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Infrared reflectivity spectroscopy of nitrogen-substituted alkaline earth aluminosilicate glasses
I O U R N A L OF
ELSEVIER
Journal of Non-Crystalline Solids 176 (1994) 69-75
Infrared reflectivity spectroscopy of nitrogen-substituted alkaline earth aluminosilicate glasses Franqois Gervais a, Annie Blin a, Corinne Gamier b, Patrick Verdier b,,, Yves Laurent b a Centre de Recherches sur la Physique des Hautes Temperatures, CNRS, 45071 Orldans c~dex 2, France b Laboratoire de Chimie des Matdriaux, URA CNRS 1496, Universit~ de Rennes, Campus Beaulieu, 35042 Rennes c~dex, France
Received 2 February 1994; revised manuscript received 20 April 1994
Abstract
Infrared reflectivity spectra of the series Mo.37Sio.53AIo.loO1.ss_3x/2N x with M = Mg, Ca and Ba and x varying between 0 and 0.09 are reported. The far-infrared vibrational frequency of the M cation mainly depends on cationic mass. In the case of magnesium, the vibrational frequency is also sensitive to the coordination implying the glass-former (tetrahedral) or glass-modifier (likely octahedral). The anionic effective charge measured from the Coulombic contribution to the infrared reflection bands is smaller in nitrogenated glasses than in oxide glasses, showing an increased average bond hybrization, consistent with hardening of the structure indicated by macroscopic properties.
1. Introduction
The partial substitution of oxygen by nitrogen in silica glasses is expected to modify the chemical bonding. This may explain the changes of properties observed experimentally: hardness, elastic moduli, brittleness, viscosity and transition temperatures [16]. To describe the change of average chemical bonding and structural arrangement, information deduced from the analysis of infrared spectra may be useful. Only one infrared study has been reported, to our knowledge, concerning the series M0.37Si0.53Alo.loO1.58_3x/2N x ( M = Mg, Ca or Ba) [7]. The experiments of Ref. [7] were performed on powdered
* Corresponding author. Telefax: +33 99 28 62 58.
samples dispersed in nujol. The spectra were limited to the range above 250 cm -1. But most information concerning the M cations is expected at smaller wavenumbers than this. It is instructive, therefore, to complement this study in the far-infrared. More generally, information deduced from the transmission spectra of glasses raises specific problems related to the broadening of lines which is inherent to glass spectra. In particular, the response of M cations is not well resolved in the absorption coefficient profiles. The problems were described in Ref. [8]. It was also shown that the difficulties may be largely avoided by measuring the reflectivity. Other specific problems are encountered in transmission experiments of powdered samples which may be greater in the case of broad-band spectra. They are due to the part of the infrared radiation which is reflected upon the surface of the grains so that the results combine
0022-3093/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved SSDI 0 0 2 2 - 3 0 9 3 ( 9 4 ) 0 0 4 2 2 - J
70
F. Gervais et al. /Journal of Non-Crystalline Solids 176 (1994) 69-75
the effects of transmission and reflection and this is generally not taken into account in the data analysis. All these reasons justified the investigation of the series Mo.37Sio.53Alo.1001.58_3x/2N x by infrared reflectivity spectroscopy, which is reported here.
0.30 --Ca4
- - - Ca3 . . . . Ca2
L
.
~
~
0,15
2. Experiment I 0
The glasses of the series investigated were prepared to have the approximate compositions M0.37Sio.53Alo.loO1.58_3x/2Nx with x = 0 (noted M1), x = 0.033 (M2), x = 0.066 (M3) and x = 0.1 (M4). The actual compositions of the samples were measured by chemical analysis. Nitrogen percentages are slightly less than expected, typically by 10%, except for M2 in which x = 0.027 instead of 0.033. The ratio N / ( N + O) thus varies between 0 and 6%. In the barium series, the samples have less barium. Instead of the expected atomic ratio B a / S i = 0.7, the actual ratio is 0.59 in sample Bal, 0.53 in Ba2, 0.56 in Ba3 and only 0.4 in Ba4. Oxynitride aluminosilicate glasses were prepared by reaction of various oxides with a nitriding agent. In order to avoid handling of hygroscopic alkaline earth oxides, the silicates M - S i O 3 (where M = Ca, Mg) were prepared by reacting MCO 3 with SiO 2 at 1250°C for 24 h. Conversely, barium is introduced as Ba2SiO 4 prepared in the same way at 1000°C for 24 h. Aluminium nitride was chosen as the nitrogen source for its greater reactivity compared with that of silicon nitride Si3N 4 and for its greater convenience in handling compared with alkaline earth nitrides. The initial mixtures (50 g) were put in a molybdenum crucible and melted in a high-frequency furnace under nitrogen atmosphere. After heating at 1400°C, the glasses were poured and annealed in a molybdenum boat. In all cases, the structural state of the glasses were measured by X-ray diffraction. Spectra were recorded with a Fourier-transform spectrometer Bruker IFS 307 which covers the wavenumber range 10 to 45000 cm -1. The surface of the polished samples studied by reflectivity spectroscopy (near normal incidence) was approximately 0.5 cm 2. The nitrogen concentration dependence of the reflectivity spectra of the Mg, Ca and Ba glasses are shown in Figs. I to 3.
200
I
I
I
400 600 800 FREI]UENEY (r-m-l)
I
~
"1000 1200
Fig. 1. Infrared reflection spectra of calcium glasses Ca0.37Si0.53AlO.1001.58_3x/2N x with x = 0 (noted Ca1), x=0.03 (Ca2),
x = 0.06 (Ca3) and x = 0.09 (Ca4).
0.25 --
Mg4
"193 492 ,~ 019
0
I 200
I I I 400 600 800 FREO,UEN[Y (cm-1)
I 1000
i~ 1200
Fig. 2. Infrared reflection spectra of magnesium glasses Mgo.37Si0.53Alo.1oO1.ss_3x/2Nx with x = 0.03 (Mg2), x=0.06 (Mg3) and x = 0.09 (Mg4).
0,30
--
Ba4
,. . . . . . . Ba2,
~
0.15
I 00
200
I
'1
I
400 600 800 FREQUENEY (cm-1)
I 1000
~ 1200
Fig. 3. Infrared reflection spectra of barium glasses Ba0.37Sio.53A10.1001.ss-3x/2Nx- Instead of the expected ratio Ba/Si = 0.7 in the tentative formula, the checked ratio is 0.59 in sample Bal, 0.53 in Ba2, 0.56 in Ba3 and only 0.40 in Ba4. x = 0 in sample Bal, x = 0.03 in Ba2, x = 0.06 in Ba3, x = 0.09 in Ba4.
F. Gervais et aL /Journal of Non-Crystalline Solids 176 (1994) 69-75 [
6
o[ A
Ca4
5 I-
. . . . Ca3
I
. . . . . . Ca2
71
l-l.l 5 J ~1'~
..
. . . . Ba3 .-. . . . . . Ba2
i E
E
.
2 1 0
200
Fig. 4. Nitrogen concentration dependence of the dielectric response of calcium glasses. The absorption coefficient, K, is related to the imaginary part of the dielectric response, e", plotted here, via K(~o)= 21re"(os)/n(oJ), where n(w) is the refractive index. The dispersion of n(to) may explain additional shoulders in K(oJ) for wide-band spectra.
3. D a t a
0
400 600 800 1 0 0 0 1200 FREQUENCY (cm-1)
analysis
Since the spectra are recorded from nearly zero up to a frequency which lies in a spectral range where there is no dispersion, a K r a m e r s - K r 6 n i g analysis of reflectivity data may be performed, safely. Results in terms of the polar mode response (imaginary part of the dielectric function) are shown in Figs. 4 to 7. 3.1. Calcium glasses The results of Fig. 4 show that: (i) the intensity, frequency and linewidth of the v4-type rocking mode at 475 cm -1 is weakly sensitive to the nitrogen content; (ii) the calcium response is also weakly
S -
Mg4 - - - Mg3
4
. . . . . Mg2
0
200
400 600 800 1000 1200 FREQUENCY (crn-1)
Fig. 6. Nitrogen concentration dependence of the dielectric re-
sponse of barium glasses.
sensitive to nitrogen, as may be expected; (iii) conversely, the v3-type stretching mode at 931 cm -1 and the weak bending mode at 707 cm -1 show a clear down-shift with nitrogen addition. A similar down-shift was reported in transmission experiments [7]. One may attempt to understand these shifts in terms of mass defect, restricted to the atoms within the Si(O,N) 4 tetrahedra for the sake of simplicity. Based on a harmonic oscillator analysis of the v3-type stretching mode,
O0=
1/2,
(1)
where /20 is the oscillator frequency, k the shortrange force constant and /x the reduced mass of the ions, one then expect an up-shift of the mode by 0.65% in the Ca4 glass compared with Cal, simply because nitrogen is lighter than oxygen. This is the opposite of the down-shift by 1% that is found
A
.... Ca4 ..... Mg4
5
2
.,;/ \..
1 0
200
400 600 800 FREQUENCY (r-m-l)
1000 1200
Fig. 5. Nitrogen concentration dependence of the dielectric response of magnesium glasses.
00
I 200
I r J i ~ 400 600 800 1 0 0 0 1200 FREQUENCY (cm-1)
Fig. 7. Glass-modifier cation dependence(M = Ca, Mg, Ba) of
glasses M0.37Si 0.53A10.1001.44No.09.
72
F. Gervais et aL /Journal of Non-Crystalline Solids 176 (1994) 69-75
experimentally. Also the general hardening of the lattice, related to the more covalent bonding Si-N compared with Si-O, which is the origin of higher hardness of nitrogen glasses, is expected to increase the force constant and, therefore, increase the frequency. The reverse is actually observed. We will report results on other glasses prior to a common discussion.
3.2. Magnesium glasses Spectra of magnesium glasses are shown in Fig. 5. We found the same trends as for calcium glasses, namely a net down-shift of the highest-frequency stretching mode with addition of nitrogen. We recover the exceptional broadening of the spectra usually observed in magnesium alumino-silicate glasses investigated previously [8]. Note that this broadening requires the presence of aluminium since no such effect is observed in magnesium silicate glasses [9]. Because of broadening, the band observed around 800 cm -1 in silicate glasses is hardly resolved in this case and it is not possible to decide whether the shift observed for calcium glasses is present.
3.3. Barium glasses As indicated in Section 2, the actual barium concentrations in our samples differ from those expected. As a result, the intensity of the barium band around 115 cm -1 is sample-dependent, consistent with the measured Ba/Si ratios. As already shown and discussed in previous papers [8,10], the concentration of glass modifier cations is related to a shift and/or splitting of the highest-frequency band. This effect manifests itself here and masks the behaviour related to nitrogen substitution.
4. Discussion
The Ba4 sample is the one in which the departure of the actual barium concentration from the expected one is the smallest. It is then instructive to compare the response of glasses of same composition (or nearly so) by just varying the type of glass modifier cation. This variation is shown in Fig. 7. If we make the assumption that the vibration of the modifier
cation is localized, in other words that the cation vibrates against the glass former network taken as an entity, the mass of which is much larger than that of the cation, we arrive at the following approximation for the frequency of the M cation O 0 at m~ 2.
(2)
The maxima of the peaks of barium and calcium in Fig. 7 obey Eq. (2) within _+3%. Conversely, the magnesium peak is found 50% higher than expected from Eq. (2). Note that in another series of glasses without aluminium which was investigated recently [9], it was concluded that magnesium plays the role of glass modifier and the magnesium response was found at lower frequency, 340 cm -1, a value which is then only 20% higher than the value of Eq. (2) parameterized with barium and calcium frequencies. The 20% difference is attributable to the smaller size of magnesium compared with calcium and barium which reduces the cation-anion bond length and induces a larger force constant. The 50% increase in the present series means that the force constant, k, is further increased , probably because the coordination number of magnesium is smaller and that the bonding of magnesium with oxygen is more hybridized. In particular, some of the magnesium atoms might be tetrahedrally coordinated. The partial contribution of magnesium to the glass-former network would then explain the broadening of all bands in the magnesium spectra compared with other series without magnesium. The fact that no broadening was observed in the series where magnesium is a glass modifier [9] supports this argument. Fig. 7 also shows that the highest-frequency band is insensitive to the type of M cation, provided that they all carry the same nominal charge. On the other hand, we observe in the barium series that the frequency of the u3-type stretching band (or group of bands) is highly dependent on the modifier-cation concentration. This confirms previous reports [8,1114]. This clear-out behaviour allows us to conclude that the structure of the former network is much more dependent upon the balance of respective ionic charges, rather than structural concepts like ionic radii and coordination of modifier cations. Note that there are a number of shoulders that have been observed by transmission [7] that are not observed in the present spectra of the same glasses.
F. Gervais et al. /Journal of Non-Crystalline Solids 176 (1994) 69-75
The frequency dependence of the absorption coefficient indicates that the penetration depth of the infrared radiation in these glasses varies between 1 and 20 ~m, depending on frequency. It appears then that one should be careful with the size and shape of the grains studied by transmission, and their eventual distribution of sizes, because it seems to give additional shoulders in broad-band spectra. Some of them may be related to the frequency dependence of the refractive index. Others also may result from the combination of multiple reflections upon the grains and partial transmission.
4.1. The effect of nitrogen Concerning the effect of nitrogen, the behaviour is the same in all glasses investigated here, namely a down-shift of vibrational frequencies with nitrogen concentration, at least if we discard spurious effects due to different barium concentrations. No additional band attributable to localized modes of nitrogen is observed. This is not surprising since no additional band attributable to aluminium is observed either. In the latter case, the absence of additional set of bands has been understood in terms of coupling of vibrations of AI and Si to yield common modes internal to the aluminosilica network [8,11,15]. The neighbour masses of nitrogen and oxygen favour a similar
73
conclusion in the present case. Actually, the mode coupling of Si-O and Si-N vibrations has been observed experimentally by Chu et al. [17], who measured the concentration dependence of the absorption in S i 3 N 4 - S i O 2 amorphous films and found a continuous shift from 1080 cm-1 in SiO 2 down to 850 cm-1 in Si3N 4. The A1-N vibrational frequency is even lower at 646 cm -1 in aluminium nitride [18]. The present observation that the coupled mode Si(AI)-O(N) frequency decreases with nitrogen addition is consistent, therefore, with the data summarized above. At first sight, the decrease with nitrogen content of the force constant that connects Si(AI) and O(N) may appear inconsistent with the hardening and related properties of the glass. The actual situation is, however, more subtle. Indeed, to have a better insight, we use a property inherent to infrared spectroscopy of probing the instantaneous dipole moment created by the respective motions of atoms charged positively against atoms charged negatively. In practice, we use the relationship that connects the poles and zeros of the dielectric response to the effective charges localized at the atomic sites [19]: 1
2
E(O2LO--g2}To)= ~vVE(Ze),/m ,. j
(3)
k
These concepts are independent of the presence or absence of periodicity [20]. The frequencies of com-
Table 1 An example of shifts of vibrational frequencies (in c m - 1 ) with nitrogen concentration, x, in the case of calcium glasses. Frequencies are parameters which yield best fit of the factorized form of the dielectric function, Eq. (4), to reflectivity data. In the fight hand column are listed the contribution of each band (or double band for the highest-frequency group) to the difference of both values given in the bottom line. Mode
Type
Cal ( x = 0)
Ca4 ( x = 0.09)
Contribution of the modes to
TO LO
Ca (external)
267 347
248 328
3 X 10 3
TO LO
rocking v4
475 533
484 534
8 X 10 3
TO LO
bending
707 724
694 708
5 X 10 3
TO LO TO LO
stretching v 3
931 974 1015 1120
922 962 1005 1111
6 X 10 3
438 X 103
416 X 103
-- ~'~jTO)Ca I
•j(/22LO -- ~2flro)
-- ~ j T O ) C a 4
74
F. Gervais et al. /Journal of Non-Crystalline Solids 176 (1994) 69-75
plex poles and zeros of the dielectric function are denoted TO and LO, respectively, by analogy with the propagation of waves in periodic media that can be transverse or longitudinal with respect to the eigenvectors of the vibrational modes. ~v is the dielectric constant of vacuum. The summation in the right-hand side of Eq. (3) is over all atoms, k, of respective mass, m k, contained in the elementary volume, v (determined from the glass density). An example of shifts of vibrational frequencies with nitrogen concentration is given in Table 1 in the case of calcium glasses. Frequencies are parameters which yield best fit of the factorized form of the dielectric function to reflectivity data: E(09) = ~H
~'~7L O - e°2 q- i3/jLO09
J O2~o
°)2 + i~/jToto"
(4)
The procedure of data fitting with Eq. (4) was detailed in Refs. [8] and [11]. In the example of the calcium glass series, the results of Table 1 shows that the left-hand side of Eq. (3) is found to decrease by 5% in the compound with the highest nitrogen content compared with the one without nitrogen. All groups of modes contribute to this decrease as detailed in Table 1. The same is true for magnesium glasses. The determination has not been attempted in the series of barium glasses because of the dispersion of barium concentrations. Thus, even if nitrogen carries a nominal charge the modulus of which is larger than that of oxygen, present measurements show that nitrogen addition decreases the average anionic effective charge. The average cationic effective charge is decreased in the same amount because of the electronic neutrality of the system. This indicates an increased average bond hybridization, consistent with the change of mechanical properties which is observed. The weakening of the Si-O(N) average radial bonding strength expressed via the short-range force constant with nitrogen addition is thus counterbalanced by a a hardening of the network which probably involves an increase of the Si-O(N)-Si average angular bonding strength. The bond angles indeed are expected to be rigidified by the hybridization of the third atomic p orbital of nitrogen. This hardening of the network does not manifest itself by an increase of the radial force constant of infrared-active modes as observed here,
but via an increase of average bond hybridization also measured in the same experiment via the decrease of E(OfLo -- O fTo).
5. Summary Infrared reflectivity spectra of the series M0.37Sio.53Alo.1oO1.58_3x/2Nx with M = Mg, Ca and Ba and x varying between 0 and 0.09, have been reported. The main conclusions are the following. (i) The vibrational frequencies of the aluminosilica glass-former network are insensitive to the type of M glass-modifier cation, provided that the nominal valence and the respective concentrations are the same. (ii) The far-infrared vibrational frequency of the M cation is found to depend mainly on its mass as expected from a simple description in terms of local oscillator, and also to depend on coordination, at least when it implies a drastic change of bond hybridization (passage from coordination 6 to 4). (iii) The highest-frequency band shifts down on addition of nitrogen, consistent with the lower frequency of Si3N 4 compared with SiO 2. (iv) This down-shift is not inconsistent with the hardening of the lattice since the average effective charge is indeed found to be lower in nitrogenated glasses compared with the reference without nitrogen, showing the increased bond hybridization. The technical help of Nicole Raimboux is acknowledged.
References [1] P. Verdier, V. Lemarchand and R. Pastuszak, Ann. Chim. Fr. 7 (1982) 293. [2] R.E. Loehman, J. Non-Cryst. Solids 42 (1980) 433. [3] P. Verdier, R. Pastuszak, V. Lemarchand and J. Lang, Rev. Chim. Min6r. 18 (1980) 361. [4] S. Sakka, K. Kamiya and T. Yoko, J. Non-Cryst. Solids 56 (1983) 147. [5] P. Verdier, L. Cohen, S. Mariotti and R. Marchand, Rev. Chim. Min6r. 20 (1983) 1. [6] W.K. Tredway and S.H. Risbud, J. Non-Cryst. Solids 56 (1983) 135. [7] J.J. Videau, J. Etourneau, C. Gamier, P. Verdier and Y. Laurent, Mater. Sci. Eng. B15 (1992) 249.
F. Gervais et al. /Journal of Non-Crystalline Solids 176 (1994) 69-75 [8] F. Gervais, A. Blin, D. Massiot, J.P. Coutures, M.H. Chopinet and F. Naudin, J. Non-Cryst. Solids 89 (1987) 384. [9] G. Hanret, Y. Vaills, Y. Luspin, F. Gervais and B. Cot6, J. Non-Cryst. Solids 170 (1994) 175. [10] F. Gervais, C. Lagrange, A. Blin, M. Aliari, G. Hauret and J.P. Coutures, J. Non-Cryst. Solids 119 (1990) 79. [11] J. Zarzycki and F. Naudin, Verres R6fract. 3 (1960) 1. [12] J.R. Sweet and W.B. White, Phys. Chem. Glasses 10 (1969) 246. [13] B.O. Mysen, D. Virgo and F. Seifert, Rev. Geophys. Space Phys. 20 (1982) 353. [14] P.F. Mc Millan and B. Piriou, Bull. Min6r. 106 (1983) 57.
75
[15] C.I. Merzbacher and W.B. White, J. Non-Cryst. Solids 130 (1991) 18. [16] T. Parot-Rajaona, B. Cot6, C. Bessada, D. Massiot and F. Gervais, J. Non-Cryst. Solids 169 (1994) 1. [17] T.L Chu, C.H. Lee and G.A. Gruber, J. Electrochem. Soc. 114 (1967) 717. [18] P. Echegut, private communication. [19] F. Gervais, Solid State Commun. 18 (1976) 191; F. Gervais and H. Arend, Z. Phys. B50 (1983) 17. [20] M.C. Payne and J.C. Inkson, J. Non-Cryst. Solids 68 (1984) 351.
ELSEVIER
Journal of Non-Crystalline Solids 176 (1994) 69-75
Infrared reflectivity spectroscopy of nitrogen-substituted alkaline earth aluminosilicate glasses Franqois Gervais a, Annie Blin a, Corinne Gamier b, Patrick Verdier b,,, Yves Laurent b a Centre de Recherches sur la Physique des Hautes Temperatures, CNRS, 45071 Orldans c~dex 2, France b Laboratoire de Chimie des Matdriaux, URA CNRS 1496, Universit~ de Rennes, Campus Beaulieu, 35042 Rennes c~dex, France
Received 2 February 1994; revised manuscript received 20 April 1994
Abstract
Infrared reflectivity spectra of the series Mo.37Sio.53AIo.loO1.ss_3x/2N x with M = Mg, Ca and Ba and x varying between 0 and 0.09 are reported. The far-infrared vibrational frequency of the M cation mainly depends on cationic mass. In the case of magnesium, the vibrational frequency is also sensitive to the coordination implying the glass-former (tetrahedral) or glass-modifier (likely octahedral). The anionic effective charge measured from the Coulombic contribution to the infrared reflection bands is smaller in nitrogenated glasses than in oxide glasses, showing an increased average bond hybrization, consistent with hardening of the structure indicated by macroscopic properties.
1. Introduction
The partial substitution of oxygen by nitrogen in silica glasses is expected to modify the chemical bonding. This may explain the changes of properties observed experimentally: hardness, elastic moduli, brittleness, viscosity and transition temperatures [16]. To describe the change of average chemical bonding and structural arrangement, information deduced from the analysis of infrared spectra may be useful. Only one infrared study has been reported, to our knowledge, concerning the series M0.37Si0.53Alo.loO1.58_3x/2N x ( M = Mg, Ca or Ba) [7]. The experiments of Ref. [7] were performed on powdered
* Corresponding author. Telefax: +33 99 28 62 58.
samples dispersed in nujol. The spectra were limited to the range above 250 cm -1. But most information concerning the M cations is expected at smaller wavenumbers than this. It is instructive, therefore, to complement this study in the far-infrared. More generally, information deduced from the transmission spectra of glasses raises specific problems related to the broadening of lines which is inherent to glass spectra. In particular, the response of M cations is not well resolved in the absorption coefficient profiles. The problems were described in Ref. [8]. It was also shown that the difficulties may be largely avoided by measuring the reflectivity. Other specific problems are encountered in transmission experiments of powdered samples which may be greater in the case of broad-band spectra. They are due to the part of the infrared radiation which is reflected upon the surface of the grains so that the results combine
0022-3093/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved SSDI 0 0 2 2 - 3 0 9 3 ( 9 4 ) 0 0 4 2 2 - J
70
F. Gervais et al. /Journal of Non-Crystalline Solids 176 (1994) 69-75
the effects of transmission and reflection and this is generally not taken into account in the data analysis. All these reasons justified the investigation of the series Mo.37Sio.53Alo.1001.58_3x/2N x by infrared reflectivity spectroscopy, which is reported here.
0.30 --Ca4
- - - Ca3 . . . . Ca2
L
.
~
~
0,15
2. Experiment I 0
The glasses of the series investigated were prepared to have the approximate compositions M0.37Sio.53Alo.loO1.58_3x/2Nx with x = 0 (noted M1), x = 0.033 (M2), x = 0.066 (M3) and x = 0.1 (M4). The actual compositions of the samples were measured by chemical analysis. Nitrogen percentages are slightly less than expected, typically by 10%, except for M2 in which x = 0.027 instead of 0.033. The ratio N / ( N + O) thus varies between 0 and 6%. In the barium series, the samples have less barium. Instead of the expected atomic ratio B a / S i = 0.7, the actual ratio is 0.59 in sample Bal, 0.53 in Ba2, 0.56 in Ba3 and only 0.4 in Ba4. Oxynitride aluminosilicate glasses were prepared by reaction of various oxides with a nitriding agent. In order to avoid handling of hygroscopic alkaline earth oxides, the silicates M - S i O 3 (where M = Ca, Mg) were prepared by reacting MCO 3 with SiO 2 at 1250°C for 24 h. Conversely, barium is introduced as Ba2SiO 4 prepared in the same way at 1000°C for 24 h. Aluminium nitride was chosen as the nitrogen source for its greater reactivity compared with that of silicon nitride Si3N 4 and for its greater convenience in handling compared with alkaline earth nitrides. The initial mixtures (50 g) were put in a molybdenum crucible and melted in a high-frequency furnace under nitrogen atmosphere. After heating at 1400°C, the glasses were poured and annealed in a molybdenum boat. In all cases, the structural state of the glasses were measured by X-ray diffraction. Spectra were recorded with a Fourier-transform spectrometer Bruker IFS 307 which covers the wavenumber range 10 to 45000 cm -1. The surface of the polished samples studied by reflectivity spectroscopy (near normal incidence) was approximately 0.5 cm 2. The nitrogen concentration dependence of the reflectivity spectra of the Mg, Ca and Ba glasses are shown in Figs. I to 3.
200
I
I
I
400 600 800 FREI]UENEY (r-m-l)
I
~
"1000 1200
Fig. 1. Infrared reflection spectra of calcium glasses Ca0.37Si0.53AlO.1001.58_3x/2N x with x = 0 (noted Ca1), x=0.03 (Ca2),
x = 0.06 (Ca3) and x = 0.09 (Ca4).
0.25 --
Mg4
"193 492 ,~ 019
0
I 200
I I I 400 600 800 FREO,UEN[Y (cm-1)
I 1000
i~ 1200
Fig. 2. Infrared reflection spectra of magnesium glasses Mgo.37Si0.53Alo.1oO1.ss_3x/2Nx with x = 0.03 (Mg2), x=0.06 (Mg3) and x = 0.09 (Mg4).
0,30
--
Ba4
,. . . . . . . Ba2,
~
0.15
I 00
200
I
'1
I
400 600 800 FREQUENEY (cm-1)
I 1000
~ 1200
Fig. 3. Infrared reflection spectra of barium glasses Ba0.37Sio.53A10.1001.ss-3x/2Nx- Instead of the expected ratio Ba/Si = 0.7 in the tentative formula, the checked ratio is 0.59 in sample Bal, 0.53 in Ba2, 0.56 in Ba3 and only 0.40 in Ba4. x = 0 in sample Bal, x = 0.03 in Ba2, x = 0.06 in Ba3, x = 0.09 in Ba4.
F. Gervais et aL /Journal of Non-Crystalline Solids 176 (1994) 69-75 [
6
o[ A
Ca4
5 I-
. . . . Ca3
I
. . . . . . Ca2
71
l-l.l 5 J ~1'~
..
. . . . Ba3 .-. . . . . . Ba2
i E
E
.
2 1 0
200
Fig. 4. Nitrogen concentration dependence of the dielectric response of calcium glasses. The absorption coefficient, K, is related to the imaginary part of the dielectric response, e", plotted here, via K(~o)= 21re"(os)/n(oJ), where n(w) is the refractive index. The dispersion of n(to) may explain additional shoulders in K(oJ) for wide-band spectra.
3. D a t a
0
400 600 800 1 0 0 0 1200 FREQUENCY (cm-1)
analysis
Since the spectra are recorded from nearly zero up to a frequency which lies in a spectral range where there is no dispersion, a K r a m e r s - K r 6 n i g analysis of reflectivity data may be performed, safely. Results in terms of the polar mode response (imaginary part of the dielectric function) are shown in Figs. 4 to 7. 3.1. Calcium glasses The results of Fig. 4 show that: (i) the intensity, frequency and linewidth of the v4-type rocking mode at 475 cm -1 is weakly sensitive to the nitrogen content; (ii) the calcium response is also weakly
S -
Mg4 - - - Mg3
4
. . . . . Mg2
0
200
400 600 800 1000 1200 FREQUENCY (crn-1)
Fig. 6. Nitrogen concentration dependence of the dielectric re-
sponse of barium glasses.
sensitive to nitrogen, as may be expected; (iii) conversely, the v3-type stretching mode at 931 cm -1 and the weak bending mode at 707 cm -1 show a clear down-shift with nitrogen addition. A similar down-shift was reported in transmission experiments [7]. One may attempt to understand these shifts in terms of mass defect, restricted to the atoms within the Si(O,N) 4 tetrahedra for the sake of simplicity. Based on a harmonic oscillator analysis of the v3-type stretching mode,
O0=
1/2,
(1)
where /20 is the oscillator frequency, k the shortrange force constant and /x the reduced mass of the ions, one then expect an up-shift of the mode by 0.65% in the Ca4 glass compared with Cal, simply because nitrogen is lighter than oxygen. This is the opposite of the down-shift by 1% that is found
A
.... Ca4 ..... Mg4
5
2
.,;/ \..
1 0
200
400 600 800 FREQUENCY (r-m-l)
1000 1200
Fig. 5. Nitrogen concentration dependence of the dielectric response of magnesium glasses.
00
I 200
I r J i ~ 400 600 800 1 0 0 0 1200 FREQUENCY (cm-1)
Fig. 7. Glass-modifier cation dependence(M = Ca, Mg, Ba) of
glasses M0.37Si 0.53A10.1001.44No.09.
72
F. Gervais et aL /Journal of Non-Crystalline Solids 176 (1994) 69-75
experimentally. Also the general hardening of the lattice, related to the more covalent bonding Si-N compared with Si-O, which is the origin of higher hardness of nitrogen glasses, is expected to increase the force constant and, therefore, increase the frequency. The reverse is actually observed. We will report results on other glasses prior to a common discussion.
3.2. Magnesium glasses Spectra of magnesium glasses are shown in Fig. 5. We found the same trends as for calcium glasses, namely a net down-shift of the highest-frequency stretching mode with addition of nitrogen. We recover the exceptional broadening of the spectra usually observed in magnesium alumino-silicate glasses investigated previously [8]. Note that this broadening requires the presence of aluminium since no such effect is observed in magnesium silicate glasses [9]. Because of broadening, the band observed around 800 cm -1 in silicate glasses is hardly resolved in this case and it is not possible to decide whether the shift observed for calcium glasses is present.
3.3. Barium glasses As indicated in Section 2, the actual barium concentrations in our samples differ from those expected. As a result, the intensity of the barium band around 115 cm -1 is sample-dependent, consistent with the measured Ba/Si ratios. As already shown and discussed in previous papers [8,10], the concentration of glass modifier cations is related to a shift and/or splitting of the highest-frequency band. This effect manifests itself here and masks the behaviour related to nitrogen substitution.
4. Discussion
The Ba4 sample is the one in which the departure of the actual barium concentration from the expected one is the smallest. It is then instructive to compare the response of glasses of same composition (or nearly so) by just varying the type of glass modifier cation. This variation is shown in Fig. 7. If we make the assumption that the vibration of the modifier
cation is localized, in other words that the cation vibrates against the glass former network taken as an entity, the mass of which is much larger than that of the cation, we arrive at the following approximation for the frequency of the M cation O 0 at m~ 2.
(2)
The maxima of the peaks of barium and calcium in Fig. 7 obey Eq. (2) within _+3%. Conversely, the magnesium peak is found 50% higher than expected from Eq. (2). Note that in another series of glasses without aluminium which was investigated recently [9], it was concluded that magnesium plays the role of glass modifier and the magnesium response was found at lower frequency, 340 cm -1, a value which is then only 20% higher than the value of Eq. (2) parameterized with barium and calcium frequencies. The 20% difference is attributable to the smaller size of magnesium compared with calcium and barium which reduces the cation-anion bond length and induces a larger force constant. The 50% increase in the present series means that the force constant, k, is further increased , probably because the coordination number of magnesium is smaller and that the bonding of magnesium with oxygen is more hybridized. In particular, some of the magnesium atoms might be tetrahedrally coordinated. The partial contribution of magnesium to the glass-former network would then explain the broadening of all bands in the magnesium spectra compared with other series without magnesium. The fact that no broadening was observed in the series where magnesium is a glass modifier [9] supports this argument. Fig. 7 also shows that the highest-frequency band is insensitive to the type of M cation, provided that they all carry the same nominal charge. On the other hand, we observe in the barium series that the frequency of the u3-type stretching band (or group of bands) is highly dependent on the modifier-cation concentration. This confirms previous reports [8,1114]. This clear-out behaviour allows us to conclude that the structure of the former network is much more dependent upon the balance of respective ionic charges, rather than structural concepts like ionic radii and coordination of modifier cations. Note that there are a number of shoulders that have been observed by transmission [7] that are not observed in the present spectra of the same glasses.
F. Gervais et al. /Journal of Non-Crystalline Solids 176 (1994) 69-75
The frequency dependence of the absorption coefficient indicates that the penetration depth of the infrared radiation in these glasses varies between 1 and 20 ~m, depending on frequency. It appears then that one should be careful with the size and shape of the grains studied by transmission, and their eventual distribution of sizes, because it seems to give additional shoulders in broad-band spectra. Some of them may be related to the frequency dependence of the refractive index. Others also may result from the combination of multiple reflections upon the grains and partial transmission.
4.1. The effect of nitrogen Concerning the effect of nitrogen, the behaviour is the same in all glasses investigated here, namely a down-shift of vibrational frequencies with nitrogen concentration, at least if we discard spurious effects due to different barium concentrations. No additional band attributable to localized modes of nitrogen is observed. This is not surprising since no additional band attributable to aluminium is observed either. In the latter case, the absence of additional set of bands has been understood in terms of coupling of vibrations of AI and Si to yield common modes internal to the aluminosilica network [8,11,15]. The neighbour masses of nitrogen and oxygen favour a similar
73
conclusion in the present case. Actually, the mode coupling of Si-O and Si-N vibrations has been observed experimentally by Chu et al. [17], who measured the concentration dependence of the absorption in S i 3 N 4 - S i O 2 amorphous films and found a continuous shift from 1080 cm-1 in SiO 2 down to 850 cm-1 in Si3N 4. The A1-N vibrational frequency is even lower at 646 cm -1 in aluminium nitride [18]. The present observation that the coupled mode Si(AI)-O(N) frequency decreases with nitrogen addition is consistent, therefore, with the data summarized above. At first sight, the decrease with nitrogen content of the force constant that connects Si(AI) and O(N) may appear inconsistent with the hardening and related properties of the glass. The actual situation is, however, more subtle. Indeed, to have a better insight, we use a property inherent to infrared spectroscopy of probing the instantaneous dipole moment created by the respective motions of atoms charged positively against atoms charged negatively. In practice, we use the relationship that connects the poles and zeros of the dielectric response to the effective charges localized at the atomic sites [19]: 1
2
E(O2LO--g2}To)= ~vVE(Ze),/m ,. j
(3)
k
These concepts are independent of the presence or absence of periodicity [20]. The frequencies of com-
Table 1 An example of shifts of vibrational frequencies (in c m - 1 ) with nitrogen concentration, x, in the case of calcium glasses. Frequencies are parameters which yield best fit of the factorized form of the dielectric function, Eq. (4), to reflectivity data. In the fight hand column are listed the contribution of each band (or double band for the highest-frequency group) to the difference of both values given in the bottom line. Mode
Type
Cal ( x = 0)
Ca4 ( x = 0.09)
Contribution of the modes to
TO LO
Ca (external)
267 347
248 328
3 X 10 3
TO LO
rocking v4
475 533
484 534
8 X 10 3
TO LO
bending
707 724
694 708
5 X 10 3
TO LO TO LO
stretching v 3
931 974 1015 1120
922 962 1005 1111
6 X 10 3
438 X 103
416 X 103
-- ~'~jTO)Ca I
•j(/22LO -- ~2flro)
-- ~ j T O ) C a 4
74
F. Gervais et al. /Journal of Non-Crystalline Solids 176 (1994) 69-75
plex poles and zeros of the dielectric function are denoted TO and LO, respectively, by analogy with the propagation of waves in periodic media that can be transverse or longitudinal with respect to the eigenvectors of the vibrational modes. ~v is the dielectric constant of vacuum. The summation in the right-hand side of Eq. (3) is over all atoms, k, of respective mass, m k, contained in the elementary volume, v (determined from the glass density). An example of shifts of vibrational frequencies with nitrogen concentration is given in Table 1 in the case of calcium glasses. Frequencies are parameters which yield best fit of the factorized form of the dielectric function to reflectivity data: E(09) = ~H
~'~7L O - e°2 q- i3/jLO09
J O2~o
°)2 + i~/jToto"
(4)
The procedure of data fitting with Eq. (4) was detailed in Refs. [8] and [11]. In the example of the calcium glass series, the results of Table 1 shows that the left-hand side of Eq. (3) is found to decrease by 5% in the compound with the highest nitrogen content compared with the one without nitrogen. All groups of modes contribute to this decrease as detailed in Table 1. The same is true for magnesium glasses. The determination has not been attempted in the series of barium glasses because of the dispersion of barium concentrations. Thus, even if nitrogen carries a nominal charge the modulus of which is larger than that of oxygen, present measurements show that nitrogen addition decreases the average anionic effective charge. The average cationic effective charge is decreased in the same amount because of the electronic neutrality of the system. This indicates an increased average bond hybridization, consistent with the change of mechanical properties which is observed. The weakening of the Si-O(N) average radial bonding strength expressed via the short-range force constant with nitrogen addition is thus counterbalanced by a a hardening of the network which probably involves an increase of the Si-O(N)-Si average angular bonding strength. The bond angles indeed are expected to be rigidified by the hybridization of the third atomic p orbital of nitrogen. This hardening of the network does not manifest itself by an increase of the radial force constant of infrared-active modes as observed here,
but via an increase of average bond hybridization also measured in the same experiment via the decrease of E(OfLo -- O fTo).
5. Summary Infrared reflectivity spectra of the series M0.37Sio.53Alo.1oO1.58_3x/2Nx with M = Mg, Ca and Ba and x varying between 0 and 0.09, have been reported. The main conclusions are the following. (i) The vibrational frequencies of the aluminosilica glass-former network are insensitive to the type of M glass-modifier cation, provided that the nominal valence and the respective concentrations are the same. (ii) The far-infrared vibrational frequency of the M cation is found to depend mainly on its mass as expected from a simple description in terms of local oscillator, and also to depend on coordination, at least when it implies a drastic change of bond hybridization (passage from coordination 6 to 4). (iii) The highest-frequency band shifts down on addition of nitrogen, consistent with the lower frequency of Si3N 4 compared with SiO 2. (iv) This down-shift is not inconsistent with the hardening of the lattice since the average effective charge is indeed found to be lower in nitrogenated glasses compared with the reference without nitrogen, showing the increased bond hybridization. The technical help of Nicole Raimboux is acknowledged.
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[15] C.I. Merzbacher and W.B. White, J. Non-Cryst. Solids 130 (1991) 18. [16] T. Parot-Rajaona, B. Cot6, C. Bessada, D. Massiot and F. Gervais, J. Non-Cryst. Solids 169 (1994) 1. [17] T.L Chu, C.H. Lee and G.A. Gruber, J. Electrochem. Soc. 114 (1967) 717. [18] P. Echegut, private communication. [19] F. Gervais, Solid State Commun. 18 (1976) 191; F. Gervais and H. Arend, Z. Phys. B50 (1983) 17. [20] M.C. Payne and J.C. Inkson, J. Non-Cryst. Solids 68 (1984) 351.