Mixed valence effect of Se6+ and Zr4+ on structural, thermal, physical, and optical properties of B2O3–Bi2O3–SeO2–ZrO2 glasses

Optical Materials 96 (2019) 109338

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Mixed valence effect of Se6+ and Zr4+ on structural, thermal, physical, and optical properties of B2O3–Bi2O3–SeO2–ZrO2 glasses

T

Seema Thakur, Anumeet Kaur, Lakhwant Singh∗ Department of Physics, Guru Nanak Dev University, Amritsar, 143005, Punjab, India

A R T I C LE I N FO

A B S T R A C T

Keywords: Glasses X-ray diffraction DSC FTIR spectroscopy Raman spectroscopy UV–Vis spectroscopy

Conventional melt quench technique has been used to synthesize the highly polarizable glass system with composition (70B2O3–30Bi2O3), (70B2O3–25Bi2O3–5SeO2) and (70B2O3–23Bi2O3–5SeO2–2ZrO2)in (wt%)abbreviated as BB, BBS and BBSZ respectively. Structural, thermal, physical and optical properties of the modified glass system have been studiedusing XRD, DSC, FTIR, Raman and UV–Vis Spectroscopy. XRD have been performed to confirm the amorphous nature of samples. The calculated OPD values and DSC analysis confirmed decrease in Tg on the addition of SeO2 and ZrO2 content. FTIR and Raman data revealed the existence of characteristic bands of [BiO3], [BiO6], [BO3], [BO4], [SeO42−], [SeO32−] units and Zr–O–Zr linkages. The effect of ZrO2 on the structural stability has been observed using density and molar volume. Adecrease in band gap resulting from the increase in NBO's has been confirmed by UV–Vis spectroscopy as well as from physical parameters like metallization criterion and urbach energy. An increase in refractive index is observed on the addition of SeO2. The parameters electronic polarizability, optical basicity and electronegativity confirms the ionic character of the studied glass samples.

1. Introduction

4 to 3 which may be due to the conversion of oxygen rich selenate phase to oxygen deficit selenite phase. Selenite phase [SeO32−] mostly acts as modifier having three co-ordinated oxygen and one lone pair of electron, whereas selenate [SeO42−] having four coordinated oxygen tries to occupy the tetrahedral sites in the glass network and hence acts as network former. At low content, SeO2 has been found to get dissolved into the network structure whereas at the higher content, its tendency to act as network provider increases thus effecting the network structure. A continuous increase in SeO2 content has noticeable change on the network structure i. e from layers to chains [10,11]. Dimitrev and his coworkers [1,5,12,13] has studied the selenite glasses with varying composition in order to investigate its glass forming ability and its effect on the structure of the glass [3]. A. BachvarovaNedelcheva et al. has reported the importance of introducing selenium oxide due to its ability to decrease the melting temperature and to enhance the optical properties of the glass system [7,14].Q. Chen et al. has reported that Bi2O3 modified glasses having high optical basicity and low melting temperature are acceptable in preserving SeO2 content for the formation of highly polarizable lone-pair of Se4+ ions. It has been recently reported that doping of SeO2 in lead borate quaternary glass system improves its optical, electrical and thermal properties [2,15]. Although lead borate glasses show wonderful properties but usage of lead being toxic for environment and human health, it must be

Many oxides such as MoO3, CuO, PbO, Ag2O, Bi2O3, V2O5 and TeO2not only work as network formers but can be employed to improve the various properties of many multicomponent glass systems. Among these oxides, SeO2belongs to one of those non-traditional class of glass formers which have not been broadly studied yet [1]. Selenium modified glasses have many applications in the field of electrical and magneto optical sensors, photoluminescence and catalysis due to the existence of its special structure in which 4p-electrons occupy two bonding orbitals (covalent bonds) and one orbital carries a lone pair of electrons. Due to this, SeO2 possesses high (electronic) polarizability, small band gap (3.77 eV) and high electrical conductivity [2]. Few reports are available in which reserchers have reported on the doping ofSeO2 in glass systems such as boron [3–5], phosphate [6]and tellurite [7–9]. SeO2 acts as network modifier in B2O3 rich glasses and has network formation process in those glasses which are rich in SeO2 content and thus helps in depolymerization [4]. It has been reported thatSeO2 has the tendency to form mostly mixed Se–O type clusters during the formation of amorphous network. The selenium modified glasses show higher ion conductivity at room temperature in the range of 10−3–10−4 S/cm due to high polarizability of Se ions. It is reported that, selenium changes its co-ordination number from ∗

Corresponding author. E-mail address: [email protected] (L. Singh).

https://doi.org/10.1016/j.optmat.2019.109338 Received 2 July 2019; Received in revised form 18 August 2019; Accepted 20 August 2019 0925-3467/ © 2019 Elsevier B.V. All rights reserved.

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avoided. So, attempts are made to synthesize such glass systems which can show similar properties as shown by lead borate glasses [16]. Bismuth borate glassesbeing enviormental friendly have been extensively used for their technological applications in opto electronics, photonic devices, fiber optic amplifiers, efficient lasers, ultrafast optical switches, optical isolators and infra red transmission components [17–22]. Looking at the versatile properties of SeO2 (discussed above), we have synthesized multicomponent glass system in which Bi2O3and SeO2will act as glass network modifiers. On the other hand, the addition of ZrO2 has deliberatedly done in SeO2 modified glass system as incorporation of ZrO2enhancesthe mechanical strength, refractive index, fracture toughness, ionic conductivity, chemical and electrical resistivity of the glasses. Zirconium doped glasses are best suited as host materials for luminescent rare earth and transition metal ions. It has tendency to increase the glass transition and melting temperature [23–25]. It has also been reported that, the addition of ZrO2 sometimes helps in enhancing the optical properties of the glasses [26]. So, in the present work we have investigated the effect of Se6+ and 4+ Zr doping to modify some of structural, thermal and optical properties of bismuth borate glass system.

Fig. 1. XRD patterns of the glass samples BB, BBS and BBSZ.

all the glass samples [18,28]. 3.2. Density and molar volume The density of the prepared samples have been calculated at room temperature with lab made set up based on Archimedes Principle using the formula given below.

2. Experimental procedure

ρ=

The glass samples with composition (70B2O3–30Bi2O3), (70B2O3–25Bi2O3–5SeO2), (70B2O3–23Bi2O3–5SeO2–2ZrO2) in (wt%) abbreviated as BB, BBS and BBSZ have been synthesized via Melt quenching technique. The weighed amount of chemicals B2O3 (boron trioxide), Bi2O3 (Bismuth oxide), SeO2 (selenium dioxide) and ZrO2(zirconium dioxide) were thoroughly mixed using agate pastle mortar. The mixture was poured into alumina crucibles and kept in a muffle furnace whose temperature was increased steadily to 1200 °C. The mixture was kept for melting at this temperature for 30 min and continuous stirring was performed to maintain homogeneity of the glass samples. The prepared melted mixture was poured onto a preheated stainless steel at 300 °C and pressed immediately by another steel plate to obtain disc shaped flat samples. These quenched samples were characterized using different techniques. The X-ray diffraction of the powdered samples was performed using Cu K α radiation in MAXima_XRD-7000 Shimadzu X-ray Diffractrometer. The values for the density have been calculated using the lab made setup based on the Archimedes Principle with xylene as the buoyant liquid. DSC of the samples have been performed using NETZSCH DSC 204F1 Phoenix. The Fourier Transform Infra Red absorption spectra of the samples has been recorded using the PerkinElmer Fourier Transform spectrophotometer at room temperature in the range of 2500–400 cm−1 using KBr as the reference material. Raman spectra of the desired samples have been performed using Renishaw In-Via Reflex micro-Raman spectrometer with 2400 lines mm−1 diffraction grating, an edge filter for stokes spectra and a CCD detector with Ar ion laser (514.5 nm wavelength). The deconvolution of FTIR and Raman spectra was required to resolve the peaks, which was performed using ‘peak fit pro’ module of Origin Pro 8.5 software as discussed elsewhere [18,27]. The grinded and polished glass samples have been used for the optical absorption measurements using Shimadzu-1601 double beam UV–Visible spectrophotometer in the wavelength range of 200–900 nm.

wa ρ wa − wl l

(1)

where wa is the weight of the sample in air, wl is the weight of sample in xylene (liquid), ρl is the density of immersion liquid xylene which is 0.865 g/cm3. The calculated values show that there is sharp increase in the density on the addition of SeO2 into the glass system which may be due to the occurrence of reversible transformation in which light selenite groups (SeO32−) are transformed into more dense selenate groups (SeO42−) having four coordinated oxygen occupying tetrahedral sites in the glass network. An increase in density is due to the replacement of lighter metal ions B2O3 [69.622 g/mol] by heavy metal ions SeO2 [110.96 g/mol] and ZrO2 [123.22 g/mol] which might be responsible for the effective changes occurring in the glass network. The structural stability can be related with the increase in density on the addition of SeO2 and ZrO2as it indicates the compactness in the glass network [29,30]. The values of the calculated density has been tabulated in Table 1. The molecular weight and molar volume has been estimated using the formulas given below and are found to be gradually decreasing. The values are tabulated in Table 1.

Vm =

Mw ρ

(2)

Mw =

∑ xiMi

(3)

Table 1 Physical parameters.

3. Results and discussions 3.1. X-ray diffraction The X-Ray Diffraction patterns of the samples BB, BBS and BBSZ have been recorded at room temperature in the range of 20°–80° and at scan speed of 2°/min as shown in Fig. 1. The glass samples have shown only broad diffuse scattering at low angles in the recorded X-Ray Diffraction patterns. This scattering gives an idea regarding the presence of long range structural disorder which confirms the amorphous nature in 2

Sample

BB

BBS

BBSZ

Average molecular weight Mw(g/mole) Density ρ(gm/cm3) Molar volume Vm(cm3/mole) Boron-boron separation ( < dB-B > ) (nm) Oxygen packing density OPD (g-atom/1) Urbach energy Eu(eV) Indirect optical band gap energy Eg1(eV)[r = 2] Eg1(eV)[r = 3] Molar refractivity Rm(cm3/mole) Molar polarizability αm(A°3) Metallization criterion M Reflection loss RL Refractive index n Electronegativity χ Electron polarizability α° Optical basicity ˄

189.620 22.030 8.557 0.287 350.57 0.163 3.226 3.432 5.123 2.032 0.401 0.598 2.340 0.865 2.721 1.267

170.770 41.991 4.066 0.011 725.39 0.270 2.767 3.208 2.556 1.013 0.371 0.628 2.464 0.741 2.832 1.329

163.915 93.682 1.749 0.004 1674.58 0.277 2.878 3.274 1.085 0.430 0.379 0.620 2.430 0.771 2.805 1.314

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where x i is the molar fraction, Mi is the molecular weight of the ith component of the sample. The molecular weight has been found to be decreased for BBS and BBSZ. Valence ions like Se6+ and Zr4+ has contracting effect on the presently studied glass system which has lead to an increase in the density, decrease in interatomic spacing as well as decrease in bond length which leads to an increase in the stretching force constants [31]. All these factors are responsible for the decrease in molar volume of the prepared glass system which can further be confirmed from average Boron–Boron separation (< dB-B >) and has been calculated using the following relation. B

1

V < dB−B > = ⎛⎜ m ⎞⎟ ⎝ N ⎠

VmB

3

(4)

Vm 2(1 − xn)

= where is the boron molar volume which contains 1 mol of boron atoms within the given glass structure, x n is considered as the molar fraction of B2 O3 and N is the Avogadro's number. The values of VmB depend only on the cation species. They show a decreasing trend which leads to considerable compression of the glass network and helps in densification of the structure. 3.3. Oxygen packing density (OPD) Oxygen packing density measures the tightness of oxygen atoms packed in the glass network. The calculation of OPD values can be done using the formula given below.

OPD =

ρ × n× 1000 Mw

(5)

where ρ is the density, ‘n’ is the no. of oxygen atoms in each oxide and Mw is the molecular weight of the prepared glass samples. A substantial increase in the OPD values of the prepared glass samples have been observed (listed in Table 1)with the addition of SeO2 and ZrO2content in the base bismuth borate glass system confirming the increase in number of non bridging oxygen atoms. The replacement of B2O3 by SeO2 and ZrO2 create more and more negatively charged tetrahedral BO−4/2 units, which are responsible for the diversification occurring across trigonal BO3/2and the anionic tetrahedral BO−4/2 units of the glass network [30]. Consequently, the dopants start acting as structural modifiers by breaking the local symmetry and by introducing the coordinated bond defects into the system leading to less compact glass structure.

Fig. 2. DSC of the glass samples BB, BBS and BBSZ. Table 2 Thermal properties and crystallization kinetic parameters for all glass samples.

3.4. Differential scanning calorimetry (DSC) DSC thermograms for the quenched glass sample BBS and BBSZ are shown in Fig. 2. DSC measurements have been performed using 38.9 mg of reference (alumina powder) and 15.82 mg of powdered glass samples in the temperature range of 35 °C–600 °C with an estimated error of ± 1 °C.The values of glass transition temperature (Tg ) and crystallization temperature (Tc ) obtained from DSC thermograms (shown in Table 2) demonstrating endothermic minima (Tg ) represents strength and rigidity of the glass matrix and exothermic maxima required for amorphous-crystalline transformation. The Tg for BB glass exists at 672 °C [17]. A drastic down shift in Tg occurs in BBS and BBSZ glass at 129 °C and 132 °C respectively. There are various reasons which can justify this decrease in glass transition temperature a) Selenium ions act as network modifier for the formation of selenite (SeO32−) with three coordinated oxygen and selenate (SeO42−) with four coordinated oxygen in the glass system in comparison to B–O–B Band [10]b) enhancement in the depolymerization with the increase in NBO's [4] (c) difference in binding energies i.e. Se–O bond (343 kJ/mol) has weaker binding energy in comparison to B–O bond (523 kJ/mol) [4] (d) Tg dependence on OPD values explaining the structural degradation as rise in OPD values lead to less compact glass structure [17]. Thermal data can be used to provide Hruby's parameter for stability

Sample

BB

BBS

BBSZ

Glass Transition Temp. (Tg) (°C) Glass Cr. Temp. (Tcr.) (°C) Thermal Stability factor ΔT (°C) = (Tc − Tg)

672 871 199

129 509 380

132 515 383

0.2961

2.9457

2.9015

Hruby's parameter H ′ =

ΔT Tg

analysis.

H′ =

ΔT Tg

(6)

Where ΔT = (Tc − Tg) is the difference between crystallization temperature (Tc) and glass transition temperature (Tg), used to calculate thermal stability of glass system [17]. Hruby's parameter provides the measure of glass forming tendency of material, if H′ ≥0.1 then it's a good estimate for glass forming ability of the material. It defines as higher the value of H′ more stable the glass and less critical would be the quenching rate [10]. The values of ΔT and Hruby's values (shown in Table 2) has been found to be increasing on the addition of SeO2 and ZrO2which signifies about the thermal stability of the glass system. Tg characteristically reveals about the structural relaxations occurring in the glass network which ultimately depends on the nature of 3

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Fig. 3. FTIR and deconvoluted spectra of the glass samples BBS and BBSZ.

connectivity of the structural units constituting the glass network. The addition of ZrO2 into the glass network provides thermal stability as structural units get more ordered [32,33].

Table 3 Peak position xc (cm−1), amplitude A (a.u.) and full width at half maximum W (cm−1) of deconvoluted peaks of FTIR spectra with composition BBS (70B2O3–25Bi2O3–5SeO2), BBSZ (70B2O3–23Bi2O3–5SeO2–2ZrO2)in (wt%) of glass system for different values of x.

3.5. Fourier transform infrared spectroscopy

Peak No.

FTIR spectra of BB, BBS and BBSZ samples were recorded at room temperature in the spectral range of 400–2500 cm−1and have been shown in Fig. 3.Table 3 depicts the parameters like peak position (xc), amplitude (A) and full width at half maxima (w) of the peaks obtained from the deconvolution process for FTIR spectra. The peaks originated after deconvolution associated to the bonds and stretching vibrations of base Bismuth Borate glass system has already been reported elsewhere [17]. However, for reference the brief description of stretching vibrations occurring in the glass system have been tabulated in Table 4. The IR spectra of the BBS glass shows the same distinctive and characteristic peaks as the base Bismuth Borate glasses but with obvious changes [3]. With the introduction of SeO2 sometimes there occur a slight shift in wavenumbers corresponding to the symmetric Bi–O stretching vibrations of polyhedral [BiO3], as observed in BBS from 700 cm−1 to 691 cm−1. The introduction of SeO2 into the base network influences the structural intensities of borate vibrational units due to the replacement of B2O3 by SeO2. There are little shifts in wavenumber on the introduction of SeO2 as a consequence of higher Se–O bond force constant and the smaller average of B–O–B bond angle [2,15]. There are various peaks in the deconvoluted data which may arise due to selenium ions and its different bondings with oxygen. The peaks positioned in the range from 448 cm−1 to 537 cm−1 are due to the vibrations of isolated [SeO3] units [1]. In the presently studied samples, the peaks assigned at 691 cm−1 and 745 cm−1 may be due to the bending vibrations of the selenium linkages [10]. The wavenumbers which are

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23

4

BBS

BBSZ

xc

A

W

xc

A

W

451 487 537 592 627 651 691 745 799 843 893 943 1022 1083 1130 1184 1235 1303 1366 1422 1483 1556 1628

0.021 0.006 0.041 0.006 0.018 0.029 0.112 0.028 0.018 0.088 0.077 0.127 0.188 0.086 0.110 0.052 0.358 0.071 0.386 0.089 0.305 0.112 0.057

94 77 93 42 65 129 71 56 124 95 85 95 108 82 103 66 127 79 122 65 106 93 105

448 507 580 641 694 749 805 865 925 984 1061 1126 1183 1246 1316 1381 1427 1487 1549 1606 1679 -

0.001 0.025 0.024 0.033 0.099 0.010 0.012 0.113 0.067 0.102 0.138 0.092 0.072 0.289 0.108 0.311 0.065 0.269 0.114 0.088 0.024 -

33 92 77 75 77 43 139 125 90 101 131 125 90 101 131 148 76 126 86 115 63 -

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Table 4 Assignments of peaks for FTIR spectra obtained during the investigation of glass samples. FTIR peak position(cm−1)

IR Assignments

448–537 575–592 592 627–700 691,745 700 507,749 745,799 843–893 805 843, 845, 865 943–984 893–1235 914 1184 1200–1680

Arises due to the vibrations of isolated [SeO3] units. Bending vibrations of Bi-O in octahedral units [BiO6]. Stretching vibrations in Bi–O−1 bands occurring due to Non bridging oxygen atoms. Bending vibrations occurring due to B–O–B linkages. Bending vibrations of selenium linkages. Superposition of Bi–O bands of [BiO3] pyramidal units to the B–O–B linkages. stretching vibrations arising from Zr–O–Zr linkages. Bending vibrations produced due to O3B-O-BO4 units. Due to symmetrical stretching vibrations of isolated pyramidal [SeO3]2Stretching vibrations produced due to [BO4] units. Bending vibrations due to [BiO3] polyhedra. Due to selenite group [SeO4]2Stretching and Overlapping of vibrations in [BO] and B–O units due to tri, tetra and penta borate groups. Stretching B–O vibrations in [BO4] units from diborate groups. Asymmetric stretching vibrations due to pyro and ortho Borate groups in B–O bonds in [BO4] units. Stretching vibrations in boroxal rings of [BO3] units due to B–O bond.

considerably noticed at 507 cm−1 and 749 cm−1in BBSZ may be due to the bending or stretching vibrations arising from Zr–O–Zr linkages [24]. The peak positioned at ~805 cm−1 can be acredited to the vibrations occurring in [BO4] units due to various arrangements [34]. The peaks at ~843 cm−1, ~845 cm−1 and 865 cm−1are considered as the characteristic vibrational band of Bi–O bond in [BiO3] polyhedral or it may be sometimes due to the vibration of isolated [SeO3] units in the BBS and BBSZ samples [1,21,34–37]. The peaks in the range 843–893 cm−1 can be due to the symmetrical stretching vibrations of isolated pyramidal [SeO3]2- groups which further leads to the formation of Se–O–Se linkages. The peaks in the range from 943 to 984 cm−1 can be due to the selenite group [SeO4]2- with four co-ordinated oxygen occupying the tetrahedral sites. On the expense of decomposition of [SeO3] to [SeO4] there occurs an appearance of Se]O bonds in the range 843–984 cm−1 [2,10,15]. The peak around 893 cm−1 arises mainly due to the transformation of [SeO2] to [SeO3] [12]. There is an observed slight shift in wavenumbers from 1366 cm−1 to 1381 cm−1due to the higher Se–O bond force constant and smaller average of B–O–B bond angle [15].

4. UV-VIS spectroscopy Fig. 5 shows the UV–Visible absorption spectra of glass samples BB, BBS and BBSZ recorded at room temperature in the range from 350 nm to 700 nm. The absorption coefficient α(ν) is given by Ref. [40].

α(ν) =

A t

(7)

where A is the absorbance obtained from the ratio of intensities of incident (I°) and transmitted (I ) beams and ‘t’ corresponds to the thickness of each sample. BBS sample has shown higher absorption in which a sharp cut-off wavelength more than 500 nm with large red shift have been observed. Normally, when selenium is used in large amount in glass samples, steep and flat peaks are observedat 500 nm and 750 nm respectively. However, In the presently studied glass system the amount of selenium is relatively low due to which only steep peak has been observed in the cut-off wavelength with noticeable red shifts at absorption edge [15]. The aborption coefficient α(ν) is related to the optical band gap (Eg) and can be calculated using the formula given by E. Davis and N. Mott [40]. The information about electronic states can be gathered from the higher energy parts of the spectra which are particularly associated with the integrand electronic transitions. During this process, an electron gets excited with photon absorption and shifts from a filled band to an empty band consequently resulting in marked increase in the absorption coefficient α(ν) [19]. The onset of this rapid change in α(ν) is known as fundamental absorption edge and the energy cooresponding to that edge is known as energy gap.

3.6. Raman spectroscopy Raman studies are conducted on glass samples named as BB, BBS and BBSZ by Raman spectrometer in the Raman shift range of 30–2000 cm−1 with spectral resolution of 1 cm−1 having backscattering geometry of 50 × objective lens. The deconvolution of the obtained broad Raman spectra has been performed to find the exact modes of vibration and Raman shifts of the peaks. Fig. 4 displays the deconvoluted Raman scattering spectra of the prepared glass and their corresponding peak position (xc), amplitude (A), full width at half maxima (w) are listed in Table 5. The peaks attributing to the bonds and stretching vibrations of base bismuth borate glass system after deconvolution has already been discussed elsewhere [17]. Howerever, for reference the brief description of stretching vibrations occurring in the glass system have been tabulated in Table 6. In Raman analysis for BBS and BBSZ the peaks originating in the range 495–741 cm−1 can be due to of symmetrical and unsymmetrical stretching modes of vibrations arsing in bridging oxygens of Se–O–Se bonds [2]. The peak emerging around 798 cm−1can be due to the existence of selenium trioxide triangles Se]O with three equivalent Se–O bands of isolated pyramidal [SeO3]2- groups [2]. The peaks which are appearing at the positions like 151 cm−1, 284 cm−1, 457 cm−1 and 541 cm−1are due to the vibrational mode occurring at atomic arrangement for zirconium and oxygen atomsin BBSZ sample [38,39].

αhν = [B(hν-Eg)]r

(8)

where constant B is constant independent of energy known as band tailing parameter and hν is the incident photon energy [17,28]. Only the indirect transitions (allowed and forbidden) are observed for amorphous materials i. e for r = 2 and 3 respectively [41]. Tauc's plot relation between (αhν)1/r and (hν) can be used to find the optical band gap energies of the glass samplesby extrapolation of the linear region of the curves to meet (hν) axis i.e. where (αhν)1/r = 0. The values are listed in Table 1 and the variation has been shown in Fig. 6. Optical band gap energy of BBS decreases whereas it increase in the case of BBSZ. A decrease in BBS glass for allowed indirect transitions from 3.22 to 2.76 eV and for indirect forbidden transitions from 2.96 to 2.64eV clearly indicates the structural changes occurring in the glass samples. As the polarization power or the cation polarizability of the Zr4+(0.357 A°3) and Se6+ (0.073 A°3) ions are stronger as compared to the base glass material B3+(0.002A°3) ions, it results in the weakening of metaloxygen bond strength and the formation of the non bridging oxygens atoms. The basic reason behind the smaller observed band gap or the 5

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Fig. 4.

Raman and deconvoluted spectra of the glass samples BB, BBS and BBSZ.

of the reasons of observed cut-off red shifts [2,42,51,52].

Table 5 Peak position xc (cm−1), amplitude A (a.u.) and full width at half maximum W (cm−1) of deconvoluted peaks of Raman spectra with composition BBS (70B2O3–25Bi2O3–5SeO2), BBSZ (70B2O3–23Bi2O3–5SeO2–2ZrO2)in (wt%). Peak No.

1 2. 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

BBS

4.1. Urbach energy

BBSZ

xc

A

W

xc

A

W

64 127 161 229 332 411 495 582 676 741 921 1023 1084 1245 1369 -

36428 34930 8507 11133 9795 7014 3572 1854 3983 175 616 681 195 1882 3645 -

51 80 77 121 165 207 202 117 165 10 90 107 2 113 197 -

56 99 151 201 284 369 457 541 639 716 798 898 997 1078 1222 1308 1405 1471

29589 36732 27318 15964 9395 14950 5297 6787 3000 3008 635 614 773 285 1529 3291 2181 782

38 64 46 103 97 127 90 139 105 121 14 89 118 72 82 135 114 81

Urbach energy (Eu), which is a measure for concentration of defects, can be estimated from the plot of natural logarithm of the absorption coefficents ln(α) and incident photon energy hν (as shown in Fig. 7). The values of urbach energy (Eu) were calculated by taking the reciprocals of the slopes of linear portion of curves. The relation between Eu and α(ν) is given by empirical urbach rule [18].

hν α(V) = β exp ⎛ ⎞ E ⎝ u⎠ ⎜

⎟

(9)

where β is constant, Eu the urbach energy interpreting the width of tail of localized states in the band gap and hν is the incident photon energy. Table 1 gives the calculated values of Eu and a gradual increase in the values of Eu have been noticed in BBS and BBSZ. E. Davis and N. Mott [40] has reported the range 0.045–0.66 for amorphous semi-conductors and for presently studied glass system the values lie between the reported range. Urbach energy helps in estimating the structural stability i. e smaller values decide the greater structural stability of the glass system [45]. Higher value of urbach energy for BBS and BBSZ glass system is due to the creation of defects and delocalization of the electrons which somehow decreases the structural stability of the glass system. On the other hand, also the bond strength of B–O (193.3 kcal/ mol) is much stronger than Bi–O (80.5 kcal/mol), Se–O (111.08 kcal/ mol) and Zr–O (185.49 kcal/mol) due to which NBO's are created and result in decrease in the structural stability as well as band gap energy Eg Ref. [19].

cut-off red shifts in BBS sample is the replacement of largerband gap of B2O3(8eV) by much smaller band gap ofSeO2(3.77eV). ZrO2(5eV) has comparatively higher band gap than SeO2(3.77 eV)due to which there occurs alittle increase in band gap in BBSZ sample from 2.76 to 2.87eV for indirect transitions and from 2.64 to 2.73 for indirect forbidden transitions. Moreover higher molecular mass of ZrO2 (123.22 g/mol) and SeO2 (110.96 g/mol) as compared to B2O3 (69.6182 g/mol) is one 6

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Table 6 Assignments of peaks for Raman spectra obtained during the investigation of glass samples. Raman shift (cm−1)

Raman Assignments

56–99 135 209–229 127–229 332–430 495–741 639–676 798 898–997 1023–1471

Vibrations produced due to Eg modes of Bismuth ions. Stretching vibrations due to heavy metal Bi3+ ions in [BiO3] and [BiO6]units. Vibrations due Bi–O bending in[BiO3] and [BiO6] units. Symmetric stretching vibrations of Bi–O linkages. Bi–O–Bi stretching vibrations in [BiO6] octahedral unit. Stretching modes arising in bridging oxygens of Se–O–Se. Bi–O- bending vibrations in [BiO6] units, B–O vibrations [BO4] units and arising due to metaborate groups of ring type structure. Isolated pyramidal [SeO3]2- groups. Vibrations due to B–O–B andB-O bendings pyro and ortho borate groups of [BO3] units. Bending vibrations due to B–O- bonds causing NBO's.

4.2. Reflection loss and metallization criterion The Molar polarizability and Molar refractivity are calculated using Lorentz-lorentz equation which is valid for large number of oxides [46] i. e

3 ⎞ Rm αm = ⎛ ⎝ 4πN ⎠

(10)

Eg ⎤ where Rm = Vm⎡1 − 20 ⎦ ⎣ The non metallic nature of solids can be estimated from metallization criterion using the formula given below

Fig. 5. Absorption spectra for different glass samples BB, BBS and BBSZ.

M=1−

Rm Vm

(11) Rm Vm

is known as reflection loss. where RL = The Reflection loss (RL) has increased for BBS and BBSZ in comparison to the base glass BB. According to K. F. Herzfeld's theory larger the reflection loss smaller is the metallization criterion [47], If RL approaches to 1 then its metal Rm > 1 (metal) andif RL deviates from Vm

1then its non metal Rm < 1 (non metal). For BBS and BBSZ value apVm proaches towards 1 indicating the reduction in non metallic nature of the prepared glass system as shown in Table 1. Consequently, the band gap between valence band and conduction reduced which is in consistence with the obtained results of band gap energies [17].V.Dimitrov et al. [48,49] has reported that the material owns high non-linear refractive index if the metallization criterion values lie in the range of 0.30–0.45. The calculated values are found to be in the range 0.401 to 0.379 which are in best agreement with the reported values. Thus, anticipating new non-linear optical materials having applications in the various fields like optical switching, optical computing, optical data storage etc. [17,18].

Fig. 6. Tauc's plot for glass samples having composition BB, BBS and BBSZ where r = 2 for indirect band gap.

4.3. Refractive index Refractive index is an important variable that can be used to investigate applications of glass materials in optical devices. It is calculated using Fresnel's formula and is shown in Table 1.

RL =

n2 − 1 n2 + 2

(12)

In the presently studied glass system, the refractive index increases due to the substitution of low molecular mass material B2O by high molecular mass material SeO2 and ZrO2. It is seen that the BBS and BBSZ glasses have higher refractive index which is either due to higher polarizability of NBO's as compared to BO's or may be due to increase in electronic polarization in high frequency, inter facial polarization in low frequency, ionic and dipolar polarization [15,43,44,50]. 4.4. Electronegativity and electron polarizability Fig. 7. Urbach's plot for the glass samples BB, BBS and BBSZ.

Electronegativity defines as an estimate of an ion that how strongly 7

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UV–Vis Spectroscopy. A contracting effect on the glass system has been made by the addition of SeO2 and ZrO2 which leads to decrease in density, interatomic spacing, bond length and molar volume and helps in densification of glass structure. The increase in OPD values on the introduction of SeO2 and ZrO2 due to the replacement of strong trigonal linkages of BO3/2 by much weaker anionic tetrahedral BO−4/2 linkages creates NBO's and hence helps in the generation of defects. The decrease in Tg also confirms the increase in NBO's and hence supports the depolymerization in the glass network. Thermal stability has been improved with the additions of SeO2 and ZrO2 into the glass system. FTIR and Raman data has confirmed the role of Se and Zr as structural modifiers transforming BO3 → BO4 and [SeO3] to [SeO4] units. The generation of NBO's has lead to the structural instability in the glass system which helped in enhancing the optical properties. From UV–Vis spectroscopy, it has been observed that there is a decrease in optical band gap which is confirmed from the behaviours of metallization criterion and urbach energy. The increase in refractive index has been observed due to the addition of highly polarizable SeO2 which increases the electronic polarizability of the glass system. Thus, the behaviours of parameters like electronegativity, optical basicity and electronic polarizability supports the ionic character of glass system and fits well as a suitable candidate from application point of view.

it attracts the electrons from the oxide ions bonded to it i. e higher the electronegativity, more strongly it will attract the bonded oxide ion. Thus resulted tight bonding enhances the covalent character of the material. Using the values of the band gap (Eg) electronegativity (χ) of the prepared samples can be calculated.

χ = 0.2688Eg

(13)

Electronic or cationic polarziability (α°) and optical basicity ( ∧ ) can further be calculated from electronegativity using following equations [43,51] (as shown in Table 1).

α° = −0.9χ + 3.5

(14)

∧ = −0.5χ + 1.7

(15)

V.Dimitrov, T. Komatsu [44] explains electronic polarizability as the distortion of electronic cloud of ions by the application of an electric field. It is connected to many properties of materials like refraction, ferroelectricity, conductivity, electro-optical effect and optical nonlinearity. Generally, electronic polarizabilty and optical basicity follows opposite trend to the electronegativity. In our case, electronegativity (χ) is found to be decreasing for BBS and BBSZ and consequently, the values of electronic polarizabilty (α°) and optical basicity (˄) increases. The B3+ions carry high positive charge, small ionic radius, high electronegativity of 3.189 and low polarizabilty of 0.002A°3in contrast to Se6+and Zr4+cations which carries high polarizabilty 0.073A°3and 0.357A°3and low electronegativity of 2.977 and 1.518 respectively. Therefore, B3+ ions hold the oxide ions very tightly and there occurs a great overlap between O (2p) and valence metal orbitals of B2O3, thus enhance the covalent character vigorously between the B–O bond. Such strong overlap is comparatively weaker in case of SeO2 and ZrO2 which weakens the bond strength and hence triggers the ionic character among the chemical bonds. These results are in favour with previous reports by Zhao, Dimitrov, Sakka and Komatsu [42–44,51–53]. Q. Chen et al. [2] has explained that the polarizability of glass system enhances on the introduction of Se6+ ions. The polarizability of Se atom in glass is around 2.8 times higher than the polarizability of isolated Se atoms. As in glass, valence electrons help in electronic bond polarization by involving Se atoms in bonding that's why there occurs a conversion of co-ordination number from BO4/SeO4 to BO3/SeO3thus creating B–Oand Se–O- bonds. There are two groups selenite [SeO32] and selenate [SeO42−] having two lone pair of electrons along with three or four coordinated oxygen. These negative sites on the groups are very much responsible for the increase in polarizability.

Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgement Authors are thankful to the Department of physics, GNDU for providing the necessary facilities. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.optmat.2019.109338. References [1] Yanko Dimitrev, Albena Bachvarova-Nedelcheva, Reni Iordanava, Glass formation tendency in the system SeO2-Ag2O-B2O3, Mater. Res. Bull. 43 (2008) 1905–1910. [2] Quiling Chen, Kai Su, Yantao Li, Zhiwei Zhao, Structure, spectra and thermal, mechanical, Faraday rotation properties of novel diamagnetic SeO2-PbO-Bi2O3B2O3 glasses, Opt. Mater. 80 (2018) 216–224. [3] F.H. ElBatal, S.Y. Marzouk, F.M. Ezz-ElDin, UV-Visible and infrared spectroscopy of gamma- irradiated lithium diborate glasses containing SeO2, J. Mol. Struct. 986 (2011) 22–29. [4] C.-H. Lee, K.H. Joo, J.H. Kim, S.G. Woo, H.J. Sohn, T. Kang, Y. Kang, Y. Park, J.Y. Characterizations of new lithium ion conducting Li2O-SeO2-B2O3 glass electrolyte, Solid State Ion. 149 (2002) 59–65. [5] Y. Dimitrev, St Yordanov, L. Lakov, The structure of oxide glasses containing SeO2, J. Non-Cryst. Solids 293–295 (2001) 410–415. [6] R. Ciceo-Lucacel, T. Radu, O. Ponta, V. Simon, Novel selenium containing borophosphate glasses: preparation and structural study, Mater. Sci. Eng. C 39 (2014) 61–66. [7] A. Bachvarova-Nedelcheva, R. Iordanova, K.L. Kostov, St Yordanov, V. Ganev, Structure and properties of a non-traditional glass containing TeO2, SeO2 and MoO3, Opt. Mater. 34 (2012) 1781–1787. [8] P. Satya Gopal Rao, Rajesh Siripuram, Sripada Suresh, Raman and IR studies on tellurite based lithium selenite glasses, Int. J. Eng. Res. Online 3 (2015) S3, ISSN: 2321-7758. [9] A. Palui, A. Ghosh, Mixed glass former effect in Ag2O−SeO2−TeO2 glasses: dependence on characteristic displacement of mobile ions and relative population of bond vibrations, J. Phys. Chem. C 121 (2017) 8738–8745. [10] Ch V. Koti Reddy, R. Balaji Rao, K. Chandra Mouli, D.V. Rama Koti Reddy, M.V. Ramana Reddy, Studies on lithium alumino phosphate glasses doped with selenium ions for hard electrolytes, J. Mater. Sci. 47 (2012) 6254–6262. [11] B. Deb, A. Ghosh, Correlation of structure and dielectric properties of silver selenomolybdate glasses, J. Appl. Phys. 112 (2012) 024102. [12] Yanko B. Dimitrev, Stancho I. Yordanov, Luben I. Lakov, Formation and structure of glasses containing SeO2, J. Non-Cryst. Solids 192–193 (1995) 179–182.

4.5. Optical basicity Optical basicity is the tendency of an oxide ion to contribute its electron density to the neighbouring metal ions. By understanding the optical basicity of the material, novel functional optical materials can be designed which are capable of showing high performance in optical devices. The available reports on actual optical basicities of B2O3, Bi2O3, SeO2 and ZrO2 are 0.420, 1.19, 0.95 and 0.79 respectively. Therefore, BBS and BBSZ are showing the highest optical basicity of 1.329 and 1.314 respectively in comparison to the base glass BB 1.267 (as shown in Table 1) due to the creation of NBO's thus supporting ionic character. The redox state of selenium ions can get influenced due to its high optical basicity leading to the generation of higher oxidation state of these ions i. e Se4+during melting which helps in increasing its ionic conductivity [15,17,43]. 5. Conclusions The highly polarizable glass on the addition of SeO2 has been synthesized via melt quench technique, which is further doped with ZrO2 to enhance its structural strength. The prepared glass system has been subjected to various techniques like XRD, DSC, FTIR, Raman and 8

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