Polarizability, optical basicity and electric susceptibility of Er3 + doped silicate borotellurite glasses

Polarizability, optical basicity and electric susceptibility of Er3 + doped silicate borotellurite glasses

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Journal of Non-Crystalline Solids xxx (xxxx) xxx–xxx

Contents lists available at ScienceDirect

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

Polarizability, optical basicity and electric susceptibility of Er3 + doped silicate borotellurite glasses S.A. Umara,b, M.K. Halimaha,⁎, K.T. Chana, A.A. Latifa a b

Department of Physics, Faculty of Science, Universiti Putra Malaysia, 43400, UPM, Serdang, Selangor, Malaysia Department of Physics, Faculty of Science, Bauchi State University Gadau, Nigeria

A R T I C L E I N F O

A B S T R A C T

Keywords: Erbium Polarizability Optical basicity Linear electric susceptibility and metallization criterion

Glasses system was fabricated using the chemical composition {[(TeO2)0.7 (B2O3)0.3]0.8 (SiO2)0.2}1 − x (Er2O3)x with x = 0.01, 0.02, 0.03, 0.04 and 0.05 by melt-quenching method. The glasses were subjected to FTIR and XRD to study the glass structural changes and amorphous nature respectively. The absorption spectrum of the glasses were obtained from UV–Vis spectroscopy and used to calculate the energy band gap. Using the Archimedes principle, the density and the molar volume were determined. From the density, molar volume, and energy band gap, other parameters such as refractive index, molar refractive index, metallization criterion, reflection loss, transmission coefficient, polarizability, optical basicity, polaron radius, dielectric constant, optical dielectric constant, electric susceptibility, average electronegativity and others parameters were obtained by calculation. The polarizability values and the optical basicity were found to increase with Er3 + ions concentration increase. The dielectric constant, optical dielectric constant and the linear electric susceptibility decreased with increase in Er3 + ions concentration. The properties studied for the erbium doped glass system suggest the glass system has a potential in the EDFA application.

1. Introduction Unlike crystalline materials that are anisotropic in nature, glass is an isotropic material with flexibility in terms of chemical composition [1]. In glass technology, tellurium oxide (TeO2) as conditional glass former is used to give high refractive index, low melting point and low phonon maxima and so needs modifying ions to easily form glass [2]. Whereas, boron oxide (B2O3) is excellent material for combination with TeO2 as it improves the glass quality in terms of transparency, RE ions solubility and hardness [3]. To achieve mechanical quality, silicon oxide (SiO2) has been used as substrates for electronic displays, optical fibers, optical disc, medical and dental implants and radiation shielding [4]. The erbium among the rare earth elements has attracted much attention in the field of glass science and technology for its excellence as doping element in various glass applications in the recent years [5]. With solubility of up to 10%, erbium ions are famously known for fiber optics application [2]. The polarizability (αm) and other parameters as molar refractivity (Rm), reflection loss (R), Transmission coefficient (T), dielectric constants (ε) and electric susceptibility (χ) are normally obtained from the obtained values of refractive index (n) [6]. For non-linear glasses, study of polarizability is necessary as it affects its conductivity, refractive



index, electro-optical effect, ferroelectricity, optical non-linearity, and optical basicity. Kesavulu et al., [7] report suggests that the Judd-Ofelt parameters are connected to the polarizability of ligand ions in glasses [7]. Because of their applications in the development of communication and information technology, non-linear optical glasses attract very much attention in research [8]. In photonic applications, the ability of glass matrix to donate electron controls the effectiveness of such cation hosting sites. The theoretical optical basicity of a glass quantifies the electron donating ability of the glass. The theoretical optical basicity (λth) was proposed by proposed by Duffy and Ingram [9]. In this paper, we present a study on the erbium doped silicate borotellurite glasses. The composition of the Er3 + ions host material was selected to improve the glass quality in terms of good near infrared transmission, optical transparency, high RE solubility, high refractive index and easy fabrication. In this study, the optical energy band gap was obtained from the UV–Vis analysis and density/molar volume to calculate optical and electrical properties of the glass system. The paper tried to calculate the refractive index, molar polarizability, molar electronic polarizability, electronic polarizability, optical basicity, dielectric constant, optical dielectric constant, electric susceptibility polaron radius and other structural properties like erbium inter-ionic

Corresponding author. E-mail address: [email protected] (M.K. Halimah).

http://dx.doi.org/10.1016/j.jnoncrysol.2017.05.018 Received 6 March 2017; Received in revised form 8 May 2017; Accepted 15 May 2017 0022-3093/ © 2017 Published by Elsevier B.V.

Please cite this article as: Umar, S.A., Journal of Non-Crystalline Solids (2017), http://dx.doi.org/10.1016/j.jnoncrysol.2017.05.018

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samples. The absorption bands shown are 1385–1585 cm− 1, 1150–1350 cm− 1 and 600–700 cm− 1. The absorptions peaks in the range of 1350–1378 cm− 1 and 1220–1240 cm− 1 are assigned to stretching vibrations of BeO in an isolated trigonal BO3 units [13] and BO3 units in boroxol rings [12] respectively. While the characteristic band 600–700 cm− 1 may be attributed to both stretching vibrations of TeeO bonds in TeO3 and TeO4, with 600–650 cm− 1 absorption representing TeeO bond vibration in TeO4 units. The IR absorption in the range of 650–700 cm− 1 may be due to the TeeO bending vibrations in the TeO3 and TeO6 units [10,23]. The frequency of vibration in TeO3 bonds is higher than those in TeO4. The absorption intensity is observed to have increased with the increase in Er3 + ion concentration in the glass network. The increasing IR absorption intensity indicates increase in the concentration of non-bridging oxygen in the network [15].

distance, erbium ions concentration, field strength of Er3 + ions yield and other structural characteristics. Although alumina crucible was used in the glass fabrication, the glass composition was not investigated and hence aluminum leaching into the glass was not considered in the study. 2. Experimental procedure A series of erbium doped silicate borotellurite glasses were fabricated by using the conventional melt-quenching technique using the chemical formula {[(TeO2)0.7 (B2O3)0.3]0.8 (SiO2)0.2}1 − x (Er2O3)x with x = 0.01, 0.02, 0.03, 0.04 and 0.05. The chemical reagents TeO2 (Alfar Aeser, 99.9%), B2O3 (Alfar Aeser, 99.9%), Er2O3 (Alfar Aeser, 99.9%) and SiO2 (Rice Husk extracted, 98.548%) were used in the glass fabrication. A total of 10 g of the powdered chemicals was weighed based on the molar proportion presented by the above chemical formula using a high precision digital weighing machine ( ± 0.0001) for each sample. To achieve chemical mixture homogeneity, the weighed chemicals were mixed and stirred for 30 min in a cleaned alumina crucible with a glass rode [10]. The homogeneous mixture was then pre-heated for an hour in a furnace at 400 °C [11] and then transferred to another furnace at temperatures 900 °C for another one to 2 h for melting. After the melting process and glass casting, the glass was allowed to anneal at 400 °C for 1 h to remove thermal stress and bubbles [12]. The prepared glasses were then cut to a thickness of 2 mm and then polished using silicon carbide to achieve glass parallelism [13]. The density of the glass samples were measured based on the famous Archimedes principle using an electronic densimeter MD-300S (Alfa Mirage). For each sample, the density measurement was carried out ten (10) times and average value taken.

ρsample =

wair ρwater wwater

3.2. Energy band gap The optical band gap (Indirect) were obtained from the absorption spectrum data obtained from UV–Vis analysis using the Davis and Mott expression for the absorption coefficient α (υ) as;

α (υ ) = B

(hυ − Eopt )n hυ

(3)

where Eopt is the optical band gap, n is a number, with n = 2 for indirect allowed transitions and B is a constant. The value of α(υ) is the absorption coefficient obtained using the expression;

α (υ) = 2.303

A t

(4)

where t is the sample thickness, A is the value of the corresponding optical absorbance. Tauc's plots were drawn between (αћω)1/n and the photon energy ћω. The optical band gap values were obtained by extrapolating the linear part of the curves at (αћω)1/2 = 0.

(1)

The molar volume was obtained for each sample using the equation.

Vm =

3.3. Refractive index

mw ρsample

(2)

The refractive index (n) of the Er3 + doped RHSBT glasses was calculated from the optical energy band gap values (Eopt) using the equation in.

where ρsample, ρwater, wair, wwater and ww are the sample density, water density, weight of sample in air, weight of sample in water and molar weight of the glass sample respectively. The optical absorption spectrum for the glass samples were determined using UV–Visible spectrometer Shidmatsu Model UV-1650PC. On the powdered portion of the glasses, FTIR spectroscopy was carried out to study the structure of the glass based on the behavior of its functional groups [14]. This was carried out at the wave number range of 280–4000 cm− 1. The XRD spectroscopy was carried out at normal room temperature on the powdered portion of the glass samples between 20 < 2θ < 80 to determine the crystalline or amorphous nature of the glass [3].

n2 − 1 =1− n2 + 2

Eopt (5)

20 3+

ions concentration The refractive index decreases with Er increase. The effects of large ionic size and concentration of Er3 + ions may be responsible for the decrease [16]. 3.4. Molar refractive index The molar refractive index was calculated from molar volume (Vm) and refractive index (n) using the Volf and Lorentz-Lorenz formula [17]

⎛ n2 − 1 ⎞ Rm = ⎜ 2 ⎟ Vm ⎝n + 2 ⎠

3. Results 3.1. Density, molar volume and XRD

(6)

3.5. Reflection loss

Fig. 1 presents the density values of the RHSBT glasses against the Er3 + ions concentration. The density was observed to increase from 3.481 ± 0.036 to 3.694 ± 0.036 g cm− 3 as the molar fraction is increased from 0.01 to 0.05 Mol of Er2O3. The molar volume increased from 34.689 ± 0.175 to 35.549 ± 0.175 cm3 as the Er2O3 molar concentration increased as can be seen in Fig. 2. The Figs. 2 and 3 represent the molar volume and the XRD pattern of the RHSBT glasses studied. The XRD pattern indicates the glasses are amorphous, as there are no sharp Bragg's peaks throughout the resolution angles probed. Fig. 4 presents the FTIR plots for the erbium doped RHSBT glass

The reflection loss (RL) from the glass surface can be calculated using Fresnel's formula [18]

⎡ n − 1 ⎤2 RL = ⎢, ⎣ n + 1 ⎥⎦

(7)

3.6. Transmission coefficient The transmission coefficient (T) can be obtained using the expres2

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Fig. 1. Density against Er3 + molar concentration of RHSBT glasses.

3.9. Erbium inter-ionic distance

sion in [19]

T=

2n n2 + 1

The inter-ionic distance of erbium ions (Ri) in the glass was calculated using the formula [24]

(9)

Fig. 6 presents the relationship between the Reflection loss and the transmission coefficient shows a partial inverse proportionality between the two quantities [20].

⎛ ⎞ 1 ⎛ 1 ⎞1 3 Ri ⎜A ⎟ = ⎜, ⎟ ⎝ ⎠ 2 ⎝ N⎠

̊

(12)

3.7. Metallization criterion 3.10. Polaron radius (Rp)

Metallization criterion of a material tells about the non-metallic nature of the material on the basis of its band gap energy and is obtained as reported by [21] as

M=1−

Rm Vm

Polaron is a quasi-particle formed by a conduction electron or hole in a polar semiconductors, ionic crystals or alkali oxides/halides with its self-induced polarization. Polarons are studied to describe the polar interaction between an electron and the longitudinal optical phonons [18,19]. The polaron radius is the linear atomic/ionic displacement field of a polaron. When the polaron radius is of the order of the lattice constant, the polaron is called small polaron. When the radius is much larger than the lattice constant of the material, the polaron is termed as large polaron [25]. Normally, the decrease in the polaron radius causes increase in the polarizability of a material [26]. The RHSBT glass system also showed the same relationship between polaron radius and polarizability. The polaron radius in the glass samples were calculated using the formula used by

(10)

The metallization criterion as shown in Table 1 shows an increase with increase in the concentration of Er3 + ions in the glass. The increase indicates the glass is becoming more non-metallic [22]. 3.8. Erbium ion concentration (N) The ion concentration of erbium was calculated using the expression in [23]

N=

XEr x ρxNA Mw

(11)

Rp =

where NA, MW, ρ, and XEr are the Avogadro's number, molar weight of the glass samples, density and molar fraction of erbium respectively.

1 ⎛⎜ π ⎞⎟1 3 , 2 ⎝ 6N ⎠

Fig. 2. Molar volume against Er3 + molar concentration of RHSBT glasses.

3

(13)

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Fig. 3. The XRD pattern of Er doped RHSBT glasses.

⎛ 3 ⎞ αm = ⎜ ⎟ Rm ⎝ 4πNA ⎠

3.11. Erbium yield field strength The field strength of erbium yield (F) was calculated using the formula suggested by [27]

F=

Z Rp2

3.14. Molar electronic polarizability (14) Molar electronic polarizability can be related with the molar refractive index by an expression given by [17]

3.12. Oxygen packing density

αme =

The oxygen packing density was calculated using the formula reported by [15]

⎛ ρ ⎞ OPD = X ⎜ ⎟ ⎝ mw ⎠

(16)

Rm 2.52

(17)

3.15. Electronic polarizability

(15)

where ρ = density, mw = molar weight of the glass sample and X = number of oxygen atoms in a unit formula. Table 1 and Fig. 7 present the values of the OPD for different Er3+ concentration and OPD plot against molar refractive index of the glass respectively. The OPD value increased in the beginning from 64.793 ± 0.175 to 64.806 ± 0.175 and then decreased to 64.081 ± 0.175 with increasing Er3 + ions in the network (Table 2).

The electronic polarizability was calculated from the refractive index

αe =

3 (n2 − 1) 4πNA (n2 + 2)

(18)

3.16. Dielectric constant 3.13. Molar polarizability The dielectric function (constant) is a directly related to the energy band gap. And the dielectric constant in this work was obtained using the formula reported by [22]

Polarizability presents the degree of response to an electromagnetic field by electrons in a material [28]. The molar polarizability was calculated from the molar refractive index (Rm) and Avogadro's number (NA) as reported by [21]

ε = n2

Fig. 4. FTIR spectra of Er doped RHSBT glasses.

4

(19)

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Table 1 Optical band gap (E), refractive index (n), molar refractive index (Rm), optical parking density (OPD), metallization criterion (M), molar polarizability (αm), molar electronic polarizability (αme) and electronic polarizability (αe). E (eV) ( ± 0.041)

n ( ± 0.028)

Rm ( ± 0.039)

OPD ( ± 0.175)

M ( ± 0.002)

αm ( ± 0.015)

αme ( ± 0.015)

αe ( ± 0.001)

2.577 2.596 2.657 2.706 2.804

2.521 2.515 2.496 2.481 2.452

22.237 22.262 22.327 22.440 22.238

64.793 64.806 64.408 63.961 64.081

0.359 0.360 0.364 0.368 0.374

8.814 8.824 8.850 8.895 8.815

8.824 8.834 8.860 8.905 8.825

0.254 0.254 0.252 0.252 0.248

Table 2 Erbium ion concentration (N), polaron radius (Rp), erbium inter-ionic distance (Ri), erbium yield field strength (F), transmission coefficient (T) and the reflection loss (RL). M. fraction

N (× 1020)

Rp (× 108)

Ri (× 107)

F (× 1016 cm− 2)

T

RL

Er Er Er Er Er

1.7360 3.4611 5.1424 6.7862 8.4703

7.2262 5.7415 5.0316 4.5873 4.2605

1.7929 1.4245 1.2484 1.1381 1.0571

1.3023 2.0628 2.6860 3.2315 3.7461

0.6854 0.6866 0.6904 0.6934 0.6994

0.1867 0.1858 0.1831 0.1810 0.1769

0.01 0.02 0.03 0.04 0.05

3.17. Optical dielectric constant

Table 3 Optical basicity (Λth), refractive index based oxide polarizability (αO2 − (n)), energy band gap based oxide polarizability (αO2 − (E)), dielectric constant (ε), optical dielectric constant (εopt) and linear electric/dielectric susceptibility (χ) of the Er doped RHSBT glasses. M. Fraction

Λth

αO2 − (n)

αO2 − (E)

ε

εopt

The relationship between the optical dielectric constant and the refractive index, polarization (p) and the static dielectric as reported by [17] was used.

Χ

εopt = p 0.01 0.02 0.03 0.04 0.05

0.7377 0.7397 0.7416 0.7436 0.7455

1.9719 1.9653 1.9627 1.9920 1.9719

2.9778 2.9773 2.9933 3.0134 2.9998

6.3576 6.3269 6.2308 6.1559 6.0121

5.3576 5.3269 5.2308 5.1559 5.0121

0.4263 0.4238 0.4162 0.4102 0.3988

χlav

Λ (χlav)

αO2 − (χla)

Λ(α)

TeO2 B2 O3 SiO2 Er2O3

2.9933 2.8800 2.9267 2.5360

0.9078 0.9336 0.9230 1.0120

2.3583 2.4441 2.4088 2.7045

0.9618 0.9867 0.9767 1.0525

(20)

The dielectric and optical dielectric constants as presented in Fig. 8 decreased from 6.3576 to 6.012 and 5.3576 to 5.0121 respectively with increase in the concentration of Er3 + ions in the glass. 3.18. The linear dielectric/electric susceptibility

Table 4 Average electronegativity (χlav), electronegativity based optical basicity (λ (χlav)), electronegativity based oxide ion polarizability (αo2 − (χla)) and optical basicity based on oxide ion polarizability. Oxide

dt = ε − 1 = n2 − 1 dp

The linear electric or dielectric susceptibility (χ) measures the ability of a material to become transiently or completely polarized. The electric susceptibility of the erbium doped RHSBT glasses was calculated from the dielectric constant values using the relation given by [29]

χ=

ε−1 4π

(21)

Fig. 10 presents the variation of the electric susceptibility (χ) with the electronic polarizability, while Table 3 presents the values for different concentrations of Er3 + ions. The value decreased from 0.4263 to 0.3988 with increase in the Er2O3 concentration from 0.01 to 0.05 Mol.

Fig. 5. Optical absorption spectra of Er doped RHSBT glasses.

5

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Fig. 6. Reflection loss (RL) variation with Transmission coefficient (T).

Fig. 7. Variation of oxygen packing density against refractive index.

Fig. 8. Dielectric and optical dielectric constants variation with Er molar concentration.

∑ Xi Λi

3.19. Optical basicity

Λth =

Optical basicity is the ability of oxide ions to donate electrons to the surrounding cations [17]. This is related to the optical properties material. For oxide mediums, the numerical expression of the average power of electron donation of the constituting oxide species in the medium gives the bulk optical basicity of the material or medium [30]. The theoretical bulk optical basicity of the multi-component glass was calculated using the formula proposed by Duff and Ingram as reported by [31]

where Λi is the individual optical basicity of each of the constituting oxides (Er2O3, TeO2, B2O3 and SiO2) as obtained from [6,27–30]. And Xi is the molar concentration of the respective constituent oxide. Table 3 presents the calculated values of the optical basicity of the RHSBT glass system studied. The value increased from 0.7377 to 0.7455 with introduction of more Er3 + ions into the glass network from 0.01 to 0.05 Mol. The optical basicity of an oxide can be calculated using the estimation from the Pauling electronegativity values of each of the 6

(22)

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Fig. 9. Variation of molar polarizabilities and molar electronic polarizability with molar refractive index.

Fig. 10. Electric susceptibility against the electronic polarizability.

Fig. 11. Variation of optical basicity with metallization criterion.

be used to obtained both the refractive index (n) based and the band gap (Eg) based optical basicity of a material [35].

constituting atoms in the oxide. And the correlation proposed by Duff and Ingram gives the value of the oxide optical basicity as [32]

Λ=

0.75 χIav − 0.25

⎛ 1 ⎞ Λ = 1.67 ⎜1 − ⎟ ⎝ αO 2− ⎠

(23)

Reddy et al. [33] also proposed a relation between oxide optical basicity and average oxide electronegativity (χIav).

Λ = 1.59–0.2279 χIav

(25)

αO2 − is the refractive index or energy band gap based oxide ion polarizability. The refractive index and band gap based optical basicity basically regarded as the experimental optical basicity values.

(24)

The relationship between the oxide ion polarizability and optical basicity of a material was proposed by Duff [34]. The relationship can 7

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system [39]. The optical energy band gap (E) as shown in Table 1 increased when the concentration of Er3 + ion was increased. The increase may be due to more number of TeO3 units formed with more Er3 + ions in the network [40]. The refractive index as presented in Table 1 showed a decreasing pattern from 2.521 ± 0.028 to 2.452 ± 0.028 with increase in the amount of Er3 + ions in the glass network. This may be due to substitution of a Te4 + with Er3 + ions which has higher polarizability [19,16]. High refractive index of the erbium doped RHBT glass system gives the glass an added advantage for application as an EDFA core material [41]. The increase in the Rm value in Table 1 from 22.237 ± 0.039 to 22.440 ± 0.039 with the concentration of erbium may be due to the increase in the value of the polarizability as the values in the table have shown. Table 1 and Fig. 7 showed an initial increase in the OPD value from 64.793 ± 0.175 to 64.806 ± 0.175 for increase in Er3 + molar fraction from 0.01 to 0.02. The value then decreased to 64.081 ± 0.175 as the Er3 + molar concentration is increased from 0.02 to 0.05. The increase may be related to an increase in the glass density of the glass [6]. While the decrease in the OPD recorded afterwards may be due to a possible increase in the interstitial space in the glass network with more introduction of erbium atoms with large atomic and ionic radii. This indicates a less tighter arrangement of atoms [42]. The molar polarizability and the electronic molar polarizability relationship against the molar refractive index are presented in Fig. 9. The relationship showed partially a linear proportionality between the polarizabilities with the refractive index. Hence the relationship indicates clearly the linear dependence of Rm on the polarizability [16]. The high polarizability value of the erbium doped RHSBT glass system suggests the glass has a potential in non-linear optical applications such as the EDFA as it affects the electro-optical behavior of the glass material [30]. The decrease in the value of the two dielectric constants as shown in Fig. 8 and Table 3, indicates a decrease in the optical absorption and extinction coefficient of the glasses with increasing molar concentration of Er2O3. The decrease in the dielectric constant increase the optical transmission capacity of the glasses [43]. Fig. 10 presents a relationship between linear dielectric susceptibility and the electronic polarizability of erbium dope RHSBT glass system. The relationship confirmed the theoretical submission of linear dependence of the dielectric susceptibility on the electronic polarizability and refractive index [44]. The linear susceptibility is an important parameter in the EDFA technology as it affects the optical non-linearity of the glass material [30]. The presentation of the values of the theoretical optical basicity against the metallization criterion was made in Fig. 11. Tables 3 and 1 present respectively the values of the optical basicity and metallization criterion. The range of values for the optical basicity and metallization criterion are 0.7377 to 0.7455 and 0.359 to 0.374 respectively. The values obtained is an indication that the erbium doped RHSBT glass system has a good optical non-linearity as suggested by Berwal and coworkers [21]. The positive values of the metallization criterion suggest the glasses are non-metallic and have relatively large refractive index [30].

3.20. Refractive index based oxide ion polarizability The oxide ion polarizability based on refractive index was calculated as reported by Meena and Bhatia [22]

⎡ R ⎤ α(n)2− = ⎢ m − Σαi⎥ (NO2−)−1 O ⎣ 2.52 ⎦

(26)

3.21. Energy band gap based oxide ion polarizability

⎡ V ⎛ α E2− = ⎢ m ⎜⎜1 − O ⎢⎣ 2.52 ⎝

⎤ Eg ⎞ ⎟⎟ − Σαi ⎥ (N O2−)−1 20 ⎠ ⎦⎥

(27)

where in the equation, Ʃαi is the molar cation polarizability and NO2 − is the number of oxide ions in the chemical formula. The values of the molar cation polarizability (α) was obtained as reported in [24,34] as αEr = 2.253, αTe = 1.595, αB = 0.002 and αSi = 0.033 for erbium, tellurium, boron and silicon respectively. 3.22. Electronegativity based oxide ion polarizability Reddy et al. [33] proposed a relationship between oxide ion polarizability (αO2 −) of an oxide and average electronegativity (χIav). The relation suggests a dependence of αO2 − on χIav as reported by [28]

α O2− = 4.624–0.7569 χIav

(28)

3.23. Average electronegativity The average electronegativity of constituting oxides in the glass was calculated using the formula proposed by Aso Kamany and Manjula [35] N

χIav =

∑ χi ni i =1

N (30)

where χi is the Pauli electronegativity, ni is the number of atom of the ith element and N is the number of elements present in the compound. 4. Discussions The glass density increase observed in Fig. 1 with increasing concentration of Er2O3 may be associated with the substitution of lighter atoms of Si, Te and B with heavier Er atoms in the glass network [36]. The increase in the molar volume with more Er3 + ions in the network as shown in Fig. 2 may be due to the substitution of atoms with smaller atomic radii with erbium atoms with larger atomic/ionic radius. The increase may also be associated with the increase in the augmentation bond length which results in an increase in the interatomic separation (Table 4) [37,36]. The UV–Vis absorption spectra of Er3 + doped RHSBT glass system in the wavelength range of 200 to 2000 nm are presented in Fig. 5. The characteristic absorption bands corresponding to the transitions from the ground states to excited states of Er3 + are 1528, 651, 522,488, 448.5 and 403 nm are attributed to transition from the ground state of 4 I15/2 to the excited states of 4I13/2,4F9/2, 2H1/2, 4F7/2, 4F3/2 and 2G11/2 respectively. The absorption intensity is highest around 521 nm and decreases in the order of 651, 488, 1528, 403 and 448.5 nm [2,10,24]. The weak absorption recorded around 534 nm may be due to the overlap of 4S3/2 with the upper 2H11/2 excited states [2]. The effective bandwidth in the S and C-bands region for the RHSBT glass system increased from 76 to 106 nm as the Er2O3 molar fraction increased from 0.01 to 0.05 Mol. The large effective bandwidths suggest the glass is a potential candidate [38]. For EDFA applications in the optical communication system, effective bandwidth is an important parameter, especially in the wavelength division multiplexing (WDM) network

5. Conclusion Erbium doped rice husk silicate borotellurite (RHSBT) glasses were prepared using the melt-quenching method with chemical composition {[(TeO2)0.7 (B2O3)0.3]0.8 (SiO2)0.2}1 − x (Er2O3)x with x = 0.01, 0.02, 0.03, 0.04 and 0.05. The glass composition was selected to achieve high refractive index, high optical transparency, high rare earth ion solubility and ease of fabrication. Part of the aim is also to achieve large effective bandwidths in the C and L-band region which is important in the EDFA application. The glass samples were subjected to FTIR, XRD characterizations and UV–Vis spectroscopy for structural and optical 8

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analysis. The glass density was obtained using the famous Archimedes principle and the values were used to obtain the molar volume. Refractive Index of the glass was calculated using the indirect optical band gap values obtained from the UV–Vis data. From the density, molar volume, refractive index and the energy band gap, we calculate the oxygen packing density, erbium ion concentration, polaron radius, erbium inter-nuclear distance and field strength of erbium yield. Molar polarizability, molar electronic polarizability, electronic polarizability, oxide ion polarizability, optical basicity, reflection loss, transmission coefficient, dielectric constant, optical dielectric constant and electric susceptibility were also calculated. The investigation revealed that the density, molar volume, energy band gap, molar refractive index, transmission coefficient, erbium yield field strength, metallization criterion, molar polarizability, molar electronic polarizability, oxide ion polarizability and optical basicity increased with Er3 + ions concentration. While the refractive index, OPD, reflection loss, polaron radius, erbium inter-ionic distance, dielectric constant, optical dielectric constant and linear electric susceptibility decreased with increasing concentration of Er3 + ions in the glass network. The high refractive index, large effective bandwidth in the C and L-band region, high polarizability and high linear dielectric susceptibility and good optical transparency of the RHSBT glass system presents the glass potential in the EDFA application. Acknowledgement The authors appreciate the contribution of the reviewers in improving the quality of the paper. The authors also appreciate the financial support for the work from the Ministry of Higher Education of Malaysia and University Putra Malaysia through FRGS 5524817. References [1] R.A.H. El-mallawany, Tellurite Glasses Handbook: Physical Properties and Data, CRC Press LLC, 2002. [2] Z.A. Said Mahraz, M.R. Sahar, S.K. Ghoshal, Band gap and polarizability of borotellurite glass: influence of erbium ions, J. Mol. Struct. 1072 (1) (2014) 238–241. [3] A.V. Deepa, M. Priya, S. Suresh, Influence of samarium oxide ions on structural and optical properties of borate glasses, Sci. Res. Essays 11 (5) (2016) 57–63. [4] A. Pedone, G. Malavasi, A.N. Cormack, U. Segre, M.C. Menziani, Insight into elastic properties of binary alkali silicate glasses; prediction and interpretation through atomistic simulation techniques, Chem. Mater. 19 (13) (2007) 3144–3154. [5] K. Annapoorani, N.S. Murthy, T.R. Ravindran, K. Marimuthu, Influence of Er3 + ion concentration on spectroscopic properties and luminescence behavior in Er3 + doped strontium telluroborate glasses, J. Lumin. 171 (2016) 19–26. [6] P.P. Pawar, S.R. Munishwar, S. Gautam, R.S. Gedam, Physical, thermal, structural and optical properties of Dy3 + doped lithium alumino-borate glasses for bright WLED, J. Lumin. 183 (2017) 79–88. [7] C.R. Kesavulu, H.J. Kim, S.W. Lee, J. Kaewkhao, N. Wantana, S. Kothan, S. Kaewjaeng, Influence of Er3 + ion concentration on optical and photoluminescence properties of Er3 + -doped gadolinium-calcium silica borate glasses, J. Alloys Compd. 683 (2016) 590–598. [8] V. Dimitrov, T. Komatsu, R. Sato, Optical basicity and O1s binding energy of simple oxides, J. Ceram. Soc. Japan 107 (1) (1999) 21–26. [9] M.K. Narayanan, H.D. Shashikala, Thermal and optical properties of BaO–CaF 2–P 2 O 5 glasses, J. Non-Cryst. Solids 422 (2015) 6–11. [10] M.K. Halimah, W.M. Daud, H.A.A. Sidek, A.W. Zaidan, A.S. Zainal, Optical properties of ternary tellurite glasses, Mater. Sci. 28 (1) (2010). [11] S.H.J. Abdul Aziz, R. El-Mallawany, S. Shawaliza Badaron, H. Mohamed Kamari, K. Amin Matori, Optical properties of erbium zinc tellurite glass system, Adv. Mater. Sci. Eng. 628954 (2015), http://dx.doi.org/10.1155/2015/628954. [12] A. Azuraida, M.K. Halimah, A.A. Sidek, C.A.C. Azurahanim, S.M. Iskandar, M. Ishak, A. Nurazlin, Comparative studies of bismuth and barium boro-tellurite glass system: structural and optical properties, Chalcogenide Lett. 12 (10) (2015) 497–503. [13] M.N. Azlan, M.K. Halimah, S.S. Zulkefly, D.W. Mohamad, Effect of erbium nanoparticles on optical properties of zinc borotellurite glass system, J. Nanomater. 8 (2) (2013) 49–59. [14] F.H. Elbatal, M.A. Marzouk, H.A. Elbatal, Optical and crystallization studies of titanium dioxide doped sodium and potassium silicate glasses, J. Mol. Struct. 1121 (2016) 54–59.

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