Paramagnetic defect centers of SiO2GeO2 gels and gel glasses

160

Journal of Non-Crystalline Solids 82 (1986) 160-164 North-Holland, Amsterdam

P A R A M A G N E T I C DEFECT CENTERS OF S i O 2 - G e O 2 G E L S AND GEL GLASSES George K O R D A S Vanderbilt University, Nashville, TN 37235, USA

S h y a m a P. M U K H E R J E E * Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA

Gamma-ray induced paramagnetic centers were detected at 9.5 GHz and 300 K in supercritically dried 94 SIO2-6 GeO2 gels after heat treatment at 462°C. The ESR-spectrum of the gel consisted of a narrow (Hpp = 1.6 G, geff = 2.003) and an asymmetric (gl = 2.001, g2 = 1.995, and g3 1.994) line. The asymmetric line was attributed to the Ge-E]-center. The defect center generating the narrow line has not been identified yet. The supercritically dried gels after heat treatment (TH) at 1000, 1150, and 1280°C were also exposed to "t-ray irradiation. In the gel with TH = 1000°C, the Ge-E]-center and the Ge(1)-center were detected. An increase in the densification temperature of 150°C (from 1000°C to 1150°C) causes the disappearance of the Ge-E]-center. At TH = 1150 and 1280°C the signal of the Ge(1)-defect center dominated the ESR-spectra of these samples. The Ge(1)-defect center was described by an electron trapped by a germanium atom co-ordinated through bridging oxygens to two silicon and one germanium atoms. =

1. Introduction I n the sol-gel m e t h o d of glass m a k i n g [1,2], the glass f o r m a t i o n occurs at relative low temperatures by hydrolytic p o l y c o n d e n s a t i o n of metal alkoxides, thus it is anticipated that the c o n c e n t r a t i o n a n d n a t u r e of structural defect centers in gel-derived glasses might be significantly different from that of m o l t e n glasses in which the melt-structure a n d the thermal history play i m p o r t a n t roles in controlling the defect centers. I n gel-derived glasses, the chemical processes such as the p o l y c o n d e n s a t i o n , d e h y d r o x y l a t i o n a n d pyrolysis could play i m p o r t a n t roles in controlling the c o n c e n t r a t i o n a n d n a t u r e of structural defect centers. Hence, it is i m p o r t a n t to investigate the d e v e l o p m e n t a n d disappearance of structural defect centers at various stages of gel-to-glass t r a n s f o r m a t i o n . Recently, the L a s e r - R a m a n Spectroscope technique [3,4] a n d the E l e c t r o n - S p i n - R e s o n a n c e (ESR) technique [5] have been successfully used for investigating the structural changes a n d the structural defect centers occurring at various stages of the gel-to-glass transformation. The defect centers in SiO 2 gel-glasses prepared with various H 2 0 / S i ( O C 2 H s ) 4 ratios * Present address: IBM Corp., Endicott, NY 13760, USA 0022-3093/86/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

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have been investigated by the ESR technique [5-7]. O~--ions probably in interstitial positions were observed in gel-glass prepared with 16 mol. H 2 0 per mol. Si(OC2Hs) 4. CO 2 ions and Eq-centers were detected in gels prepared with 4 mol. H 2 0 per mol. Si(OC2Hs) 4 and densified at 1000°C [6,7]. The present work deals with paramagnetic states of 60Co-y-ray irradiated 94 SiO 2 - 6GeO 2 gels after different stages of densification. This system was selected because of its importance in the production of optical wave guide fibers.

2. Experimental Silica-germania gel-monoliths with 6 mol.% G e O 2 were used for the present study. The hydrolytic polycondensation process of Si(OCH3) 4 with Ge(OC 2H s ) at p H around 3 was used to prepare gel-monoliths. The molar ratio of H 2 0 / M ( O R ) 4 was 5 (for procedural details see ref. 8). Gel-monoliths were dried under supercritical conditions at around 250°C and under a pressure in the range 1100 to 1200 psi. The supercritically dried gels were subsequently heat treated in flowing air atmosphere at different temperatures (see table 1). The ESR-spectra were recorded using an IBM-Bruker ER 200 D-SRC spectrometer operating at 9.5 G H z and 300 K. The magnetic field was measured with an IBM-Bruker ER 035 M N M R gaussmeter. The ESR-spectra were measured in a double cavity. The g-values of the paramagnetic center were detected by comparing its resonance position with the signal of the V A R I A N strong pitch. The signals were recorded using an IBM 9001 computer and analyzed with software developed by IBM.

3. Results Fig. 1 shows the ESR spectrum of an irradiated supercritically dried gel heat treated at 462°C. The spectrum of this gel consists of a narrow line (Sl-compo-

Table 1 Heat treatment schedule of 94SIO 6GeO2 (mol.%)gels Sample no.

Heat treatment Time (min)

Temperature (°C)

1 2 3 4

240 15 10 10

462 1000 1150 1280

G. Kordas, S.P. Mukherjee / Paramagnetic defect centres of Si02-GeO 2 gels

162

S1-COMPONENT

S2-COMPONENT

2

~ II

,.

T I 3408

i

I 342 •

i

H[G]

Fig. 1. ESR-spectrum of a y-ray irradiated supercritically dried 94SiO2.6GeO 2 gel after heat treatment at 462°C; Recorded at 20.3 m W and 300 K.

nent, AHpp= 1.6 G, geff = 2.003) and an asymmetric line (S2-component, gl = 2.001 g2 ---- 1.995 and g3 = 1.994). The signals of these components were recorded at various microwave power levels. These measurements revealed different saturation behavior for the S1- and S2-components indicating that these components are generated by different defect centers. Figs. 2a and b show the spectrum of a gel (sample 2) densified at 1000°C and recorded at 20.3 mW and 20.3 nW. One can notice from the figures that the line shape of the spectrum of this sample depends on the microwave power level. The spectrum of this material recorded at 20.3 nW is similar to the spectrum (fig. 2c) of the GeO2-glass produced by conventional melting technique [9]. Fig. 3a shows the ESR-spectrum of the gel-glass (TH = 1280°C) recorded at 300 K and 20.3 roW. The ESR-spectrum of this sample is characterized by the

a I

~

20.3mW

b

~

C

~ !

3406

I

.

20.3nW Ge02.gla~0.3 nW I

I

I

3426 b H[G]

Fig. 2. (a) and (b) ESR spectra of a supercritically dried 94SiO2-6GeO 2 gel after heat treatment at 1000°C; (a) Recorded at 20.3 mW, (b) S2-component, at 20.3 nW, (c) spectrum of a GeO2-glass produced by oxide melting.

G. Kordas, S.P. Mukherjee / Paramagnetic defect centres of SiOe-GeOe gels

163

2.0008

TH = 1280°C S3-COMPONENT 20.3 mW

1.999~, 1.9934

~o3

~3

-T

T = 1150°C

3~8

'

:~

'

20.3 nW

~'28

'

• G [H]

Fig. 3. (a) Spectrum of a supercritically dried 94SIO2-6GeO 2 gel after heat treatment at T H = 1280°C; recorded at 20.3 nW and 300 K. (b) Signal obtained by the subtraction of the spectrum in fig. 2b from the spectrum in fig. 2a. (c) Spectrum of supercritically dried 94SIO2. 6GeO 2 after heat treatment at Trl = 1150°C; Recorded at 20.3 nW and 300 K.

total g-value anisotropy ( g l = 2.0008, g2 = 1.9994 and g3 = 1.9934, S 3 - c o m p o nent, fig. 3a). In the same figure this spectrum (fig. 3a) was compared with the difference spectrum (fig. 3b) generated by the subtraction of the signals of the gel densified at T H = 1000°C recorded at 20.3 nW (fig. 2b) and 20.3 mW (fig. 2a). The spectra of the gel-glasses with Tn = 1150 and 1280°C were recorded at 20.3 nW (fig. 3c). The signal of the Ge-E'~-center was not evident from these measurements (fig. 3c).

4. Discussion

In SiO2-gels produced with 4 mol. water/mol. Si(OC2Hs) 4 densified at 500°C the Si-OC2H ~ and CH 3 radicals were detected [5]. The ethyl and methyl radical give a set of twelve hfs-lines and four hfs-lines respectively [5]. These hfs-splittings were not detected in the supercritically dried 94SIO 2 • 6 GeO 2 gel heat treated at 462°C. Since other possible contaminants [6,7] of gels have different g-factors from those determined for the S2- and S3-components, we assume that they are due to intrinsic defect centers. The S2-component has the same g-values as those reported for the GeO2-E'~-center (fig. 2c) [9,10]. We describe this defect with an electron located on a sp3-orbital of a germanium

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G. Kordas, S.P. Mukherjee /Paramagnetic defect centres of SiO2-GeO 2 gels

a t o m facing an oxygen vacancy [9,10]. The nature of the defect center causing the Sl-component has not been identified yet. The Sl-component was not detected at T H = 1000°C. At Trt = 1000°C the signal of the Ge-E~-center (S2-component, fig. 2b) and the signal of a new asymmetric line (S3-component, fig. 3b) were isolated. The line shape and the g-values (gl = 2.0008, g2 = 1.9991, g3 = 1.9934) of the S3-component are, within the errors of the measurements, the same as those reported for the Ge(1)-center ( & = 2.0007, g2 = 1.9994, g3 = 1.993) [10]. This center was described by an oxygen trapped by a germanium a t o m co-ordinated through bridging oxygens to two silicon and one germanium atoms [10]. The Ge(1)-center was the only one center detected in the gels with T H higher than 1150°C. The formation of a germanium atom co-ordinated through bridging oxygens to two silicon and one germanium atoms might be due to the condensation reaction of = G e - O - S i = bonds. Hence, the GeO2-rich clusters in gels containing the E~-centers might be transforming into Ge(1)-centers on thermal treatment at >/1000°C when dehydroxylation and condensation leading to - = G e - O - S i = formation takes place simultaneously. Publication support for the paper was provided by the IBM Corporation, Endicott, New York. The samples were prepared at the Jet Propulsion Laboratory, California Institute of Technology under contract with the National Aeronautics and Space Administration. One of us (G.K.) gratefully acknowl-. edge the financial support of the US A r m y Research O f f i c e / D u r h a m under contract No. D A A G . 29-81-K-0143.

References [1] S.P. Mukherjee, J. Non-Cryst. Solids 42 (1980) 477. [2] S. Sakka, Treatise on Materials Science, Glass III, Vol. 22, ed., H. Herman (Academic Press, London, 1982) p. 129. [3] V. Gottardi, M. Guglielmi, A. Bertoluzza, G. Fagnano and M.A. Morelli, J. Non-Cryst. Solids 63 (1984) 71. [4] S.P. Mukherjee and S.K. Sharma, J. Non-Cryst. Solids 71 (1985) 317. [5] G. Kordas, R.A. Weeks and L.C. Klein, J. Non-Cryst. Solids 71 (1985) 327. [6] G. Kordas and D.C. Klein, Ultrastructure Processing of Ceramics, Glasses, and composites, eds., L.L. Hench, D. Ulrich, to be published. [7] G. Kordas and L.C. Klein, J. Non-Cryst. Solids, in press. [8] S.P. Mukherjee, in: Better Cheramies Through Chemistry, eds., C.J. Brinker, D.E. Clark, D. Ulrich, Mat. Res. Soc. Symp. Vol. 32 (Elsevier, Amsterdam, 1984) p. 111. [9] G. Kordas, R.A. Weeks and D.L. Kinser, J. Appl. Phys. 54 (1983) 9. [10] E.J. Friebele, D.L. Griscom and G.H. Sigel Jr, J. Appl. Phys. 45, 8 (1974) 3424.