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3M2O3·2SiO2 (M=Y, La, In, Al, Ga) glasses
Journal of Non-Crystalline Solids 246 (1999) 27±33
Tg and FTIR of (2.5 ÿ x)CaOáx/3M2O3á2SiO2 (M Y, La, In, Al, Ga) glasses F. Branda *, F. Arcobello-Varlese, A. Costantini, G. Luciani Dipartimento di Ingegneria dei Materiali e della Produzione, P.le Tecchio, Napoli, Italy Received 22 June 1998; received in revised form 6 January 1999
Abstract A comparative study of the eect of substituting oxides of (M Y, La, In, Al, Ga) to CaO in 2.5CaO2SiO2 glass in the narrow compositional range of the binary CaO±SiO2 system in which homogeneous glasses can be obtained (0.4 < CaO/(CaO + SiO2 ) < 0.55). A plot of glass transformation temperature, Tg , vs the ionic ®eld strength, Z/r2 , where Z and r are the charge and the radius of the cation, is useful in discussing the role of oxides in the glassy structure. It is hypothesized that, in the composition range, studied Al2 O3 and Ga2 O3 act as network forming oxides, while La2 O3 , Y2 O3 and In2 O3 act as network modifying oxides. FTIR spectra agree with this hypothesis and with the expectations based on the criteria reported in the literature, in particular with the one proposed by McMillan that the network modi®er cations have an ionic ®eld strength Z/r2 < 5 Aÿ2 . Ó 1999 Elsevier Science B.V. All rights reserved.
1. Introduction CaO±Al2 O3 (Ga2 O3 )±SiO2 systems have been extensively studied and the glass-forming regions were determined [1,2]. Properties such as density, refractive index, thermal expansion coecient and dilatometric softening temperature were measured [1±3]. Exceptionally refractory glasses or gradient optical elements can be obtained [4,5]. Several papers have been published in which the eect of adding oxides of other trivalent elements to silicate glasses has been studied. Y2 O3 and La2 O3 added to silicate and aluminosilicate glasses improved alkaline durability [6] increased glass transformation temperatures, increase refractive indices, low electrical conductiv*
Corresponding author. Tel.: +39-081 768 2672; fax: +39-081 768 2595; e-mail: [email protected]
ities and decrease thermal expansion, that make them a good alternative to borosilicate glasses for sealing glass to tungsten and molybdenum [7]. The glass formation region in the system Na2 O± In2 O3 ±SiO2 has been investigated and the optical, thermal and electrical properties of the glasses of this system measured [8]. In this paper a comparision of the eect on Tg of substituting oxides of trivalent elements M2 O3 (M Y, La, In, Al, Ga) to CaO in 2.5CaO2SiO2 glass is reported; the composition of this glass lies [1,2] in the narrow compositional range of the binary CaO±SiO2 system in which homogeneous glasses can be obtained (0.4 < xCaO < 0.55). Glasses of composition 2:5ÿxCa OX =3M2 O3 2SiO2 M Y; La; In; Al; Ga and
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 9 ) 0 0 0 3 9 - 3
I
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F. Branda et al. / Journal of Non-Crystalline Solids 246 (1999) 27±33
2:5 ÿ xCaO 2 0:5xSiO2
II
were prepared An important property of the general formula of series (I), useful for the discussion is: the numerical ratio of the oxygen to silicon atoms is constant throughout all substitutions. This means that, as long as modi®er oxides are added, the degree of covalent cross linking does not change. In order to compare results, the composition of binary calcium silicate glasses was expressed in terms of formula (II). The two formulas are, in fact, related to the more general one: 2:5 ÿ xCaOX =zMy Oz 2SiO2 : Hypothesis on the structural role of the added oxides are advanced on the basis of the glass transformation temperature, Tg , data and Fourier transform infra red (FTIR) spectra. 2. Experimental procedure Glasses of composition 2:5ÿxCaOX =3M2 O3 2SiO2 M Y; La; In; Al; Ga
I
and 2:5 ÿ xCaO 2 0:5xSiO2
II
were prepared by melting analytical grade reagents Al2 O3 , Ga2 O3 , Y2 O3 , La2 O3 , In2 O3 , CaCO3 , and SiO2 in a platinum crucible in an electric oven for 4 h at 1550°C. Melts were quenched by plunging the bottom of the crucible into cold water. Dierential scanning calorimetry was carried out by means of a Netzsch heat ¯ux apparatus model 404M on about 50 mg powdered samples (63±90 mm) at a heating rate 10°C/min. Powdered Al2 O3 was used as reference material. Fourier transform infrared (FTIR) transmittance spectra were recorded in the 400±1200 cmÿ1 region using a Mattson 5020 system, equipped with a DTGS KBr (deuterated triglycine sulphate with potassium bromide windows) detector, with a resolution of 2 cmÿ1 (20 scans). KBr pelletised disks containing 20 mg of sample and 200 mg KBr were made. The FTIR spectra have been elaborated by means of a Mattson software (FIRST Macros).
3. Results As in previous papers [9] the minimum appearing on the derivative of the DTA curve recorded at 10°C/min was taken as the glass transformation temperature, Tg . Tg are plotted in Fig. 1 as a function of composition expressed by means of the x values of the above reported general formulas (I) and (II). Dierent trends are observed depending on the type of substituent. In the case of yttrium and indium, Tg linearly increases as CaO is substituted. Non-linear trends with minima are instead observed in the case of La2 O3 , Al2 O3 and Ga2 O3 containing glasses. As can be seen a decreasing trend is observed for the glasses of the series (II) with values of Tg comparable to those of the Al2 O3 and Ga2 O3 cntaining glasses. Fig. 2 shows the FTIR spectra of the La2 O3 containing glasses, relative to dierent substitutions. In the case of the Y2 O3 and In2 O3 containing glasses very similar spectra were recorded; therefore, they are not reported. As can be seen, features of FTIR spectra are not greatly aected by the substitution. As in the case of fused silica, the SiO4 stretching vibration modes give rise to a sharp band at 1100 cmÿ1 [10,11]. When a network modifying oxide is added, it progressively shifts towards lower wavenumbers and broadens [10,12].
Fig. 1. Glass transformation temperature Tg (2°C) vs composition of the glasses expressed as the x values of the formulas of the (2.5 ÿ x)CaOX /3M2 O3 2SiO2 (M Y, La, In, Al, Ga) (I series) and(2.5 ÿ x)CaO(2 + 0.5x)SiO2 (II series).
F. Branda et al. / Journal of Non-Crystalline Solids 246 (1999) 27±33
29
Fig. 3. FTIR spectra of (2.5 ÿ x)CaOX /3Al2 O3 2SiO2 glasses. Fig. 2. FTIR spectra of (2.5 ÿ x)CaOX /3La2 O3 2SiO2 glasses.
This is due [10,12] to the build up of SiO4 tetrahedral units bearing a progressively higher number of non-bridging oxygens. In the spectrum of 2.5CaO2SiO2 glass, therefore, the broad band in the higher wavenumber range (800±1100 cmÿ1 ), is linked to the stretching vibration modes of the SiO4 tetrahedra; it partially overlaps the band at 700 cmÿ1 , due to bending vibration modes of Si± O±Si [13,14]. In the course of the substitution no substantial changes are observed. The band at 500 cmÿ1 , that in the binary base glass is due to Si±O±Si bending [10±12], is also slightly aected. Modi®cations are instead induced by CaO substitution with Al2 O3 , Ga2 O3 and SiO2 . In the Al2 O3 containing glasses (Fig. 3) the peak at 700 cmÿ1 appears not to be greatly aected. However, as the substitution proceeds, overlap with the broad band at 800±1100 cmÿ1 , due to the tetrahedral units stretching modes, progressively reduces owing to a sharpening of the latter. The band at 500 cmÿ1 is progressively reduced and shifts towards
lower wavenumbers. Analogous changes of the 800±1100 cmÿ1 and 500 cmÿ1 bands were recorded in the case of the glasses of series (II) (Fig. 5). The trend in the case of the Ga2 O3 containing glasses (Fig. 4) is similar but for two bands at 600 and 570 cmÿ1 due to the Ga±O±Si and Ga± O±Ga bending modes, respectively [14]. The 700 cmÿ1 band progressively reduces and shifts towards greater wavenumbers. 4. Discussion When an oxide of a trivalent element is introduced in a silicatic structure, MO4 tetrahedral units can be formed substituting the SiO4 ones [15,16]; alternatively, the cations introduced can be allocated in the holes of the structure as `network modifying ions'. Al2 O3 is a well-known example of the ®rst behavior. Owing to the need of charge compensation of the AlO4 tetrahedron Ca2 cations are subtracted to their network modifying role; in particular when the ratio Al2 O3 /CaO 1 a
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F. Branda et al. / Journal of Non-Crystalline Solids 246 (1999) 27±33
Fig. 4. FTIR spectra of (2.5 ÿ x)CaOX /3Ga2 O3 2SiO2 glasses.
silica glass type structure is obtained in which all the oxygen atoms are bridging. Several criteria, predicting the role of oxides, are available [15,16]. McMillan found [16] that the network modi®er cations have Z/r2 < 5 Aÿ2 . In Table 1 the radius [17], the coordination number in the oxide [15] and the ionic ®eld strength of the cations are reported. Therefore only Al3 and Ga3 should enter as network forming cations. As it is known the glass transformation temperature Tg is a structural sensitive parameter. Following Ray [18] it depends on (a) the density of covalent cross-linking, (b) the number and strength of cross-links between the cation and oxygens. In order to better analyze the results reported in Fig. 1, the Tg are plotted, in Figs. 6±9, as a function of the ionic ®eld strength of the cations, Z/r2 , which is a measure of the electrostatic force the ion can exert upon neighboring oxygens. In these plots two types of behaviors can be distin-
Fig. 5. FTIR spectra of (2.5 ÿ x)CaO (2 + 0.5x)SiO2 glasses.
guished. When Z/r2 > 5 the Tg fall to lower values instead of growing as expected. An explanation can be found if it is admitted that Al3 and Ga3 act as network forming ions and La3 , Y3 and In3 as network modifying ions. When entering the structure as a network modifying ion, a mean coordination number close to the one in the pure oxide structure, reported in Table 1, can be expected. Therefore, according to Ray, greater valTable 1 Coordination number in the oxide, CN (from Ref. [15]), radius, r (from Ref. [17]) and ionic ®eld strength of cations, Z/r2 . ÿ2 ) M CN r (A) Z/r2 (A Si (+4) Al (+3) Ga (+3) In (+3) Y (+3) La (+3) Ca (+2)
4 6 6 6 8 7 8
0.42 0.51 0.62 0.81 0.89 1.02 0.99
22.7 11.53 7.80 4.57 3.78 2.90 2.04
F. Branda et al. / Journal of Non-Crystalline Solids 246 (1999) 27±33
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Fig. 6. Glass transformation temperature, Tg , (2°C) vs the ionic ®eld strength, Z/r2 , of the glasses of composition 2.3CaO(0.2/3)M2 O3 2SiO2 (M Y, La, In, Al, Ga).
Fig. 8. Glass transformation temperature, Tg , (2°C) vs the ionic ®eld strength, Z/r2 , of the glasses of composition 1.5CaO(1/3)M2 O3 2SiO2 (M Y, La, In, Al, Ga).
Fig. 7. Glass transformation temperature, Tg , (2°C) vs the ionic ®eld strength, Z/r2 , of the glasses of composition 1.9CaO(0.6/3)M2 O3 2SiO2 (M Y, La, In, Al, Ga).
Fig. 9. Glass transformation temperature, Tg , (2°C) vs the ionic ®eld strength, Z/r2 , of the glasses of composition (2.5 ÿ x)CaOX /3M2 O3 2SiO2 (M In, Al, Ga): x 1.4 full symbols; x 1.8 open symbols.
ues of Tg would be expected for La3 , Y3 and In3 than for the ions entering the silicate glass structure as network forming ions, that is in fourfold coordination. The curves reported in Fig. 1 show that the initial decreasing trend of the curves of the Al2 O3 and Ga2 O3 containing glasses is reversed at high values of substitution. This should be due to the increase of covalent cross-linking density. Therefore, the lower is the non-bridging oxygen atoms number, the greater the degree of covalent crosslinking aects Tg values. Observe that in the case
of the binary CaO±SiO2 system, the curve of Tg vs composition shows trend similar to the Ga2 O3 and Al2 O3 glasses. In all three cases the initial decreasing trend could be the result of the substitution of the network modifying cation Ca2 , of higher coordination number, with a network forming cation Si4 , in fourfold coordination, in a composition range of relative insensitivity to changes of covalent cross-linking density. As the substitution proceeds the increase of the covalent cross linking density appears to become important and makes Tg increase.
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F. Branda et al. / Journal of Non-Crystalline Solids 246 (1999) 27±33
As the general formula of glasses is studied such that the molar ratio O/Si is constant, no change in the covalent cross-linking is expected when M2 O3 is a network modifying oxide. The non-linear trend observed in the case of La2 O3 glasses can be explained on the basis of previous ®ndings [19]. Non-linear trends in the Tg vs composition curves of glasses of base composition Na2 O±SiO2 and Li2 O±SiO2 were related [19] to dierences in the ionic ®eld strength of the substituting and substituted network modifying cation. It was explained with the `diculty' for the substituting cation to impose its own coordination when its ®eld strength is close to the substituted cation one. It must be taken into account that the substitution of Ca2 with M3 cation requires a topological rearrangement: three non-bridging oxygens, instead of two, must be coordinated. A `diculty' in imposing its own coordination, at the beginning of substitution, could therefore be recognized to La3 having a Z/r2 2.90 Aÿ2 not suciently greater then the Ca2 one, Z/r2 2.04 Aÿ2 thus justifying the nonlinear trend reported in Fig. 1. The observed FTIR spectra modi®cations along the substitution are consistent with the above reported interpretation of the Tg curves. One must remember the similarity of the spectra of La2 O3 , Y2 O3 and In2 O3 containing glasses on one side and those of Al2 O3 and Ga2 O3 containing glasses on the other side. The variation of the band in the range 800± 1100 cmÿ1 appears relevant for the above reported discussion. It is known that SiO4 stretching vibration modes of fused silica gives rise to a sharp band at 1100 cmÿ1 [10,11]. When a network modifying oxide is added, it progressively shifts towards lower wavenumber and broadens [10,12]. This is due [10,12] to the build up of SiO4 tetrahedral units bearing a progressively higher number of non-bridging oxygens. Therefore, the lack of modi®cation, when La2 O3 , Y2 O3 and In2 O3 are substituted, well agrees with the hypothesis that they enter as network modifying oxides. Since the substitution is such that the ratio O/Si is not changed, no changes in the number of the non-bridging oxygens is expected and in the distribution among the tetrahedra. The case of the M2 O3 oxides entering as network
forming oxides is, instead, dierent. In particular when CaO/M2 O3 1 all oxygens should become bridging and the stretching band is expected to be the result of the overlapping of the ones of SiO4 and MO4 tetrahedra bearing only bridging oxygens. Therefore, a progressive sharpening is expected as it is observed for Ga2 O3 and Al2 O3 containing glasses and for the glasses of the (II) series.
5. Conclusions The plot of Tg vs Z/r2 proved to be useful in discussing the role of oxides in the glassy structure. On this basis it can be hypothesized that, at least in the studied composition range, Al2 O3 and Ga2 O3 act as network forming oxides, while La2 O3 , Y2 O3 and In2 O3 as network modifying oxides. FTIR spectra agree with this hypothesis which is consistent with expectations based on the literature criteria.
References [1] J.E. Shelby, J. Am. Ceram. Soc. 68 (3) (1985) 155. [2] J.E. Shelby, Phys. Chem. Glasses 35 (4) (1994) 147. [3] P.L. Higby, L.E. Busse, I.D. Aggarwal, Mater. Sci. Forum 67/68 (1991) 155. [4] V.Ya. Alaev, Yu.A. Zhatkin, G.O. Karapetyan, V.V. Moiseev, N.V. Remizov, Soviet J. Glass Phys. Chem. 15 (2) (1989) 128. [5] Yu.A. Kozlov, Soviet J. Glass Phys. Chem. 16 (5) (1990) 424. [6] A. Makimishima T. Shimoira, J. Non-Cryst. Solids 38/39 (2) (1980) 661. [7] J.E. Shelby, S.M. Minton, C.E. Lord, M.R. Tuzzolo, Phys. Chem. Glasses 33 (3) (1992) 93. [8] J.E. Shelby, J.J. Noonan, Phys. Chem. Glasses 39 (1) (1998) 36. [9] F. Branda, A. Buri, D. Caferra, A. Marotta, Phys. Chem. Glasses 22 (3) (1981) 68. [10] I. Simon, H.O. McMahon, J. Am. Ceram. Soc. 36 (1953) 160. [11] Y. Kim, A.E. Clark, L.L. Hench, J. Non-Cryst. Solids 113 (1989) 195. [12] I. Simon, in: J.D. Mackenzie (Ed.), Modern Aspects of the Vitreous State, Butterworth, London, 1960, p. 120. [13] D.E. Day, G.E. Rindone, J. Am. Ceram. Soc. 45 (10) (1962) 489.
F. Branda et al. / Journal of Non-Crystalline Solids 246 (1999) 27±33 [14] P.L. Higby, J.E. Shelby, R.A. Condrate Sr, Phys. and Chem. Glasses 28 (3) (1987) 115. [15] H. Rawson, Inorganic Glass-forming Systems, Academic Press, New York, 1967. [16] P.W. McMillan, Glass-ceramics, Academic Press, New York, 1964.
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[17] R.C. Weast, CRC Handbook of Chemistry and Physics, 58th ed., CRC, Ohio, 1977±1978. [18] N.H. Ray, J. Non-Cryst. Solids 15 (1974) 423. [19] F. Branda, A. Buri, D. Caferra, A. Marotta, J. Non-Cryst. Solids 54 (1/2) (1983) 193.
Tg and FTIR of (2.5 ÿ x)CaOáx/3M2O3á2SiO2 (M Y, La, In, Al, Ga) glasses F. Branda *, F. Arcobello-Varlese, A. Costantini, G. Luciani Dipartimento di Ingegneria dei Materiali e della Produzione, P.le Tecchio, Napoli, Italy Received 22 June 1998; received in revised form 6 January 1999
Abstract A comparative study of the eect of substituting oxides of (M Y, La, In, Al, Ga) to CaO in 2.5CaO2SiO2 glass in the narrow compositional range of the binary CaO±SiO2 system in which homogeneous glasses can be obtained (0.4 < CaO/(CaO + SiO2 ) < 0.55). A plot of glass transformation temperature, Tg , vs the ionic ®eld strength, Z/r2 , where Z and r are the charge and the radius of the cation, is useful in discussing the role of oxides in the glassy structure. It is hypothesized that, in the composition range, studied Al2 O3 and Ga2 O3 act as network forming oxides, while La2 O3 , Y2 O3 and In2 O3 act as network modifying oxides. FTIR spectra agree with this hypothesis and with the expectations based on the criteria reported in the literature, in particular with the one proposed by McMillan that the network modi®er cations have an ionic ®eld strength Z/r2 < 5 Aÿ2 . Ó 1999 Elsevier Science B.V. All rights reserved.
1. Introduction CaO±Al2 O3 (Ga2 O3 )±SiO2 systems have been extensively studied and the glass-forming regions were determined [1,2]. Properties such as density, refractive index, thermal expansion coecient and dilatometric softening temperature were measured [1±3]. Exceptionally refractory glasses or gradient optical elements can be obtained [4,5]. Several papers have been published in which the eect of adding oxides of other trivalent elements to silicate glasses has been studied. Y2 O3 and La2 O3 added to silicate and aluminosilicate glasses improved alkaline durability [6] increased glass transformation temperatures, increase refractive indices, low electrical conductiv*
Corresponding author. Tel.: +39-081 768 2672; fax: +39-081 768 2595; e-mail: [email protected]
ities and decrease thermal expansion, that make them a good alternative to borosilicate glasses for sealing glass to tungsten and molybdenum [7]. The glass formation region in the system Na2 O± In2 O3 ±SiO2 has been investigated and the optical, thermal and electrical properties of the glasses of this system measured [8]. In this paper a comparision of the eect on Tg of substituting oxides of trivalent elements M2 O3 (M Y, La, In, Al, Ga) to CaO in 2.5CaO2SiO2 glass is reported; the composition of this glass lies [1,2] in the narrow compositional range of the binary CaO±SiO2 system in which homogeneous glasses can be obtained (0.4 < xCaO < 0.55). Glasses of composition 2:5ÿxCa OX =3M2 O3 2SiO2 M Y; La; In; Al; Ga and
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 9 ) 0 0 0 3 9 - 3
I
28
F. Branda et al. / Journal of Non-Crystalline Solids 246 (1999) 27±33
2:5 ÿ xCaO 2 0:5xSiO2
II
were prepared An important property of the general formula of series (I), useful for the discussion is: the numerical ratio of the oxygen to silicon atoms is constant throughout all substitutions. This means that, as long as modi®er oxides are added, the degree of covalent cross linking does not change. In order to compare results, the composition of binary calcium silicate glasses was expressed in terms of formula (II). The two formulas are, in fact, related to the more general one: 2:5 ÿ xCaOX =zMy Oz 2SiO2 : Hypothesis on the structural role of the added oxides are advanced on the basis of the glass transformation temperature, Tg , data and Fourier transform infra red (FTIR) spectra. 2. Experimental procedure Glasses of composition 2:5ÿxCaOX =3M2 O3 2SiO2 M Y; La; In; Al; Ga
I
and 2:5 ÿ xCaO 2 0:5xSiO2
II
were prepared by melting analytical grade reagents Al2 O3 , Ga2 O3 , Y2 O3 , La2 O3 , In2 O3 , CaCO3 , and SiO2 in a platinum crucible in an electric oven for 4 h at 1550°C. Melts were quenched by plunging the bottom of the crucible into cold water. Dierential scanning calorimetry was carried out by means of a Netzsch heat ¯ux apparatus model 404M on about 50 mg powdered samples (63±90 mm) at a heating rate 10°C/min. Powdered Al2 O3 was used as reference material. Fourier transform infrared (FTIR) transmittance spectra were recorded in the 400±1200 cmÿ1 region using a Mattson 5020 system, equipped with a DTGS KBr (deuterated triglycine sulphate with potassium bromide windows) detector, with a resolution of 2 cmÿ1 (20 scans). KBr pelletised disks containing 20 mg of sample and 200 mg KBr were made. The FTIR spectra have been elaborated by means of a Mattson software (FIRST Macros).
3. Results As in previous papers [9] the minimum appearing on the derivative of the DTA curve recorded at 10°C/min was taken as the glass transformation temperature, Tg . Tg are plotted in Fig. 1 as a function of composition expressed by means of the x values of the above reported general formulas (I) and (II). Dierent trends are observed depending on the type of substituent. In the case of yttrium and indium, Tg linearly increases as CaO is substituted. Non-linear trends with minima are instead observed in the case of La2 O3 , Al2 O3 and Ga2 O3 containing glasses. As can be seen a decreasing trend is observed for the glasses of the series (II) with values of Tg comparable to those of the Al2 O3 and Ga2 O3 cntaining glasses. Fig. 2 shows the FTIR spectra of the La2 O3 containing glasses, relative to dierent substitutions. In the case of the Y2 O3 and In2 O3 containing glasses very similar spectra were recorded; therefore, they are not reported. As can be seen, features of FTIR spectra are not greatly aected by the substitution. As in the case of fused silica, the SiO4 stretching vibration modes give rise to a sharp band at 1100 cmÿ1 [10,11]. When a network modifying oxide is added, it progressively shifts towards lower wavenumbers and broadens [10,12].
Fig. 1. Glass transformation temperature Tg (2°C) vs composition of the glasses expressed as the x values of the formulas of the (2.5 ÿ x)CaOX /3M2 O3 2SiO2 (M Y, La, In, Al, Ga) (I series) and(2.5 ÿ x)CaO(2 + 0.5x)SiO2 (II series).
F. Branda et al. / Journal of Non-Crystalline Solids 246 (1999) 27±33
29
Fig. 3. FTIR spectra of (2.5 ÿ x)CaOX /3Al2 O3 2SiO2 glasses. Fig. 2. FTIR spectra of (2.5 ÿ x)CaOX /3La2 O3 2SiO2 glasses.
This is due [10,12] to the build up of SiO4 tetrahedral units bearing a progressively higher number of non-bridging oxygens. In the spectrum of 2.5CaO2SiO2 glass, therefore, the broad band in the higher wavenumber range (800±1100 cmÿ1 ), is linked to the stretching vibration modes of the SiO4 tetrahedra; it partially overlaps the band at 700 cmÿ1 , due to bending vibration modes of Si± O±Si [13,14]. In the course of the substitution no substantial changes are observed. The band at 500 cmÿ1 , that in the binary base glass is due to Si±O±Si bending [10±12], is also slightly aected. Modi®cations are instead induced by CaO substitution with Al2 O3 , Ga2 O3 and SiO2 . In the Al2 O3 containing glasses (Fig. 3) the peak at 700 cmÿ1 appears not to be greatly aected. However, as the substitution proceeds, overlap with the broad band at 800±1100 cmÿ1 , due to the tetrahedral units stretching modes, progressively reduces owing to a sharpening of the latter. The band at 500 cmÿ1 is progressively reduced and shifts towards
lower wavenumbers. Analogous changes of the 800±1100 cmÿ1 and 500 cmÿ1 bands were recorded in the case of the glasses of series (II) (Fig. 5). The trend in the case of the Ga2 O3 containing glasses (Fig. 4) is similar but for two bands at 600 and 570 cmÿ1 due to the Ga±O±Si and Ga± O±Ga bending modes, respectively [14]. The 700 cmÿ1 band progressively reduces and shifts towards greater wavenumbers. 4. Discussion When an oxide of a trivalent element is introduced in a silicatic structure, MO4 tetrahedral units can be formed substituting the SiO4 ones [15,16]; alternatively, the cations introduced can be allocated in the holes of the structure as `network modifying ions'. Al2 O3 is a well-known example of the ®rst behavior. Owing to the need of charge compensation of the AlO4 tetrahedron Ca2 cations are subtracted to their network modifying role; in particular when the ratio Al2 O3 /CaO 1 a
30
F. Branda et al. / Journal of Non-Crystalline Solids 246 (1999) 27±33
Fig. 4. FTIR spectra of (2.5 ÿ x)CaOX /3Ga2 O3 2SiO2 glasses.
silica glass type structure is obtained in which all the oxygen atoms are bridging. Several criteria, predicting the role of oxides, are available [15,16]. McMillan found [16] that the network modi®er cations have Z/r2 < 5 Aÿ2 . In Table 1 the radius [17], the coordination number in the oxide [15] and the ionic ®eld strength of the cations are reported. Therefore only Al3 and Ga3 should enter as network forming cations. As it is known the glass transformation temperature Tg is a structural sensitive parameter. Following Ray [18] it depends on (a) the density of covalent cross-linking, (b) the number and strength of cross-links between the cation and oxygens. In order to better analyze the results reported in Fig. 1, the Tg are plotted, in Figs. 6±9, as a function of the ionic ®eld strength of the cations, Z/r2 , which is a measure of the electrostatic force the ion can exert upon neighboring oxygens. In these plots two types of behaviors can be distin-
Fig. 5. FTIR spectra of (2.5 ÿ x)CaO (2 + 0.5x)SiO2 glasses.
guished. When Z/r2 > 5 the Tg fall to lower values instead of growing as expected. An explanation can be found if it is admitted that Al3 and Ga3 act as network forming ions and La3 , Y3 and In3 as network modifying ions. When entering the structure as a network modifying ion, a mean coordination number close to the one in the pure oxide structure, reported in Table 1, can be expected. Therefore, according to Ray, greater valTable 1 Coordination number in the oxide, CN (from Ref. [15]), radius, r (from Ref. [17]) and ionic ®eld strength of cations, Z/r2 . ÿ2 ) M CN r (A) Z/r2 (A Si (+4) Al (+3) Ga (+3) In (+3) Y (+3) La (+3) Ca (+2)
4 6 6 6 8 7 8
0.42 0.51 0.62 0.81 0.89 1.02 0.99
22.7 11.53 7.80 4.57 3.78 2.90 2.04
F. Branda et al. / Journal of Non-Crystalline Solids 246 (1999) 27±33
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Fig. 6. Glass transformation temperature, Tg , (2°C) vs the ionic ®eld strength, Z/r2 , of the glasses of composition 2.3CaO(0.2/3)M2 O3 2SiO2 (M Y, La, In, Al, Ga).
Fig. 8. Glass transformation temperature, Tg , (2°C) vs the ionic ®eld strength, Z/r2 , of the glasses of composition 1.5CaO(1/3)M2 O3 2SiO2 (M Y, La, In, Al, Ga).
Fig. 7. Glass transformation temperature, Tg , (2°C) vs the ionic ®eld strength, Z/r2 , of the glasses of composition 1.9CaO(0.6/3)M2 O3 2SiO2 (M Y, La, In, Al, Ga).
Fig. 9. Glass transformation temperature, Tg , (2°C) vs the ionic ®eld strength, Z/r2 , of the glasses of composition (2.5 ÿ x)CaOX /3M2 O3 2SiO2 (M In, Al, Ga): x 1.4 full symbols; x 1.8 open symbols.
ues of Tg would be expected for La3 , Y3 and In3 than for the ions entering the silicate glass structure as network forming ions, that is in fourfold coordination. The curves reported in Fig. 1 show that the initial decreasing trend of the curves of the Al2 O3 and Ga2 O3 containing glasses is reversed at high values of substitution. This should be due to the increase of covalent cross-linking density. Therefore, the lower is the non-bridging oxygen atoms number, the greater the degree of covalent crosslinking aects Tg values. Observe that in the case
of the binary CaO±SiO2 system, the curve of Tg vs composition shows trend similar to the Ga2 O3 and Al2 O3 glasses. In all three cases the initial decreasing trend could be the result of the substitution of the network modifying cation Ca2 , of higher coordination number, with a network forming cation Si4 , in fourfold coordination, in a composition range of relative insensitivity to changes of covalent cross-linking density. As the substitution proceeds the increase of the covalent cross linking density appears to become important and makes Tg increase.
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F. Branda et al. / Journal of Non-Crystalline Solids 246 (1999) 27±33
As the general formula of glasses is studied such that the molar ratio O/Si is constant, no change in the covalent cross-linking is expected when M2 O3 is a network modifying oxide. The non-linear trend observed in the case of La2 O3 glasses can be explained on the basis of previous ®ndings [19]. Non-linear trends in the Tg vs composition curves of glasses of base composition Na2 O±SiO2 and Li2 O±SiO2 were related [19] to dierences in the ionic ®eld strength of the substituting and substituted network modifying cation. It was explained with the `diculty' for the substituting cation to impose its own coordination when its ®eld strength is close to the substituted cation one. It must be taken into account that the substitution of Ca2 with M3 cation requires a topological rearrangement: three non-bridging oxygens, instead of two, must be coordinated. A `diculty' in imposing its own coordination, at the beginning of substitution, could therefore be recognized to La3 having a Z/r2 2.90 Aÿ2 not suciently greater then the Ca2 one, Z/r2 2.04 Aÿ2 thus justifying the nonlinear trend reported in Fig. 1. The observed FTIR spectra modi®cations along the substitution are consistent with the above reported interpretation of the Tg curves. One must remember the similarity of the spectra of La2 O3 , Y2 O3 and In2 O3 containing glasses on one side and those of Al2 O3 and Ga2 O3 containing glasses on the other side. The variation of the band in the range 800± 1100 cmÿ1 appears relevant for the above reported discussion. It is known that SiO4 stretching vibration modes of fused silica gives rise to a sharp band at 1100 cmÿ1 [10,11]. When a network modifying oxide is added, it progressively shifts towards lower wavenumber and broadens [10,12]. This is due [10,12] to the build up of SiO4 tetrahedral units bearing a progressively higher number of non-bridging oxygens. Therefore, the lack of modi®cation, when La2 O3 , Y2 O3 and In2 O3 are substituted, well agrees with the hypothesis that they enter as network modifying oxides. Since the substitution is such that the ratio O/Si is not changed, no changes in the number of the non-bridging oxygens is expected and in the distribution among the tetrahedra. The case of the M2 O3 oxides entering as network
forming oxides is, instead, dierent. In particular when CaO/M2 O3 1 all oxygens should become bridging and the stretching band is expected to be the result of the overlapping of the ones of SiO4 and MO4 tetrahedra bearing only bridging oxygens. Therefore, a progressive sharpening is expected as it is observed for Ga2 O3 and Al2 O3 containing glasses and for the glasses of the (II) series.
5. Conclusions The plot of Tg vs Z/r2 proved to be useful in discussing the role of oxides in the glassy structure. On this basis it can be hypothesized that, at least in the studied composition range, Al2 O3 and Ga2 O3 act as network forming oxides, while La2 O3 , Y2 O3 and In2 O3 as network modifying oxides. FTIR spectra agree with this hypothesis which is consistent with expectations based on the literature criteria.
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[17] R.C. Weast, CRC Handbook of Chemistry and Physics, 58th ed., CRC, Ohio, 1977±1978. [18] N.H. Ray, J. Non-Cryst. Solids 15 (1974) 423. [19] F. Branda, A. Buri, D. Caferra, A. Marotta, J. Non-Cryst. Solids 54 (1/2) (1983) 193.