density of the starting material, because (1) the Raman spectrum of the quenched sample is similar to that of the starting sample and (2) Arndt et al. (1971) have reported that the refractive index of shocked SiO2 glass densified by 10% decreased to the value observed for thermal glass after a few minutes of heating to 1173 K. They also noticed that the refractive index of shocked glasses started to decrease for heating at 773 K. We observe here, at about the same temperature, the onset of a significant decrease of the 495 cm1 broad band frequency (Fig. 5a) which is continuous up to the highest temperature. This frequency decrease corresponds to an
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increase of the average SiOSi angle, consistent with a decrease of the sample density and refractive index (Arndt et al. 1971). The other main modification observed with increasing temperature is the decrease in intensity of the D1 and D2 bands which starts at a higher temperature (about 973 K) than the frequency decrease of the broad band. Thus, this change does not seem to be directly related to the decrease of refractive index. Interestingly enough, the frequency decrease of the broad band begins at lower temperatures than the intensity decrease of the defect bands. Thus the spectra in the intermediate temperature range are similar to those observed on samples recovered from shock pressures of 32.0 and 43.4 GPa, i.e. with a position of the broad band intermediate between that of the most densified glasses and that of the starting sample, and high intensities of the defect bands. We thus infer from these annealing experiments that the structure of the samples recovered from shock at 32.0 and 43.4 GPa is the result of partial relaxation of the structural modifications at high pressures due to increasing residual temperatures. These residual temperatures are high enough (Mashimo et al. 1980) to allow for the relaxation of the SiOSi bond angle within the time scale of the post-shock stage in the experiment, hence for the relaxation of the density increase. They are, however, too low to allow for the complete relaxation of the siloxane defects even in the sample shocked at 43.4 GPa. It is worth noticing, however, that the high siloxane ring concentration in that sample is associated with a density that is very close to that of the starting material. This indicates that siloxane ring concentration cannot be directly related to density changes, but are rather indicative of a stronger variation of ring statistics in that sample than in the starting material at nearly constant density. Also, the high siloxane defect concentrations are the fingerprint of structural modifications responsible for the densification during the high pressure high-temperature shock event. These unquenched modifications can be of two types: either a strong change in the ring statistics (smaller rings being favoured at high pressure, Hemley et al. 1986) or coordination changes of Si at high pressure (e.g. Meade et al. 1992), siloxane rings being formed during or after decompression through the relaxation of five- or six-fold silicon polyhedra. The latter hypothesis is consistent with the mechanism invoked for the increase of D1 and D2 band intensities with increasing fictive temperature (Geissberger and Galeener 1983) and attributed to the formation of transient five- or six-fold Si responsible for the viscous flow in the melt (McMillan et al. 1994).
Conclusions The structural evolution of SiO2 glasses recovered from shock-wave experiments can be summarized as follows. For shock pressures up to 18 GPa, a small densification (4%) is observed, accompanied by a small decrease of the average SiOSi angle. No significant increase of the siloxane defect concentrations is observed at this stage.
Similarly, Walrafen and Krishnan (1981) observed little change on the Raman spectrum of SiO2 glass densified by 8.1% at 23 C under 9.0 GPa when compared with that of normal SiO2 glass. This first densification stage is attributed to the collapse of the largest ring cavities. Above shock pressures of 18 GPa, the densification increases rapidly to reach about 11% at 26 GPa. Changes in the Raman spectrum indicate a further decrease of SiOSi average angle, in consistency with X-ray diffraction results, and an increase in the siloxane defect concentration. These changes advocate for a densification mechanism by a global reduction of the ring size. The maximum densification is recovered for a shock pressure of about 26 GPa. Above 26 GPa, the densities of the recovered samples decrease with increasing shock pressure. Raman spectra indicate an increase of the SiOSi angles with respect to those in the most dense sample. The intensity variations of the siloxane defect bands are not strictly correlated to that of density. They continue to increase in the sample shocked near 32 GPa before slightly decreasing in the sample shocked near 43 GPa. This structural evolution is similar to that observed by in situ Raman spectroscopy at high temperature using the sample recovered from 26.3 GPa. These variations are thus attributed to the relatively high shock and residual temperatures above 26 GPa, which allow the partial relaxation of the structural changes during decompression. Finally, the variety of transformations undergone by vitreous silica under shock compression suggests that it can provide a useful tool for deciphering shock histories in naturally shocked rocks that commonly contain vitrified quartz. Acknowledgements We thank Mr. K. Fukuoka for carrying out shock-wave experiments. A part of this work was carried out under the Visiting Researcher's Program of the IMR, Tohoku University of Japan. This work was partially supported by Grant-in-Aid for International Scientific Research (09044069) of the Ministry of Education, Science, Sports and Culture of Japan and by Foundation for Promotion of Material Science and Technology of Japan (MST Foundation).
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