CdO layers on glass prepared by the sol-gel method

JOURNAL OF

ELSEVIER

Journal of Non-CrystallineSolids 195 (1996) 54-63

Infrared study of SiO2/TiO2/CdO layers on glass prepared by the sol-gel method H. Hobert a,*, B. Seltmann b a Institute of Physical Chemistry, Friedrich-Schiller-UniversityJena, Lessingstrasse 10,07743 Jena, Germany b Institute of Glass Chemistry, Friedrich-Schiller-UniversityJena, Lessingstrasse lO, 07743 Jena, Germany

Received 8 May 1995; revised 21 July 1995

Abstract Fourier transform infrared reflection spectra of a set of SiO2-TiO2-based glass layers with different admixtures of CdO obtained by sol-gel procedures were evaluated by dispersion analysis. The pure SiO 2 layer shows the features of vitreous quartz. A small amount of CdO caused a broadening of the bands; a larger amount formed a glass-like silicate spectrum with a new band at 997 cm- 1. The TiO2-containing samples also showed glass-like silicate spectra with prominent bands at 965 and 1030 cm- 1 and an increase of absorption in the region from 400 to 900 cm- i, indicating the existence of TiO 2 phases. e~ and the thickness of the layers can be computed but with less accuracy than by interferometric determinations.

1. Introduction Thin glass layers produced by sol-gel methods [1] can be used as matrices of materials with non-linear optical properties. They need a thorough structural description, including the determination of the optical and dielectric properties. We report and discuss the infrared reflection spectra of such layers on the base of SiO2-TiO 2 glasses with different admixtures of CdO, and derive their dielectric functions by a dispersion analysis with the aim to determine the specific effects of titanium and cadmium ions on the structure of those glasses and to contribute to the controversial discussions about the structure of the SiO2-TiO 2 glasses [2]. Because the glasses were prepared as thin layers on a substrate (slides of soda

* Corresponding author. Tel: +49-3641 635 873. Telefax: + 49-3641 636 118. E-mail: [email protected].

lime glass), we tested, also, the abilities of such measurements to evaluate the thickness of the layers and their index of refraction.

2. Experimental 2.1. Sol and film preparation

To prepare sol-gel films, we used a basic set of three sols with different SiO2-TiO 2 ratios, denoted here as Sil00, SiSOTi20 and Si60Ti40 (the numbers give the molar parts of SiO 2 and TiO2). Further sols were derived from these by adding 5 or 15 molar parts of CdO (denoted by ...Cd5 or ...Cdl5). Sols were prepared utilizing N-(2-aminoethyl) 3aminopropyl methyl dimethoxysilane (NMDMOS, from ABCR), methyltriethoxysilane (MTEOS), titanium isopropoxide (TTIP), C d ( A c ) 2 . H 2 0 , pure methanol and ethanol (all from Merck) and acety-

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H. Hobert, B. Seltmann / Journal of Non-Crystalline Solids 195 (1996)54-63

lacetone (acac, from Riedel-deHaen) as received without further purification. The preparation steps were as follows: (i) mixing of the Cd salt (if necessary) with the Si-containing components (MTEOS:NMDMOS = 2:1) in methanol under magnetic stirring (NMDMOS acts as chelating agent for the cadmium); (ii) dropwise addition of deionized water and some 4N HC1 in methanol for prehydrolysis of the S i - O - R bonds; (iii) for the Ti containing sols, the T r I P was mixed with acac (ratio 1:1) in ethanol under stirring. The reaction was exothermic and yields a clear, yellow solution; (iv) dropwise addition of the Si-containing solution under stirring. In all cases, the molar ratio, r w, of water to alkoxides was 8, the ratio of alcohols to alkoxides, rj, was 5 with equimolar parts of ethanol and methanol, except for the sol Si80Ti20 for which r w was 3 and r I was 10 for better stability of the sol. After reaction, the sois were filtered and poured in closed vessels until the start of the dip coating procedure. Film deposition was done using ordinary microscopy slides as substrates. They were cleaned before coating in ammoniacal water, in water and ethanol and finally dried in hot air. The substrate was then dipped in the coating solution and withdrawn at a rate of 5 cm min- ~. The films were placed into a furnace and the glassy films were formed by heating in air to 770 K using a step regime: heating in 4 h to 390 K, holding 4 h at 390 K, heating in 4 h to 520 K, holding 4 h at 520 K, heating in 4 h to 650 K, holding 4 h at 650 K, heating in 4 h to 770 K and holding 4 h at 770 K.

2.2. Properties The chemical composition of the films after the baking process corresponds to the theoretical compositions which were verified by Rutherford back scattering (RBS) measurements. Auger and SIMS measurements detected no composition changes along a depth profile. The calculations of refractive indices and thickness of the films are based on interference spectra derived from spectroscopic comparison of coated and uncoated substrates in the spectral region from 400 to 2000 nm (Shimadzu UV3101PC spec-

55

trometer). For data analysis, we used a program [3] based on the formula of Abelbs [4] and a two-oscillator model [5]. The thicknesses were examined in some cases by RBS and ellipsometric measurements and showed good agreement with the interferometric data. Indices of refraction computed by analysis of waveguide experiments with such films on oxidized Si wafers showed deviations in the range of 1-2 × 10 -2 units.

2.3. IR measurements The IR spectra were obtained by a FT-IR spectrometer (Bruker IFS 66) with a resolution of 4 cm- 1 and with 128 scans in a reflection cell (Harrick Scientific Corp.). The angle of incidence was 10°. The IR radiation was polarized perpendicular to the plane of incidence. The effectivity of the metal/KRS5 polarizer was determined to be better than 98% in the spectral region of interest. The single beam spectra were ratioed against the spectrum of a gold film evaporated on a glass slice, the reflectivity of which was assumed to be 100%. To avoid distortions by radiation reflected on the rear side of the substrate, the spectra were limited on 800 points in the region from 400 to 1942 cm -~. Smoothing by 11-point boxcar filter eliminated small noise effects.

2.4. Dispersion analysis Because reflection spectra depend on experimental conditions, it is desirable to describe the samples by their optical or dielectric functions which express the inherent properties of a material. The dispersion analysis derives such dielectric functions by formulation and optimization of an optical model of the system with analytical expressions of the dielectric functions. The parameters of these functions are improved by non-linear regression which fits the computed spectra to the experimental spectra. The optical model was a simple, three-media system of air/layer/substrate. The dielectric constant of air was set to 1. The substrate was treated as a semi-infinite layer. Its dielectric function was determined in a previous study [6] and introduced into the computations as a set of complex numbers. The dielectric function of the layer was the object of optimization.

56

H. Hobert, B. Seltmann/ Journal of Non-Crystalline Solids 195 (1996) 54-63

parameter (in cm -~ ) and 7 is the damping constant (in c m - l). This function, E *, was then folded with a distribution function, g, to simulate the observed broadening of the bands caused by the microheterogeneity of the environment of the vibrating atoms. The applied Gaussian distribution function introduced the broadening parameter, o-, into the model, which describes

It was computed at first as a sum of damped, harmonic oscillators, supplemented by the high frequency part of the dielectric function, ~ , J v~

+

E

j= l v2j- v2 + i T Y

,

(1)

where v is the wavenumber (in cm-1), v0is the resonance wavenumber (in c m - 1), vp is the intensity

O O

quartz glass (20 dgr)

SilO0 SilOOCd5 SilOOCd15

Si80Ti20 Si80Ti2OCd5 Si80Ti20Cd15

Si60Ti40 Si6OTi4OCd5 Si6OTi4OCd15 x substrat

; / c m -1 Fig. 1. FT-IR reflection spectra of Si02/TiO2/CdO layers on a soda lime glass (angle of incidence 10°, TE polarization); reflection spectra of the substrate and quartz glass are added for comparison.

H. Hobert, B. Seltmann / Journal of Non-Crystalline Solids 195 (1996) 54-63

the change of the band shape from the profile of a harmonic oscillator to the profile of a Gaussian function = g * E*.

(2)

The broadening function, g, was

g= f~-~ (1/tr )e -~2/~2,

(3)

where N is the number of points and cr is the broadening parameter. This procedure [6] is equivalent to functions proposed by Efimov and Makarova [7] or Brendel and Bormann [8]. If the dielectric functions were known, the reflection spectra were computed applying the Fresnel equations for layered systems [9].

3. Results

The reflection spectra of the SiO2-TiO 2 layers, of the substrate and of quartz glass (measured at 0 =

20 °) are collected in Fig. 1. These spectra represent two important features: all samples containing TiO 2 have similar spectra. The different contents of TiO 2 and CdO have only a small effect. The differences between these spectra and the spectrum of the substrate glass are relatively small. The samples without TiO 2 have distinct spectra. The samples Sil00 and Sil00Cd5 develop features which can be explained by the formation of a layer of vitreous silica. The sample Sil00Cdl5 has a spectrum which is intermediate between the spectra of quartz glass and substrate. The dispersion analysis was carried out assuming eight damped, harmonic oscillators in each sample initially. The 24 parameters connected with this model were completed by two further parameters: E~ and the thickness, d. In the TiO2-free samples, some oscillators had small differences in their wavenumbers, v 0, such that they could be replaced by a single oscillator. Because of that, the samples Sil00 and

35

35

:I

~2* 30

57

c

25 2O

20

15

1St

10

10~

5 0

6oo

8oo

looo

0 i'

12oo

35

600

860

1000

1200

1400

800

lO00

1200

1400

35 I 3O

E2* 30 25

~

b

I d

251

20

20 !

15

15:

10 5F

5 0

600

800

lO00

1200

1400

/ cm -1

0

600

~ / cm -1

Fig. 2. Imaginary part of the dielectric function before the folding procedure of the samples (a) Sil00, (b) Sil00Cdl5, (c) Si60Ti40 and (d) Si60Ti4OCd 15.

H. Hobert, B. Seltmann / Journal of Non-Crystalline Solids 195 (1996) 54-63

58 10

lOr

8

8F

4

4

r~ c

2

600

~2

800

1000

12111)

14(11)

600

10

lOt

8

81-

8013

101)0

12(1(I

1,,oo

800

I000

1200

1400

d

6

600

800

1000

1200

1400

u'

600

v / cm -1

~ / cm "1

Fig. 3. Imaginary part of the dielectric function after the folding for samples (a) Sil00, (b) Sil00Cdl5, (c) Si60Ti40 and (d) Si60Ti40Cdl5.

Sil00Cd5 were modelled with six and the sample Sil00Cdl5 with seven oscillators. Fig. 2 gives the imaginary part of the dielectric functions, E2*, of the samples with the smallest and the largest contents of TiO 2 and CdO: Sil00, Sil00Cdl5, Si60Ti40 and Si60Ti40Cdl5. The next step was the convolution of this e * function with a Gaussian distribution func-

tion. The broadening parameter of this function is another parameter of these systems. The imaginary part of the broadened dielectric functions, E2, is shown in Fig. 3. The parameters for the three most distinct samples, S i l 0 0 , S i l 0 0 C d l 5 and Si60Ti40Cdl5, are given in Table 1. Fig. 4 finally shows the experimental reflection spectrum, the

Table 1 Parameters, characterizing the gel glass layers (the first line shows ~ , thickness, d (nm) and o- ( c m - i ); the following lines give v o, vp and 3' for each oscillator (cm- i )) SilO0

Sil00Cdl5

Si60Ti40Cdl5

1.85

305

15.3

2.13

334

24.0

2.51

539

33.0

453 507 . 796 1047 1078 1164

323 102

10.0 10.0

459 520 . 785 997 1048 1087 1158

353 116

10.0 11.1

142 429 478 279 193

10.0 32.4 21.1 12.2 17.7

452 525 635 768 959 1025 1100 1155

391 245 260 207 522 497 235 166

10.0 54.5 117 35.8 30.6 25.0 25.7 10.0

.

. 131 472 531 305

. I0.0 23.4 15.5 81.6

H. Hobert, B. Seltmann // Jourt~tl of Non-Crystalline Solids 195 (1996) 54-63

0.4 [

a

59

0.4

R

C

0.3

0.3

0.2

0.2

0.1

0.1

0 0.02

0. o

o

V v .

.

.

.

.

..~

r~

.

X./

-O.02

-0.02

t~ 0.4

R 0"41

b

0.3

0.3

0.2

0.2

0.1

0.1 J

0.020

.

.

.

.

"=

d

0 0.02 0 ~t- ^ _ ~ V . . .

0

.

v

A

v

~-~ ~

-

V

-0.02 v / c m -1

v / c m -1

Fig. 4. Spectral fit for samples (a) Sil00, (b) Sil00Cdl5, (c) Si60Ti40 and (d) Si60Ti40Cdl5. computed spectrum. The lower part shows the difference spectrum.

Table 2 Comparison of the e'~ and d values from interferences in UV-VIS spectra and IR reflection spectra (the last column gives the broadening parameters) Sample

SilO0 Sil00Cd5 Sil00Cdl5 Si80Ti20 Si80Ti20Cd5 Si80TiO20Cd 15 Si60Ti40 Si60Ti40Cd5 Si60Ti40Cd 15

Interferences

2.13 2.16 2.25 2.59 2.53 2.59 2.85 2.82 2.85

IR reflection

d

~

d

o"

291 454 294 252 541 405 275 204 301

1.85 2.13 2.13 2.30 2.37 2.44 2.47 2.45 2.51

305 409 334 275 551 495 400 297 538

15.2 15.9 24.0 31.4 31.0 33.0 31.6 32.0 33.2

-

, experimental spectrum; • - -,

spectrum computed from a and the difference spectrum of both for the selected samples. These curves give an impression of the fit quality. Similar degrees of approximation were achieved with the other sampies. The spectra give no hints on the presence of S i - O H groups or residues of organic materials in the glass layers.

4. Discussion

The unfolded E * functions, especially their imaginary part, are a suitable base for the discussion of the main effects. These functions are obtained as

60

H. Hobert, B. Seltmann / Journal of Non-Crystalline Solids 195 (1996)54-63

intermediate results on the path from the set of parameters to the computed spectrum, and they represent the dielectric function, as it could appear without the broadening of bands due to the disorder in the environment of the oscillators. They give a more resolved representation of the oscillators and allow a better comparison between the spectra of different samples than the • functions (which are of physical relevance). Because the broadening parameters of the various samples are different (see Table 2, final column), such a comparison should be limited to the position and the relative areas of the oscillators but should not include the amplitude or band width of the oscillators. Fig. 5 gives the collection of all E2* curves including the spectra of the substrate and quartz glass. The distinctness of the TiOE-free samples and the similarity of all TiO2-containing samples, already seen in the spectra, are confirmed. The small shifts in the band positions in the group of the three Si80Ti20 samples or the different appearance of the band at 640 cm-1 in the Si60Ti40 group give an impression of the variability of these results which depends on certain factors: sample preparation, accuracy of the spectra registration, quality of the model and the choice of the initial values of the parameters. Because the main source of uncertainty is the adequacy of the model, it makes little sense to perform a detailed analysis of quantitative errors in the parameter values. The similarity between the spectra of the Sil00 and Sil00Cd5 samples and the spectrum of quartz glass is remarkable; there are only differences in the width of the bands which indicate a higher degree of structural disorder in the glass layers, especially in the sample Sil00Cd5. However, the sample Sil00Cdl5 shows a replacement of the characteristics of vitreous silica by new features, especially the appearance of a new oscillator at 997 cm -~, and its • 2" curve is similar to the curve of a silicate glass. The six TiO2-containing samples represent relatively uniform spectra. The region of the asymmetric Si-O valence vibrations between 900 and 1300 cmis dominated by two strong oscillators at 965 and 1030 cm -I and two weak oscillators at 1100 and 1160, respectively, 1155 cm -1. This part of the spectrum indicates a clear structural change from silica to a silicate glass. This is caused mainly by the

quartz glass

SilO0 SilOOCd5 Si100Cd15

Si8OTi20 Si80Ti2OCd5

J~-___.A

Si80Ti2OCd15

Si60Ti40 Si60Ti4OCd5 Si60Ti4OCd15

600

800

1OO0

1200

1400

substrate

Fig. 5. Comparison of the imaginary parts of the unfolded dielectric functions, E~, of the sol-gel glass layers, quartz glass and substrate.

TiO 2 component because it is also observed in the samples Si80Ti20 and Si60Ti40, which are free of CdO. The bands in the region between 950 and 1050 cm-1 are considered to be caused by the vibrations of non-bridging oxygen, and it is obvious that at least a part of the titanium ions acts as network modifiers. Titanium as a network former is not excluded, but its vibrations are probably synchronized with the vibrational frequencies of the silicon atoms by mode coupling. There are no reasons to explain the band at 965 cm- l with S i - O - T i vibrations or vibrations of TiO 4 tetrahedrons as is sometimes done [10-12]. This interpretation is usually coupled with the hint of the

H. Hobert, B. Seltmann / Journal of Non-Crystalline Solids 195 (1996) 54-63

tetrahedral coordination of the Ti 4+ ions which should substitute for the Si 4+ ions in the glass network in the samples with low titania contents. Such tetrahedral coordination was concluded from the appearance of a specific peak in the X-ray absorption near edge spectroscopy (XANES) curves [13,14]. Transitions causing this peak are forbidden for atoms in a centrosymmetric, octahedral environment. Therefore, the observation of this peak indicates the absence of centrosymmetry but not uniquely the presence of tetrahedral coordination. An alternative interpretation of the spectra could be the assumption of SiO 2 microregions beside TiO 2 microregions. Such an explanation was supported by XPS observations [2,15-17], showing in SiO2-TiO 2 the existence of only two types of oxygen: oxygen bonded to two silica or to two titanium atoms but not a third type involved in S i - O - T i bonds. 29Si-NMR measurements on SiO2-TiO2-ZrO2 sol-gel systems assert an extremely weak copolymerisation of the three constituents and the formation of homogeneous nanodispersions of SiO 2 and TiO2/ZrO 2 regions [18]. A Raman study [19] on diphasic gels prepared by a colloidal sol-gel method showed the same Raman spectra as the corresponding melted glasses, which could be explained only with the assumption of molecular-scale titania-rich clusters within a network of silica. Keeping in mind that bands around 950 c m - 1 are observed in many other silicate glasses [20,21], we propose to consider the occurrence of the band around 965 cm-~ in the SiO2-TiO 2 mainly as a hint of the formation of non-bridging oxygens in the silica network and not as a specific sign of S i - O - T i bonds. The increase of the TiO 2 content is connected with small but significant changes (Fig. 6): an increase of E2 takes place in the region between 400 and 900 c m - ~, a decrease at 960 c m - ~ and a small increase at 1120 cm -1. The difference spectrum reminds us of transmission spectrum of TiO 2 powder and can be understood as a hint of the existence of TiO 2 microregions in the layers. This conclusion is in avoid with many other observations of amorphous or crystalline TiO z phases in samples with more than 10 mol% T i O 2 [10,17]. One can suppose that the formation of separate microregions is favored by the use of chelating reagents in the sol production [22]. CdO transforms the spectra of the SiO2 samples

61

~2

600 A~2

800

1000

1200

1400

It i

I

ii O -1

~icm -I Fig. 6. Effect of the T i O 2 content on the ~2 functions: Si80Ti20; • - . , Si60Ti40; and the difference curve (below).

from the silica spectrum to the spectrum of a silicate glass. The band at 997 c m - t indicates the formation of non-bridging oxygen. The effect of an increase of the CdO content in the SiO2/TiO 2 samples is demonstrated in Fig. 7: the oscillator at 460 c m - ~ is increased and shifted to 452 cm - l , the oscillator at

~2

600

-'

800

I000

1200

1400

1 ~/cm-1

Fig. 7. Effect of the CdO content on the e 2 functions: ~ Si60Ti40; • • . , Si60Ti40Cdl5; and the difference curve (below).

H. Hobert, B. Seltmann / Journal of Non-Crystalline Solids 195 (1996) 54-63

62 0.40

/\

0.30 , .\,

, .~, '%\ ',~\

"" 0.20

0. I0 0.00 4O0

600

800

t 000

1200

I., / c m

a::

0.40

[

0.30

k

"

"

'

"

"

'

'

"

"

"

1400

600

-1

'

"

.

.

.

.

.

.

'

"

"

0.20

0.10

...-. ~ .

o oo 400

.

.

600

800

.

.

.

.

.

1000

1.' / c m

1200

1400

600

-1

Fig. 8. Simulated reflection spectra demonstrating the effect of the thickness of the layers. (a) SilO0, pure substrate ( ), d = 100 nrn (. • • ), d = 200 nm (---), d = 300 nrn ( - . -). (b) Si60Ti40, pure substrate ( ), d = 1 0 0 nm ( - - .), d = 200 nm (---), d = 300 nrn ( - - - ) .

960 cm-1 increased, and in the region around 1120 cm-1 is a diminution of ~2. These changes are in a certain sense opposite to the effects of TiO 2. An interesting aspect of the optimization is the behavior of the parameters e~ and d which are not connected with an individual oscillator but influence the whole spectrum. These values, represented in Table 2, can be compared with the interferometric results. It is seen that their accuracy and the degree of accord are not satisfactory. A tendency exists that the e~ values grow with increasing contents of TiO2, but the individual values show a large variance and should be discussed with caution. This variance can be understood considering that these values are determined mainly by the transparent part of the spectrum which is limited in our experiments to the small region between 1300 and 1800 cm - l , too small for the development of clear interference patterns. A second reason is the similarity between the spectra of the layer and the substrate, especially in the case of the TiOE-Containing layers. Fig. 8

illustrates this aspect with some computed spectra for layers of different thicknesses assuming the spectral characteristics of the samples Sil00 and Si60Ti40. The broadening parameter, or, of the folding function is also given in Table 2. This parameter is an indicator of the microheterogeneity around the oscillators. The differences between the quartzglass-like samples, Sil00 and Sil00Cd5 (bulk quartz glass gives t r = 11.8), and the glass-like samples which contain TiO 2 are obvious. The sample Sil00Cdl5 shows this heterogeneity to a medium degree.

5. Conclusions

(a) Infrared reflection spectra show structural peculiarities of the SiO2-TiO 2 layers dependent on their composition. The sample containing only SiO 2 shows a spectrum like vitreous quartz but with lower values of ~. Admixture of 5% CdO causes a broad-

H. Hobert, B. Seltmann / Journal of Non-Crystalline Solids 195 (1996) 54-63

ening of the bands (coupled modes); 15% CdO produces a silicate glass spectrum with a new intense band at 997 cm-1 connected with non-bridging oxygen. Titania admixtures give spectra with silicate bands at 1030 and 965 cm -t and an increased absorption in the region between 900 and 400 cm-1 which show the coexistence of a silicate network and a TiO2-rich phase. (b) e~ and thickness values derived by dispersion analysis from the region-limited IR spectra can be obtained but are not as accurate as the results of interferometric measurements.

Acknowledgements

This study was supported by Deutsche Forschungsgemeinschaft (Project SFB 196).

References [1] C.J. Brinker and G.W. Scherer, Sol-Gel Science (Academic Press, New York, 1990). [2] M. Guglielmi, in: Proc. 16th Int. Congr. on Glass, Vol. lnaug. Lect., Bul. Soc. Esp. Ceram. Vid. Madrid (1992) 17. [3] W. Richter, Beitr. Opt. Quantenelektron. 11 (1986) 92. [4] F. Abel,s, in: Progress in Optics, Vol. 2 (North-Holland, Amsterdam, 1963) p. 249.

63

[5] S.H. Wemple and M. DiDomenico Jr., Phys. Rev. Lett. 23 (1969) 1156. [6] H. Hobert and H.H. Dunken, this issue, p. 64. [7] A.N. Efimov and E.G. Makarova, Fiz. Khim. Stekla 15 (1989) 366. [8] R. Brendel and D. Bormann, J. Appl. Phys. 71 (1992) 1. [9] W.N. Hansen, J. Opt. Soc. Am. 58 (1968) 380. [10] M.F. Best and R.A. Condrate, J. Mater. Sci. Lett. 4 (1985) 994. [11] Lin Yang, S.S. Saavedra, N.R. Armstrong and J. Hayes, Anal. Chem. 66 (1994) 1254. [12] K. Kusabiraki, J. Non-Cryst. Solids, 52 (1982) 115. [13] R.B. Greegor, F.W. Lytle, D.R. Sandstorm, J. Wong and P. Schultz, J. Non-Cryst. Solids 55 (1983) 27. [14] M. Emili. L. Incoccia, S. Mobilio, G. Fagheazzi and M. Guglielmi, J. Non-Cryst. Solids 74 (1985) 129. [15] S.M. Mukhopadhyay and S.H. Garofalini, J. Non-Cryst. Solids 126 (1990) 202. [16] H. Yamanaka, K. Nakahata and R. Terai, J. Non-Cryst. Solids 95&96 (1987) 405. [17] L. Armelao, P. Colombo, G. Granozzi and M. Gugliemi, J. Non-Cryst. Solids 139 (1992) 198. [18] C. Wies, K. Meise-Gresch, W. Miiller-Warmuth, W. Beier, A.A. G~ktas and G.H. Frischat, Phys. Chem. Glasses 31 (1990) 138. [19] D.S. Knight, G.C. Pantano and W.B. White, Mater. Lett. 8 (1989) 156. [20] F. Gervais, A. Blin, D. Massiot, J.P. Coutures, V. Chopinet and F. Naudin, J. Non-Cryst. Solids 89 (1987) 384. [21] P.P. Bihuniak and R.A. Condrate, J. Non-Cryst. Solids 44 (1981) 331. [22] Y. Sorek, R. Reisfeld and R. Tenne, Chem. Phys. Lett. 227 (1994) 235.