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Raman spectroscopic characterization of tellurite glasses containing heavy metal oxides
ARTICLE IN PRESS Physica B 405 (2010) 1269–1273
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Raman spectroscopic characterization of tellurite glasses containing heavy metal oxides G. Upender a, Vasant G. Sathe b, V. Chandra Mouli a, a b
Glassy Materials Research Laboratory (GMRL), Department of Physics, Osmania University, Hyderabad 500 007, Andhra Pradesh, India Consortium of Scientific Research, Khandwa Road, Indore 452 017, India
a r t i c l e in fo
abstract
Article history: Received 29 June 2009 Received in revised form 9 October 2009 Accepted 23 November 2009
Raman spectroscopy was used for the characterization of TW (100 x)TeO2 xWO3, TWP 60TeO2– (40 x)WO3–xPbO and TAW (85 x)TeO2–15Ag2O–xWO3 system. The Raman spectra of the glasses were interpreted in terms of the structural transformations produced by the modifiers. The main results show the progressive transformation of trigonal bipyramids TeO4 to TeO3 + 1 polyhedron for TeO2–WO3 glasses with increasing WO3 content, the Te environment changes from trigonal bipyramids TeO3 to TeO3 + 1 polyhedral for TeO2–Ag2O–WO3 glasses with increasing WO3 content. On the other hand, the Te environment changes from TeO3/TeO3 + 1 to TeO4 for TeO2–WO3–PbO glasses with increasing PbO content. Raman measurements reveal the dual role of lead atoms in the ternary system TeO2–WO3– PbO. WO3 addition was found to promote the formation of TeO3/TeO3 + 1 units and WO6 units in all the glasses at the expense of TeO4 units and WO4 units. Results show that in all the systems, the coordination state of W ion change from 4 to 6 when WO3 concentration increases beyond 30 mol%. & 2009 Elsevier B.V. All rights reserved.
Keywords: Tellurite glasses Glass structure Raman spectroscopy Heavy metals
1. Introduction Glasses with heavy metal oxides (TeO2, GeO2, Bi2O3, WO3, PbO, Ag2O, etc.) are promising materials for IR technologies, non-linear optics and design of laser devices [1]. Tellurite-based glasses are the subject of intense current research because of the interesting electrical and optical properties. Main features include extended Infrared transmittance [2], high non-linear 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. Tungsten oxide-tellurite glasses have been obtained showing an extremely high refractive index with low dispersion value, low crystallization ability and good chemical resistance [5,6]. The high non-linear refractive index of Te + 4 containing glasses is attributed to the non-bonding lone electron pair 5s2, of tellurium [7]. For these reasons, TeO2–WO3 glasses have become the subject of several investigations. It is well known that the addition of network modifying oxide (Ag2O) to TeO2 glass, results in the breakage of Te–O–Te linkages, thereby resulting in systematic conversion of TeO4 to TeO3 structural units [8,9]. Due to its unique structural configurations, TeO2 is unique different from other
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glass forming oxides like SiO2, B2O3 and P2O5, etc. Hence it is of interest to study the structural changes brought about by heavy metal oxides (WO3, PbO, Ag2O) incorporation in TeO2 glass, which may help in predicting the physico-chemical properties of these glasses. The aim of the present work was to study the short range structure and structural changes with composition of (100 x)TeO2 xWO3, 60TeO2–(40–x)WO3–xPbO and (85 x) TeO2–15Ag2O–xWO3 glass systems by Raman spectroscopy. These systems are of interest because WO3 is one of the transition metal oxides and is also a conditional glass former, whose structure is formed by WO4 and WO6 units and PbO has ability to form stable glasses and it stands out as unique because of its dual role one as modifier, if Pb–O is ionic and the other as glass former with PbO4 structural units, if Pb–O is covalent [10].
2. Experimental The tellurium-based glasses (100 x)TeO2–xWO3, 60TeO2– (40 x) WO3 x PbO and (85 x)TeO2–15Ag2O–xWO3 were prepared from 99.9% purity-grade oxides (Aldrich). Powders of TeO2, WO3, PbO and Ag2O were weighted to get the required composition and ground in a mortar with a pestle for 1 h to obtain homogeneous mixtures. Each batch was then transferred to a platinum crucible and melted at about 800–950 1C in an electric furnace. This melt was held at this temperature for 30 min until a bubble free liquid was formed. The melts were stirred to achieve desirable homogeneity. The homogeneous melt was
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Table 1 Glass compositions. Oxides (mol%) Samples
TeO2
WO3
PbO
Ag2O
TW1 TW2 TW3 TW4 TWP1 TWP2 TWP3 TWP4 TWP5 TAW1 TAW2 TAW3 TAW4 TAW5
90 80 70 60 60 60 60 60 60 85 75 65 55 45
10 20 30 40 40 30 20 10 0 0 10 20 30 40
– – – – 0 10 20 30 40 – – – – –
– – – – – – – – – 15 15 15 15 15
rapidly quenched by pouring it on to a preheated stainless steel mould to avoid excess thermal shocks. The glasses were annealed for 8 h at 100 1C 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 reco˚ on a Philips PW (1140) rded using a copper target (ka =1.54 A) diffractometer at room temperature. Raman spectra of the present glass samples were recorded at room temperature using JOBIN–YVON HORIBA (LABRAM HR-800), spectrometer in the wavelength range 200–1100 cm 1. The excitation wavelength used was 488 nm (Ar + laser), with a power of 10 mW. The incident laser power is focused in a diameter of 1–2 mm and a notch filter is used to suppress Rayleigh light. Samples used for the measurement were of 1 mm thickness and 1 cm in diameter. Raman shifts are measured with a precision of 0.3 cm 1 and the spectra resolution is of the order 1 cm 1.
Fig. 1. XRD spectra of the samples TW1, TW3, TAW1, TAW3, TWP1 and TWP3.
3. Results and discussion 3.1. XRD and Raman spectra 3.1.1. (100 x)TeO2–xWO3 glasses The X-ray diffraction spectra (Fig. 1) show no peaks, indicating that the samples are amorphous. Fig. 2 shows the Raman spectra of the glass samples (100 x)TeO2–xWO3 with 10 rxr40 mol% and crystalline a-TeO2. The assignments of Raman bands of glass samples are given in Table 2 and Raman band positions are summarized in Table 3. They are characterized by five bands: three medium bands around 355, 460 and 745 cm 1 and two strong bands around 663–730 and 925–930 cm 1. The Raman spectrum of crystalline a-TeO2 (Fig. 2) displays well resolved bands at 390, 589 and 643 cm 1. By comparison with a-TeO2 and other references [9,11,12], it is clear from Fig. 2 that the strongest sharp band observed at 643 cm 1 in the Raman spectrum of crystalline a-TeO2 has broadened and shifted to higher frequencies ( = 663 cm 1) in the spectra of TW1 glass. This broadening could be related to chemical perturbation of the vibrational energies arising from glass former-glass modifier binding and this broadening is a result of the distribution of bond angles and average nearest neighbor distances in the glass matrix. The strong band at 663 cm 1 and shoulder around 745 cm 1 (Fig. 2) for sample TW1 correspond to the vibrational oscillations of the TeO4 tetrahedra in the glass matrix [12,13]. It is known from the literature that TeO2 crystalline samples exist in two forms, the tetragonal a-TeO2 and orthorhombic b-TeO2. Both
Fig. 2. Raman spectra of TeO–WO3 glasses and crystalline a-TeO2.
Table 2 Assignments of Raman bands of glasses. Raman band (cm 1) observed in the glass
Assignment
890–930
Stretching vibrations of W–O and WQO bonds in WO4 or WO6 units Stretching vibrations of W–O–W in WO4 or WO6 units Bending vibrations of W–O–W in WO6 units Stretching vibrations of TeO3/TeO3 + 1 units Stretching vibrations of TeO4 tbp Stretching vibrations of TeO4 tbp Stretching vibrations of Te–O–W or Te–O–Te linkages Vibrations of Pb–O in PbO4 units
820–870 340–360 720–735 745–760 650–670 440–490 320–338
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Table 3 Observed Raman band positions in the (100 errors 71 cm 1. Sample ID
Raman bands (cm
TW1 TW2 TW3 TW4
355 355 355 355
460 465 472 479
x)TeO2–xWO3 glass system, with
1
) 663 668 – –
745 743 701 730
845 849 855 858
925 926 928 930
structures can be described by a network made up of TeO4 tbps, which consist of two equatorial and two axial oxygen atoms [14]. The trigonal bipyramids (TeO4) are deformed where the Te atom is not at the center of the equatorial plane and third position in this plane is occupied with the free electron pair of the Te atom. The local structure of tellurite glasses is similarly built up of TeO4 tbps. As shown in Fig. 2 the band at 663 cm 1 and the shoulder around 745 cm 1 gradually overlap as WO3 concentration increases from 10 to 30 mol% (TW1–TW3). As WO3 concentration reaches 30 mol% or more (TW4), the broad band at 663 cm 1 and a small shoulder at 745 cm 1 does not appear independently but rather is merged and form a new band at 730 cm 1 (TW4). The Raman band at 730 cm 1 is due to the vibrational mode of the TeO3 tp or TeO3 + 1 units that dominate the structural units of the 60TeO2–40WO3 sample (Fig. 2). We now return to the assignment of the rest of the Raman bands of the glasses shown in Fig. 2. The band at around 355 cm 1 is due to the bending vibrations of W–O–W in the WO6 units [15]. As shown in Fig. 2, the weak band at 355 cm 1 increases gradually as WO3 concentration up to 30 mol%. The band at around 460 cm 1 is suggested to be the stretching vibrations in Te–O–W linkages and the formation of Te–O–W linkages is expected because both W and Te atoms have comparable electro negativity values and can therefore substitute for each other in bonding with O atoms [16]. However, the band around 460 cm 1 was observed in many tellurite glasses and is most often assigned to bending vibrations of Te–O–Te linkages. The band around 925 cm 1 is assigned to the symmetric stretching vibrations of W–O and WQO bonds associated with WO4 or WO6 units [9,12,17]. The Raman band at 845 cm 1 is assigned to stretching vibrations of W–O–W in WO4 or WO6 units [9,12,17]. It is clear from Fig. 2 that the relative intensity of the bands around 925 cm 1 increases with respect to the prominent band at 663 cm 1 as WO3 content increases from 10 to 30 mol%. Since the intensity of the Raman bands is proportional to the number of scattering units and their scattering efficiency (Raman cross-section), it may be concluded that the number of WO4 tetrahedra increase as WO3 concentration increase up to 30 mol% (TW1–TW3). As shown in Fig. 2, the relative intensity of the band around 925 cm 1 increases and shifts from 925 to 930 cm 1 as WO3 concentration increases above 30 mol% (TW4 glass) and the band at 845 cm 1 shifts to higher wave numbers; from 845 to 858 cm 1. Also, as WO3 concentration increases the broad bands around 460 cm 1 gradually decreases and shifts from 460 to 479 cm 1 and in addition a band around 355 cm 1 decreases and become very weak when WO3 concentration reaches 40 mol% (TW4 glass). All these observations indicate that the W ion coordination state changes from four-fold coordination [WO4] with 10 rxr30 mol% (TW1–TW3 glasses) to six-fold coordination [WO6] with x Z30 mol%. Although W and Te ions have approximately the same electronegativity values, W ions possess a higher electronic polarizability (ionic charge/radius) than the Te ions and therefore, when xZ30 mol%, more oxygen can be attracted to W ions which in turn convert the WO4 units to WO6 units. Addition of WO3 content above 20 mol% brings about a
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shift in the Raman band at 663 cm 1 towards the 730 cm 1, which indicates on increasing WO3 concentration, tellurium ion coordination state changes from four-fold coordination with 10 rxr20 mol% to three-fold or 3+ 1 coordination with xZ30. As shown in Fig. 2, on increasing WO3 concentration, a gradual broadening and overlapping of the respective bands are observed. This reflects the gradual incorporation of the constituent polyhedra and complexes (TeO3, WO4, WO6) forming the glass network. 3.1.2. 60TeO2–(40 x)WO3–xPbO glasses Fig. 3 shows the Raman spectra of the glass samples 60TeO2– (40 x)WO3–xPbO with 0rx Z40 mol%. The assignments of Raman bands of TeO2–WO3–PbO glasses are given in Table 2 and observed Raman band positions for all the compositions are summarized in Table 4. The Raman spectrum of 60TeO2–40WO3 (TWP1) glass consists of five bands at 930, 858, 730, 479 and 355 cm 1. The Raman studies on tungsten tellurite glasses have shown that observed band around 930 cm 1 is assigned to symmetric stretching vibrations of W–O and WQO bonds associated with WO6 units, the Raman band at 858 cm 1 is assigned to stretching vibrations of W–O–W in WO4 or WO6 units [9,12,17]. The band around 730 cm 1 is typical for tellurite glasses prepared from TeO2 and heavy metal oxides and the band around 730 cm 1 is assigned to symmetric stretching vibrations between Te and non-bridging oxygen (NBO) in TeO3 + 1 units (The TeO3 + 1 unit can be thought of a distorted trigonal bipyramidal (TeO4) unit with one oxygen further away from the central tellurium than the remaining three oxygens) or possibly to stretching mode of TeO3 unit [18] most likely to TeQO bond stretching in OQTeO2 units
Fig. 3. Raman spectra of TeO2–WO3–PbO glasses.
Table 4 Observed Raman band positions in the 60TeO2–(40 with errors 7 1 cm 1. Sample ID
Raman bands (cm
TWP1 TWP2 TWP3 TWP4 TWP5
355 348 343 336 320
479 475 471 456 445
x)WO3–xPbO glass system,
1
) – – 658 660 662
730 729 730 731 730
858 853 842 831 –
930 922 907 895 –
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[17]. The Raman band observed at 479 cm 1 can be attributed to the stretching vibrations of Te–O–W or Te–O–Te linkages. The band at 355 cm 1 represents the characteristic bonds of tungsten glasses which may be attributed to bending vibrations of W–O–W in WO6 units [9,12,17]. Equimolar substitution of WO3 by PbO causes significant changes in the Raman spectra arising from tungsten network and tellurite network. It can be seen from Fig. 3 that the intensity of the bands at around 479–445 cm 1 decreases and shift in the Raman band from 479 (TWP1) to 445 cm 1 (TWP5) with the concentration of PbO up to 40 mol%, this indicates that Te–O–W or Te–O–Te linkages are broken and the formation of some Te–O–Pb linkages at the expense of Te–O–Te or Te–O–W linkages [19,20]. As the PbO content increases from 10 to 20 mol%, as shown in Fig. 3, the band at 355 cm 1 (TWP1) shifts to 343 cm 1 (TWP3) and the Raman band shifts from 343 to 320 cm 1 as PbO content increases from 30 to 40 mol%, furthermore the intensity of this band increases with increasing PbO content from 10 to 40 mol% indicating that that PbO acts both as a network former and network modifier, depending up on its relative concentration. This receives support from the appearance of the Raman band near 320–336 cm 1 and is due to Pb–O vibration in PbO4 units [21]. By substitution of PbO for WO3, as shown in Fig. 3, the intensity of band at 730 cm 1 decreases with increasing PbO content, when the PbO content increases from 10 to 20 mol%, a new band at around 658 cm 1 (TWP3) is observed in addition to that of 730 cm 1 band, which indicates the decrease of Te–O bonds with non-bridging oxygens (NBO) and the formation of Te–O–Pb bonds in the glass network and the small shoulder at around 658–662 cm 1 is assigned to symmetric stretch of Te–O in TeO4 units or to Te–O–Te linkage between two fourfold coordinated Te atoms [22]. The intensity of shoulder at 658 cm 1 increase at the expense of the band at 730 cm 1, but this band (730 cm 1) does not vary its position with PbO content. This suggests that the PbO up to 20 mol% plays a network modifier role and at x Z30 it plays a network former role in the present system. At low proportion of PbO (up to 20 mol%), it enters the glass network by breaking up the Te–O–Te, Te–O–W bonds (normally the oxygen of PbO break the local symmetry while Pb2 + ions occupy interstitial positions) and introduces coordinate defects known as dangling bonds along with non-bridging oxygen ions (Te–O yPb2 + y O–Te) which in turn neutralizes the negative charge of non-bridging oxygen atoms and decrease of Te–O bonds with non-bridging oxygens (NBO) by forming TeO4 units [22], while as PbO increases (from 30 to 40 mol%), a considerable proportion may be acts as a double bridges between adjacent TeO4 and WO4 units such as QTe–O–Pb–O–WQ which can be formed beside the formation of PbO4 units [23]. Therefore, for PbOZ30 mol%, Pb2 + acts as glass forming agent and is incorporated in the glass structure in the form of PbO4 units. As the PbO content increases from 10 to 30 mol% causes a shift in the Raman band due to WO6 units toward lower frequencies; from 930 to 922, 907 and then to 895 cm 1 and the intensity of the band around 930–895 cm 1 region decreases as PbO content increases upto 30 mol% .The observed shift in the 930 cm 1 band toward lower wave numbers and variation in the intensity of the bands is may be an indication of the transformation of WO6 units to WO4 units. This receives support from the appearance of band at 895 cm 1 in 60TeO2–10WO3–30PbO (TWP4) and is due to WO4 units [9]. The variation of intensity of the band at around 858– 831 cm 1 and shift in the Raman band from 858 (TWP1) to 831 (TWP4) with the concentration of PbO upto 30 mol% can be understood as follows. When WO3 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 the formation of WO4 units at high PbO content. That is why the band at 858–831 cm 1 and the
band at 930–895 cm 1 shift towards lower wave numbers with the addition of PbO content. 3.1.3. (85 x)TeO2–15Ag2O–xWO3 glasses The Raman spectra of the (85 x)TeO2–15Ag2O–xWO3 glasses are shown in Fig. 4 The assignments of Raman bands are given in Table 2 and observed Raman band positions for all the compositions are summarized in Table 5. The Raman spectrum of TAW1 (85TeO2–15Ag2O) glass consists of two bands at 689 and 460 cm 1. The Raman studies on silver tellurite glasses have shown that the observed band around 689 cm 1 is due to vibrational mode of the TeO3 tp units that dominate the structural units of the TAW1 sample and the band around 460 cm 1 is assigned to bending vibrations of Te–O–Te linkages [9,12,17]. It can be seen from Fig. 4 that, the addition of WO3 results in a new strong band around 907 cm 1, a band around 824 cm 1 and a weak band at 344 cm 1 in addition to those bands around 689–701 cm 1 and 460–463 cm 1. The observed sharp band at 907 cm 1 and a weak band at 344 cm 1 represent the characteristic bands of tungsten glasses which can be attributed to symmetric stretching vibrations of W–O and WQO bonds in the WO4 or WO6 units and bending vibrations of W–O–W in the WO6 octahedra, respectively. The Raman band at 824 cm 1 is assigned to stretching vibrations of W–O–W in WO4 or WO6 units. The intensities of the bands observed at 344, 689 cm 1 and at 907 cm 1 increases as the WO3 content increases from 10 to 40 mol% with constant Ag2O (TAW2–TAW5), where as that of other peak at 460 cm 1 decrease as the WO3 content increases from 10 to 40 mol%. It can be seen from Fig. 4 that all the Raman bands move to higher wavenumbers with an
Fig. 4. Raman spectra of TeO2–Ag2O–WO3 glasses.
Table 5 Observed Raman band positions in the (85 with errors 7 1 cm 1. Sample ID
Raman bands (cm
TAW1 TAW2 TAW3 TAW4 TAW5
– 344 348 350 352
460 463 471 477 482
x)TeO2–15Ag2O–xWO3 glass system,
1
) 689 701 714 728 739
– 824 829 834 841
– 907 908 910 915
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increase in the WO3 content. It is also noticeable that band at 460 cm 1 is fairly strong for all the samples (TAW1–TAW5). The position and intensity of the band at 689 cm 1 is sensitive to the type and concentration of the metal oxides. In Li2O–TeO2 glasses [24], Raman bands due to TeO4 tbp and TeO3 tp were found at 770 and 670 cm 1, respectively, but in our work no band corresponding to TeO4 tbp units is observed, moreover the band corresponding to TeO3 tp units observed at 689 cm 1 which is at higher Raman wavenumber than that observed for the TeO3 tp band of the Li2O–TeO2 glasses. We attribute the disappearance of the band due to TeO4 units and the appearance of the band due to TeO3 tp units at higher wavenumbers due to the influence of Ag + ions on the tellurite network. The Ag + ions break the Te–eqOax–Te bridging bonds forming TeO2 glass network and form the Te–eqO– Ag + or Te–axO–Ag + species and also Ag + ions break the tungsten network by forming W–O–Ag + species [9,15,25]. All these species prevents the formation of TeO4 tbp units in all samples (TAW1– TAW5). The intensity of the band at 460 cm 1 decrease with an increase in WO3 content as expected since there are fewer Te–O– Te linkages and more Te–O–W linkages [26]. As shown in Fig. 4, the intensity of the band at 907 cm 1 due to WO4 units increases as WO3 concentration increases from 10 to 30 mol%, it may be concluded that the number of WO4 units increases as WO3 concentration increases and reaches a maximum at 30 mol% (TAW2–TAW4). As the WO3 concentration increases from 30 to 40 mol%, the intensity the band at 907 cm 1 increases while this band is shifted to higher frequencies (915 cm 1). The intensity of the band at 344 cm 1 increases and it shifts to 352 cm 1 as the WO3 concentration increases from 10 to 40 mol% (Fig. 4). For constant Ag2O, the substitution of TeO2 by WO3 causes an increase of networking oxygens, according to the proportion 3/2 and these oxygens form new Te–O–W and W–O– W linkages [26]. All these observations suggest that the W ion coordination state changes from tetrahedral coordination (WO4) with 10 rx r30 mol% (TAW2–TAW4) to octahedral coordination (WO6) with x Z30 mol (TAW5). Formation of WO6 octahedra groups in the structure of the glasses causes a change in the coordination state of tellurium ions and their partial conversion from 3 coordination (TeO3) to 3+ 1 coordination (TeO3 + 1). On the other hand, W6 + is a heavy metal ion; therefore its addition to the glass may distort and strain the TeO2 network. The TeO3 tp is distorted into a TeO3 + 1 polyhedron because of the formation of W–O–Te linkages. This is supported by the appearance of Raman bands in the 728–740 cm 1 region corresponding to TeO3 + 1 polyhedron. As the WO3 content increases, the major band shifts from 689 cm 1 (x= 0) to 739 cm 1 (x = 40). This may be related to the increase of number of TeO3 + 1 units at the expense of number of TeO3 units. 4. Conclusions Transparent and stable glasses were obtained in the TeO2– WO3, TeO2–WO3–PbO and TeO2–Ag2O–WO3 systems. The Raman spectra of TeO2–WO3 glasses reveal that the glass network consists of TeO4, [TeO3]/[TeO3 + 1], WO4 and WO6 units. The Raman spectra of TeO2–WO3–PbO glasses reveal that the glass network consists of TeO4, [TeO3]/[TeO3 + 1], WO4 and WO6 and PbO4 units. The Raman spectra of TeO2–Ag2O–WO3 glasses reveal
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that the glass network consists of TeO3, TeO3 + 1, WO4 and WO6 units. Addition of WO3 increases the population of lower coordination units [TeO3]/[TeO3 + 1] in the glass network at the expense of higher coordination units [TeO4]. In the case of TeO2– WO3–PbO glasses, the TeO3/TeO3 + 1 units are transformed to TeO4 units with increasing PbO content and the Raman band around 320–336 cm 1 is due to Pb–O vibration in PbO4 units. Addition of WO3 to all the glass systems results in the formation of W–O–W and W–O–Te linkages, while that of the Te–O–Te linkages decreases as WO3 increases. The formation of Te–O–W linkages increases the connectivity of the glass network. Tungsten atoms participate in all the glasses as WO4 units up to the WO3 concentration r30 mol%. With the increase in the WO3 content beyond 30 mol% tungsten atoms participate in all the glasses as WO6 units.
Acknowledgments One of the authors, G. Upender is grateful to UGC (University Grants Commission), New Delhi, for providing financial assistance under the scheme of RFSMS (Research Fellowships in Science for Meritorious Students) and the authors also thank Mr. Manoj (CSR, Indore) for his help in recording Raman measurements.
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Physica B journal homepage: www.elsevier.com/locate/physb
Raman spectroscopic characterization of tellurite glasses containing heavy metal oxides G. Upender a, Vasant G. Sathe b, V. Chandra Mouli a, a b
Glassy Materials Research Laboratory (GMRL), Department of Physics, Osmania University, Hyderabad 500 007, Andhra Pradesh, India Consortium of Scientific Research, Khandwa Road, Indore 452 017, India
a r t i c l e in fo
abstract
Article history: Received 29 June 2009 Received in revised form 9 October 2009 Accepted 23 November 2009
Raman spectroscopy was used for the characterization of TW (100 x)TeO2 xWO3, TWP 60TeO2– (40 x)WO3–xPbO and TAW (85 x)TeO2–15Ag2O–xWO3 system. The Raman spectra of the glasses were interpreted in terms of the structural transformations produced by the modifiers. The main results show the progressive transformation of trigonal bipyramids TeO4 to TeO3 + 1 polyhedron for TeO2–WO3 glasses with increasing WO3 content, the Te environment changes from trigonal bipyramids TeO3 to TeO3 + 1 polyhedral for TeO2–Ag2O–WO3 glasses with increasing WO3 content. On the other hand, the Te environment changes from TeO3/TeO3 + 1 to TeO4 for TeO2–WO3–PbO glasses with increasing PbO content. Raman measurements reveal the dual role of lead atoms in the ternary system TeO2–WO3– PbO. WO3 addition was found to promote the formation of TeO3/TeO3 + 1 units and WO6 units in all the glasses at the expense of TeO4 units and WO4 units. Results show that in all the systems, the coordination state of W ion change from 4 to 6 when WO3 concentration increases beyond 30 mol%. & 2009 Elsevier B.V. All rights reserved.
Keywords: Tellurite glasses Glass structure Raman spectroscopy Heavy metals
1. Introduction Glasses with heavy metal oxides (TeO2, GeO2, Bi2O3, WO3, PbO, Ag2O, etc.) are promising materials for IR technologies, non-linear optics and design of laser devices [1]. Tellurite-based glasses are the subject of intense current research because of the interesting electrical and optical properties. Main features include extended Infrared transmittance [2], high non-linear 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. Tungsten oxide-tellurite glasses have been obtained showing an extremely high refractive index with low dispersion value, low crystallization ability and good chemical resistance [5,6]. The high non-linear refractive index of Te + 4 containing glasses is attributed to the non-bonding lone electron pair 5s2, of tellurium [7]. For these reasons, TeO2–WO3 glasses have become the subject of several investigations. It is well known that the addition of network modifying oxide (Ag2O) to TeO2 glass, results in the breakage of Te–O–Te linkages, thereby resulting in systematic conversion of TeO4 to TeO3 structural units [8,9]. Due to its unique structural configurations, TeO2 is unique different from other
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E-mail address: [email protected] (V.C. Mouli). 0921-4526/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2009.11.063
glass forming oxides like SiO2, B2O3 and P2O5, etc. Hence it is of interest to study the structural changes brought about by heavy metal oxides (WO3, PbO, Ag2O) incorporation in TeO2 glass, which may help in predicting the physico-chemical properties of these glasses. The aim of the present work was to study the short range structure and structural changes with composition of (100 x)TeO2 xWO3, 60TeO2–(40–x)WO3–xPbO and (85 x) TeO2–15Ag2O–xWO3 glass systems by Raman spectroscopy. These systems are of interest because WO3 is one of the transition metal oxides and is also a conditional glass former, whose structure is formed by WO4 and WO6 units and PbO has ability to form stable glasses and it stands out as unique because of its dual role one as modifier, if Pb–O is ionic and the other as glass former with PbO4 structural units, if Pb–O is covalent [10].
2. Experimental The tellurium-based glasses (100 x)TeO2–xWO3, 60TeO2– (40 x) WO3 x PbO and (85 x)TeO2–15Ag2O–xWO3 were prepared from 99.9% purity-grade oxides (Aldrich). Powders of TeO2, WO3, PbO and Ag2O were weighted to get the required composition and ground in a mortar with a pestle for 1 h to obtain homogeneous mixtures. Each batch was then transferred to a platinum crucible and melted at about 800–950 1C in an electric furnace. This melt was held at this temperature for 30 min until a bubble free liquid was formed. The melts were stirred to achieve desirable homogeneity. The homogeneous melt was
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Table 1 Glass compositions. Oxides (mol%) Samples
TeO2
WO3
PbO
Ag2O
TW1 TW2 TW3 TW4 TWP1 TWP2 TWP3 TWP4 TWP5 TAW1 TAW2 TAW3 TAW4 TAW5
90 80 70 60 60 60 60 60 60 85 75 65 55 45
10 20 30 40 40 30 20 10 0 0 10 20 30 40
– – – – 0 10 20 30 40 – – – – –
– – – – – – – – – 15 15 15 15 15
rapidly quenched by pouring it on to a preheated stainless steel mould to avoid excess thermal shocks. The glasses were annealed for 8 h at 100 1C 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 reco˚ on a Philips PW (1140) rded using a copper target (ka =1.54 A) diffractometer at room temperature. Raman spectra of the present glass samples were recorded at room temperature using JOBIN–YVON HORIBA (LABRAM HR-800), spectrometer in the wavelength range 200–1100 cm 1. The excitation wavelength used was 488 nm (Ar + laser), with a power of 10 mW. The incident laser power is focused in a diameter of 1–2 mm and a notch filter is used to suppress Rayleigh light. Samples used for the measurement were of 1 mm thickness and 1 cm in diameter. Raman shifts are measured with a precision of 0.3 cm 1 and the spectra resolution is of the order 1 cm 1.
Fig. 1. XRD spectra of the samples TW1, TW3, TAW1, TAW3, TWP1 and TWP3.
3. Results and discussion 3.1. XRD and Raman spectra 3.1.1. (100 x)TeO2–xWO3 glasses The X-ray diffraction spectra (Fig. 1) show no peaks, indicating that the samples are amorphous. Fig. 2 shows the Raman spectra of the glass samples (100 x)TeO2–xWO3 with 10 rxr40 mol% and crystalline a-TeO2. The assignments of Raman bands of glass samples are given in Table 2 and Raman band positions are summarized in Table 3. They are characterized by five bands: three medium bands around 355, 460 and 745 cm 1 and two strong bands around 663–730 and 925–930 cm 1. The Raman spectrum of crystalline a-TeO2 (Fig. 2) displays well resolved bands at 390, 589 and 643 cm 1. By comparison with a-TeO2 and other references [9,11,12], it is clear from Fig. 2 that the strongest sharp band observed at 643 cm 1 in the Raman spectrum of crystalline a-TeO2 has broadened and shifted to higher frequencies ( = 663 cm 1) in the spectra of TW1 glass. This broadening could be related to chemical perturbation of the vibrational energies arising from glass former-glass modifier binding and this broadening is a result of the distribution of bond angles and average nearest neighbor distances in the glass matrix. The strong band at 663 cm 1 and shoulder around 745 cm 1 (Fig. 2) for sample TW1 correspond to the vibrational oscillations of the TeO4 tetrahedra in the glass matrix [12,13]. It is known from the literature that TeO2 crystalline samples exist in two forms, the tetragonal a-TeO2 and orthorhombic b-TeO2. Both
Fig. 2. Raman spectra of TeO–WO3 glasses and crystalline a-TeO2.
Table 2 Assignments of Raman bands of glasses. Raman band (cm 1) observed in the glass
Assignment
890–930
Stretching vibrations of W–O and WQO bonds in WO4 or WO6 units Stretching vibrations of W–O–W in WO4 or WO6 units Bending vibrations of W–O–W in WO6 units Stretching vibrations of TeO3/TeO3 + 1 units Stretching vibrations of TeO4 tbp Stretching vibrations of TeO4 tbp Stretching vibrations of Te–O–W or Te–O–Te linkages Vibrations of Pb–O in PbO4 units
820–870 340–360 720–735 745–760 650–670 440–490 320–338
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Table 3 Observed Raman band positions in the (100 errors 71 cm 1. Sample ID
Raman bands (cm
TW1 TW2 TW3 TW4
355 355 355 355
460 465 472 479
x)TeO2–xWO3 glass system, with
1
) 663 668 – –
745 743 701 730
845 849 855 858
925 926 928 930
structures can be described by a network made up of TeO4 tbps, which consist of two equatorial and two axial oxygen atoms [14]. The trigonal bipyramids (TeO4) are deformed where the Te atom is not at the center of the equatorial plane and third position in this plane is occupied with the free electron pair of the Te atom. The local structure of tellurite glasses is similarly built up of TeO4 tbps. As shown in Fig. 2 the band at 663 cm 1 and the shoulder around 745 cm 1 gradually overlap as WO3 concentration increases from 10 to 30 mol% (TW1–TW3). As WO3 concentration reaches 30 mol% or more (TW4), the broad band at 663 cm 1 and a small shoulder at 745 cm 1 does not appear independently but rather is merged and form a new band at 730 cm 1 (TW4). The Raman band at 730 cm 1 is due to the vibrational mode of the TeO3 tp or TeO3 + 1 units that dominate the structural units of the 60TeO2–40WO3 sample (Fig. 2). We now return to the assignment of the rest of the Raman bands of the glasses shown in Fig. 2. The band at around 355 cm 1 is due to the bending vibrations of W–O–W in the WO6 units [15]. As shown in Fig. 2, the weak band at 355 cm 1 increases gradually as WO3 concentration up to 30 mol%. The band at around 460 cm 1 is suggested to be the stretching vibrations in Te–O–W linkages and the formation of Te–O–W linkages is expected because both W and Te atoms have comparable electro negativity values and can therefore substitute for each other in bonding with O atoms [16]. However, the band around 460 cm 1 was observed in many tellurite glasses and is most often assigned to bending vibrations of Te–O–Te linkages. The band around 925 cm 1 is assigned to the symmetric stretching vibrations of W–O and WQO bonds associated with WO4 or WO6 units [9,12,17]. The Raman band at 845 cm 1 is assigned to stretching vibrations of W–O–W in WO4 or WO6 units [9,12,17]. It is clear from Fig. 2 that the relative intensity of the bands around 925 cm 1 increases with respect to the prominent band at 663 cm 1 as WO3 content increases from 10 to 30 mol%. Since the intensity of the Raman bands is proportional to the number of scattering units and their scattering efficiency (Raman cross-section), it may be concluded that the number of WO4 tetrahedra increase as WO3 concentration increase up to 30 mol% (TW1–TW3). As shown in Fig. 2, the relative intensity of the band around 925 cm 1 increases and shifts from 925 to 930 cm 1 as WO3 concentration increases above 30 mol% (TW4 glass) and the band at 845 cm 1 shifts to higher wave numbers; from 845 to 858 cm 1. Also, as WO3 concentration increases the broad bands around 460 cm 1 gradually decreases and shifts from 460 to 479 cm 1 and in addition a band around 355 cm 1 decreases and become very weak when WO3 concentration reaches 40 mol% (TW4 glass). All these observations indicate that the W ion coordination state changes from four-fold coordination [WO4] with 10 rxr30 mol% (TW1–TW3 glasses) to six-fold coordination [WO6] with x Z30 mol%. Although W and Te ions have approximately the same electronegativity values, W ions possess a higher electronic polarizability (ionic charge/radius) than the Te ions and therefore, when xZ30 mol%, more oxygen can be attracted to W ions which in turn convert the WO4 units to WO6 units. Addition of WO3 content above 20 mol% brings about a
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shift in the Raman band at 663 cm 1 towards the 730 cm 1, which indicates on increasing WO3 concentration, tellurium ion coordination state changes from four-fold coordination with 10 rxr20 mol% to three-fold or 3+ 1 coordination with xZ30. As shown in Fig. 2, on increasing WO3 concentration, a gradual broadening and overlapping of the respective bands are observed. This reflects the gradual incorporation of the constituent polyhedra and complexes (TeO3, WO4, WO6) forming the glass network. 3.1.2. 60TeO2–(40 x)WO3–xPbO glasses Fig. 3 shows the Raman spectra of the glass samples 60TeO2– (40 x)WO3–xPbO with 0rx Z40 mol%. The assignments of Raman bands of TeO2–WO3–PbO glasses are given in Table 2 and observed Raman band positions for all the compositions are summarized in Table 4. The Raman spectrum of 60TeO2–40WO3 (TWP1) glass consists of five bands at 930, 858, 730, 479 and 355 cm 1. The Raman studies on tungsten tellurite glasses have shown that observed band around 930 cm 1 is assigned to symmetric stretching vibrations of W–O and WQO bonds associated with WO6 units, the Raman band at 858 cm 1 is assigned to stretching vibrations of W–O–W in WO4 or WO6 units [9,12,17]. The band around 730 cm 1 is typical for tellurite glasses prepared from TeO2 and heavy metal oxides and the band around 730 cm 1 is assigned to symmetric stretching vibrations between Te and non-bridging oxygen (NBO) in TeO3 + 1 units (The TeO3 + 1 unit can be thought of a distorted trigonal bipyramidal (TeO4) unit with one oxygen further away from the central tellurium than the remaining three oxygens) or possibly to stretching mode of TeO3 unit [18] most likely to TeQO bond stretching in OQTeO2 units
Fig. 3. Raman spectra of TeO2–WO3–PbO glasses.
Table 4 Observed Raman band positions in the 60TeO2–(40 with errors 7 1 cm 1. Sample ID
Raman bands (cm
TWP1 TWP2 TWP3 TWP4 TWP5
355 348 343 336 320
479 475 471 456 445
x)WO3–xPbO glass system,
1
) – – 658 660 662
730 729 730 731 730
858 853 842 831 –
930 922 907 895 –
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[17]. The Raman band observed at 479 cm 1 can be attributed to the stretching vibrations of Te–O–W or Te–O–Te linkages. The band at 355 cm 1 represents the characteristic bonds of tungsten glasses which may be attributed to bending vibrations of W–O–W in WO6 units [9,12,17]. Equimolar substitution of WO3 by PbO causes significant changes in the Raman spectra arising from tungsten network and tellurite network. It can be seen from Fig. 3 that the intensity of the bands at around 479–445 cm 1 decreases and shift in the Raman band from 479 (TWP1) to 445 cm 1 (TWP5) with the concentration of PbO up to 40 mol%, this indicates that Te–O–W or Te–O–Te linkages are broken and the formation of some Te–O–Pb linkages at the expense of Te–O–Te or Te–O–W linkages [19,20]. As the PbO content increases from 10 to 20 mol%, as shown in Fig. 3, the band at 355 cm 1 (TWP1) shifts to 343 cm 1 (TWP3) and the Raman band shifts from 343 to 320 cm 1 as PbO content increases from 30 to 40 mol%, furthermore the intensity of this band increases with increasing PbO content from 10 to 40 mol% indicating that that PbO acts both as a network former and network modifier, depending up on its relative concentration. This receives support from the appearance of the Raman band near 320–336 cm 1 and is due to Pb–O vibration in PbO4 units [21]. By substitution of PbO for WO3, as shown in Fig. 3, the intensity of band at 730 cm 1 decreases with increasing PbO content, when the PbO content increases from 10 to 20 mol%, a new band at around 658 cm 1 (TWP3) is observed in addition to that of 730 cm 1 band, which indicates the decrease of Te–O bonds with non-bridging oxygens (NBO) and the formation of Te–O–Pb bonds in the glass network and the small shoulder at around 658–662 cm 1 is assigned to symmetric stretch of Te–O in TeO4 units or to Te–O–Te linkage between two fourfold coordinated Te atoms [22]. The intensity of shoulder at 658 cm 1 increase at the expense of the band at 730 cm 1, but this band (730 cm 1) does not vary its position with PbO content. This suggests that the PbO up to 20 mol% plays a network modifier role and at x Z30 it plays a network former role in the present system. At low proportion of PbO (up to 20 mol%), it enters the glass network by breaking up the Te–O–Te, Te–O–W bonds (normally the oxygen of PbO break the local symmetry while Pb2 + ions occupy interstitial positions) and introduces coordinate defects known as dangling bonds along with non-bridging oxygen ions (Te–O yPb2 + y O–Te) which in turn neutralizes the negative charge of non-bridging oxygen atoms and decrease of Te–O bonds with non-bridging oxygens (NBO) by forming TeO4 units [22], while as PbO increases (from 30 to 40 mol%), a considerable proportion may be acts as a double bridges between adjacent TeO4 and WO4 units such as QTe–O–Pb–O–WQ which can be formed beside the formation of PbO4 units [23]. Therefore, for PbOZ30 mol%, Pb2 + acts as glass forming agent and is incorporated in the glass structure in the form of PbO4 units. As the PbO content increases from 10 to 30 mol% causes a shift in the Raman band due to WO6 units toward lower frequencies; from 930 to 922, 907 and then to 895 cm 1 and the intensity of the band around 930–895 cm 1 region decreases as PbO content increases upto 30 mol% .The observed shift in the 930 cm 1 band toward lower wave numbers and variation in the intensity of the bands is may be an indication of the transformation of WO6 units to WO4 units. This receives support from the appearance of band at 895 cm 1 in 60TeO2–10WO3–30PbO (TWP4) and is due to WO4 units [9]. The variation of intensity of the band at around 858– 831 cm 1 and shift in the Raman band from 858 (TWP1) to 831 (TWP4) with the concentration of PbO upto 30 mol% can be understood as follows. When WO3 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 the formation of WO4 units at high PbO content. That is why the band at 858–831 cm 1 and the
band at 930–895 cm 1 shift towards lower wave numbers with the addition of PbO content. 3.1.3. (85 x)TeO2–15Ag2O–xWO3 glasses The Raman spectra of the (85 x)TeO2–15Ag2O–xWO3 glasses are shown in Fig. 4 The assignments of Raman bands are given in Table 2 and observed Raman band positions for all the compositions are summarized in Table 5. The Raman spectrum of TAW1 (85TeO2–15Ag2O) glass consists of two bands at 689 and 460 cm 1. The Raman studies on silver tellurite glasses have shown that the observed band around 689 cm 1 is due to vibrational mode of the TeO3 tp units that dominate the structural units of the TAW1 sample and the band around 460 cm 1 is assigned to bending vibrations of Te–O–Te linkages [9,12,17]. It can be seen from Fig. 4 that, the addition of WO3 results in a new strong band around 907 cm 1, a band around 824 cm 1 and a weak band at 344 cm 1 in addition to those bands around 689–701 cm 1 and 460–463 cm 1. The observed sharp band at 907 cm 1 and a weak band at 344 cm 1 represent the characteristic bands of tungsten glasses which can be attributed to symmetric stretching vibrations of W–O and WQO bonds in the WO4 or WO6 units and bending vibrations of W–O–W in the WO6 octahedra, respectively. The Raman band at 824 cm 1 is assigned to stretching vibrations of W–O–W in WO4 or WO6 units. The intensities of the bands observed at 344, 689 cm 1 and at 907 cm 1 increases as the WO3 content increases from 10 to 40 mol% with constant Ag2O (TAW2–TAW5), where as that of other peak at 460 cm 1 decrease as the WO3 content increases from 10 to 40 mol%. It can be seen from Fig. 4 that all the Raman bands move to higher wavenumbers with an
Fig. 4. Raman spectra of TeO2–Ag2O–WO3 glasses.
Table 5 Observed Raman band positions in the (85 with errors 7 1 cm 1. Sample ID
Raman bands (cm
TAW1 TAW2 TAW3 TAW4 TAW5
– 344 348 350 352
460 463 471 477 482
x)TeO2–15Ag2O–xWO3 glass system,
1
) 689 701 714 728 739
– 824 829 834 841
– 907 908 910 915
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increase in the WO3 content. It is also noticeable that band at 460 cm 1 is fairly strong for all the samples (TAW1–TAW5). The position and intensity of the band at 689 cm 1 is sensitive to the type and concentration of the metal oxides. In Li2O–TeO2 glasses [24], Raman bands due to TeO4 tbp and TeO3 tp were found at 770 and 670 cm 1, respectively, but in our work no band corresponding to TeO4 tbp units is observed, moreover the band corresponding to TeO3 tp units observed at 689 cm 1 which is at higher Raman wavenumber than that observed for the TeO3 tp band of the Li2O–TeO2 glasses. We attribute the disappearance of the band due to TeO4 units and the appearance of the band due to TeO3 tp units at higher wavenumbers due to the influence of Ag + ions on the tellurite network. The Ag + ions break the Te–eqOax–Te bridging bonds forming TeO2 glass network and form the Te–eqO– Ag + or Te–axO–Ag + species and also Ag + ions break the tungsten network by forming W–O–Ag + species [9,15,25]. All these species prevents the formation of TeO4 tbp units in all samples (TAW1– TAW5). The intensity of the band at 460 cm 1 decrease with an increase in WO3 content as expected since there are fewer Te–O– Te linkages and more Te–O–W linkages [26]. As shown in Fig. 4, the intensity of the band at 907 cm 1 due to WO4 units increases as WO3 concentration increases from 10 to 30 mol%, it may be concluded that the number of WO4 units increases as WO3 concentration increases and reaches a maximum at 30 mol% (TAW2–TAW4). As the WO3 concentration increases from 30 to 40 mol%, the intensity the band at 907 cm 1 increases while this band is shifted to higher frequencies (915 cm 1). The intensity of the band at 344 cm 1 increases and it shifts to 352 cm 1 as the WO3 concentration increases from 10 to 40 mol% (Fig. 4). For constant Ag2O, the substitution of TeO2 by WO3 causes an increase of networking oxygens, according to the proportion 3/2 and these oxygens form new Te–O–W and W–O– W linkages [26]. All these observations suggest that the W ion coordination state changes from tetrahedral coordination (WO4) with 10 rx r30 mol% (TAW2–TAW4) to octahedral coordination (WO6) with x Z30 mol (TAW5). Formation of WO6 octahedra groups in the structure of the glasses causes a change in the coordination state of tellurium ions and their partial conversion from 3 coordination (TeO3) to 3+ 1 coordination (TeO3 + 1). On the other hand, W6 + is a heavy metal ion; therefore its addition to the glass may distort and strain the TeO2 network. The TeO3 tp is distorted into a TeO3 + 1 polyhedron because of the formation of W–O–Te linkages. This is supported by the appearance of Raman bands in the 728–740 cm 1 region corresponding to TeO3 + 1 polyhedron. As the WO3 content increases, the major band shifts from 689 cm 1 (x= 0) to 739 cm 1 (x = 40). This may be related to the increase of number of TeO3 + 1 units at the expense of number of TeO3 units. 4. Conclusions Transparent and stable glasses were obtained in the TeO2– WO3, TeO2–WO3–PbO and TeO2–Ag2O–WO3 systems. The Raman spectra of TeO2–WO3 glasses reveal that the glass network consists of TeO4, [TeO3]/[TeO3 + 1], WO4 and WO6 units. The Raman spectra of TeO2–WO3–PbO glasses reveal that the glass network consists of TeO4, [TeO3]/[TeO3 + 1], WO4 and WO6 and PbO4 units. The Raman spectra of TeO2–Ag2O–WO3 glasses reveal
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that the glass network consists of TeO3, TeO3 + 1, WO4 and WO6 units. Addition of WO3 increases the population of lower coordination units [TeO3]/[TeO3 + 1] in the glass network at the expense of higher coordination units [TeO4]. In the case of TeO2– WO3–PbO glasses, the TeO3/TeO3 + 1 units are transformed to TeO4 units with increasing PbO content and the Raman band around 320–336 cm 1 is due to Pb–O vibration in PbO4 units. Addition of WO3 to all the glass systems results in the formation of W–O–W and W–O–Te linkages, while that of the Te–O–Te linkages decreases as WO3 increases. The formation of Te–O–W linkages increases the connectivity of the glass network. Tungsten atoms participate in all the glasses as WO4 units up to the WO3 concentration r30 mol%. With the increase in the WO3 content beyond 30 mol% tungsten atoms participate in all the glasses as WO6 units.
Acknowledgments One of the authors, G. Upender is grateful to UGC (University Grants Commission), New Delhi, for providing financial assistance under the scheme of RFSMS (Research Fellowships in Science for Meritorious Students) and the authors also thank Mr. Manoj (CSR, Indore) for his help in recording Raman measurements.
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