Optical, physical and structural studies of boro-zinc tellurite glasses

Optical, physical and structural studies of boro-zinc tellurite glasses

Physica B 406 (2011) 1242–1247 Contents lists available at ScienceDirect Physica B journal homepage: www.elsevier.com/locate/physb Optical, physica...

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Physica B 406 (2011) 1242–1247

Contents lists available at ScienceDirect

Physica B journal homepage: www.elsevier.com/locate/physb

Optical, physical and structural studies of boro-zinc tellurite glasses P. Gayathri Pavani a,n, K. Sadhana b, V. Chandra Mouli a a b

Glassy Material Research Laboratory, Department of Physics, Osmania University, Hyderabad 500 007, India School of Physics, Hyderabad Central University, Hyderabad 500 046, India

a r t i c l e i n f o

a b s t r a c t

Article history: Received 27 September 2010 Received in revised form 3 January 2011 Accepted 4 January 2011 Available online 6 January 2011

To investigate the modification effect of the modifier ZnO on boro-tellurite glass, a series of glasses with compositions 50B2O3–(50  x)ZnO–xTeO2 have been prepared by conventional melt quenching technique. Amorphous nature of the samples was confirmed through X-ray diffraction technique. Optical absorption and IR structural studies are carried out on the glass system. The optical absorption studies revealed that the cutoff wavelength increases while optical band gap (Eopt) and Urbach energy decreases with an increase of ZnO content. Refractive index evaluated from Eopt was found to increase with an increase of ZnO content. The compositional dependence of different physical parameters such as density, molar volume, oxygen packing density, optical basicity, have been analyzed and discussed. The IR studies showed that the structure of glass consists of TeO4, TeO3/TeO3 + 1, BO3, BO4 and ZnO4 units. & 2011 Elsevier B.V. All rights reserved.

Keywords: Tellurite glasses Optical energy gap Non-bridging oxygens Optical basicity IR

1. Introduction

2. Experimental procedure

There has been an increasing interest in synthesis, structural and physical properties of heavy metal oxide glasses due to their high refractive index, high density and high IR transparency [1,2]. Tellurium oxide (TeO2) based glasses are of scientific and technical interest on account of their various unique properties and have been considered as promising materials for use in optical amplifiers because of their large third order non-linear susceptibility [3,4]. Optically transparent TeO2 based glass–ceramics showing second harmonic generation have been discovered [5,6]. B2O3 is one of the best and well known glass former. Addition of small amount of TeO2 into borate glass network enhances glass quality with an improvement in transparency, refractive index. Addition of ZnO into boro-tellurite glass network produces low rates of crystallization and increase glass forming ability. The objective of the present work is to study the effect of modifier ZnO on boro-tellurite glasses. Optical parameters and their variation with modifier have been discussed. Variation in glass structure is observed by carrying out density measurement and correlating different physical parameters with ZnO in zinc boro-tellurite glass system. The zinc boro-tellurite network structure is studied by means of FT-IR characterization.

2.1. Glass preparation

n

Corresponding author. Tel.: +91 40 27682242; fax: + 91 40 27099020. E-mail address: [email protected] (P. Gayathri Pavani).

0921-4526/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2011.01.006

Zinc boro-tellurite glasses of compositions 50B2O3–xZnO– (50 x)TeO2 (x in mol% ranging from 10 to 50 in steps of 10) were prepared by melt quenching technique. The stoichometric amounts of analar grades zinc oxide, boric acid, tellurium dioxide are thoroughly mixed using spectral grade acetone and loaded into porcelain crucibles. They are heated in high temperature furnace and melted in the range 800–950 1C depending on the glass composition. The melt was stirred occasionally for proper mixing. After the disappearance of bubbles the melt was quickly poured and pressed between two stainless steel plates maintained at 200 1C. The glass samples were annealed for about 12 h at 300 1C to remove thermal strains. The composition of the glass samples employed in the present study are tabulated (Table 1).

2.2. Glass characterization To check the non-crystallinity of the glass samples, X-ray measurements were performed using copper target (Ka ¼1.54 A1) on Philips PW (1140) diffractometer at room temperature. The results showed that XRD patterns of the glasses (shown in Fig. 1) did not reveal any discrete or sharp peaks, but broad humps, the characteristic of amorphous materials. The optical absorption spectrum of the present glass samples was recorded at room temperature using a double beam Schimadzu Spectrophotometer

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3. Results and discussion

Table 1 Composition for 50B2O3–xZnO–(50  x)–TeO2. S. no.

Sample

B2O3 (mol%)

ZnO (mol%)

TeO2 (mol%)

1 2 3 4 5 6

T0 T1 T2 T3 T4 T5

50 40 30 20 10 0

50 40 30 20 10 0

0 10 20 30 40 50

3.1. Determination of optical band gap, Urbach energy and refractive index Fig. 2 shows optical absorption spectrum of present boro-zinc tellurite glass system. It is clear that there is no sharp absorption edge which corresponds to characteristic of glassy state. It is also observed from Table 1 that the fundamental absorption edge shifts to longer wavelength with increase in ZnO; this may be due to the less rigidity of the glass system as the ZnO content increases. At 0 mol% of ZnO lowest cutoff wavelength or highest band gap is observed due to the formation of TeO4 units that are altering with ZnO4 units. This is supported by means of IR characterization. Absorption coefficient near the edge of each curve was determined at wavelength of 5 nm using the relation

aðoÞ ¼

2:303A t

ð3Þ

where t is the thickness of the sample and A corresponds to absorbance. The relation between a(o) and the photon energy of incident radiation ho is given by the relation

aðoÞ ¼

BðhuEopt Þ2 hu

ð4Þ

where Eopt is the optical energy gap, hn is the photon energy and B is a constant called band tailing parameter. The relation can be written as ðahuÞ1=2 ¼ BðhuEopt Þ

Fig. 1. XRD pattern of boro-zinc tellurite glass system.

in the wavelength region 300–800 nm. Thickness of the glass specimens was measured by means of micrometer gage. The density of the glass samples was determined by standard Archimedes principle. These measurements were done using single pan balance and xylene as an immersion liquid.



a r ab x

Using this relation Eopt values are determined by extrapolation of linear region of the plots of ðahuÞ1=2 against hn to ðahuÞ1=2 ¼ 0 as in Fig. 3. The values of Eopt thus obtained for all glass samples are given in Table 2. In ZnO doped boro-tellurite glasses optical energy gap decreases with increase of ZnO (decrease of TeO2) for constant B2O3. In boro-tellurite glasses Eopt decreases with increase of TeO2 [10]. Results of optical energy gap are quite close with those found in literature of tellurite glasses [11–13]. Addition of TeO2 to ZnO is to produce a break down of continuous ZnO network, which is reflected in the absorption spectra by a significant shifting of absorption edge to longer wavelengths. The

ð1Þ

where a is weight of the glass sample in air and b is the weight of the sample when immersed in xylene of density (rx) 0.865 gm/cm3. Theoretical optical basicity for the glass system under study has been calculated using basicities assigned to the individual oxides on the basis of the following equation proposed by Duffy and Ingram [7,8]:

Lth ¼ xðZnOÞLðZnOÞ þ xðB2 O3 ÞLðB2 O3 Þ þ xðTeO2 ÞLðTeO2 Þ

ð2Þ

where x(ZnO), x(B2O3), x(TeO2) are the equivalent fractions of different oxides, i.e. the proportion of the oxide atom they contribute to the glass system and L(ZnO), L(B2O3), L(TeO2) are optical basicity values assigned to the constituent oxides taken from Ref. [9]. IR spectroscopy is the most advantageous tool and has been extensively employed over the years to investigate the structure of glasses; we have used it to determine structure of tellurite glasses containing various amounts of ZnO and TeO2. The vibration spectra of the obtained glass samples was recorded at room temperature using KBr pellet technique in the range 400–4000 cm  1 on FT-IR Spectrophotometer.

ð5Þ

Fig. 2. Wavelength vs. absorbance for B2O3–ZnO–TeO2 glass system.

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shifts of absorption edge are more likely related to structural rearrangements of the glass and relative concentrations of various fundamental units.

Stevels [14] suggested that the movement of absorption band to lower energy corresponds to the transition from the nonbridging oxygen, which binds an electron more loosely than bridging oxygen. The variation of Eopt with ZnO composition in the glass system is shown in Fig. 4. The results in the figure can be explained by suggesting that non-bridging oxygen ion content increases with increase of ZnO shifting band edge to lower energy leading to a decrease in optical energy gap. Due to this increase of non-bridging oxygens the glass structure becomes more randomized; the addition of ZnO causes the breaking of the regular structure of borate and tellurite as a result of this the band gap decreases. Band tailing parameter is determined form the slope of (ahn)1/2 against hn (Fig. 3). Its value lies between 5.71 and 12.643 (cm eV)  1. Refractive index is determined from optical energy gap using the relation proposed by Dimitrov and Sakka [15] rffiffiffiffiffiffiffiffi Eopt n2 1 ¼ 1 20 n2 þ 2

Fig. 3. (ahn)1/2 as a function of energy for (hn) B2O3–ZnO–TeO2.

Table 2 Optical parameters of boro-zinc tellurite glass. Sample Cut off wavelength (nm)

Optical energy gap Eopt (eV)

Band tailing Urbach energy DE parameter B (cm eV)  1 (eV)

Refractive index (n)

T0 T1 T2 T3 T4 T5

2.55 2.599 2.623 2.649 2.675 2.73

0.2443 0.248 0.292 0.296 0.325 0.352

2.53 2.514 2.507 2.499 2.491 2.474

443 441 439 436 430 426

5.71 6.284 6.811 9.641 12.31 12.643

ð6Þ

From Fig. 4 it is clear that refractive index decreases with increase of Eopt. Increase of ZnO increased the refractive index from 2.474 to 2.53. This value is greater than of zinc tellurite glass. The increase of refractive index may be due to lower cation polarizability of Zn2 + ions than TeO4 + ions. Urbach plots are the plots where natural logarithm of absorption coefficients (ln a) is plotted against photon energy (hn). Such an Urbach plot for present boro-zinc tellurite glass system is shown in Fig. 5. The values of Urbach energy were calculated by determining slopes of the linear regions of the curves at lower phonon energies and taking their reciprocals. The values of Urbach energy for a range of amorphous semiconductors [16] lies between 0.045 and 0.567 eV. For the glasses investigated in the present study the value of Urbach energy lies between 0.244 and 0.352 eV. The estimated values are consistent with the reported values of V2O5–P2O5–TeO2 glasses (0.31–0.68 eV) and of P2O5–TeO2, Bi2O3–P2O5–TeO2 (0.17–0.67 eV) depending on glass composition [17–19]. It is found that Urbach energy decreases with increase of ZnO content decreasing the fragility nature of the glass network.

Fig. 4. Variation of Eopt and n with composition of ZnO (mol%).

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3.2. Density, molar volume, oxygen packing density and optical basicity: Addition of ZnO in present glass system decreases the density however in Zinc tellurite glass system increase of ZnO increased

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the density [20]. This behavior is generally observed when content of modifier oxide is increased in borate glass. The change in density of such system is related to density of formed structural units when introducing modifier oxide. Because density is an additive property it can be proposed that each oxide in glass would have its contribution in density. Density at x¼50 mol% of ZnO is 3.8 gm/cm3 (Table 3) and its value increases as content of ZnO into glass network decreases (Fig. 6). The decrease in density on increasing the modifier content ZnO is related to the addition of ZnO (atomic mass 81.35) at the expense of TeO2 (atomic mass 159.599). When higher molecular weight substance is substituted by lower molecular weight substance net molecular weight decreases resulting in weak connectivity in glass network [21] and hence decrease in density with increase of ZnO at expense of TeO2 is observed. Molar volume (Vm) and oxygen packing density (O) are calculated using the relation Molar volume ðVm Þ ¼

M

ð7Þ

r

M is molecular weight and r is density. Oxygen packing density ðOÞ ¼

Fig. 5. Urbach plot of B2O3–ZnO–TeO2. Table 3 Physical parameters of boro-zinc tellurite glass system. Sample Density r Molar volume (g/cc) Vm (cc/mole)

T0 T1 T2 T3 T4 T5

3.85 4.19 4.321 4.422 4.444 4.698

26.629 26.335 27.347 28.491 30.11 30.47

Oxygen packing density (O.P.D.) (g atm/l)

Optical basicity (L)

75.107 79.743 80.449 80.728 79.708 82.045

0.775 0.758 0.741 0.724 0.707 0.69

rO M

ð8Þ

O is number of oxygen atoms per formula units and these values are included in Table 3. Fig. 6 shows variation of Vm and r with ZnO. It is clear that with decrease of ZnO, both density and molar volume decrease. In general it is expected that the density and molar volume should show opposite behavior to each other, but in the present glasses the behavior is different. However, this anomalous behavior was earlier reported for many glass systems, for example, TeO2–NbO– Bi2O3 [22], Na2O–B2O3–TeO2 [23], Bi2O3–Li2O–B2O3 [24]. Here molar volume is directly proportional to glass density and molecular weight. The increase in molar volume from 26.629 to 30.47 cc/mole indicates an increase in free volume with decrease of ZnO. The larger values of radii and bond length of TeO2 compared to ZnO caused increase of molar volume at decrease of ZnO. Variation of oxygen packing density with the composition of ZnO is shown in Fig. 7 and values are in Table 3. Decrease in oxygen packing density with increase of ZnO indicates that formation of

Fig. 6. Variation of density and molar volume with composition of ZnO.

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Fig. 7. Variation of oxygen packing density with composition of ZnO. Fig. 9. IR spectra of boro-zinc tellurite glass.

Table 4 IR band assignments for B2O3–ZnO–TeO2 glass system. Characteristic band Assignment (cm  1) 418 660–665 680–728 694 921–1007 1200–1253 1388–1410 2500–3500

Fig. 8. Variation of optical basicity with composition of ZnO.

TeO4 and BO4 units is prevented revealing that NBOs increase in number. This can be interpreted in terms of formation of more rigid and highly cross linked network resulting in tightly packed glass structure. The calculated values of optical basicity Lth are presented in Table 3. The values of optical basicity vary from 0.707 to 0.775 for the present glass system. From Fig. 8 it can be concluded that basicity increases with increase of ZnO. This is because ZnO is having larger basicity (1.13) than TeO2 (0.96). Replacement of higher basicity by lower will increase the optical basicity and covalency nature of the glasses decreases. Moreover with increase of ZnO the number of non-bridging-oxygens increase, which cause a decrease in Eopt and increase in optical basicity. 3.3. FT-IR spectral studies The FT-IR spectra of the present glasses (Fig. 9) is characterized by intense absorption bands in the frequency regions 600–665, 680–728, 921–1007, 1200–1253, 1388–1410, 2500–3500 cm  1and an additional peak at 418 for T2 .Observed absorption peak for all compositions are presented in Table 4. The absorption band

Vibrations of ZnO tetrahedral (ZnO4). Te–O bonds stretching vibrations in TeO4 units. Te–O bending vibrations in TeO3 units and TeO6 units. Assigned to bending vibrations of B–O linkages in the borate network. B–O–B bending vibrations in BO4 units. B–O stretching vibrations in BO3 units from boroxol rings. B–O stretching vibrations in BO3 units from varied types of borate groups. H–O–H or water groups.

centered at 640 cm  1 is the characteristic of pure TeO2 glass [25–27]. The structure pattern of tellurium containing glasses is determined by trigonal pyramid TeO3 and bipyramid TeO4. The absorption in the range 600–700 cm  1in such glasses is determined by stretching vibrations of Te–O bonds in TeO3 and TeO4 groups. The absorption of TeO3 group has a high frequency position than TeO4 group. In general case the absorption band range of TeO3 group correlates with frequency of 650–700 cm  1 and that of TeO4 group correlated with 600–650 cm  1. The band shifts of these groups depend on changes in the composition of the glass network. Analyzing obtained results of the present glass system it is clear that TeO3 groups are present in all the tellurite containing glasses because of the appearance of the band at 680–728 cm  1 [28,29]. In the present glass system when ZnO mol% decreased from 40 to 20 mol% a band at 619 cm  1 is observed in addition to those at 681 and 748 cm  1, which indicate the formation of Te–O–Zn bonds in the glass network; this suggests the formation of TeO4 units at the expense of TeO3 units. At low proportion of ZnO from 10–20 mol% it enters the glass network by breaking up of Te–O–Te, Te–O–B bonds and introduces coordination effects called as dangling bonds (Te–O  yZn + 2yO–Te), which in turn decrease TeO3 units by forming TeO4 units. When ZnO content is increased from 20 to 40 mol% a considerable proportion may act as double bridge between TeO4 and BO4 units such as¼Te–O–Zn–O–B¼that can be formed beside the formation of ZnO4 units. At 30 mol% ZnO, ZnO

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participates in the glass network with ZnO4 structural units and alternate with TeO4 units that is supported by the appearance of absorption peak at 418 cm  1 the characteristic of ZnO4 unit [30–32]. ZnO in this case acts as a glass forming agent and is incorporated in the glass structure in the formation of ZnO4 units. Therefore up to 20 mol% it places role of network modifier. In all the other glasses normal ZnO band does not appear in the region of 400–550 cm  1. This means that zinc lattice is completely broken down and T0 glass may be approximately formulated as ZnB4 O7 [31]. There is a good agreement between the data of sample T0 and those from literature concerning IR spectra of Zinc borate glass. The absorption peak at 694 cm  1 is attributed to vibrations of B–O–B linkage in which both boron atoms are triangularly coordinated [31]. In pure B2O3 glass, the 806 cm  1 frequency is a characteristic of boroxol ring. In the present glass system the vanishing of 806 cm  1 means no boroxol ring in the glass structure; ultimately it consists of BO3 and BO4 groups. These groups may be attached in the form of random network. This corresponds to the progressive substitution of boroxol ring by BO3 and BO4 groups. For the present glass system the peaks located in the 921–1007 cm  1 range are assigned to [BO4] units while those at 1200–1253 cm  1 are assigned to stretching vibrations of BO3 units from boroxol rings and 1388–1410 cm  1 B–O stretching vibrations in [BO3] from varied types of borate groups [28,33]. The bands between 2953 and 3610 cm  1 correspond to stretching vibrations of hydrogen group in hydrogen bond with incorporation of ZnO. Increase in ZnO content of studied glasses generates (i) a significant increase of band at 921 and the shift of this band to 1007 cm  1 (attributed to the vibrations of [BO4] structural units), (ii) the shift of band from 600 to 728 cm  1 is assigned to TeO4 and TeO3 units, respectively, and (iii) the shift of band from 1404 to 1397 cm  1 is attributed to vibrations of BO3 units from varied types of borate groups. Taking into consideration these structural changes of IR spectra we assume that increase of ZnO in the glass structure leads to the following: (i) the disintegration of some boroxol units and the transformation of some trigonal [BO3] units into tetrahedral [BO4] units and (ii) the number of TeO4 groups with nonbridging oxygen decrease because some trigonal bipyramidal [TeO4] units were transformed to trigonal [TeO3] pyramidal units. (iii) At 30 mol% of ZnO, TeO4 is replaced by ZnO4 indicating the presence of ZnO4 structure. 4. Conclusion Ternary boro-zinc tellurite glasses have been successfully synthesized. The different optical, physical and structural analysis of each glass sample was carried out. It could be concluded that increase of ZnO results in decrease of optical energy gap owing to increase in Non-bridging oxygen ions shifting the band edge to longer wavelength. Refractive index increases with increase of ZnO due to decrease in optical energy gap. The optical basicity values are found to be in range 0.707–0.775 for the present glass system. Density of boro-zinc tellurite glasses is decreased with

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the addition of ZnO as lower molecular weight (ZnO 81.35 gm) is substituted by a higher molecular weight (TeO2–159.599 gm). IR studies revealed that the incorporation of modifier oxide (ZnO) in place of TeO2 increased the number of non-bridging oxygens by gradually replacing trigonal bipyramids [TeO4] units with trigonal pyramids [TeO3] through [TeO3 + 1]. Also BO4 units are replaced by BO3 units. At 30 mol% of ZnO, ZnO4 units are observed and in all other glasses this group is absent concluding that zinc lattice is completely broken.

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