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Studies on boro cadmium tellurite glasses
Optical Materials 34 (2011) 215–220
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Studies on boro cadmium tellurite glasses P. Gayathri Pavani ⇑, S. Suresh, V. Chandra Mouli Glassy Material Research Laboratory, Department of Physics, Osmania University, Hyderabad 500 007, India
a r t i c l e
i n f o
Article history: Received 18 April 2011 Received in revised form 1 August 2011 Accepted 12 August 2011 Available online 22 September 2011 Keywords: Tellurite glasses Optical energy gap Refractive index IR and Raman
a b s t r a c t To investigate the modification effect of the modifier CdO on boro tellurite glass, a series of glasses with compositions (50 x) CdO–xTeO2–50B2O3 have been prepared by conventional melt quenching technique. Optical absorption, IR and Raman structural studies are carried out on the glass system. The optical absorption studies revealed that the cutoff wave length and refractive index increase while optical band gap (Eopt) and Urbach energy decreases with increase of CdO content. The IR and Raman studies revealed that structure of glass network consists of [TeO3]/[TeO3+1], [TeO4], [BO3], [BO4] and [Cd–Te] linkages .The compositional dependence of different physical parameters such as density, molar volume, oxygen packing density, optical basicity, have been analyzed and discussed. Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction Structural studies in glasses are of important owing to the interrelation between the atomic arrangement and properties. To comprehend structural details, a common strategy is to change the glass structure either by adding network modifiers or by interrogating the material with some external stimuli such as high heat or pressure. Tellurium oxide is an interesting case where the basic building block of structure experiences considerable changes either by modifiers or by increasing temperature, though being of three-dimensional geometry. Tellurite glasses possess interesting glass-forming ability, glass structure, no hygroscopic properties and low melting point [1,2]. Tellurium oxide-based glasses are of scientific and technological interest on account of their low melting temperatures [3,4] large thermal expansion [5] high refractive index refractive indices, high dielectric constants and good IR transmissions, thus they have been considered as promising materials for use in optical fibers and non-linear optical devices [6–10]. Crystalline TeO2 has a unique structural unit that is an asymmetrical TeO4 trigonal bipyramid (tbp), in which there are two different kinds of sites, two axial and three equatorial positions. One of the latter is occupied by a lone pair of electrons. The structural of TeO2 based glasses is of interest, because there are two types of basis structural unit, i.e. TeO4 tbp with a lone pair electron in an equatorial position and TeO3 trigonal pyramid (tp). The tellurite glass system is an important amorphous system considered to have important commercial applications in optical communication
⇑ Corresponding author. Tel.: +91 40 27682242; fax: +91 40 27099020. E-mail address: [email protected] (P. Gayathri Pavani). 0925-3467/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2011.08.016
due to its high refractive index, good infrared transmittance and high optical non-linearity [11–15].These peculiar characteristics of tellurium based glasses attracted researchers towards preparation of combination of glass formers like B2O3, V2O5, etc. for its desired optical properties. Boro tellurite glasses lead to complex specification in glass structure [16]. Keeping these objectives in view B2O3–TeO2–K2O [17], ZnO–TeO2, [18] 50B2O3–(50 x) ZnO– xTeO2 [19],TeO2–V2O5–MoO3 [20]The boric oxide (B2O3) which is used as glass formers in many glass systems is incorporated into various kind of glass systems as a flux material in order to attain good physical and chemical properties. In their crystalline form, borates with various compositions are of exceptional importance due to their interesting linear and nonlinear optical properties [21].The change in structure and physical properties of binary and ternary tellurite glasses such as B2O3–TeO2 [22], V2O5–CdO– B2O3 [23], Ag2O3–B2O3–TeO2 [24], TeO2–B2O3–PbO2 [25], mixed alkali effect in boro tellurite [26], cadmium borate glasses [27] have been widely studied. But to the knowledge of the author no study was reported on cadmium based boro tellurite glass system. Hence it is found to be interesting to study the structural, physical and optical properties of the novel ternary (50 x) CdO–xTeO2– 50B2O3 glass system.
2. Experimental procedure 2.1. Glass preparation Boro cadmium tellurite glasses of compositions (50 x) CdO– xTeO2–50B2O3 (x in mol% ranging from 10 to 50 in steps of 10) were prepared by usual melt quenching technique. The stoicho-
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metric amounts of analytical grades of cadmium oxide, boric acid and tellurium dioxide were thoroughly mixed using spectral grade acetone and loaded into platinum crucible. The mixtures were melted in platinum crucible at 800–950 °C depending on the glass composition. The melt was stirred occasionally for homogeneous mixing. After the disappearance of bubbles the melt was quenched on stainless steel plates maintained at 200 °C. The glass samples were annealed at 300 °C/2 h to remove thermal strains. The composition of the glass samples employed in the present study are shown in Table 1.
2.2. Glass characterization To check the non-crystallinity of the glass samples, X-ray diffraction measurements were performed at room temperature using copper target (Ka = 1.5406 Å) on Philips PW (1140) diffractometer. Fig. 1 shows the XRD patterns of boro cadmium tellurite glass system at room temperature. The diffraction patterns of the glasses did not reveal any discrete or sharp peaks, but broad humps, which reveals the characteristic of amorphous materials. The optical absorption spectrum of the present glass samples was recorded at room temperature using a double beam spectrophotometer (Schimadzu) in the wave length region 300–700 nm. Thickness of the glass specimens was measured by using micrometer gauge. The density of the samples was determined by standard Archimedes principle. These measurements were done using single pan balance and xylene as an immersion liquid.
q¼
a q ab x
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 (qx) 0.865 gm/cm3. Theoretical optical basicity for the glass system has been calculated by using basicity assigned to the individual oxides on the basis of the following equation proposed by Duffy and Ingram [28,29].
Kth ¼ xðCdOÞKðCdOÞ þ xðB2 O3 ÞKðB2 O3 Þ þ xðTeO2 ÞKðTeO2 Þ
ð2Þ
Fig. 1. XRD pattern of boro cadmium tellurite glass system.
3. Results and discussion 3.1. Determination of optical band gap, Urbach energy and refractive index Fig. 2 shows the optical absorption spectra of boro cadmium tellurite glass system. The absence of sharp peaks in the figure corresponds to characteristic of amorphous nature. It is also observed from Table 2 that the fundamental absorption edge shifts to longer wavelength side with an increase of CdO; this may be due to less rigidity of the glass system. The lowest cutoff wave length or highest band gap is observed in 0 mol% of CdO glass system which is due to the formation of TeO4 units which are altered with CdO4 units. Absorption coefficient near the edge of each curve was determined at wave length of 5 nm by using the relation.
aðxÞ ¼
2:303A t
ð3Þ
where t is the thickness of the sample and A corresponds to absorbance. The relation between a(x) and the photon energy of incident radiation hm is given by the relation
where x(CdO), 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 K(CdO), K(B2O3), K(TeO2) are optical basicity values assigned to the constituent oxides taken from the literature [30]. Fourier transform infrared spectra (FTIR) and Raman spectroscopy are the most advantageous tools to determine the structure of tellurite glasses containing various amounts of CdO and TeO2. The vibration spectra of the glass samples were recorded at room temperature using KBr pellet technique in the range 400– 4000 cm1. The Raman spectra were recorded in the range of 0– 1200 cm1.
Table 1 Composition of X CdO–(50 x) TeO2–50B2O3. S.No.
Sample
B2O3 Mol%
CdO Mol%
TeO2 Mol%
1 2 3 4 5 6
C0 C1 C2 C3 C4 C5
50 50 50 50 50 50
0 10 20 30 40 50
50 40 30 20 10 0
Fig. 2. Wavelength vs absorbance for boro cadmium tellurite glass system.
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Table 2 Optical parameters of boro cadmium tellurite glass. Sample
Cut Off wave length (nm)
Optical energy gap Eopt (eV)
Urbach energy DE (eV)
Band tailing parameter B (cm eV)1
Refractive index (n)
C0 C1 C2 C3 C4 C5
345 346 352 369 373 426
3.170 2.990 2.970 2.828 2.805 2.73
0.309 0.337 0.238 0.249 0.327 0.352
5.535 5.759 5.705 5.984 6.011 6.120
2.353 2.400 2.405 2.446 2.452 2.474
aðxÞ ¼
Bðht Eopt Þ2 ht
ð4Þ
where Eopt is the optical energy gap, hm is the photon energy and B is a constant called band tailing parameter. The relation can be written as 1
ðahtÞ2 ¼ Bðht Eopt Þ
ð5Þ
Using this relation Eopt values were determined by extrapolation of 1 1 linear region of the plots of ðahtÞ2 against hm to ðahtÞ2 ¼ 0 as shown in Fig 3. The values of Eopt thus obtained for all glass samples are given in Table 2. In boro tellurite glasses Eopt decreases with an increase of TeO2 [5]. In the present CdO based boro tellurite glasses optical energy gap decreases with increase of CdO (decrease of TeO2 mol%) content. Results of the optical energy gap are quite close with those found in literature of tellurite glasses [31–33,19]. Increase of CdO in the glass system is to produce a breakdown of continuous CdO network which is reflected in the absorption spectra by a significant shifting of absorption edge to longer wavelength side. The shift of absorption edge is related to structural rearrangements of relative concentrations of various fundamental units. The variation of Eopt with CdO composition in the glass system is shown in Fig 4. It was observed that with increase in CdO concentration, the optical absorption spectra shifts towards lower wavelength side which does not necessarily mean that number of non-bridging oxygen’s (NBO’s) decrease. Stevels [34] suggested that the movement of absorption band to lower energy corresponds to the transition from the non-bridging oxygen which binds an electron more loosely than bridging oxygen. Band tailing parameter is determined from the slope of (ahm)1/2 against hm (Fig. 3). Its value lies between 5.535 and 6.120 (cm eV)1. Refractive index is determined from optical energy gap using the relation proposed by Dimitrov and Sakka [35].
n2 1 ¼1 n2 þ 2
rffiffiffiffiffiffiffiffi Eopt 20
Fig. 3. (aht)1/2 as a function of energy for boro cadmium tellurite glass system.
Fig. 4. Variation of optical band gap and refractive index with composition of CdO.
ð6Þ
Fig. 4 shows the plot of Eopt vs refractive index. It is clear from figure that refractive index decreases with an increase of Eopt. With an increase of CdO mol% the values of refractive index increased from 2.353 to 2.474, may be due to lower cation polarizability of Cd2+ ions. This value is found to be greater than of zinc tellurite glasses. Urbach plots are the plots where natural logarithm of absorption coefficients (lna) is plotted against photon energy (hm).Such an Urbach plot for present boro cadmium 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. It is found that Urbach energy decreases with increase of CdO content which is attributed to decrease in fragility nature of the glass network. In the present glass system the value of Urbach energy lies between 0.238 eV and 0.352 eV. The values of Urbach energy for a range of amorphous semiconductors [36] lies between 0.045 and
Fig. 5. Urbach plot of boro cadmium tellurite glass system.
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0.567 eV. The estimated values are consistent with the reported values of Ag2O3–B2O3–TeO2, (0.09 and 0.26 eV)glasses, B2O3– ZnO–TeO2 glasses, and of P2O5–TeO2, Bi2O3–P2O5–TeO2 (0.17– 0.67 eV) depending on glass composition [24,19,37,18]. 3.2. Density, molar volume, oxygen packing density, optical basicity Addition of CdO at the expense of TeO2 in present glass system decreases the density. In Zinc tellurite glass system with an increase of ZnO content density is found to increases [34] and in boro tellurite glasses the density increases with an increase of TeO2 [22]. This behavior is generally observed when content of modifier oxide is increased in borate glasses. The addition of modifier oxide in the glass system changes the density of the structural units by changing the density of the glass system. Because density is an additive property it can be proposed that each oxide in glass would have its contribution in density. Density at x = 50 mol% of CdO is 4.288 gm/ cm3 (Table 3) and its value increases as content of CdO into glass network decreases (Fig 6). The decrease in density on increasing the modifier content CdO is due to the substitution of higher molecular weight substance TeO2 (atomic mass 159.599 gm/cm3) by lower molecular weight substance CdO (atomic mass 128.4 gm/cm3) there by net molecular weight decreases resulting in weak connectivity in glass network [38]. Molar volume (Vm) and oxygen packing density (O) are calculated using the relation.
Molar volumeðV m Þ ¼
M
Table 3 Physical parameters of boro cadmium tellurite glass system. Sample
Density q (g/cm3)
Molar volume Vm (cm3/mole)
Oxygen packing density (O.P.D.) (g atm/l)
Optical basicity (K)
C0 C1 C2 C3 C4 C5
4.698 4.608 4.548 4.427 4.380 4.218
25.644 25.468 25.117 25.097 24.658 24.458
97.490 94.237 91.571 87.660 85.167 81.774
0.71 0.721 0.732 0.743 0.754 0.765
ð7Þ
q
M is molecular weight and q is density.
Oxygen packing densityðOÞ ¼
qO M
Fig. 6. Variation of density and molar volume with composition of CdO.
ð8Þ
O is number of oxygen atoms per formula units and these values are tabulated in Table 3. Fig. 6. Shows the variation of Vm and q with CdO. It is clear from the figure that with decrease of CdO, both density and molar volume decreases. In general, the density and molar volume are inversely related. But in the present glasses the behavior is different. However, this anomalous behavior was earlier reported in many glass systems, TeO2–NbO–Bi2O3 [39], Na2O–B2O3–TeO2 [40], Bi2O3–Li2O–B2O3 [41]. In the present study molar volume is directly proportional to glass density and molecular weight. The increase in molar volume from 24.458 to 25.644 cm3mole with decrease of CdO indicates an increase in free volume. The larger values of radii and bond length of TeO2 compared to CdO gave rise to an increase of molar volume. Variation of oxygen packing density with the composition of CdO is shown in Fig 7 and the values are given in Table 3. Decrease in oxygen packing density with increase of CdO indicates that the formation of TeO4 and BO4 units is prevented revealing that nonbridging oxygens (NBO) increase in number. This can be interpreted in terms of formation of more rigid and highly cross linked network resulting in closely packed glass structure. The calculated values of optical basicity Kth are presented in Table 3. The values of optical basicity vary from 0.710 to 0.765. From Fig 8 it can be observed that basicity increases with an increase of CdO, it may be due to the fact that CdO is having larger basicity (1.13) than TeO2 (0.96). Replacement of higher basicity by lower will increase the optical basicity and decreases the covalence nature of the glasses. 4. FTIR Spectroscopy Fig. 9 shows the FTIR spectrum of the present glass system. The bands located around 460 cm1, in the range of 610–680 cm1 and
Fig. 7. Variation of oxygen packing density with composition of CdO.
720–780 cm1 are assigned to the bending mode of Te–O–Te or O– Te–O linkages, the stretching mode of [TeO4] trigonal pyramidal with bridging oxygen and the stretching mode of [TeO3] trigonal pyramidal with non-bridging oxygen, respectively [42–44]. The structural bands of the present glass system is characterized by IR absorption bands in the wave numbers region 436– 446 cm1, 633–646 cm1, 677–688 cm1, 756–767 cm1, 910– 933 cm1, 1245–1252 cm1 and 1342–1360 cm1. The IR band positions are summarized in Table 4. Intense absorption bands or weak shoulders in the region 633–767 cm1 corresponding to TeO4, TeO3/TeO3+1 units are preserved in all the studied glasses
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Table 4 IR band assignments for boro cadmium tellurite glass system. Characteristic IR band (cm1)
Assignment
436–446 633–646 677–688 756–767 910–933 1245–1252
Vibration of tetrahedral CdO Te–O–Te linkages Stretching vibrations of TeO4 trigonal bi pyramidal Stretching vibrations of TeO3 tp B–O–B bending vibrations in BO4 units Asymmetric stretching vibration of B–O bonds from ortho borate groups B–O bonds stretching vibration of BO3 units from various borate groups
1342–1360
Fig. 8. Variation of optical basicity with composition of CdO.
Fig. 10. Raman band assignment of boro cadmium tellurite glass system.
pyramidal [TeO3] units (ii) absence of absorption peak 805 cm1 indicates the absence of boroxol ring and the glass system consists of BO3 and BO4 units attached in the form of random network. (iii) The absorption at the region 840 cm1 is not observed in these glasses which suggests that formation of tetrahedral coordination of Cd (i.e. CdO4) is absent. Fig. 9. IR spectra of boro cadmium tellurite glass system.
5. Raman spectroscopy except x = 0. In the region 600–800 cm1, B–O–B bending vibrations manifest them selves [47]. In the present system for x = 0 glass, 688 cm1 band is assigned to B–O–B bending vibrations [23]. The peaks in the region 910–933 cm1 are assigned to [BO4] units, those at 1245–1252 cm1 are assigned to B–O stretching vibrations in BO3 units from boroxol rings and 1342–1360 cm1 B–O stretching vibrations in [BO3] units from varied types of borate groups [45–49]. Increase of CdO content of studied glasses shows a significant shift of band position from 933 cm1 to 905 cm1 (attributed to vibrations of [BO4] structural units), 767 to 670 cm1 (assigned to bending vibrations of [TeO3] units), 1252 to 1246 cm1 (assigned to B–O stretching vibrations in [BO3] units from varied types of boroxol rings), 1342–1362 cm1 (attributed to vibrations of [BO3] units from varied types of borate groups) and 436– 446 cm1 (stretching vibrations of Cd–O bonds) [50,51]. Considering these changes of the IR spectral features, we assume that the addition of Cadmium oxide in the boro tellurite glass structure leads to the following: (i) the number of the [TeO4] groups with non-bridging oxygen decreases because some trigonal bi pyramidal [TeO4] structural units were transformed into trigonal
The Raman spectrum of the glass samples is shown in Fig 10. The assignment of observed Raman band positions for all compositions is summarized in Table 5. The Raman spectra of boro cadmium tellurite glass system consists of mainly five intense Raman bands at around 320, 450, 630, 720, 1330 cm1. The assigned bands were compared with the literature [52–57] as shown in Table 6. The band centered at 450 cm1 is assigned to a-TeO2 structure in which tellurium atoms adopt different polyhedral coordination. When CdO content increases the intensity of this band decreases due to the breakage of Te–O–Te bonds, thus implying that the modifier reduces the hardness of relevant bonds and the rigidity of the glassy network. The strong Raman band observed at 650 cm1 in crystalline a-TeO2 is red shifted to 663 cm1 in boro cadmium tellurite glass. This broadening of the band and shift could be related to chemical perturbation of the vibrational energies arising from glass former-glass modifier binding. This broadening is a result of the distribution of bond angles and average nearest neighbor distances in the glass matrix. The strong peak in the region 750–762 cm1 is due to the formation of TeO3+1 or TeO3 pyramids associated with NBO in glass samples
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Table 5 Raman band assignments of boro cadmium tellurite glass system. Characteristics raman bands (cm1)
Assignment
320–330 431–500 630–682
Phonon vibrations of CdTe. Te–O–Te linkages Stretching vibrations of TeO4 trigonal bipyramidal and TeO3+1 polyhedra. Stretching vibrations of Te–O from TeO3 tp units or TeO3+1 polyhedra and symmetric breathing vibration of six member ring with a BO3 triangle replaced by a BO4-tetrahedra
745–762
[2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13]
Table 6 Literature review of Raman band assignments for Te, CdTe and binary tellurite glasses [48–54]. Wave number (cm1)
Vibration mode
325 400–500 600–615 650–670 720–760
Phonon vibrations of – CdTe structure Bending vibrations –[Te–O–Te] or [O–Te–O] linkages Stretching vibrations of TeO4 tbp Stretching vibrations of TeO4 tbp Stretching vibrations of Te–O from TeO3 tp units or TeO3+1 Polyhedra and symmetric breathing vibrations of six membered ring with a BO3 triangle replaced by a BO 4 tetrahedral. Stretching vibrations of TeO3 tp.
770–780
[14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25]
and symmetric breathing vibration of six members ring with one BO4. Narrow peaks observed around 325 cm1 are assigned to phonon vibrations of Cd–Te structure (Ref Table 6). In present glass system it could be concluded that CdO changes (i) the four fold coordinated Te ions to three fold coordinated and (ii) symmetric breathing vibrations of six metaborate with one or two BO4 units. 6. Conclusion Ternary boro cadmium 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 CdO 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 CdO 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 cadmium tellurite glasses decreased with the addition of CdO as lower molecular weight (CdO – 128.4) is substituted by a higher molecular weight (TeO2 – 159.599). IR studies revealed that the incorporation of modifier oxide (CdO) in place of TeO2 increased the number of non-bridging oxygen by gradually replacing trigonal bipyramids [TeO4] units with trigonal pyramids [TeO3] through [TeO3+1]. The structure consists of randomly connected BO3 and BO4 groups. Raman studies revealed presence of CdTe network in the glass system. Acknowledgment The author is thankful to DRDO for the financial support. References [1] M.A. Sidkey, A. Abd El – Moneim, L. Abd El-Latif, Mater. Chem. Phys. 61 (1999) 103.
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Optical Materials journal homepage: www.elsevier.com/locate/optmat
Studies on boro cadmium tellurite glasses P. Gayathri Pavani ⇑, S. Suresh, V. Chandra Mouli Glassy Material Research Laboratory, Department of Physics, Osmania University, Hyderabad 500 007, India
a r t i c l e
i n f o
Article history: Received 18 April 2011 Received in revised form 1 August 2011 Accepted 12 August 2011 Available online 22 September 2011 Keywords: Tellurite glasses Optical energy gap Refractive index IR and Raman
a b s t r a c t To investigate the modification effect of the modifier CdO on boro tellurite glass, a series of glasses with compositions (50 x) CdO–xTeO2–50B2O3 have been prepared by conventional melt quenching technique. Optical absorption, IR and Raman structural studies are carried out on the glass system. The optical absorption studies revealed that the cutoff wave length and refractive index increase while optical band gap (Eopt) and Urbach energy decreases with increase of CdO content. The IR and Raman studies revealed that structure of glass network consists of [TeO3]/[TeO3+1], [TeO4], [BO3], [BO4] and [Cd–Te] linkages .The compositional dependence of different physical parameters such as density, molar volume, oxygen packing density, optical basicity, have been analyzed and discussed. Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction Structural studies in glasses are of important owing to the interrelation between the atomic arrangement and properties. To comprehend structural details, a common strategy is to change the glass structure either by adding network modifiers or by interrogating the material with some external stimuli such as high heat or pressure. Tellurium oxide is an interesting case where the basic building block of structure experiences considerable changes either by modifiers or by increasing temperature, though being of three-dimensional geometry. Tellurite glasses possess interesting glass-forming ability, glass structure, no hygroscopic properties and low melting point [1,2]. Tellurium oxide-based glasses are of scientific and technological interest on account of their low melting temperatures [3,4] large thermal expansion [5] high refractive index refractive indices, high dielectric constants and good IR transmissions, thus they have been considered as promising materials for use in optical fibers and non-linear optical devices [6–10]. Crystalline TeO2 has a unique structural unit that is an asymmetrical TeO4 trigonal bipyramid (tbp), in which there are two different kinds of sites, two axial and three equatorial positions. One of the latter is occupied by a lone pair of electrons. The structural of TeO2 based glasses is of interest, because there are two types of basis structural unit, i.e. TeO4 tbp with a lone pair electron in an equatorial position and TeO3 trigonal pyramid (tp). The tellurite glass system is an important amorphous system considered to have important commercial applications in optical communication
⇑ Corresponding author. Tel.: +91 40 27682242; fax: +91 40 27099020. E-mail address: [email protected] (P. Gayathri Pavani). 0925-3467/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2011.08.016
due to its high refractive index, good infrared transmittance and high optical non-linearity [11–15].These peculiar characteristics of tellurium based glasses attracted researchers towards preparation of combination of glass formers like B2O3, V2O5, etc. for its desired optical properties. Boro tellurite glasses lead to complex specification in glass structure [16]. Keeping these objectives in view B2O3–TeO2–K2O [17], ZnO–TeO2, [18] 50B2O3–(50 x) ZnO– xTeO2 [19],TeO2–V2O5–MoO3 [20]The boric oxide (B2O3) which is used as glass formers in many glass systems is incorporated into various kind of glass systems as a flux material in order to attain good physical and chemical properties. In their crystalline form, borates with various compositions are of exceptional importance due to their interesting linear and nonlinear optical properties [21].The change in structure and physical properties of binary and ternary tellurite glasses such as B2O3–TeO2 [22], V2O5–CdO– B2O3 [23], Ag2O3–B2O3–TeO2 [24], TeO2–B2O3–PbO2 [25], mixed alkali effect in boro tellurite [26], cadmium borate glasses [27] have been widely studied. But to the knowledge of the author no study was reported on cadmium based boro tellurite glass system. Hence it is found to be interesting to study the structural, physical and optical properties of the novel ternary (50 x) CdO–xTeO2– 50B2O3 glass system.
2. Experimental procedure 2.1. Glass preparation Boro cadmium tellurite glasses of compositions (50 x) CdO– xTeO2–50B2O3 (x in mol% ranging from 10 to 50 in steps of 10) were prepared by usual melt quenching technique. The stoicho-
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metric amounts of analytical grades of cadmium oxide, boric acid and tellurium dioxide were thoroughly mixed using spectral grade acetone and loaded into platinum crucible. The mixtures were melted in platinum crucible at 800–950 °C depending on the glass composition. The melt was stirred occasionally for homogeneous mixing. After the disappearance of bubbles the melt was quenched on stainless steel plates maintained at 200 °C. The glass samples were annealed at 300 °C/2 h to remove thermal strains. The composition of the glass samples employed in the present study are shown in Table 1.
2.2. Glass characterization To check the non-crystallinity of the glass samples, X-ray diffraction measurements were performed at room temperature using copper target (Ka = 1.5406 Å) on Philips PW (1140) diffractometer. Fig. 1 shows the XRD patterns of boro cadmium tellurite glass system at room temperature. The diffraction patterns of the glasses did not reveal any discrete or sharp peaks, but broad humps, which reveals the characteristic of amorphous materials. The optical absorption spectrum of the present glass samples was recorded at room temperature using a double beam spectrophotometer (Schimadzu) in the wave length region 300–700 nm. Thickness of the glass specimens was measured by using micrometer gauge. The density of the samples was determined by standard Archimedes principle. These measurements were done using single pan balance and xylene as an immersion liquid.
q¼
a q ab x
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 (qx) 0.865 gm/cm3. Theoretical optical basicity for the glass system has been calculated by using basicity assigned to the individual oxides on the basis of the following equation proposed by Duffy and Ingram [28,29].
Kth ¼ xðCdOÞKðCdOÞ þ xðB2 O3 ÞKðB2 O3 Þ þ xðTeO2 ÞKðTeO2 Þ
ð2Þ
Fig. 1. XRD pattern of boro cadmium tellurite glass system.
3. Results and discussion 3.1. Determination of optical band gap, Urbach energy and refractive index Fig. 2 shows the optical absorption spectra of boro cadmium tellurite glass system. The absence of sharp peaks in the figure corresponds to characteristic of amorphous nature. It is also observed from Table 2 that the fundamental absorption edge shifts to longer wavelength side with an increase of CdO; this may be due to less rigidity of the glass system. The lowest cutoff wave length or highest band gap is observed in 0 mol% of CdO glass system which is due to the formation of TeO4 units which are altered with CdO4 units. Absorption coefficient near the edge of each curve was determined at wave length of 5 nm by using the relation.
aðxÞ ¼
2:303A t
ð3Þ
where t is the thickness of the sample and A corresponds to absorbance. The relation between a(x) and the photon energy of incident radiation hm is given by the relation
where x(CdO), 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 K(CdO), K(B2O3), K(TeO2) are optical basicity values assigned to the constituent oxides taken from the literature [30]. Fourier transform infrared spectra (FTIR) and Raman spectroscopy are the most advantageous tools to determine the structure of tellurite glasses containing various amounts of CdO and TeO2. The vibration spectra of the glass samples were recorded at room temperature using KBr pellet technique in the range 400– 4000 cm1. The Raman spectra were recorded in the range of 0– 1200 cm1.
Table 1 Composition of X CdO–(50 x) TeO2–50B2O3. S.No.
Sample
B2O3 Mol%
CdO Mol%
TeO2 Mol%
1 2 3 4 5 6
C0 C1 C2 C3 C4 C5
50 50 50 50 50 50
0 10 20 30 40 50
50 40 30 20 10 0
Fig. 2. Wavelength vs absorbance for boro cadmium tellurite glass system.
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Table 2 Optical parameters of boro cadmium tellurite glass. Sample
Cut Off wave length (nm)
Optical energy gap Eopt (eV)
Urbach energy DE (eV)
Band tailing parameter B (cm eV)1
Refractive index (n)
C0 C1 C2 C3 C4 C5
345 346 352 369 373 426
3.170 2.990 2.970 2.828 2.805 2.73
0.309 0.337 0.238 0.249 0.327 0.352
5.535 5.759 5.705 5.984 6.011 6.120
2.353 2.400 2.405 2.446 2.452 2.474
aðxÞ ¼
Bðht Eopt Þ2 ht
ð4Þ
where Eopt is the optical energy gap, hm is the photon energy and B is a constant called band tailing parameter. The relation can be written as 1
ðahtÞ2 ¼ Bðht Eopt Þ
ð5Þ
Using this relation Eopt values were determined by extrapolation of 1 1 linear region of the plots of ðahtÞ2 against hm to ðahtÞ2 ¼ 0 as shown in Fig 3. The values of Eopt thus obtained for all glass samples are given in Table 2. In boro tellurite glasses Eopt decreases with an increase of TeO2 [5]. In the present CdO based boro tellurite glasses optical energy gap decreases with increase of CdO (decrease of TeO2 mol%) content. Results of the optical energy gap are quite close with those found in literature of tellurite glasses [31–33,19]. Increase of CdO in the glass system is to produce a breakdown of continuous CdO network which is reflected in the absorption spectra by a significant shifting of absorption edge to longer wavelength side. The shift of absorption edge is related to structural rearrangements of relative concentrations of various fundamental units. The variation of Eopt with CdO composition in the glass system is shown in Fig 4. It was observed that with increase in CdO concentration, the optical absorption spectra shifts towards lower wavelength side which does not necessarily mean that number of non-bridging oxygen’s (NBO’s) decrease. Stevels [34] suggested that the movement of absorption band to lower energy corresponds to the transition from the non-bridging oxygen which binds an electron more loosely than bridging oxygen. Band tailing parameter is determined from the slope of (ahm)1/2 against hm (Fig. 3). Its value lies between 5.535 and 6.120 (cm eV)1. Refractive index is determined from optical energy gap using the relation proposed by Dimitrov and Sakka [35].
n2 1 ¼1 n2 þ 2
rffiffiffiffiffiffiffiffi Eopt 20
Fig. 3. (aht)1/2 as a function of energy for boro cadmium tellurite glass system.
Fig. 4. Variation of optical band gap and refractive index with composition of CdO.
ð6Þ
Fig. 4 shows the plot of Eopt vs refractive index. It is clear from figure that refractive index decreases with an increase of Eopt. With an increase of CdO mol% the values of refractive index increased from 2.353 to 2.474, may be due to lower cation polarizability of Cd2+ ions. This value is found to be greater than of zinc tellurite glasses. Urbach plots are the plots where natural logarithm of absorption coefficients (lna) is plotted against photon energy (hm).Such an Urbach plot for present boro cadmium 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. It is found that Urbach energy decreases with increase of CdO content which is attributed to decrease in fragility nature of the glass network. In the present glass system the value of Urbach energy lies between 0.238 eV and 0.352 eV. The values of Urbach energy for a range of amorphous semiconductors [36] lies between 0.045 and
Fig. 5. Urbach plot of boro cadmium tellurite glass system.
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0.567 eV. The estimated values are consistent with the reported values of Ag2O3–B2O3–TeO2, (0.09 and 0.26 eV)glasses, B2O3– ZnO–TeO2 glasses, and of P2O5–TeO2, Bi2O3–P2O5–TeO2 (0.17– 0.67 eV) depending on glass composition [24,19,37,18]. 3.2. Density, molar volume, oxygen packing density, optical basicity Addition of CdO at the expense of TeO2 in present glass system decreases the density. In Zinc tellurite glass system with an increase of ZnO content density is found to increases [34] and in boro tellurite glasses the density increases with an increase of TeO2 [22]. This behavior is generally observed when content of modifier oxide is increased in borate glasses. The addition of modifier oxide in the glass system changes the density of the structural units by changing the density of the glass system. Because density is an additive property it can be proposed that each oxide in glass would have its contribution in density. Density at x = 50 mol% of CdO is 4.288 gm/ cm3 (Table 3) and its value increases as content of CdO into glass network decreases (Fig 6). The decrease in density on increasing the modifier content CdO is due to the substitution of higher molecular weight substance TeO2 (atomic mass 159.599 gm/cm3) by lower molecular weight substance CdO (atomic mass 128.4 gm/cm3) there by net molecular weight decreases resulting in weak connectivity in glass network [38]. Molar volume (Vm) and oxygen packing density (O) are calculated using the relation.
Molar volumeðV m Þ ¼
M
Table 3 Physical parameters of boro cadmium tellurite glass system. Sample
Density q (g/cm3)
Molar volume Vm (cm3/mole)
Oxygen packing density (O.P.D.) (g atm/l)
Optical basicity (K)
C0 C1 C2 C3 C4 C5
4.698 4.608 4.548 4.427 4.380 4.218
25.644 25.468 25.117 25.097 24.658 24.458
97.490 94.237 91.571 87.660 85.167 81.774
0.71 0.721 0.732 0.743 0.754 0.765
ð7Þ
q
M is molecular weight and q is density.
Oxygen packing densityðOÞ ¼
qO M
Fig. 6. Variation of density and molar volume with composition of CdO.
ð8Þ
O is number of oxygen atoms per formula units and these values are tabulated in Table 3. Fig. 6. Shows the variation of Vm and q with CdO. It is clear from the figure that with decrease of CdO, both density and molar volume decreases. In general, the density and molar volume are inversely related. But in the present glasses the behavior is different. However, this anomalous behavior was earlier reported in many glass systems, TeO2–NbO–Bi2O3 [39], Na2O–B2O3–TeO2 [40], Bi2O3–Li2O–B2O3 [41]. In the present study molar volume is directly proportional to glass density and molecular weight. The increase in molar volume from 24.458 to 25.644 cm3mole with decrease of CdO indicates an increase in free volume. The larger values of radii and bond length of TeO2 compared to CdO gave rise to an increase of molar volume. Variation of oxygen packing density with the composition of CdO is shown in Fig 7 and the values are given in Table 3. Decrease in oxygen packing density with increase of CdO indicates that the formation of TeO4 and BO4 units is prevented revealing that nonbridging oxygens (NBO) increase in number. This can be interpreted in terms of formation of more rigid and highly cross linked network resulting in closely packed glass structure. The calculated values of optical basicity Kth are presented in Table 3. The values of optical basicity vary from 0.710 to 0.765. From Fig 8 it can be observed that basicity increases with an increase of CdO, it may be due to the fact that CdO is having larger basicity (1.13) than TeO2 (0.96). Replacement of higher basicity by lower will increase the optical basicity and decreases the covalence nature of the glasses. 4. FTIR Spectroscopy Fig. 9 shows the FTIR spectrum of the present glass system. The bands located around 460 cm1, in the range of 610–680 cm1 and
Fig. 7. Variation of oxygen packing density with composition of CdO.
720–780 cm1 are assigned to the bending mode of Te–O–Te or O– Te–O linkages, the stretching mode of [TeO4] trigonal pyramidal with bridging oxygen and the stretching mode of [TeO3] trigonal pyramidal with non-bridging oxygen, respectively [42–44]. The structural bands of the present glass system is characterized by IR absorption bands in the wave numbers region 436– 446 cm1, 633–646 cm1, 677–688 cm1, 756–767 cm1, 910– 933 cm1, 1245–1252 cm1 and 1342–1360 cm1. The IR band positions are summarized in Table 4. Intense absorption bands or weak shoulders in the region 633–767 cm1 corresponding to TeO4, TeO3/TeO3+1 units are preserved in all the studied glasses
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Table 4 IR band assignments for boro cadmium tellurite glass system. Characteristic IR band (cm1)
Assignment
436–446 633–646 677–688 756–767 910–933 1245–1252
Vibration of tetrahedral CdO Te–O–Te linkages Stretching vibrations of TeO4 trigonal bi pyramidal Stretching vibrations of TeO3 tp B–O–B bending vibrations in BO4 units Asymmetric stretching vibration of B–O bonds from ortho borate groups B–O bonds stretching vibration of BO3 units from various borate groups
1342–1360
Fig. 8. Variation of optical basicity with composition of CdO.
Fig. 10. Raman band assignment of boro cadmium tellurite glass system.
pyramidal [TeO3] units (ii) absence of absorption peak 805 cm1 indicates the absence of boroxol ring and the glass system consists of BO3 and BO4 units attached in the form of random network. (iii) The absorption at the region 840 cm1 is not observed in these glasses which suggests that formation of tetrahedral coordination of Cd (i.e. CdO4) is absent. Fig. 9. IR spectra of boro cadmium tellurite glass system.
5. Raman spectroscopy except x = 0. In the region 600–800 cm1, B–O–B bending vibrations manifest them selves [47]. In the present system for x = 0 glass, 688 cm1 band is assigned to B–O–B bending vibrations [23]. The peaks in the region 910–933 cm1 are assigned to [BO4] units, those at 1245–1252 cm1 are assigned to B–O stretching vibrations in BO3 units from boroxol rings and 1342–1360 cm1 B–O stretching vibrations in [BO3] units from varied types of borate groups [45–49]. Increase of CdO content of studied glasses shows a significant shift of band position from 933 cm1 to 905 cm1 (attributed to vibrations of [BO4] structural units), 767 to 670 cm1 (assigned to bending vibrations of [TeO3] units), 1252 to 1246 cm1 (assigned to B–O stretching vibrations in [BO3] units from varied types of boroxol rings), 1342–1362 cm1 (attributed to vibrations of [BO3] units from varied types of borate groups) and 436– 446 cm1 (stretching vibrations of Cd–O bonds) [50,51]. Considering these changes of the IR spectral features, we assume that the addition of Cadmium oxide in the boro tellurite glass structure leads to the following: (i) the number of the [TeO4] groups with non-bridging oxygen decreases because some trigonal bi pyramidal [TeO4] structural units were transformed into trigonal
The Raman spectrum of the glass samples is shown in Fig 10. The assignment of observed Raman band positions for all compositions is summarized in Table 5. The Raman spectra of boro cadmium tellurite glass system consists of mainly five intense Raman bands at around 320, 450, 630, 720, 1330 cm1. The assigned bands were compared with the literature [52–57] as shown in Table 6. The band centered at 450 cm1 is assigned to a-TeO2 structure in which tellurium atoms adopt different polyhedral coordination. When CdO content increases the intensity of this band decreases due to the breakage of Te–O–Te bonds, thus implying that the modifier reduces the hardness of relevant bonds and the rigidity of the glassy network. The strong Raman band observed at 650 cm1 in crystalline a-TeO2 is red shifted to 663 cm1 in boro cadmium tellurite glass. This broadening of the band and shift could be related to chemical perturbation of the vibrational energies arising from glass former-glass modifier binding. This broadening is a result of the distribution of bond angles and average nearest neighbor distances in the glass matrix. The strong peak in the region 750–762 cm1 is due to the formation of TeO3+1 or TeO3 pyramids associated with NBO in glass samples
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Table 5 Raman band assignments of boro cadmium tellurite glass system. Characteristics raman bands (cm1)
Assignment
320–330 431–500 630–682
Phonon vibrations of CdTe. Te–O–Te linkages Stretching vibrations of TeO4 trigonal bipyramidal and TeO3+1 polyhedra. Stretching vibrations of Te–O from TeO3 tp units or TeO3+1 polyhedra and symmetric breathing vibration of six member ring with a BO3 triangle replaced by a BO4-tetrahedra
745–762
[2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13]
Table 6 Literature review of Raman band assignments for Te, CdTe and binary tellurite glasses [48–54]. Wave number (cm1)
Vibration mode
325 400–500 600–615 650–670 720–760
Phonon vibrations of – CdTe structure Bending vibrations –[Te–O–Te] or [O–Te–O] linkages Stretching vibrations of TeO4 tbp Stretching vibrations of TeO4 tbp Stretching vibrations of Te–O from TeO3 tp units or TeO3+1 Polyhedra and symmetric breathing vibrations of six membered ring with a BO3 triangle replaced by a BO 4 tetrahedral. Stretching vibrations of TeO3 tp.
770–780
[14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25]
and symmetric breathing vibration of six members ring with one BO4. Narrow peaks observed around 325 cm1 are assigned to phonon vibrations of Cd–Te structure (Ref Table 6). In present glass system it could be concluded that CdO changes (i) the four fold coordinated Te ions to three fold coordinated and (ii) symmetric breathing vibrations of six metaborate with one or two BO4 units. 6. Conclusion Ternary boro cadmium 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 CdO 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 CdO 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 cadmium tellurite glasses decreased with the addition of CdO as lower molecular weight (CdO – 128.4) is substituted by a higher molecular weight (TeO2 – 159.599). IR studies revealed that the incorporation of modifier oxide (CdO) in place of TeO2 increased the number of non-bridging oxygen by gradually replacing trigonal bipyramids [TeO4] units with trigonal pyramids [TeO3] through [TeO3+1]. The structure consists of randomly connected BO3 and BO4 groups. Raman studies revealed presence of CdTe network in the glass system. Acknowledgment The author is thankful to DRDO for the financial support. References [1] M.A. Sidkey, A. Abd El – Moneim, L. Abd El-Latif, Mater. Chem. Phys. 61 (1999) 103.
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