Infrared Spectroscopic Characterization of Tellurite Glasses Containing Heavy Metal Oxides

The infrared (IR) spectra of (100-x)TeO 2 –xWO 3 glasses reveal that the glass network consists of [TeO 3 ]/[TeO 3+1 ], [TeO 4 ], [WO 4 ], and [WO 6 ] groups as basic structural units. Addition of WO 3 oxide to the binary TeO 2 –WO 3 glasses increases the amount of lower coordination of [TeO 3 ]/[TeO 3+1 ] units and decreases the higher coordination of [TeO 4 ] units and also the formation of Te–O–W linkages at the expense of Te–O–Te linkages. The IR spectra of 60TeO 2 –(40-x)WO 3 –xPbO glasses reveal that the glass network consists of [TeO 3 ], [TeO 4 ], [WO 4 ], [WO 6 ], and [PbO 4 ] units. Changes in the coordination state of tellurium and tungsten ions occur when the PbO and WO 3 concentrations are varied. The dual role of the lead ions is conﬁrmed in 60TeO 2 –(40-x)WO 3 –xPbO glass system. The W ion coordination state changes from 4 to 6 when WO 3 concentration increases beyond 30 mol% in both (100-x)TeO 2 –xWO 3 and 60TeO 2 –(40-x)WO 3 –xPbO glass systems. The IR spectra of 60B 2 O 3 –10TeO 2 –(30-x)ZnO–xPbO glasses reveal that the glass network consists of [TeO 3 ], [BO 3 ], and [BO 4 ] groups.


Introduction
Glasses with heavy metal oxides (TeO 2 , GeO 2 , Bi 2 O 3 , WO 3 , PbO, Ag 2 O, etc.) are promising materials for IR technologies, nonlinear optics, and design of laser devices [1]. Telluritebased glasses are the subject of intense current research because of the interesting electrical and optical properties. Main features include extended Infrared transmittance [2], high nonlinear optical indices [3], low fusion temperature, and they constitute an excellent matrix for active element doping, justifying a continuous technological interest [4]. The synthesis of glasses with high refractive index values is of great importance in the glass science and the optical industries. Among tellurite glasses, the glasses based on WO 3 , PbO, and other heavy-metal oxides (HMO) are known to have high linear refractive indices (n > 2.1) [5][6][7]. The high linear refractive index of Te +4 containing glasses is attributed to the nonbonding lone electron pair 5s 2 of tellurium [8].
The PbO is unique in its influence on the glass structure and is widely used in glasses because it enhances the resistance against devitrification, improves the chemical durability, and lowers the melting temperature [9,10]. PbO could act both as glass network former and as modifier depending on its concentration in the glasses [11,12]. B 2 O 3 is one of the best and well-known glass former. Addition of small amount of TeO 2 into the borate glass network enhances the glass quality with an improvement in transparency, refractive index. Addition of ZnO into the boro-tellurite glass network produces low rates of crystallization and increases the glass-forming ability [13]. Hence it is of interest to study the structural changes brought about by PbO in tungsto-tellurite and boro-tellurite glasses, which may help in predicting the physico-chemical properties.

Experimental
The tellurium-based glasses (100-x)TeO 2 -xWO 3 , 60TeO 2 -(40-x)WO 3 -xPbO, and 60B 2 O 3 -10TeO 2 -(30-x)ZnO-xPbO were prepared from 99.9% purity-grade oxides (Aldrich). Powders of TeO 2 , WO 3 , B 2 O 3 , ZnO, and PbO were weighted to get the required composition and ground in a mortar  B 2 O 3  ZnO  TW1  90  10  ---TW2  80  20  ---TW3  70  30  ---TW4  60  40  ---TWP1  60  40  0  --TWP2  60  30  10  --TWP3  60  20  20  --TWP4  60  10  30  --TWP5  60  0  40  --BTZP1  10  -0  60  30  BTZP2  10  -10  60  20  BTZP3  10  -20  60  10  BTZP4  10  -30  60  0 with a pestle for 1 hour to obtain homogeneous mixtures. Each batch was then transferred to a platinum crucible and melted at about 800-950 • C in an electric furnace. This melt was held at this temperature for 30 minutes until a bubble-free liquid was formed. The melts were stirred to achieve desirable homogeneity. The homogeneous melt was quenched by pouring it on to a preheated stainless steel mould to avoid excess thermal shocks. The glasses were annealed for 8 hours at 100 • C to relieve the mechanical strains. The compositions of the glass samples employed in the present study are given in Table 1. X-ray diffractograms of powdered glass samples were recorded using a copper target (λ(k α ) = 1.54 A • ) on a Philips PW (1140) diffractometer at room temperature. The IR spectra of the glass samples were recorded at room temperature using a Perkin-Elmer FT-IS spectrometer model 1605 using KBr disc technique. The investigated samples were ground to fine particles and then mixed with KBr in the ratio (0.002 : 0.2 g) glass to KBr, respectively. The weighted mixture was then subjected to a pressure of 5 tons/cm 2 . The transmission spectra were measured immediately after preparing the desired disks.

Results and Discussion
3.1. XRD and IR Spectra of (100-x)TeO 2 -xWO 3 Glasses. The X-ray diffraction spectra show no peaks, indicating that the samples are amorphous.
The IR spectra of (100-x)TeO 2 -xWO 3 glass system recorded in the wave number region 1150-440 cm −1 are shown in Figure 1. The IR spectra of these glasses are characterized by IR absorption bands in the wave number regions 925-945 cm −1 , 850-865 cm −1 , 730-790 cm −1 , 600-680 cm −1 , and 460-490 cm −1 . The IR band positions are summarized in Table 2. The IR absorption in the range 600-680 cm −1 in the tellurium containing glasses is due to stretching vibrations of the Te-O bond in the TeO 4 tbp (trigonal bipyramids) and TeO 3 tp (trigonal pyramids) units [14]. The band observed at 640 cm −1 in 90TeO 2 -10WO 3   [15][16][17]. The band at around 730-790 cm −1 is due to Te-O eq bond vibrations of distorted TeO 4 units [17,18]. The shoulder at 865 cm −1 is assigned to vibration of W-O-W linkages [19,20]. The IR band in the region 460-490 cm −1 is assigned to Te-O-W linkages, which would increase the glass network connectivity and this assignment is made in agreement with the theoretical model for vibrations of mixed bridge bonds containing heavy metal and glass former atoms [21]. The formation of Te-O-W linkages is expected because both W and Te atoms have comparable electro negativity and can therefore substitute for each other in bonding with O atoms [21]. That is, there is a fraction of W cations that have partial covalent bonding and are incorporated in the binary TeO 2 -WO 3 glass network [17]. Equimolecular substitution of WO 3 for TeO 2 causes changes in the structure of the TeO 2 -WO 3 glasses, which is apparent in the IR spectra of glasses ( Figure 1). As the WO 3 content increases from 10 mol% to 40 mol%, the band at 640 cm −1 in 90TeO 2 -10WO 3 (TW1) shifts towards higher wave numbers: from to 640 to 645, 665, and then to 675 cm −1 , while the band at 730 cm −1 shifts to 790 cm −1 . This observed shift may be due to higher field intensity of mixed Te-O-W and W-O-W linkages, in which the oxygen is highly polarized, compared with Te-O-Te linkages, since W 6+ -ions possess higher field intensity than Te 4+ -ions [18], and this observed shift may be related to the apparition of TeO 3 units concomitant to a reduction in the number of TeO 4 units [22]. Thus the effect of addition of WO 3 content to the TeO 2 glass matrix is to transform the TeO 4 basic structural units forming the TeO 2 glass to TeO 3 units. This is supported by the appearance of IR band in the region 665-680 cm −1 of 70TeO 2 -30WO 3 (TW3) and 60TeO 2 -40WO 3 (TW4) glass systems corresponding to TeO 3 units.
The shoulder at about 850-865 cm −1 relative to the existence of vibration of W-O-W linkages is present for all the glass samples. As the WO 3 content increases from 10 to 40 mol%, this causes a shift in the absorption band at 925 cm −1 due to WO 4 Table 3. The band in the region 670-680 cm −1 in 60TeO 2 -40WO 3 (TWP1) glass is assigned to  6 ] units in the structure of the glass [17]. The shoulder at around 810-850 cm −1 is due to vibration of W-O-W linkages and the band in the region 460-490 cm −1 is assigned to Te-O-W linkages [17]. Equimolecular substitution of PbO for WO 3 causes changes in the structure of the TWP glasses, which is apparent in the IR spectra of glasses ( Figure 2). As the PbO content increases from 10 to 30 mol%, this causes a shift in the IR band due to W-O-W linkages towards lower wave numbers: from 850 to 843, 820, and then to 810 cm −1 and also causes a shift in the band 945 cm −1 due to WO 6 octahedra toward lower wave numbers: from 945 to 929, 911 and then to 894 cm −1 . The observed shift in the 945 cm −1 band and in the band 850 cm −1 toward lower wave number in the composition range from 10 to 30 mol% PbO may be an indication of the transformation of WO 6 units to WO 4 units. This receives support from the appearance of band at 894 cm −1 in 60TeO 2 -30PbO-10WO 3 (TWP4) and is due to WO 4 units [23]. When WO 3 is substituted mol by mol by PbO the number of oxygens in the glass network diminishes according to the ratio 3/1. Thus the glass network becomes less distorted and this also suggests that the formation of WO 4  Therefore, for PbO up to 20-mol%, it plays a network modifier role, and at x ≥ 30, it plays a network former role in the present system. The study performed clarified the structural species of lead-tungsten tellurite glasses and confirmed the dual structural role of lead ions.
The IR band absorption in the high wave number region of the spectrum with maximum in the 3436-3450 cm −1 region belongs to O-H stretching vibrations [26]. In the present glass system (BTZP), the disappearance of the absorption band at 806 cm − [14,30]. Analyzing the obtained results and  -668  1022  1333  3448  BTZP2  -668  1081  1344  3448  BTZP3  458  687  1085  1342  3448  BTZP4  458  667  1022  1342  3450 comparing them to the published data [12], it is clear that trigonal