The structure and optical dispersion of the refractive index of tellurite glass

Optical Materials 33 (2011) 1569–1572

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The structure and optical dispersion of the refractive index of tellurite glass W.A. Capanema a, K. Yukimitu a, J.C.S. Moraes a,⇑, F.A. Santos a, M.S. Figueiredo a, S.M. Sidel a, V.C.S. Reynoso a, O.A. Sakai b, A.N. Medina b a b

Faculdade de Engenharia, UNESP – Univ Estadual Paulista, Ilha Solteira – SP, Brazil Departamento de Física, Universidade Estadual de Maringá (UEM), Maringá-PR, Brazil

a r t i c l e

i n f o

Article history: Received 4 November 2010 Received in revised form 9 March 2011 Accepted 9 March 2011 Available online 17 May 2011 Keywords: Tellurite glass Optical dispersion Infrared spectroscopy Refractive index

a b s t r a c t The structure and optical properties of a 80TeO2–(20x)Li2O–xTiO2 glass system where x = 0, 5, 10, and 15 mol% has been investigated using FTIR spectroscopy and Brewster angle measurements. The sample preparation, linear refractive index and density measurements, and infrared spectroscopic analysis are described. The refractive index and density of the studied tellurite glass samples increase when the amount of Ti in the glass is increased. The dispersion of the phase refractive index was analyzed using Wemple’s model. The dispersion energy Ed is significantly affected by the addition of Ti to TeO2-based glass. The analysis of FTIR spectra indicate a Te coordination change that is in agreement with the increase of the Te coordination number determined from dispersion data using Wemple’s equation. Ó 2011 Elsevier B.V. All rights reserved.

1. Introduction Tellurium oxide (TeO2)-based optical glass has the potential to integrate optical devices such as all-optical waveguide switching [1] and modern photonic crystal fibers [2]. This is due to its interesting properties, including high linear refractive index values (higher than 2.0 in most cases), high non-linear refractive indices, high transmittance in a wide spectral band, good chemical durability, and a low melting temperature. It is well known that pure tellurium oxide exhibits serious restrictions to become glass [3,4]. Such difficulty is attributed to the presence of a pair of electrons in the equatorial region of the TeO4 trigonal bipyramidal (tbp) found in the tellurium oxide network which limits the structural arrangement necessary for glass formation. Thus, it is extremely difficult to obtain pure tellurium oxide glass using the traditional melt-quenching method. On the other hand, tellurite glass can easily be obtained by incorporating an alkaline oxide as a net modifier. However, such incorporation affects the optical properties, mainly the linear and non-linear refractive indices. When the ratio of the modifying oxide increases, the dominant TeO4 tbp progressively converts into TeO3 trigonal pyramids (tp) [5]. The substitution of alkaline oxide with transition

⇑ Corresponding author. Address: Departamento de Física e Química – UNESP, Av. Brazil 56, (15.385-000) Ilha Solteira, São Paulo – Brazil. Tel.: +55 18 3743 1029; fax: +55 18 3742 4868. E-mail address: [email protected] (J.C.S. Moraes). 0925-3467/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2011.03.053

metals in a ternary system tends to restore these optical properties [6,7]. The main objective of this work was investigated how the structural changes affect the refractive index of the 80TeO2– (20x)Li2O–xTiO2 glass system. For this, density and infrared absorption measurements were performed and it was applied the Wemple’s model to analyze the dispersion of the phase refractive index. Furthermore, the thermal stability of the glasses was evaluated by DT = TxTg, whose parameter is important to optical fiber development. 2. Wemple’s model The Wemple’s model suggests that the refractive index dispersion is given in the following equation [8]:

n2  1 ¼

E0 Ed E20  E2

;

ð1Þ

where E0 is Sellmeier’s gap energy, E is the photon energy, and Ed is the dispersion energy. For crystals, Ed obeys the simple empirical relationship

Ed ¼ bNc Z a Ne ;

ð2Þ

where b = (0.37 ± 0.04) or (0.26 ± 0.03) eV in covalent and ionic materials, respectively [9]; Nc is the coordination number of the nearest cation to the anion, Za is the valency of the anion, and Ne is the total number of valence electrons per anion [9]. The Nc of vitreous material can be determined by the relationship proposed by Wemple [9],

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Ead qa Nac ; cry ¼ cry q Ncry Ed c

W.A. Capanema et al. / Optical Materials 33 (2011) 1569–1572

ð3Þ

where q is the density and the superscripts cry and a refer to crystalline and amorphous forms, respectively.

3. Experimental procedures In this study, a series of four samples of 80TeO2–(20x)Li2 O–xTiO2 glass, for which x = 0, 5, 10, and 15 mol%, were prepared using the conventional melt-quenching method. Well-mixed powder in a Pt–Au 5% crucible was melted at 850 °C for 30 min, and was then quenched into a preheated brass mold. Each quenched sample was annealed around the glass transition temperature for 2 h. The 80TeO2–20Li2O sample was used as a reference to understand how the substitution of Li with Ti affects the structure and optical properties of the glass system. The nomenclature used in this work for these glass samples is TL, TLT5, TLT10, and TLT15 for x = 0, 5, 10, and 15 respectively. The characterization of the samples was achieved using X-ray diffraction (XRD), differential scanning calorimetry (DSC), and density measurements. The XRD patterns were obtained at room temperature using Cu Ka radiation and an Ultima IV X-ray diffractometer (Rigaku Corporation, Osaka, Japan). The DSC curves were obtained using a TA Instruments – DSC 2920 and 10 mg of a powdered sample in an aluminum pan with dry nitrogen streaming through the heating chamber. A heating rate of 10 °C/min was used in the room temperature to 450 °C range. The densities (q) of the glass samples were measured using the Archimedes principle and a sensitive high precision analytical balance with distillated water as the immersion liquid. The density was determined through the relationship q = [ma/(maml)]ql, where ma and ml are the sample mass in air and water, respectively, and ql is the water density obtained using a Anton Paar densimeter model DMA 5000. For the phase refractive index (n) measurements, blade-form samples of approximately 1 mm thick and 1 cm2 surface area were prepared. The n value for various radiation wavelength was obtained using the system in Fig. 1, which basically consists of a white light source (250 W halogen lamp), monochromator (Spectra Pro 150, Acton Research Corporation, Acton, MA, USA), a h/2h horizontal goniometer (HZB4, Freiberger Präzisionsmechanik, Germany), and a detector (Model 2151, Newport Corporation, Irvine, CA, USA). The system is controlled by a LabView program.

Table 1 Thermal parameters of studied glasses: glassy transition (Tg) and melting transition temperatures (Tx). Glass

Tg (°C)

Tx (°C)

DT = TxTg (°C)

TL TLT5 TLT10 TLT15

264 284 315 338

348 372 407 434

84 88 92 96

Table 2 Dispersion energy (Ed), Sellmeier’s gap energy (Eo), density (q), and coordination number (Nc) of crystals and tellurite glasses. Material

Ed (eV)

q (g/cm3)

Nc

Reference

TeO2 (crystal) Li2O (crystal) TiO2 (crystal) TL (crystal) TLT5 (crystal) TLT10 (crystal) TLT15 (crystal) TL (glass) TLT5 (glass) TLT10 (glass) TLT15 (glass)

23.2 16.6 25.7 33.2 ± 3.6 33.7 ± 3.6 34.3 ± 3.7

5.99 2.01 4.26 5.19 5.30 5.42

6 4 6 5.6 5.7 5.8

[8,15,16] [15,16] [8,15,16] this work this work this work

34.9 ± 3.8

5.53

5.9

this work

5.05 ± 0.01 5.19 ± 0.01 5.25 ± 0.01 5.30 ± 0.01

4.2 ± 0.5 4.0 ± 0.5 5.5 ± 0.7 6.4 ± 0.9

this this this this

23.9 ± 0.2 22.9 ± 0.2 31.6 ± 0.4 36.5 ± 0.6

Eo (eV)

7.60 ± 0.07 6.71 ± 0.05 8.7 ± 0.1 9.8 ± 0.2

work work work work

Stepper motors control the rotation of the sample (h) and of the detector (2h), with a resolution of 0.01 grade/step. The structural changes were observed by Fourier transform infrared (FTIR) spectroscopy using samples dissolved in KBr tablets were prepared. The measurements were carried out using a Nicolet Nexus 670 FTIR spectrometer with 64 scans and a resolution of 4 cm1. 4. Results and discussion The XRD pattern of all samples showed the characteristic halo of an amorphous material. Table 1 shows the thermal parameters obtained from DSC curves. The glass densities are presented in Table 2. The difference between crystallization and glassy temperature (DT = TxTg) has been frequently used to estimate of glass stability.

Fig. 1. System used to measure the refractive index vs. wavelength by Brewster’s angle method. L1–L2: lens; D: detector; ACI: acquisition and control interfaces.

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Absorbance (a. u.)

4

1 2

3

d c b a 400

500

600

700

800

900

1000

-1

Wavenumber (cm ) Fig. 2. Infrared spectra of the 80TeO2–(20x)Li2O–xTiO2 glass: (a) TL, (b) TLT5, (c) TLT10, and (d) TLT15.

2.26

TLT15 TLT10 TLT5 TL

2.24

Refractive index (n)

2.22 2.20 2.18 2.16 2.14 2.12 2.10 2.08 450

500

550

600

650

700

750

800

850

Wavelength (nm) Fig. 3. Refractive index as a function of the wavelength of the four studied glass samples.

To achieve a large working range during operation such as fiber drawing, it is desirable to have DT value as large as possible [10]. As shown in Table 1, the difference DT increased with the substitution of Li by Ti in the glass composition, showing that this substitution is advantageous for the improvement of thermal stability. The FTIR baseline-corrected absorption spectra of glass in the 500–950 cm1 range are presented in Fig. 2. The spectra basically show two intense absorption peaks, one centered at 640–660 cm1 (peak 1 in the figure) and the other at 770 cm1 (peak 3). These two absorptions are ascribed [11] to the stretching mode of the TeO4 trigonal bipyramid (tbp) and of the TeO3 trigonal pyramid (tp), respectively. The spectrum of the glass containing Ti

presents a shoulder at 850 cm1 (peak 4), whose absorption might be due to stretching vibrations of Ti–O–Ti bridges, as observed in the structure of WO3–TeO2 glass [12]. Based on the spectra, a progressive increase in the relative intensity can be observed between the 640 and 770 cm1 absorption peaks, when the Ti content in the glass increased. The addition of an alkaline metal modifier to TeO2-based glass favors the formation of TeO3 tp structural units [13,14]. On the other hand, the formation of TeO4 tbp structural units occurs when a modifier transition metal is added to the glass [13]. Thus, the relative increase observed between the two peaks is indicative of a structural change from TeO3 to TeO4. Furthermore, it can be observed that the

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TeO3 to TeO4 units. The atomic packing factor is higher in the TeO4 structural units than in the TeO3 units. This fact explains the increase observed in the density and refractive index of the glass samples when the Li atoms were substituted with Ti atoms.

0.31 TL TLT5 TLT10 TLT15

0.30

2

1/(n -1)

0.29

5. Conclusions

0.28 0.27 0.26 0.25 2

3

4

5 2

6

7

The structural and refractive index dispersion properties of 80TeO2–(20x)Li2O–xTiO2 glasses were studied by using FTIR and Brewster angle techniques, respectively. Our results show that the replacement of Li by Ti in the glass composition increases the refractive index and improves the thermal stability. The observed increase of the refractive index is due to change of coordination from TeO3 tp to TeO4 tbp. This conclusion is supported by infrared data analysis and the assessment of the Te coordination number using Wemple’s relationships.

2

E (eV)

Fig. 4. Fitting of the experimental data using Wemple’s equation for TL, TLT5, TLT10, and TLT15 glass samples.

TL glass spectrum presents a shoulder at 680 cm1 (peak 2) that is also ascribed to the stretching mode of the TeO3 tp units [13]. This shoulder decreases when the Ti content is increased, disappearing completely for the glass containing 15 mol% of TiO2. This fact also indicates a coordination change of tellurium atoms (TeO3 tp ? TeO4 tbp) with the partial substitution of alkaline metal oxide (Li2O) with transition metal oxide (TiO2). The phase refractive index values obtained for different wavelengths are presented in Fig. 3. It can be observed that the values of the refractive index increase significantly with increasing Ti content. Fig. 4 shows the plot of 1/(n21) versus the square of photon energy (E2) for each glass. The Ed and Eo values of each glass sample were obtained by fitting Eq. (1) to the experimental dispersion curves of Fig. 4; these are presented in Table 2. The average Te coordination number was determined starting from the density and Nc values of the TeO2, Li2O, and TiO2 in crystalline forms (Table 2) using Eqs. (2) and (3). The Ed values of the TL and TLT in crystalline forms were determined using Eq. (2), taking into account the glass stoichiometry. For this, we have supposed that the crystalline form of the TL and TLT are constituted by TeO2, Li2O, and TiO2 crystals in the same proportion in vitreous form. Therefore, the Te coordination numbers obtained in this way are shown in Table 2. The increase of the Ti content in the glass composition caused the rising of Nc of Te atoms. This result is in accordance with the analysis of infrared data, which suggests a structural change from

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