Journal of Non-Crystalline Solids 404 (2014) 116–123

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Journal of Non-Crystalline Solids

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Structural characterization of borotellurite andalumino-borotellurite glasses

Nirmal Kaur, Atul Khanna ⁎Department of Physics, Guru Nanak Dev University, Amritsar, Punjab 143005, India

⁎ Corresponding author. Tel.: +91 183 225 8802x3168E-mail address: akphysics@yahoo.com (A. Khanna).

http://dx.doi.org/10.1016/j.jnoncrysol.2014.08.0020022-3093/© 2014 Elsevier B.V. All rights reserved.

a b s t r a c t

a r t i c l e i n f o

Article history:Received 30 April 2014Received in revised form 27 June 2014Available online xxxx

Keywords:Borotellurite and alumino-borotellurite glasses;Phase separation;Crystalline precipitates;Infrared and Raman spectroscopy;Chemical durability

Borotellurite (xB2O3–(100 − x)TeO2; x = 15, 20, 25, 30 and 35 mol.%) and alumino-borotellurite (yAl2O3–

10B2O3–(100− y)TeO2; y = 5, 10, 20 and 30 mol.%) glasses are prepared and studied by X-ray diffraction, den-sity, differential scanning calorimetry, thermo-gravimetric analysis, UV–visible, infrared and Raman spectrosco-py. Borotellurite glasses are hygroscopic and on crushing into powder, they absorb atmospheric water vapors toform crystalline precipitates of TeO2 in an amorphous matrix; the chemical durability of these glasses deterio-rates with increasing B2O3 concentration. The refractive index of borotellurite glasses decreases from 2.29 to2.26 while the glass transition temperature increases from 339 to 366 °C with an increase in B2O3 mol.%. Theshort-range structure of glasses consists of TeO4, TeO3, BO4 and BO3 structural units. Using the ratio of theareas under TeO4/TeO3 and BO4/BO3 Raman and infrared bands respectively, the Te\O coordination is foundto decrease from 3.63 to 3.56 and B\O co-ordination from 3.34 to 3.17 respectively on increasing B2O3 concen-tration from 5 to 30-mol.%. Alumino-borotellurite glasses show an amorphous–amorphous phase separation atan alumina concentration of 10-mol.% and above, while alumino-borotellurite sample with 30-mol.% of Al2O3

forms crystalline precipitates of α-Al2O3 in an amorphous matrix.© 2014 Elsevier B.V. All rights reserved.

1. Introduction

TeO2-based glasses have wide glass forming range and useful prop-erties such as low melting temperature, high dielectric constant, highrefractive index, low phonon energy and good transmittance of infraredradiation (0.4–6 μm); therefore, tellurite glasses have attracted signifi-cant interest over many years for application in non-linear optical de-vices, optical fiber amplifiers, lasers, solid state batteries, etc. [1–5].Tellurite glasses have been reported to exhibit 30 times higher Ramangain coefficients than silica glass, and find application in Raman ampli-fiers and non-linear optical waveguides [6,7]. Silver–vanadium telluriteglasses show ionic conduction due to silver cations andmixed electron-ic–ionic conductionwhen dopedwith transitionmetal ions [8]. The syn-thesis of pure TeO2 glasses requires melt-quenching rates of ~105 K s−1

[9]. The short-range structure of pure TeO2 glass has been studied byneutron diffraction and Raman spectroscopy and it is reported that64% of Te ions in the glass network exist in TeO4 bipyramidal units.The addition of metal oxides modifies the Te\O network and simulta-neously enhances the glass forming ability [1,2]. An interesting featureof the structure of tellurite glasses is that when the environment ismore ionic, the addition of a modifier favors the formation of trigonalpyramidal, [TeO3/2] (tp) units at the expense of trigonal bipyramidal,[TeO4/2] (tbp) units [1,2,5,9–11]. Study of potassium tellurite glasses

; fax: +91 183 225 8820.

has shown that up to a K2O molar concentration of 14.3%, non-bridging oxygens (NBO) are generated and TeO4 tetrahedra do nottransform into TeO3 units, but at higher K2O content, TeO4 transformsinto TeO3 units [9]. Incorporation of TeO2 in the alkali borate glasses de-creases the latter's hygroscopic nature and enhances its infrared trans-mittance [12]. Borate glasses have a random network consisting oftetrahedral (BO4) and trigonal boron (BO3) units and their combinationform diborate, triborate, tetraborate and pentaborate groups [13].

The presence of both B2O3 and TeO2 in borotellurite glasses leads tocomplex speciation in the glass structure. Also a glass containing twonetwork formers such as TeO2 and B2O3 can lead to the formation ofmixed structural units as in borosilicate glasses [14]. It is interesting tostudy the effect of TeO2 on the fraction of tetrahedral borons in theglass network. Borotellurite glasses have potential application especial-ly inmicro-electronics and opto-acoustics owing to their favorable opti-cal and electrical properties [15]. Raman study of B2O3–TeO2 glasses bySekiya et al. concluded that in glasses with low B2O3 content, there ex-ists a continuous network of TeO4, BO4 and BO3 groups [16]. On increas-ing B2O3 content, condensation of isolated BO4, B2O5 and BO3 units takesplace and causes phase-separation. Glasses containing nearly 50-mol.%of B2O3 consist of sharing vertices of tellurite clusters, isolated BO4,B2O5, BO3 and condensed borate groups. Dimitriev et al. determinedthe phase diagramof a binary borotellurite system and reported the for-mation of two phases in glasses, one with high TeO2 content was atransparent glass and the other phase richer in B2O3 was opaque andits structure consists of a complicated micro-heterogeneous network

Table 1Composition, density andmolar volume of borotellurite and alumino-borotellurite glasses.

Sample code Composition Molar mass,M(g mol−1)

Density, d(g cm−3)

Molar volume,Vm

(cm3 mol−1)B2O3 TeO2 Al2O3

15BTe 15 85 – 146.04 5.110 ± 0.003 28.5920BTe 20 80 – 141.59 4.906 ± 0.002 28.8625BTe 25 75 – 137.09 4.728 ± 0.001 28.9930BTe 30 70 – 132.59 4.472 ± 0.002 29.655Al–10BTe 10 85 5 147.72 5.047 ± 0.027 29.2710Al–10BTe 10 80 10 144.84 4.811 ± 0.015 30.1020Al–10BTe 10 70 20 139.07 4.416 ± 0.003 31.4930Al–10BTe 10 60 30 133.31 4.303 ± 0.002 30.98

Fig. 2. XRD patterns of alumino-borotellurite glasses. All samples are amorphous exceptthe sample with 30-mol.% of Al2O3, this sample shows sharp peaks due to crystalline pre-cipitates of α-Al2O3.

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[17]. The binary borotellurite system is a simple eutectic and does notform any crystalline compounds. A wide stable miscibility gap was de-termined with TeO2 concentration from 3.9 to 73.6-mol.% by Burgeret al. [18]. Phase separation is also reported byYardımcı et al.whodeter-mined the glass forming range of B2O3–TeO2 system to be 5–25 mol.% ofB2O3 [15]. Halimah et al. studied the ultrasonic and physical propertiesof the system: B2O3–TeO2 and found an increase in the rigidity of theglass structure with the addition of TeO2 [19].

The incorporation of a third component such as Al2O3 inborotellurite glasses influences B\O and Te\O coordinations and pro-vides an opportunity to tailor glass properties for applications. Additionof Al2O3 in borotellurite glasses enhances their chemical durability [20].In alumino-borotellurite glasses, there are reports of microcrystal distri-bution in an amorphous matrix [21].

The objective of this work is to elucidate changes that occur in glassshort-range structure, in particular B\O and Te\O coordination as afunction of B2O3 molar concentration in binary borotellurite glassesand the effect of addition of Al2O3 on glass structure and properties. Co-ordination numbers of boron and tellurium ions with oxygen are stud-ied by FTIR and Raman spectroscopy respectively. Glass properties likedensity, optical band gap, refractive index and thermal properties aremeasured and structure–property correlations are made.

2. Experimental work

2.1. Glass preparation

Samples of borotellurite glasses: xB2O3–(100− x)TeO2with x=15,20, 25, 30 and 35 mol.% and alumino-borotellurite glasses: yAl2O3–

10B2O3–(100−y)TeO2: y = 5, 10, 20, 30 mol.% were prepared usingH3BO3 (Aldrich, 99.9%), α-Al2O3 (CDH, India 99.9%) and TeO2 (Aldrich,

Fig. 1. XRD patterns of xB2O3–(100− x)TeO2 glasses.

99%) as startingmaterials. The chemicalsweremixed together in appro-priate molar ratios in agate mortar pestle for about 30 min and thentransferred to a platinum (Pt) crucible. The batch mixture was sinteredat 250 °C for 24 h and melted at 800 °C for about 30 min in an electricfurnace. The temperature of Pt crucible containing the melt was mea-sured by a chromel–alumel thermocouple positioned close to it. Foreach borotellurite composition, a glass sample was prepared by normalquenching; in which the melt was poured on brass block and a buttonshaped sample was obtained and immediately transferred to anotherfurnace where it was annealed at 300 °C for 30 min.

One sample containing B2O3 concentration of 35-mol.% was pre-pared but it was milky on its upper surface. Clear transparent glasseswere obtained up to B2O3 concentration of 30-mol.%.

Alumino-borotellurite samples with a fixed B2O3 concentration of10-mol.% and variable Al2O3 concentration of 5 to 30-mol.% were pre-pared by splat quenching. A small amount of melt was poured on aheavy stainless steel plate and then pressed by a massive brass block.Alumino-borotellurite glasses required higher quenching rates for theformation of glassy phase. Splat quenched samples were not given anyannealing treatment. The composition, molar mass, density and molarvolume of samples are given in Table 1.

2.2. X-ray diffraction

X-ray diffraction (XRD) studies were performed on powderedsamples using Bruker D8 Focus X-ray diffractometer with Cu Kα

Fig. 3.Density andmolar volume of borotellurite glasses as a function of B2O3mol.% (errorbars are of the size of the symbols).

Fig. 4. Density and molar volume of alumino-borotellurite glasses as a function of Al2O3

mol.% (error bars are of the size of the symbols).

Table 2Optical properties of B2O3–TeO2 glasses.

Sample code Absorption cut-offwavelength,λo [nm]

Energy gapEo [eV]

χ n αo Λ

15BTe 369 3.37 0.905 2.29 2.68 1.2520BTe 361 3.44 0.925 2.28 2.67 1.2425BTe 358 3.47 0.933 2.27 2.66 1.2330BTe 354 3.51 0.944 2.26 2.65 1.23

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radiation (λ= 1.54056 Å) in the 2θ range of 10°–70°. The X-ray tubewas operated at 40 kV and 30 mA and the scattered X-ray intensitywas measured with a scintillation detector.

2.3. Density measurements

The density of glasses was measured by Archimedes method on anelectronic balance of sensitivity 10−4 g using turpentine oil as the im-mersion fluid. Measurement on each sample was repeated three times.

2.4. DSC and TG analysis

Glass transition temperature (mid-point), crystallization (peakpoint) and liquidus temperatures (peak point) were measured by dif-ferential scanning calorimetry (DSC). Thermal studies were performedon SETARAM SETSYS Evolution-1750 system in the temperature rangeof 200–850 °C at a heating rate of 10 °C/min, in an air flow rate of20 ml/min. Measurements were carried out on powdered samples inplatinum pans. Sample amounts of 20–30mgwere used. Themaximumuncertainty in the glass transition, crystallization and liquidus tempera-tures was ±1 °C. Themogravimetric analysis (TGA) was performed si-multaneously with DSC.

2.5. Fourier transform infrared spectroscopy (FTIR)

FTIR spectra of samples were recorded in the wavenumber range of400 cm−1 to 1600 cm−1 at room temperature on Perkin-Elmer FrontierFT-IR spectrometer using KBr disc technique. Samples were ground tofine particles and thenmixedwith KBr. Themixture was then subjected

Fig. 5. Optical absorption spectra of borotellurite glasses.

to a pressure of 10 tons cm−2 to prepare thin pellets. The FTIR transmis-sion spectra were measured immediately after preparing the pellets.

2.6. Raman spectroscopy

Raman scattering studies were performed on samples on RenishawInVia Reflex Micro Raman spectrometer using 488 nm argon ion laser(5 mW), 2400 lines/mm diffraction grating, a suitable edge filter and aPeltier cooled CCD detector. Measurements were done in unpolarisedmode, at room temperature in the backscattering geometry, in thewave number range of 300 to 1000 cm−1 and at a spectral resolutionof 1 cm−1.

2.7. UV–visible spectroscopy

Optical absorption spectra of polished disk shaped borotelluriteglasses were measured at room temperature on Shimadzu 1601 doublebeam UV–visible spectrophotometer in a wavelength range of 200–1100 nm. The optical absorption coefficient α(λ) was calculated by di-viding absorbance A, with sample thickness. Since the glass sampleswere thick (2–3 mm), the true optical bandgap of glasses could not bedetermined by Tauc plots or by Mott–Davis models which requiresthe measurement of absorption spectra at photon wavelengths whereα ~103–104 cm−1 [22,23]. Absorption cut-off wavelength, λo was arbi-trarily taken as the wavelength at which the absorption coefficientreaches ~12 cm−1. The optical band gap Eg of glasses was determinedfrom λo. The optical bandgap Eg was used to calculate the optical elec-tronegativity χ, by the following relationship [24]:

χ ¼ 0:2688Eg ð1Þ

The values of χwere used to determine the refractive index n, elec-tronic polarizability αo and optical basicity Λ, of glasses from the follow-ing theoretical formulae [24]:

n ¼ −0:73 ln 0:102χ½ � þ 0:5511 ð2Þ

Fig. 6. DSC spectra of borotellurite glasses.

Table 3Glass transition (mid point value), crystallization (peak point) and liquidus (peak point) temperatures of B2O3–TeO2 and Al2O3–B2O3–TeO2 glasses.

Sample code Glass transitiontemperature(°C)

Crystallizationtemperature(°C)

Liquidus temperature(°C)

Tg1 Tg2 Tc1 Tc2 Tm1 Tm2 Tm3

15BTe 339 – 427 – 682 706 –

20BTe 352 – 447 491 684 689 69625BTe 355 – 445 463 686 691 –

30BTe 366 – 474 – 685 – –

5Al–10BTe 346 – 446 499 690 – –

10Al–10BTe 330 371 493 – 751 – –

20Al–10BTe 331 400 616 – 789 – –

30Al–10BTe 335 399 635 – 809 – –

Fig. 7. TGA curves of borotellurite glasses showing a decrease in sampleweight at ~100 °C.

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αo ¼ −0:9χþ 3:5 ð3Þ

Λ ¼ −0:5χ þ 1:7 ð4Þ

2.8. Chemical stability

The chemical stability of borotellurite and alumino-borotellurtiteglasses was studied by crushing samples into a fine powder and expos-ing it to laboratory ambient conditions for about one month; the

Fig. 8. DSC spectra of alumino-borotellurite glasses.

exposed samples were afterwards analyzed by XRD and Raman spec-troscopy to study changes in the glass short-range structure.

3. Results

3.1. Structure

Fig. 1 shows the broad peaks in the XRD patterns of borotelluritesamples, which confirmed their amorphous structure.

Alumino-borotellurite samples are also amorphous except the sam-ple with 30-mol.% of Al2O3, which show crystalline peaks [Fig. 2].

3.2. Density

The density of borotellurite glasses decreases from 5.110 ± 0.003 to4.472± 0.002 g cm−3 as the concentration of B2O3 is increased from 15to 30-mol.%. Molar volume increases slightly from 28.59 to29.65 cm3 mol−1 [Table 1 & Fig. 3].

Fig. 4 shows changes in density and molar volume in alumino-borotellurite glasses. The density of alumino-borotellurite glasses de-creases from5.047± 0.027 to 4.303± 0.002 g cm−3 andmolar volumevaries from 29.27 to 31.49 cm3 mol−1 as Al2O3 content is increased.

3.3. Optical properties

The optical absorption cut-off wavelength decreases from 369 to354 nm with an increase in B2O3 concentration from 15 to 30-mol.% inborotellurite glasses [Fig. 5]. Refractive indices are in the range of 2.29to 2.26. Electronic polarizability αo decreases from 2.68 to 2.65 and op-tical basicity Λ decreases from 1.25 to 1.23 respectively [Table 2].

Fig. 9. FTIR absorption spectra of borotellurite glasses with B2O3 concentration varyingfrom 15 to 30-mol.%.

Table 4IR band assignments in B2O3–TeO2 glasses.

Characteristic bands(cm−1)

Assignment

500–800 Stretching vibration modes of Te\O and bending modes ofB\O\B linkages

800–1150 B\O bond stretching vibrations in BO4

1150–1550 Asymmetric stretching of B\O bond in BO3

Table 6IR band assignments in Al2O3–B2O3–TeO2 glasses.

Characteristicbands(cm−1)

Assignment

~460 Al\O vibrations in AlO6

600–700 Te\O stretching vibrations in TeO4

700–800 Te\O stretching vibrations in TeO3 and Al\O vibrations in AlO4

900–1150 Al\O vibrations in AlO4 and B\O vibrations in BO4

1150–1550 B\O vibrations in BO3

120 N. Kaur, A. Khanna / Journal of Non-Crystalline Solids 404 (2014) 116–123

3.4. Thermal properties

The DSC spectra of borotellurite glasses are shown in Fig. 6. Glasstransition temperature (Tg) increases from 339 °C to 366 °C on increas-ing B2O3 concentration from 15 to 30-mol.%. The values of glass transi-tion temperature (Tg), crystallization temperature (Tc) and liquidustemperature (Tm) are given in Table 3. TGA curves show a large decreasein sampleweight in the temperature range of 90 °C to 120 °C frompow-dered samples on heat treatment [Fig. 7].

Alumino-borotellurite glass with 5-mol.% Al2O3 has a single glasstransition at 346 °C. However glasses with higher alumina mol.% showmultiple glass transitions in DSC patterns [Fig. 8].

3.5. Short range structure by FTIR

The FTIR spectra of borotellurite glasses have three broad absorptionbands in the wavenumber ranges of 500–800 cm−1, 800–1150 cm−1

and 1150–1550 cm−1 [Fig. 9]. These bands are characteristics of vibra-tions of tellurite and borate structural units [Table 4].

Alumino-borotellurite glasses show two broad infrared absorptionbands in the wavenumber ranges of 400–1100 and 1100–1600 cm−1

[Fig. 10]. A small peak at ~460 cm−1 (marked as * in Fig. 10) is observedin sample 30Al–B10Te.

3.6. Short range structure by Raman spectroscopy

Raman studies were performed on xB2O3–(100− x)TeO2 with x =15, 20, 25 and 30 mol.%; the spectra are shown in Fig. 11. All sam-ples have two broad bands in the ranges of 350 to 550 cm−1 and550 to 900 cm−1. The deconvoluted spectra of one sample contain-ing 15-mol.% B2O3 (sample code: 15BTe) are given in Fig. 12. Thedeconvoluted peaks are centered at 450, 503, 615, 665, 718, 762and 902 cm−1.

Alumino-borotellurite glasses have Raman bands at 430, 660,750 cm−1 and a shoulder near 900 cm−1 [Fig. 13]; these bands weredeconvoluted with peaks at 337, 450, 505, 567, 606, 666, 724, 754 and864 cm−1 [Fig. 14]. The Raman spectra of the sample 30Al–B0Te has asmall peak at ~430 cm−1 (marked as * in Fig. 13).

3.7. Chemical stability

Fig. 15 shows the XRD patterns of borotellurite glass with a B2O3

concentration of 30-mol.% (sample code: 30BTe) before and after its

Table 5Te\O co-ordination, NTe\O determined from the ratio of areas under Raman bands at~614, 660, 717 and 762 cm−1 and boron–oxygen co-ordination number, NB\O deter-mined from the area ratio of FTIR bands in the wave number ranges of 800–1150 cm−1

and 1150–1550 cm−1.

Sample code Raman bands area ratioA614þA660

A614þA660þA717þA762

NTe\O FTIR bands area ratioA800−1150

A800−1150þA1150−1550

NB\O

15BTe 0.63 3.63 0.34 3.3420BTe 0.60 3.60 0.27 3.2725BTe 0.59 3.59 0.25 3.2530BTe 0.56 3.56 0.17 3.17

exposure to atmospheric water vapors for a period of one month. TheXRD pattern of the initial sample is amorphouswhich crystallizes partlyon exposure to ambient atmosphere.

Fig. 16 shows the Raman spectra of the same glass sample (30BTe)before andafter its exposure to ambient atmospheric vapors. The as pre-pared sample is amorphous and has broad Raman bands; however, theexposed sample (30BTe*) exhibits sharp Raman peaks.

4. Discussion

4.1. Borotellurite glasses

The density of borotellurite glasses decreases from 5.110 ±0.003 g cm−3 to 4.472 ± 0.002 g cm−3 as the concentration ofB2O3 is increased from 15 to 30-mol.%. This is in agreement withthe molecular weight of the constituent oxides. The molecularmass of B2O3 (69.62 amu) is less than TeO2 (159.60 amu); hence,the molar mass of the glasses decreases with an increase in B2O3

concentration, which causes density to decrease. There is a small in-crease in molar volume (about 3.7%) with an increase in B2O3 con-tent as compared to the decrease in molecular weight (about 10%).The values of density and molar volume are in the range reportedby El-Mallawany [5].

Optical absorption cut-off wavelength shifts towards shorter wave-lengths when B2O3 concentration is increased upto 30 mol.% in binaryborotellurite glasses [Fig. 5 & Table 2]. Refractive index n is calculatedfrom optical band gap Eg using Eq. (1) and shows dependence on densi-ty, which is the case formost of the glass systems although some glassesdo not follow this general behavior [25]. The decrease in the refractiveindex from 2.29 to 2.26 is due to decrease in density and decrease inaverage ionic polarizability [25]. Optical basicity decreases with an in-crease in B2O3 concentration; this is expected because the polarizabilityof B3+ is less than Te4+.

An increase in Tg with B2O3 concentration is due to higher enthalpyof B\O bond (808.8 ± 20.9 kJ mol−1) as compared to Te\O bond

Fig. 10. FTIR absorption spectra of alumino-borotellurite glasses.

Fig. 11. Raman spectra of borotellurite glasses.

Fig. 13. Raman spectra of alumino-borotellurite glasses.

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enthalpy of 376.1 ± 20.9 kJ mol−1 [26]. There is a significant loss inweight at ~100 °C in the TGA curves of borotellurite glasses, due to evo-lution of adsorbed water [Fig. 7]. TGA therefore confirms thatborotellurite glasses are hygroscopic and pick up atmospheric watermolecules on exposure to ambient air. The chemical degradation ofborotellurite glasses occurs rapidly on crushing the samples into a pow-der; however, as prepared button shaped glasses are stable over a peri-od of several months and do not show degradation on exposure toambient atmosphere.

The FTIR spectra of borotellurite glasses show three vibrationalbands in the ranges of 500–800 cm−1, 800–1150 cm−1 and 1150–1550 cm−1 [Fig. 9 and Table 4]. The band at ~640 cm−1 is due toTe\O bond vibrations in TeO4 units which is a characteristic feature ofpure TeO2 glass. The infrared absorption band of TeO3 unit is in therange of 650–700 cm−1 and that of TeO4 is in the range of 600–650 cm−1[27]. The exact positions of infrared bands depend on theglass composition. The infrared band 800–1150 cm−1 is due to asym-metric stretching vibrations in [BO4], while the band 1150–1550 cm−1

is due to asymmetric stretching vibrations in BO3 units [28].The areas under the bands due to BO4 and BO3 units are useful to

quantitatively estimate the concentration of two borate structuralunits in glasses [29,30]. The ratio of area under the band 800–1150 cm−1 and area under the band 1150–1550 cm−1 i.e.

A800−1150A800−1150þA1150−1550

is the measure of fraction of tetrahedral borons in

Fig. 12. Deconvoluted Raman spectra of bor

the glass structure and gives B\O co-ordination number, NB\O inthe glass by the following relationship:

NB‐O ¼ 3 þ A800−1150

A800−1150þA1150−1550

� �ð5Þ

The values of NB\O in borotellurite glasses are presented in Table 5and its variation with B2O3 concentration is shown in Fig. 17. Clearlythe fraction of tetrahedral boron decreases with an increase in B2O3

mol.%, a finding that is consistent with earlier results [18,29]. There isa decrease in fraction of tetrahedral borons with an increase in B2O3

mol.% that limits glass-forming ability of the B2O3–TeO2 system.The overlapping peaks in broad Raman bands of glasses [Fig. 11]

were deconvoluted with Peakfit software. Prior to deconvolution,two-point baseline correction was performed and Raman spectrawere fitted with Gaussian peaks at 450, 503, 615, 665, 718, 762 and902 cm−1 [Fig. 12]. The peaks at 615 cm−1 and 665 cm−1 are dueto Te\O stretching vibrations in TeO4 units while the Raman peaksat 718 cm−1 and 762 cm−1 are due to Te\O vibrations in TeO3

and/or TeO3+ 1 units respectively. The peak at 450 cm−1 is due tobond bending vibrations of Te\O\Te linkages [31]. Similar peak po-sitions are reported by Sekiya et al. for B2O3–TeO2 glasses [16], and inpure TeO2 glass at ~450 cm−1, 611 cm−1, 659 cm−1, 716 cm−1 and773 cm−1 [10].

otellurite glass with 15-mol.% of B2O3.

Fig. 14. Deconvoluted Raman spectra of alumino-borotellurite glass with 5-mol.% of Al2O3.

122 N. Kaur, A. Khanna / Journal of Non-Crystalline Solids 404 (2014) 116–123

Neutron diffraction study of pure TeO2 glass found that Te\Oco-ordination number is 3.64 [9]. This result matched exactlywith the value calculated from the area ratio under the Ramanbands: ( A614þA660

A614þA660þA717þA762). Therefore Raman spectroscopy directly

gives Te\O co-ordination in glasses. The Raman spectra of fourborotellurite glasses were deconvoluted using Peakfit software andthe areas under four Raman peaks were determined; the Te\O co-ordination number is given byNTe\O=3+( A614þA660

A614þA660þA717þA762), and it de-

creases from 3.63 to 3.56 on increasing B2O3 content in B2O3–TeO2

glasses [Table 5].It should be noted that NB\O andNTe\O are determined by two inde-

pendent techniques i.e. FTIR and Raman spectroscopy, yet their varia-tion with B2O3 concentration is exactly same [Fig. 17]; this shows thatthe same mechanism is responsible for the structural transformations:TeO4➔TeO3 and BO4➔BO3.

4.2. Alumino-borotellurite glasses

X-ray diffraction shows that alumino-borotellurite samples areamorphous except the sample with 30-mol.% of Al2O3. The XRD patternof this sample has sharp peaks at 35.1°, 43.3°, 52.6° and 57.5° [Fig. 2]. Allthese peaks match with α-phase of aluminum oxide [32]. Therefore athigh Al2O3 concentration true glasses do not form but an amorphousphase containing phase-separated α-Al2O3 crystals is produced onsplat quenching. The sample with 30-mol.% Al2O3 sample is semi-

Fig. 15. Changes in theXRD patterns of borotellurite glass sample (30BTe) as prepared andafter exposing its fine powder to ambient atmosphere for one month (30BTe*). The pat-tern at the top is of crystalline TeO2.

transparent due to inclusions of crystals ofα-Al2O3,while other sampleswith lower Al2O3 concentration are clear and transparent.

The density of alumino-borotellurite glasses decreases with an in-crease in Al2O3 mol.%. [Table 1 & Fig. 4]. Tg in alumino-borotelluriteglasses increases with an increase in Al2O3 concentration due to an in-crease in the number of stronger Al\O bonds (enthalpy of formation:512 kJ mol−1) compared to Te\O bonds (enthalpy: 376.1 kJ mol−1)[33].

DSC scans reveal that single-phase alumino-borotellurite glass formsonly with 5-mol.% Al2O3 since this sample exhibits single glass transi-tion at 346 °C. However glasses with 10, 20 and 30-mol.% Al2O3 showmultiple glass transitions due to amorphous–amorphous phase separa-tion [20,34,35] [Fig. 8]. Hence there ismiscibility gap in the Al2O3–B2O3–

TeO2 system above 5-mol.% of Al2O3 but the chemical durability ofglasses is enhanced due to the reduction of hygroscopic nature [20].Crystallization temperature increases from 446 °C to 635 °C, andliquidus temperature increases from 690 °C to 809 °C as Al2O3 contentis increased. Sampleswith 30-mol.% of Al2O3 do not show crystallizationpeaks in DSC spectra, due to separation of Al2O3 as crystalline phase inan amorphous matrix.

The FT-IR spectra of alumino-borotellurite glasses can be roughly di-vided into two absorption bands: 400–1100 and 1100–1600 cm−1

[Fig. 10]. Gaussian fitting deconvoluted these bands into peaks at 493,601, 705, 775, 806, 892, 991, 1213, 1363, 1440 and 1586 cm−1 whichcorrespond to different structural units of BO4, BO3, AlO4 and AlO6

[Table 6]. Unlike borotellurites, quantification of NB\O is not possiblefrom FTIR spectra since the band at ~900 cm−1 can be attributed due

Fig. 16. Raman spectra of crystalline α-TeO2, glass (sample 30BTe) and that of samplecrystallized by absorption of water vapors (30BTe*).

Fig. 17. Variation in NTe\O and NB\O with B2O3 mol.% in B2O3–TeO2 glasses.

123N. Kaur, A. Khanna / Journal of Non-Crystalline Solids 404 (2014) 116–123

to vibrations of both AlO4 and BO4 units [28]. The FTIR spectra of thesample 30Al–10BTe shows a peak at 460 cm−1 (labeled * in Fig. 10)due to vibrations of AlO6 units [36], which are present in α-Al2O3. Thissmall FTIR peak is an indication of the crystalline structure of the precip-itates, as confirmed by XRD.

The Raman spectra of alumino-borotellurite glasses have broadbands in the ranges of 350 to 550 cm−1 and 550 to 850 cm−1

[Fig. 13]. The bands were deconvoluted with peaks at 337, 450,505, 567, 606, 666, 724, 754 and 864 cm−1 in sample 5Al–B10Te[Fig. 14]. These peaks correspond to vibrations in different borate,aluminate and tellurite structural units. Peaks at 450, 606, 666, 724and 754 cm−1 correspond to vibrations in Te\O units as explainedearlier for borotellurite glasses [16]. The peak at 505 cm−1 can be at-tributed to symmetric motion about the bridge of oxygen inAl\O\Al linkages, while the peak at 567 cm−1 corresponds to vibra-tions of Al\O linkages in AlO6 [37]. The shoulder at ~900 cm−1 canbe assigned to vibrations of AlO4 units [38–40]. It is not possible todetermine quantitatively Te\O and Al\O co-ordination inalumino-borotellurite glasses from Raman studies since the broadband 700 to 900 cm−1 contains overlapping peaks due to both AlO4

and TeO3 units.

5. Conclusions

Borotellurite glasses are prepared and characterized by differenttechniques, and these glasses are hygroscopic and on crushing intopowder they adsorb water from atmosphere to form crystalline precip-itates of α-TeO2 in an amorphous matrix. The glass forming ability ofB2O3–TeO2 is limited to about 30 mol.% of B2O3, above this concentra-tion, B2O3 separates out from the melt to form milky precipitates onthe top part of the quenched sample. On incorporating Al2O3 into theB2O3–TeO2 system, the chemical durability of glasses increases but theglass forming ability falls and single-phase Al2O3–B2O3–TeO2 glassesform at low Al2O3 concentration of 5-mol.%; at higher Al2O3 concentra-tion, samples show amorphous–amorphous phase separation andcrystalline-amorphous phase separation at 30-mol.% of Al2O3.

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