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Characterization of new tellurite glasses and crystalline phases in the TeO2–PbO–Bi2O3–B2O3 system
Journal of Non-Crystalline Solids 386 (2014) 90–94
Contents lists available at ScienceDirect
Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/ locate/ jnoncrysol
Characterization of new tellurite glasses and crystalline phases in the TeO2–PbO–Bi2O3–B2O3 system☆ Yongtao Zhou, Yunxia Yang ⁎, Feihong Huang, Jing Ren, Shuanglong Yuan, Guorong Chen Key Laboratory for Ultrafine Materials of Ministry of Education, School of Materials Science and Engineering, East China University of Science and Technology, Shanghai 200237, China
a r t i c l e
i n f o
Article history: Received 5 September 2013 Received in revised form 20 November 2013 Available online 20 December 2013 Keywords: Tellurite glasses; Structure; Thermal properties; Optical spectroscopy
a b s t r a c t A new kind of tellurite glass based on TeO2–B2O3–Bi2O3–PbO system is prepared. It is found that B2O3 is the component that contributes most to increase the thermal stability of the glasses, while Bi2O3 is the most contributing oxide to decrease the thermal stability. The structural origins for the variations of the thermal stability with the glass composition are discussed based on the Fourier-transform infrared (FTIR) spectrometry measurement. The present glasses show potential to be used for photonic devices and low melting point sealing glasses. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Among glass families investigated for photonic applications, tellurite glasses are of great interest because of their promising optical properties (high linear and nonlinear refractive indices, low phonon energy, and wide transmission in the infrared range) [1]. The unique optical properties of tellurite glasses are mainly due to the high polarisability of the Te4+-ions, which possess a non-bonding electron lone pair 5s2. However, TeO2 is hard to form glass alone, so other network former or modifier oxides are necessary [2,3]. Heavy metal oxides with empty d orbitals, such as PbO and Bi2O3 can be incorporated into tellurite glasses. These glasses possess very large refractive index and third order nonlinearity, which may find applications for ultrafast switching in optical devices. Moreover, they could be used as laser medium for broadband fibre amplifier or laser in optical communication because they exhibit very broad luminescence. Silva et al. [4] reported TeO2–PbO binary glass forming region and structure. Tomoharu Hasegawa [5] studied TeO2–B2O3–Bi2O3 ternary glass structures. Munoz-Martín et al. [6] mentioned that ternary tellurite–tungstate glass systems with alkaline oxides, zinc oxide, bismuth oxide or lanthanum oxide as third components, or other complex multicomponent glasses have demonstrated to be excellent hosts for Er3 + and Yb3+-ions and very promising materials for developing broadband integrated optical amplifiers. Following the research line of a former work [4], the purpose of this research is the study of properties of glasses in the TeO2–B2O3–Bi2O3–PbO quaternary system, on the basis of their possible application as base materials for photonic devices and low melting ☆ Project of Shanghai Science and Technology Committee. Project Number: 13dz1201103. ⁎ Corresponding author. Tel.: +86 21 64252647; fax: +86 21 64253395. E-mail address: [email protected] (Y. Yang). 0022-3093/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jnoncrysol.2013.11.037
point sealing glasses. Due to the toxicity and contamination of PbO, reducing the use of PbO is necessary, so it is important to know the possibility of obtaining glasses with good properties when PbO is partly substituted by Bi2O3. It is found that the incorporation of Bi2O3 makes glasses easily devitrified and B2O3 can overcome this drawback. The detailed structural origins for the variations of the glass thermal stability are discussed based on the FTIR spectrometry measurement. 2. Experimental 2.1. Glass preparation Glasses of nominal compositions summarised in Table 1 were obtained by traditional melt-quenching method. Reagent grade chemicals TeO2, B2O3, Bi2O3 and PbO were used as starting raw materials and well mixed in an agate mortar for 10 min. Each batch was then melted by using a platinum crucible in the range of 850–900 °C for 35 min. For each composition, the bulk glass was prepared by pouring the melt onto the preheated brass mould. The obtained glass blocks were immediately introduced into the annealing furnace, kept for 1 h at temperatures ranging from 300 to 350 °C and then cooled down to room temperature. From the glass blocks parallel slabs of about 1 mm thick were cut and optically polished. 2.2. Characterisation Crystalline phases of the devitrified glass samples were identified by X-ray diffraction (XRD) spectroscopy. A Siemens D 5000 equipment was used (Kα of Cu, 40 kV, 30 mA, 2θ = 10–80°). Glass transition temperature (Tg) and thermal stability were estimated by Differential Scanning Calorimetry (DSC), which was carried
Y. Zhou et al. / Journal of Non-Crystalline Solids 386 (2014) 90–94 Table 1 Composition (mol%) and thermal properties of the prepared glasses. Number
TeO2
B2O3
Bi2O3
PbO
Tg
G1 G2 G3 G4 G5 G6 G7 G8 G9 G10 G11 G12 G13 G14 G15 G16 G17 G18 G19 G20 G21
60 60 60 60 60 60 60 60 60 60 60 60 60 60 60 60 60 60 60 60 60
5 5 5 5 5 5 10 10 10 10 10 15 15 15 15 20 20 20 25 25 30
5 10 15 20 25 30 5 10 15 20 25 5 10 15 20 5 10 15 5 10 5
30 25 20 15 10 5 25 20 15 10 5 20 15 10 5 15 10 5 10 5 5
Partially crystallised Crystallised Crystallised Crystallised Crystallised Crystallised 314.81 387.06 Partially crystallised Crystallised Crystallised Crystallised 330.71 415.87 342.42 410.30 347.27 404.08 Partially crystallised 344.67 355.50 433.52 360.03 429.95 363.28 367.47 367.15
Tc
ΔT
72.25
XRD patterns of G1 and G2 are shown in Fig. 2. The G1, corresponding to the formula 60TeO2–5B2O3–5Bi2O3–30PbO, exhibits only broad features characteristic of glassy and amorphous materials. Narrow Bragg peaks with high intensity appear in the G2 glass sample corresponding to the formula 60TeO2–5B2O3–10Bi2O3–25PbO, which means that the crystallisation tendency of the glasses increases dramatically with the incorporation of Bi2O3. The observed crystalline phases were identified as Bi2Te4O11 both in G1 and in G2 samples. We can get that a chemical reaction was taking place in the glass-forming melt, which reads as: Bi2 O3 þ 4TeO2 →Bi2 Te4 O11 :
85.16 67.88 56.81
78.02 69.92
91
ð1Þ
This reaction led to the formation of these Bi2Te4O11 crystalline phases. 3.2. FTIR spectra
The glass-forming region of the pseudo-quaternary system TeO2– B2O3–Bi2O3–PbO is presented in Fig. 1. This domain is located around the binary TeO2–B2O3 area where glassy compositions are observed up to 30 mol% of PbO, and 15 mol% of Bi2O3 in the 60TeO2–B2O3 side. The
Both in crystalline TeO2 and in tellurite glasses, Te4 +-ions form [TeO4] groups in trigonal bipyramids (tbps) [7]. In such structures two oxygen ions are located in the axial vertices, while the other two oxygen ions and the lone electron pair of tellurium are located in the three equatorial positions. The equatorial Te\O bonds are slightly shorter than the axial bonds, as indicated by Kim and Yoko [8]. Trigonal bipyramids are linked to each other by sharing their vertices and forming a continuous three-dimensional structure. The spectrum of crystalline TeO2 is dominated by two main bands (Table 2): one is a wide asymmetric band at 650–660 cm−1 and the other is a narrow symmetric band at 775 cm−1 [9]. In the spectra of tellurite glasses, the band at 650 cm−1 appears very much broader, due to their disordered structure, which causes the 775 cm−1 band to appear as a shoulder [10,11]. All FTIR spectra of the quaternary glasses show the same features illustrated in Figs. 3–5. On one hand, the characteristic main band at 650 cm−1 always appears, although its exact position depends on the composition. This indicates that the structure based on the [TeO4] tbps is maintained in all the glass samples. On the other hand, the band at around 900 cm− 1 indicates that [BiO3] and [BiO6] polyhedral groups also form in the glasses. Besides, the shoulders at 680–720 cm−1 and around 1100 cm− 1 are attributed to bending vibrations of B\O in [BO3] triangles and stretching vibrations of B\O in [BO4] tetrahedrons, respectively.
Fig. 1. Ternary diagram for the 60TeO2–B2O3–Bi2O3–PbO system showing the composition of the samples prepared. Hollow (○) points correspond to homogeneous glasses and full (●) points to devitrified samples.
(a) Glasses with constant B2O3 content in 60TeO2–15B2O3– xBi2O3–(25 − x)PbO system. In the glass series 60TeO2– 15B2O3–xBi2O3–(25 − x)PbO, the band at 1200–1300 cm− 1 assigned to [BO3] groups shifts towards a lower wave number, i.e. longer wavelength, when PbO is substituted by Bi2O3 (Fig. 3). This is due to the lower field intensity of the mixed Te\O\Bi and B\O\Bi linkages, in which the oxygen is less polarised, compared with Te\O\Te linkages, since Te 4 +ions possess higher field intensity than Bi3 +-ions. (b) Glasses with constant PbO content in 60TeO2–xB2O3–(30 − x) Bi2 O3 –10PbO system. For constant [PbO], the decreasing of the [Bi2O3]/[B2O3] ratio, i.e. the substitution of Bi2O3 by B2O3, causes an increase in the B\O\B and Te\O\B linkages and a decrease in the B\O\Bi, Te\O\Bi and Bi\O\Bi linkages. Thus, the incorporation of B 2O 3 yields a network distortion that can be monitored by the progressive broadening of the bands at around 650 cm − 1 and 1200 cm − 1 (Fig. 4). Meanwhile, the band positions of the two bands shift towards a higher wave number, i.e. shorter wavelength. This is due to the higher field intensity of the B\O\B and Te\O\B linkages, in which the oxygen is highly polarised, compared with B\O\Bi, Te\O\Bi and Bi\O\Bi linkages, since Bi 3 +-ions have larger ion radius and possess lower
out under N2 atmosphere at a heating rate of 10 °C min−1, using a TA Instruments/Q200 system, with a maximum error of 3 °C. The optical absorption/transmission spectra were recorded in the 300–1000 nm wavelength range, using an Ocean Optics spectrophotometer model HR4000CG-UV-NIR. Transmittance to be further used for the energy gap (Eg) calculations was corrected for the standard thickness of 1 mm. Fourier-transform infrared (FTIR) spectra were recorded with a 20SXC FT-IR spectrometer with a resolution of 2 cm−1 in the 400–1800 cm−1 range. KBr pellets technique was used for recording spectra of glasses. 3. Results and discussion 3.1. Glass forming region
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Y. Zhou et al. / Journal of Non-Crystalline Solids 386 (2014) 90–94
Fig. 2. X-ray diffraction pattern of samples G1 and G2.
field intensity. In other words, the decreasing of the [Bi2O3]/ [B2O3] ratio makes the network become more rigid. (c) Glasses with constant Bi 2 O3 content in 60TeO2 –xB2 O3 – 10Bi 2 O3 –(30 − x)PbO system. For constant [Bi 2 O 3], the decreasing of the [PbO]/[B2O3] ratio, i.e. the substitution of PbO by B2 O3 , yields a network distortion that can be also monitored by the progressive broadening of the bands at around 650 cm− 1 and 1200 cm− 1 (Fig. 5). As aforementioned, this is due to the formation of new B\O\B and Te\O\B linkages with the increasing of B2O3 content. Here again, the decreasing of the [PbO]/[B 2O 3 ] ratio makes the network become more rigid.
3.3. Thermal properties 3.3.1. Glass transition temperature The transition temperature (Tg) gives information on both the strength of interatomic bonds and the glass network connectivity, in a similar way that the melting temperature does for crystalline solids. Tg of the glasses studied in the present work varies linearly with the molar fraction of each one of the three components (Fig. 6). The maximum variation occurs when B2O3 is substituted by PbO in the glass series 60TeO2–xB2O3–5Bi2O3–(35 − x)PbO. The corresponding ΔTg increases from 11 to 19 °C for each 5 mol% PbO substituted by 5 mol%
B2O3. The results indicate that B2O3 is the most contributing oxide to increase the glass transition temperature. The transition temperature always decreases when B2O3 or Bi2O3 is substituted by PbO. So PbO is the most contributing oxide to decrease the glass transition temperature. The incorporation of PbO causes the formation of new non-bridging oxygens in Te\O\…Pb2+…\O\Te, Bi\O\…Pb2 +…\O\Bi and B\O\…Pb2 +…\O\B linkages. The higher polarisability of such new non-bridging oxygens explains the Tg shift towards lower temperature, due to the lower field intensity of Pb2+-ions (0.27) compared with that of Te4+-ions (0.71).
3.3.2. Thermal stability The difference between the onset crystallisation temperature Tc and the glass transition temperature Tg is usually taken as an index to estimate the glass thermal stability. The wider the thermal stability range, the more favoured is the glass forming process [12].
Table 2 Vibration types of different IR wave numbers. Range of wave numbers Vibration types (cm−1) 650–660 775 Around 900 1200–1300
Asymmetric bending vibrations of Te\O bonds in [TeO4] trigonal bipyramids Symmetric stretching vibrations of Te\O bonds in [TeO4] trigonal bipyramids Symmetric stretching vibrations of [BiO3] and [BiO6] polyhedral Stretching vibrations of B\O bonds in [BO3] triangles
Fig. 3. FTIR spectra of the quaternary glass series 60TeO2–15B2O3–xBi2O3–(25 − x)PbO.
Y. Zhou et al. / Journal of Non-Crystalline Solids 386 (2014) 90–94
93
Table 3 Cut-off wavelength and energy gap of TeO2–B2O3–Bi2O3–PbO glasses. Group
Number
TeO2
B2O3
Bi2O3
PbO
λo (nm)
Eg (eV)
I
G12 G13 G14 G14 G17 G19 G13 G17 G20
60 60 60 60 60 60 60 60 60
15 15 15 15 20 25 15 20 25
5 10 15 15 10 5 10 10 10
20 15 10 10 10 10 15 10 5
377 380 384 384 376 369 380 376 371
3.291 3.265 3.231 3.231 3.300 3.363 3.265 3.300 3.344
II
III
thermal stability. A careful tuning of the glass composition could lead to the maximum substitution of PbO by Bi2O3 while keeping the good glass forming ability and thermal stability. 3.4. Optical absorption Fig. 4. FTIR spectra of the quaternary glass series 60TeO2–xB2O3–(30 − x)Bi2O3–10PbO.
The bandgaps (Eg) of the glasses can be approximately estimated from the wavelength of the ultraviolet cut-off (λo) (Table 3). In all the cases, with the substitution of B2O3 by Bi2O3 or PbO, the energy gap Eg diminishes. The incorporation of network modifier ions (such as Pb2+) into a tellurite glass leads to the breaking of the glass network tbps and the formation of non-bridging oxygens. The more weakened the glass network and higher the number of non-bridging oxygens, the lower the Eg values [13]. 4. Conclusions
Fig. 5. FTIR spectra of the quaternary glass series 60TeO2–xB2O3–10Bi2O3–(30 − x)PbO.
Calculated thermal stability of the glasses studied is summarised in Table 1. Bi2O3 is the most contributing oxide to decrease the thermal stability, while B2O3 is the most contributing oxide to increase the
The basic trigonal bipyramid [TeO4] (tbp) structure, as well as the [BiO3 ] and [BiO 6 ] polyhedral groups and [BO3 ] triangle groups is maintained in TeO2 –B2 O 3 –Bi2 O 3–PbO system studied, even for a low content of B2O3. The broadening of the main FTIR band at around 650 cm− 1 and 1200 cm− 1 indicates a deformation of the glass network. Such deformation increases as the B 2 O3 content increases. Pb 2 + -ion incorporation does not produce new bands in FTIR and their presence does not affect the position of the main wide band at around 650 cm − 1 and 1200 cm− 1, indicating that Pb 2 + -ions exist as glass modifiers and do not take part in the formation of glass network. The transition temperature Tg of all the glasses studied varies linearly with the molar fraction of each of three components: B2O3 is the oxide that contributes most to increase the glass transition temperature, while PbO is the oxide that contributes most to decrease the glass transition temperature. The weaker network connectivity and higher polarisability of non-bridging oxygens formed with the introduction of PbO explain the Tg shift towards lower temperature. From the point of view of thermal stability, the glasses with wider temperature range to be formed without crystallisation risk are those with high content of B2O3 and low content of Bi2O3. Finally, the energy gap Eg decreases when B2O3 is substituted by Bi2O3 or PbO, which is also the result of the weakened network connectivity. References
Fig. 6. Variation of the transition temperature Tg of glasses in 60TeO2–B2O3–Bi2O3–PbO system as a function of the B2O3 molar fraction.
[1] M.A. Villegas, J.M. Fernández Navarro, Physical and structural properties of glasses in the TeO2–TiO2-Nb2O5 system, J. Eur. Ceram. Soc. 27 (2007) 2715–2723. [2] X. Shen, Q.H. Nie, T.F. Xu, Y. Gao, Optical transitions of Er3+/Yb3+ codoped TeO2– WO3–Bi2O3glass, Spectrochim. Acta A Mol. Biomol. Spectrosc. 61 (2005) 2827–2831. [3] Abdeslam Chagraoui, Abdelmjid Tairi, Kaltoum Ajebli, Hanane Bensaid, New tellurite glasses and crystalline phases in the Bi2O3–CaO–TeO2system: synthesis and characterization, J. Alloys Compd. 495 (2010) 67–71. [4] M.A.P. Silva, Y. Messaddeq, S.J.L. Ribeiro, M. Poulain, F. Villain, V. Briois, Structural studies on TeO2–PbO glasses, J. Phys. Chem. Solids 62 (2001) 1055–1060. [5] Tomoharu Hasegawa, Optical properties of Bi2O3–TeO2–B2O3 glasses, J. Non-Cryst. Solids 357 (2011) 2857–2862. [6] D. Munoz-Martín, M.A. Villegas, J. Gonzalo, J.M. Fernández-Navarro, Characterisation of glasses in the TeO2–WO3–PbO system, J. Eur. Ceram. Soc. 29 (2009) 2903–2913.
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[7] V. Dimitrov, Phase-diagram and IR spectral investigations of the 2TeO2·V2O5–Li2O· V2O5·2TeO2 system, J. Solid State Chem. 66 (1987) 256–262. [8] S.H. Kim, T. Yoko, Nonlinear optical properties of TeO2 based glasses: MOx–TeO2 (M = Sc, Ti, V, Nb, Mo, Ta and W) binary glasses, J. Am. Ceram. Soc. 78 (1995) 1061–1065. [9] P. Armand, P. Charton, New ternary tellurite glasses: TeO2–WO3–Sb2O4 and TeO2– WO3–Ga2O3, Phys. Chem. Glasses 43 (2002) 291–295. [10] Y. Dimitriev, V. Dimitrov, M. Arnaudov, IR-spectra and structures of tellurite glasses, J. Mater. Sci. 18 (1983) 1353–1358.
[11] I. Shaltout, Y. Tang, R. Braunstein, E.E. Shaisha, FTIR spectra and some optical properties of tungstate–tellurite glasses, J. Phys. Chem. Solids 57 (1996) 1223–1230. [12] L. Zhu, T.F. Xu, Q.H. Nie, X. Shen, Spectral properties and thermal stability of erbium TeO2–WO3–La2O3 glass, J. Inorg. Mater. 21 (2006) 351–356. [13] J.A. Duffy, M.D. Aberdeen, Optical properties of oxide glasses, in: D.R. Uhlman, N.J. Kreidl (Eds.), Optical Properties of Glass, American Ceramic Society, Westerville, OH, 1991.
Contents lists available at ScienceDirect
Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/ locate/ jnoncrysol
Characterization of new tellurite glasses and crystalline phases in the TeO2–PbO–Bi2O3–B2O3 system☆ Yongtao Zhou, Yunxia Yang ⁎, Feihong Huang, Jing Ren, Shuanglong Yuan, Guorong Chen Key Laboratory for Ultrafine Materials of Ministry of Education, School of Materials Science and Engineering, East China University of Science and Technology, Shanghai 200237, China
a r t i c l e
i n f o
Article history: Received 5 September 2013 Received in revised form 20 November 2013 Available online 20 December 2013 Keywords: Tellurite glasses; Structure; Thermal properties; Optical spectroscopy
a b s t r a c t A new kind of tellurite glass based on TeO2–B2O3–Bi2O3–PbO system is prepared. It is found that B2O3 is the component that contributes most to increase the thermal stability of the glasses, while Bi2O3 is the most contributing oxide to decrease the thermal stability. The structural origins for the variations of the thermal stability with the glass composition are discussed based on the Fourier-transform infrared (FTIR) spectrometry measurement. The present glasses show potential to be used for photonic devices and low melting point sealing glasses. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Among glass families investigated for photonic applications, tellurite glasses are of great interest because of their promising optical properties (high linear and nonlinear refractive indices, low phonon energy, and wide transmission in the infrared range) [1]. The unique optical properties of tellurite glasses are mainly due to the high polarisability of the Te4+-ions, which possess a non-bonding electron lone pair 5s2. However, TeO2 is hard to form glass alone, so other network former or modifier oxides are necessary [2,3]. Heavy metal oxides with empty d orbitals, such as PbO and Bi2O3 can be incorporated into tellurite glasses. These glasses possess very large refractive index and third order nonlinearity, which may find applications for ultrafast switching in optical devices. Moreover, they could be used as laser medium for broadband fibre amplifier or laser in optical communication because they exhibit very broad luminescence. Silva et al. [4] reported TeO2–PbO binary glass forming region and structure. Tomoharu Hasegawa [5] studied TeO2–B2O3–Bi2O3 ternary glass structures. Munoz-Martín et al. [6] mentioned that ternary tellurite–tungstate glass systems with alkaline oxides, zinc oxide, bismuth oxide or lanthanum oxide as third components, or other complex multicomponent glasses have demonstrated to be excellent hosts for Er3 + and Yb3+-ions and very promising materials for developing broadband integrated optical amplifiers. Following the research line of a former work [4], the purpose of this research is the study of properties of glasses in the TeO2–B2O3–Bi2O3–PbO quaternary system, on the basis of their possible application as base materials for photonic devices and low melting ☆ Project of Shanghai Science and Technology Committee. Project Number: 13dz1201103. ⁎ Corresponding author. Tel.: +86 21 64252647; fax: +86 21 64253395. E-mail address: [email protected] (Y. Yang). 0022-3093/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jnoncrysol.2013.11.037
point sealing glasses. Due to the toxicity and contamination of PbO, reducing the use of PbO is necessary, so it is important to know the possibility of obtaining glasses with good properties when PbO is partly substituted by Bi2O3. It is found that the incorporation of Bi2O3 makes glasses easily devitrified and B2O3 can overcome this drawback. The detailed structural origins for the variations of the glass thermal stability are discussed based on the FTIR spectrometry measurement. 2. Experimental 2.1. Glass preparation Glasses of nominal compositions summarised in Table 1 were obtained by traditional melt-quenching method. Reagent grade chemicals TeO2, B2O3, Bi2O3 and PbO were used as starting raw materials and well mixed in an agate mortar for 10 min. Each batch was then melted by using a platinum crucible in the range of 850–900 °C for 35 min. For each composition, the bulk glass was prepared by pouring the melt onto the preheated brass mould. The obtained glass blocks were immediately introduced into the annealing furnace, kept for 1 h at temperatures ranging from 300 to 350 °C and then cooled down to room temperature. From the glass blocks parallel slabs of about 1 mm thick were cut and optically polished. 2.2. Characterisation Crystalline phases of the devitrified glass samples were identified by X-ray diffraction (XRD) spectroscopy. A Siemens D 5000 equipment was used (Kα of Cu, 40 kV, 30 mA, 2θ = 10–80°). Glass transition temperature (Tg) and thermal stability were estimated by Differential Scanning Calorimetry (DSC), which was carried
Y. Zhou et al. / Journal of Non-Crystalline Solids 386 (2014) 90–94 Table 1 Composition (mol%) and thermal properties of the prepared glasses. Number
TeO2
B2O3
Bi2O3
PbO
Tg
G1 G2 G3 G4 G5 G6 G7 G8 G9 G10 G11 G12 G13 G14 G15 G16 G17 G18 G19 G20 G21
60 60 60 60 60 60 60 60 60 60 60 60 60 60 60 60 60 60 60 60 60
5 5 5 5 5 5 10 10 10 10 10 15 15 15 15 20 20 20 25 25 30
5 10 15 20 25 30 5 10 15 20 25 5 10 15 20 5 10 15 5 10 5
30 25 20 15 10 5 25 20 15 10 5 20 15 10 5 15 10 5 10 5 5
Partially crystallised Crystallised Crystallised Crystallised Crystallised Crystallised 314.81 387.06 Partially crystallised Crystallised Crystallised Crystallised 330.71 415.87 342.42 410.30 347.27 404.08 Partially crystallised 344.67 355.50 433.52 360.03 429.95 363.28 367.47 367.15
Tc
ΔT
72.25
XRD patterns of G1 and G2 are shown in Fig. 2. The G1, corresponding to the formula 60TeO2–5B2O3–5Bi2O3–30PbO, exhibits only broad features characteristic of glassy and amorphous materials. Narrow Bragg peaks with high intensity appear in the G2 glass sample corresponding to the formula 60TeO2–5B2O3–10Bi2O3–25PbO, which means that the crystallisation tendency of the glasses increases dramatically with the incorporation of Bi2O3. The observed crystalline phases were identified as Bi2Te4O11 both in G1 and in G2 samples. We can get that a chemical reaction was taking place in the glass-forming melt, which reads as: Bi2 O3 þ 4TeO2 →Bi2 Te4 O11 :
85.16 67.88 56.81
78.02 69.92
91
ð1Þ
This reaction led to the formation of these Bi2Te4O11 crystalline phases. 3.2. FTIR spectra
The glass-forming region of the pseudo-quaternary system TeO2– B2O3–Bi2O3–PbO is presented in Fig. 1. This domain is located around the binary TeO2–B2O3 area where glassy compositions are observed up to 30 mol% of PbO, and 15 mol% of Bi2O3 in the 60TeO2–B2O3 side. The
Both in crystalline TeO2 and in tellurite glasses, Te4 +-ions form [TeO4] groups in trigonal bipyramids (tbps) [7]. In such structures two oxygen ions are located in the axial vertices, while the other two oxygen ions and the lone electron pair of tellurium are located in the three equatorial positions. The equatorial Te\O bonds are slightly shorter than the axial bonds, as indicated by Kim and Yoko [8]. Trigonal bipyramids are linked to each other by sharing their vertices and forming a continuous three-dimensional structure. The spectrum of crystalline TeO2 is dominated by two main bands (Table 2): one is a wide asymmetric band at 650–660 cm−1 and the other is a narrow symmetric band at 775 cm−1 [9]. In the spectra of tellurite glasses, the band at 650 cm−1 appears very much broader, due to their disordered structure, which causes the 775 cm−1 band to appear as a shoulder [10,11]. All FTIR spectra of the quaternary glasses show the same features illustrated in Figs. 3–5. On one hand, the characteristic main band at 650 cm−1 always appears, although its exact position depends on the composition. This indicates that the structure based on the [TeO4] tbps is maintained in all the glass samples. On the other hand, the band at around 900 cm− 1 indicates that [BiO3] and [BiO6] polyhedral groups also form in the glasses. Besides, the shoulders at 680–720 cm−1 and around 1100 cm− 1 are attributed to bending vibrations of B\O in [BO3] triangles and stretching vibrations of B\O in [BO4] tetrahedrons, respectively.
Fig. 1. Ternary diagram for the 60TeO2–B2O3–Bi2O3–PbO system showing the composition of the samples prepared. Hollow (○) points correspond to homogeneous glasses and full (●) points to devitrified samples.
(a) Glasses with constant B2O3 content in 60TeO2–15B2O3– xBi2O3–(25 − x)PbO system. In the glass series 60TeO2– 15B2O3–xBi2O3–(25 − x)PbO, the band at 1200–1300 cm− 1 assigned to [BO3] groups shifts towards a lower wave number, i.e. longer wavelength, when PbO is substituted by Bi2O3 (Fig. 3). This is due to the lower field intensity of the mixed Te\O\Bi and B\O\Bi linkages, in which the oxygen is less polarised, compared with Te\O\Te linkages, since Te 4 +ions possess higher field intensity than Bi3 +-ions. (b) Glasses with constant PbO content in 60TeO2–xB2O3–(30 − x) Bi2 O3 –10PbO system. For constant [PbO], the decreasing of the [Bi2O3]/[B2O3] ratio, i.e. the substitution of Bi2O3 by B2O3, causes an increase in the B\O\B and Te\O\B linkages and a decrease in the B\O\Bi, Te\O\Bi and Bi\O\Bi linkages. Thus, the incorporation of B 2O 3 yields a network distortion that can be monitored by the progressive broadening of the bands at around 650 cm − 1 and 1200 cm − 1 (Fig. 4). Meanwhile, the band positions of the two bands shift towards a higher wave number, i.e. shorter wavelength. This is due to the higher field intensity of the B\O\B and Te\O\B linkages, in which the oxygen is highly polarised, compared with B\O\Bi, Te\O\Bi and Bi\O\Bi linkages, since Bi 3 +-ions have larger ion radius and possess lower
out under N2 atmosphere at a heating rate of 10 °C min−1, using a TA Instruments/Q200 system, with a maximum error of 3 °C. The optical absorption/transmission spectra were recorded in the 300–1000 nm wavelength range, using an Ocean Optics spectrophotometer model HR4000CG-UV-NIR. Transmittance to be further used for the energy gap (Eg) calculations was corrected for the standard thickness of 1 mm. Fourier-transform infrared (FTIR) spectra were recorded with a 20SXC FT-IR spectrometer with a resolution of 2 cm−1 in the 400–1800 cm−1 range. KBr pellets technique was used for recording spectra of glasses. 3. Results and discussion 3.1. Glass forming region
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Fig. 2. X-ray diffraction pattern of samples G1 and G2.
field intensity. In other words, the decreasing of the [Bi2O3]/ [B2O3] ratio makes the network become more rigid. (c) Glasses with constant Bi 2 O3 content in 60TeO2 –xB2 O3 – 10Bi 2 O3 –(30 − x)PbO system. For constant [Bi 2 O 3], the decreasing of the [PbO]/[B2O3] ratio, i.e. the substitution of PbO by B2 O3 , yields a network distortion that can be also monitored by the progressive broadening of the bands at around 650 cm− 1 and 1200 cm− 1 (Fig. 5). As aforementioned, this is due to the formation of new B\O\B and Te\O\B linkages with the increasing of B2O3 content. Here again, the decreasing of the [PbO]/[B 2O 3 ] ratio makes the network become more rigid.
3.3. Thermal properties 3.3.1. Glass transition temperature The transition temperature (Tg) gives information on both the strength of interatomic bonds and the glass network connectivity, in a similar way that the melting temperature does for crystalline solids. Tg of the glasses studied in the present work varies linearly with the molar fraction of each one of the three components (Fig. 6). The maximum variation occurs when B2O3 is substituted by PbO in the glass series 60TeO2–xB2O3–5Bi2O3–(35 − x)PbO. The corresponding ΔTg increases from 11 to 19 °C for each 5 mol% PbO substituted by 5 mol%
B2O3. The results indicate that B2O3 is the most contributing oxide to increase the glass transition temperature. The transition temperature always decreases when B2O3 or Bi2O3 is substituted by PbO. So PbO is the most contributing oxide to decrease the glass transition temperature. The incorporation of PbO causes the formation of new non-bridging oxygens in Te\O\…Pb2+…\O\Te, Bi\O\…Pb2 +…\O\Bi and B\O\…Pb2 +…\O\B linkages. The higher polarisability of such new non-bridging oxygens explains the Tg shift towards lower temperature, due to the lower field intensity of Pb2+-ions (0.27) compared with that of Te4+-ions (0.71).
3.3.2. Thermal stability The difference between the onset crystallisation temperature Tc and the glass transition temperature Tg is usually taken as an index to estimate the glass thermal stability. The wider the thermal stability range, the more favoured is the glass forming process [12].
Table 2 Vibration types of different IR wave numbers. Range of wave numbers Vibration types (cm−1) 650–660 775 Around 900 1200–1300
Asymmetric bending vibrations of Te\O bonds in [TeO4] trigonal bipyramids Symmetric stretching vibrations of Te\O bonds in [TeO4] trigonal bipyramids Symmetric stretching vibrations of [BiO3] and [BiO6] polyhedral Stretching vibrations of B\O bonds in [BO3] triangles
Fig. 3. FTIR spectra of the quaternary glass series 60TeO2–15B2O3–xBi2O3–(25 − x)PbO.
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Table 3 Cut-off wavelength and energy gap of TeO2–B2O3–Bi2O3–PbO glasses. Group
Number
TeO2
B2O3
Bi2O3
PbO
λo (nm)
Eg (eV)
I
G12 G13 G14 G14 G17 G19 G13 G17 G20
60 60 60 60 60 60 60 60 60
15 15 15 15 20 25 15 20 25
5 10 15 15 10 5 10 10 10
20 15 10 10 10 10 15 10 5
377 380 384 384 376 369 380 376 371
3.291 3.265 3.231 3.231 3.300 3.363 3.265 3.300 3.344
II
III
thermal stability. A careful tuning of the glass composition could lead to the maximum substitution of PbO by Bi2O3 while keeping the good glass forming ability and thermal stability. 3.4. Optical absorption Fig. 4. FTIR spectra of the quaternary glass series 60TeO2–xB2O3–(30 − x)Bi2O3–10PbO.
The bandgaps (Eg) of the glasses can be approximately estimated from the wavelength of the ultraviolet cut-off (λo) (Table 3). In all the cases, with the substitution of B2O3 by Bi2O3 or PbO, the energy gap Eg diminishes. The incorporation of network modifier ions (such as Pb2+) into a tellurite glass leads to the breaking of the glass network tbps and the formation of non-bridging oxygens. The more weakened the glass network and higher the number of non-bridging oxygens, the lower the Eg values [13]. 4. Conclusions
Fig. 5. FTIR spectra of the quaternary glass series 60TeO2–xB2O3–10Bi2O3–(30 − x)PbO.
Calculated thermal stability of the glasses studied is summarised in Table 1. Bi2O3 is the most contributing oxide to decrease the thermal stability, while B2O3 is the most contributing oxide to increase the
The basic trigonal bipyramid [TeO4] (tbp) structure, as well as the [BiO3 ] and [BiO 6 ] polyhedral groups and [BO3 ] triangle groups is maintained in TeO2 –B2 O 3 –Bi2 O 3–PbO system studied, even for a low content of B2O3. The broadening of the main FTIR band at around 650 cm− 1 and 1200 cm− 1 indicates a deformation of the glass network. Such deformation increases as the B 2 O3 content increases. Pb 2 + -ion incorporation does not produce new bands in FTIR and their presence does not affect the position of the main wide band at around 650 cm − 1 and 1200 cm− 1, indicating that Pb 2 + -ions exist as glass modifiers and do not take part in the formation of glass network. The transition temperature Tg of all the glasses studied varies linearly with the molar fraction of each of three components: B2O3 is the oxide that contributes most to increase the glass transition temperature, while PbO is the oxide that contributes most to decrease the glass transition temperature. The weaker network connectivity and higher polarisability of non-bridging oxygens formed with the introduction of PbO explain the Tg shift towards lower temperature. From the point of view of thermal stability, the glasses with wider temperature range to be formed without crystallisation risk are those with high content of B2O3 and low content of Bi2O3. Finally, the energy gap Eg decreases when B2O3 is substituted by Bi2O3 or PbO, which is also the result of the weakened network connectivity. References
Fig. 6. Variation of the transition temperature Tg of glasses in 60TeO2–B2O3–Bi2O3–PbO system as a function of the B2O3 molar fraction.
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