68

CHAPTER 3

STRUCTURE-PROPERTY CORRELATIONS IN PbO-TeO2 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) TeO2 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

3.2.1 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

27.4o, 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 PbTeO3 [Powder diffraction files #721462, #780448,

#350644], Pb2Te3O8 [Powder diffraction file #440568] and TeO2 [Beyer (1967)]

crystalline phases. Splat-quenched lead tellurites are mostly amorphous except for few

sharp at 27.4o, 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 PbTeO3 [Powder

diffraction files #721462, #780448, #350644], Pb2Te3O8 [Powder diffraction file

#440568] and TeO2 [Beyer (1967)] crystalline phases.

69

Fig. 3.1 XRD patterns of normal-quenched lead tellurite samples (‘o’, PbTeO3; ‘$’,

Pb2Te3O8; ‘+’, TeO2 crystalline phases; broad hump in graphs is not visible

since graphs are stacked and intensities of crystalline peaks are high).

70

Fig. 3.2 XRD patterns of splat-quenched lead tellurite glasses (‘o’, PbTeO3; ‘$’,

Pb2Te3O8; ‘+’, TeO2 crystalline phases).

71

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 PbTeO3, Pb2Te3O8

and TeO2 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 PbTeO3, Pb2Te3O8 and TeO2 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)].

72

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 Pb2Te3O8

31.4, 45.1, 45.3, 48.6 PbTeO3

27.2, 27.3 TeO2

15PbTe-n

31.5, 45.2, 53.7 Pb2Te3O8

31.7, 45.2, 56.3 PbTeO3

27.2 TeO2

17PbTe-n

31.5, 31.7, 45.4, 53.7 Pb2Te3O8

30.8, 31.5, 45.2, 56.3, 65.9, 66.2 PbTeO3

27.2 TeO2

19PbTe-n

27.4, 31.7, 45.4, 53.8 Pb2Te3O8

27.4, 31.7, 45.4, 54.0, 56.4, 56.6, 66.3, 72.9, 73.2, 75.4,

83.9, 84.0 PbTeO3

27.4, 31.7, 54.0, 56.6, 66.3, 72.9, 73.2, 75.2, 75.4, 83.9,

84.0 TeO2

21PbTe-n

27.4, 31.7, 45.4, 53.8 Pb2Te3O8

27.4, 31.7, 45.4, 54.0, 56.4, 56.6, 66.3, 72.9, 73.2, 75.4,

83.9, 84.0 PbTeO3

27.4, 31.7, 54.0, 56.6, 66.3, 72.9, 73.2, 75.2, 75.4, 83.9,

84.0 TeO2

15PbTe-s

31.7 Pb2Te3O8

31.7 PbTeO3

31.7 TeO2

19PbTe-s

27.4 Pb2Te3O8

27.4 PbTeO3

27.4 TeO2

21PbTe-s

27.4, 31.7, 53.8 Pb2Te3O8

27.4, 31.7 PbTeO3

27.4, 31.7 TeO2

73

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 6.132 ± 0.002

to 6.405 ± 0.004 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 (VM) of lead tellurite glasses.

74

For splat-quenched lead tellurite glasses, a similar increase in density from

6.056 ± 0.007 to 6.389 ± 0.022 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 = 223.21 amu) in the place of lighter TeO2 (Molecular weight =

159.60 amu).

Molar volume (VM) 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

ni is molar fraction of the oxide component, i,

Mi its molecular weight,

and d is the glass density.

Molar volume decreases slightly from 27.38 to 27.00 cm3 mol

-1 with the

increase in PbO concentration from 13 to 21-mol%. Eraiah prepared 40PbO-60TeO2

glass and found its density to be 5.60 g cm-3

and molar volume to be 33 cm3 mol

-1

[Eraiah (2010)]. Vithal et al. prepared 70PbO-30TeO2 glass and found density and

molar volume to be 6.79 g cm-3

and 30.1 cm3 mol

-1, respectively [Vithal et al. (1997)].

Assuming the mixture of oxide components to be an ideal solution the additive

crystalline oxide volume, Vo for each composition is calculated using the following

relationship:

…(3.8)

where,

di is the density,

Mi is the molecular weight,

and ni, the mol fraction of the i

th

component (crystalline oxide).

75

Table 3.2 Density (d), molar volume (VM), additive crystalline oxide volume (Vo), excess volume (Vx) and oxygen packing density

(OPD) data of PbO-TeO2 glasses.

Sample

Code

PbO mol%

d

(g cm-3

)

VM

(cm3

mol-1

)

Vo

(cm3

mol-1

)

VX

(cm3

mol-1

)

OPD

(g atom liter-1

)

13PbTe-n 13 6.132 ± 0.002 27.38 27.54 -0.16 68.30

15PbTe-n 15 6.207 ± 0.005 27.25 27.44 -0.19 67.89

17PbTe-n 17 6.254 ± 0.003 27.25 27.35 -0.10 67.16

19PbTe-n 19 6.297 ± 0.004 27.26 27.25 0.01 66.40

21PbTe-n 21 6.405 ± 0.004 27.00 27.16 -0.16 66.30

13PbTe-s 13 6.056 ± 0.007 27.72 27.54 0.18 67.46

15PbTe-s 15 6.112 ± 0.005 27.68 27.44 0.24 66.84

17PbTe-s 17 6.155 ± 0.020 27.69 27.35 0.34 66.09

19PbTe-s 19 6.228 ± 0.026 27.57 27.25 0.32 65.65

21PbTe-s 21 6.389 ± 0.022 27.07 27.16 -0.09 66.12

76

Excess volume, Vx is defined as the difference of VM

and Vo.

...(3.9)

where,

VM is glass molar volume

and Vo 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 normal-

quenched 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

3.2.4.), 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

Molecular

weight

(amu)

Density

(g cm-3

)

Molar Volume

(cm3 mol

-1)

PbO

(Tetragonal) 223.21 9.530 23.42

TeO2

(Tetragonal) 159.60 5.670 28.15

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:

...(3.10)

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)].

77

OPD decreases from 68.30 g atom liter-1

to 66.30 g atom liter-1

in normal

quenched glasses, and in splat quenched glasses, it decrease from 67.46 g atom liter-1

to

66.12 g atom liter-1

with the increase in PbO from 13 to 21-mol%, which indicates the

loosening of the glass network.

3.2.3 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, λo 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.

78

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)

13 407

15 403

17 400

19 398

21 395

The absorption coefficient, α (cm-1) is calculated by dividing absorbance with

sample thickness. The glass samples are relatively thick (0.6-1.4 mm) and the maximum

79

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 ~ 103-10

4 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 λo 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.

80

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 290oC (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 329oC. Normally quenched samples

containing 19 and 21-mol % of PbO (samples 19PbTe-n and 21PbTe-n, respectively)

show a glass transition at 310oC (mid-point value).

The splat-quenched lead tellurites of same composition exhibit clear and abrupt

glass transition, which decreases systematically from 290oC to 279

oC 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)TeO2 (x=10, 30, 50 mol%) glasses and found the

decrease in glass transition from 289oC to 231

oC [Silva et al. (2001)]. Table 3.5 gives

the values of glass transition (Tg), crystallization (Tc) and liquidus temperatures (Tm) in

all lead tellurite samples. Normal and splat-quenched lead tellurite glass with 13 mol %

of PbO have same glass transition temperature of 290oC (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 Tg at 329oC (samples 15PbTe-n and 17PbTe-n)

compared to the values of 287oC (sample 15PbTe-s) and 285

oC (sample 17PbTe-s) for

splat-quenched glasses of equal composition. Similarly, for normal quenched glasses

containing 19 and 21 mol% PbO, Tg is observed to be 310oC (samples 19PbTe-n and

21PbTe-n) which is very large compared to the Tg values of 282oC (sample 19PbTe-s)

and 279oC (sample 21PbTe-s) in splat-quenched glasses of equal composition. A priori,

higher Tg in samples prepared at higher quenching rates is expected. The opposite

behaviour is probably due to presence of crystalline phases of PbTeO3, Pb2Te3O8 and

TeO2 in normally quenched (slowly cooled) samples, as confirmed by our XRD

measurements (discussed earlier in section 3.2.1).

81

Table 3.5 Glass transition temperature (Tg), crystallization temperatures (Tc), melting temperatures (Tm), thermal stability range (S)

and glass formation factor (K) for lead tellurite glasses (All temperatures are in oC and 1, 2, 3, 4 in subscripts represent

multiple peaks).

Sample

code

PbO

mol %

Tg Tc1 Tc2 Tc3 Tm1 Tm2 Tm3 Tm4 S K

13PbTe-n 13 290 314 324 379 516 665 - - 24 0.12

15PbTe-n 15 329 377 - - 517 646 - - 48 0.34

17PbTe-n 17 329 374 - - 522 627 - - 45 0.30

19PbTe-n 19 310 366 - - 498 532 604 - 56 0.42

21PbTe-n 21 310 369 - - 500 512 533 585 59 0.45

13PbTe-s 13 290 319 376 - 515 664 - - 29 0.15

15PbTe-s 15 287 317 323 381 519 647 - - 30 0.15

17PbTe-s 17 285 320 376 - 523 624 - - 35 0.17

19PbTe-s 19 282 318 353 361 499 536 605 - 36 0.20

21PbTe-s 21 279 312 346 353 502 513 534 582 33 0.17

82

Fig. 3.7 DSC patterns of normal-quenched lead tellurite samples.

Fig. 3.8 DSC patterns of splat-quenched lead tellurite samples.

83

Normally quenched samples are not true glasses but glass-ceramics, the glass

transition temperature of amorphous phase in the composite material dramatically

enhances to 329oC, 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

PbTeO3, TeO2 and Pb2Te3O8 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 Tc is crystallization and Tg 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 Tg, Tc, Tm are glass transition, crystallization and melting temperatures,

respectively [El-Mallawany and Ahmed (2008)].

Thermal stability of normal quenched samples increases from 24oC to 59

oC

while that of splat-quenched glasses increases from 30oC to 36

oC, with the increase in

PbO from 13 to 19- mol%, and then decreases to 33oC 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%.

84

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-SiO2 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).

85

Fig. 3.10 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-TeO2) and

TeO2 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 TeO4 trigonal bypyramids (tbp’s), TeO3+1

polyhedra and TeO3 trigonal planar units. The existence of this peak in our glass

samples indicates the presence of continuous network consisting of TeOn (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 TeO4 tbps into TeO3 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 TeO4 tbp units, while the peaks at 730 cm-1

and 775

cm-1

are due to the vibrations from a continuous network composed of TeO3 tp units

(Table 3.6). Raman study on lead tellurite glasses by Silva et al. found the existence of

86

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 TeO4 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.

The NBOs, which represents the oxygen atoms forming Te=O and Te-O- and

their resonating bonds, are formed in TeO3+1 polyhedron or TeO3 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. 3.11 Deconvoluted Raman spectra of 13 mol% PbO glass sample (sample code-

13PbTe-n).

Raman band frequencies

(cm-1

) Vibrational band assignments

408-430 Te-O-Te linkages

586-603 TeO4 tbp

646-662 TeO4 tbp

702-730 TeO3 tp

759-775 TeO3 tp

87

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 TeO4 tbp leading to

the formation of TeO3 units via the formation of TeO3+1 units and the formation of

NBOs. Similar variation in intensities for 646 cm-1

and 775 cm-1

peaks is also observed

for 10Li2O-xNb2O5-(89-x)TeO2 glasses [Babu and Mouli (2009)], La2O3-TeO2 and

Y2O3-TeO2 glasses [Sekiya et al. (1995)]. Fig. 3.12 shows the intensities variation of all

Raman bands with PbO concentration.

The structural unit making up pure TeO2 glass is an asymmetrical TeO4 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 TeO4 tbp units by means of an intermediary TeO3+1 polyhedron to TeO3

tp units, and concentration of non-bridging oxygen increases [Balaya and Sunandana

(1994)]. Pb2+

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 TeO3+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 TeO3+1 polyhedra. Further elongation of the Te-O bond of TeO3+1

polyhedra breaks this bond and lead to the formation of trigonal TeO3 units and NBOs

[Pan and Morgan (1997); Caricato et al. (2003)].

The conversion of TeO4 tbp units to TeO3 tp units is found using the procedure

adopted by Kalampounias et al., where the relative amount of TeO3 tp and TeO4 tbp

units are estimated by calculating I774/I662 intensity ratio, which is considered

proportional to the concentration ratio R, where R= [TeO3]/[TeO4] of these units

[Kalampounias et al. (2011)]. Fig. 3.13 displays the intensity ratio, I774/I662 with the

increase in PbO concentration.

88

Fig. 3.12 Variation of intensities of Raman bands with PbO mol%.

Fig. 3.13 Variation of intensity ratio, I774/I662 with varying PbO mol%.

89

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 TeO4 units and

increase in concentration of TeO3 units at the expense of TeO4 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 TeO3 and TeO4 units

and hence significant changes are expected in the structure of these glasses at 15 mol%

of PbO. The addition of PbO to TeO2 network breaks the Te-O-Te bonds and maximum

disruption of TeO4 units is observed at PbO concentration above 15 mol%.

90

Summary

Lead tellurite glasses of composition xPbO-(100-x)TeO2 are prepared and

characterized by XRD, density, UV-visible spectroscopy, DSC and Raman

spectroscopy. Density increases from 6.132 ± 0.002 to 6.405 ± 0.004 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 =

223.21 amu) in the place of lighter TeO2 (Molecular weight = 159.60 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 ~290oC, 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 PbTeO3,

Pb2Te3O8 and TeO2 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 PbTeO3,

Pb2Te3O8 and TeO2 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

91

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 TeO4 units, with the creation of TeO3+1 polyhedron and/or TeO3 units.

The intensity ratio, I774/I662 increases from 0.25 to 0.93 with the increase in PbO

concentration from 13 to 21 mol%.

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