Structural, optical, dielectric and thermal properties of molybdenum tellurite and borotellurite glasses

Structural, optical, dielectric and thermal properties of molybdenum tellurite and borotellurite glasses

Journal of Non-Crystalline Solids 444 (2016) 1–10 Contents lists available at ScienceDirect Journal of Non-Crystalline Solids journal homepage: www...

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Journal of Non-Crystalline Solids 444 (2016) 1–10

Contents lists available at ScienceDirect

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

Structural, optical, dielectric and thermal properties of molybdenum tellurite and borotellurite glasses Amandeep Kaur a, Atul Khanna a,⁎, Fernando González b, Carmen Pesquera b, Banghao Chen c a b c

Department of Physics, Guru Nanak Dev University, Amritsar, Punjab 143005, India Department of Chemistry and Process & Recourse Engineering, University of Cantabria, Spain Chemistry & Biochemistry Department, Florida State University, Tallahassee, FL 32306, USA

a r t i c l e

i n f o

Article history: Received 19 December 2015 Received in revised form 9 April 2016 Accepted 15 April 2016 Available online xxxx Keywords: Molybdenum tellurite and borotellurite glasses Short-range order Raman spectroscopy B11 MAS-NMR Tellurium coordination Boron coordination Structural relaxation

a b s t r a c t Molybdenum tellurite and borotellurite glasses were prepared and structure-property correlations were carried out by density, X-ray diffraction, dielectric measurements, differential scanning calorimetry, UV–visible, infrared, Raman and B11 Magic Angle Spinning Nuclear Magnetic Resonance studies. The short-range structure of molybdenum tellurite glasses consists of TeO4, TeO3 and MoO6 structural units. Increase in MoO3 concentration from 20 to 50 mol% decreases the Te\O \ coordination from 3.48 to 3.26 and lowers the glass transition temperature (Tg) due to increase in the concentration of weaker Mo\\O bonds at the expense of stronger Te\O \ bonds. Refractive index of molybdenum tellurite glasses increases while the dielectric constant decreases with increase in MoO3 concentration. The addition of B2O3 in the tellurite network enhances Tg and suppresses the tendency towards crystallization. The effects of B2O3 are similar to that of MoO3 and it produces structural transformations: TeO4 → TeO3 and BO4 → BO3. The addition of B2O3 does not significantly modify the optical properties but the dielectric constant decreases by a small amount. Glass sample of 20MoO3-80TeO2 was annealed at 280 °C for ~500 h and changes in its density and thermal properties were studied; it was found that the annealing increases the glass density slightly, but it causes a drastic enhancement of Tg by 10 °C, due to the structural rearrangements in the intermediate range order without effecting Te\O and Mo\\O speciation. © 2016 Elsevier B.V. All rights reserved.

1. Introduction TeO2 based glasses have attracted considerable scientific interest due to their several useful properties such as good glass stability and durability, wide optical transmission window, low melting point, nonhygroscopic nature, high refractive indices and exceptional non-linear optical properties. Tellurite glasses find applications as gas sensors, memory switching devices and optical waveguides [1–5]. Crystalline α-TeO2 contains Te4 + only in tetrahedral coordination with oxygen (i.e. NTe-O = 4), but glassy TeO2 has NTe-O b4 [6]. Glassy TeO2 can be synthesized by twin roller quenching at melt-cooling rates of ~ 105 K s−1. Ab initio molecular dynamic simulation studies on amorphous TeO2 by Pietrucci et al. found NTe-O to be 3.69 [7] and neutron diffraction studies on glassy TeO2 by Gulenko et al. [8] and by Barney et al. [6] determined NTe-O to be 3.73 and 3.68 respectively. Therefore experimental findings match well with theoretical predictions on the short range structure of glassy TeO2. Further NTe-O from neutron diffraction analysis show good agreement with the values determined from Raman studies on tellurite glasses [6]. NTe-O decreases and the glass forming ability of TeO2 enhances significantly on mixing it with alkali, alkaline-earth, ⁎ Corresponding author. E-mail address: [email protected] (A. Khanna).

http://dx.doi.org/10.1016/j.jnoncrysol.2016.04.033 0022-3093/© 2016 Elsevier B.V. All rights reserved.

heavy metal, rare earth and transition metal oxides [1]. The addition of metal oxides in tellurite glasses improves the functionality of glasses for optical applications [9,10]. MoO3 has excellent optoelectronic properties [11]. It can act as a network former [12], and also as network modifier in the presence of other glass formers such as TeO2 [13] and B2O3 [14]. On mixing it with TeO2 it forms glasses in the composition range of 12.5 to 58.5 mol% of MoO3 [15]. MoO3 has the ability to control phase separation in glasses [16]. It produces structural modification in the tellurite network similar to WO3 and V2O5 in WO3-TeO2 [17,18] and V2O5TeO2 systems [19] respectively. In TeO2-MoO3 glasses the basic structural units are fourfold coordinated TeO4 tetrahedra, TeO3 + 1, TeO3 and six-fold coordinated single and paired MoO6 octahedra [20,21]. The short-range atomic order in molybdenum tellurite glasses has been analyzed by variety of techniques: neutron and X-ray diffraction [22,23], X-ray photoelectron spectroscopy (XPS) [24] and Extended Xray Absorption Fine Structure (EXAFS) [16] and it is found that the addition of MoO3 decreases Te4 + coordination from 4 to 3 and that of Mo6+ from 6 to 4 [22]. Neov et al. [22] and Manisha et al. [25] reported that MoO6 units transform into MoO4 with increase in MoO3 concentration in molybdenum tellurite glasses. Whereas Sokolov et al. [20] analyzed the structure of molybdenum tellurite glasses by quantum mechanical

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calculations and Raman spectroscopy and concluded that only TeO4, O_TeO 2, single octahedral (O_MoO 5) and paired octahedral (2[O_MoO5]) units exist in the glass network. Moreover according to Sokolov et al. MoO6 units with two double bonds and MoO4 tetrahedra are unstable and do not exist in the glass network. Sekiya et al. [21] and Dimitriev et al. [26] also concluded from Raman and FTIR studies that at low MoO3 concentration (b30 mol%) the intensity of the Raman peak at 920 cm− 1 (due to Mo_O bond vibrations of single or paired MoO6 ) is higher than the intensity of Raman peak at 870 cm− 1 (attributed to vibrations of Mo\\O\\Mo linkages in MoO6 ). On increasing MoO 3 mol%, the concentration of Mo_O bonds decreases and the peak at 870 cm− 1 becomes more prominent due to the formation of Mo\\O\\Mo linkages. Dimitriev et al. found from X-ray diffraction radial distribution function analysis that NTe-O decreases with increase in MoO3 concentration and that these glasses contain MoO6 units [23]. Calas et al. [16] concluded from Mo\\K edge EXAFS that isolated MoO4 exist in molybdenum tellurite glasses which are not directly connected to the glass network. Mekki et al. [24] found from X-ray photoelectron spectroscopy (XPS) studies that the binding energies of 3d electrons of Te4+ in MoO3-TeO2 glasses is equal to that in αTeO2 crystals, similarly the binding energy of 3d electrons of Mo6+ in glasses is equal to that in α-MoO3 crystals, hence these authors concluded that there exist only TeO4 and MoO6 units in molybdenum tellurite glasses containing 10 to 40 mol% of MoO3 and that the oxidation state of Mo ions is only 6+ and there are no Mo ions in 4+ and 5+ states. Therefore, there are contradictory findings on Mo\\O and Te\\O speciation in these glasses and it is an unresolved issue that whether Mo6+ coordination changes or remains constant with MoO3 concentration. It is necessary to carry out comprehensive studies on the thermal, optical and structural properties of MoO3-TeO2 glasses to resolve the questions on NTe-O and NMo-O. B2O3 is the best oxide glass former [27], and is incorporated in silicate glasses to increase its chemical and thermal stability. Basic structural units of borate glasses are BO4 and BO3. An increase in the concentration of B2O3 in borotellurite glasses causes the transformation of BO4 into BO3 and decrease in boron oxygen coordination (NB-O) [28, 29]. Decrease in the fraction of tetrahedral borons (N4) in the glass network lowers the glass forming ability (GFA) of borotellurite glasses. The thermal stability and GFA of borate glasses depends on N4 value in the glass network. Higher the N4, more is its glass forming range [29], while in tellurite glasses, the opposite is true; it are the triangularly coordinated TeO3 units which are the feature of the glassy phase and TeO4 units are a feature of crystalline TeO2. Borate and tellurite units in borotellurite glasses can connect with each other to form mixed structural units such as BTeO3 and BTeO5 which enhance the electrical conductivity of borotellurite glasses [30]. Multi-component tellurite glasses have good optical and electrical properties because of high refractive index and lower ability to devitrify as compared to binary tellurite glass system [31]. Tellurite glasses in the systems such as TeO2–WO3, TeO2–Nb2O5 [18], TeO2–Nb2O5-Bi2O3 [32], TeO2–Nb2O5–ZnO [33], TeO2–Nb2O5–ZnO-Gd2O3 [34] and TeO2–TiO2– Bi2O3 [3] have been prepared and characterized for their excellent nonlinear optical properties, high refractive indices and good electrical conductivity. It is the objective of this work to analyze the changes in shortrange structure of molybdenum and molybdenum borotellurite glasses and their thermal, optical and dielectric properties with varying MoO3 and B2O3 concentrations in respective glasses. B11 Magic Angle Spinning Nuclear Magnetic Resonance (MAS-NMR), Raman and FTIR methods are used to study the effects of addition of B2O3 and MoO 3 on B\\O, Te\\O and Mo\\O speciation. Finally, the effects of long duration annealing on the density, thermal, optical, shortrange and medium-range structure of one molybdenum tellurite glass (20MoTe) is studied.

2. Experimental 2.1 Glass preparation Molybdenum tellurite and borotellurite glasses of composition: xMoO3-(100-x) TeO2 with x = 20, 30, 35, 40, 45 and 50 mol% and 20MoO3-xB2O3-(80-x) TeO2 with x = 5 and 10 mol% respectively were prepared using MoO3 (Otto Kemi, India, 99%), H3BO3 (Aldrich India, 99.9%) and TeO2 (Aldrich India, 99%) as starting materials. Appropriate amounts of chemicals were weighed and mixed together in agate mortar pestle for about 30 min and then transferred to a platinum crucible. The batch mixture was melted at 850 °C for 30 min in an electric furnace. For each composition a glass sample was prepared by normal quenching method in which a small quantity of the melt was poured on a heavy brass plate and a disk-shaped sample was obtained and annealed at 300 °C for 30 min. Bubble free, clear and dark-brown colored samples were obtained, the color of glasses darkened with increase in the MoO3 concentration. The composition, density and molar volume of samples are given in Table 1. 2.2 X-ray diffraction (XRD) XRD measurements were performed on powdered glass samples on Bruker D8 Focus X-ray diffractometer with Cu Kα radiation (λ = 1.54056 Å) in the 2θ range of 10°–65°. The X-ray tube was operated at 40 kV and 30 mA and the scattered X-ray intensity was measured with a scintillation detector. 2.3 Density measurement Density of glasses was measured by Archimedes method using dibutylphatalate (DBP) as the immersion fluid. The error in density was calculated from the precision of measurement of mass by electronic balance (10−4 g) and it was in the range of ±0.002 to ±0.004 g cm−3. 2.4 Differential Scanning Calorimetry (DSC) DSC studies were carried out on a SETARAM SETYS 16 TG-DSC system in temperature range of 200–800 °C at heating rate of 10 °C min−1. Measurements were performed on powdered samples in platinum pans. Samples amounts of 20–50 mg were used for DSC analysis. Maximum uncertainty in the measurement of glass transition (midpoint), crystallization (peak point) and melting temperatures (peak point) is ±1 °C. 2.5 Fourier transform infrared spectroscopy (FTIR) FTIR spectra of molybdenum borotellurite samples were recorded on Perkin-Elmer Frontier FTIR spectrometer using KBr disk technique in the wavenumber range of 400 cm−1 to 2000 cm−1 at room temperature. The mixture of powdered glass sample and spectroscopic grade KBr (1:100 by weight) was subjected to pressure of 10 tons cm−2 to prepare thin pellets. The FTIR absorption spectra were measured immediately after preparing the pellets. 2.6 Raman spectroscopy Raman scattering studies were performed on samples with Renishaw In-Via Reflex micro-Raman spectrometer using 514.5 nm argon ion laser (50 mW) as excitation source, diffraction grating having 2400 lines mm−1, an edge filter and a Peltier cooled CCD detector. Measurements were carried out in an unpolarized mode, at room temperature in the backscattering geometry, in the wave number range of 30 to 1000 cm−1 at a spectral resolution of 1 cm−1.

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Table 1 Density, optical, thermal, dielectric and structural properties of molybdenum tellurite and borotellurite glasses. Sample Code

20MoTe 25MoTe 30MoTe 35MoTe 40MoTe 45MoTe 50MoTe 20Mo5BTe 20Mo10BTe

Composition (mol %) MoO3

TeO2

B2O3

20 25 30 35 40 45 50 20 20

80 75 70 65 60 55 50 75 70

– – – – – – – 5 10

Density ρ (g cm−3)

5.251 ± 0.002 5.255 ± 0.002 5.176 ± 0.003 5.102 ± 0.002 5.018 ± 0.003 4.964 ± 0.004 4.858 ± 0.004 5.189 ± 0.002 4.994 ± 0.002

VM (cm3 mol−1)

29.80 ± 0.01 29.63 ± 0.01 29.93 ± 0.02 30.21 ± 0.01 30.56 ± 0.01 30.77 ± 0.02 31.24 ± 0.02 29.29 ± 0.01 29.53 ± 0.01

Eg (eV)

n

(±0.01)

(±0.01)

2.50 2.45 2.41 2.24 2.02 1.48 1.14 2.52 2.53

2.51 2.52 2.54 2.59 2.66 2.89 3.08 2.50 2.50

20.0 ± 2.0 18.6 ± 0.6 18.8 ± 1.2 17.9 ± 0.6 17.0 ± 0.7 16.2 ± 1.0 15.7 ± 1.3 17.6 ± 0.5 15.8 ± 0.7

Tg (°C) (±1°C)

323 320 319 318 315 313 309 327 332

Tc (°C) (±1°C)

Tm (°C) (±1°C)

Tc1

Tc2

Tm1

Tm2

415 438 435 490 – – 444 444 462

501 – 500 – – – – 490 –

545 – 548 549 – 502 528 533 532

647 – – – – – – – 596

NTe-O

NB-O (FTIR)

NB-O (NMR)

(±0.01)

(±0.01)

(±0.01)

3.48 3.43 3.40 3.37 3.35 3.28 3.26 3.44 3.41

– – – – – – – 3.67 3.52

– – – – – – – 3.64 3.54

study the structural relaxation and the effects of annealing on the density, thermal, optical and the structural properties of glasses.

2.7 UV–visible spectroscopy The optical absorption spectra of all polished samples were performed at room temperature on Shimadzu 1601 double beam UV–visible spectrophotometer in the wavelength range: 400 to 1100 nm. The absorption coefficient α(λ) was calculated by dividing the absorption A, with the thickness of glass samples, which is ~1.5 to 2.5 mm. The optical band gap, Eg was calculated from cut-off wavelength λo, which was arbitrarily taken as the wavelength at which absorption coefficient becomes 12.5 cm−1. The electronegativity, optical polarizability and basicity were calculated from the following empirical relationships [35]: χ ¼ 0:2688Eg

εr (1 kHz)

:

ð1Þ

η ¼ −0:73In½0:102χ þ 0:5511:

ð2Þ

α o ¼ −0:9χ þ 3:5

ð3Þ

Λ ¼ −0:5χ þ 1:7:

ð4Þ

3. Results 31. Structure XRD patterns of molybdenum tellurite and borotellurite glasses exhibit broad humps without any sharp peaks which confirm the amorphous structure of samples (Fig. 1). 3.2. Density Density, ρ of molybdenum tellurite glasses decreases from 5.251 ± 0.002 to 4.858 ± 0.004 g cm− 3 as the concentration of MoO3 is increased from 20 to 50 mol% and molar volume increases from 29.80 ± 0.01 to 31.24 ± 0.02 cm3 mol−1. Upon adding 10 mol% of B2O3 into molybdenum tellurite glass, density decreases from 5.251 ± 0.002 to 4.994 ± 0.002 g cm−3 (Table 1 & Fig. 2). 3.3. Optical properties

2.8 Dielectric spectroscopy Dielectric measurements were carried out on polished, disk-shaped samples of thickness ~1.5 mm on the impedance analyzer (Wayn Kerr 6500B) in the frequency range of 300 Hz to 1 MHz at room temperature. Sliver electrodes were coated on both sides of samples and dielectric constant (εr) was calculated by the following formula: εr ¼

Ct εo A

The optical absorption cut-off wavelength increases from 497 to 1088 nm, refractive index, n increases from 2.51 to 3.08 and optical band gap decreases from 2.50 eV to 1.14 eV with increase in MoO3 mol. % from 20 to 50 mol% (Fig. 3 and Table 1). In molybdenum borotellurite glasses the optical cut-off absorption wavelength shifts slightly to lower wavelength from 497 nm to 492 nm with the addition of 10 mol% B2O3 (Fig. 4 and Table 1).

ð5Þ

where εo is the permittivity of vacuum (8.85 × 10−12 F m−1), C is the capacitance measured to an accuracy of ±0.1 pF, t is the sample thickness measured to an accuracy of ± 0.01 mm and A is sample surface area measured to an accuracy of ±5 mm2. 2.9 B11 MAS-NMR B11 MAS-NMR studies were performed on two molybdenum borotellurite glasses on a Bruker AVIII HD NMR spectrometer operating at magnetic field of 11.74 T with a 4 mm Bruker MAS probe at Larmor frequency of 160.5299 MHz for B11 nuclei. Sample spinning rate was 14 kHz. A short RF pulses (b15°) with recycle delay of 20s were used. Spectra were collected after 4096 scans and referenced to solid NaBH4 at −42.16 ppm. 2.10 Annealing Molybdenum tellurite glass (Sample: 20MoTe) was annealed at 220 °C for 63 h and then subsequently heated at 280 °C for 459 h to

3.4. Thermal properties DSC patterns of molybdenum tellurite glasses are shown in Fig. 5. Tg decreases steadily from 323 °C to 309 °C with increase in MoO3 concentration from 20 to 50 mol%. On adding 10 mol% of B2O3 into molybdenum tellurite glass, Tg increases to 332 °C (Sample: 20Mo10BTe) (Fig.6). The values of glass transition temperature (Tg), crystallization temperature (Tc) and liquidus temperature (Tm) are given in Table 1. Samples containing 40 and 45 mol% of MoO3 (Samples: 40MoTe and 45MoTe) do not show crystallization and melting peaks. 3.5. B\\O speciation by FTIR FTIR absorption spectra of molybdenum borotellurite glasses show broad bands in wavenumber ranges: 535 to 720 cm−1, 720 to 840 cm− 1, 840 to 920 cm− 1, 920 to 980 cm−1, 990 to 1290 cm−1, 1290 to 1490 cm−1 and 1490 to 1770 cm−1. These bands are due to vibrational modes of tellurite, borate and molybdate structural units (Table 2). Fig. 7 shows the FTIR spectra of molybdenum borotellurite glasses. The coordination of B4+ with oxygen (NB-O) in molybdenum

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Fig. 1. XRD patterns of molybdenum tellurite and molybdenum borotellurite glasses.

borotellurite glasses was calculated by the following formula [36]: N4 ¼

A990 þ A1290 : A990 þ A1290 þ A1490 þ A1770

NB−O ðFTIRÞ ¼ 3 þ N4

ð6Þ ð7Þ

where A is the area under Gaussian peaks centered at 990, 1290, 1490 and 1770 cm−1. NB-O(FTIR) decreases from 3.67 ± 0.01 to 3.52 ± 0.01 on increasing B2O3 from 5 to 10 mol% (Table 1). 3.6. Te\\O speciation by Raman spectroscopy Raman studies were used to determine the coordination number of Te4+. Raman spectra of α-TeO2, α-MoO3, molybdenum tellurite glasses and borotellurite glasses are shown in Figs. 8 and 9. Raman spectra of αMoO3 have sharp peaks at 996, 823, 667, 293, 159, 129, 115 and 84 cm−1, and weak peaks at 473, 380, 366, 246, 218 and 198 cm−1. Similarly crystalline α-TeO2 has five strong peaks at 645, 392, 150, 122 and 62 cm−1 and very weak peaks at 770, 720, 588 and 173 cm−1 (Table 3). All glasses show four broad bands from 30 to 300 cm− 1, 300 to 570 cm−1, 570 cm− 1 to 820 cm − 1 and 820 cm− 1 to 1000 cm− 1

Fig. 2. Variation of density and molar volume with MoO3 mol%.

(Table 3). Raman spectra were baseline corrected and deconvoluted with Gaussian peaks using Peakfit software. The deconvoluted spectrum of one sample (20MoTe) in the Raman shift range of 550 to 1000 cm−1 is shown in Fig. 10. Deconvoluted peaks are centered at ~ 609, 663, 718, 777, 871 and 925 cm−1 and the areas (A) under these peaks were used to calculate the Te\\O coordination (NTe-O) by the following relationship [29]: NTe−o ¼ 3 þ

A609 þ A663 : A609 þ A663 þ A718 þ A777

ð8Þ

NTe-O decreases steadily from 3.48 ± 0.01 to 3.26 ± 0.01 on increasing MoO3 concentration from 20 to 50 mol% (Fig. 11). In case of molybdenum borotellurite glasses, NTe-O decreases from 3.48 to 3.41 on increasing B2O3 from 5 to 10 mol%. 3.7 B\\O speciation by B11 MAS NMR B11 MAS-NMR spectra of molybdenum borotellurite glasses are shown in Fig.12. These spectra show one sharp peak at 0.2 ppm and a second broader peak at 9 ppm. The spectra were fitted with Gaussian

Fig. 3. Optical absorption spectra of MoO3-TeO2 glasses.

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Fig. 4. Optical spectra of MoO3-B2O3-TeO2 glasses.

Fig. 6. DSC patterns of MoO3-B2O3-TeO2 glasses.

peaks and the integrated areas under these peaks A4 and A3 due to BO4 and BO3 units respectively, were used to calculate the NB-O:

NB−OðNMRÞ ¼ 3 þ

A4 : A3 þ3 A4

5

ð9Þ

NB-O(NMR) decreases from 3.64 ± 0.01 to 3.54 ± 0.01 on increasing B2O3 from 5 to 10 mol% in molybdenum borotellurite glasses.

3.8 Dielectric constant Dielectric constant (εr) of molybdenum tellurite glasses decreases from 18.6 ± 0.6 to 15.7 ± 1.3 on increasing MoO3 concentration from 25 to 50 mol% (Fig. 13). Similarly in molybdenum borotellurite glasses εr decreases from 20.0 ± 2.0 to 15.8 ± 0.7 with the addition of 10 mol% of B2O3.

3.9. Effects of annealing on glass structure and properties X-ray diffraction pattern of molybdenum tellurite glass with 20 mol% of MoO3 (Sample: 20MoTe) and for the sample annealed for ~500 h (20MoTe-H) show broad humps which confirmed that the sample did not crystallize after long duration heat treatment (~ 500 h) (Fig. 14). Density increases from 5.251 ± 0.002 to 5.294 ± 0.002 g cm−3 and the value of Tg increases drastically, by 10 °C from 323 °C to 333 °C after annealing treatment of the glass sample (Fig. 15 & Table 4). Fig. 16 shows that the optical cut-off wavelength of annealed glass shifts slightly, from 497 to 500 nm. The Raman spectra of the annealed sample (20MoTe-H) and the initial sample (20MTe) shown in Fig. 17 are nearly same, which indicates that NTe-O remains constant at 3.48 after annealing (Table 4). 4. Discussion 4.1 Molybdenum tellurite glasses Density of glasses decreases with the increase in MoO3 concentration because molar mass of MoO3 (143.94 u.) is lower than that of TeO2 (159.6 u.). On increasing MoO3 concentration, molar mass decreases and hence density falls. Mallawany et al. [37] reported density of MoO3-TeO2 glasses to be in the range of 5.01 to 4.60 g cm−3 and Neov et al. [22] in the range: 5.28 to 4.70 g cm−3 (MoO3 concentration of 20 to 50 mol%) which are quite closer to the density of glasses characterized in the present work.

Table 2 Assignments of FTIR bands in glasses.

Fig. 5. DSC patterns of MoO3-TeO2 glasses.

Absorption bands (cm−1)

Assignments

530–670 720–800 840–920 920–980 990–1290 1290–1770

Asymmetrical stretching vibration of Te\O \ linkages in TeO4 Asymmetrical stretching vibration of Te\ \O linkages in TeO3 units Mo\O \\\Mo linkages MoO bond vibrations in single and paired MoO6 Asymmetrical stretching vibrations in BO4 Asymmetrical stretching vibrations in BO3

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Fig.9. Raman spectra of MoO3-B2O3-TeO2 glasses. Fig.7. FTIR spectra of MoO3-B2O3-TeO2 glasses. FTIR spectrum of 20MoTe is displayed for comparison.

Optical absorption cut-off wavelength shifts to longer wavelengths with increase in MoO3 mol. %. Refractive index optical polarizability increases with MoO3 concentration due to the formation of highly polar MoO6 octahedra in MoO3-TeO2 system [38,39]. Glass transition temperature decreases steadily on adding MoO3, this is because the single bond enthalpy of Mo\\O (386 kJ mol−1) is lower than the single bond enthalpy of Te\\O (428 kJ mol− 1). Sekiya et al. [21] reported that Tg of MoO3-TeO2 system first increases up to 20 mol% of MoO3, reaches a maxima at ~ 35 mol% MoO3 and then decreases with further increase in MoO3 concentration. The present DSC analysis show that Tg decreases monotonically with increase in MoO3 from 20 to 50 mol% and there is no local maxima in Tg. These findings rule out the transformation: MoO6 → MoO4, since the change in Mo6+ coordination would enhance Tg because Mo\\O bonds in MoO4 are stronger than these bonds in MoO6.

In all probability there is only breakage of Mo_O double bonds (bond length ~ 167 pm) and the formation of Mo\\O\\Mo linkages (bond length ~ 195 pm) [40] on adding more and more MoO3, this conclusion is supported by Raman studies which confirm that the peak at ~873 cm−1 grows at the expense of the peak at ~920 cm−1. Raman spectra of α-TeO2 shows a strong peak at 645 cm−1 due to anti-symmetric stretching vibrations of Te\\axOeq\Te bonds in TeO4 units, the peaks at 392 and 336 cm− 1 are due to symmetric vibrations of Te\\O\\Te bridges [41]. Weak peaks at 770 and 720 cm− 1 reveal that small amounts of TeO3 units exist even in crystalline α-TeO2. The peak at 588 cm− 1 is due to symmetric and anti-symmetric stretching modes of TeO4 units. Strong peaks in the low wavenumber region:

Table 3 Assignments of Raman bands in glasses. Raman bands (cm−1) α-TeO2 60, 122, 148 336, 392 588 720, 770 α-MoO3 84, 115, 129, 159 198, 246 285 337, 366, 380, 473 667 820 996

Fig.8. Raman spectra of MoO3-TeO2 glasses. Raman spectra of crystalline α-MoO3 and αTeO2 are also shown.

Assignments

Longitudinal optical mode vibrations (LO) of TeO4 around the bridging oxygens Symmetric vibrations of Te\O \\\Te bridges Symmetric and antisymmetric stretching modes of TeO4 units Symmetric and antisymmetric stretching modes of TeO3 units Rotational and translation chain modes of Mo\\O linkages Twisting vibrations O_Mo_O O_Mo_O bond vibrations Scissoring and bending vibrations of O\Mo\ \ \O linkages Antisymmetric stretching vibrations of O\Mo \ \O \ linkages Stretching vibrations of Mo\ \O\Mo \ linkages Antisymmetric stretching vibrations of Mo_O bonds

MoO3-TeO2 and MoO3-B2O3-TeO2 glasses 56 Boson peak 380 Assigned to corner-shared MoO6 octahedra units 400 to 500 Bending vibration of Te\ \O\\Te or O\Te \ \O \ linkages 570 to 700 Stretching vibrations of TeO4 tetrahedra 700 to 820 Stretching vibrations of TeO3 units 870 Vibrations of Mo\O \\Mo \ linkages 923 to 940 Vibrations of Mo\O \ and Mo_O bonds in single and paired MoO6 units

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Fig.10. Deconvoluted Raman spectrum of molybdenum tellurite glass containing 20 mol%. of MoO3.

148, 122 and 60 cm−1 are due to longitudinal optical mode vibrations of TeO4 around the bridging oxygens [42–45]. Raman spectra of α-MoO3 has two prominent peaks at 996 cm−1 and 820 cm−1 due to anti-symmetric stretching vibrations of Mo_O terminal bonds and stretching vibrations of Mo\\O\\Mo linkages respectively [46], the peak at 667 cm−1 is due to antisymmetric stretching vibrations of O\\Mo\\O linkages, the peak at 285 cm− 1 is due to the O_Mo_O bond vibrations, finally the sharp peaks in low wavenumbers at 159, 129, 115 and 84 cm−1 are due to rotational and translation chain modes of Mo\\O linkages. Weak peaks at 473, 380, 366 and 337 cm−1 are due to the scissoring and bending vibrations of O\\Mo\\O linkages, the peaks at 246 and 198 cm−1 are due to O_Mo_O twisting vibrations [40]. The Raman spectra of molybdenum tellurite glasses show the strongest peak at 923 cm−1 due to vibrations of Mo\\O\\and Mo_O bonds in MoO6 units [21]. The shoulder at 870 cm− 1 is due to vibrations of Mo\\O\\Mo linkages [20]. The band in the wavenumber range: 700 to 820 cm− 1 is due to stretching vibrations of TeO3 units, while that from 570 and 700 cm− 1 is due to stretching vibrations of TeO4. The band from 400 to 500 cm−1 is due to bending vibration of Te\\O\\Te or O\\Te\\O linkages and the shoulder at 380 cm−1 is assigned to cornershared MoO6 octahedra units [47,48]. The first sample with 20 mol% MoO3 has a peak at 923 cm− 1, it shifts to 940 cm− 1 on increasing

MoO3 concentration to 50 mol%. On the other hand, the band at 870 cm−1 grows in intensity with increase in MoO3 mol. % and indicates the formation of Mo-shortOlong-Mo linkages associated with MoO6 units having one Mo_O double bond [21]. In molybdenum tellurite glasses the intensity of the Raman peak at 665 cm−1 decreases steadily with increase in MoO3 mol. %, despite the fact that α-MoO3 has a very strong peak at 665 cm−1. Therefore the band at 665 cm−1 in molybdenum tellurite glasses is mostly due to vibrations in TeO4 and there is little or no contribution from O\\Mo\\O vibrations of MoO6 units. Further the shoulder at 770 cm−1 in the first glass sample (20MoTe) becomes a prominent peak when MoO3 concentration reaches 50 mol%, this indicates the steady conversion of TeO4 into TeO3 units. The contributions of vibrations of MoO6 units are insignificant in the wavenumber range of 600 to 790 cm− 1 and the MoO6 units contribute mostly in the high frequency regions from 800 to 1000 cm−1. The decrease in NTe-O from Raman spectroscopy confirms the conversion of TeO4 into TeO3+1/TeO3 units through the breaking of Te\\O\\Te bonds on adding MoO3 [13]. Coordination of Mo6+ with oxygen does not seem to change from 6 to 4 with increase in MoO3 content because as discussed above, Tg decreases monotonically and hence the conversion of MoO6 into MoO4 is ruled out.

Fig.11. Variation of NTe-O with MoO3 mol. % in MoO3-TeO2 glasses.

Fig.12. B11 MAS-NMR spectra of molybdenum borotellurite glasses.

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A. Kaur et al. / Journal of Non-Crystalline Solids 444 (2016) 1–10

Fig.13. Variation of dielectric constant, εr with MoO3 mol% and signal frequency in. molybdenum tellurite glasses.

The Raman spectra in the low wavenumber region from 30 to 300 cm−1 consists of shoulder at ~ 200 cm−1 due to stretching vibrations of TeO4 units and the strong peak at 56 cm− 1, the latter is the boson peak and is an ubiquitous feature of glasses [49], the intensity of the peak at ~56 cm−1 decreases with increase in MoO3 mol%. Dielectric constant of molybdenum tellurite glasses decreases from 20.0 to 15.8 (at 1 kHz) on increasing MoO3 concentration up to 50 mol%. The decrease in dielectric constant can be either due to decrease in glass density, or due to decrease in electronic polarizability, or both. The causes of its variation can be understood from the analysis of Clausius-Mosotti equation: εr −1 4παρNA ¼ 3M g εr þ 2

ð10Þ

where α, NA, ρ and Mg are the polarizability, Avogadro number, density and molar mass respectively. The variation of ðεεrr−1 þ2 ÞMg with ρ is linear A (Fig. 18) with constant slope ( 4παN = 21.1 cm3) and polarizability 3 −24 3 (α = 8.5 × 10 cm ). Therefore εr depends only upon density and α remains constant. Mallawany et al. [50] reported that the graph between ðεεrr−1 þ2 ÞMg and

ρ is non-linear due to variation of α, the dielectric constant values

Fig. 15. DSC patterns of the initial and annealed molybdenum tellurite glass.

reported by Mallawany et al. were also significantly lower in the range of 9.8 to 5.5 for glasses containing 20 to 45 mol% MoO3. The present study has found a linear variation between the two parameters (Fig. 18) and significantly higher εr in the range: 20.0 ± 2.0 to 15.7 ± 1.3. Finally the decrease of εr with increase in frequency is due to decrease in electronic polarizability (Fig. 13).

4.2 Molybdenum borotellurite glasses On adding B2O3 into molybdenum tellurite glasses, density decreases because molar mass of B2O3 (69.62 u) is significantly lower than the molar mass of TeO2 (159.6 u). Molar volume also decreases; this anomalous behavior has been reported in TeO2–Li2O–B2O3 glasses by Saddek et al. [51] and in TeO2–Na2O–B2O3 by Halimah et al. [52]. Tg increases from 323 °C to 332 °C on adding B2O3 into the glass network because B\\O bond enthalpy (808 kJ mol−1) is significantly higher than Te\\O bond enthalpy (428 kJ mol−1) [29,53]. Increase in Tg of molybdenum borotellurite glasses and the suppression of crystallization peaks shows that the thermal stability of molybdenum borotellurite glass enhances upon adding B2O3. The FTIR band from 990 to 1290 cm−1 in molybdenum borotellurite glasses is due to anti-symmetric stretching vibrations of B\\O bonds in BO4 units and the band from 1290 to 1770 cm−1 is due to asymmetric stretching vibrations of B\\O stretching vibrations in triangular BO3 units [29]. NB-O decreases with increase in B2O3 concentration which shows that BO4 units are unstable in the tellurite network [36]. The values of NB-O(NMR) are in excellent agreement with NB-O(FTIR). Raman spectra of molybdenum borotellurite glasses are similar to that of molybdenum tellurite glasses. B2O3 acts like a modifier in the tellurite network and has the same effect as MoO3 in producing the structural transformations: TeO4➔TeO3 and BO4➔BO3 [17,24].

Table 4 Density and glass transition temperature of 20MoO3-80TeO2 glass before and after annealing.

Fig. 14. XRD patterns of initial and annealed molybdenum tellurite glass containing 20 mol% MoO3.

Sample code

ρ (g cm−3)

Tg (°C)

20MoTe 20MoTe-H

5.251 ± 0.002 5.294 ± 0.002

323 333

A. Kaur et al. / Journal of Non-Crystalline Solids 444 (2016) 1–10

Fig. 18. Linear correlation between ðεεrr −1 þ2 ÞMg and ρ.

Fig. 16. Optical absorption spectra of the initial and annealed molybdenum tellurite glass.

4.3 Structural relaxation in molybdenum tellurite glass Long duration annealing of molybdenum tellurite glass produces a small increase in density but a drastic enhancement of Tg. Raman spectra of the initial and annealed glasses are very similar, and hence rule out any significant change in the short-range structural features such as NTe-O and NMo-O. The short-range structure of MoO3-TeO2 glasses is very similar to that of WO3-TeO2 glasses and it is reported that glass transition temperature of WO3-TeO2 glasses depends mostly on the changes in Te\\O\\Te correlations in the intermediate range distances of ~ 0.385 nm [17]. Nishida et al. reported that value of Tg in potassium, barium and magnesium tellurite glasses increases with increase in distortion of network forming polyhedra such as TeO4 [54]. Annealing can produce distortion effects and modify bond angle distributions in the tellurite network. Therefore it is proposed that drastic enhancement in the glass transition temperature of the molybdenum tellurite glass is due to structural rearrangements in the intermediate range atomic order. The network connectivity enhances by the consumption of non-

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bridging oxygens and by forming linkages among the structural units. This enhanced network connectivity increases density and Tg. 5. Conclusion Short-range structure, density, thermal, optical and dielectric properties of molybdenum tellurite glasses containing 20 to 50 mol% MoO3 were studied. Molybdenum tellurite glasses have high refractive indices and dielectric constants. On adding B2O3 into tellurite network, the optical and dielectric constant remains nearly same but the thermal stability against devitrification increases considerably. Raman studies confirm that MoO3 transform TeO4 into TeO3 units and our findings rule out the earlier conclusions arrived by XPS analysis that Te\\O coordination remains constant at 4 in MoO3-TeO2 glasses. Raman spectroscopy is more useful for determining the Te\\O speciation and confirms a continuous decrease in NTe-O with increase in MoO3 content in glasses. The combined Raman and DSC studies also indicate that Mo\\O coordination remains constant at 6. Long duration annealing does not produce any significant changes in the glass short-range structural properties such as NTe-O and NMo-O but enhance the network connectivity via changes in the intermediate range order. These structural modifications drastically increase the glass transition temperature. Acknowledgement Financial support from Inter-university Accelerator Center (UFR53304), New Delhi and UGC-DAE — Consortium for Scientific Research, Mumbai (CRS-M-215), India is acknowledged. References

Fig. 17. Raman spectra of the initial and annealed molybdenum tellurite glass.

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