ELSEVIER Journal of Non-Crystalline Solids 171 (1994) 134-140

]OURNAL OF

NON-CI NINI

Vibrational spectra of bismuth silicate glasses and hydrogen-induced reduction effects

Z. Pan, D.O. Henderson, S.H. Morgan * Physics Department, Fisk University, Box 15, Nashville, TN 37208, USA

Received 27 August 1993; revised manuscript received 20 December 1993

Abstract

Raman (10-1200 cm 1) and infrared reflectance (100-2000 cm -1) spectra are reported for bismuth silicate glasses of the formula xBi20 3 (10-x )S iO 2 with x = 9, 8, 7, 6, 5, 4. The basic vibrational characteristics and the possible glass structure are discussed. The results provide evidence that bismuth oxide behaves as the network former while silicon oxide is present as isolated SiO 4 tetrahedra. Hydrogen treatment was performed at 300C on a bismuth silicate glass with x = 5. The intensity of most Raman bands decreased with increasing exposure time. The infrared spectrum after reduction showed vibrational bands at 1124 and 1220 cm i which are characteristic of S i -O-S i stretching vibrational modes in a fused-silica network. The spectral changes observed indicate the formation of bismuth metallic colloids as well as the formation of the bridged S i -O-S i structure in the glass surface layer during reduction.

1. Introduction

Glasses formed by mixing heavy metal oxides with GeO 2 or SiO 2 have been invest igated [1-5] for their possible appl icat ion as low-loss f iber optic mater ia ls in the infrared region [6] as well as the active med ium of Raman-act ive f iber opti- cal ampli f iers [7]. Bismuth is probably the heavi- est e lement whose oxide forms glasses with GeO 2 and SiO 2 [8]. A l though a Raman investigation of the xB i20 3 "yGeO 2 glass system has been con- ducted recently [9], the vibrat ional spectra of the xB i20 3 .yS iO 2 system remains to be studied. The miscible contents of B i20 3 in the xB i203 .yS iO 2

* Corresponding author. Tel: + 1-615 329 8621. Telefax: + 1- 615 329 8634.

system are significantly di f ferent from that in the xB i20 3 .yGeO 2 system [10]; therefore, the struc- tures of the two glass systems are expected to be different. V ibrat ional spectra have proved to be informative for probing the structure of glasses.

The t reatment of meta l -ox ide -conta in ing glasses with hydrogen leads to metal cation re- duction at the glass surface, changes the composi- tion and structure of the surface layer, and pro- vides a method to modify the optical propert ies of glasses for desired applications. A series of studies have been repor ted on reduced heavy- metal -contain ing sil icate glasses [11-15].

In this paper, the Raman and infrared re- f lectance spectra of the xB i20 3 (10 -x )S iO z glass system with x = 4 to 9 are presented. The basic vibrat ional characterist ics and the possible glass structure are discussed. At tempts to make

0022-3093/94/$07.00 1994 Elsevier Science B.V. All rights reserved SSDI 0022-3093(94)00138-D

z. Pan et al. /Journal of Non-CrystaUine Sofids 171 (1994) 134-140 135

glasses for Bi20 3 content < 40 mol% were un- successful. The limiting O/S i ratio corresponds to a composition where one would expect to have isolated SiO 4 tetrahedra in the glass network [16]. This composition is 40 mol% Bi20 3 in the Bi203-SiO 2 system. At this limiting composition, oxygen atoms are evenly shared by Bi and Si. Therefore, the presence of silicon oxide as iso- lated SiO 4 tetrahedra is only possible for a com- position x >/4. The hydrogen-induced reduction effects on a Bi203-SiO 2 glass sample are also reported. The systematic changes of Raman spec- tra as well as infrared reflectance spectra were observed with increase of hydrogen treatment time. These spectral changes are interpreted as the result of the formation of bismuth metallic colloids accompanied by the formation of a more fused-silica-like network in the surface layer.

2. Exper imental

Glasses with the formula xB i20 3 (10- x)SiO2, where x = 4, 5, 6, 7, 8 and 9, were pre- pared from Bi20 3 (99.8%, Fisher Scientific Co.) and SiO 2 (99.5%, Alfa Products) in 50 g batches. The mixtures were melted in fused silica crucibles at a temperature of 1100-1400C for 20 min. The melted liquids were then quenched on a room- temperature copper plate to form glasses and subsequently annealed for 1 h just below the glass transition temperature, Tg. The glass transition temperature was determined by DSC measure- ment using a Perkin-Elmer model DSC-4 instru- ment. The density measurement was performed

Table 1 The densities of bismuth silicate glasses with different Bi203 contents

Bi203 Density (tool%) (g/cm 3)

40 6.35 5{) 7.03 60 7.18 70 7.20 80 7.29 90 7.33

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136 Z. Pan et at/Journal of Non-Crystalline Solids 171 (1994) 134-140

system with x = 9 to 4. The density as a function of molar percentage of Bi20 3 is listed in Table 1. The molecular weight of Bi20 3 is 7.8 times that of SiO2 and the corresponding weight percent change is from 98.6 to 88.6% for these glasses

The DSC measurements were performed from 200 to 550C. Fig. 1 shows the differential scan- ning calorimetry analysis for a 5Bi203.5SiO 2 glass sample. The glass transition temperature as was determined Tg = 425C. An exothermal peak was observed at 486C resulting from crystalliza- tion. Other samples with different concentration, x, have similar DSC results with a Tg range from 420 to 430C and an exothermal peak around 490C.

The HH Raman spectra for 8Bi203-2SiO2, 6Bi20 3 4SiO 2, and 4Bi20 3 6SiO 2 are shown in Fig. 2. The profiles of these spectra bear a resem-

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(b)

(c)

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200 400 600 800 1000 1200

Frequency (cm -1)

Fig. 2. The HH Raman spectra of xB i203 . (10-x )S iO 2 glasses with different compositions: (a) x = 8, (b) x = 6, (c) x=4.

60

40

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Frequency (cm -1)

Fig. 3. The infrared reflectance spectra of xB i203 . (10- x)SiO z glasses with different compositions: (a) x = 8, (b) x =6, (c) x= 4,

blance to the spectra of 5PbO'5S iO 2 glasses reported previously [1,21] In the low-frequency region, there is a strong, highly polarized band at 132 cm -1 and a rounded, depolarized peak around 56 cm-1. These bands are superimposed on a polarized broad plateau which extends to above 400 cm-1, and then gradually drops Two Raman bands at about 380 and 570 cm-1 can be recognized. In the high-frequency region, a broad band is observed near 920 cm -1. The Raman spectra from the samples with different concen- trations (x = 5-9) are very similar; only the spec- trum of 4Bi20 3 6SiO 2 glass shows some changes. The intensity of the 132 cm 1 peak is reduced and the 920 cm- I peak is blue shifted to about 935 cm- 1.

The infrared reflectance spectra for 8Bi20 3

Z. Pan et aL /Journal of Non-Crystalline Solids 171 (1994) 134-140 137

2SIO2, 6Bi20 3. 4SIO2, and 4Bi20 3 .6SiO 2 are shown in Fig. 3. These spectra show strong ab- sorptions in the far infrared region below 250 cm ~ with peaks at about 135, 191 and 245 cm- and a shoulder extending to 500 cm-1. A broad, strong band is observed around 880 cm- ~, similar to the peak observed for the lead silicate glass with a high lead content [1]. Fig. 4 illustrates the successive Raman spectra of the 5Bi203.5SiO 2 sample with different hydrogen treatment time. The sample became opaque after the treatment. The scattered light was then collected from the sample surface. These Raman spectra have a

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200 400

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Frequency (cm -1) Fig. 4. Raman spectra as a function of hydrogen treatment time for a 5Bi203 .5SiO 2 sample: (a) before reduction, (b) 1 h reduction, (c) 2 h reduction, (d) 4 b reduction. The treatment temperature is 300C.

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soo 750 1000 1250 1500 1750 2000

Frequeney (era -1)

Fig. 5. Infrared reflectance spectra as a function of hydrogen treatment time for a 5Bi203.5SiO 2 sample: (a) before reduc- tion, (b) 1 h reduction, (c) 2 h reduction, (d) 4 h reduction. The treatment temperature is 300C.

systematic change with the increase of the treat- ment time: (1) the intensity of Raman scattering decreases with increasing treatment time; (2) the Rayleigh scattering tail enhanced after hydrogen treatment. The sharp peak at 81 cm -~ which appears after treatment was determined to be an artifact.

Fig. 5 illustrates successive mid-infrared re- flectance spectra of the 5Bi203.5SiO ~ samples with different hydrogen treatment time. The spectra gradually change: the 880 cm -~ band reduces and shifts to a shorter wavenumber while the 1124 and 1220 cm -~ bands appear and in-

138 Z. Pan et al. /Journal of Non-Crystalline Solids 171 (1994) 134-140

crease with increasing hydrogen treatment time. These two latter bands are similar to that ob- served from a fused-silica sample [1,17].

4. Discussion

4.1. Raman and infrared reflectance spectra before reduction

The Raman band near 56 cm-1 is attributed to acoustic Raman scattering combined with other low-frequency Raman modes which appear in the spectrum of the bismuth-silicon-oxide crystalline form [18] and are broadened in the glass. The strong, highly polarized band near 132 cm-t is assigned to the B i -O stretching mode. The heavy-metal atom has a high polarizability; the vibrations of the heavy-metal cations produce a strong modulation of the localized polarizability which causes a high-intensity Raman scattering. This heavy-metal band is comparable with the 135 cm 1 Raman peak in the spectrum of lead silicate glass with similar metal concentration [1]. The 135 cm -1 band was assigned to the Pb-O stretching in the PbO 4 pyramid [1,2,19]. Consid- ering that the Bi + has the same electronic config- uration as Pb and similar atomic mass, we suggest that the 132 cm -1 Raman band possibly comes from the B i -O stretching in the Bi+O4 pyramid configuration. The shoulder near 380 cm -~ is highly polarized, possibly associated with angular deformations involving the oxygen atoms [2]. The band centered near 920 cm ~ is depolarized, which indicates that it is not from a totally sym- metric form. This band is very possibly from the asymmetric stretch modes of B i -O-S i configura- tions [20].

The far-infrared reflectance spectra show three peaks at about 137, 191 and 245 cm -1 from different B i -O stretching vibrations. Analogous to the structure of bismuth oxide crystals and some bismuthate glasses, the B i -O distances vary within a wide range [5,21,22]. These B i -O bonds are expected to have different degrees of covalent or ionic nature, the more covalent one corre- sponding to higher frequencies [23]. The infrared bands in the 420 to 480 cm-~ region were re-

ported to be characteristic absorption bands of [BiO 6] polyhedra [24]. In the mid-infrared region, the strong and broad band centered about 880 cm-1 has a small blue shift when the bismuth content decreases. We think this band is related to B i -O-S i stretch vibrations.

Both the Raman and infrared spectra were found to exhibit almost no change when the glass composition changed from x = 5 to x = 9. This implies that the structure of the xBi20 3 (10- x)SiO 2 system does not change much when the composition changes. It was also found that there is no infrared band which corresponds to the S i -O-S i stretch vibrations which are at 1124 and 1220 cm-~. We note that the Bi/Si ratio is from 18 : 1 to 4 : 3 in our glass system and the bismuth oxide is dominant over silicon oxide. These exper- imental observations suggest that the bismuth- oxygen polyhedra behave as the network formers in our glass system. The structures of bismuth- oxygen polyhedra are more flexible owing to the more ionic nature of the B i -O bond; therefore this network can incorporate the S iO 4 tetrahedra to a certain extent. The change of the composi- tion from x = 5 to x = 9 only modifies the density of SiO 4 tetrahedra in the network. This interpre- tation explains why our attempt to make glass with composition less than x = 4 was not success- ful. As Heynes et al. reported [16], the limiting O/S i ratio in a Bi203-SiO 2 system corresponds to a composition where one would expect isolated SiO 4 tetrahedra in the melt. The limiting ratio was given as O/S i = 4 which corresponds to x = 4 in our glass formula. Our experimental observa- tions and results are consistent with this early report.

4.2. Hydrogen-induced reduction effects

The original Bi203-SiO 2 glass is brown. The samples gradually darken during exposure to hy- drogen at 300C. Heat treatment extending to 4 h leads to the formation of visible Bi particles on the surface. It has been reported that the overall process is controlled by the initial reduction of the metal ion to the atomic state and subse- quently the atoms diffuse to growing nuclei to form colloids or clusters [14]. The successive Ra-

Z. Pan et al. /Journal of Non-Crystalline Solids 171 (1994) 134-140 139

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0

0

09

40

20

0

40

20

0

80

40

I I I I I

880 (~)

I I I I I

1124

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750 1000 1250

(b)

I I

(c)

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500 1500 1750 2000

for hydrogen-treated samples. These spectra show a systematic change with increase of reduction time (Fig. 5). The 1124 and 1220 cm -1 bands appeared after treatment and increased with treatment time. These two bands from a sample with 4 h treatment are similar to corresponding bands from a fused silica sample (Fig. 6). The 1124 and 1220 cm i bands are well known, and attributed to the TO and LO modes of S i -O-S i stretching in the fused-silica network [24]. Before the hydrogen treatment, the SiO 4 tetrahedra ap- pear to be separated in the glass network and there are no bridged S i -O-S i bonds. During hydrogen treatment, part of the Bi ions are re- duced to Bi atoms, and diffuse to form metal clusters or diffuse out from the surface. This process leads to a decrease of the bismuth oxide content in the network, and separated SiO 4 tetra- hedra reorganize to form a more fused-silica-like network. This observation is very interesting, be- cause it indicates that, by exercising control over the reducing conditions, one can control the near-surface composition of these glasses.

F requency (cm -1)

Fig. 6. Infrared reflectance spectra from: (a) a 5Bi203-5SiO 2 sample before reduction, (b) a 5Bi203"5SIO 2 sample after 4 h reduction, (c) a fused silica sample.

man spectra upon hydrogen treatment (Fig. 4) show significant changes. First, the Raman inten- sity decreases with the increase of treatment time. The metal clusters and/or metallic colloids in the surface layer strongly scatter and reflect the laser light, which leads to a reduction of the scattering volume. The gradual decrease of the Raman in- tensity with increasing treatment time indicates that the number and size of metal clusters and/or metallic colloids increase with hydrogen treat- ment time. Second, the Rayleigh scattering in- creased significantly after reduction. The strong Rayleigh scattering is attributed to particle scat- tering, i.e., scattering by metal clusters and col- loidal particles which are suspended in the sur- face layer. Rayleigh scattering by particles is usu- ally much stronger than that by fluctuations [25].

Infrared reflectance spectra were also recorded

5. Conc lus ion

The Raman and infrared spectra for bismuth silicate glasses of the formula xB i203 . (10- x)SiO 2 with x=4-9 have been reported. A strong, highly polarized Raman band at 132 cm- was observed and assigned to the Bi -O symmet- ric stretching mode in the Bi +O4 pyramid config- uration. The spectra observed suggest that these glasses are formed by bismuth-oxygen polyhedra with isolated SiO 4 tetrahedra.

The spectral changes upon hydrogen treat- ment reflect the structural change in the surface layer of the glass. These changes are interpreted as a result of the formation of bismuth metallic colloids accompanied by the formation of a more fused-silica-like network in the surface layer. Hy- drogen treatment may become a useful method to modify the optical properties of glasses.

The authors acknowledge the support of NASA through Grant NAGW-2925.

140 Z. Pan et al. /Journal of Non-Crystalline Solids 171 (1994) 134-140

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