J. MATER. CHEM., 1994, 4(4), 529-532 Preparation and Characterization of Heavy-metal Oxide Glasses: Bi,O,-PbO-B,O,-GeO, System Victor C. Solano Reynosot Luiz C. Barbosa: Oswaldo L. AIves,b Norbert0 Aranhaa and Carlos L. Cesara a lnstituto de Fisica Gleb Wataghin, UNICAMP, P.O. Box 6165, Campinas, SP, 13081, Brazil Laboratorio de Quimica do Estado Solido, lnstituto de Quimica, UNICAMP, P.O. Box 6154, Campinas, SP, 13081, Brazil High-refractive-index glasses of heavy-metal oxides have large optical non-linearities which make them promising for optoelectronic applications. This paper describes the fabrication and characterization of the xBi203-Pb0-B203-(52 -x)Ge02 (BPBG) glass system with x =10, 15, 20, 30 and 35.The BPBG glasses were melted in a high-purity alumina crucible placed in a Super Khantal resistance furnace at 1000 "C for 30 min and then poured onto a steel plate.The characterization was done by X-ray diffraction (XRD), density, dilatometry, light absorption (UV-VIS-IR) and linear refractive-index measurements. The infrared spectra show the presence of BiO, and GeO, in the glass structure suggesting a network former role for the bismuth oxide. These measurements allowed us to estimate the non-linear refractive index n2, using Line's formula, to be as high as 1.5 x 10-l8m2W-'. The recent interest in high-refractive-index glass systems is due to their large optical non-linearities. This property can be enhanced further by the presence of heavy-metal oxides, such as Bi203 and PbO, in the glass structure, making these heavy-metal oxide glasses promising for optoelectronics appli- cations, non-linear optics' and other applications based on high refractive index.2 Furthermore, these glasses show acou- sto-optical and magneto-optical proper tie^.^ The chemical and physical properties of the glasses are intimately related to their structure^.^ This work describes the preparation of the quaternary system Bi,0,-PbO-B,03-Ge02 (BPBG) glasses and their characterization using X-ray diffraction (XRD), density, dila- tometry, light absorption (UV-VIS-IR) and linear refractive- index measurements. Experimental The compositions of the BPBG glasses used in this study are listed in Table 1.Batches of ca. 50 g of glass have been prepared by mixing reagent-grade bismuth(u1) oxide, lead@) oxide (both from Riedel j, boric acid (Merck) and extra pure germanium dioxide (Kawecki Berylco).This mixture was melted in a high-purity alumina crucible placed in a Super Khantal electrical resistance furnace at 1000"C for 30 min and then poured onto a cold stainless-steel plate, followed by 5 h annealing at 350°C. We made samples in three forms: 0.5mm thickness slabs for the UV-VIS-IR, powder for the XRD and IR, and blow thin films for absorption-coefficient measurements. The XRD patterns were obtained with a Model 3X-A Shimadzu diffractometer with an Ni filter using Cu-Ka radiation (1.5418A). Infrared spectra were recorded in the 4000-400cm-' region for the slabs or KBr pellets on a Nicolet 60SX-B Fourier transform spectrometer.Absorption Table 1 BPBG glass system compositions (mol%) glass ~~~~~ Bi,03 PbO B203 GeO, BPBG-1 10 40 8 42 BPBG-2 15 40 8 37 BPBG-3 20 40 8 32 BPBG-4 30 40 8 22 BPBG-5 35 40 8 17 spectra (200-900 nm) were obtained on a Cary-Varian 2300 spectrophotometer. Density measurements were carried out on Micromeritics Multivolume Pycnometer model 1305 using helium as the displacement gas. The thermal dilatation data were obtained with a Harrop dilatometer. The linear refractive index was measured by the Brewster angle method at 632.8 nm with an He-Ne laser. Results and Discussion Glass Formation Using the procedure described in the Experimental we obtained bubble-free transparent glasses with a yellou colour- ation and high homogeneity.All the compositions showed the typical halo (28=28"j in the XRD patterns, indicating that the samples are amorphous. There was no sign of crystallization even for the compositions containing 3 5 mol% of Bi203 and 40 mol% of PbO. This result confirms Beck and Taylor's idea about the minimum bismuth and lead oxide concentrations to the stability of the amorphous phase.' Density Fig. 1shows the density dependence on the glass compositions. The density value increases with the Bi,03 concentration. The substitution of the GeO, by the heavier Bi203 oxide could explain this behaviour. However, the bonds with Bi,03 are more covalent than the bonds with GeO,, which tends to increase the density also.The density tends to saturate for the higher Bi203 concentrations (BPBG-4 and BPBG-5 samples). The weight of Bi203 compared to GeO, and the increase in the covalent bond character can explain only the increase in the density and not the saturation. The only mechanism which can oppose continuous density enhancement is the increase in the Bi-0 bond distance. It seems that this increase in the Bi-0 bond distance happens after the 25 mol% Bi2O3 con- centration. A hypothesis for this is that initially the Bi203 stands in an interstitial site of the glass network. As these sites saturate, Bi203 is forced to a substitutional site where the Bi-0 bond distance is affected. This picture is coherent with the estimates for the non-linear index n2 discussed later.The BPBG densities are small compared to other bismuth and lead glass systems.6 530 6.8 -7 6.6-5ln \-x .-c u)C4 6.4-10 15 20 25 30 35 Bi203(mol %) Fig. 1 Effect of Bi,03 content (mol%) on the density of the Bi,O,-PbO-B,O,-GeO, glass system (see Table 1) Dilatometry Fig. 2 shows the thermal expansion coefficient, in the (130-160) x lop7 K-' range, as a function of the Bi20, con- centration. Similar orders of magnitude have been reported in the literature for other quaternary heavy-metal oxide gla~ses.~The thermal expansion coefficient shows small vari- ations with the Bi,03 concentration, but the glass-transition temperature (T,) and the softening point temperature (Td)are very sensitive to the Bi,O, concentrations, as shown in Fig.3. The decrease in Tgusually means a more open glass network, while the increase in the thermal expansion coefficient usually means weaker bonds. The difference between Td and the T,, for the whole composition range, is roughly constant, as shown by Fig. 3. Linear Refractive Index We expected a high refractive index for these heavy-metal oxide glasses due to the large Bi and Pb p~larizability'~~ and, indeed, Fig. 4 shows a 1.93-2.20 BPBG refractive index n. The increase of refractive index with Bi203 content (Fig. 4) was also observed by other authors for similar systems." Canale et studying the binary system Bi,O,-GeO, and PbO-GeO,, attributed this change to the lead or bismuth oxides region on the glass network.On the other hand, it is well known that Ge02 or PbO tends to increase the refractive index, making it hard to understand the role of the Bi203 on this index. -1601 J. MATER. CHEM., 1994, VOL. 4 10 15 20 25 30 35 Bi203(mol %) Fig. 3 Effect of Bi203 content (mol%) on the T, (0)and Td (B) temperatures of the Bi,0,-Pb0-B20,-Ge0, glass system (see Table 1) 2.5 I ---I 1.5 10 15 20 25 30 35 Bi203(mol Yo) Fig.4 Linear refractive index uersus Bi,O, content (mol%) of the Bi,O3-Pb0-B2O3-GeO, glass system (see Table 1 ) Spectral Characteristics U V-VIS Spectra Fig. 5 shows the effect of the Bi,O, content on the UV-VTS cut-off for BPBG glasses. We assign the cut-off value for the wavelength where the extrapolation of the decay in trans- mission curve in the UV region reaches the zero transmission. The red shift in the cut-off with the Bi,03 concentration was also observed for other heavy-metal glasses, for example, lead 100 I10 15 20 25 30 35 10 15 20 25 30 35 Bi203(mol %) Bj2O3(mol Yo) Fig.2 Effect of Bi,O, content (mol%) on the thermal expansion Fig. 5 Effect of Bi203 content (mol%) on the ultraviolet cut-off coefficient of the Bi,O,-PbO-B,O,-GeO, glass (see Table 1) wavelength of the Bi,O,-PbO-B,O,-GeO, glass system J. MATER. CHEM., 1994, VOL. 4 gallobismuthates.' The electronic transitions from the valence to the conduction bands are in the range 0.35-0.45 pm, depending on the glass composition.The absorption coefficient cx was measured using the thin blown glass film samples. Fig. 6 shows the plot of J(rE) us. E, where E is the photon energy in eV. We determined the optical gap Eoptas the energy where the extrapolation of J(zE) crosses the JcxE=O axis (Fig. 7). The Urbach tail AE was obtained from the slope of the In a us. E in the UV region. There is clearly a red shift with the Bi203 concentration as shown in Fig. 6 and 7. According to Lines'' the Sellmeier gap (E,) is given by E, 5 2(Eopt+AE),and it can be fed into his formula, n2= { 3.4(n2+2)(n-l)d2/(nEz)}x lo-,' m2 W-', to estimate the non-linear refractive index (n,). The parameter d is tbe metal -oxygen bond distance and was estimated as 2.3-2.5 A. Fig. 8 shows these estimates as a function of Bi,O, concentration.The values are in the range (60-150) x m2 W-', 20-50 times larger than the nz in silica, and agree with published results for other heavy-metal oxide glasses.12 There are two regions of non-linear index n,: a low value region, -IN 'G -IN .2-1% UI 8 Y {j //I/// I/0 I 2.50 2.75 3.00 3.25 3.50 energytev Fig. 6 ,'YE versus E plot for xBi203-40Pb0-8B,03-( 52 -x)GeO,. (a) u=10; (b)x=15; (c) x=20; (d) x=30 and (e) x=35 3.1 2.9 2.8 i Fig.7 Optical gap Eopt uersus Bi203 content (mol%) for the Bi,0,-PbO-B,0,-Ge02 glass system 531 -140 7 120-3 c\I E 0Y 100-z--..c" c8ol60 10 15 20 25 30 35 Bi203(mol "A) Fig. 8 Estimated values of non-linear refractive index as fiinction of Bi,O, content (mol%) of the Bi,O,-PbO-B,O,-GeO, glass (see text) (60-80) x m2W-' for a Bi203 content of 10-20mol% and a high value region, (140-15O)x 10-20m2 W -'for a Bi203 content of 30-35 mol%.The change in refractive index from 1.93 to 2.2 increases the n2 by 34%, and the change in the Sellmeier gap is responsible for another 32% increase. The remainder (19Y0) comes from changes in ihe bis-muth-oxygen bond distance, d, which has increased This is consistent with the density saturation for high Bi203 content. Infrared Spectra The BPBG glasses infrared spectra in the regioiz 1400-400 cm-' are presented in Fig. 9. The strong broad band with a minimum at 750 cm-' for the BPBG-1 sample, can be attributed to the Ge-0-Ge stretching mode.13 The other broad band observed for all compositions, situated in the range 1350-1100 cm-', are related to B203 vibration^.'^ The behaviour with the Bi203 content has three main features: (i) the broad band assigned to Ge-0-Ge stretching mode shows a shift from 750 to 700 cm-l and a smaller baridwidth; (ii) the shoulder at 800 cm-' appears as a defined we<tk band at 860 cm-'; (iii) the band at 550 cm-I practically disappears and another one with a minimum near 450 cm-l is observed, see Fig.9 insert. The band at 750cm-' is shifted to lower wavenumber in this glass family when compared to the pure GeO I (tetra-hedral) band minimum at 890 cm-'. This shift h<is been interpreted as due to the appearance of non-bridging <)xygens (depolymerization) in the germanium oxide network13 and the consequent change in the germanium coordination frc )m four (tetrahedral) to six (octahedral).The fact that there is a minimum at 750 cm-' even for the Bi203 poorest com~sosition suggests the presence of GeO, octahedra in this glass system. The small changes from 700 to 750cm-' indicate that the Pb2+ (constant at 40 mol%) already made the depol ymeriz- ation of GeO, network which makes the Bi3+ conti ibution to the formation of this non-bridging oxygen insignificant. The weak band observed at 860cm-' has been assigned to the distorted Bi06 octahedra, and has been taken as a sign of the network forming role of bismuth oxide.15.16 Finally, some comments about the 450 cm-' band.This band has been assigned to 0-Ge-0 and frequently ,ippears in GeO, g1a~ses.l~The disappearance, or shift tc lower wavenumbers, of the 450cm-' band for the bismuth-rich wavenumberlcm-’ I,,I*I*I,IIlIII I 1 1 1400 1200 1000 800 600 400 wavenurnber/cm-’ Fig. 9 IR spectra (1400-4OO cm-’) of xBi203-40PbO-8B203-(52-x)Ge02. (a) x =10; (b)x = 15; (c)x =20; (d) x =30 and (e)x =35. Inset shows 450 cm-’ band region. composition can be explained by the decrease in the GeO, content, which would explain the disappearance, and/or by a vibrational coupling involving the Bi- 0 stretching and defor- mation vibrations of the germanium oxide networks, which would explain the shift to lower wavenumbers.17 A similar effect was observed in the binary system Bi2O,-Ge0,.l3 Conclusions The Bi,03-PbO-B,O,-GeO, system forms glasses with great homogeneity and stability.Thermal expansion coefficient, J. MATER. CHEM.. 1994, VOL. 4 density, linear refractive index and UV-VIS spectra are depen- dent on the Bi,O, content. The estimated value of non-linear refractive index, as high as 150 x lo-,’ m2 W-l. suggests the potential use of these glasses in optoelectronic applications. Infrared spectra show features that can be interpreted as due to the network-forming role of bismuth oxide and the presence of Ge06 groups. At the moment, systematic studies for other glass compositions, Raman and non-linear properties measurements are in progress in our laboratory.The authors acknowledge the Conselho Nacional de Desenvolvimento Cientifico e Tecnologico (CNPq),Fundaqiio de Amparo a Pesquisa do Estado de Siio Paulo (FAPESP), Programa de Apoio ao Desenvolvimento Cientifico e Tecnologico (PADCT) and the Telecomunica@es Brasileiras S/A (Telebras) for financial support. References 1 D. W. Hall, M. A. Newhouse, N. F. Borelli, W. H. Dumbaugh and D. L. Weidman. Appl. Phj’s. Lett., 1989,54, 1293 2 0. H. El-Bayoumi, A. A. Said, R. J. Andrews, hl. J. Suscanage, T. P. Swiller, J. H. Simmons and Van Styland, Bol. Soc. Esp. Ceram. Vid., 1992,31,3-c, 9. 3 J. C. Lapp, H. Dumbaugh and M. L. Powley, Rit.. Sftaz. Sper. Vetro, 1989, 1,91. 4 J. E. Canale, R. A. Condrate, Sr., K. Nassau and B. C. Cornilsen, J. Can. Ceram. Soc., 1986,55, 50. 5 W. R. Beck and N. W. Taylor, USPut 2853393, 23 September, 1958. 6 J. A. Ruller and J. 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