Preparation and characterization of new fluorotellurite glasses for photonics application

Journal of Non-Crystalline Solids 355 (2009) 447–452

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Preparation and characterization of new fluorotellurite glasses for photonics application Guihua Liao, Qiuping Chen *, Jianjun Xing, Hrvoje Gebavi, Daniel Milanese, Michael Fokine, Monica Ferraris Dipartimento di Scienza dei Materiali ed Ingegneria Chimica, Politecnico di Torino, C.so Duca degli Abruzzi 24, I-10129 Torino, Italy

a r t i c l e

i n f o

Article history: Received 8 February 2008 Received in revised form 10 January 2009

PACS: 42.70.a 65.60.+a 78.30.j 78.40.q

a b s t r a c t Glasses based on (85  x)TeO2–xZnF2–12PbO–3Nb2O5 (x = 0–40) system have been studied for the first time for fabricating mid-infrared optical fiber lasers. The thermal and optical properties including UV– Vis, Raman as well as FTIR spectra are reported. It is demonstrated that increasing the ZnF2 concentration to 30 mol% significantly increased the thermal stability of the glass. Adding ZnF2 also reduced the hydroxyl (OH) content of the glass resulting in lower optical absorption in the mid-infrared region, which is crucial for infrared laser applications. The glass absorption cut-off edge near 400 nm blue-shifts with increasing ZnF2 addition. Raman spectra show a depolymerization of the glass network with increasing transformation of TeO3+1 to TeO3 structures. Ó 2009 Elsevier B.V. All rights reserved.

Keywords: Infrared glasses Fluorides Heavy metal oxides Optical properties Absorption Optical spectroscopy FTIR measurements Raman spectroscopy Oxide glasses Tellurites Thermal properties Calorimetry Glass transition

1. Introduction Heavy metal oxide and oxyfluoride glasses have received considerable attention for their potential application in fiber lasers for the mid-infrared region, as well as for nonlinear optics. Among these glasses, tellurite glasses have been extensively investigated over the last two decades, due to their optical properties as well as for fundamental scientific research. Tellurite glasses are characterized by good chemical durability, good thermal stability, high refractive index, high transmittance in near to middle infrared regions, and also high solubility for rare earth ions [1]. Fluoride glasses, on the other hand, have lower phonon energy (550 cm1) with a broader optical transmittance region between

* Corresponding author. E-mail address: [email protected] (Q. Chen). 0022-3093/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2009.01.011

0.3 and 7 lm [2,3], but require a more complex fabrication route than tellurite glasses. For these reasons, fluorotellurite glasses, a combination of the two glass systems, have received increasing attention [4–8] with the goal to find a good compromise between the pure fluoride and tellurite glass properties. Specifically, a decrease of the unwanted hydroxyl concentration, which is expected for oxyfluoride glasses due to chemical interaction between fluorine and hydroxyl ions, should subsequently reduce optical absorption loss in infrared region as well as quenching effect on the excited levels of rare earth ions, since improved fluorescence lifetimes for rare earth ions were observed [7]. In this paper, the properties of a new fluorotellurite glass system based on TeO2–ZnF2–PbO–Nb2O5 compositions are reported. The main influences on the thermal and optical properties by ZnF2 addition to tellurite glasses are discussed. Raman spectra have been recorded in order to analyze the corresponding changes in glass structure.

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2. Experimental 2.1. Glass fabrication Bulk glasses with composition of (85  x) TeO2–xZnF2–12PbO– 3Nb2O5, where x = 0, 10, 20, 30 and 40 mol%, were fabricated using anhydrous zinc fluoride and metal oxides as starting materials. The glass samples are labeled correspondingly as TZF0–TZF4. The starting materials have purity of 99+% for TeO2 and ZnF2 and 99.99% for PbO and Nb2O5. Mixed batches were melted in platinum crucibles with a platinum cover from 730 to 825 °C for 30 min, depending on the composition of glass. In order to improve the homogeneity of the obtained glass, a stirring operation was carried out by opening the furnace and shaking the crucible during the melting process of glass. The melt was poured on a brass mould preheated at approximately the glass transition temperature (Tg) and subsequently annealed at around 10 °C above Tg for 3 h. The glass samples were then cooled down slowly to room temperature. The entire process was carried out in a glove-box purged with dry air. 2.2. Thermal properties, density and refractive index measurements Differential scanning calorimetry (DSC, Perkin–Elmer DSC7) was used to determine the glass transition temperature, Tg, and the crystallization onset temperature, Tx, of the samples. DSC measurements were performed under an argon atmosphere using a constant heating rate of 10 °C/min and a constant gas flux of 20 ml/min. Approximately 35 mg of fine grain sample were used for each measurement. For further discussion of the results we also calculated a value of the thermal stability of the glass (DT), here defined as DT = Tx  Tg. DT typically indicates the thermal stability against crystallization of the glass. Thermomechanical analysis (TMA, Perkin–Elmer TMA7) was used to measure the thermal expansion properties. The samples used for TMA measurement were prepared with dimension of (4–5)  (4–5)  (10 ± 0.5) mm3 with parallel top and bottom surfaces. The TMA measurements were performed in helium atmosphere at a constant heating rate of 10 °C/min. The density of each sample was determined at room temperature using the Archimedes’ principle. Small bulk glass samples without flaws were used with distilled water as the immersion liquid. In addition, the sample refractive indices were measured using the prism coupling method (Metricon Model 2010 M) at 633, 1312 and 1533 nm, respectively, with a resolution of ±0.0001.

The samples used for spectroscopic measurements were cut to a dimension of 15  10  1.5 mm3 and optically polished. The absorption spectra in UV–Vis region (200–800 nm) were recorded for each sample using a UV–Vis-IR spectrophotometer (Varian Cary 500). The absorption cut-off wavelength in UV–Vis region of the glasses were determined by the obtained absorption spectra, and its error was extrapolated within ±2% according to the measurement error of sample thickness. In addition, a Bruker Tensor 37 Table 1 Thermal property parameters and densities of the glasses.

TZF0 TZF1 TZF2 TZF3 TZF4

Tg ±5 (°C)

Tx ±5 (°C)

DT (°C)

314 282 263 270 268

459 464 380 447 411

145 182 117 177 143

3. Experimental results 3.1. Thermal properties and densities of the glasses Density and thermal properties data of all glass samples are reported in Table 1. Two features can be found from Table 1: (i) Tg of all prepared fluorotellurite glasses monotonously decreases with increasing concentration of ZnF2 in the batch; (ii) DT reaches a maximum of 182 °C for sample containing an amount of ZnF2 of 10 mol% (TZF1). So it can be inferred that the thermal stability value (DT) of the present fluorotellurite glass (TZF1) is significantly improved. TZF1, therefore, seems to be a promising candidate for optical fiber fabrication, due to excellent thermal stability against devitrification. Table 1 also reveals that the thermal expansion coefficient (a) of the glasses slightly increases with increasing ZnF2 content. It can be noted that the densities of the fluorotellurite glasses decrease with increasing ZnF2 content. 3.2. Refractive index of the glasses Table 2 lists the refractive index (n) of all glasses measured at the wavelengths of 632.8, 1312 and 1533 nm. The refractive index of fluorotellurite glasses decreased with increasing ZnF2 content. 3.3. UV–Vis absorption spectra The measured absorption spectra in the UV–Vis region of all glasses are showed in Fig. 1. The UV absorption edge of all glasses is blue-shifted with increasing ZnF2 content from approximately 416 nm for TZF0 to 357 nm for TZF4, see Fig. 2. 3.4. FTIR spectra of the glasses

2.3. Spectroscopic measurements

Glass

FTIR Spectrophotometer was used to record the infrared (IR) transmittance spectra in the range from 700 to 4000 cm1 for each sample. All measurements on spectra were performed at room temperature, and the wavelength accuracies of the two instruments were the same (±1 nm). Raman spectra were measured at room temperature using a MKI Renishaw Ramanscope equipped with a BH2-UMA Olympus microscope. The excitation source was provided by an Ar ion laser, tunable in the 350–600 nm wavelength region. The laser power and wavelength used for the measurements were set to 22 mW and 514.5 nm, respectively. Moreover, the intensity data was recorded at intervals of 1 cm1 and the offset of the system is 1 cm1

a(80–240 °C) ±0.1(106/K)

Density ±0.01 (g/cm3)

16.2 17.0 17.7 17.8 18.7

5.90 5.81 5.76 5.71 5.65

Fig. 3 shows the FTIR spectra, measured from 1600 to 4000 cm1, of all the glasses. Of these spectra curve, the peak at around 2900 cm1 was identified as absorption bands of CO2 and CH/CH2/CH3 originating from glass polishing process [9]. 3.5. Raman spectra of the glasses In order to further understand the change of glass properties with the addition of ZnF2 in glass, Raman spectra of all the glasses Table 2 Refractive indices of the various glasses. Glass

632.8 nm ±3%

1312 nm ±3%

1533 nm ±4%

TZF0 TZF1 TZF2 TZF3 TZF4

2.1900 2.1078 2.0276 1.9550 1.8865

2.1267 2.0533 1.9813 1.9160 1.8532

2.1205 2.0480 1.9768 1.9119 1.8493

G. Liao et al. / Journal of Non-Crystalline Solids 355 (2009) 447–452

Fig. 1. UV–Vis absorption spectra of all the glasses.

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Fig. 4. Raman spectra of the glasses TZF0–TZF4.

Fig. 2. Cut-off edge in UV–Vis region of the glasses TZF0–TZF4. The line in the figure is guide for the eye.

Fig. 5. Gaussian deconvolution of Raman spectra of the glass TZF0 and TZF4. Fig. 3. FTIR spectra of the glasses TZF0–TZF4.

were measured, and the results are shown in Fig. 4. Fig. 5 shows Gaussian deconvolution results of Raman spectra of the glass TZF0 and TZF4. From Fig. 5 one can see that there are five bands

in the present glasses, namely, band A peaking at around 320 cm1; band B peaking at around 450 cm1; band C peaking at around 665 cm1; band D peaking at around 760 cm1; and

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band E peaking at around 860 cm1. Moreover, it can be noted by Fig. 4 that when compared with the spectrum of glass TZF0, the ZnF2 containing samples TZF1–TZF4 show a significant modification in spectrum shape. With the increase of ZnF2 content in glass, the intensities of band A, B and C decreased relatively to that of band D.

4. Discussion 4.1. Thermal properties, density and refractive index of the glasses The fact that Tg decreases with the addition of ZnF2 is in agreement with similar studies of fluoride addition to other glass systems, e.g. oxyfluoride glasses as reported in Refs. [4–6]. Yang et al. [10] studied the effect of lead fluoride addition on germanate glass and suggested that when fluoride is added into germanium dioxide glass, oxygen ions can simply be replaced by fluorine ions; and that the fluorine ions then act as either non-bridging or bridging species between structural units. Hayakawa et al. [11] studied the effect of fluorine ion on the properties of lead silicate glasses, and certified that the fluorine ions prefer to be bonded to lead ions rather than to the network-forming ions. Moreover, Yang et al. [10] also suggested that the role of fluorine introduced in various oxide glasses such as silicate and germanate is very similar. Based on these study results, therefore, it can be inferred that in the case of fluorotellurite glass, fluorine and zinc ions are typically located within the three-dimensional Te–O network forming ionic Zn–F bonds, and hence act to depolymerize the glass structure. In addition, even though fluorine ions serve to connect two structural units, these bonds are weaker compared to bridging oxygen bonds [10], as the field strength of the fluorine ion is less than that of the oxygen ion (F: 0.565 Å2; O2: 1.02 Å2). Both these effects should lead to a lowering of Tg, with the addition of ZnF2 to the oxide glass. It is interesting to note, that while changes in Tg follows a similar trend as for oxyfluoride glasses when adding ZnF2, the onset crystallization temperature (Tx), and consequently DT show quite different behaviors from Refs [4–6]. A possible reason for this discrepancy may be explained as follows: addition of ZnF2 into the TeO2–PbO–Nb2O5 system would form different structural units from the TeO4 and TeO3 polyhedra, consequently, the different structural units compete with each other, this could tend to retard the nucleation and crystallization and to stabilize the glass [8], and shifting the onset temperature of crystallization (Tx) to higher values. However, the introduction of ZnF2 into the oxide glass plays a depolymerization role towards the original oxide glass network after all, therefore, with increasing concentration of ZnF2, the concentration of non-bridging species increases, resulting in a decrease of viscosity as well as activation energy for nucleation and crystal growth. At sufficiently high ZnF2 addition Tx would then decrease. The increase of the thermal expansion coefficients (a) of the glasses with increasing ZnF2 content obtained in this study agreed well with that reported previously [6], and it has been widely accepted that the thermal expansion coefficient increases when the softening point and transition temperature of the glass decrease. Regarding glass densities, its decrease with increasing zinc fluoride content is not surprising as: (i) substitution of ZnF2 for TeO2 results in a decrease of mass/unit-volume of the glass due to different molar mass; (ii) substitution of ZnF2 for TeO2 depolymerizes the glass network resulting in a less dense structure [7,8]. With respect to the refractive index of the glass, as the polarizability of ZnF2 is lower than that of TeO2 [5,8], a decrease in refractive index is expected with an increase in ZnF2 content. In addition, since the refractive index of a glass is also related to its density, the lowering of the glass density induced by the introduction of ZnF2

concentration will also contribute to the decrease in refractive index: this result is in good agreement with other similar studies performed on silica based glasses [12,13]. 4.2. UV–Vis absorption spectra of the glasses The UV absorption edge of all glasses fabricated in this study is blue-shifted with increasing ZnF2 content (see Figs. 1 and 2). This trend agrees with other results [4,6] on oxyhalide tellurite glasses and the same behavior was observed for silica glass [14], but differs from that observed for some oxide glasses reported in the literature [15,16], where a typical red-shift of the absorption edge with the addition of glass modifiers was reported. Some previous studies [4,17] have implied that the changes in the glass structure units contributed to the blue shift of the UV absorption edge mentioned above. Inoue et al. [17] studied Na2O–TeO2 glass system and confirmed the blue-shift of the absorption edge with the addition of Na2O, and consequently concluded that the structural conversion from TeO4 towards TeO3+1 and TeO3 should be responsible for this blue-shift. As for TeO2– ZnO–ZnF2 oxyfluoride system glass [4], when ZnF2 substituted for ZnO at a constant TeO2 content, the absorption cut-off wavelength of the studied glass also decreased continuously: it was also suggested that Te–F bonds were probably present in TeO2–ZnO–ZnF2 glass and that the blue-shift of the UV–Vis absorption cut-off wavelength resulted from the replacement of O2 with F in TeO4 units in the fluorotellurite glass. According to previous studies on fluorotellurite glasses [4,18,19], it can be inferred that, in the present glass system, when ZnF2 is introduced into oxide glass (TZF0), the substitution of F ions for O2 ions in TeO4 units, or the formation of Te–F bonds should take place. Generally, the bonding of an anion with cations depends on the positive ion field strength, and the field strength can be expressed as Fs = Zc/(Rc + Ra)2, here Zc is the valence of the cation, R the ionic radius of the cation (c) or anion (a) [9]. In the present fluorotellurite glasses, the radii of cations are, respectively, 0.97 Å for Te4+, 1.19 Å for Pb2+, 0.74 Å for Zn2+ and 0.64 Å for Nb5+, and the radius of F is 1.33 Å [20]. The calculated field strengths are 0.756 Å2 for Te4+, 0.315 Å2 for Pb2+, 0.467 Å2 for Zn2+ and 1.288 Å2 for Nb5+, respectively. Thus, the probability of binding of F ions with these positive ions should be Nb5+ > Te4+ > Zn2+ > Pb2+. Since the batching amount of Nb2O5 in the present glasses was little when compared with that of TeO2, ZnF2 and PbO, the influence of Nb–F bond on the structure of the fabricated glass was very small and consequently ignored. Therefore, it could be deduced that F ions introduced by ZnF2 into TeO2–PbO–Nb2O5 system glass would preferentially bind Te ions in TeOn units by replacing O ions in the TeOn units, and consequently form Te(O,F)n, such as Te(O,F)3+1 and Te(O,F)3. Yuan et al. [21] and Zhang et al. [22] investigated the structures of TeO2– AlF3 glass and TeO2–BaO–BaF2–La2O3–LaF3 glass by Raman spectra, respectively, and also considered that with the addition of fluoride, the structural units of the glasses were converted from TeO4 to Te(O,F)4, and then to Te(O,F)3. Based on the all discussions above, it could be concluded that the decrease of UV–Vis absorption cut-off wavelength of the present fluorotellurite glass with the increase of ZnF2 was also resulted from the conversion of structural unit in the glass from TeO4 to Te(O,F)4, and then to Te(O,F)3. On the other hand, the absorption of glass in UV and Vis spectral ranges is caused by electron transitions from unexcited to excited states [1]. The absorption edge in oxide glasses corresponds to the transition of an electron belonging to an oxygen ion to an excited state. The more weakly these electrons are bound, the more easily the absorption occurs at longer wavelength [23]. Yousef [24] studied optical properties of zinc halotellurite glasses (TeO2–ZnX2, X = F, Cl, Br), and suggested that, after ZnX2 was introduced into

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the tellurite glass, the Te–Oax bond became stronger when X ion occupied the equatorial position than when O2– ion did. According to the suggestions mentioned above, it may be reasonably speculated on that the electrons of Oax ion in Te(O,F)3+1, Te (O,F)3 groups were bound more tightly than that in TeO3+1, TeO3 groups, and consequently the electron transition from unexcited to excited states would require more energy, which lead to the shift of optical absorption edge from lower to higher energy with the increase of ZnF2 content and Te (O,F)3+1 or/and Te (O,F)3 units in the glass structure. 4.3. FTIR spectra of the glasses It is clearly noted from Fig. 3 that the absorption in the 2000– 3600 cm1 region, which is associated with hydroxyl (OH) groups, is much larger in the TZF0 glass than for the other glasses containing ZnF2. The OH absorption consists of three different absorption bands, and their positions depend on the degree of hydrogen bonding of the OH groups in the glass. These bands have been reported as strongly H-bonded OH groups (at around 2300 cm1), weakly Hbonded OH groups (at around 3000 cm1); and free OH groups (at around 3300 cm1) [8,25]. In addition, studies on TeO2–ZnF2–Na2O glass systems performed by O’Donnell et. al. [8] suggested that the absorption band at approximately 3300 cm1 is attributed to the stretching mode of free Te–OH groups and/or the stretching mode of molecular water, while the absorption band locating at around 3060 cm1 was attributed to the stretching mode of weak hydrogen-bonded Te–OH groups. They also attributed the absorption band located near 2290 cm1 to a stretching mode of strong hydrogen bonded Te–OH groups. It is, therefore, reasonable to assume that the difference in the absorption spectra of sample TZF0 is related to the OH groups. Fig. 3 also shows that ZnF2 is very efficient in removing OH groups, or ‘drying’ these glasses during the melting process. The significant reduction of OH groups in the glass with addition of ZnF2 can be attributed to the following reactions [8,9]:

2½BTe—OH þ ZnF2 ! BTe—O—TeB þ ZnO þ 2HFðgÞ ;

ð1Þ

ZnF2 þ H2 O ! ZnOs þ 2HFðgÞ :

ð2Þ

As HF is volatile and subsequently removed from the system, OH groups in the fluorotellurite glasses will be reduced continuously with the addition of ZnF2.

Fig. 6. Variation of integrated area of various absorption bands of the glasses. The lines in the figure are guides for the eye.

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The small figure inserted in Fig. 6 is derived from Gaussian fitting disposal to the absorption band in 2000–3600 cm1 region of FTIR spectra curves in Fig. 3. Before Gaussian fitting, the absorption bands of CO2 and CH/CH2/CH3 was removed from the spectra curves. The curves with symbol in Fig. 6 show the variation in the integrated area of each OH absorption band and the sum of integrated area of the three absorption bands as a function of ZnF2 content in batch. From the insert in Fig. 6 it can be clearly seen that there are three OH absorption bands (denoted as A1, A2 and A3, respectively). The peak positions of these bands were respectively determined at 2260, 2980 and 3300 cm1; therefore, they should correspond to stretching mode of the strongly H-bonded OH groups, weakly H-bonded OH groups and free OH groups, respectively. Of course, A2 is the biggest absorption band among them. In addition, it can also be seen from Fig. 6 that the addition of ZnF2 leaded to disappearance of absorption band A1, a mild decrease of absorption band A3 and a sharp decrease of the biggest absorption band, A2, in the prepared fluorotellurite glasses (TZF1–TZF4). Consequently, the sum of integrated area of three absorption bands of fluorotellurite glasses was remarkably reduced, which means the total absorption loss derived from stretching mode of OH groups in the glass was drastically reduced. Moreover, Fig. 6 illustrates that the sum of integrated area of the three absorption bands of the glasses initially decreased with increasing ZnF2 content and reached a minimum at 30 mol% ZnF2 content. When the ZnF2 content further increased to 40%, however, the absorption loss of in the glass increased again. In fact, according to Fig. 3, by calculating absorption coefficient at 2980 cm1 of the fluorotellurite glasses, it could be found that the addition of 10% ZnF2 in batch rapidly decreased absorption coefficient of the studied glass from 1.22 cm1 for TZF0 glass to about 0.061 cm1 for TZF1 glass, and then reached a minimum of 4.67  103 cm1 for TZF3 glass. For TZF4 glass, however, the absorption coefficient of glass increased again to 0.037 cm1, which suggested that the content of 30 mol% ZnF2 should be the most appropriate concentration for the studied fluorotellurite glass system. 4.4. Raman spectra and structure evolution of the glasses It is well-known that the structure of tellurite glasses with high content of TeO2 mostly consists of structural units of asymmetrical, trigonal bipyramids, TeO4, (tbp). The previous studies [4,26] have certified that the band C in Fig. 5 should be ascribed to the anti-symmetric vibration modes of TeO4 entities and the band A and B should been assigned to the bending vibration modes of Te–O–Te or O–Te–O linkages of TeO4 entities. The increasing concentration of modifiers in the glass would result in the formation of TeO3 trigonal pyramids (tp) through an intermediate stage formation of TeO3+1 polyhedra [1]. The proportion of TeO3 and TeO3+1 structural units within the glass network depends on the chemical nature and concentration of the modifier. Some previous studies [4,27,28] have ascribed the band D to the stretching vibration modes of TeO3+1 and TeO3 units, involving non-bridging oxygen (NBO); Furthermore, in the present study a low intensity band E at approximately 860 cm1 was observed, which can be associated to the Nb–O bond in the NbO6 structural unit [29]. The bands attributed to the vibration mode of Pb–O bonds is located in the low frequency region (<200 cm1) [30], and hence were not shown in Fig. 5. Fig. 7 shows the normalized variation of the intensity ratio between the bands located at 665 and 760 cm1 (i.e. band C and band D), denoted by I665/I760. It can be seen that the intensity ratio (I665/I760) decreased with increasing ZnF2 concentration in the glass, and there is an approximately 75% decrease for the TZF4

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Fig. 7. Variation of intensity ratio (I665/I760) of Raman Spectra of the glasses. The line in the figure is guide for the eye.

sample containing 40 mol% ZnF2, which means some structural changes occurred in the glass. Fig. 5 reveals that the TZF0 glass was mainly composed of volumes of TeO4. Besides, small amount of TeO3/TeO3+1 and a few of PbO4 and NbO6 also existed in the structure of TZF0. When ZnF2 was introduced into the glass, the intensity of the band C at around 665 cm1 ascribed to the antisymmetric vibration modes of TeO4 units decreased significantly, and hence led to a sharp decrease of I665/I760 value in Fig. 7. Meantime, the band A and B ascribed to the bending vibration modes of TeO4 entities also decreased gradually. These should confirm a decrease of the TeO4 units concentration accompanying with the formation of more TeO3+1, TeO3 and/or Te(O,F)3+1, Te(O,F)3 units in the fluorotellurite glasses. Undoubtedly, this change in the glass structure resulted from the breakage effect of F ions on Te–O–Te linkages in the glass network. With the substitution of ZnF2 for TeO2, more and more O2 bridging Te4+ might be effectively replaced by F because of their comparable ionic radius, and that formally one oxygen ion may be replaced by two fluorine ions, which would break Te–O–Te linkages in the glass structure and depolymerize the structure network accompanied by the formation of more TeO3+1, TeO3 and/or Te(O,F)3+1, Te(O,F)3 groups which their Raman signatures could be hidden within the broad bands in a large wavenumber range from 600 cm1 to 800 cm1[4,21,22]. Nevertheless, it could be found from Fig. 4 that the peak position of band D ascribed to the stretching vibration modes of TeO3+1 and TeO3 units slightly shifted to higher wavenumber with increase of ZnF2 in the glasses. Maybe this could be explained as follows: due to the substitution of F for O in TeO4 to form Te(O,F)3+1 or Te(O,F)3, the Te–O–Te bonds in Te(O,F)3+1 or Te(O,F)3 units became stronger than those in TeO3+1 or TeO3 units, which hence make the needed energy used for inducing the Te–O–Te bonds to do stretching vibration larger.

5. Conclusions New glasses based on TeO2–ZnF2–PbO–Nb2O5 system were fabricated and characterized. The addition of ZnF2 in the tellurite glass changed significantly the glass properties such as Tg, a, density and refractive index, as well as the glass optical properties as UV–Vis absorption, etc. The glass transition temperature (Tg), density and refractive index decreased with increasing ZnF2 content. It is noted that the glass thermal stability in terms of DT (Tx – Tg) was significantly increased, particularly when the ZnF2 concentration was

10% (TZF1), which indicates TZF1 could be a promising candidate for optical fiber fabrication against devitrification. The absorption loss in visible and infrared regions of the fluorotellurite glasses (TZF1–4) was much reduced compared with that of the tellurite glass (TZF0), which was because the hydroxide (OH) groups decreased markedly. With increasing of ZnF2, the glass absorption cut-off edge at around 400 nm shifted to shorter UV wavelength. The obtained results are promising, since these glasses were fabricated for optical laser application in the infrared region and the reduced absorption loss would allow them to be good candidate materials for this application. The increase of the [ZnF2]/[TeO2] ratio resulted in a significant change of the Raman spectra of the fluorotellurite glass. The intensities of two bands at around 665 cm1 and at 320–450 cm1 assigned to TeO4 groups decreased remarkably. The intensity ratio (I665/I760) decreased from 100% of TZF0 to about 25% of TZF4. These confirm a decrease of TeO4 groups and the formation of more TeO3+1, TeO3 and/or Te(O,F)3+1, Te(O,F)3 groups with increasing ZnF2 content in the fluorotellurite glasses. Acknowledgments The authors would express their sincere thanks to Dr Jiang Shibin and Dr Wu Jianfeng of NPP Photonics, USA, for their kind and valuable suggestions and helps with this work. They thank Dr Sergio Ferrero and Dr Alberto Venturello of DISMIC, Politecnico di Torino, for performing Raman and FTIR spectroscopy measurements on all the samples. The authors are also grateful to the EU for funding the Asia-Link program (ASIA-LINK-CN/ASIA-LINK/004 (81206)). References [1] R.A.H. El-Mallawany, Tellurite Glasses Handbook: Physical Properties and Data, CRC, Boca Raton, FL, 2001. [2] J. Lucas, Curr. Opin. Solid State Mater. 4 (1999) 181. [3] T. Miyashita, T. Manabe, IEEE Transactions on Microwave Theory and Techniques MTT-30 (10) (1982) 1420. [4] V. Nazabal, S. Todoroki, A. Nukui, T. Matsumoto, S. Suehara, T. Hondo, T. Araki, S. Inoue, C. Rivero, T. Cardinal, J. Non-Cryst. Solids 325 (2003) 85. [5] M.D. O’Donnell, K. Richardson, R. Stolen, J. Am. Ceram. Soc. 90 (2007) 1448. [6] El Sayed S. Yousef, J. Phys. D: Appl. Phys. 38 (2005) 3970. [7] G. Wang, S. Dai, J. Zhang, S. Xu, L. Hu, Z. Jiang, J. Lumin. 113 (2005) 27. [8] M.D. O’Donnell, C.A. Miller, D. Furniss, V.K. Tikhomirov, A.B. Seddon, J. NonCryst. Solids 331 (2003) 48–57. [9] M.D. O’Donnell, PhD thesis, University of Nottingham, UK, 2004. [10] Z. Yang, S. Xu, L. Hu, Z. Jiang, J. Opt. Soc. Am. B 21 (5) (2004) 951. [11] S. Hayakawa, A. Osaka, H. Nishioka, S. Matsumoto, Y. Miura, J. Non-Cryst. Solids 272 (2000) 103. [12] H. Kakiuchida, E.H. Sekiya, N. Shimodaira, K. Saito, A.J. Ikushima, J. Non-Cryst. Solids 353 (2007) 568. [13] C.Z. Tan, J. Arndt, H.S. Xie, Physica B 252 (1998) 28. [14] K. Saito, A.J. Ikushima, J. Appl. Phys. 91 (8) (2002) 4886. [15] H. Jia, G. Chen, Z. Hou, J. Non-Cryst. Solids 319 (2003) 322. [16] D. Lezal, J. Pedlikova, J. Horak, J. Non-Cryst. Solids 196 (1996) 178. [17] S. Inoue, Y.Shimizugawa, A. Nukui, T. Maeseto, J. Non-Cryst. Solids 189 (1995) 36. [18] D.L. Sidebottom, M.A. Hruschka, B.G. Potter, R.K. Brow, J. Non-Cryst. Solids 222 (1997) 282. [19] A. Nishara Begum, V. Rajendran, J. Phy. Chem. Solids 67 (2006) 1697. [20] N.N. Greenwood, A. Earnshaw, Chemistry of the Elements, 2nd Ed., Butterworth, Heinemann, Linacre House, Jordan Hill, Oxford, 1997. [21] X. Yuan, Y. Fan, H. Hu, J. Chinese Ceram. Soc. 33 (12) (2005) 1494. [22] J. Zhang, J. Qiu, Y. Kawamoto, Mater. Lett. 55 (2002) 77. [23] S. Sebastian, M. Khadar, Bull. Mater. Sci. 27 (2) (2004) 207. [24] El Sayed Said Yousef, J. Mater. Sci. 42 (2007) 4502. [25] X. Feng, S. Tanabe, T. Hanada, J. Non-Cryst. Solids 281 (2001) 48. [26] S. Marjanovic, J. Toulous, H. Jain, C. Sandmann, V. Dierolf, A.R. Kortan, N. Kopylov, R.G. Ahrens, J. Non-Cryst. Solids 322 (2003) 311. [27] H.Li, Y. Su, S.K. Sundaram, J. Non-Cryst. Solids 293–295 (2001) 402. [28] D.L. Sidebottom, M.A. Hruschka, B.G. Potter, R.K. Brow, J. Non-Cryst. Solids 222 (1997) 282. [29] F.C. Cassanjes, Y. Messaddeq, L.F.C. de Oliveira, L.C. Courrol, L. Gomes, S.J.L. Ribeiro, J. Non-Cryst. Solids 247 (1999) 58. [30] Z. Pan, S.H. Morgan, J. Non-Cryst. Solids 210 (1997) 130.