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Raman spectroscopy study of phase transformations in titania-containing lithium aluminosilicate glasses doped with CoO
Journal of Non-Crystalline Solids 351 (2005) 2969–2978 www.elsevier.com/locate/jnoncrysol
Raman spectroscopy study of phase transformations in titania-containing lithium aluminosilicate glasses doped with CoO S.J. Bae a, Uk Kang a, O. Dymshits
b,*
, A. Shashkin b, M. Tsenter b, A. Zhilin
b
a
b
Korea Electrotechnology Research Institute, Kyonggi-Do 437-808, South Korea Research and Technological Institute of Optical Materials Science, 36/1 Babushkin Street, Saint-Petersburg 192171, Russia Received 15 December 2004; received in revised form 20 May 2005
Abstract The influence of small additions of CoO (0.05–2.0 mol%) on phase decomposition and further crystallization in glasses of the Li2O– Al2O3–SiO2–TiO2 system were studied by comparison of variations in corresponding Raman scattering spectra, optical absorption spectra and X-ray diffraction patterns. It is found out that at early stages of phase decomposition at heat-treatment temperatures of 660–700 C, cobalt oxide enters the alurninotitanate phase, increases temperature range of its existence and influences the kinetics of formation of crystalline phases with anatase and anosovite structure precipitated in this phase upon heat-treatment. Upon heattreatments at higher temperatures (720–750 C), crystallization of aluminocobalt spinel and of small amounts of cobalt titanate is revealed. Addition of CoO facilitates crystallization of b-eucryptite solid solution with b-quartz structure, which takes place at the same temperatures. With increasing heat-treatment temperature, anosovite, spinel, and cobalt titanate are decomposed. At 950 C, a part of TiO2 precipitates in the form of aluminum titanate, tialite, which leads to slowing down the crystallization of anatase. 2005 Elsevier B.V. All rights reserved. PACS: 61.10.E; 61.43.F; 81.05.P
1. Introduction It is well known that phase transformations that take place upon heat-treatments of titania-containing lithium aluminosilicate glasses are reflected in corresponding Raman spectra [1,2]. These spectroscopic findings in combination with X-ray data provide rich information about compositions, structures, and amounts of precipitating phases as well as about sizes of regions of inhomogeneity (low-frequency Raman scattering) [3–6]. Small additions of alkali, alkali earth, rare earth, and transition metal oxides in compositions of glass–ceramics are used as technological additives useful for glass manufacturing or for achieving special spectral–lumi*
Corresponding author. Tel.: +7 812 560 1911; fax: +7 812 560 1022. E-mail address: [email protected] (O. Dymshits). 0022-3093/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2005.07.019
nescent properties. However, added even in small amounts, these dopants influence significantly the sequence of phase transformations and structure of final material. Previously, with the help of Raman and optical spectroscopy we demonstrated that addition of alkali, alkali earth, and transition metals (Na, Mg, and Ni) changes significantly both kinetics of phase separation and crystallization of titania-containing and silicate phase in lithium aluminosilicate glasses [1,2]. Such influence of small additions of doping ions we explained either by their impact on glass viscosity (the case of Na2O) or by stabilization of liquid phase-separated regions (the case of MgO and NiO). Earlier [2], by Raman spectroscopy we studied in detail only the influence of NiO on phase transformations in lithium aluminosilicate glasses and demonstrated that NiO enters amorphous aluminotitanate phase, crystallizes there in the form of NiAl2O4 and retards crystallization of the main
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crystalline lithium aluminosilicate phase with the structure of b-quartz. The objective of the present paper is to study the effect of CoO addition on phase separation and crystallization of lithium aluminosilicate glasses.
2. Experimental procedure We have studied lithium aluminosilicate glasses with excess of Al2O3 over Li2O nucleated by TiO2 with no dopants and doped by 0.05–2.0 mol% CoO. Initial glasses 400 g in weight were melted in a laboratory electric furnace with Globar heating elements in silica crucibles at 1560 C for 4 h with stirring, poured onto a metal plate, and annealed at 600 C. Glasses were heat-treated in isothermal conditions at 660–950 C for 24 h. The exact heat-treatment schedules are listed in figure captions. Raman spectra were excited with argon ion laser emission at wavelengths of 488.0 and 514.5 nm and recorded by double monochromator in right angle geometry. X-ray diffraction (XRD) patterns of powdered samples were measured using Cu Ka radiation with a Ni filter.
3. Results and discussion We have studied changes in Raman spectra of heattreated glasses in dependence of CoO content. The results obtained are shown in Figs. 1–3, 5–9 and in Table 1. All initial glasses are X-ray amorphous. Addition of CoO does not influence their Raman spectra that have the shape shown in Fig. 1, curve 1. The spectrum of the initial glass has two bands corresponding to vibrations of silicate glass network, an intense one at 460 cm1 and a weaker one at 800 cm1. The intense band at 910 cm1 corresponds to vibrations of [TiO4] tetrahedra entered into this network [7]. The wide boson peak has a maximum at 90 cm1. Heat-treatment at 660 C leads to liquid phase separation, which modifies spectra of all glasses under study (Fig. 1, curves 2–6): the band at 910 cm1 slightly shifts to 890–880 cm1 and weakens, which reflects titanium leaving the silicate network and the formation of aluminotitanate phase. New Raman bands at 150 cm1 and 250 cm1 correspond to disordered crystalline phases with anatase and anosovite1 structures, respectively [1]. As it was demonstrated, low-frequency Raman spectroscopy reveals phase separation in heat-treated samples [1,4–6]. Previously, we studied the evolution of a low-frequency Raman spectrum in the course of heattreatment of a lithium aluminosilicate glass with titania.
1 Solid solution xR2O3 Æ yTiO2 with anosovite structure (R = A13+, traces of Ti3+ and Fe3+), x y.
Fig. 1. Raman spectra of initial (1) and heat-treated at 660 C for 24 h (2–6) glasses with varied CoO content (mol%). 1 and 2—0.0CoO; 3— 0.1CoO; 4—0.5CoO; 5—1.0CoO; 6—2.0CoO. kexit = 488.0 nm.
The boson peak in Raman spectra of lithium aluminosilicate glasses was found to consist of two closely spaced differently polarized components of a broad boson peak. Their frequencies, shapes, and widths are pronouncedly changed with heat-treatment. A narrow low-frequency band two components of which are associated with spheroidal and torsional modes of elastic vibrations of the regions of inhomogeneity enriched in TiO2 was observed as a result [1,4,6]. Sizes of inhomogeneous regions formed in heat-treated glasses were calculated from the position of the low-frequency band in Raman spectrum,
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Fig. 3. Raman spectra of glasses with varied CoO content (mol%) heat-treated at 720 C for 24 h. 1—0.0CoO; 2—0.05 CoO; 3—0.1CoO; 4—1.0CoO; 5—2.0CoO (mol%). kexit = 514.5 nm.
140
6 120
5
I, a. u.
100
Fig. 2. Raman spectra of glasses with varied CoO content (mol%) heat-treated at 700 C for 24 h. 1—0.0CoO; 2—0.05CoO; 3—0.1CoO; 4—1.0CoO; 5—2.0CoO (mol%). kexit = 488.0 nm.
4 3
80 60 40
2
20
1
which arises from acoustic resonance in these regions [1,4,6,8]. There is a correlation between its position and sizes of inhomogeneous regions [8] ms02
ns m l ¼ 02 ; 2pRc
ð1Þ
where ms02 is a frequency of one of the spheroidal vibration modes, which is mostly active in Rainan scattering, coefficient ns02 is calculated by equations listed in [8] at fixed relationship between the transversal mt, and longi-
0 24.5
25.0
25.5
26.0
26.5
2θ, degrees
Fig. 4. Relative intensity of the main peak of b-eucryptite solid solution for glass–ceramics with various CoO content (mol%) heattreated at 720 C for 24 h. 1—0.0CoO; 2—0.05CoO; 3—0.1CoO; 4— 0.5CoO; 5—1.0CoO; 6—2.0CoO.
tudinal ml, velocities of sound and depends on conditions on the interface between phases, R is the radius
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Fig. 6. Raman spectra of glasses with varied CoO content (mol%) heat-treated at 800 C for 24 h. 1—0.0CoO; 2—0.05 CoO; 3—0.1CoO; 4—1.0CoO; 5—2.0CoO (mol%). kexit = 514.5 nm. Fig. 5. Raman spectra of glasses with varied CoO content (mol%) heat-treated at 750 C for 24 h. 1—0.0CoO; 2—0.05CoO; 3—0.1CoO; 4—1.0CoO; 5—2.0CoO (mol%). kexit = 514.5 nm.
of spherical particles, and c is the velocity of light in vacuum. According to [9], for a large number of crystals ns02 insignificantly changes with mt/ml, then an approximate Eq. (1) can be used, in which ns02 =p 0:8 : 2R
0:8ml cms02
ð2Þ
To determine sizes of inhomogeneous regions, one should know their composition so to choose the proper velocity of sound. For glasses under study with regions of inhomogeneity having composition of TiO2, we used velocity of sound for rutile2 ml = 9.02 · 105 cm/s because 2
Rutile is a stable form of TiO2. Velocities of sound for anatase, a metastable form of TiO2 and for solid solution of anosovite are not known.
previously we demonstrated a good fit of sizes of inhomogeneous regions calculated by this means to the data of small angle X-ray scattering [1,6]. In the spectrum of glass heat-treated at 660 C the boson peak disappears while a narrow low-frequency band is formed (Fig. 1). Its position was found to depend on the CoO content in the initial glass. It shifts to lower frequencies with increasing the CoO concentration (ms02 = 68 cm1 for glass with no CoO and 50 cm1 for glass with 2 mol% CoO). It implies that increasing the CoO content leads to an increase of sizes of titania-containing inhomogeneous regions from 3.5 to 4.7 nm. After heat-treatment at 700 C these sizes become equal to 5.0 nm for samples with any CoO content (the position of the low-frequency band is within the range of 46–49 cm1, see Fig. 2). The behavior of the band at 800 cm1 is of special interest. In initial glasses this band arises from vibra-
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Fig. 7. Raman spectra of initial (1) and heat-treated glass doped by 0.1 mol% CoO. Heat-treatment schedules: 2—660 C for 24 h; 3— 720 C for 24 h; 4—750 C for 24 h; 5—800 C for 24 h; 6—950 C for 24 h. kexit = 514.5 nm.
tions of SiO4 tetrahedra of the glass network, its intensity is low, noticeably smaller than the intensity of band at 910 cm1. It is independent of CoO content (compare curves 1 in Figs. 1 and 7–9). However, in spectra of samples heat-treated at 660 and 700 C intensity of the band at 800 cm1 increases with CoO content (Fig. 1, curves 4–6, Table 1 and Fig. 2, curves 2–5). Previously, we dem-
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Fig. 8. Raman spectra of initial (1) and heat-treated glass doped by 0.5 mol% CoO. Heat-treatment schedules: 2—660 C for 24 h; 3— 720 C for 24 h; 4—750 C for 24 h; 5—800 C for 24 h; 6—950 C for 24 h. kexit = 514.5 nm.
onstrated that in this spectral range there are also vibrations of Ti–O bonds of [TiO6] octahedra located in aluminotitanate amorphous phase provided that the amount of this phase is high (for instance, in glasses with high contents of TiO2 and Al2O3) [1]. Small additions of transition metal ions are known to stabilize liquid aluminotitanate phase [1,2]. An increase of intensity of the band at 800 cm1 in Raman spectra of heat-treated
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Fig. 9. Raman spectra of initial (1) and heat-treated glass doped by 1.0 mol% CoO. Heat-treatment schedules: 2—660 C for 24 h; 3— 720 C for 24 h; 4—750 C for 24 h; 5—800 C for 24 h; 6—950 C for 24 h.
glasses doped by NiO we explained by this reason [2]. Judging from the increase of intensity of the same band in spectra shown in Figs. 1 and 2, CoO stabilizes amorphous aluminotitanate phase in the same manner as NiO does, which is the most pronounced at early stages of phase separation, at 660 and 700 C. At these stages intensities of bands at 150 and 250 cm1 characteristic of crystallizing aluminotitanate phase behave differently with CoO content (Fig. 1, curves 2–6). Intensity of the former band originated from vibrations of Ti–O bonds in anatase structure increases only slightly (Fig. 1, curves 2–6). Intensity of the latter band due to Ti–O vibrations in anosovite crystals decreases to such extent that after heat-treatment at 660 C this band cannot be
found in Raman spectrum of the sample with 2 mol% CoO (Fig. 1, curve 6). After heating at 700 C it disappears even from the spectrum of sample with 1 mol% CoO (Fig. 2, curves 4 and 5). Similar behavior of these bands we observed in glasses of similar base compositions with no dopants at higher temperatures and explained it by recrystallization of anosovite to anatase [1]. The influence of CoO on Raman spectra becomes even more pronounced after heat-treatment at 720 C. With increasing the CoO content, the bands at 250 and 800 cm1 behave at the same way as at temperature of 700 C (see Fig. 3, curves 4 and 5). However, an increase of intensity of the band at 800 cm1 characteristic of the amorphous part of aluminotitanate phase correlates with a decrease of intensity of the band located at 150 cm1 and originated from the crystalline part of this phase (compare curves 2 and 5 in Fig. 3). The shape of a broad band at 460 cm1 originated from the silicate phase gradually changes with CoO. An appearance of its narrow component at 480 cm1 and of a weak high-frequency band at 1100 cm1, characteristic of b-eucryptite solid solution [1], manifests crystallization of the main phase (Fig. 3, curves 2–5). The X-ray diffraction data confirms an acceleration of crystallization of b-eucryptite solid solution with increasing the CoO content in the initial glass, Fig. 4. Low frequency band is observed only in the spectrum of the sample with no CoO, which has a minimum background scattering. Its position shifts to 40 cm1, which corresponds to sizes of inhomogeneity regions equal to 6.0 nm. In Raman spectra of samples starting with dopant content of 0.1 mol% CoO and heated at 720C, a weak band appears at 680 cm1. Its intensity increases with CoO (Fig. 3, curves 3–5). This band is probably due to the formation of a small amount of cobalt titanate, as its frequency corresponds to that of the single intense Raman band (690 cm1) of a specially prepared model compound, CoTiO3. An absence of CoTiO3 peaks on XRD patterns of our samples is probably due to a small amount of this phase. Raman spectra of samples heat-treated at 750 C for 24 h are shown in Fig. 5. The spectra prove crystallization of b-eucryptite solid solutions in all samples, including the sample with no CoO. With increasing heat-treatment temperature, the amount of titanate phase crystallized in the form of anatase also increases, which is confirmed by an appearance of its band with a low intensity located at 640 cm1. The Raman band at 250 cm1, a spectral attribute of anosovite solid solution, disappears. It implies that at this heat-treatment schedule anosovite transforms into anatase in samples with any content of CoO. The intensities of bands at 150 and 800 cm1 characteristic of anatase crystals and of amorphous part of aluminotitanate phase, respectively, are practically independent of CoO content. The only exception is the sample with the
Table 1 Dependence of relative intensities of Raman bands of heat-treated glasses on content of CoO Heat-treatment, C/h
Band position, cm1
CoO content, mol% 0.0 1
1
Initial glass
2
660, 24
3
700, 24
4
720, 24
5
750, 24
6
800, 24
7
950, 24
910 800 890 800 150 880 800 150 880 800 150 880 800 150 880 800 150 880 800 150
0.1 2
3
1.0 0.2 0.40 0.20 0.30 0.26 0.26 0.50
1.0 }
4.3 0.8 11.6
}
}
2
3
1.0 0.2 0.45 0.26 0.26 0.29 0.36 0.40 0.8
}
1
0.5
1.0 0.06
2
3
1.0 0.2 0.38 0.26 0.27 0.27 0.32 0.41 1.0
0.06
1
1.0
2.9 0.8 6.0
?
0.08 0.5 0.07
1 1.0 0.2 0.43 0.43 0.30 0.30 0.50 0.30
? 1.1 7.8
0.08 0.6 0.07
2.0 2
3
R1.2 1.8 1.3 2.5 1.2 7.2
1 1.0 0.2 0.46 0.60 0.37 0.30 0.70 ?
0.08 0.7 0.07
1.5 0.14
1.6 0.11
1.9 0.12
2.1 0.2
2.0
1.9
3.3
2.6
2
3
R2.0 ? 2.0 1.4 1.6 3.4
0.14 0.5 0.09 1.7
(1) Intensities of bands listed in the columns 1, 2, and 3 are measured in respect of the intensity in the maximum of the band at 460 cm1, in respect of the integrated intensity of the band at 460 cm1 and in respect of the intensity in the maximum of the Raman band located at 480 cm1, respectively. (2) Columns 2 of line 3 show relative total intensities of bands at 880–800 cm1. (3) ? denotes that we failed to estimate the intensity of the band.
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maximum CoO content of 2 mol%. In Raman spectrum of such sample the band at 800 cm1 is much more intensive than in the sample with no CoO while the band at 150 cm1 is noticeably weaker (Fig. 5, curves 1 and 5). With increasing heat-treatment temperature to 800 C, similar dependences are found in Raman spectra of samples with various CoO contents, Fig. 6. After this heat-treatment in Raman spectra there is the whole set of anatase bands: 150, 400, 520, and 640 cm1, which means that the amount of these crystals increases. Let us discuss in detail the behavior of high-frequency bands in Raman spectra. At early stages of phase separation (Figs. 1–3) as we already mentioned, there is a redistribution of intensities between bands at 910 and 800 cm1 in favor of the latter band, which corresponds to titanium leaving the glass forming network and participating in the formation of amorphous aluminotitanate phase. The intensity of this band increases with CoO because Co ions enter this phase. After heat-treatments at 750–800 C, intensities of these bands become comparable at any CoO content and in Raman spectrum we see only one band with hardly visible maxima at 880 and 800 cm1 (Figs. 5 and 6). We explained an appearance of two maxima in a high-frequency band by vibrations of two types of titanium–oxygen polyhedrons forming amorphous aluminotitanate phase together with aluminium–oxygen polyhedrons [1]. They can be [TiO4] tetrahedrons and [TiO6] octahedrons both with or without distortions, which leads to a variation in length of Ti–O bonds and, consequently, frequencies of corresponding vibrations. When temperature increases to 950 C, in spectra of samples with any CoO content we see redistribution of intensities in a high-frequency band. Intensity in the maximum 880 cm1 increases and sometimes the position of maximum shifts to 900 cm1 (Figs. 7–9, curves 6). The similar behavior we observed in other glass of lithium aluminosilicate system, in which aluminotitanate amorphous phase preserves upon crystallization of the main phase. Intensification of this band in the spectrum after high-temperature heat-treatments of the sample manifests beginning of Al2TiO5, tialite, crystallization [1]. In Raman spectrum of tialite the most intense is a broad band at 900 cm1 [10], which appears the first. At further temperature increase to 1150 C, tialite continue crystallizing and in Raman spectrum all its bands appear [1]. It is interesting not only to describe qualitatively the influence of CoO addition on kinetics of phase separation and crystallization in glasses under study but to estimate amounts of precipitated phases as well. It is a complex task as it is very difficult to find a single internal standard in Raman spectra of glasses heat-treated at different stages. For instance, for initial glasses and glasses heat-treated at 660 and 700 C, intensity in the maxi-
mum of the band at 460 cm1 can be such an internal standard. Then comparing relative intensities of bands at 900, 800, and 150 cm1 we find how does the TiO2 content in amorphous silicate network, in aluminotitanate phase and in anatase change with CoO (Table 1, lines 1–3, columns 1–3). When heat-treatment temperature increases to 720 C, intensity in the maximum of the band at 460 cm1 does not play the role of an internal standard any more because it changes with CoO, as CoO facilitates crystallization of the main phase. In this case, we used an integral intensity of the complex band nearby 460 cm1, in which a narrow band appears at 480 cm1 due to crystallization of b-eucryptite solid solution, as an internal standard (line 4, column 2). Previously, it was demonstrated [5] that integral intensity of this band practically does not change (with accuracy of ±10%) in the course of glass crystallization, there is only some intensity redistribution between broad and narrow components, corresponding to vibrations in amorphous and crystalline parts of the glass matrix. To the accuracy of this observation, the band can play the role of required standard. It should be also pointed out that starting from spectra of samples heat-treated at 720 C for 24 h, we compared integrated intensity of the complex band at 880–800 cm1 as it is just this band which characterizes the total amount of aluminotitanate phase in samples.3 In samples heat-treated at 750, 800, and 950 C, according to X-ray diffraction analysis, crystallization of the main phase is practically accomplished. For quantitative characterization of their Raman spectra we used intensity in the maximum of the complex band at 480 cm1 as an internal standard (Table 1, lines 5–7, column 3). We changed the internal standard, because the error of its estimation is less than that for integrated band complicated by superposition with anatase bands at 400 and 520 cm1. Consequently, relative intensities of a series of Raman bands listed in Table 1 allow for a rough estimation of amounts of some phases precipitated in samples upon their heat-treatment depending on the CoO content. It is seen from Table 1 that in spectra of initial glasses intensity of band at 910 cm1 is independent of CoO content. The band at 800 cm1 is as weak as in spectra of regular silicate glasses (Table 1, line 1). It means that addition of CoO does not stimulate phase separation of
3 We presume that during heat-treatment at 720 C for 24 h practically all titanium leaves the aluminosilicate network and Raman spectra do not exhibit any more the band nearby 910 cm1 originated from [TiO4] tetrahedrons built into this network. The origin of the high-frequency shoulder at 880 cm1, which locates nearby the band at 800 cm1, was discussed above.
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initial glass. Optical absorption spectra are similar for all initial glasses [11]. They are due to absorption of Co(II) ions predominantly in octahedral coordination with a small share of tetrahedral coordination [11]. This spectrum is characteristic for Co(II) in silicate glasses and thoroughly discussed in the literature (see Ref. [12]). In the course of heat-treatment CoO does not influence the rate with which titanium leaves the silicate network. The rate of phase separation is governed solely by heat-treatment schedule. It is proved by the same weakening of intensity of the band at 910 cm1 by a factor of 2.5 after heat-treatment at 660 C for 24 h (Table 1, line 2) and by a factor of 3.5–4.0 after further temperature increase to 700 C (line 3) for samples with any CoO content. It should be noted that at early stages of heat-treatment the amount of aluminotitanate phase crystallized directly upon phase separation is practically independent of CoO content. This is evident from the constancy of intensity of the anatase band at 150 cm1 (Table 1, line 2). Its small increase in the spectrum of sample with 2.0 mol% CoO is probably due to crystallization of the titanate phase solely in the form of anatase. Spectral attribute of anosovite, the band at 250 cm1, is not found in the corresponding spectrum (Fig. 1, curve 6). Hence, at the very beginning of phase separation, addition of CoO leads only to increase of the temperature interval of existence of aluminotitanate amorphous phase and increases its amount (in the spectrum of the sample with 2.0 mol% CoO intensity of characteristic band at 800 cm1 is three times as great as in sample with no CoO, Table 1, lines 2 and 3). However, at the next stage of the process, at 700 C, with increasing the CoO content, crystallization of amorphous aluminotitanate phase retards just due to its stabilization by CoO. In fact, the anatase band in Raman spectrum weakens by a factor of 1.7 with CoO content increase from 0 to 1.0 mol% (Table 1, line 3). At 720 and 750 C, this correlation between an increase of the amount of stabilized amorphous aluminotitanate phase and a decrease of amount of anatase reveals the most pronouncedly. Indeed, the integral intensity of the band at 800–880 cm1 in the spectrum of the sample with 2.0 mol% is two times as great as in the spectrum of the sample with no CoO, while intensity of the anatase band at 150 cm1 is three times less than in the spectrum of the sample with no CoO (Table 1, lines 4 and 5, column 2). It should be noted that some deviation from this correlation for samples with 0.1, and 0.5 mol% CoO and an increase of the anatase band intensity at constant intensity of the integrated band 800–880 cm1 (Table 1, line 5, columns 2 and 3) is most likely to be connected with anosovite and cobalt titanate recrystallization into anatase which takes place at heat-treatment 750 C for 24 h.
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It is possible to follow the variation of intensities of Raman bands with temperature.4 The band at 800– 880 cm1 intensifies in spectra of samples heat-treated not over 720 C, and then it weakens and does it the quicker, the higher is the CoO content in the sample (Table 1, lines 4–6). In the process, intensity of the band at 150 cm1 grows faster for samples with higher CoO content and becomes comparable in spectra of all samples (Table 1, lines 6 and 7). For instance, in the temperature range 720–800 C intensity of anatase band in spectra of glasses with no CoO increases by a factor of 4 and by a factor of 9 for samples with 1.0 and 2.0 mol% CoO. It means that upon high-temperature heat-treatments the rate of aluminotitanate phase crystallization increases with increasing the CoO content. Probably, CoO does not stabilize this phase any more. This assumption is correlated with XRD and optical spectroscopy findings. According to XRD data, aluminocobalt spinel appears in samples starting from 720 C, which manifests CoO leaving the aluminotitanate phase. Absorption spectra of these samples are similar to spectra of aluminocobalt spinel [12]. It is worth mentioning that at the beginning of phase separation anatase, on the contrary, rapidly crystallizes in glass with no CoO. For this sample intensity of the band at 150 cm1 increases by the factor of 3.3 in the temperature range 660–720 C while for sample with 1.0 mol% CoO it increases only by the factor of 1.4 (Table 1, lines 2 and 4, columns 1 and 3). Thus, if at the beginning of phase separation CoO enters aluminotitanate phase, stabilizes it and helps retarding its crystallization, after heating at 720 C the situation changes. Cobalt ions in the form of CoAl2O4 crystals leave aluminotitanate phase speeding up its crystallization. Then in samples with any CoO content the same amount of anatase is to crystallize. The amount of precipitated anatase corresponds only to heat-treatment temperature (800–950 C). It develops the faster, the higher is the content of aluminotitanate phase stored in the sample with the help of CoO at early stages of heat-treatments. When temperature increases to 950 C, in spectra of samples with any CoO content we see a redistribution of intensities in a complex band at 800–880 cm1. Intensity in the maximum 880 cm1 increases and the position of maximum sometimes shifts to 900 cm1 (Figs. 7–9, curves 6). Intensification of this band manifests beginning of Al2TiO5, tialite, crystallization [1] (the most intense band of tialite is 900 cm1). The CoO somehow speeds up tialite crystallization (Table 1, line 7). At 950 C anatase continue to crystallize, however, the process slows down. As it is seen from Table 1, with heat-treatment temperature increase from 800 to 4 In lines 3–7 of Table 1 only relative intensities of bands listed in the same column are comparable.
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950 C intensity of the band at 150 cm1 increases 1.5 times when compared with its increase to 3 times at temperature increase from 750 to 800 C.
4. Conclusions By the use of combination of Raman spectroscopy, optical absorption spectra and X-ray diffraction analysis we demonstrated that addition of CoO influences kinetics of phase transformations of titania-containing lithium aluminosilicate glasses. It was found that addition of CoO into initial glass does not facilitate its phase separation as it does not influence the rate with which titanium leaves the silicate network. At low-temperature heat-treatments (660– 700 C), CoO enters aluminotitanate amorphous phase, which results in increased temperature interval of existence and increased amount of this phase. It retards anatase crystallization and speeds up transformation of anosovite into anatase. Formation of minor phases of cobalt titanate and spinel is revealed in a narrow temperature range. With formation of these phases aluminotitanate amorphous phase decomposes and the amount of anatase precipitated at elevated temperatures increases with CoO. Addition of CoO decreases the temperature of crystallization of the main crystalline phase, b-eucryptite solid solution.
Acknowledgments This work was partly supported by the Russian Foundation for Basic Researches (Project Nos. 04-0332657 and 05-03-08013-ofi_p) and by Grant NSH1405.2003.3 of President of Russian Federation.
References [1] O.S. Dymshits, A.A. Zhilin, V.I. Petrov, M.Ya. Tsenter, T.I. Chuvaeva, V.V. Golubkov, Glass Phys. Chem. 24 (2) (1998) 114. [2] O.S. Dymshits, A.A. Zhilin, M.Ya. Tsenter, T.I. Chuvaeva, Glass Phys. Chem. 17 (1) (1991) 87. [3] E. Duval, A. Boukenter, B. Champagnon, Phys. Rev. Lett. 56 (19) (1986) 2052. [4] V.I. Petrov, Ya.S. Bobovich, Opt. Spectrosc. 67 (1989) 363. [5] Ya.S. Bobovich, V.I. Petrov, M.Ya. Tsenter, A.A. Zhilin, T.I. Chuvaeva, Opt. Spectrosc. 72 (1992) 758. [6] T.I. Chuvaeva, O.S. Dymshits, V.I. Petrov, M.Ya. Tsenter, A.A. Zhilin, V.V. Golubkov, J. Non-Cryst. Solids 243 (1999) 244. [7] Ya.S. Bobovich, Opt. Spektrosk. 14 (5) (1963) 647 [Opt. Spectrosc. (Engl. Transl.)]. [8] A. Tamura, K. Higeta, T. Ichinokava, J. Phys. C 15 (24) (1982) 4975. [9] B. Andrianasolo, B. Champagnon, C. Esnouf, J. Non-Cryst. Solids 126 (1) (1990) 103. [10] S.A. Azimov, D.D. Gulamova, N.N. Melnik, M.Kh. Sarkisov, S.S. Sulejmanov, L.M. Tsapenko, Inorg. Mater. 20 (3) (1984) 469. [11] Uk Kang, A.A. Zhilin, G.T. Petrovsky, O.S. Dymshits, T.I. Chuvaeva, J. Non-Cryst. Solids 258 (1999) 216. [12] Uk Kang, O.S. Dymshits, A.A. Zhilin, T.I. Chuvaeva, G.T. Petrovsky, J. Non-Cryst. Solids 204 (1996) 151.
Raman spectroscopy study of phase transformations in titania-containing lithium aluminosilicate glasses doped with CoO S.J. Bae a, Uk Kang a, O. Dymshits
b,*
, A. Shashkin b, M. Tsenter b, A. Zhilin
b
a
b
Korea Electrotechnology Research Institute, Kyonggi-Do 437-808, South Korea Research and Technological Institute of Optical Materials Science, 36/1 Babushkin Street, Saint-Petersburg 192171, Russia Received 15 December 2004; received in revised form 20 May 2005
Abstract The influence of small additions of CoO (0.05–2.0 mol%) on phase decomposition and further crystallization in glasses of the Li2O– Al2O3–SiO2–TiO2 system were studied by comparison of variations in corresponding Raman scattering spectra, optical absorption spectra and X-ray diffraction patterns. It is found out that at early stages of phase decomposition at heat-treatment temperatures of 660–700 C, cobalt oxide enters the alurninotitanate phase, increases temperature range of its existence and influences the kinetics of formation of crystalline phases with anatase and anosovite structure precipitated in this phase upon heat-treatment. Upon heattreatments at higher temperatures (720–750 C), crystallization of aluminocobalt spinel and of small amounts of cobalt titanate is revealed. Addition of CoO facilitates crystallization of b-eucryptite solid solution with b-quartz structure, which takes place at the same temperatures. With increasing heat-treatment temperature, anosovite, spinel, and cobalt titanate are decomposed. At 950 C, a part of TiO2 precipitates in the form of aluminum titanate, tialite, which leads to slowing down the crystallization of anatase. 2005 Elsevier B.V. All rights reserved. PACS: 61.10.E; 61.43.F; 81.05.P
1. Introduction It is well known that phase transformations that take place upon heat-treatments of titania-containing lithium aluminosilicate glasses are reflected in corresponding Raman spectra [1,2]. These spectroscopic findings in combination with X-ray data provide rich information about compositions, structures, and amounts of precipitating phases as well as about sizes of regions of inhomogeneity (low-frequency Raman scattering) [3–6]. Small additions of alkali, alkali earth, rare earth, and transition metal oxides in compositions of glass–ceramics are used as technological additives useful for glass manufacturing or for achieving special spectral–lumi*
Corresponding author. Tel.: +7 812 560 1911; fax: +7 812 560 1022. E-mail address: [email protected] (O. Dymshits). 0022-3093/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2005.07.019
nescent properties. However, added even in small amounts, these dopants influence significantly the sequence of phase transformations and structure of final material. Previously, with the help of Raman and optical spectroscopy we demonstrated that addition of alkali, alkali earth, and transition metals (Na, Mg, and Ni) changes significantly both kinetics of phase separation and crystallization of titania-containing and silicate phase in lithium aluminosilicate glasses [1,2]. Such influence of small additions of doping ions we explained either by their impact on glass viscosity (the case of Na2O) or by stabilization of liquid phase-separated regions (the case of MgO and NiO). Earlier [2], by Raman spectroscopy we studied in detail only the influence of NiO on phase transformations in lithium aluminosilicate glasses and demonstrated that NiO enters amorphous aluminotitanate phase, crystallizes there in the form of NiAl2O4 and retards crystallization of the main
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crystalline lithium aluminosilicate phase with the structure of b-quartz. The objective of the present paper is to study the effect of CoO addition on phase separation and crystallization of lithium aluminosilicate glasses.
2. Experimental procedure We have studied lithium aluminosilicate glasses with excess of Al2O3 over Li2O nucleated by TiO2 with no dopants and doped by 0.05–2.0 mol% CoO. Initial glasses 400 g in weight were melted in a laboratory electric furnace with Globar heating elements in silica crucibles at 1560 C for 4 h with stirring, poured onto a metal plate, and annealed at 600 C. Glasses were heat-treated in isothermal conditions at 660–950 C for 24 h. The exact heat-treatment schedules are listed in figure captions. Raman spectra were excited with argon ion laser emission at wavelengths of 488.0 and 514.5 nm and recorded by double monochromator in right angle geometry. X-ray diffraction (XRD) patterns of powdered samples were measured using Cu Ka radiation with a Ni filter.
3. Results and discussion We have studied changes in Raman spectra of heattreated glasses in dependence of CoO content. The results obtained are shown in Figs. 1–3, 5–9 and in Table 1. All initial glasses are X-ray amorphous. Addition of CoO does not influence their Raman spectra that have the shape shown in Fig. 1, curve 1. The spectrum of the initial glass has two bands corresponding to vibrations of silicate glass network, an intense one at 460 cm1 and a weaker one at 800 cm1. The intense band at 910 cm1 corresponds to vibrations of [TiO4] tetrahedra entered into this network [7]. The wide boson peak has a maximum at 90 cm1. Heat-treatment at 660 C leads to liquid phase separation, which modifies spectra of all glasses under study (Fig. 1, curves 2–6): the band at 910 cm1 slightly shifts to 890–880 cm1 and weakens, which reflects titanium leaving the silicate network and the formation of aluminotitanate phase. New Raman bands at 150 cm1 and 250 cm1 correspond to disordered crystalline phases with anatase and anosovite1 structures, respectively [1]. As it was demonstrated, low-frequency Raman spectroscopy reveals phase separation in heat-treated samples [1,4–6]. Previously, we studied the evolution of a low-frequency Raman spectrum in the course of heattreatment of a lithium aluminosilicate glass with titania.
1 Solid solution xR2O3 Æ yTiO2 with anosovite structure (R = A13+, traces of Ti3+ and Fe3+), x y.
Fig. 1. Raman spectra of initial (1) and heat-treated at 660 C for 24 h (2–6) glasses with varied CoO content (mol%). 1 and 2—0.0CoO; 3— 0.1CoO; 4—0.5CoO; 5—1.0CoO; 6—2.0CoO. kexit = 488.0 nm.
The boson peak in Raman spectra of lithium aluminosilicate glasses was found to consist of two closely spaced differently polarized components of a broad boson peak. Their frequencies, shapes, and widths are pronouncedly changed with heat-treatment. A narrow low-frequency band two components of which are associated with spheroidal and torsional modes of elastic vibrations of the regions of inhomogeneity enriched in TiO2 was observed as a result [1,4,6]. Sizes of inhomogeneous regions formed in heat-treated glasses were calculated from the position of the low-frequency band in Raman spectrum,
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Fig. 3. Raman spectra of glasses with varied CoO content (mol%) heat-treated at 720 C for 24 h. 1—0.0CoO; 2—0.05 CoO; 3—0.1CoO; 4—1.0CoO; 5—2.0CoO (mol%). kexit = 514.5 nm.
140
6 120
5
I, a. u.
100
Fig. 2. Raman spectra of glasses with varied CoO content (mol%) heat-treated at 700 C for 24 h. 1—0.0CoO; 2—0.05CoO; 3—0.1CoO; 4—1.0CoO; 5—2.0CoO (mol%). kexit = 488.0 nm.
4 3
80 60 40
2
20
1
which arises from acoustic resonance in these regions [1,4,6,8]. There is a correlation between its position and sizes of inhomogeneous regions [8] ms02
ns m l ¼ 02 ; 2pRc
ð1Þ
where ms02 is a frequency of one of the spheroidal vibration modes, which is mostly active in Rainan scattering, coefficient ns02 is calculated by equations listed in [8] at fixed relationship between the transversal mt, and longi-
0 24.5
25.0
25.5
26.0
26.5
2θ, degrees
Fig. 4. Relative intensity of the main peak of b-eucryptite solid solution for glass–ceramics with various CoO content (mol%) heattreated at 720 C for 24 h. 1—0.0CoO; 2—0.05CoO; 3—0.1CoO; 4— 0.5CoO; 5—1.0CoO; 6—2.0CoO.
tudinal ml, velocities of sound and depends on conditions on the interface between phases, R is the radius
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Fig. 6. Raman spectra of glasses with varied CoO content (mol%) heat-treated at 800 C for 24 h. 1—0.0CoO; 2—0.05 CoO; 3—0.1CoO; 4—1.0CoO; 5—2.0CoO (mol%). kexit = 514.5 nm. Fig. 5. Raman spectra of glasses with varied CoO content (mol%) heat-treated at 750 C for 24 h. 1—0.0CoO; 2—0.05CoO; 3—0.1CoO; 4—1.0CoO; 5—2.0CoO (mol%). kexit = 514.5 nm.
of spherical particles, and c is the velocity of light in vacuum. According to [9], for a large number of crystals ns02 insignificantly changes with mt/ml, then an approximate Eq. (1) can be used, in which ns02 =p 0:8 : 2R
0:8ml cms02
ð2Þ
To determine sizes of inhomogeneous regions, one should know their composition so to choose the proper velocity of sound. For glasses under study with regions of inhomogeneity having composition of TiO2, we used velocity of sound for rutile2 ml = 9.02 · 105 cm/s because 2
Rutile is a stable form of TiO2. Velocities of sound for anatase, a metastable form of TiO2 and for solid solution of anosovite are not known.
previously we demonstrated a good fit of sizes of inhomogeneous regions calculated by this means to the data of small angle X-ray scattering [1,6]. In the spectrum of glass heat-treated at 660 C the boson peak disappears while a narrow low-frequency band is formed (Fig. 1). Its position was found to depend on the CoO content in the initial glass. It shifts to lower frequencies with increasing the CoO concentration (ms02 = 68 cm1 for glass with no CoO and 50 cm1 for glass with 2 mol% CoO). It implies that increasing the CoO content leads to an increase of sizes of titania-containing inhomogeneous regions from 3.5 to 4.7 nm. After heat-treatment at 700 C these sizes become equal to 5.0 nm for samples with any CoO content (the position of the low-frequency band is within the range of 46–49 cm1, see Fig. 2). The behavior of the band at 800 cm1 is of special interest. In initial glasses this band arises from vibra-
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Fig. 7. Raman spectra of initial (1) and heat-treated glass doped by 0.1 mol% CoO. Heat-treatment schedules: 2—660 C for 24 h; 3— 720 C for 24 h; 4—750 C for 24 h; 5—800 C for 24 h; 6—950 C for 24 h. kexit = 514.5 nm.
tions of SiO4 tetrahedra of the glass network, its intensity is low, noticeably smaller than the intensity of band at 910 cm1. It is independent of CoO content (compare curves 1 in Figs. 1 and 7–9). However, in spectra of samples heat-treated at 660 and 700 C intensity of the band at 800 cm1 increases with CoO content (Fig. 1, curves 4–6, Table 1 and Fig. 2, curves 2–5). Previously, we dem-
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Fig. 8. Raman spectra of initial (1) and heat-treated glass doped by 0.5 mol% CoO. Heat-treatment schedules: 2—660 C for 24 h; 3— 720 C for 24 h; 4—750 C for 24 h; 5—800 C for 24 h; 6—950 C for 24 h. kexit = 514.5 nm.
onstrated that in this spectral range there are also vibrations of Ti–O bonds of [TiO6] octahedra located in aluminotitanate amorphous phase provided that the amount of this phase is high (for instance, in glasses with high contents of TiO2 and Al2O3) [1]. Small additions of transition metal ions are known to stabilize liquid aluminotitanate phase [1,2]. An increase of intensity of the band at 800 cm1 in Raman spectra of heat-treated
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Fig. 9. Raman spectra of initial (1) and heat-treated glass doped by 1.0 mol% CoO. Heat-treatment schedules: 2—660 C for 24 h; 3— 720 C for 24 h; 4—750 C for 24 h; 5—800 C for 24 h; 6—950 C for 24 h.
glasses doped by NiO we explained by this reason [2]. Judging from the increase of intensity of the same band in spectra shown in Figs. 1 and 2, CoO stabilizes amorphous aluminotitanate phase in the same manner as NiO does, which is the most pronounced at early stages of phase separation, at 660 and 700 C. At these stages intensities of bands at 150 and 250 cm1 characteristic of crystallizing aluminotitanate phase behave differently with CoO content (Fig. 1, curves 2–6). Intensity of the former band originated from vibrations of Ti–O bonds in anatase structure increases only slightly (Fig. 1, curves 2–6). Intensity of the latter band due to Ti–O vibrations in anosovite crystals decreases to such extent that after heat-treatment at 660 C this band cannot be
found in Raman spectrum of the sample with 2 mol% CoO (Fig. 1, curve 6). After heating at 700 C it disappears even from the spectrum of sample with 1 mol% CoO (Fig. 2, curves 4 and 5). Similar behavior of these bands we observed in glasses of similar base compositions with no dopants at higher temperatures and explained it by recrystallization of anosovite to anatase [1]. The influence of CoO on Raman spectra becomes even more pronounced after heat-treatment at 720 C. With increasing the CoO content, the bands at 250 and 800 cm1 behave at the same way as at temperature of 700 C (see Fig. 3, curves 4 and 5). However, an increase of intensity of the band at 800 cm1 characteristic of the amorphous part of aluminotitanate phase correlates with a decrease of intensity of the band located at 150 cm1 and originated from the crystalline part of this phase (compare curves 2 and 5 in Fig. 3). The shape of a broad band at 460 cm1 originated from the silicate phase gradually changes with CoO. An appearance of its narrow component at 480 cm1 and of a weak high-frequency band at 1100 cm1, characteristic of b-eucryptite solid solution [1], manifests crystallization of the main phase (Fig. 3, curves 2–5). The X-ray diffraction data confirms an acceleration of crystallization of b-eucryptite solid solution with increasing the CoO content in the initial glass, Fig. 4. Low frequency band is observed only in the spectrum of the sample with no CoO, which has a minimum background scattering. Its position shifts to 40 cm1, which corresponds to sizes of inhomogeneity regions equal to 6.0 nm. In Raman spectra of samples starting with dopant content of 0.1 mol% CoO and heated at 720C, a weak band appears at 680 cm1. Its intensity increases with CoO (Fig. 3, curves 3–5). This band is probably due to the formation of a small amount of cobalt titanate, as its frequency corresponds to that of the single intense Raman band (690 cm1) of a specially prepared model compound, CoTiO3. An absence of CoTiO3 peaks on XRD patterns of our samples is probably due to a small amount of this phase. Raman spectra of samples heat-treated at 750 C for 24 h are shown in Fig. 5. The spectra prove crystallization of b-eucryptite solid solutions in all samples, including the sample with no CoO. With increasing heat-treatment temperature, the amount of titanate phase crystallized in the form of anatase also increases, which is confirmed by an appearance of its band with a low intensity located at 640 cm1. The Raman band at 250 cm1, a spectral attribute of anosovite solid solution, disappears. It implies that at this heat-treatment schedule anosovite transforms into anatase in samples with any content of CoO. The intensities of bands at 150 and 800 cm1 characteristic of anatase crystals and of amorphous part of aluminotitanate phase, respectively, are practically independent of CoO content. The only exception is the sample with the
Table 1 Dependence of relative intensities of Raman bands of heat-treated glasses on content of CoO Heat-treatment, C/h
Band position, cm1
CoO content, mol% 0.0 1
1
Initial glass
2
660, 24
3
700, 24
4
720, 24
5
750, 24
6
800, 24
7
950, 24
910 800 890 800 150 880 800 150 880 800 150 880 800 150 880 800 150 880 800 150
0.1 2
3
1.0 0.2 0.40 0.20 0.30 0.26 0.26 0.50
1.0 }
4.3 0.8 11.6
}
}
2
3
1.0 0.2 0.45 0.26 0.26 0.29 0.36 0.40 0.8
}
1
0.5
1.0 0.06
2
3
1.0 0.2 0.38 0.26 0.27 0.27 0.32 0.41 1.0
0.06
1
1.0
2.9 0.8 6.0
?
0.08 0.5 0.07
1 1.0 0.2 0.43 0.43 0.30 0.30 0.50 0.30
? 1.1 7.8
0.08 0.6 0.07
2.0 2
3
R1.2 1.8 1.3 2.5 1.2 7.2
1 1.0 0.2 0.46 0.60 0.37 0.30 0.70 ?
0.08 0.7 0.07
1.5 0.14
1.6 0.11
1.9 0.12
2.1 0.2
2.0
1.9
3.3
2.6
2
3
R2.0 ? 2.0 1.4 1.6 3.4
0.14 0.5 0.09 1.7
(1) Intensities of bands listed in the columns 1, 2, and 3 are measured in respect of the intensity in the maximum of the band at 460 cm1, in respect of the integrated intensity of the band at 460 cm1 and in respect of the intensity in the maximum of the Raman band located at 480 cm1, respectively. (2) Columns 2 of line 3 show relative total intensities of bands at 880–800 cm1. (3) ? denotes that we failed to estimate the intensity of the band.
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maximum CoO content of 2 mol%. In Raman spectrum of such sample the band at 800 cm1 is much more intensive than in the sample with no CoO while the band at 150 cm1 is noticeably weaker (Fig. 5, curves 1 and 5). With increasing heat-treatment temperature to 800 C, similar dependences are found in Raman spectra of samples with various CoO contents, Fig. 6. After this heat-treatment in Raman spectra there is the whole set of anatase bands: 150, 400, 520, and 640 cm1, which means that the amount of these crystals increases. Let us discuss in detail the behavior of high-frequency bands in Raman spectra. At early stages of phase separation (Figs. 1–3) as we already mentioned, there is a redistribution of intensities between bands at 910 and 800 cm1 in favor of the latter band, which corresponds to titanium leaving the glass forming network and participating in the formation of amorphous aluminotitanate phase. The intensity of this band increases with CoO because Co ions enter this phase. After heat-treatments at 750–800 C, intensities of these bands become comparable at any CoO content and in Raman spectrum we see only one band with hardly visible maxima at 880 and 800 cm1 (Figs. 5 and 6). We explained an appearance of two maxima in a high-frequency band by vibrations of two types of titanium–oxygen polyhedrons forming amorphous aluminotitanate phase together with aluminium–oxygen polyhedrons [1]. They can be [TiO4] tetrahedrons and [TiO6] octahedrons both with or without distortions, which leads to a variation in length of Ti–O bonds and, consequently, frequencies of corresponding vibrations. When temperature increases to 950 C, in spectra of samples with any CoO content we see redistribution of intensities in a high-frequency band. Intensity in the maximum 880 cm1 increases and sometimes the position of maximum shifts to 900 cm1 (Figs. 7–9, curves 6). The similar behavior we observed in other glass of lithium aluminosilicate system, in which aluminotitanate amorphous phase preserves upon crystallization of the main phase. Intensification of this band in the spectrum after high-temperature heat-treatments of the sample manifests beginning of Al2TiO5, tialite, crystallization [1]. In Raman spectrum of tialite the most intense is a broad band at 900 cm1 [10], which appears the first. At further temperature increase to 1150 C, tialite continue crystallizing and in Raman spectrum all its bands appear [1]. It is interesting not only to describe qualitatively the influence of CoO addition on kinetics of phase separation and crystallization in glasses under study but to estimate amounts of precipitated phases as well. It is a complex task as it is very difficult to find a single internal standard in Raman spectra of glasses heat-treated at different stages. For instance, for initial glasses and glasses heat-treated at 660 and 700 C, intensity in the maxi-
mum of the band at 460 cm1 can be such an internal standard. Then comparing relative intensities of bands at 900, 800, and 150 cm1 we find how does the TiO2 content in amorphous silicate network, in aluminotitanate phase and in anatase change with CoO (Table 1, lines 1–3, columns 1–3). When heat-treatment temperature increases to 720 C, intensity in the maximum of the band at 460 cm1 does not play the role of an internal standard any more because it changes with CoO, as CoO facilitates crystallization of the main phase. In this case, we used an integral intensity of the complex band nearby 460 cm1, in which a narrow band appears at 480 cm1 due to crystallization of b-eucryptite solid solution, as an internal standard (line 4, column 2). Previously, it was demonstrated [5] that integral intensity of this band practically does not change (with accuracy of ±10%) in the course of glass crystallization, there is only some intensity redistribution between broad and narrow components, corresponding to vibrations in amorphous and crystalline parts of the glass matrix. To the accuracy of this observation, the band can play the role of required standard. It should be also pointed out that starting from spectra of samples heat-treated at 720 C for 24 h, we compared integrated intensity of the complex band at 880–800 cm1 as it is just this band which characterizes the total amount of aluminotitanate phase in samples.3 In samples heat-treated at 750, 800, and 950 C, according to X-ray diffraction analysis, crystallization of the main phase is practically accomplished. For quantitative characterization of their Raman spectra we used intensity in the maximum of the complex band at 480 cm1 as an internal standard (Table 1, lines 5–7, column 3). We changed the internal standard, because the error of its estimation is less than that for integrated band complicated by superposition with anatase bands at 400 and 520 cm1. Consequently, relative intensities of a series of Raman bands listed in Table 1 allow for a rough estimation of amounts of some phases precipitated in samples upon their heat-treatment depending on the CoO content. It is seen from Table 1 that in spectra of initial glasses intensity of band at 910 cm1 is independent of CoO content. The band at 800 cm1 is as weak as in spectra of regular silicate glasses (Table 1, line 1). It means that addition of CoO does not stimulate phase separation of
3 We presume that during heat-treatment at 720 C for 24 h practically all titanium leaves the aluminosilicate network and Raman spectra do not exhibit any more the band nearby 910 cm1 originated from [TiO4] tetrahedrons built into this network. The origin of the high-frequency shoulder at 880 cm1, which locates nearby the band at 800 cm1, was discussed above.
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initial glass. Optical absorption spectra are similar for all initial glasses [11]. They are due to absorption of Co(II) ions predominantly in octahedral coordination with a small share of tetrahedral coordination [11]. This spectrum is characteristic for Co(II) in silicate glasses and thoroughly discussed in the literature (see Ref. [12]). In the course of heat-treatment CoO does not influence the rate with which titanium leaves the silicate network. The rate of phase separation is governed solely by heat-treatment schedule. It is proved by the same weakening of intensity of the band at 910 cm1 by a factor of 2.5 after heat-treatment at 660 C for 24 h (Table 1, line 2) and by a factor of 3.5–4.0 after further temperature increase to 700 C (line 3) for samples with any CoO content. It should be noted that at early stages of heat-treatment the amount of aluminotitanate phase crystallized directly upon phase separation is practically independent of CoO content. This is evident from the constancy of intensity of the anatase band at 150 cm1 (Table 1, line 2). Its small increase in the spectrum of sample with 2.0 mol% CoO is probably due to crystallization of the titanate phase solely in the form of anatase. Spectral attribute of anosovite, the band at 250 cm1, is not found in the corresponding spectrum (Fig. 1, curve 6). Hence, at the very beginning of phase separation, addition of CoO leads only to increase of the temperature interval of existence of aluminotitanate amorphous phase and increases its amount (in the spectrum of the sample with 2.0 mol% CoO intensity of characteristic band at 800 cm1 is three times as great as in sample with no CoO, Table 1, lines 2 and 3). However, at the next stage of the process, at 700 C, with increasing the CoO content, crystallization of amorphous aluminotitanate phase retards just due to its stabilization by CoO. In fact, the anatase band in Raman spectrum weakens by a factor of 1.7 with CoO content increase from 0 to 1.0 mol% (Table 1, line 3). At 720 and 750 C, this correlation between an increase of the amount of stabilized amorphous aluminotitanate phase and a decrease of amount of anatase reveals the most pronouncedly. Indeed, the integral intensity of the band at 800–880 cm1 in the spectrum of the sample with 2.0 mol% is two times as great as in the spectrum of the sample with no CoO, while intensity of the anatase band at 150 cm1 is three times less than in the spectrum of the sample with no CoO (Table 1, lines 4 and 5, column 2). It should be noted that some deviation from this correlation for samples with 0.1, and 0.5 mol% CoO and an increase of the anatase band intensity at constant intensity of the integrated band 800–880 cm1 (Table 1, line 5, columns 2 and 3) is most likely to be connected with anosovite and cobalt titanate recrystallization into anatase which takes place at heat-treatment 750 C for 24 h.
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It is possible to follow the variation of intensities of Raman bands with temperature.4 The band at 800– 880 cm1 intensifies in spectra of samples heat-treated not over 720 C, and then it weakens and does it the quicker, the higher is the CoO content in the sample (Table 1, lines 4–6). In the process, intensity of the band at 150 cm1 grows faster for samples with higher CoO content and becomes comparable in spectra of all samples (Table 1, lines 6 and 7). For instance, in the temperature range 720–800 C intensity of anatase band in spectra of glasses with no CoO increases by a factor of 4 and by a factor of 9 for samples with 1.0 and 2.0 mol% CoO. It means that upon high-temperature heat-treatments the rate of aluminotitanate phase crystallization increases with increasing the CoO content. Probably, CoO does not stabilize this phase any more. This assumption is correlated with XRD and optical spectroscopy findings. According to XRD data, aluminocobalt spinel appears in samples starting from 720 C, which manifests CoO leaving the aluminotitanate phase. Absorption spectra of these samples are similar to spectra of aluminocobalt spinel [12]. It is worth mentioning that at the beginning of phase separation anatase, on the contrary, rapidly crystallizes in glass with no CoO. For this sample intensity of the band at 150 cm1 increases by the factor of 3.3 in the temperature range 660–720 C while for sample with 1.0 mol% CoO it increases only by the factor of 1.4 (Table 1, lines 2 and 4, columns 1 and 3). Thus, if at the beginning of phase separation CoO enters aluminotitanate phase, stabilizes it and helps retarding its crystallization, after heating at 720 C the situation changes. Cobalt ions in the form of CoAl2O4 crystals leave aluminotitanate phase speeding up its crystallization. Then in samples with any CoO content the same amount of anatase is to crystallize. The amount of precipitated anatase corresponds only to heat-treatment temperature (800–950 C). It develops the faster, the higher is the content of aluminotitanate phase stored in the sample with the help of CoO at early stages of heat-treatments. When temperature increases to 950 C, in spectra of samples with any CoO content we see a redistribution of intensities in a complex band at 800–880 cm1. Intensity in the maximum 880 cm1 increases and the position of maximum sometimes shifts to 900 cm1 (Figs. 7–9, curves 6). Intensification of this band manifests beginning of Al2TiO5, tialite, crystallization [1] (the most intense band of tialite is 900 cm1). The CoO somehow speeds up tialite crystallization (Table 1, line 7). At 950 C anatase continue to crystallize, however, the process slows down. As it is seen from Table 1, with heat-treatment temperature increase from 800 to 4 In lines 3–7 of Table 1 only relative intensities of bands listed in the same column are comparable.
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950 C intensity of the band at 150 cm1 increases 1.5 times when compared with its increase to 3 times at temperature increase from 750 to 800 C.
4. Conclusions By the use of combination of Raman spectroscopy, optical absorption spectra and X-ray diffraction analysis we demonstrated that addition of CoO influences kinetics of phase transformations of titania-containing lithium aluminosilicate glasses. It was found that addition of CoO into initial glass does not facilitate its phase separation as it does not influence the rate with which titanium leaves the silicate network. At low-temperature heat-treatments (660– 700 C), CoO enters aluminotitanate amorphous phase, which results in increased temperature interval of existence and increased amount of this phase. It retards anatase crystallization and speeds up transformation of anosovite into anatase. Formation of minor phases of cobalt titanate and spinel is revealed in a narrow temperature range. With formation of these phases aluminotitanate amorphous phase decomposes and the amount of anatase precipitated at elevated temperatures increases with CoO. Addition of CoO decreases the temperature of crystallization of the main crystalline phase, b-eucryptite solid solution.
Acknowledgments This work was partly supported by the Russian Foundation for Basic Researches (Project Nos. 04-0332657 and 05-03-08013-ofi_p) and by Grant NSH1405.2003.3 of President of Russian Federation.
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