Effect of Bi2O3 on physical, optical and structural studies of ZnO–Bi2O3–B2O3 glasses

Effect of Bi2O3 on physical, optical and structural studies of ZnO–Bi2O3–B2O3 glasses

LETTER TO THE EDITOR Journal of Non-Crystalline Solids 354 (2008) 5573–5579 Contents lists available at ScienceDirect Journal of Non-Crystalline So...

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LETTER TO THE EDITOR

Journal of Non-Crystalline Solids 354 (2008) 5573–5579

Contents lists available at ScienceDirect

Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/locate/jnoncrysol

Letter to the Editor

Effect of Bi2O3 on physical, optical and structural studies of ZnO–Bi2O3–B2O3 glasses D. Saritha a, Y. Markandeya a, M. Salagram b, M. Vithal c, A.K. Singh d, G. Bhikshamaiah a,* a

Department of Physics, Nizam college, Osmania University, Hyderabad, India Department of Physics, Osmania University, Hyderabad, India c Department of Chemistry, Osmania University, Hyderabad, India d Defence Metallurgical Research Laboratory, Hyderabad, India b

a r t i c l e

i n f o

Article history: Received 16 March 2008 Received in revised form 28 August 2008 Available online 25 October 2008 PACS: 32.30.Rj 33.20.Ea 61.05.cp 63.50.Lm 74.25.Gz 74.62.c 96.12.Ma

a b s t r a c t Zinc bismuth borate glasses with composition 10ZnO–xBi2O3–(90  x)B2O3 (where x is in mol%, ranging from 25 to 50 in steps of 5) have been prepared using conventional melt quenching technique. Differential scanning calorimetry (DSC) studies showed that the glass transition temperature (Tg) decreases from 473 °C to 449 °C as the content of Bi2O3 increases. The optical absorption studies revealed that the cutoff wavelength increases while optical band gap energy (Eopt) and Urbach energy (DE) decreases with increase of Bi2O3 content. The Eopt values of these glasses are found to be in the range 3.464–3.169 eV where as the values of DE lies in the range 0.503–0.178 eV. The DSC and optical absorption studies revealed that the glass network becomes less tightly packed and degree of disorder increases with increase in Bi2O3 concentration in the present glass system. The IR studies indicate that these glasses are made up of [BiO6], [BO3] and [BO4] basic structural units. The values of optical basicity evaluated using oxide ion polarizability ab2 ðEopt Þ obtained from optical band gap are in agreement with those values calculated from optical basicity values of the constituent oxides. Ó 2008 Elsevier B.V. All rights reserved.

Keywords: Amorphous semiconductors Differential scanning calorimetry Optical absorption Glass transition Oxide glasses Borates Optical band gap Optical basicity Oxide ion polarizability X-ray diffraction

1. Introduction There has been an increasing interest in the synthesis, structure and physical properties of heavy metal oxide (HMO) glasses due to their high refractive index, high infrared transparency and high density [1,2]. Glasses based on heavy metal oxide such as Bi2O3 have wide applications in the field of glass ceramics, layers for optical and electronic devices, thermal and mechanical sensors, reflecting windows, etc. [3,4]. Bismuth oxide cannot be considered as network former due to small field strength of Bi3+ ion. However, in combination with B2O3, glass formation is possible in a relatively large compositional range [5]. B2O3 is one of the most common glass former. According to KroghMoe [6] the structure of vitreous B2O3 consists of a random network of boroxyl rings and BO3 triangles connected by B–O–B linkages. It * Corresponding author. Tel.: +91 4027035281; fax: +91 4023240806. E-mail address: [email protected] (G. Bhikshamaiah). /$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2008.09.017

was reported that addition of a network modifier in borate glasses could produce the conversion of the triangular BO3 structural units to BO4 tetrahedra with a coordination number four [7]. A survey of literature shows that there are many reports available on ternary bismuth borate glasses [8–13]. However the studies with zinc addition are very limited [14,15]. The present work is taken up with an objective to characterize the glass system with higher B2O3 content and constant ZnO concentration by means of physical, optical, and infra red studies and to understand structural details.

2. Experimental Bismuth borate glasses of compositions 10ZnO–xBi2O3– (90  x)B2O3 (x in mol% and ranging from 25 to 50 in steps of 5 mol%) were prepared by melt quenching method. The stoichiometric amounts of ZnO, Bi2O3, H3BO3 (AR grade) are thoroughly mixed using spectral grade acetone and loaded into porcelain

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Table 1 Physical parameters of the glass system 10ZnO–xBi2O3–(90  x)B2O3 Parameter (± error limits)

ZnBB1

ZnBB2

ZnBB3

ZnBB4

ZnBB5

ZnBB6

Average molecular weight, M (g/mole) Density, q(g/cc) (±0.001) Molar volume, Vm (cc/mole) (±0.01) Oxygen packing density, O (g atm/l) (±0.01) Zn2+ ion concentration, N (1021/cc) (±0.01) Inter ionic distance, R (Å´) (±0.01) Glass transition temperature, Tg (°C) (±1) Cutoff wavelength, kcutoff (nm) (±1) Optical band gap energy, Eopt (eV) (±0.005) Urbach energy, DE (eV) (±0.001) Oxide ion polarizability, ao2 (Eopt) (Å´3) (±0.005) Optical basicity, K(Eopt) (±0.005) Theoretical optical basicity, Kth

169.88 4.265 39.83 70.29 3.02 6.91 473 344 3.464 0.422 1.320 0.405 0.617

189.7 4.468 42.45 65.96 2.84 7.06 463 353 3.357 0.503 1.514 0.567 0.625

209.52 4.675 44.82 62.47 2.69 7.19 460 358 3.267 0.274 1.691 0.682 0.632

229.34 4.972 46.12 60.71 2.61 7.26 455 365 3.243 0.232 1.733 0.706 0.639

249.16 5.213 47.79 58.59 2.52 7.35 453 371 3.209 0.236 1.809 0.747 0.647

268.97 5.313 50.62 55.31 2.38 7.49 449 375 3.169 0.178 1.958 0.817 0.654

a  qx ; ða  bÞ

where a is the weight of the glass sample in air, b is the weight of the glass sample when immersed in xylene of density (qx) 0.865 gm/cm3. The glass transition temperature (Tg), was evaluated for all the glass samples from thermograms recorded using a Differential Scanning Calorimeter (DSC 2920) supplied by TA Instruments. A small amount (5–15 mg) of the material was taken in the aluminum pan of the DSC setup and scanned at a heating rate of 10 °C/min. The optical absorption spectra of the glass samples were recorded at room temperature using a double beam Shimadzu spec-

3. Results 3.1. Physical parameters The densities (q) of the glass samples determined in the present study are given in Table 1 with probable error of ±0.001. The molar volume (Vm) of the glass samples, was calculated using the molecular weight (M) and density (q) with the following relation:

V m ¼ M=q

ð1Þ

and these values are included in Table 1. Oxygen packing density (O), ionic concentration (N) of Zn2+ ion and inter ionic distance (R) are calculated using the following relations and are presented in Table 1:

O ¼ ðq=MÞ  n;

ð2Þ

where n is the no. of oxygen atoms per formula unit:

N ¼ ð6:023  mol% of cation  valency of cationÞ=V m ;

ð3Þ

R ¼ ð1=NÞ1=3 :

ð4Þ

5.4

5.2

5.0 3



trometer (model UV-3101 PC) in the wavelength range 300– 450 nm. The uncertainty in the observed wave length is found to be ±1 nm. Infrared spectra of the powdered glass samples were recorded at room temperature in the range 400–2000 cm1 using a Perkin–Elmer FT-IS spectrometer (model 1605). These measurements were made on glass powder dispersed in KBr pellets.

density (gm/cm )

crucibles. They are heated in a muffle furnace at 400 °C for 30 min and then melted in the range 1050–1100 °C, depending up on the composition, for about 1 h. The melt was stirred occasionally for proper mixing. After the disappearance of the bubbles, the melt was quickly poured and pressed between two stainless steel plates maintained at 200 °C. The glasses thus obtained were found to be transparent and yellowish in color. These glass samples were annealed for about 15 h at 200 °C to remove thermal strains. The glass samples are labeled as ZnBB1, ZnBB2, ZnBB3, ZnBB4, ZnBB5 and ZnBB6 for x = 25, 30, 35, 40, 45 and 50 mol% of Bi2O3, respectively, as shown in Table 1. Powder X-ray diffraction (XRD) spectra for all the glass samples in the present investigation were recorded at room temperature using a PW 1830 diffractometer with Cu Ka radiation (40 KV X 25 mA) and a graphite monocromator with 2h (h being Bragg angle) from 10° to 90°. XRD spectra of a typical glass sample ZnBB1 is shown in Fig. 1. The absence of Bragg peaks confirms the amorphous nature of the glass. Similar XRD patterns were observed for other glass samples. The density (q) of the glass samples was determined to an accuracy of 0.001 by the standard Archimedes principle. These measurements were done using single pan balance and xylene as an inert immersion liquid. The density (q) was obtained from the relation

4.8

4.6

4.4

4.2 20

25

30

35

40

45

composition of Bi2O3 (mol %) Fig. 1. XRD spectra of ZnBB1 glass sample.

Fig. 2. Variation of density with composition of Bi2O3.

50

55

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52

2

ZnBB6 50

Heatflow (mW) Exo.

3

molar volume (cm /mol)

1 48 46 44 42

0

-1

-2 40 38 20

-3 25

30

35

40

45

50

100

55

200

500

Fig. 5. DSC thermogram of ZnBB6 glass sample.

475

72

glass transition temperature, T g (oC)

70

oxygen packing density, O (g-atm/l)

400

Temperature ( C)

Fig. 3. Variation of molar volume with composition of Bi2O3.

68 66 64 62 60 58 56 54 20

300 o

composition of Bi2O3 (mol %)

25

30 35 40 45 composition of Bi2O3 (mol%)

50

55

470

465

460

455

450

445 20

25

30

35

40

45

50

55

composition of Bi2O3 (mol%)

Fig. 4. Variation of oxygen packing density (O) with composition of Bi2O3. Fig. 6. Variation of glass transition temperature (Tg) with composition of Bi2O3.

The variation of density, molar volume and oxygen packing density with composition of Bi2O3 is shown in Figs. 2–4, respectively. It is evident from Figs. 2–4 and Table 1 that the density and molar volume of the present glass system increases where as the oxygen packing density decreases with increase in the content of Bi2O3. The probable errors in molar volume, oxygen packing density, ionic concentration of Zn2+ ion and inter ionic distance have been calculated and included in Table 1. 3.2. Differential scanning calorimetry (DSC) Fig. 5 shows a DSC thermogram for a typical glass sample ZnBB6. The glass transition temperatures (Tg) determined from DSC thermograms for all the samples are presented in Table 1 along with probable error of ±1 °C. The variation of Tg with composition of Bi2O3 is shown in Fig. 6. 3.3. Optical absorption Fig. 7 shows the optical absorption spectrum of a typical glass ZnBB2. The absorption coefficient, a(x), near the edge of each

curve was determined at wavelength intervals of 5 nm for linear region and 2 nm for the non-linear region, using the relation [16]

aðxÞ ¼ ð1=tÞ lnðI=Io Þ;

ð5Þ

where t is the thickness of each sample and ln (I/Io) corresponds to absorbance. The relation between a(x) and the photon energy of the incident radiation, h  x is given by the following relation [17]:

aðxÞ ¼ const:ðhx  Eopt Þ2 =hx;

ð6Þ

where Eopt is the energy of the optical band gap. The relation (6) can be written as

ðah  xÞ1=2 ¼ const:ðhx  Eopt Þ:

ð7Þ

Using the relation (7) the Eopt values were determined by the hx)1/2 against extrapolation of the linear region of the plots of (a 1/2 hx) = 0 as shown in Fig. 8. The values of the Eopt thus obhx to (a  tained for all the glass samples are given in Table 1 along with the probable error of ±0.005 eV. The relation between a(x) and Urbach energy (DE) is given by the well known Urbach law given by the relation

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5.0

ZnBB2

4.5

Absorbance (arb. units)

4.0 3.5 3.0 2.5 2.0 1.5 1.0 345

360

375

390

405

wavelength λ (nm) Fig. 9. ln a plotted against photon energy,  hx, for ZnBB2 glass sample.

Fig. 7. Optical absorption spectrum of ZnBB2 glass sample.

tion of Bi2O3 is shown in Fig. 10. It can be seen from Fig. 10 that Eopt decreases with the increase of Bi2O3 content. 3.4. Oxide ion polarizability (ao2 ) Dimitrov and Sakka [18] have derived the relationship between

ao2 and Eopt using the relationship proposed by Duffy [19]. This relationship has been modified by Banu et al. [20] and found to be applicable for many glass systems. The relation is given as

ao2 ðEopt Þ ¼

! #  1=2  X Eopt  0:98 Vm 1  pai q1 : 2:52 1:23 i

"

ð10Þ

The oxide ion polarisability values ao2 (Eopt) obtained using Eopt for the present glass system are calculated using Eq. (10) and given in Table 1. The probable error in ao2 was found to be ±0.005 Å´3 and the same is included in Table 1. The variation of ao2 with the composition of Bi2O3 is shown in Fig. 11. It is evident from Fig. 11 that ao2 increases with increase in the content of Bi2O3. Fig. 8. (a hx)1/2 as a function of photon energy,  hx, for ZnBB2 glass sample.

3.50

DE is usually interpreted as the width of the tail of the localized states in the band gap. The relation (8) can be rewritten as

3.45

ln aðxÞ ¼ ð hx=DEÞ  const:

ð9Þ

Urbach plots are the plots where the natural logarithm of absorption coefficients, ln a, is plotted against photon energy, hx. In the present study such an Urbach plot for a typical glass  ZnBB2 is shown in Fig. 9. The values of Urbach energy (DE) were calculated by determining slopes of the linear regions of the curves and taking their reciprocals. The values of DE thus determined are presented in Table 1. The probable error in DE is found to be ±0.001 eV and is included in Table 1. It is found that Urbach energy, DE decreases with increase in the content of Bi2O3 in this glass system. The cutoff wavelength obtained for various glass samples from optical absorption spectra are given in Table 1 along with the probable error (±1 nm). It can be seen from the Table 1 that the cutoff wavelength shifts to longer wavelength with increase in Bi2O3 content. The variation of optical band gap energy (Eopt) with composi-

optical band gap energy, E opt (eV)

ð8Þ

aðxÞ ¼ const: expðhx=DEÞ;

3.40 3.35 3.30 3.25 3.20 3.15 25

30

35

40

45

50

composition of Bi2O3 (mol%) Fig. 10. Variation of optical band gap energy (Eopt) with composition of Bi2O3.

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is ±0.005 and included in Table 1. The optical basicity values calculated using the Eq. (11) correlate well with the values calculated using Eq. (12). The variation of optical basicity, K(Eopt) with the composition of Bi2O3 is shown in Fig. 12.

2.0 1.9

oxide ion polarizability

5577

1.8

3.6. IR spectra

1.7

The infrared spectra recorded for all the glass samples are shown in Fig. 13. All the glass compositions show bands at 450 cm1(w), 470–490 cm1 (w), 520 cm1 (w), 700 cm1 (s), 1020–1030 cm1 (bs), 1310–1340 cm1 (bs), 1450 cm1 (w), 1560 cm1 (w) and 1650–1690 cm1 (w). Shoulders around 840 cm1 and 1234 cm1 were also observed.

1.6 1.5 1.4

4. Discussion

1.3 25

30

35

40

45

4.1. Density, glass transition temperature and oxygen packing density

50

composition of Bi2O3 (mol%) Fig. 11. Variation of oxide ion polarizability(ao2 Þ with composition of Bi2O3.

3.5. Theoretical optical basicity The theoretical optical basicity (Kth) for the glass system under study has been calculated using the relation [21]

Kth ¼ XðZnOÞKðZnOÞ þ XðBi2 O3 ÞKðBi2 O3 Þ þ XðB2 O3 ÞKðB2 O3 Þ;

ð11Þ

where X(ZnO), X(Bi2O3), X(B2O3) are the equivalent fractions of the different oxides, i.e., the proportion of the oxide atom that contributes to the glass system and K(ZnO), K(Bi2O3), K(B2O3) are the optical basicity values of the constituent oxides. Here the values of K(ZnO) = 0.82, K(Bi2O3) = 1.19 and K(B2O3) = 0.425 have been taken from the literature [18]. The calculated values of Kth are presented in Table 1. The values of optical basicity can also be calculated using oxide ion polarizability ao2 (Eopt) obtained from optical band gap by the relation suggested by Duffy [22]

 KðEopt Þ ¼ 1:67 1 

 1 : ao2 ðEopt Þ

ð12Þ

Substituting the values of ao2 (Eopt) into Eq. (12) the optical basicity values of the present glass system are calculated and tabulated in Table 1. The probable error evaluated in ao2 (Eopt) values

0.8

optical basicity

0.7

It can be seen from the Fig. 2 and Table 1 that the density of ZnBB glasses increases as the Bi2O3 content increases. This is due to the high relative molecular mass of Bi2O3 compared to other glass constituents. Fig. 6 shows that the glass transition temperature (Tg) decreases with increase in the Bi2O3 content. The decrease in Tg may be due to the increasing number of non-bridging oxygen atoms as Bi2O3 content increases. It is obvious that the decrease in Tg is due to increase in the number of Bi–O linkages which are weaker than B–O linkages. It may be noted that the bond strength of Bi–O is 81.9 kcal mol1 and the bond strength of B–O is 192.7 kcal mol1 [23]. Oxygen packing density which is a measure of the tightness of packing of the oxide network can also be used to explain the decrease in Tg with the increase in Bi2O3 content. It can be seen from Fig. 4 that oxygen packing density decreases as the concentration of Bi2O3 increases. This indicates that the structure becomes loosely packed with increase in the concentration of Bi2O3. A looser macromolecular structure requires smaller internal energy for the chain mobility which is needed for the glass transition. Thus, the addition of Bi2O3 indicates the formation of a more open macromolecular chain in the present glass system leading to decrease in Tg. 4.2. Theoretical optical basicity It can be observed from Table 1 that the theoretical optical basicity (Kth) values increase with increase in Bi2O3 content. This may be understood according to Eq. (12). This equation shows that the basicity increases with increase in polarizability. It is well known in the literature that Bi3+ ions are highly polarizable. Therefore, increase in Bi2O3 content causes an increase in optical basicity in these glasses as shown in Fig. 12 [8]. 4.3. IR spectra

0.6

0.5

0.4 25

30

35

40

45

50

Composition of Bi2O3 (mol%) Fig. 12. Variation of optical basicity, K(Eopt) with composition of Bi2O3.

Bi2O3 containing glasses have four fundamental vibrations in the IR spectral regions at 830, 620, 450 and 350 cm1. Boron also has three vibrational bands at 1200–1600, 800–1200 and at 700 cm1. The boron–oxygen network can be in the form of planar BO3 and/or tetrahedral BO4. The planar BO3 gives four fundamental bands around 950 (m1), 750 (m2), 1250 (m3) and 600 (m4) cm1. Tetrahedral BO4 unit also gives four bands around 1000 (m1), 900 (m2), 600 (m3) and 550 (m4) cm1. The weak band observed at 1650–1690 cm1 is due to OH bending mode of vibration [23]. The origin of this weak band may be due to the trapping of water molecules in the glass matrix

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Fig. 13. Infrared spectra of 10ZnO–xBi2O3–(90  x)B2O3 glasses.

Table 2 Band positions and corresponding assignments of IR spectra of all glass compositions Bands and shoulders

Assignment

450 cm1 (w) and 470–490 cm1 (w) 520 cm1 (w) 700 cm1 (s) Shoulder at 840 cm1 1020–1030 cm1 (bs) Shoulder at 1234 cm1 1310–1340 cm1 (bs) 1450 cm1(w) and 1560 cm1 (w) 1650–1690 cm1 (w)

Bi–O vibrations m4 vibration of BO4 tetrahedra The bending vibration of B–O–B linkages of BO3 units Symmetrical stretching vibration of the Bi–O bonds in the [BiO3] groups m1 mode of BO4 tetrahedra B–O stretching vibrations of (BO)3 units m3 mode of planar BO3 group Asymmetrical stretching of the [BO3]units OH bending mode of vibration

during the decomposition of boric acid. The weak bands observed around 1560 cm1 and 1450 cm1 are due to asymmetrical stretching of the [BO3] units [9]. The broad and strong band observed in the range 1310–1340 cm1 is due to m3 mode of planar BO3 group while the broad and strong band at 1020–1030 cm1 is due to m1 mode of BO4 tetrahedra. These bands become more intense with increase in Bi2O3 content.The shoulder around 1234 cm1 found in this glass system is assigned to the B–O stretching vibrations of (BO)3 units with non-bridging oxygen atoms [9,23,24]. It can be seen from Fig. 13 that this shoulder becomes more predominant as the content of Bi2O3 increases. This result indicates that the number of non-bridging oxygen atoms increases with increase in the content of Bi2O3. The shoulder around 840 cm1 becomes more predominant as the Bi2O3 content increases and is related to the symmetri-

cal stretching vibration of the Bi–O bonds in the [BiO3] groups [11]. The strong band at 700 cm1 is due to the bending vibration of B–O–B linkages of BO3 units [9,25]. The weak band observed at 520 cm1 is due to m4 vibrations of BO4 tetrahedra and the weak bands below 500 cm1 are due to Bi–O vibrations [10,11]. The increase in Bi2O3 content in the present glass system transforms the structure into a bismuthate one formed by [BiO6] and [BiO3] groups. The band positions and the corresponding assignments are given in Table 2 for all the glass compositions. 4.4. Optical absorption It can be seen from Table 1 that the position of the fundamental absorption edge or cutoff wavelength in these glasses shifts from

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344 nm to 375 nm as the content of Bi2O3 increases from 25 to 50 mol%. The absorption edge in disordered materials at the higher levels of absorption coefficient (P10 cm1) is usually interpreted in terms of indirect transitions across an optical gap according to the theory of electronic structure of amorphous materials [17]. For such absorption by indirect transitions, the absorption coefficient, a(x), is given by the relation (6) which is used for determination of Eopt given in Table 1. It is seen from the Fig. 10 and Table 1 that the optical band gap decreases with increase in Bi2O3 content in the glass. It is to be noted that for lower values of the absorption coefficient, a, i.e., for the absorption in the region of lower photon energy of the edge the relation (6) gradually changes into an exponential dependence, as given by the relation (8). As observed by Chopra and Bahl [26], the exponential tail observed in various materials and in their different structures must have the same physical origin. This origin can be attributed to the phonon-assisted indirect electronic transitions. Since the basic building units of zinc bismuth borate glasses are known to be BO4 tetrahedra, probably the internal vibrations of the molecular ion groups BO4 take part in the indirect transitions. Urbach energy, DE, is usually interpreted as the width of the tail of localized states in the band gap. In the present work, DE, obtained from the equivalent Urbach plots corresponds to lower energy part of the absorption edge. In the present glass system, the shift of the absorption edge or cutoff wavelength to longer wavelength and the decrease of Eopt to lower energies with increase in Bi2O3 content are related to the progressive increase in the concentration of non-bridging oxygen (NBO) atoms. This increase in turn gives rise to a possible decrease in the (B–O–B) bridging oxygens (BO). The shift is attributed to the structural changes which are as a result of the differing site occupations, i.e., interstitial or substitutional, of the Bi3+ ions which add to the zinc borate matrix and modify the network. We assume that as the cation concentration increases, the bridging oxygens (BO) develop bonds with Bi3+ which in turn lead to the gradual breakdown of the glass network. This breakdown seems to account for the decrease in the Eopt value, i.e., edge shifts to longer wavelengths, as Bi2O3 content is increased from 25 to 50 mol %. Such a decrease in Eopt can thus be attributed to decrease in the photon-assisted indirect transitions. The value of Eopt in the present glass system lies in the range 3.169–3.464 eV. A recent study [15] of Bi2O3–B2O3 glass system with addition of ZnO showed that the value of Eopt lies in the range 2.951–2.994 eV and does not vary much even though the ZnO is added a maximum of 40 mol %. The higher values of Eopt found in the present glass system, therefore may be due to the higher content of B2O3. 5. Conclusions The decrease in the values of glass transition temperature (Tg) from DSC studies and optical band gap (Eopt) from optical absorption analysis indicate that the glass network becomes less tightly packed and degree of disorder increases with increase of concentration of Bi2O3. This fact has been supported by IR studies in which the number of non-bridging oxygens (NBOs) increases which leads to loosening of structure with increase in the content of Bi2O3.

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The value of Eopt depends on the concentration of B2O3 irrespective of ZnO concentration present in ZnO–Bi2O3–B2O3 glasses. The infrared spectral analysis of the present glass system shows that Bi3+ cations are incorporated in the glass network as [BiO3] pyramidal and [BiO6] octahedral units. The band below 500 cm1 is due to vibrations of Bi–O bonds in the [BiO6] polyhedra while the shoulder at 840 cm1 is related to symmetrical stretching vibrations of the Bi–O bonds in the [BiO3] groups. Oxide ion polarizability ao2 of these glasses increases with increase in the content of Bi2O3. The values of optical basicity evaluated using oxide ion polarizability ao2 (Eopt) obtained from optical band gap are in agreement with those values calculated from optical basicity values of the constituent oxides. Acknowledgments The authors (D. Saritha, Y. Markandeya and G. Bhikshamaiah) wishes to thank the Principal, Nizam College, Osmania University, Hyderabad for his encouragement and providing necessary facilities. They also thank Head, Department of Physics, Osmania University, for providing experimental facilities. One of the authors (D. Saritha) thanks the Principal, University College for Women, Osmania University, Hyderabad for her encouragement.

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