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Raman spectroscopic investigations of Mn2+ doping effects on the densification of acid-catalyzed silica xerogels
Journal of Non-Crystalline Solids 243 (1999) 209±219
Raman spectroscopic investigations of Mn2 doping eects on the densi®cation of acid-catalyzed silica xerogels Jean-Marie Nedelec a, Mohamed Bouazaoui b, Sylvia Turrell a
a,*
Laboratoire de Spectrochimie Infrarouge et Raman, CNRS UPR 2631, B^ at. C8, Centre d'Etudes et de Recherches Laser et Applications, Universit e des Sciences et Technologies de Lille, 59655 Villeneuve d'Ascq cedex, France b Laboratoire de Physique des Lasers, Atomes et Mol ecules, CNRS UMR, B^ at. P5, Centre d'Etudes et de Recherches Laser et Applications, Universit e des Sciences et Technologies de Lille, 59655 Villeneuve d'Ascq cedex, France Received 13 March 1998; received in revised form 25 August 1998
Abstract Undoped and Mn2 -doped silica xerogels were prepared from hydrolysis and condensation of tetramethyl orthosilicate (TMOS). The xerogels were characterised by density measurements and ¯uorescence and Raman spectroscopies. Raman measurements over the range 4±1200 cmÿ1 showed that the number of three- and four-membered rings in the xerogel network depends on the thermal treatment and on the concentration of Mn2 ions. Indeed, both structures are found to be more numerous in the gel network of the doped samples than in the undoped one, showing that doping with Mn2 hampers the destruction of three- and four-membered rings. In the low-wave number region (4±100 cmÿ1 ), doping with manganese ions was found to aect the position of the boson peak. The boson peak pro®les were used to deduce that the sizes of the cohesive domains in the gel-derived silica network are much larger for doped samples (11 nm for 500 ppm) than for undoped ones (2.1 nm). Ó 1999 Elsevier Science B.V. All rights reserved.
1. Introduction In recent years, preparation of active glasses using the sol±gel method has attracted a considerable interest [1,2]. A great deal of work has been devoted to the study of the structural changes occurring at various stages of the gel-to-glass transformation as well as to the comparison of the structures of gel-derived glasses and of glasses prepared by the conventional melting technique [3±5]. Most of these studies concerned silica gels and gel-derived silica glasses owing to their potential applications in various areas (coating, optics,. . .).
* Corresponding author. Tel.: +33-3 20 43 49 20; fax: +33-3 20 43 67 55; e-mail: [email protected]
These materials have been doped with organic and inorganic species for use as active optical devices such as lasers and ampli®ers [6,7]. Transition metal ions have been used in glasses for their luminescence properties or as probes to follow the structural evolution of the host matrix as a function of annealing temperature or of sintering atmosphere [8,9]. The majority of these studies assume that probes, whether transition metal or rare-earth ions [10±14], do not modify the gel structure when they are incorporated in the gel in small amounts. The purpose of the present work is to use Raman spectroscopy to investigate the eects of doping with Mn2 ions on the structure of silica gels and gel-derived silica glasses. The Raman data over the low-frequency range is analyzed, using the non-continuous model proposed by Duval et al.
0022-3093/99/$ ± see front matter Ó 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 ( 9 8 ) 0 0 8 3 9 - 4
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[15,16], to determine the sizes of cohesive domains (blobs) within the doped and undoped gel-derived silica glasses. These results will be compared to those obtained for fused silica. 2. Experimental Mn2 -doped silica gels were prepared by hydrolysis and condensation of tetramethyl orthosilicate (TMOS). The silicon alkoxide was diluted in methanol in the presence of water in a molar ratio H2 O : TMOS 10, which allows the development of monolithic gels. A small amount of HNO3 was used as catalyst. The Mn2 ions were introduced by adding appropriate quantities of Mn(NO3 )2 á6H2 O to the initial mixture. The ®nal concentration of manganese ions ranged from 0 to 1000 ppm. The sol thus obtained was ®ltered, cast in plastic molds, and kept at 60°C for a gelation time of twenty hours. The resulting wet gels were aged for 3 weeks at 30°C and then heat-treated for 24 h at various annealing temperatures ranging from 300°C to 940°C. Densities of heat-treated gels were measured using a helium gas pycnometer. This technique is non-destructive and more accurate than the methods based on Archimedes' principle. Typical dimensions of monolithic xerogels were 1 ´ 0.5 ´ 0.4 cm3 . Ninety-degree Raman scattering measurements were obtained with a triple-monochromator using the 514.5 nm line of an Argon-ion laser as an excitation source with a typical power of 300 mW. The signal was detected with a cooled photomultiplier. The spectral range investigated was 4±1200 cmÿ1 , with a spectral slit width of 1 cmÿ1 . A similar experimental set-up was used to record ¯uorescence spectra in the visible region. In this case, the excitation source was the 457.9 nm line of an Ar laser. 3. Results 3.1. Density measurements Variations of the densities of Mn2 -doped silica gels as a function of annealing temperature are
Fig. 1. Densities of: (a) 200 ppm, (b) 500 ppm and (c) 1000 ppm Mn2 -doped xerogels as a function of annealing temperature.
shown in Fig. 1. The densities for the gels doped at 500 and 1000 ppm were found to be almost identical for all annealing temperatures. For both samples, the density increases gradually from 1.7 to 1.9 g/cm3 for the temperature range 300±800°C. However, for this same temperature range, the density of the 200 ppm doped sample varies from 1.85 to 2.1 g/cm3 . The fact that the values are consistently higher for the latter case indicates that Mn2 -doping hampers the densi®cation of silica gels. Fig. 1 also shows that the complete densi®cation of xerogels which corresponds to a density of 2.23 g/cm3 for 500 and 1000 ppm doped samples and 2.32 g/cm3 for the 200 ppm doped sample, occurs for annealing temperatures ranging from 800°C to 940°C. It might be noted that densities of samples heat-treated at 940°C are slightly higher than that of melt-fused silica (2.20 g/cm3 ). High density values like these have been reported previously for undoped silica xerogels [17,18]. Araujo et al. explained these high values by a structure with fewer defects in the gel-derived silica [18]. 3.2. Emission spectra Emission spectra of 1000 ppm doped silica xerogels heat-treated at 800°C and 940°C are shown in Fig. 2. At 800°C, the spectrum exhibits a slightly asymmetric broad band ranging from 500 to 700 nm which is characteristic of the emission of
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211
3.3. Raman spectra
Fig. 2. Fluorescence spectra of 1000 ppm Mn2 -doped xerogels annealed at: (a) 800°C and (b) 940°C, under excitation at k 457.9 nm.
Mn2 ions in a silica-rich environment. This band consists of two components, a maximum around 550 nm and a shoulder at 650 nm, both of which correspond to the transition from the ®rst excited state 4 T1 to the ground state 6 A1 [9,19,20]. The ®rst component around 550 nm is generally assigned to the emission of Mn2 ions when they are in tetrahedral sites, while the second component at 650 nm is attributed to the emission coming from octahedrally coordinated Mn2 . Hence, at an annealing temperature of 800°C, it is evident from Fig. 2 that Mn2 ions occupy both tetrahedral and octahedral sites. For xerogels annealed at 940°C, the shoulder at 650 nm nearly disappears, indicating that the majority of Mn2 ions are now in tetrahedral sites. It might also be noted that the maximum of the emission band shifts from 550 to 570 nm with an increase in temperature. This shift to higher wavelengths implies a stronger crystal ®eld according to the Tanabe±Sugano diagram, since the energy of the ®rst excited level 4 T1 decreases when the ligand ®eld increases [9]. In fact, within the gel network of densi®ed xerogels annealed at 940°C, the Mn2 ion is expected to be closer to its surrounding oxygen ions than in the case of xerogels heated at 800°C which are less densi®ed. This eect results in an increase of the crystal ®eld leading to a decrease of the energy of the ®rst excited level 4 T1 and consequently to a red shift of the emission line.
Raman spectra of undoped and 500 ppm Mn2 doped xerogels heat-treated at various temperatures are shown in Figs. 3 and 4, respectively. All the spectra exhibit the same bands except for the 500 ppm Mn2 -doped xerogel annealed at 300°C. In this spectrum a supplementary band at 1054 cmÿ1 is observed, which corresponds to symmetric stretching vibrations of C±O bonds of residual organic groups. Since these organic groups are eliminated at higher temperatures, this band is no longer observed for temperatures above 300°C. Nevertheless, the spectra exhibit several changes with annealing temperature and Mn2 -doping.
Fig. 3. Room-temperature Raman spectra of undoped xerogels annealed at various temperatures.
Fig. 4. Room-temperature Raman spectra of 500 ppm Mn2 doped xerogels annealed at various temperatures.
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(1) The main Raman band around 430 cmÿ1 broadens and the intensity of the high-wave number wing above 500 cmÿ1 decreases with annealing temperature. These changes, which are more pronounced for undoped samples, are more clearly observed in Fig. 5, where the 200±700 cmÿ1 region has been presented for three temperatures. (2) At high annealing temperatures, the shape of the band around 800 cmÿ1 is aected by doping with Mn2 . In eect, while for undoped samples annealed at 940°C the intensities of the high-frequency components decrease with respect to those in the low-frequency region, no change is observed for doped samples (Fig. 6). (3) The intensities of the sharp peaks at 490 and 606 cmÿ1 , generally called `defect' lines, vary continuously with heat-treatment when compared to that of the 430 cmÿ1 band. These changes are obvious in spectra of both doped and undoped samples. Nevertheless, it can be noticed that at the Fig. 6. Raman spectra in the region around 800 cmÿ1 for undoped (solid lines) and 500 ppm Mn2 doped (dashed lines) xerogels annealed at 900°C and 940°C.
Fig. 5. Raman spectra in the region around 430 cmÿ1 for undoped samples annealed at (a) 650°C, (b) 800°C and (c) 940°C.
annealing temperature T 940°C, the band at 606 cmÿ1 has nearly disappeared for undoped samples (Fig. 3), while it is still clearly observed for doped ones (Fig. 4). (4) In Fig. 3, Raman spectra show that the band at 980 cmÿ1 associated with the Si±OH stretching mode drops in intensity with increasing temperature and almost vanishes for undoped samples heated to 940°C (Fig. 3). On the other hand, as can be seen in Fig. 4, for doped samples this band is still clearly observed at 940°C, indicating that the polycondensation process is not completely achieved for doped samples, and water is still present in the glass. (5) The low-frequency regions below 100 cmÿ1 of the spectra of undoped and doped samples heattreated at 940°C are shown in Fig. 7. For the undoped samples, a band appears around 40 cmÿ1 . However, this band is located at much lower frequencies for doped xerogels. In eect, it is located at 11 and 18 cmÿ1 for 500 and 200 ppm Mn-doped samples, respectively (Fig. 7). It is also worthy to
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Fig. 7. Low-frequency Raman spectra of undoped, 200 and 500 ppm Mn2 doped xerogels annealed at 940°C. The spectrum of Suprasil is shown for comparison.
note that no band is observed in this frequency region for xerogels doped with a concentration higher than 500 ppm. The low-frequency Raman spectrum of fused silica is shown in Fig. 7 for comparison.
4.1. High-wave number region ( m > 200 cmÿ1 ) 4.1.1. The 430 cmÿ1 Raman contour The 430 cmÿ1 band is associated with a network Si±O±Si bending vibration. It involves a motion of the oxygen atom along the line bisecting the Si±O±Si angle (h), and is called a symmetricstretch (SS). According to Sen et al. [21] and Galeener et al. [22], the frequency (x1 ) of this peak, calculated using the nearest-neighbour central-force ®eld for an ideal continuous-randomnetwork (NN-CF-ICRN), is given by x21
a 1 cos h ; MOx
force constant for vitreous silica [23] (a 545 N/m) and MOx 16, one arrives at a distribution of h ranging from 150° to 124° (Dh 26°) which corresponds to 300 and 500 cmÿ1 , respectively. The decrease in intensity on the high-frequency side (>500 cmÿ1 ), with an increase in annealing temperature (Fig. 5), indicates that the Si±O±Si angular distribution changes so as to disfavour small angles. This shift towards angles which are more energetically favourable is in agreement with Walrafen et al. [24,25] and Galeener et al. [26] who showed that the increase in energy of formation per Si±O±Si bridge in densi®ed vitreous silica is high for angles less than 120°. 4.1.2. The 800 cmÿ1 band The band at 800 cmÿ1 was ®rst assigned by Bell and Dean [27,28] to a complex vibration involving substantial silicon motion in addition to a bending movement of oxygen. Sen and Thorpe [21] used the NN-CF-ICRN model to obtain the following expression for the frequency (x3 ) of this mode x23
4. Discussion of Raman spectra
1
where a is the central-force constant, and MOx is the oxygen mass. Hence, according to this simpli®ed relation, which takes into account only central forces and assumes that a is independent of h, the change in width of the 430 cmÿ1 band with heat treatment is due entirely to a spread in the Si±O±Si angular distribution. Using Eq. (1) with a typical
213
a 1 cos h 4a ; MOx 3MSi
2
where MSi is the mass of silicon. Raman spectra presented in Fig. 6 show that the shape of this band is almost the same for both doped and undoped samples for annealing temperatures up to 900°C. But at 940°C, the intensity of the highfrequency components (around 840 cmÿ1 ) of this band decreases by comparison with that of the low-frequency region (around 790 cmÿ1 ) for undoped samples. According to Eq. (2), this behaviour could be related to a distortion of the Si±O±Si bridging angle with increasing temperature, which would lead to a structure in which the majority of the network Si±O±Si bonds are oriented at large h values. For doped xerogels no changes were observed with heat-treatments up to 940°C (Fig. 6), indicating that even at this temperature, the presence of Mn2 maintains a relatively constant general distribution of Si±O±Si angles. 4.1.3. The 490 and 606 cmÿ1 lines The sharp lines observed in vitreous silica (v-SiO2 ) at 490 and 606 cmÿ1 were ®rst tentatively related to network defects and labelled D1 and D2 ,
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respectively. Galeener et al. [26,29,30] and more recently Barrio et al. [31] assigned the D1 line to the symmetric breathing mode of regular fourmembered silica rings, due to a movement of oxygen atoms. The D2 line was assigned to a similar motion of three-membered planar rings. The spectra of Figs. 3 and 4 show that both of these defects are present in both the doped and undoped xerogels, but with varying concentrations. It is also evident that with a change of annealing temperature, the intensities of both D1 and D2 bands vary with respect to that of the 430 cmÿ1 band. These intensities have been estimated from the corresponding band amplitudes. Curve (a) of Fig. 8 shows the variation of the intensity ratio ID1 /I430 , as a function of annealing temperature for an undoped silica xerogel (the error bars shown on the graph indicate an uncertainty of about 0.02 for the calculated ratio). This ratio decreases continuously with temperature up to 900°C, after which there is a sharp drop in the range of 900±940°C. This variation indicates that fourfold rings initially present in the wet gel network, are gradually destroyed up to 900°C, after which there is a very sharp increase in the rate of their destruction. Curves (b) and (c) of Fig. 8 present the ratio ID1 /I430 for samples doped with Mn2 at 200 and 500 ppm. It can be noticed that up to 900°C, ID1 /I430 varies in a manner similar to that of the undoped xerogels. However, above 900°C this ratio remains almost
constant. Indeed, for these two doped xerogels, the intensity ratio at 940°C is greater than that for undoped silica xerogels, and the absence of a sharp drop shows that doping with Mn2 impedes the destruction of fourfold rings in the gel structure. Curves (a±c) of Fig. 9 show the ratio of the intensity of the Raman D2 line with respect to that of the 430 cmÿ1 band, ID2 /I430 , as a function of annealing temperatures for undoped and doped xerogels. The error bars shown on the graphs are about 0.03. For all samples, the ratio ®rst increases up to an annealing temperature of about 650°C, in agreement with previous works [32]. Above 800°C the ratio ID2 /I430 decreases sharply for the undoped xerogels, but only slightly for the 500 ppm doped sample. In the case of the sample doped at 200 ppm, the intensity change for temperatures above 800°C is intermediate between that observed for the two previous cases. Hence, we can conclude that doping with Mn2 stabilizes the three-membered rings in the gel network.
Fig. 9. Intensity ratio ID2 /I430 as a function of annealing temperature for: (a) undoped xerogels, (b) 200 ppm doped xerogels, (c) 500 ppm doped xerogels. (Lines are only drawn as guides for the eye.) Table 1 Intensity ratios ID1 /I430 and ID2 /I430 as a function of the concentration of Mn2 for samples heated at 940°C.
Fig. 8. Intensity ratio ID1 /I430 as a function of annealing temperature for: (a) undoped xerogels, (b) 200 ppm doped xerogels, (c) 500 ppm doped xerogels. (Lines are only drawn as guides for the eye.)
[Mn2 ] ppm
ID1 /I430 (0.002)
ID2 /I430 (0.003)
0 200 500 1000
0.170 0.392 0.474 0.735
0.213 0.255 0.500 0.507
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Table 1 gives the intensity ratios ID1 /I430 and ID2 /I430 as a function of the concentration of Mn2 for samples heated at 940°C. These values clearly show that doping with manganese ions hampers the destruction of three- and four-membered silica rings. Furthermore, the fact that the intensity ratio ID1 /I430 is over ®ve times greater than ID2 /I430 for undoped samples at high temperatures, is in agreement with Brinker et al. [33] who found that three-membered rings are much more unstable than their four-membered ring counterparts with thermal treatment. 4.2. Low-wave number region (4 < m < 100 cmÿ1 ) Fig. 7 shows Raman spectra in the 4±100 cmÿ1 spectral range for gel-derived silica annealed at 940°C and for fused silica (Suprasil 1). In this region, bands are observed at 11, 25 and 48 cmÿ1 , for 500, 200 ppm Mn2 -doped and undoped gel-derived silica, respectively (the Raman spectrum of fused silica is shown for comparison). This band called the boson peak (BP), is universally characteristic of glasses. Several models have been used to interpret the low-frequency Raman scattering of light from glasses and consequently the boson peak [15,34±40]. According to Shuker and Gammon [40], the Raman intensity I(x,T) (where x is the frequency expressed in cmÿ1 and T is the temperature) can be written as follows, for Stokes scattering g xc x n x 1 ; 3 x where (n(x) + 1) is the Bose factor for Stokes scattering, g(x) is the vibrational density of states (VDOS) and c(x) the light-vibration coupling coecient. According to Eq. (3), the maximum of the BP could result either from a maximum of c(x) or of g(x) or from a combination of these two parameters. Nevertheless, comparison of data from Raman scattering and from inelastic neutron scattering [41±43] or heat capacity measurements [38] for several glasses (especially silicates) has shown that the coupling coecient has linear frequency dependence [38] (c x / x), and hence that the reduced Raman scattering intensity is proportional to the VDOS
I x; T
IR
I x; T g xc x / g x: n x 1 x
215
4
Consequently, the shape of the BP on a reduced intensity scale is due to the low-energy VDOS. In this paper, we will assume that, as in the case of fused silica, the coupling coecient also has a linear frequency dependence for gel-derived silica (c x / x), and we will interpret our results in terms of a model developed by Duval et al. which has been relatively successfully applied to silica [15,16] and to other glass systems [44]. In this model, the glass is assumed to be inhomogeneous on the nanometer scale and these inhomogeneities consist of regions of cohesive structures or blobs in which vibrational excitations are localized. In this context, Duval et al. expressed the observed eective vibrational density of states geff as follows: Zx geff x g x F x0 dx0 ;
5
0
where g(x) is the real density of vibrational states within the blobs and the frequency distribution F(x0 ) is such that F(x0 ) dx0 is the ratio of the volume occupied in the glass by domains whose fundamental-mode frequencies lie between x0 and x0 + dx0 to the total volume of the glass sample. Hence, a determination of F(x0 ) will lead to the size distribution of domains [15,16]. The shape of the low-frequency spectrum for the 200 ppm Mn2 -doped xerogel annealed at 940°C seems to result from the superposition of two bands, with the ®rst component appearing at a frequency close to that of the BP in the spectrum of the 500 ppm-doped sample. This observation could suggest the coexistence of two kinds of cohesive domains with two dierent size distributions. However, since the interpretation of the mathematical treatment of this complex band is dicult, the spectrum of the 200 ppm-doped sample will not be discussed in the following section. Fig. 10 shows a ln±ln plot of reduced intensities as a function of x, for Raman scattering from the 500 ppm Mn2 -doped gel-derived silica, undoped gel-derived silica and for fused silica (Suprasil 1). For the three curves, a linear dependence of
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geff x xc
Zx F x0 dx0 :
6
0
The derivative of Eq. (6) gives the frequency distribution F(x0 ). Fig. 11 shows these derivatives as a function of x. The curves have been ®tted by a log-normal distribution 2 ÿ ln x ÿ ln x 0 1 exp ; 7 F x0 p 2r2 2prx0 where r is characteristic of the full width at half maximum. Duval et al. [15,45] showed that the fundamental vibrational mode frequency x0 of a given domain is inversely proportional to its size 2a 2a
Fig. 10. ln±ln plot of Raman reduced intensity Ired as a function of frequency, for undoped and 500 ppm doped xerogels annealed at 940°C and for Suprasil.
ln IR / geff x versus ln(x) is observed for frequencies higher than a certain value which we will denote as the cut-o frequency xc . Since this cuto frequency corresponds to the maximum of the density of states and consequently to the maximum of the BP, xc is ®xed at a value around the observed BP frequency 11, 48 and 55 cmÿ1 for doped, undoped gel-derived silica and fused silica, respectively. Above xc , the reduced intensity and consequently geff (x) vary as xc . The exact value of xc is not so important, as a slight change in the value of xc has little eect on the slope c. However the value of the slope is critical since for x > xc , which is the upper limit for x0 , geff (x) is proportional to g(x) which is in turn proportional to xc . Once g(x) is determined experimentally (in this work g x / x0:7 ), for the BP spectral region, Eq. (5) leads to
Sv ; x0 c
8
where S is a domain shape factor (generally equal to 0.65) [16], v the transverse sound velocity for the material and c the velocity of light in the vacuum. of the maximum of F(x0 ) Hence, the frequency x corresponds to the most probable size of domains in the glassy structure. Moreover, using the full width at half maximum of F(x0 ) curves and Eq. (8), an estimate of the size distribution can be carried out.
Fig. 11. Frequency distributions F(x0 ) of cohesive domains (dotted lines) and log-normal distribution ®ts (solid line), for undoped and 500 ppm doped xerogels annealed at 940°C and for Suprasil.
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Choosing the sound velocity of vitreous silica of the v 4100 m/s [46], and using the frequency x maximum of F(x0 ), it is found that the most probable size of domains 2a is equal to 11.1 nm (8 cmÿ1 ), 2.9 nm (30 cmÿ1 ) and 2.24 nm (36 cmÿ1 ), for 500 Mn2 -doped, undoped gel-derived silica and fused silica, respectively. These values are obtained assuming the same sound velocity for all samples, an assumption which is supported by density measurements which indicate that all samples heat-treated at 940°C have densities which are only slightly higher than that of vitreous silica (Fig. 1). Hence, the great dierence in 2a in Mndoped samples cannot be due to a reduced sound velocity which would lead to a lower density. This assumption is in agreement with Chmel et al. [47] who showed that the change observed in the boson peak position in the Raman spectra of gel-derived silica is not caused by a change in sound velocity. Indeed, they pointed out that in the annealingtemperature range where the BP appears in the Raman spectra of silica xerogels, the transverse sound velocities measured by Brillouin scattering were almost constant and equal to about 4000 m/s. The spread in domain size estimated using Eq. (7) is found to be equal to 2.1 nm for the doped xerogels and 1.2 nm for undoped samples. Therefore, it is clear that the distribution of domain sizes compared with the most probable size 2a is much narrower for doped materials D 2a=2a 0:2 than for undoped ones and fused silica D 2a=2a 0:5. The results presented above, indicate that at an annealing temperature of 940°C, doping with Mn2 ions allows the formation of larger cohesive entities in the gel network, with a smaller spread in domain size than for undoped samples. Furthermore, one might note that for more highly doped samples (>500 ppm) the size of domains should be larger than that obtained for the 500 ppm sample and thus unobservable by Raman spectroscopy. This is certainly the reason why the boson peak is absent in Raman spectra of highly doped samples. For undoped gel-derived silica, the domain sizes are much smaller and exhibit a wider spread around the most probable value, a behaviour which is similar to that observed for fused silica.
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5. Conclusion Density measurements of silica xerogels reported in Section 3.1 show that complete densi®cation occurs for annealing temperatures in the range 800±940°C. These results indicate that at 940°C, the doped xerogels have attained the density of fused silica. Therefore, according to this characterization, silica xerogels should have macroscopic properties which are similar to those of vitreous silica prepared by melting. Fluorescence results discussed in Section 3.2 show that the manganese ions incorporated into silica xerogels are in a divalent state and occupy both octahedral and tetrahedral sites. Investigations by Raman spectroscopy show that the doping of silica xerogels with Mn2 has a marked eect on the gel structure at the microscopic level. The intensity ratios ID1 /I430 and ID2 /I430 which are related to three- and four-membered silica rings within the gel structure are found to vary both with the thermal treatment and with the amount of Mn2 dopant. For temperatures up to 650°C, fourfold rings are destroyed to form both threefold rings and the three-dimensional network of SiO4 tetrahedra. This process is witnessed by a decrease of the ID1 / I430 ratio and an increase of ID2 /I430 . For the 650± 800°C range, the number of both three- and fourmembered rings is almost stable, but above 800°C, both structures are destroyed to form the three-dimensional network of SiO4 tetrahedra. For undoped xerogels, this destruction becomes drastic in the short temperature range 900±940°C. On the other hand, in the case of doped samples in this temperature range, not only does this drastic destruction seem to be delayed for three-membered rings, but it is not observed for fourfold rings. This dierence indicates that doping with Mn2 , even in small amounts (less than 0.1%), maintains the presence of three- and four-membered rings in the gel-derived silica structure. On the other hand, results indicate that since the more highly strained three-membered rings are considerably less stable than their four-membered counterparts, there are signi®cantly fewer in the ®nal network structure. The temperatures at which the destruction of three and fourfold rings is found to be more important for undoped xerogels (900 to 940°C)
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also correspond to temperatures at which there is an appearance of the Boson peak (BP). Calculations based on the pro®le of the BP indicate that the size of cohesive domains in the gel-derived silica network depends on the concentration of Mn2 ions. In eect, for the sample doped at 500 ppm, the domain size is much larger (11 nm) than that of undoped samples (2.1 nm). This latter value is close to that obtained for fused silica. Hence, contrary to information provided by density measurements, one can conclude that on the nanometer scale, the densi®cation of doped xerogels is not completely achieved. Moreover, if we consider that the occurrence of the Boson peak is indicative of a glassy structure, then we can deduce that the conversion of these xerogels to the glassy state is achieved within a very short temperature range (between 930°C and 940°C). The correlation between the appearance and the position of the boson peak on the one hand and the number of defects remaining in the gel network on the other, requires further clari®cation. One can note that for undoped xerogels the densi®cation process induces strains that cause a distortion of the SiO4 tetrahedral network, and the destruction of three- and four-membered rings leading to the creation of small-sized cohesive domains. Doping with Mn2 slows down the densi®cation process, reducing strains and maintaining the existence of more three- and four-membered rings, thus leading to the formation of larger ordered regions. Acknowledgements The authors thank C. Armellini for her invaluable help in sample elaboration and Drs M. Ferrari and A. Boukenter for very fruitful discussions. The Centre d'Etudes et de Recherches Lasers et Applications (CERLA) is supported by the Ministere Charge de la Recherche, the Region Nord/Pas de Calais and the Fonds Europeens de Developpement Economique des Regions. References [1] L.L. Hench, J.K. West (Eds.), Chemical Processing of Advanced Materials, Wiley, New York, 1992.
[2] E. Pope, J. Mackenzie, J. Am. Ceram. Soc. 76 (5) (1993) 1325. [3] J.L. Rousset, E. Duval, A. Boukenter, B. Champagnon, A. Monteil, J. Serughetti, J. Dumas, J. Non-Cryst. Solids 107 (1988) 27. [4] K. Dahmouche, A. Boukenter, C. Bovier, J. Dumas, E. Duval, J. Serughetti, J. Mater. Sci. 30 (1995) 4149. [5] M.C. Matos, L.M. Ilharco, R.M. Almeida, J. Non-Cryst. Solids 147&148 (1992) 232. [6] M. Canva, P. Georges, J.F. Perelgritz, A. Brun, F. Chaput, J.P. Boilot, Appl. Opt. 34 (1995) 428. [7] F. Wu, D. Machewirth, E. Snitzer, G.H. Siegel Jr. J. Mater. Res. 9 (1994) 2703. [8] N. Abidi, B. Deroide, J.V. Zanchetta, D. Bourret, H. Elmkami, P. Rumori, Phys. Chem. Glasses 37 (4) (1996) 149. [9] P.E. Menassa, D.J. Simkin, P. Taylor, J. Lumin. 35 (1986) 223. [10] R. Campostrini, G. Carturan, M. Ferrari, M. Montagna, O. Pilla, J. Mater. Res. 7 (3) (1992) 745. [11] M.J. Lochhead, K.L. Bray, J. Non-Cryst. Solids 170 (1994) 143. [12] A. Bahtat, M. Bouazaoui, M. Bahtat, J. Mugnier, Opt. Comm. 111 (1994) 55. [13] D. Levy, R. Reisfeld, D. Avnir, Chem. Phys. Lett. 109 (1984) 593. [14] M. Ferrari, R. Campostrini, G. Carturan, M. Montagna, Philos. Mag. B 65 (1992) 251. [15] E. Duval, A. Boukenter, T. Achibat, J. Phys.: Condens. Matter 2 (1990) 10227. [16] A. Boukenter, E. Duval, Philos. Mag. B 77 (2) (1998) 557. [17] N. Tohge, G.S. Moore, J.D. Mackenzie, J. Non-Cryst. Solids 63 (1984) 95. [18] F.G. Araujo, G.P. LaTorre, L.L. Hench, J. Non-Cryst. Solids 185 (1995) 41. [19] B. Andrianasolo, B. Champagnon, E. Duval, Phys. Chem. Glasses 30 (6) (1989) 215. [20] C. Sumalatha, B. Sreedhar, M. Yamazaki, T. Takeuchi, K. Mori, K. Kojima, J. Non-Cryst. Solids 203 (1996) 84. [21] P.N. Sen, M.F. Thorpe, Phys. Rev. B 15 (1977) 4030. [22] F.L. Galeener, Phys. Rev. B 19 (1979) 4292. [23] F.L. Galeener, A.J. Leadbetter, M.W. Strongfellow, Phys. Rev. B 27 (2) (1983) 1052. [24] G.E. Walrafen, Y.C. Chu, M.S. Hokmabadi, J. Chem. Phys. 92 (12) (1990) 6987. [25] A.G. Revesz, G.E. Walrafen, J. Non-Cryst. Solids 54 (1983) 323. [26] F.L. Galeener, Solid State Commun. 44 (7) (1982) 1037. [27] R.J. Bell, Rep. Prog. Phys. 35 (1972) 1315. [28] P. Dean, Rev. Mod. Phys. 44 (1972) 127. [29] F.L. Galeener, Phys. Rev. B 19 (8) (1979) 4292. [30] F.L. Galeener, J.C. Mikkelsen Jr., Phys. Rev. B 23 (10) (1981) 5527. [31] R.A. Barrio, F.L. Galeener, E. Martinez, R.J. Elliott, Phys. Rev. B 48 (21) (1993) 15672. [32] A. Bertoluzza, C. Fagnano, M.A. Morelli, J. Non-Cryst. Solids 48 (1982) 117.
J.-M. Nedelec et al. / Journal of Non-Crystalline Solids 243 (1999) 209±219 [33] C.J. Brinker, B.C. Bunker, D.R. Tallant, K.J. Ward, J. Chim. Phys. 83 (1986) 851. [34] A.J. Martin, W. Brenig, Phys. Status Solidi B 64 (1974) 163. [35] J. Jackel, in: Amorphous Solids: Low Temperature Properties, Springer, Berlin, 1981. [36] R.J. Nemanich, Phys. Rev. B 16 (1977) 1665. [37] T. Pang, Phys. Rev. B 45 (1992) 2490. [38] A.P. Sokolov, A. Kislink, D. Quitman, E. Duval, Phys. Rev. B 48 (1993) 7692. [39] P. Benassi, A. Fontana, W. Frizzera, M. Montana, V. Mazzacurati, G. Signorelli, Philos. Mag. B 71 (4) (1995) 761. [40] R. Shuker, R.W. Gammon, Phys. Rev. Lett. 25 (1970) 222.
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[41] U. Buchenau, N. N ucker, A.J. Dianoux, Phys. Rev. Lett. 53 (1984) 2316. [42] A.J. Dianoux, Philos. Mag. B 59 (1989) 17. [43] V.K. Malinovski, V.N. Novikov, P.P. Parshin, A.P. Sokolov, M.G. Zemlyanov, Europhys. Lett. 11 (1990) 43. [44] A. Mermet, these de doctorat, Universite de Lyon 1, (1996). [45] E. Duval, A. Boukenter, B. Champagnon, Phys. Rev. Lett. 56 (19) (1986) 2052. [46] J.E. Graebner, B. Golding, L.C. Allen, Phys. Rev. B 34 (1986) 5696. [47] A. Chmel, T. Pesina, V.S. Shashkin, J. Non-Cryst. Solids 210 (1997) 254.
Raman spectroscopic investigations of Mn2 doping eects on the densi®cation of acid-catalyzed silica xerogels Jean-Marie Nedelec a, Mohamed Bouazaoui b, Sylvia Turrell a
a,*
Laboratoire de Spectrochimie Infrarouge et Raman, CNRS UPR 2631, B^ at. C8, Centre d'Etudes et de Recherches Laser et Applications, Universit e des Sciences et Technologies de Lille, 59655 Villeneuve d'Ascq cedex, France b Laboratoire de Physique des Lasers, Atomes et Mol ecules, CNRS UMR, B^ at. P5, Centre d'Etudes et de Recherches Laser et Applications, Universit e des Sciences et Technologies de Lille, 59655 Villeneuve d'Ascq cedex, France Received 13 March 1998; received in revised form 25 August 1998
Abstract Undoped and Mn2 -doped silica xerogels were prepared from hydrolysis and condensation of tetramethyl orthosilicate (TMOS). The xerogels were characterised by density measurements and ¯uorescence and Raman spectroscopies. Raman measurements over the range 4±1200 cmÿ1 showed that the number of three- and four-membered rings in the xerogel network depends on the thermal treatment and on the concentration of Mn2 ions. Indeed, both structures are found to be more numerous in the gel network of the doped samples than in the undoped one, showing that doping with Mn2 hampers the destruction of three- and four-membered rings. In the low-wave number region (4±100 cmÿ1 ), doping with manganese ions was found to aect the position of the boson peak. The boson peak pro®les were used to deduce that the sizes of the cohesive domains in the gel-derived silica network are much larger for doped samples (11 nm for 500 ppm) than for undoped ones (2.1 nm). Ó 1999 Elsevier Science B.V. All rights reserved.
1. Introduction In recent years, preparation of active glasses using the sol±gel method has attracted a considerable interest [1,2]. A great deal of work has been devoted to the study of the structural changes occurring at various stages of the gel-to-glass transformation as well as to the comparison of the structures of gel-derived glasses and of glasses prepared by the conventional melting technique [3±5]. Most of these studies concerned silica gels and gel-derived silica glasses owing to their potential applications in various areas (coating, optics,. . .).
* Corresponding author. Tel.: +33-3 20 43 49 20; fax: +33-3 20 43 67 55; e-mail: [email protected]
These materials have been doped with organic and inorganic species for use as active optical devices such as lasers and ampli®ers [6,7]. Transition metal ions have been used in glasses for their luminescence properties or as probes to follow the structural evolution of the host matrix as a function of annealing temperature or of sintering atmosphere [8,9]. The majority of these studies assume that probes, whether transition metal or rare-earth ions [10±14], do not modify the gel structure when they are incorporated in the gel in small amounts. The purpose of the present work is to use Raman spectroscopy to investigate the eects of doping with Mn2 ions on the structure of silica gels and gel-derived silica glasses. The Raman data over the low-frequency range is analyzed, using the non-continuous model proposed by Duval et al.
0022-3093/99/$ ± see front matter Ó 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 ( 9 8 ) 0 0 8 3 9 - 4
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[15,16], to determine the sizes of cohesive domains (blobs) within the doped and undoped gel-derived silica glasses. These results will be compared to those obtained for fused silica. 2. Experimental Mn2 -doped silica gels were prepared by hydrolysis and condensation of tetramethyl orthosilicate (TMOS). The silicon alkoxide was diluted in methanol in the presence of water in a molar ratio H2 O : TMOS 10, which allows the development of monolithic gels. A small amount of HNO3 was used as catalyst. The Mn2 ions were introduced by adding appropriate quantities of Mn(NO3 )2 á6H2 O to the initial mixture. The ®nal concentration of manganese ions ranged from 0 to 1000 ppm. The sol thus obtained was ®ltered, cast in plastic molds, and kept at 60°C for a gelation time of twenty hours. The resulting wet gels were aged for 3 weeks at 30°C and then heat-treated for 24 h at various annealing temperatures ranging from 300°C to 940°C. Densities of heat-treated gels were measured using a helium gas pycnometer. This technique is non-destructive and more accurate than the methods based on Archimedes' principle. Typical dimensions of monolithic xerogels were 1 ´ 0.5 ´ 0.4 cm3 . Ninety-degree Raman scattering measurements were obtained with a triple-monochromator using the 514.5 nm line of an Argon-ion laser as an excitation source with a typical power of 300 mW. The signal was detected with a cooled photomultiplier. The spectral range investigated was 4±1200 cmÿ1 , with a spectral slit width of 1 cmÿ1 . A similar experimental set-up was used to record ¯uorescence spectra in the visible region. In this case, the excitation source was the 457.9 nm line of an Ar laser. 3. Results 3.1. Density measurements Variations of the densities of Mn2 -doped silica gels as a function of annealing temperature are
Fig. 1. Densities of: (a) 200 ppm, (b) 500 ppm and (c) 1000 ppm Mn2 -doped xerogels as a function of annealing temperature.
shown in Fig. 1. The densities for the gels doped at 500 and 1000 ppm were found to be almost identical for all annealing temperatures. For both samples, the density increases gradually from 1.7 to 1.9 g/cm3 for the temperature range 300±800°C. However, for this same temperature range, the density of the 200 ppm doped sample varies from 1.85 to 2.1 g/cm3 . The fact that the values are consistently higher for the latter case indicates that Mn2 -doping hampers the densi®cation of silica gels. Fig. 1 also shows that the complete densi®cation of xerogels which corresponds to a density of 2.23 g/cm3 for 500 and 1000 ppm doped samples and 2.32 g/cm3 for the 200 ppm doped sample, occurs for annealing temperatures ranging from 800°C to 940°C. It might be noted that densities of samples heat-treated at 940°C are slightly higher than that of melt-fused silica (2.20 g/cm3 ). High density values like these have been reported previously for undoped silica xerogels [17,18]. Araujo et al. explained these high values by a structure with fewer defects in the gel-derived silica [18]. 3.2. Emission spectra Emission spectra of 1000 ppm doped silica xerogels heat-treated at 800°C and 940°C are shown in Fig. 2. At 800°C, the spectrum exhibits a slightly asymmetric broad band ranging from 500 to 700 nm which is characteristic of the emission of
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211
3.3. Raman spectra
Fig. 2. Fluorescence spectra of 1000 ppm Mn2 -doped xerogels annealed at: (a) 800°C and (b) 940°C, under excitation at k 457.9 nm.
Mn2 ions in a silica-rich environment. This band consists of two components, a maximum around 550 nm and a shoulder at 650 nm, both of which correspond to the transition from the ®rst excited state 4 T1 to the ground state 6 A1 [9,19,20]. The ®rst component around 550 nm is generally assigned to the emission of Mn2 ions when they are in tetrahedral sites, while the second component at 650 nm is attributed to the emission coming from octahedrally coordinated Mn2 . Hence, at an annealing temperature of 800°C, it is evident from Fig. 2 that Mn2 ions occupy both tetrahedral and octahedral sites. For xerogels annealed at 940°C, the shoulder at 650 nm nearly disappears, indicating that the majority of Mn2 ions are now in tetrahedral sites. It might also be noted that the maximum of the emission band shifts from 550 to 570 nm with an increase in temperature. This shift to higher wavelengths implies a stronger crystal ®eld according to the Tanabe±Sugano diagram, since the energy of the ®rst excited level 4 T1 decreases when the ligand ®eld increases [9]. In fact, within the gel network of densi®ed xerogels annealed at 940°C, the Mn2 ion is expected to be closer to its surrounding oxygen ions than in the case of xerogels heated at 800°C which are less densi®ed. This eect results in an increase of the crystal ®eld leading to a decrease of the energy of the ®rst excited level 4 T1 and consequently to a red shift of the emission line.
Raman spectra of undoped and 500 ppm Mn2 doped xerogels heat-treated at various temperatures are shown in Figs. 3 and 4, respectively. All the spectra exhibit the same bands except for the 500 ppm Mn2 -doped xerogel annealed at 300°C. In this spectrum a supplementary band at 1054 cmÿ1 is observed, which corresponds to symmetric stretching vibrations of C±O bonds of residual organic groups. Since these organic groups are eliminated at higher temperatures, this band is no longer observed for temperatures above 300°C. Nevertheless, the spectra exhibit several changes with annealing temperature and Mn2 -doping.
Fig. 3. Room-temperature Raman spectra of undoped xerogels annealed at various temperatures.
Fig. 4. Room-temperature Raman spectra of 500 ppm Mn2 doped xerogels annealed at various temperatures.
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(1) The main Raman band around 430 cmÿ1 broadens and the intensity of the high-wave number wing above 500 cmÿ1 decreases with annealing temperature. These changes, which are more pronounced for undoped samples, are more clearly observed in Fig. 5, where the 200±700 cmÿ1 region has been presented for three temperatures. (2) At high annealing temperatures, the shape of the band around 800 cmÿ1 is aected by doping with Mn2 . In eect, while for undoped samples annealed at 940°C the intensities of the high-frequency components decrease with respect to those in the low-frequency region, no change is observed for doped samples (Fig. 6). (3) The intensities of the sharp peaks at 490 and 606 cmÿ1 , generally called `defect' lines, vary continuously with heat-treatment when compared to that of the 430 cmÿ1 band. These changes are obvious in spectra of both doped and undoped samples. Nevertheless, it can be noticed that at the Fig. 6. Raman spectra in the region around 800 cmÿ1 for undoped (solid lines) and 500 ppm Mn2 doped (dashed lines) xerogels annealed at 900°C and 940°C.
Fig. 5. Raman spectra in the region around 430 cmÿ1 for undoped samples annealed at (a) 650°C, (b) 800°C and (c) 940°C.
annealing temperature T 940°C, the band at 606 cmÿ1 has nearly disappeared for undoped samples (Fig. 3), while it is still clearly observed for doped ones (Fig. 4). (4) In Fig. 3, Raman spectra show that the band at 980 cmÿ1 associated with the Si±OH stretching mode drops in intensity with increasing temperature and almost vanishes for undoped samples heated to 940°C (Fig. 3). On the other hand, as can be seen in Fig. 4, for doped samples this band is still clearly observed at 940°C, indicating that the polycondensation process is not completely achieved for doped samples, and water is still present in the glass. (5) The low-frequency regions below 100 cmÿ1 of the spectra of undoped and doped samples heattreated at 940°C are shown in Fig. 7. For the undoped samples, a band appears around 40 cmÿ1 . However, this band is located at much lower frequencies for doped xerogels. In eect, it is located at 11 and 18 cmÿ1 for 500 and 200 ppm Mn-doped samples, respectively (Fig. 7). It is also worthy to
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Fig. 7. Low-frequency Raman spectra of undoped, 200 and 500 ppm Mn2 doped xerogels annealed at 940°C. The spectrum of Suprasil is shown for comparison.
note that no band is observed in this frequency region for xerogels doped with a concentration higher than 500 ppm. The low-frequency Raman spectrum of fused silica is shown in Fig. 7 for comparison.
4.1. High-wave number region ( m > 200 cmÿ1 ) 4.1.1. The 430 cmÿ1 Raman contour The 430 cmÿ1 band is associated with a network Si±O±Si bending vibration. It involves a motion of the oxygen atom along the line bisecting the Si±O±Si angle (h), and is called a symmetricstretch (SS). According to Sen et al. [21] and Galeener et al. [22], the frequency (x1 ) of this peak, calculated using the nearest-neighbour central-force ®eld for an ideal continuous-randomnetwork (NN-CF-ICRN), is given by x21
a 1 cos h ; MOx
force constant for vitreous silica [23] (a 545 N/m) and MOx 16, one arrives at a distribution of h ranging from 150° to 124° (Dh 26°) which corresponds to 300 and 500 cmÿ1 , respectively. The decrease in intensity on the high-frequency side (>500 cmÿ1 ), with an increase in annealing temperature (Fig. 5), indicates that the Si±O±Si angular distribution changes so as to disfavour small angles. This shift towards angles which are more energetically favourable is in agreement with Walrafen et al. [24,25] and Galeener et al. [26] who showed that the increase in energy of formation per Si±O±Si bridge in densi®ed vitreous silica is high for angles less than 120°. 4.1.2. The 800 cmÿ1 band The band at 800 cmÿ1 was ®rst assigned by Bell and Dean [27,28] to a complex vibration involving substantial silicon motion in addition to a bending movement of oxygen. Sen and Thorpe [21] used the NN-CF-ICRN model to obtain the following expression for the frequency (x3 ) of this mode x23
4. Discussion of Raman spectra
1
where a is the central-force constant, and MOx is the oxygen mass. Hence, according to this simpli®ed relation, which takes into account only central forces and assumes that a is independent of h, the change in width of the 430 cmÿ1 band with heat treatment is due entirely to a spread in the Si±O±Si angular distribution. Using Eq. (1) with a typical
213
a 1 cos h 4a ; MOx 3MSi
2
where MSi is the mass of silicon. Raman spectra presented in Fig. 6 show that the shape of this band is almost the same for both doped and undoped samples for annealing temperatures up to 900°C. But at 940°C, the intensity of the highfrequency components (around 840 cmÿ1 ) of this band decreases by comparison with that of the low-frequency region (around 790 cmÿ1 ) for undoped samples. According to Eq. (2), this behaviour could be related to a distortion of the Si±O±Si bridging angle with increasing temperature, which would lead to a structure in which the majority of the network Si±O±Si bonds are oriented at large h values. For doped xerogels no changes were observed with heat-treatments up to 940°C (Fig. 6), indicating that even at this temperature, the presence of Mn2 maintains a relatively constant general distribution of Si±O±Si angles. 4.1.3. The 490 and 606 cmÿ1 lines The sharp lines observed in vitreous silica (v-SiO2 ) at 490 and 606 cmÿ1 were ®rst tentatively related to network defects and labelled D1 and D2 ,
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respectively. Galeener et al. [26,29,30] and more recently Barrio et al. [31] assigned the D1 line to the symmetric breathing mode of regular fourmembered silica rings, due to a movement of oxygen atoms. The D2 line was assigned to a similar motion of three-membered planar rings. The spectra of Figs. 3 and 4 show that both of these defects are present in both the doped and undoped xerogels, but with varying concentrations. It is also evident that with a change of annealing temperature, the intensities of both D1 and D2 bands vary with respect to that of the 430 cmÿ1 band. These intensities have been estimated from the corresponding band amplitudes. Curve (a) of Fig. 8 shows the variation of the intensity ratio ID1 /I430 , as a function of annealing temperature for an undoped silica xerogel (the error bars shown on the graph indicate an uncertainty of about 0.02 for the calculated ratio). This ratio decreases continuously with temperature up to 900°C, after which there is a sharp drop in the range of 900±940°C. This variation indicates that fourfold rings initially present in the wet gel network, are gradually destroyed up to 900°C, after which there is a very sharp increase in the rate of their destruction. Curves (b) and (c) of Fig. 8 present the ratio ID1 /I430 for samples doped with Mn2 at 200 and 500 ppm. It can be noticed that up to 900°C, ID1 /I430 varies in a manner similar to that of the undoped xerogels. However, above 900°C this ratio remains almost
constant. Indeed, for these two doped xerogels, the intensity ratio at 940°C is greater than that for undoped silica xerogels, and the absence of a sharp drop shows that doping with Mn2 impedes the destruction of fourfold rings in the gel structure. Curves (a±c) of Fig. 9 show the ratio of the intensity of the Raman D2 line with respect to that of the 430 cmÿ1 band, ID2 /I430 , as a function of annealing temperatures for undoped and doped xerogels. The error bars shown on the graphs are about 0.03. For all samples, the ratio ®rst increases up to an annealing temperature of about 650°C, in agreement with previous works [32]. Above 800°C the ratio ID2 /I430 decreases sharply for the undoped xerogels, but only slightly for the 500 ppm doped sample. In the case of the sample doped at 200 ppm, the intensity change for temperatures above 800°C is intermediate between that observed for the two previous cases. Hence, we can conclude that doping with Mn2 stabilizes the three-membered rings in the gel network.
Fig. 9. Intensity ratio ID2 /I430 as a function of annealing temperature for: (a) undoped xerogels, (b) 200 ppm doped xerogels, (c) 500 ppm doped xerogels. (Lines are only drawn as guides for the eye.) Table 1 Intensity ratios ID1 /I430 and ID2 /I430 as a function of the concentration of Mn2 for samples heated at 940°C.
Fig. 8. Intensity ratio ID1 /I430 as a function of annealing temperature for: (a) undoped xerogels, (b) 200 ppm doped xerogels, (c) 500 ppm doped xerogels. (Lines are only drawn as guides for the eye.)
[Mn2 ] ppm
ID1 /I430 (0.002)
ID2 /I430 (0.003)
0 200 500 1000
0.170 0.392 0.474 0.735
0.213 0.255 0.500 0.507
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Table 1 gives the intensity ratios ID1 /I430 and ID2 /I430 as a function of the concentration of Mn2 for samples heated at 940°C. These values clearly show that doping with manganese ions hampers the destruction of three- and four-membered silica rings. Furthermore, the fact that the intensity ratio ID1 /I430 is over ®ve times greater than ID2 /I430 for undoped samples at high temperatures, is in agreement with Brinker et al. [33] who found that three-membered rings are much more unstable than their four-membered ring counterparts with thermal treatment. 4.2. Low-wave number region (4 < m < 100 cmÿ1 ) Fig. 7 shows Raman spectra in the 4±100 cmÿ1 spectral range for gel-derived silica annealed at 940°C and for fused silica (Suprasil 1). In this region, bands are observed at 11, 25 and 48 cmÿ1 , for 500, 200 ppm Mn2 -doped and undoped gel-derived silica, respectively (the Raman spectrum of fused silica is shown for comparison). This band called the boson peak (BP), is universally characteristic of glasses. Several models have been used to interpret the low-frequency Raman scattering of light from glasses and consequently the boson peak [15,34±40]. According to Shuker and Gammon [40], the Raman intensity I(x,T) (where x is the frequency expressed in cmÿ1 and T is the temperature) can be written as follows, for Stokes scattering g xc x n x 1 ; 3 x where (n(x) + 1) is the Bose factor for Stokes scattering, g(x) is the vibrational density of states (VDOS) and c(x) the light-vibration coupling coecient. According to Eq. (3), the maximum of the BP could result either from a maximum of c(x) or of g(x) or from a combination of these two parameters. Nevertheless, comparison of data from Raman scattering and from inelastic neutron scattering [41±43] or heat capacity measurements [38] for several glasses (especially silicates) has shown that the coupling coecient has linear frequency dependence [38] (c x / x), and hence that the reduced Raman scattering intensity is proportional to the VDOS
I x; T
IR
I x; T g xc x / g x: n x 1 x
215
4
Consequently, the shape of the BP on a reduced intensity scale is due to the low-energy VDOS. In this paper, we will assume that, as in the case of fused silica, the coupling coecient also has a linear frequency dependence for gel-derived silica (c x / x), and we will interpret our results in terms of a model developed by Duval et al. which has been relatively successfully applied to silica [15,16] and to other glass systems [44]. In this model, the glass is assumed to be inhomogeneous on the nanometer scale and these inhomogeneities consist of regions of cohesive structures or blobs in which vibrational excitations are localized. In this context, Duval et al. expressed the observed eective vibrational density of states geff as follows: Zx geff x g x F x0 dx0 ;
5
0
where g(x) is the real density of vibrational states within the blobs and the frequency distribution F(x0 ) is such that F(x0 ) dx0 is the ratio of the volume occupied in the glass by domains whose fundamental-mode frequencies lie between x0 and x0 + dx0 to the total volume of the glass sample. Hence, a determination of F(x0 ) will lead to the size distribution of domains [15,16]. The shape of the low-frequency spectrum for the 200 ppm Mn2 -doped xerogel annealed at 940°C seems to result from the superposition of two bands, with the ®rst component appearing at a frequency close to that of the BP in the spectrum of the 500 ppm-doped sample. This observation could suggest the coexistence of two kinds of cohesive domains with two dierent size distributions. However, since the interpretation of the mathematical treatment of this complex band is dicult, the spectrum of the 200 ppm-doped sample will not be discussed in the following section. Fig. 10 shows a ln±ln plot of reduced intensities as a function of x, for Raman scattering from the 500 ppm Mn2 -doped gel-derived silica, undoped gel-derived silica and for fused silica (Suprasil 1). For the three curves, a linear dependence of
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geff x xc
Zx F x0 dx0 :
6
0
The derivative of Eq. (6) gives the frequency distribution F(x0 ). Fig. 11 shows these derivatives as a function of x. The curves have been ®tted by a log-normal distribution 2 ÿ ln x ÿ ln x 0 1 exp ; 7 F x0 p 2r2 2prx0 where r is characteristic of the full width at half maximum. Duval et al. [15,45] showed that the fundamental vibrational mode frequency x0 of a given domain is inversely proportional to its size 2a 2a
Fig. 10. ln±ln plot of Raman reduced intensity Ired as a function of frequency, for undoped and 500 ppm doped xerogels annealed at 940°C and for Suprasil.
ln IR / geff x versus ln(x) is observed for frequencies higher than a certain value which we will denote as the cut-o frequency xc . Since this cuto frequency corresponds to the maximum of the density of states and consequently to the maximum of the BP, xc is ®xed at a value around the observed BP frequency 11, 48 and 55 cmÿ1 for doped, undoped gel-derived silica and fused silica, respectively. Above xc , the reduced intensity and consequently geff (x) vary as xc . The exact value of xc is not so important, as a slight change in the value of xc has little eect on the slope c. However the value of the slope is critical since for x > xc , which is the upper limit for x0 , geff (x) is proportional to g(x) which is in turn proportional to xc . Once g(x) is determined experimentally (in this work g x / x0:7 ), for the BP spectral region, Eq. (5) leads to
Sv ; x0 c
8
where S is a domain shape factor (generally equal to 0.65) [16], v the transverse sound velocity for the material and c the velocity of light in the vacuum. of the maximum of F(x0 ) Hence, the frequency x corresponds to the most probable size of domains in the glassy structure. Moreover, using the full width at half maximum of F(x0 ) curves and Eq. (8), an estimate of the size distribution can be carried out.
Fig. 11. Frequency distributions F(x0 ) of cohesive domains (dotted lines) and log-normal distribution ®ts (solid line), for undoped and 500 ppm doped xerogels annealed at 940°C and for Suprasil.
J.-M. Nedelec et al. / Journal of Non-Crystalline Solids 243 (1999) 209±219
Choosing the sound velocity of vitreous silica of the v 4100 m/s [46], and using the frequency x maximum of F(x0 ), it is found that the most probable size of domains 2a is equal to 11.1 nm (8 cmÿ1 ), 2.9 nm (30 cmÿ1 ) and 2.24 nm (36 cmÿ1 ), for 500 Mn2 -doped, undoped gel-derived silica and fused silica, respectively. These values are obtained assuming the same sound velocity for all samples, an assumption which is supported by density measurements which indicate that all samples heat-treated at 940°C have densities which are only slightly higher than that of vitreous silica (Fig. 1). Hence, the great dierence in 2a in Mndoped samples cannot be due to a reduced sound velocity which would lead to a lower density. This assumption is in agreement with Chmel et al. [47] who showed that the change observed in the boson peak position in the Raman spectra of gel-derived silica is not caused by a change in sound velocity. Indeed, they pointed out that in the annealingtemperature range where the BP appears in the Raman spectra of silica xerogels, the transverse sound velocities measured by Brillouin scattering were almost constant and equal to about 4000 m/s. The spread in domain size estimated using Eq. (7) is found to be equal to 2.1 nm for the doped xerogels and 1.2 nm for undoped samples. Therefore, it is clear that the distribution of domain sizes compared with the most probable size 2a is much narrower for doped materials D 2a=2a 0:2 than for undoped ones and fused silica D 2a=2a 0:5. The results presented above, indicate that at an annealing temperature of 940°C, doping with Mn2 ions allows the formation of larger cohesive entities in the gel network, with a smaller spread in domain size than for undoped samples. Furthermore, one might note that for more highly doped samples (>500 ppm) the size of domains should be larger than that obtained for the 500 ppm sample and thus unobservable by Raman spectroscopy. This is certainly the reason why the boson peak is absent in Raman spectra of highly doped samples. For undoped gel-derived silica, the domain sizes are much smaller and exhibit a wider spread around the most probable value, a behaviour which is similar to that observed for fused silica.
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5. Conclusion Density measurements of silica xerogels reported in Section 3.1 show that complete densi®cation occurs for annealing temperatures in the range 800±940°C. These results indicate that at 940°C, the doped xerogels have attained the density of fused silica. Therefore, according to this characterization, silica xerogels should have macroscopic properties which are similar to those of vitreous silica prepared by melting. Fluorescence results discussed in Section 3.2 show that the manganese ions incorporated into silica xerogels are in a divalent state and occupy both octahedral and tetrahedral sites. Investigations by Raman spectroscopy show that the doping of silica xerogels with Mn2 has a marked eect on the gel structure at the microscopic level. The intensity ratios ID1 /I430 and ID2 /I430 which are related to three- and four-membered silica rings within the gel structure are found to vary both with the thermal treatment and with the amount of Mn2 dopant. For temperatures up to 650°C, fourfold rings are destroyed to form both threefold rings and the three-dimensional network of SiO4 tetrahedra. This process is witnessed by a decrease of the ID1 / I430 ratio and an increase of ID2 /I430 . For the 650± 800°C range, the number of both three- and fourmembered rings is almost stable, but above 800°C, both structures are destroyed to form the three-dimensional network of SiO4 tetrahedra. For undoped xerogels, this destruction becomes drastic in the short temperature range 900±940°C. On the other hand, in the case of doped samples in this temperature range, not only does this drastic destruction seem to be delayed for three-membered rings, but it is not observed for fourfold rings. This dierence indicates that doping with Mn2 , even in small amounts (less than 0.1%), maintains the presence of three- and four-membered rings in the gel-derived silica structure. On the other hand, results indicate that since the more highly strained three-membered rings are considerably less stable than their four-membered counterparts, there are signi®cantly fewer in the ®nal network structure. The temperatures at which the destruction of three and fourfold rings is found to be more important for undoped xerogels (900 to 940°C)
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also correspond to temperatures at which there is an appearance of the Boson peak (BP). Calculations based on the pro®le of the BP indicate that the size of cohesive domains in the gel-derived silica network depends on the concentration of Mn2 ions. In eect, for the sample doped at 500 ppm, the domain size is much larger (11 nm) than that of undoped samples (2.1 nm). This latter value is close to that obtained for fused silica. Hence, contrary to information provided by density measurements, one can conclude that on the nanometer scale, the densi®cation of doped xerogels is not completely achieved. Moreover, if we consider that the occurrence of the Boson peak is indicative of a glassy structure, then we can deduce that the conversion of these xerogels to the glassy state is achieved within a very short temperature range (between 930°C and 940°C). The correlation between the appearance and the position of the boson peak on the one hand and the number of defects remaining in the gel network on the other, requires further clari®cation. One can note that for undoped xerogels the densi®cation process induces strains that cause a distortion of the SiO4 tetrahedral network, and the destruction of three- and four-membered rings leading to the creation of small-sized cohesive domains. Doping with Mn2 slows down the densi®cation process, reducing strains and maintaining the existence of more three- and four-membered rings, thus leading to the formation of larger ordered regions. Acknowledgements The authors thank C. Armellini for her invaluable help in sample elaboration and Drs M. Ferrari and A. Boukenter for very fruitful discussions. The Centre d'Etudes et de Recherches Lasers et Applications (CERLA) is supported by the Ministere Charge de la Recherche, the Region Nord/Pas de Calais and the Fonds Europeens de Developpement Economique des Regions. References [1] L.L. Hench, J.K. West (Eds.), Chemical Processing of Advanced Materials, Wiley, New York, 1992.
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