3.1 Introduction. 3.2 Characterization Techniques CHAPTER 3. STRUCTURE-PROPERTY CORRELATIONS IN PbO-TeO 2 GLASSES

Transcription

1 CHAPTER 3 STRUCTURE-PROPERTY CORRELATIONS IN PbO-TeO 2 GLASSES 3.1 Introduction In this chapter the results of characterization studies on lead tellurite glasses are presented. Glass samples from the system: x PbO- (100-x) TeO 2 with x= 13, 15, 17, 19 and 21 mol % are prepared and characterized by X-ray diffraction, density measurements, UV-visible absorption spectroscopy, differential scanning calorimetry (DSC) and Raman spectroscopy. 3.2 Characterization Techniques X-ray Diffraction (XRD) X-ray diffraction measurements are performed on all normal and splat quenched lead tellurite samples and their diffraction patterns are shown in Figs. 3.1 and 3.2, respectively. XRD patterns of normal quenched samples show strong and sharp diffraction peaks superimposed on weak broad peaks due to the glassy phase. The background of broad peaks is small compared to sharp and intense peaks due to the presence of crystalline phases. The normal quenched samples show several peaks at 27.4 o, 31.7 o, 45.4 o, 53.8 o, 54.0 o, 66.3 o, 72.9 o, 73.2 o, 75.2 o, 75.4 o, 83.9 o and 84.0 o (Fig. 3.1), which are due to the PbTeO 3 [Powder diffraction files #721462, #780448, #350644], Pb 2 Te 3 O 8 [Powder diffraction file #440568] and TeO 2 [Beyer (1967)] crystalline phases. Splat-quenched lead tellurites are mostly amorphous except for few sharp at 27.4 o, 31.7 o and 53.9 o superimposed on broad peaks due to the majority glassy phase (Fig. 3.2). The three peaks in splat-quenched samples are due to PbTeO 3 [Powder diffraction files #721462, #780448, #350644], Pb 2 Te 3 O 8 [Powder diffraction file #440568] and TeO 2 [Beyer (1967)] crystalline phases. 68

2 Fig. 3.1 XRD patterns of normal-quenched lead tellurite samples ( o, PbTeO 3 ; $, Pb 2 Te 3 O 8 ; +, TeO 2 crystalline phases; broad hump in graphs is not visible since graphs are stacked and intensities of crystalline peaks are high). 69

3 Fig. 3.2 XRD patterns of splat-quenched lead tellurite glasses ( o, PbTeO 3 ; $, Pb 2 Te 3 O 8 ; +, TeO 2 crystalline phases). 70

4 Table 3.1 gives the XRD peaks and the corresponding crystalline phases in samples prepared at two-quenching rates. XRD patterns show that normal quenched samples are not true glasses but glass-ceramics, containing significant amount of PbTeO 3, Pb 2 Te 3 O 8 and TeO 2 crystalline phases, while splat-quenched samples are mostly amorphous. The formation of crystalline phases in the normal quenched sample containing 19 mol% PbO (19PbTe-n) is due to the following crystallization processes: (3.1) (3.2) The overall crystallization process is: (3.3) Similarly for normal quenched sample containing 21-mol % PbO, crystalline phases of PbTeO 3, Pb 2 Te 3 O 8 and TeO 2 are formed during slow cooling of the melt according to the following crystallization routes: (3.4) (3.5) The overall crystallization process is: (3.6) Similar equations can be written for samples with lower PbO mol%. Splat quenched sample with 21-mol % of PbO (21PbTe-s) is more crystalline than with 19- mol % of PbO (19PbTe-s), indicating that higher PbO concentration deteriorates the glass-forming ability of lead tellurites. The increase in PbO concentration leads to the breaking of more Te-O-Te and Te-O-Pb linkages, giving rise to non-bridging oxygens. Resulting weakly connected chains made glass formation difficult [Neov et al. (1979)]. Earlier authors reported that at higher PbO concentrations, the glass formation becomes difficult in binary glasses [Pan and Ghosh (2002); Pisarski et al. (2005)]. 71

5 Table 3.1 XRD peaks and the corresponding crystalline phases in lead tellurite glasses prepared at two quenching rates. Sample code XRD peaks positions ( o ) Crystal phases 13PbTe-n 27.2, 31.6, 45.1, 53.5, 53.7 Pb 2 Te 3 O , 45.1, 45.3, 48.6 PbTeO , 27.3 TeO 2 15PbTe-n 31.5, 45.2, 53.7 Pb 2 Te 3 O , 45.2, 56.3 PbTeO TeO 2 17PbTe-n 31.5, 31.7, 45.4, 53.7 Pb 2 Te 3 O , 31.5, 45.2, 56.3, 65.9, 66.2 PbTeO TeO 2 19PbTe-n 21PbTe-n 15PbTe-s 27.4, 31.7, 45.4, 53.8 Pb 2 Te 3 O , 31.7, 45.4, 54.0, 56.4, 56.6, 66.3, 72.9, 73.2, 75.4, 83.9, , 31.7, 54.0, 56.6, 66.3, 72.9, 73.2, 75.2, 75.4, 83.9, 84.0 PbTeO 3 TeO , 31.7, 45.4, 53.8 Pb 2 Te 3 O , 31.7, 45.4, 54.0, 56.4, 56.6, 66.3, 72.9, 73.2, 75.4, 83.9, , 31.7, 54.0, 56.6, 66.3, 72.9, 73.2, 75.2, 75.4, 83.9, 84.0 PbTeO 3 TeO Pb 2 Te 3 O PbTeO TeO 2 19PbTe-s 27.4 Pb 2 Te 3 O PbTeO TeO 2 21PbTe-s 27.4, 31.7, 53.8 Pb 2 Te 3 O , 31.7 PbTeO , 31.7 TeO 2 72

6 3.2.2 Density Density is a powerful tool capable of exploring the changes in the structure of glasses and is affected by structural compactness, changes in geometrical configurations, coordination numbers, cross-link densities, and dimensions of interstitial spaces or voids in glass [Saddeek et al. (2008)]. Density increases from ± to ± g cm -3 for normal-quenched lead tellurite glasses as PbO concentration increases from 13 to 21-mol% (Table 3.2 and Fig. 3.3). Fig. 3.3 Density (d) and molar volume (V M ) of lead tellurite glasses. 73

7 For splat-quenched lead tellurite glasses, a similar increase in density from ± to ± g cm -3 in the same composition range is observed. This increase in density of lead tellurite glasses can be due to the substitution of heavier PbO (Molecular weight = amu) in the place of lighter TeO 2 (Molecular weight = amu). Molar volume (V M ) is considered to be the better tool for studying the changes in glass structure since it eliminates mass from the density and uses equal number of particles for comparison purposes, and is calculated from density using the following relationship: (3.7) where n i is molar fraction of the oxide component, i, M its molecular weight, i and d is the glass density. Molar volume decreases slightly from to cm 3 mol -1 with the increase in PbO concentration from 13 to 21-mol%. Eraiah prepared 40PbO-60TeO 2 glass and found its density to be 5.60 g cm -3 and molar volume to be 33 cm 3 mol -1 [Eraiah (2010)]. Vithal et al. prepared 70PbO-30TeO 2 glass and found density and molar volume to be 6.79 g cm -3 and 30.1 cm 3 mol -1, respectively [Vithal et al. (1997)]. Assuming the mixture of oxide components to be an ideal solution the additive crystalline oxide volume, V o for each composition is calculated using the following relationship: (3.8) where, d i is the density, M i is the molecular weight, and n i, the mol fraction of the i th component (crystalline oxide). 74

8 Table 3.2 Density (d), molar volume (V M ), additive crystalline oxide volume (V o ), excess volume (V x ) and oxygen packing density (OPD) data of PbO-TeO 2 glasses. Sample Code PbO mol% d (g cm -3 ) V M (cm 3 mol -1 ) V o (cm 3 mol -1 ) V X (cm 3 mol -1 ) OPD (g atom liter -1 ) 13PbTe-n ± PbTe-n ± PbTe-n ± PbTe-n ± PbTe-n ± PbTe-s ± PbTe-s ± PbTe-s ± PbTe-s ± PbTe-s ±

9 Excess volume, V x is defined as the difference of V M and V o....(3.9) where, V M is glass molar volume and V o is additive crystalline oxide volume. Table 3.3 gives the density and molar volume of two crystalline oxide components The glass molar volume is lower than its crystalline counterparts in normalquenched glass samples, which indicates that the addition of PbO stiffens the network and thus lead to the increase in glass transition temperature (discussed later in section ), but in splat-quenched glasses, the glass molar volume is higher than its crystalline components which indicates that PbO introduces excess volume in the glass network. Table 3.3 Density, molecular weight and molar volume data of crystalline oxide components. Oxide component PbO (Tetragonal) TeO 2 (Tetragonal) Molecular weight (amu) Density (g cm -3 ) Molar Volume (cm 3 mol -1 ) Oxygen packing density (OPD) is a measure of the tightness of packing of the oxide network and is defined as the ratio of measured density per molecular weight and number of oxygen atoms in a formula unit [Ganguli and Rao (1999)]. OPD is obtained using the following relationship: where d is the glass density, (O) is the number of oxygen atoms per formula units and M is the molecular weight of glass [Altaf and Chaudhry (2010)] (3.10)

10 OPD decreases from g atom liter -1 to g atom liter -1 in normal quenched glasses, and in splat quenched glasses, it decrease from g atom liter -1 to g atom liter -1 with the increase in PbO from 13 to 21-mol%, which indicates the loosening of the glass network UV-visible Absorption Spectroscopy Figs. 3.4 and 3.5 display the optical absorption spectra of normal and splat quenched lead tellurite glass samples, respectively. The optical absorption spectra shift to lower wavelengths with the increase in PbO concentration. In tellurite glasses this is not necessarily an indication of decrease in the concentration of non-bridging oxygens (NBOs) unlike in borate and silicate glasses, where the absorption of UV-visible light is mostly due to the excitation of electrons of NBOs. This is due to the difference in electronic structure of tellurite glass network from borate and silicate glass network [Kowada et al. (1996)]. The optical absorption cut off wavelength, λ o is arbitrarily taken as the wavelength at which the absorption coefficient, α, increases abruptly. Fig. 3.4 UV-visible absorption spectra of normal-quenched lead tellurite glasses. 77

11 Fig. 3.5 UV-visible absorption spectra of splat-quenched lead tellurite glasses. The optical cut off wavelength decreases from 407 nm to 395 nm with the increase in PbO concentration from 13 to 21-mol% in both normal and splat-quenched glasses (Table 3.4). Table 3.4 Cut-off wavelength in normal and splat-quenched lead tellurite glasses. PbO mol% Cut-off wavelength (nm) The absorption coefficient, α (cm -1 ) is calculated by dividing absorbance with sample thickness. The glass samples are relatively thick ( mm) and the maximum 78

12 absorbance measured by spectrometer is 4.00, hence the true optical bandgap in glasses could not be determined as suggested by Tauc plots or Mott-Davis models that require the measurement of absorption spectra at photon wavelengths where absorption coefficient, α ~ cm -1 [Tauc et al. (1966); Mott and Davis (1971); Tauc (1974)]. Fig. 3.6 displays the variation of cut-off wavelength ( o ) with the PbO mol%. The linear fitted graph has the following equation:...(3.11) where x is PbO mol% and o is cut-off wavelength. Fig. 3.6 Variation of λ o with varying PbO mol% (denoted as x). Normally quenched samples show a small absorption shoulder around 477 nm, just below the absorption cut-off, which is absent in splat-quenched glasses (Figs. 3.4 and 3.5). This absorption shoulder is in all probability due to scattering and absorption of light by small crystals that coexist with glassy phase in normally (slowly) cooled samples. 79

13 3.2.4 Differential Scanning Calorimetry (DSC) Figs. 3.7 and 3.8 display the DSC patterns of lead tellurite glasses prepared by normal and splat quenching methods, respectively. The first lead tellurite glass containing 13-mol % of PbO (sample 13PbTe-n) has glass transition at 290 o C (midpoint value). The two slowly cooled samples containing 15 and 17-mol % of PbO (samples 15PbTe-n and 17PbTe-n, respectively) show a weak and broad glass transition just before the exothermic crystallization peak, at 329 o C. Normally quenched samples containing 19 and 21-mol % of PbO (samples 19PbTe-n and 21PbTe-n, respectively) show a glass transition at 310 o C (mid-point value). The splat-quenched lead tellurites of same composition exhibit clear and abrupt glass transition, which decreases systematically from 290 o C to 279 o C as PbO was increased from 13 to 21-mol %. Similar trend in glass transition was observed by Silva et al. who prepared xpbo-(100-x)teo 2 (x=10, 30, 50 mol%) glasses and found the decrease in glass transition from 289 o C to 231 o C [Silva et al. (2001)]. Table 3.5 gives the values of glass transition (T g ), crystallization (T c ) and liquidus temperatures (T m ) in all lead tellurite samples. Normal and splat-quenched lead tellurite glass with 13 mol % of PbO have same glass transition temperature of 290 o C (Table 3.5), but a large difference is observed in samples with PbO concentrations higher than 13 mol%, prepared at two melt cooling rates. Slowly cooled glasses has T g at 329 o C (samples 15PbTe-n and 17PbTe-n) compared to the values of 287 o C (sample 15PbTe-s) and 285 o C (sample 17PbTe-s) for splat-quenched glasses of equal composition. Similarly, for normal quenched glasses containing 19 and 21 mol% PbO, T g is observed to be 310 o C (samples 19PbTe-n and 21PbTe-n) which is very large compared to the T g values of 282 o C (sample 19PbTe-s) and 279 o C (sample 21PbTe-s) in splat-quenched glasses of equal composition. A priori, higher T g in samples prepared at higher quenching rates is expected. The opposite behaviour is probably due to presence of crystalline phases of PbTeO 3, Pb 2 Te 3 O 8 and TeO 2 in normally quenched (slowly cooled) samples, as confirmed by our XRD measurements (discussed earlier in section 3.2.1). 80

14 Table 3.5 Glass transition temperature (T g ), crystallization temperatures (T c ), melting temperatures (T m ), thermal stability range (S) and glass formation factor (K) for lead tellurite glasses (All temperatures are in o C and 1, 2, 3, 4 in subscripts represent multiple peaks). Sample code PbO mol % T g T c1 T c2 T c3 T m1 T m2 T m3 T m4 S K 13PbTe-n PbTe-n PbTe-n PbTe-n PbTe-n PbTe-s PbTe-s PbTe-s PbTe-s PbTe-s

15 Fig. 3.7 DSC patterns of normal-quenched lead tellurite samples. Fig. 3.8 DSC patterns of splat-quenched lead tellurite samples. 82

16 Normally quenched samples are not true glasses but glass-ceramics, the glass transition temperature of amorphous phase in the composite material dramatically enhances to 329 o C, probably due to pressure exerted by coexisting crystals [Srivastava and Basu (2007)]. All splat-quenched lead tellurite glasses have at least two broad crystallization peaks while normally quenched samples (except the first sample 13PbTe-n) show one crystallization peak. The composition of lead tellurite glasses is such that it produces PbTeO 3, TeO 2 and Pb 2 Te 3 O 8 phases on crystallization during DSC heating run by the following routes: (3.12) (3.13) Similar equations can be written for lead tellurites containing higher PbO mol %. The thermal stability range (S) for the glass is estimated by the following relationship:...(3.14) where T c is crystallization and T g is glass transition temperatures [El-Mallawany and Ahmed (2008)]. The glass-forming tendency (K), which is a useful measure of devitrification tendency for the glass, is given by:...(3.15) where T g, T c, T m are glass transition, crystallization and melting temperatures, respectively [El-Mallawany and Ahmed (2008)]. Thermal stability of normal quenched samples increases from 24 o C to 59 o C while that of splat-quenched glasses increases from 30 o C to 36 o C, with the increase in PbO from 13 to 19- mol%, and then decreases to 33 o C at 21-mol% PbO. The glass forming parameter, K increases from 0.12 to 0.45 in normal-quenched samples and in splat-quenched glasses it increases from 0.15 to 0.20 with the increase in PbO from 13 to 19- mol%, and further decreases to 0.17, with the increase in PbO to 21 mol%. 83

17 3.2.5 Raman Spectroscopy Figs. 3.9 and 3.10 display the intensity-normalized Raman spectra of normal and splat- quenched lead tellurite glasses, respectively. Raman spectra are deconvoluted to five peaks centred at 430 cm -1, 586 cm -1, 662 cm -1, 733 cm -1 and 774 cm -1 (sample code-13pbte-n) using Peakfit software and two point baseline correction method [Khalil et al. (2010)] (Fig. 3.11). The spectra of both normal and splat-quenched glasses show exactly same behaviour although normal-quenched glass samples are not true glasses but are semi-transparent glass ceramics, while splat-quenched samples are mostly amorphous (Figs. 3.9 and 3.10). This suggested that there is no difference in the molecular structure in both glasses and same vibrating structural units are present in them. McMillan found from Raman studies on CaO-MgO-SiO 2 glasses prepared at two quenching rates that there is little or no effect of quenching rate on glass short range structure [McMillan (1984)]. Fig. 3.9 Intensity-normalized Raman spectra of normal-quenched lead tellurite glasses (The graphs are shifted for the sake of clarity). 84

18 Fig Intensity-normalized Raman spectra of splat-quenched lead tellurite glasses (The graphs are shifted for the sake of clarity). It was reported earlier that the Raman spectrum of paratellurite (α-teo 2 ) and TeO 2 glass could be deconvoluted into five similar peaks centred at 450 cm -1, 611 cm -1, 659 cm -1, 716 cm -1 and 773 cm -1 [Sekiya et al. (1989)]. The peak at lower wave number, 430 cm -1 is ascribed to the symmetric stretching and bending vibrations of Te- O-Te linkages, formed by sharing vertices of TeO 4 trigonal bypyramids (tbp s), TeO 3+1 polyhedra and TeO 3 trigonal planar units. The existence of this peak in our glass samples indicates the presence of continuous network consisting of TeO n (n= 4, 3+1, 3) polyhedra. The decrease in intensity of this band in lead tellurite glasses indicates cleavage of Te-O-Te linkages and a formation of NBOs, which is consistent with the conversion of TeO 4 tbps into TeO 3 polyhedra having one NBO. The addition of PbO to tellurite network breaks Te-O-Te bonds, thus leads to a decrease in the Te-O coordination number. The peaks at 586 cm -1 and 646 cm -1 are assigned to vibrations of continuous network composed of TeO 4 tbp units, while the peaks at 730 cm -1 and 775 cm -1 are due to the vibrations from a continuous network composed of TeO 3 tp units (Table 3.6). Raman study on lead tellurite glasses by Silva et al. found the existence of 85

19 peaks at 450 cm -1, 650 cm -1 and a broad shoulder at 750 cm -1. The intensities of 450 cm -1 and 650 cm -1 peak are found to decrease with increase in PbO concentration, while intensity of shoulder at 750 cm -1 increases with increase in PbO concentration, which clearly indicates the cleavage of TeO 4 tbp network by breaking Te-O-Te linkages [Silva et al. (2001)]. These earlier findings agree with our results. Table 3.6 Raman bands assignment of deconvoluted Raman spectra of normal and splat-quenched lead tellurite glasses. Raman band frequencies (cm -1 Vibrational band assignments ) Te-O-Te linkages TeO 4 tbp TeO 4 tbp TeO 3 tp TeO 3 tp The NBOs, which represents the oxygen atoms forming Te=O and Te-O - and their resonating bonds, are formed in TeO 3+1 polyhedron or TeO 3 tp units and they interact weakly with adjacent tellurium atoms. The bond between NBO and tellurium is weakened and therefore the vibration appears in the lower wave number at 730 cm -1 [Sekiya et al. (1994)]. Fig Deconvoluted Raman spectra of 13 mol% PbO glass sample (sample code- 13PbTe-n). 86

20 The decrease in the intensities of 586 cm -1 and 646 cm -1 bands and increase in intensities of 730 cm -1 and 775 cm -1 bands indicate the distortion of TeO 4 tbp leading to the formation of TeO 3 units via the formation of TeO 3+1 units and the formation of NBOs. Similar variation in intensities for 646 cm -1 and 775 cm -1 peaks is also observed for 10Li 2 O-xNb 2 O 5 -(89-x)TeO 2 glasses [Babu and Mouli (2009)], La 2 O 3 -TeO 2 and Y 2 O 3 -TeO 2 glasses [Sekiya et al. (1995)]. Fig shows the intensities variation of all Raman bands with PbO concentration. The structural unit making up pure TeO 2 glass is an asymmetrical TeO 4 trigonal bipyramid (tbp) units in which one of the equatorial sites is occupied by a lone pair of electrons. Upon inclusion of modifiers or intermediates, the coordination state of Te changes from TeO 4 tbp units by means of an intermediary TeO 3+1 polyhedron to TeO 3 tp units, and concentration of non-bridging oxygen increases [Balaya and Sunandana (1994)]. Pb 2+ enters the glass network as a modifier and breaks the Te-O-Te bonds and results in the formation of dangling (broken) bonds. During this process there can be different ways of forming dangling bonds in the present glass: (i) the stable Te-O and (ii) the unstable Te-O bonds which are later modified to Te-O (or simply TeO 3+1 ) owing to the contraction of one Te-O and the elongation of another Te-O bond. With increasing PbO content, cleavage of continuous network leads to an increase in the concentration of TeO 3+1 polyhedra. Further elongation of the Te-O bond of TeO 3+1 polyhedra breaks this bond and lead to the formation of trigonal TeO 3 units and NBOs [Pan and Morgan (1997); Caricato et al. (2003)]. The conversion of TeO 4 tbp units to TeO 3 tp units is found using the procedure adopted by Kalampounias et al., where the relative amount of TeO 3 tp and TeO 4 tbp units are estimated by calculating I 774 /I 662 intensity ratio, which is considered proportional to the concentration ratio R, where R= [TeO 3 ]/[TeO 4 ] of these units [Kalampounias et al. (2011)]. Fig displays the intensity ratio, I 774 /I 662 with the increase in PbO concentration. 87

21 Fig Variation of intensities of Raman bands with PbO mol%. Fig Variation of intensity ratio, I 774 /I 662 with varying PbO mol%. 88

22 The intensity ratio increases from 0.25 to 0.93 with the increase in PbO concentration from 13 to 21-mol%, which indicates the distortion of TeO 4 units and increase in concentration of TeO 3 units at the expense of TeO 4 units. Similar findings were reported by Duverger et al. where this intensity ratio increased almost linearly with addition of PbO [Duverger et al. (1997)]. An abrupt increase in intensity ratio at PbO concentration of 15 mol% is observed, which indicates that at this PbO concentration there are significant changes in the concentration of TeO 3 and TeO 4 units and hence significant changes are expected in the structure of these glasses at 15 mol% of PbO. The addition of PbO to TeO 2 network breaks the Te-O-Te bonds and maximum disruption of TeO 4 units is observed at PbO concentration above 15 mol%. 89

23 Summary Lead tellurite glasses of composition xpbo-(100-x)teo 2 are prepared and characterized by XRD, density, UV-visible spectroscopy, DSC and Raman spectroscopy. Density increases from ± to ± g cm -3 in normal quenched lead tellurite glasses as PbO concentration is increased from 13 to 21- mol%. Similar trend in density is observed in splat-quenched glasses. This increase in density of lead tellurite glasses is due to the substitution of heavier PbO (Molecular weight = amu) in the place of lighter TeO 2 (Molecular weight = amu). X-ray diffraction measurements are performed on all normal and splat quenched lead tellurite samples and large difference in thermal properties is observed for samples prepared at two melt-quenching rates. XRD patterns of normal quenched samples show the significant amount of crystalline phases coexist with the glassy phase. Normal quenched lead tellurites are semi-transparent glass ceramics while splat quenched glasses are almost completely amorphous. Normal and splat quenched lead tellurite samples with 13 mol % of PbO have same glass transition temperature ~290 o C, but a large difference is observed in samples with PbO concentrations higher than 13 mol%, prepared at two melt cooling rates. The opposite behaviour is probably due to presence of crystalline phases of PbTeO 3, Pb 2 Te 3 O 8 and TeO 2 in normally quenched samples. The glass transition temperature of the amorphous phase in glass-ceramics (normal-quenched) of lead tellurites increases significantly probably due to pressure exerted by the coexisting crystallites of PbTeO 3, Pb 2 Te 3 O 8 and TeO 2 phases in the composite material. Splat quenched sample with 21- mol % of PbO (21PbTe-s) is more crystalline than with 19-mol % of PbO (19PbTe-s), indicating that higher PbO concentration deteriorates glass-forming ability of lead tellurites. The optical absorption spectra shift to lower wavelengths with the increase in PbO concentration. The optical cut off wavelength decreases from 407 nm to 395 nm with the increase in PbO concentration from 13 to 21-mol%. Slowly-cooled lead tellurite samples that contain significant amounts of crystalline phase show an absorption shoulder, just below the optical absorption cut-off in their UV-visible 90

24 spectra. This absorption shoulder is attributed to the scattering and absorption of light by small crystals that are present in the glassy matrix. Raman scattering studies found that the addition of PbO shifts in relative intensities and frequencies of Raman bands, and clearly indicates the decrease in the concentration of TeO 4 units, with the creation of TeO 3+1 polyhedron and/or TeO 3 units. The intensity ratio, I 774 /I 662 increases from 0.25 to 0.93 with the increase in PbO concentration from 13 to 21 mol%. 91

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