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X-ray diffraction and absorption studies of ion exchanged glasses
ELSEVIER
Physica B 208&209 (1995) 349 350
X-ray diffraction and absorption studies of ion exchanged glasses M. Dubiel a'*, R. Schmitz a, U. Kolb b, D. Gutwerk b, H. Bertagnolli b aDepartment of Physics, University Halle Wittenber 9, Friedemann-Bach-Platz 6, 06108 Halle, Germany bDepartment of Physical Chemistry, University Stuttyart, Pfaffenwaldrin9 55, 70550 Stuttgart, Germany
Abstract
X-ray diffraction and absorption experiments were performed to determine the structure of commercial sodium silicate glasses before and after a silver-sodium ion exchange. EXAFS measurements at the Na and Ag K-edge display the transition of the sodium environment consisting of five oxygens analogously to that of disilicate glasses to the twofold coordination of silver similar to crystalline Ag20. The existence of A g - A g correlations could be deduced from both the curve-fitting analyses of Ag K-edge oscillations and the differential distribution function of diffraction data. The influence of silver incorporation on the silicate network structure has been shown.
Silver incorporation by ion exchange processes into silicate glasses represents a suitable technique to cause refractive index profiles, e.g., for the fabrication of passive waveguides for integrated optics application. However, the substitution of sodium by silver ions cannot be described by a simple replacement. There are some results that indicate structural rearrangements during the ion exchange well below the glass transformation temperature. Therefore, systematic studies were carried out to clarify the microscopic structure of such glasses by means of X-ray experiments. EXAFS spectroscopy at the Na and Ag K-edges should give the information concerning the local environments of these ions participating directly in the exchange process. In order to complete the structural data and to make a comparison with EXAFS interpretation diffraction measurements were performed. Commercial multicomponent sodium silicate glasses containing 13.8 mol% N a 2 0 and 72 mol% SiO2 were kept in contact with nitrate melts (NaNO3/AgNO3) at
* Corresponding author.
330 °C for 310 h. Defined Ag/Na exchange ratios (5-80%) could be obtained by corresponding melt compositions. Na K-edge (1072 eV) spectra were collected by a modified total yield detection method at the synchrotron radiation source at Daresbury Laboratory. Owing to a limited range of the wave number k of the ejected photoelectrons (Mg K-edge at 1305 eV) the first quantitative determinations of structure parameters were restricted to the first coordination sphere, that means the N a - O correlation. The distances were calculated approximating the phase terms by a linear function in k. The results (for example see Fig. 1) show an analogous sodium environment of both the glassy samples and the crystalline sodium disilicate structure according to earlier results [1]. The data of the basic glass exhibit a N a - O distance of 2.1(8) A and a coordination number of nearly 5. The incorporation of silver measured up to an exchange ratio of 37% causes a slight increase of the N a - O correlation to 2.3(5) ,g, and a slight drop of the coordination number. Ag K-edge (25 514eV) spectra were performed in Novosibirsk at the storage ring VEPP-3 in the transmission mode. The data evaluation were carried out using
0921-4526/95/$09.50 (~ 1995 Elsevier Science B.V. All rights reserved SSDI 0 9 2 1 - 4 5 2 6 ( 9 4 ) 0 0 6 9 4 - 6
350
M. Dubiel et al. /Physica B 2 0 8 & 2 0 9 (1995) 3 4 9 - 3 5 0 3.0
5
Ag K-edge
No K-edge
z
"Z" (3 2.0
3
1.0
2
,/ 0.0 i
o
2
z
6
6
8
r,= (,~)
-1,0 0.0
,
,
Fig. 1. Fourier transforms of EXAFS oscillations at the Na and Ag K-edge for the glasses with an exchange ratio of 37%.
a program package for fitting the EXAFS oscillations [2]. As backscatterer of the first neighbourhood could be identified oxygen exhibiting an average A g - O distance of 2.04 ~, with a twofold coordination. The fitting procedure yields that the second peak (see Fig. 1) consists of two parts, a small contribution of a light backscatterer (,~2.64/~) and a dominating Ag-Ag correlation ( ~ 2.68 A). These results demonstrate clearly the change of the oxygen coordination during the replacement of sodium by silver within the glass network. Ag ions prefer the twofold coordination as in silver oxide. Furthermore, the EXAFS data reveal the tendency of silver ions to form aggregates or clusters in spite of their positive charge as it is known in ternary oxides [3]. This behaviour can be explained by d l ° - d l ° interactions of silver ions. Additionally, X-ray diffraction experiments were recorded at two characteristic glass compositions, the basic glass and the sample with an Ag/Na exchange ratio of 80%. The scattering intensities were collected using a position sensitive detector and Mo-K, radiation (2 = 0.7093 A) in the reflexion mode with a rotating probe. Whereas the first peak in the total correlation function, G(r) represents the Si-O separation within the SiO4 tetrahedrons (see Fig. 2) the second one at 2.48
2.0
4.0
6.0
8.0
r/A
1 0.0
Fig. 2. Total correlation functions G(r) of the basic glass (solid line) and the sample with an exchange ratio of 80% (dashed line) calculated from diffraction data.
should be a superimposition of mainly N a - O , C a - O , and O - O pair correlations in the basic glass network. The silver-sodium exchange leads to the occurrence of additional pair correlation functions at 2.10 and 2.74 detected by the differential correlation function calculated from the total correlation functions of both samples. The assignment of the peaks is in agreement with the EXAFS data, only small shifts of the peak maxima are evident. The origin of the second peak should be a A g ÷ - A g ÷ interaction as, for example, in Ag~ ÷ clusters in Ag60 2 [4]. The existence of Ag o centres can be excluded because of experimental results by optical spectroscopy and electron microscopy.
References [1] G.N. Greaves, A. Fontaine, P. Lagarde, D. Raoux and S.J. Gurman, Nature 293 (1981) 611. [2] T.S. Ertel, H. Bertagnolli, S. Hiickmann, U. Kolb and D. Peter, Appl. Spectrosc. 46 (1992) 690. [3] M. Jansen, Angew. Chemic 99 (1987) 1136. [4] W. Beesk, P.G. Jones, H. Rumpel, E. Schwarzmann and G. Sheldrick, J. Chem. Soc. Chem. Comm. (1981) 664.
Physica B 208&209 (1995) 349 350
X-ray diffraction and absorption studies of ion exchanged glasses M. Dubiel a'*, R. Schmitz a, U. Kolb b, D. Gutwerk b, H. Bertagnolli b aDepartment of Physics, University Halle Wittenber 9, Friedemann-Bach-Platz 6, 06108 Halle, Germany bDepartment of Physical Chemistry, University Stuttyart, Pfaffenwaldrin9 55, 70550 Stuttgart, Germany
Abstract
X-ray diffraction and absorption experiments were performed to determine the structure of commercial sodium silicate glasses before and after a silver-sodium ion exchange. EXAFS measurements at the Na and Ag K-edge display the transition of the sodium environment consisting of five oxygens analogously to that of disilicate glasses to the twofold coordination of silver similar to crystalline Ag20. The existence of A g - A g correlations could be deduced from both the curve-fitting analyses of Ag K-edge oscillations and the differential distribution function of diffraction data. The influence of silver incorporation on the silicate network structure has been shown.
Silver incorporation by ion exchange processes into silicate glasses represents a suitable technique to cause refractive index profiles, e.g., for the fabrication of passive waveguides for integrated optics application. However, the substitution of sodium by silver ions cannot be described by a simple replacement. There are some results that indicate structural rearrangements during the ion exchange well below the glass transformation temperature. Therefore, systematic studies were carried out to clarify the microscopic structure of such glasses by means of X-ray experiments. EXAFS spectroscopy at the Na and Ag K-edges should give the information concerning the local environments of these ions participating directly in the exchange process. In order to complete the structural data and to make a comparison with EXAFS interpretation diffraction measurements were performed. Commercial multicomponent sodium silicate glasses containing 13.8 mol% N a 2 0 and 72 mol% SiO2 were kept in contact with nitrate melts (NaNO3/AgNO3) at
* Corresponding author.
330 °C for 310 h. Defined Ag/Na exchange ratios (5-80%) could be obtained by corresponding melt compositions. Na K-edge (1072 eV) spectra were collected by a modified total yield detection method at the synchrotron radiation source at Daresbury Laboratory. Owing to a limited range of the wave number k of the ejected photoelectrons (Mg K-edge at 1305 eV) the first quantitative determinations of structure parameters were restricted to the first coordination sphere, that means the N a - O correlation. The distances were calculated approximating the phase terms by a linear function in k. The results (for example see Fig. 1) show an analogous sodium environment of both the glassy samples and the crystalline sodium disilicate structure according to earlier results [1]. The data of the basic glass exhibit a N a - O distance of 2.1(8) A and a coordination number of nearly 5. The incorporation of silver measured up to an exchange ratio of 37% causes a slight increase of the N a - O correlation to 2.3(5) ,g, and a slight drop of the coordination number. Ag K-edge (25 514eV) spectra were performed in Novosibirsk at the storage ring VEPP-3 in the transmission mode. The data evaluation were carried out using
0921-4526/95/$09.50 (~ 1995 Elsevier Science B.V. All rights reserved SSDI 0 9 2 1 - 4 5 2 6 ( 9 4 ) 0 0 6 9 4 - 6
350
M. Dubiel et al. /Physica B 2 0 8 & 2 0 9 (1995) 3 4 9 - 3 5 0 3.0
5
Ag K-edge
No K-edge
z
"Z" (3 2.0
3
1.0
2
,/ 0.0 i
o
2
z
6
6
8
r,= (,~)
-1,0 0.0
,
,
Fig. 1. Fourier transforms of EXAFS oscillations at the Na and Ag K-edge for the glasses with an exchange ratio of 37%.
a program package for fitting the EXAFS oscillations [2]. As backscatterer of the first neighbourhood could be identified oxygen exhibiting an average A g - O distance of 2.04 ~, with a twofold coordination. The fitting procedure yields that the second peak (see Fig. 1) consists of two parts, a small contribution of a light backscatterer (,~2.64/~) and a dominating Ag-Ag correlation ( ~ 2.68 A). These results demonstrate clearly the change of the oxygen coordination during the replacement of sodium by silver within the glass network. Ag ions prefer the twofold coordination as in silver oxide. Furthermore, the EXAFS data reveal the tendency of silver ions to form aggregates or clusters in spite of their positive charge as it is known in ternary oxides [3]. This behaviour can be explained by d l ° - d l ° interactions of silver ions. Additionally, X-ray diffraction experiments were recorded at two characteristic glass compositions, the basic glass and the sample with an Ag/Na exchange ratio of 80%. The scattering intensities were collected using a position sensitive detector and Mo-K, radiation (2 = 0.7093 A) in the reflexion mode with a rotating probe. Whereas the first peak in the total correlation function, G(r) represents the Si-O separation within the SiO4 tetrahedrons (see Fig. 2) the second one at 2.48
2.0
4.0
6.0
8.0
r/A
1 0.0
Fig. 2. Total correlation functions G(r) of the basic glass (solid line) and the sample with an exchange ratio of 80% (dashed line) calculated from diffraction data.
should be a superimposition of mainly N a - O , C a - O , and O - O pair correlations in the basic glass network. The silver-sodium exchange leads to the occurrence of additional pair correlation functions at 2.10 and 2.74 detected by the differential correlation function calculated from the total correlation functions of both samples. The assignment of the peaks is in agreement with the EXAFS data, only small shifts of the peak maxima are evident. The origin of the second peak should be a A g ÷ - A g ÷ interaction as, for example, in Ag~ ÷ clusters in Ag60 2 [4]. The existence of Ag o centres can be excluded because of experimental results by optical spectroscopy and electron microscopy.
References [1] G.N. Greaves, A. Fontaine, P. Lagarde, D. Raoux and S.J. Gurman, Nature 293 (1981) 611. [2] T.S. Ertel, H. Bertagnolli, S. Hiickmann, U. Kolb and D. Peter, Appl. Spectrosc. 46 (1992) 690. [3] M. Jansen, Angew. Chemic 99 (1987) 1136. [4] W. Beesk, P.G. Jones, H. Rumpel, E. Schwarzmann and G. Sheldrick, J. Chem. Soc. Chem. Comm. (1981) 664.