Correlation between non-linear optical parameter and structure of Li2B4O7 glasses doped with Er3+ ions

Journal of Non-Crystalline Solids 531 (2020) 119843

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Correlation between non-linear optical parameter and structure of Li2B4O7 glasses doped with Er 3 +ions

T

G. Chandrashekaraiaha,b, A. Jayasheelanc, Mangala Gowrid, N. Sivasankara Reddye, ⁎ C. Narayana Reddyf, a

R&D Center, Bharatiar University, Coimbatore, Tamil Nadu, 641046, India Department of Physics, Government First Grade College, Kunigal, Tumkur 572130, India c Department of Physics, Maharani Science College for Women, Bangalore 560001, India d Department of Physics, University College of Science, Tumkukr 562002, India e Department of Physics, School of Engineering, Presidency University, Bangalore 560064, India f Department of Physics, PES University, Bangalore 560050, India b

ARTICLE INFO

ABSTRACT

Keywords: Judd-Ofelt parameter Raman study IR study Photoluminescence Borate glasses

The glass system 90 Li2 B4 O7 + x Er2O3 + (10 x ) BiCl3 , where 0.1 ≤ x ≤ 0.5 mol%, has been synthesized by melt quenching technique. The Judd-Ofelt (JO) theory has been initiated for the precise analysis of the peak intensities. The JO parameter Ω2, which represents asymmetry and covalency of the emission environment increases with increase in Er3+ ions, while Ω4 and Ω6 reveal the rigidity of the host medium. Radiative parameters such as τR, AR, βR, FOM, and σR have been calculated. The photometric studies, CIE & CCT studies confirm that, for all the investigated glasses CIE coordinated diagram is found to be white light. CCT graph which gives white light emission around 10000K. Two photon absorption coefficient (TPA) of Er 3 + doped glasses lie in the range of 7.491 to 8.868. Glasses with highest TPA shows low Eg values indicating a large density of states of O2 in highest occupied molecular orbitals.

1. Introduction

as optical limiting [15], pumping lasing [16] and 3D optical data storage [4]. On the other hand, β2 can provide information about the micro-transparent flaws in the glass which can cause problems in the UV-Lasers. The TPA theory was formulated by Goppert-Mayer and verified experimentally by Kaiser and Garrett [17–19]. They have studied the applicability of TPA concept to twenty-one different glass systems and the TPA was scaled up by optical band gaps for the glasses containing transition metal oxides. This showed optical non-linearity due to the increase in refractive indexriangles. B2O3 consists of planar [BO3/2]0( ≡ B3) triangles with B O B angle lying between 120∘ and 130∘ [20,21]. The addition of alkali B4 units oxides in the B2O3 network creates [BO4/2] Alkali oxides (upto = 0.35) and thus, the dimensionality and network conB2 O3 nectivity can be enhanced. The quantification of B4 and B4 /B3 ratio is important in assigning structural origin for the trends in physical properties. The study of structure of glass can be made using Raman and infrared spectroscopy [22,23]. The infrared spectra of single alkali diborate glasses can be used to estimate the amount of B4 units and the normalized absorption area can be used to confirm the boron-oxygen tetrahedral structure. The concentration of B4 estimated from the mid-

White light emitting diodes (WLED's) play a significant role in the development of next generation solid state illuminating devices due to their reliability, energy efficiency and luminous efficiency [1–4]. The white LED's commercially available are coated with phosphor containing rare earth (RE3 +) oxides [5–7] have poor color reproducibility, low thermal stability, low color rendering index and poor chemical durability [7]. Optical properties of RE3 + doped glassy materials play a pivotal role in laser technology as optical fibers, wave guides, amplifiers, IR-visible up converters and so on [8]. By modifying the covalency and local structure, optical properties can be tailored [9]. The rareearth, Er3 +, doped glasses play a significant role in the development of lasers, optical amplifiers etc. [10]. The Bi2O3 is a conditional glass former, but it readily forms glass in combination with archetypal glass formers such as B2O3, SiO2, P2O5, GeO2 etc. [11]. The addition of glass formers increases the melting temperature and thermal stability of glass [12], while the addition of halides reduces the phonon energy and increases the transparency of glass [13,14]. It is important to study two photon absorption coefficient (TPA or β2) which has applications such

⁎

Corresponding author. E-mail address: [email protected] (C. Narayana Reddy).

https://doi.org/10.1016/j.jnoncrysol.2019.119843 Received 23 July 2019; Received in revised form 14 November 2019; Accepted 4 December 2019 0022-3093/ © 2019 Elsevier B.V. All rights reserved.

Journal of Non-Crystalline Solids 531 (2020) 119843

G. Chandrashekaraiah, et al.

IR spectra scales linearly with the fraction of B4 atoms determined from the NMR study [24]. The aim of the proposed study is Li2B4O7 base glass doped with BiCl3 and Er2O3 is (i) to calculate the J-O parameter and oscillatory strengths from the absorption spectra (ii) to analyze the luminescence characteristics of the coupled transitions 2H11/2 + 4S3/ 4 2 → I15/2 in the visible region (iii) to evaluate the covalent/ionic nature of the investigated glasses and (iv) to identify different vibrational modes of linkages present in the network structure.

420 nm [28]. The transitions from ground state to the excited states of pronounced peaks at 364, 377, 405, 442, 450, 487, 520, 542 and 650 nm corresponding to 4I15/2 → 4G9/2, 4G11/2, 2G9/2, 4F3/2, 4F5/2, 4F7/ 2 4 4 2, H11/2, S3/2 and F9/2 transitions respectively [29,30]. The absorption coefficients (σ) were calculated [25,26] using the 2.303A formula = Nt , where A is the absorbance, N is the concentration of 3 + Er ions, t is the thickness of the sample. The line strength was calculated using the formula [30]. The integrated absorption cross sectional area was determined from the absorption spectra. The mean wavelength ( ¯ ) was obtained from the absorption cross section data using equation [30].

2. Experimental The glass system with chemical compositions 90 Li2B4 O7 + x Er2 O3 + (10 x ) BiCl3 , where x (in mol%) = 0.1 (LBE1), 0.2 (LBE2), 0.3 (LBE3), 0.4 (LBE4) and 0.5 (LBE5) was prepared by melt quenching method. Analar-grade Li2CO3, H3BO3 and BiCl3 were weighed in accordance with the above formula. Detailed account of the synthesis was described in our recent publication [12].

( ) ( )

¯=

(1)

Transitions of ions in the glass are mainly due to the presence of magnetic and electric dipoles. The transitions of magnetic dipoles occur between similar parity states, whereas the transitions of electric dipoles occur between the states of odd parity. The radiative transitions leading to absorption spectra takes place due to the transition from lower lying state to higher excited states A(J'→J). The radiative transition probabilities, A( J, J ) , of all the excited states were calculated using equation [33].

2.1. Characterization Determination of density, molar volume and glass transition temperature (extracted from DSC thermos graphs) of the prepared glass were explained elsewhere [12]. The infrared spectra of LBE glasses were recorded using Vertex 70 model, Bruker Germany, IR- spectrometer in the range of 400 – 4000 cm−1 at room temperature using KBr pellet. The Raman spectra of the glass samples were recorded using high resolution Jobin-Yvon-Hariba (Model LABRAM-HR visible) spectrometer. The photoluminescence spectra of the investigated glasses were recorded using photoluminescence emission spectrometer furnished with Fluorolog-3.

AR ( J,

J)

=

n(n2 + 2)2 64 4 ˜3 SED + n3SMD 3h(2J + 1) 9

(2)

Where, n is the refractive index of glass, SED and SMD are the electric and magnetic dipole line strengths respectively. J is the total angular momentum quantum number of higher excited state. The line strengths of electric dipole transitions and magnetic dipole transitions were calculated using Eqs. (3) and (4) respectively [30].

SED =

3. Results and discussion

|{(S, L) U (

= 2,4,6

e2h2

)

(S , L)J }| 2

(3)

{(S, L)J L + 2S (S , L)J }| 2

3.1. Judd-Ofelt intensity analysis

SMD =

The Judd-Ofelt theory gives precise analogy of the intensity and accurate measurements of rare earth ion concentrations in the solids. The room temperature absorption spectra of LBE5 glass sample was recorded in the wavelength range of 320–800 nm as shown in Fig. 1 and several peaks have been identified. These observed absorption peaks are incorporated with literature of the Er3 + doped various other glasses. The strong absorption peaks correspond to the transitions between various excited levels and the ground level [25-27]. The active absorption of the glass sample in ultraviolet range are the peaks below

The magnetic dipole transitions are non-zero, only if S ] S' and L = L'. Additional selection rules for total angular momentum are J = J , J = (J + 1), J = (J 1). The estimation of magnetic dipole moment using the matrix elements in LS coupling schemes are simple and can be determined from the Eq. (5), which is derived from the concept of angular momentum [34]. The line strength as predicted by the Judd-Ofelt theory is given by Eq. (5) [31,32].

16

Scal (J

2 mc2

{(S, L)J U (

J)=

)

(S , L)J }

(4)

2

(5)

= 2,4,6 (λ)

Where, ||U || are doubly reduced unit tensor operators taken from the reference [32] and were tabulated in Table 1. The method of least square fit was employed to determine the Judd-Ofelt intensity parameters using the measured values and predicted values of line Table 1 Reduced matrix elements for absorption peaks of Er 3 + ion used in the calculations. Transition

U(2)

U(4)

U(6)

RME(MD)

4

0 0 0 0 0.021974 0.918357 0 0 0 0 0.712554 0

0.0334 0 0 0.017411 0.004095 0.526087 0.01896 0 0 0.146878 0.412365 0.535386

0.0029 0.0026 0.0172 0.116315 0.075754 0.117176 0.225554 0.1272 0.22321 0.626538 0.092467 0.461795

0.00000 0.00000 0.00000 0.00000 0.53157 0.000000 0.000000 0.000000 0.000000 0.000000 0.000000 0.000000

4

G7/2- I15/2 G5/2-4I15/2 2 P3/2-4I15/2 2 G7/2-4I15/2 2 K15/2-4I15/2 4 G11/2-4I15/2 2 G9/2-4I15/2 4 F3/2-4I15/2 4 F5/2-4I15/2 4 F7/2-4I15/2 2 H11/2-4I15/2 4 F9/2-4I15/2 4

Fig. 1. UV–Vis absorption spectra of LBE5 glass. 2

Journal of Non-Crystalline Solids 531 (2020) 119843

G. Chandrashekaraiah, et al.

the radiative lifetime can be calculated using the Eq. (8) [36].

Table 2 JO intensity parameters and spectroscopic quality (χ) for the Er 3 + doped other glasses. Glass

2

NfBEr Bismuthate Sodalime silicate Tellurite Borosilicate Lithium borate ZBE3 Phosphate LBE1 LBE2 LBE4 LBE5

× 10

20

1.125 3.86 2.72 5.22 4.13 3.24 3.02 5.0 0.308 0.0751 0.157 0.1698

4

× 10

0.646 1.52 2.31 1.55 1.83 0.92 1.32 1.5 0.204 0.0589 0.0969 0.0903

20

6

× 10

0.366 1.17 1.28 1.13 1.63 0.82 1.22 1.07 0.202 0.058 0.0905 0.0816

20

χ

Reference

1.76 1.11 1.8 1.371 1.123 1.123 1.082 1.402 1.10 1.015 1.070 1.106

[54] [56] [37] [59] [58] [59] [60] [57] Present Present Present Present

rad

J

A( J,

= work work work work

J)

=

AR ( J, AT

J)

20cm2) ,

J)

g( ¯)

(9)

rad

I( ¯) I( ¯) d ¯

(10)

The figure of merit (FOM) for optical gain, given by (σe × Δλeff), shows the band width of an amplifier. The FOM values in Table 4 match with other Er3 +doped glasses. The values of SED, AED, AMD, AT, βmeas, g( ¯) and σ are tabulated in Tables 3 and 4. 3.2. Photoluminescence studies The excitation spectra of Er3 + doped LBE glasses is obtained by exciting the sample using a wavelength of 545 nm shown in the Fig 2b. The transitions from ground state to the excited states pronounced at 369, 397, 437, 449, 446, 479 and 494 nm which are corresponding to 4 I15/2 → 4G9/2, 4G11/2, 4H9/2, 4F3/2 4F5/2, 4F7/2 transitions respectively [41]. By observing these transitions, electronic transition 4I15/2 → 4G11/ 2 is assigned to the wavelength 397 nm has maximum intensity and same has been chosen to obtain emission spectrum of LBE glasses. The obtained emission spectrum shows two peaks centered at 529 nm and 545 nm corresponding to the electronic transitions from 2H11/2 and 2S3/ 4 3+ ions respectively as shown in the 2 to the ground state I15/2 of Er 3 + Fig 2a. The Er ion goes to the excited state 4G11/2 by absorbing the wavelength of 397 nm. Anurag Pandey et al. [42] reported that the emission of blue band (between range of 474 to 481 nm), green band (between range of 510 to 580 nm) and red band (between range of 640 to 645 nm). These bands arise due to emission from the excited energy state to ground state. We confirm that investigated glasses contain basic colors of green, red and blue emissions. These color combinations could lead to obtain white color emission from the glass, which clearly confirm from the CIE color coordinate analysis explained in the proceeding section. In the way to identify the color of the emitted light, the investigation in the color emitted by the source by comparing with the standard color emitted by the white light emitting source.

(7)

The measurement of radiative life times is quite difficult, hence, it is indirectly calculated in most of the cases. The difficulty is due to existence of spontaneous emission. Also, it includes energy transfer, nonradiative relaxation and radiative trapping. As a result, direct measurement of the life time does not yield radiative lifetime at any temperature. Therefore, utilizing the reciprocal of the absorption indeed, Table 3 Line strengths (× 10

rad (J 8 cn2

g( ¯) =

The branching ratio describes the interaction of light and ions as a function of wavelength of allowed transition to an excited state, which is determined using the Eq. (7) [36]. cal ( J,

2

The radiative transition probabilities present in the LBE glasses are in good agreement with the reported values [37]. Therefore, these glasses can be treated as host materials for broad band amplifiers [38,39]. The line shape function, g( ¯) is given by [36].

(6)

J)

(8)

J)

The emission cross section depends on the composition of the glasses. By taking into the consideration of relation between radiative transition, branching ratio and refractive index, the emission cross section can be calculated using the equation [36].

strengths. Table 2 The Judd-Ofelt intensity parameters ( ( = 2, 4 & 6)) and spectroscopic quality factor ( = 4 / 6) confirms that the glass prepared in the present study can become a good material for the laser production [35]. The Judd-Ofelt intensity parameter relates the distinct collection and bonding conditions of the RE3 + ions. These parameter show not only covalence of bonds but also on the irregularity of vicinity of the RE3 + ions. The estimated JO intensity parameters (Ω2, 4, 6) and spectroscopic quality factor of the glasses were shown in the Table 2. Usually the magnitudes of Ω2 parameter depends on the asymmetry and covalency of rare-earth ions [34], while other values of Ω4 and Ω6 depend on viscosity and rigidity of the host medium. The measured values of three JO intensity parameters of the LBE glasses were in the order Ω2 > Ω4 > Ω6 as shown Table 2. The values of intensity parameters obtained shows that the Er3 + ions are established in a higher covalence situation within the glass network. The measured radiative transition rates (AMD and AED) were given in the Table 3. The total radiative transition probability (AT) is given by

AT =

1 AT (J

=

transition probabilities, branching ratios.

Transition

λ(nm)

SED (calculated)

SED (measured)

AED s

4

293 301 314 355 364 377 406 441 450 488 520 651

0.029018 0.045969 0.015570 0.041973 0.051619 0.0018929 0.0246012 0.040286 0.038282 0.044015 0.0018781 0.090967

0.0206 0.0000 0.0238 0.0309 0.0154 0.0262 0.0219 0.0057 0.0115 0.0017 0.0157 0.0082

15.605 0.008 12.806 5.741 1.325 2.712 2.177 1.046 1.395 0.122 0.618 486.902

4

G7/2– I15/2 G5/2–4I15/2 2 P3/2–4I15/2 2 G7/2–4I15/2 2 K15/2–4I15/2 4 G11/2–4I15/2 2 G9/2–4I15/2 4 F3/2–4I15/2 4 F5/2–4I15/2 4 F7/2–4I15/2 2 H11/2–4I15/2 4 F9/2–4I15/2 4

3

1

AMD s 0.000 0.000 0.000 0.000 0.010 0.201 0.000 0.000 0.000 0.150 0.000 0.0000

1

β 0.0035 0.0000 0.0006 0.0020 0.0032 0.0124 0.0183 0.0073 0.1768 0.0388 0.1432 1.000

Journal of Non-Crystalline Solids 531 (2020) 119843

G. Chandrashekaraiah, et al.

Table 4 Calculated total transition probabilities, line shape functions, emission cross sections (× 10 Transition 4

4

G7/2– I15/2 G5/2–4I15/2 2 P3/2–4I15/2 2 G7/2–4I15/2 2 K15/2–4I15/2 4 G11/2–4I15/2 2 G9/2–4I15/2 4 F3/2–4I15/2 4 F5/2–4I15/2 4 F7/2–4I15/2 2 H11/2–4I15/2 4 F9/2–4I15/2 4

AT s

1

15.605 0.008 12.806 5.741 1.335 2.913 2.177 1.046 1.395 0.272 0.618 486.90

E cm

1

34,130 33,223 31,847 28,169 27,473 26,525 24,631 22,676 22,222 20,492 19,231 15,361

22

cm2) , optical gain (×10

25

cm2s ), and gain bandwidth (× 10

28cm3)

.

Lifetime (ms)

g( ¯)

σ

Optical gain

Gain Bandwidth

0.2245 0.5751 0.0451 0.3536 2.4228 4.2500 8.4098 7.0239 126.769 142.208 231.804 2.0538

0.128276 0.079034 0.221208 0.147323 0.137324 0.153202 0.117907 0.062368 0.072953 0.076586 0.080302 0.040292

4.2583 0 7.1969 2.6047 0.59606 1.5758 1.04897 0.31267 0.51103 0.12342 0.33271 206.222

0.95599 0 0.32458 0.921013 1.44413 6.69695 8.82164 2.19617 64.7823 17.5521 77.123 423.54

3.31967 0 3.25343 1.768 0.434053 1.02855 0.889663 0.501337 0.70049 0.161159 0.41432 511.813

Fig. 3. (a) CIE chromaticity coordinates for LBE glasses under excitation wavelength 397 nm. (b) CCT coordinates of LBE glasses.

fluorescence emission spectra in the visible region under the excitation wavelength of 397 nm. The CIE chromaticity coordinates and corresponding correlated color temperature values of different concentration of Er3 + doped glasses were listed in the Table 5. Under CIE chromaticity diagram we have noticed that, the emission fluorescence spectra exhibit white light in the visible region. The estimated LBE glass spectra under excitation of 397 nm which are close to the expected value of the white light coordinates (0.333, 0.333). This confirms that, highest doped Er3 +

Fig. 2. (a) Photoluminescence emission spectra of LBE glasses under 397 nm excitation. (b) Excitation spectra of LBE5 glass excited at 545 nm.

3.2.1. Photometric analysis (CIE & CCT) The emission of white light in the Er3 + doped bismuth-borate glass has been confirmed in the commission international deI'Echairage (CIE) 1931chromaticity as shown in the Fig 3a. The CIE chromaticity diagram for different mol% of Er3 + doped glasses are obtained from the 4

Journal of Non-Crystalline Solids 531 (2020) 119843

G. Chandrashekaraiah, et al.

Table 5 CIE chromaticity coordinates and CCT values of Er 3 + doped LBE glasses under excitation wavelength of 397 nm. No

Glass composition

1

Li2O -B2O3- BiCl3; Er 3 + (0.1 mol%)

2

3

5

(0.25824, 0.294)

CCT 11,986

Li2O -B2O3 - BiCl3; Er 3 + (0.2 mol%)

(0.30809, 0.31633)

9892

Li2O -B2O3 -BiCl3; Er 3 + (0.4 mol%)

(0.25977, 0.29609)

11,658

Li2O -B2O3 - BiCl3; Er 3 + (0.3 mol%)

4

CIE chromaticity (x, y)

Li2O-B2O3 -BiCl3; Er 3 + (0.5 mol%)

(0.26584, 0.30548)

10,468

(0.25737, 0.28464)

12,916

Table 6 Literature TPA absorption coefficient and optical band gap along with present work. Sl no

Glass composition (wt%)

β2 (cm/ GW)

Eg (eV)

Reference

1 2 3 4 5 6

60SiO2 - 40Na2O 5TiO2 - 55 SiO2 - 40Na2O 15TiO2 - 45 SiO2 - 40Na2O 20TiO2 - 40 SiO2 - 40Na2O 13.38Li2O - 7.12Al2O3 - 79.5SiO2 13.38Li2O - 7.12Al2O3 79.5SiO2 + 0.2g Cr2O3 13.38Li2O - 7.12Al2O3 −79.5SiO2 + 0.4g Cr2O3 13.38Li2O - 7.12Al2O3 79.5SiO2 + 0.6g Cr2O3 75SiO2 - 25Na2O 10BaO −75SiO2-15Na2O 15BaO −75SiO2 −10Na2O 20Na2O - 80B2O3 2BaO - 18Na2O - 80B2O3 5BaO - 15Na2O - 80B2O3 10BaO - 10Na2O - 80B2O3 2MgO - 18Na2O - 80B2O3 2MgO - 18Na2O - 80B2O3 10MgO - 10Na2O - 80B2O3 60Bi2O3 - 80B2O3 70Bi2O3 - 30B2O3 80Bi2O3 - 20B2O3 LBE1 LBE2 LBE3 LBE4 LBE5

6.7 8.1 8.4 6.0 14.2 17.9

3.62 3.35 3.27 3.57 2.75 2.25

[44] [44] [44] [44] [44] [44]

17.3

2.22

[44]

15.6

2.13

[44]

6.6 4.8 4.1 7.0 7.3 7.5 8.8 7.6 7.4 7.0 12.9 15.1 16.4 7.49 7.81 8.22 8.54 8.86

3.62 3.6 3.64 3.48 3.46 3.45 3.29 3.42 3.42 3.45 2.92 2.8 2.71 3.49 3.45 3.4 3.36 3.36

[44] [44] [44] [44] [44] [44] [44] [44] [44] [44] [44] [44] [44] Present Present Present Present Present

7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25

26

work work work work work

Fig. 5. (a) FTIR spectra of LBE1-5 glasses. (b) Deconvoluted FTIR spectra of LBE5 glass.

(LBE5) glass sample can exhibit better fluorescence compared to the other samples. The correlated color temperature (CCT) values were obtained from the CIE chromaticity diagram coordinates by using the McCamy's approximate formula [42]. CCT = 449n3 + 3525n2 6823.3n + 5520.33. X X where n = Y Ye and (X e = 0.332, Ye=0.186) is the epicenter of the e

merging. The estimated CCT value for Er3 + doped glasses is 10784K,which is close to white light zone in the chromaticity order as shown in the Fig 3b. It can be noted that, as CCT values are higher, the perception of brightness is good with an improved visual perception [43]. 3.3. Two photon absorption coefficient (TPA)

The third order complex non-linear susceptibility (χ) in isotropic materials is a measure of optical non-linearity and the imaginary part of susceptibility is directly proportional to TPA [45,46]. The susceptibility is given by

Fig. 4. Linear relation between TPA and optical band gap energy of LBE glasses.

=

5

n2 0 c 2 2

(11)

Journal of Non-Crystalline Solids 531 (2020) 119843

G. Chandrashekaraiah, et al.

Table 8 Band position (cm−1) and corresponding peak assignment of the Raman spectra. Positions of band in cm−1

Assignments

Reference

1641 1478 1309

B O stretching of metaborate groups B O stretching in metaborate groups Presence of otho and pyroborate units containing B O vibrations

[49] [49] [38]

1121 948

739

Where n is the refractive index, ɛ0 is the absolute permittivity of the free space, c is the velocity of light and γ is the frequency. For all glasses susceptibility (χ) can be expressed as [46,47] ga

gb

gc

1 (Ega +

3w )(Egb

2w )(Egc

w)

1 (Ega + w )(Egb + 2w )(Egc

w)

+ +

1 (Ega + w )(Egb

2w )(Egc

[38]

[50,51] [38]

Ωga is dipole moment transition matrix element of the transition between ground state and first excited state (a), Ega is the energy difference between the ground state and excited states. a, b, and c are the excited states. If the photon energy approaches the energy difference between the ground state and any excited states, resonance absorption takes place. The term Ω is propotional to the square root of absorption coefficient (α) and Ega correspond to the band gap energy. Hence, the non-linearity absorption increases with increase in absorption coefficient or decrease in the optical band gap energy [45–48]. TPA of the Er3 + doped LBE glasses lie in the range 7.491 to 8.868 cm/GW and obey the two parabolic band model [49]. The data listed in Table 6, along with the literature values for various glass system. Fig. 4 represents the variation of TPA versus Eg of 21 glass systems along with present work. As can be seen from the Fig 4, the data points fall on a straight with negative slope. It is also clear from Table 6, that glasses with highest TPA show lowest Eg this can be ascribed to a large transition moment of O2 , indicating large density of states of O2 in highest occupied molecular orbitals which in turn reduce the optical band gap. This is also reflected in the variation of Eg with Er2O3 content [12]. The addition of Er2O3 to a network glass breaks the covalent bonds of tightly bound diborate [B4O7]2− units and converts bridging oxygens into non-bridging oxygens (NBOs). The modified action of Er2O3 is explained elsewhere [12]. Since NBO's are weakly bound to borons and are less stable, the optical non-linearity increases [32]. Besides a strong anharmonic effect arises, when the valence electrons of the NBO's are subjected to an optical electric field. Fig 4 shows the scaling of TPA to Eg (which describes the electronic structure) in Er2O3 doped glasses. The plot gives relationship between the theoretical non-linear TPA and experimental band gap energy. Structurally, lithium ion has one valence electron in the outer shell and get distorted in response to applied electric field. Consequently, a small charge displacement takes place resulting in low polarizability. The importance of monovalent alkali ions is to break the covalent bonds of archetypal glass forming oxides with the creation of NBO's. The valence electrons in NBO's can be distorted easily when subjected to field. Thus, anharmonic effect arises. As the concentration of rare earth oxide increases, the covalency of metal ligand bond and the radiative transition probabilities are increases [44].

Fig. 6. (a) Raman spectra of LBE1-5 glasses. (b) Deconvoluted Raman spectra of LBE5 glass.

abc

Presence of diborate groups [B4 O7]2 B O stretching in pyro- and ortho-borate groups B O vibration in isolated diborate groups

w)

1 (Ega + w )(Egb + 2w )(Egc + 3w ) (12)

Table 7 Band positions (in cm−1) and corresponding peak assignments of IR spectra of the LBE glasses. Sl no 1 2 3 4 5 6

Position of IR bands −1

555 cm 692 cm−1 848 cm−1 1097 cm−1 1237 cm−1 1368 cm−1

Assignment

Reference

B-O-B stretching (BO3) B-O-B bond bending vibration in supernatural units B-O stretching vibration of BO4 units Stretching vibration of B-O bond in BO4 units Asymmetric stretching vibrations of B-O bond Presence of ortho, pyroborate containing BO3ˉ units

[43,48] [40] [34–37] [34–37] [24–28] [24–28]

6

Journal of Non-Crystalline Solids 531 (2020) 119843

G. Chandrashekaraiah, et al.

Fig. 7. Structural units of Borate groups.

3.4. Structural studies

modes of B O of the tetrahedral [BO4/2] groups and in third region mainly associated with bending mode of B O B linkage present in the trigonal [BO3/2]0 groups [49,50]. The absorption band position and percentage transmission are depending on the network modification. The IR absorption bands noticed in the Figs. 5a and b of LBE glasses are in good agreement with literature results of bismuth-borate glasses [38–41, 50]. The bands and their assignment are presented in the Table 7. It is evident from Table 7, bands that are induced between 1237 cm−1and 1368 cm−1 is a result of stretching of B O band in super-structural trigonal borate group such as [BO3/2]0, [BO4/2] and [BO1/2 O2 ]2 [11,51–55]. The absorption bands appear in the range of 1050–1100 cm−1 are due to the stretching vibrational modes of boronoxygen bond from di, tri and penta borate units and overlapping of nonbridging oxygens (NBO'S) [40–43]. The bending vibrations associated with the B O B linkages of [BO3/2]0 units are induced in the region of 692 cm−1[51]. The vibrational bands seen at 368 cm−1 and 555 cm−1 can be assigned to highly distorted stretching modes of Bi O and Bi O Bi bending modes BiO6 octahedral units present in the structure [51,54,55]. It is evident from the Fig 5a and b that addition of Er2O3 did not induce any peak in the longer wavenumber

3.4.1. FT-IR analysis As pointed out in our earlier publication Li2B4O7 glasses containing Er2O3, 11B MAS NMR spectroscopic study reveals the enhancement of charged borate units due to the addition of Er2O3. Besides, Er2O3 addition opens up of the tight [B4 O7 ]2 (diborate) units, which in turn create NBOs in the structure. In order to substantiate the findings of 11B MAS NMR study vibrational (IR and Raman) spectroscopic studies have been initiated. The FTIR and Raman spectra are shown in Figs. 5a and 6a respectively. FTIR study is an important tool to analyze the structure of glass network altered by the modifiers. This will provide information about the structural motifs and various vibrational modes of structural elements. It is evident from Fig 5a. and deconvoluted plot in Fig 5b. that vibrational modes which are highly active in the Infrared region and bands are precisely classified into three sections on the basis of wavenumber. The first region assigned to the stretching vibrations of the BO3 triangle associated with the different borate units and the second region is assigned to B O bond of [BO3/2]0 groups and the stretching 7

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G. Chandrashekaraiah, et al.

region indicating that the glasses are durable and thermally stable [12]. Broad bands are due to the overlapping of a few individual bands, each band is attributed to the concentration of a particular structural group. In the deconvuluted spectra, area under each band gives the proportion B of B3 and B4 species. They are used to calculate N4 (= B +4B ) where 3 4 0 [BO4/2] B4 and [BO3/2] ≡ B3 which are comparable to N4 values calculated using MAS NMR spectra. N4 values for LBE1, LBE2, LBE3, LBE4 and LBE5 glasses are 0.31, 0.27, 0.25, 0.24 and 0.22 respectively.

Author contributions

3.4.2. Raman studies Raman spectroscopy is used to study the vibrational modes present in the structure of borate glasses, these modes are the ‘breathing modes’ of the basic structural units. Fig. 6a represents the Raman spectra of LBE glasses. The raw spectrum of investigated glasses was found to be asymmetric and overlapping of a few bands. It is difficult to get the active band position precisely and this problem has been removed by curve fitting method such as Gaussian deconvoluted spectra of a representative LBE5 glass in the Fig 6b. The bands and their assignment of LBE glasses are shown in the Table 8. The possible cation-oxygen linkages viz, B O,B O , B O B in trigonal, tetrahedral, diborate, metaborate, pentaborate and boron rings are identified in infrared spectroscopic study. The groups are further probed by Raman studies to substantiate the network linkages. The bands in the region of 1642 cm−1 and 1478 cm−1 are due to the stretching of B O and B O respectively in metaborate groups. The band at 1121 cm−1 is indication of the presence of diborate groups. B O vibrations in pyro and ortho borate groups are centered at 948 cm−1. The presence of B O vibrations in isolated diborate groups are centered at 739 cm−1. A weak band at 540 cm−1 is due to the bending vibrations of B O B in boroxyl rings. The spectra consist of a moderately strong band in the region of 181 cm−1 can be attributed to the bismuthate structural units [54]. The vibrational modes seen at 345 cm−1 can be assigned to bismuth-oxygen polyhedra [55]. The structure of B2O3 glass normally consist of planar triangle [56]. Addition of Li2O to B2O3 can create [BO4/2] units, which leads to increase in the connectivity and dimensionality of the network. The dimensionality may be expressed as D=2Y+3(1 Y) where Y is the concentration of trigonal borons while (1 Y ) is the concentration of the tetrahedral borons. In the present work Li2O and B2O3 is kept at diborate concentration (1Li2O: 2B2O3). Our earlier publication [12] reveals that when BiCl3 content varied from the 10 to 30 mol%. N4 values remained the same indicating that was not acted as network modifier while Er2O3 assume the network modifying role. When Er2O3 is added, the diborate network opened up by creating charged borate units. The possible borate units in the investigated glasses are shown in Fig. 7.

G. Chandrashekaraiah: Conceptualization, Data curation. A. Jayasheelan: Investigation, Methodology. Mangala Gowri: Formal analysis. N. Sivasankara Reddy: Software, Supervision, Validation, Visualization. C. Narayana Reddy: Writing - original draft, Writing review & editing.

CG has carried out the experimental work, calculations and analysis. MG has performed J-O analysis and discussions. JS has contributed to Photoluminescence studies. NSR contributed FTIR and Raman spectroscopic studies and discussions. CNR contributed to results, discussions and developing the entire manuscript. CRediT authorship contribution statement

Declaration of Competing Interest None. References [1] Shriya Sinha, Manoj Kumar Mahata, Kaushal Kumar, RSC. Adv. 6 (2016) 89642–89654. [2] Y.C. Lin, M. Karlson, M. Bettinelli, Chem. Mater. 31 (11) (2019) 3851–3862. [3] D. Ramachari, D. Esparza, T. Lopez-luke, V.H. Romero, L. Perez-Mayen, et al., J. Alloys Compd. 698 (2017) 433–441. [4] Hao Dong, Ling-Dong Sun, Chun-Hua, Chem. Soc. Rev. 44 (2015) 1608–1634. [5] G.A. Kumar, M. Pokhel, D.K. Sardar, Mater. Lett. 98 (2013) 63–66. [6] Y. Wang, S. Gai, N. Niu, F. He, P. Yang, T. J. Phys. Chem. Chem. Phys. 15 (2013) 16795–16805. [7] K. Annapoorani, Ch. Basavapoornima, N. Suriya Murthy, K. Marimuthu, J. NonCryst. Solids 447 (2016) 273–282. [8] J.A. Caired, A.J. Ramponi, P.R. Staver, J. Opt. Soc. Am. B 8 (1991) 1391–1403. [9] P. Babu, C.K. Jayashankar, Opt. Mater. 15 (2000) 65–79. [10] Xiang Shen, Quinhua, T. Xu, S. Dai, X. Wang, Physica B 381 (2006) 219–223. [11] K.J. Rao, Structural Chemistry of Glasses, Elsevier, North Holland, 2002. [12] G. Chandrashekaraiah, N. Sivasankara Reddy, B. Sujatha, R. Viswanatha, C. Nararaya Reddy, J. Non-Cryst. Solids. 498 (2018) 252–261. [13] S. Tanable, N. Sigimoto, S. Ito, T. Hanada, J. Lumin. 87-89 (2000) 670–672. [14] M. Laya, Krishnan, M.M. Neethish, V.V. Ravikanth kumar, J. Lumin. 201 (2008) 442–450. [15] F. Ahmadi, R. Hussain, S.K. Ghoshal, J. Alloys Compd. 711 (2017) 94–102. [16] Anita Hastir, Nipin Kohli, Ravi Chand Singh, J. Phys. Chem. Solids 105 (2017) 23–34. [17] M. Goppert-Mayer, Ann. Phys. (Berlin) 18 (2009) 466–479. [18] W. Kaiser, C.G.B. Garrett, Phys. Rev. Lett. 7 (6) (1961) 229–231. [19] Foud El-Diasty, F.A. Abdel Wahab, M. Abdel-Baki, J. Appl. Phys. 100 (2006) 093511. [20] Reni Iordanova, Milonova Margarita, Lyubomir Aleksandrov, Atul Khanna, J. NonCryst Solids 481 (2018) 254–259. [21] P.J. Bray, J. Edward, J.G. O'Keefe, J. Phys. Chem.35 (196) 443. [22] L.D. Pye, V.D. Frechtte, N.J. Kreidl, Borate glasses. Structure, Properties and Applications, Plenum Press, New York, 1978. [23] E.I. Kamitsos, G.D. Chryssikos, J. Mol. Struct. 247 (1991) 1–16. [24] E.I. Kamitsos, J. Ephysique IV2 (1992) C2–87. [25] P. Babu, H.J. Seo, C.R. Kesavulu, K.H. Jang, C.K. Jayashankar, J. Lumin. 129 (2009) 444–448. [26] K. Annapoorni, N. Suriya Murthy, T.R. Ravindran, K. Marimuthu, J. Lumin. 171 (2016) 19–26. [27] K. Lingaraju, K. Suresh, S. Ju, W.-.T. Han, et al., Opt. Mater. Express 5 (2015) 1689–1703. [28] G.S. Ofelt, J. Chem. Phys. 37 (1962) 511–520. [29] W.T. Camall, P.R. Fields, B.C. Wynboume, J. Chem. Phys. 42 (1965) 3797.23681 USA. [30] B.M. WalshNASA Langley Research Center, Hampton. VA. [31] B.R. Judd, Optical absorption intensities of rare-earth ions, Phys. Rev. 127 (1962) 750–761. [32] S. Tanabe, X. Feng, T. Hanada, Opt. Lett. 25 (2000) 817–819. [33] B.M. Walsh, Judd-Ofelt Theory: Principles and Practices in Advances in Spectroscopy for Lasers and Sensing, in: B. Di Bartolo, O. Forte (Eds.), Springer, Netherlands, 2006, pp. 403–433. [34] B.R. Judd, Operator Techniques in Atomic Spectroscopy, McGraw–Hill, New York, 1963. [35] K. Selvaraju, K. Marimutu, Physica B 407 (2012) 1086–1093. [36] D.K. Sardar, D.M. Dee, K.L. Nash, R.M. Yow, John B. Gruber, J. Appl. Phys. 100 (2006) 123106. [37] X. Zou, T. Izumiysni, J. Non-Cryst. Solids 162 (1993) 68–80. [38] R. Rolli, M. Mountaga, S. Chaussedent, A. Monteil, V.K. Tikhomitrov, M. Ferrari,

4. Conclusion Using UV–Vis absorption spectra of Li2B4O7 glasses containing Er3 + and Bi3+, Judd-Ofelt parameter such as line strength, transition probabilities, radiative life time, and stimulus cross section have been calculated. The estimated parameters Ω2 > Ω4 > Ω6, reveals that Er3 + ion is in higher covalency situation. The value of TPA of the investigated glasses lie in the range of 7.491 to 8.68 cm/GW. The scaling of TPA versus Eg have been carried out which gives relationship between nonlinear TPA & Eg. As TPA increases, the band gap between HOMO & LUMO hybrid orbitals decreases. Higher TPA of the investigated glasses with addition of Er3 +ions leads to non-linearity which is derived from hyper polarizabilities. The structural studies reveals that the creation of NBO's due to the opening up of [B4 O7 ]2 units as a result of network modification. The formation of charged B4 units at the cost of B3 leads to the increasing N4-values. These values are good agreement with the MAS NMR results. 8

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G. Chandrashekaraiah, et al. Opt. Mater. 21 (2003) 743–748. [39] L.R.P. Kassab, L.C. Courrol, R. Seragioli, N.U. Wetter, S.H. Tatmi, L. Gomes, J. NonCryst. Solids. 348 (2004) 94–97. [40] L. Vijayalakshmi, K. Naveen Kumar, K. Srinivasa Rao, Pyung Hwang, J. Mol. Struct. 1155 (2018) 394–402. [41] Anurag Pandey, S. Soma, Vijay Kumar, Vinod Kumar, Kaushal Kumar, Vineet Kumar, H.C. Swart, Sensor Actuat. B-chem 202 (2004) 1305–1312. [42] P. Remya Mohan, Subash Ramya, Viji Vidhyadharam Gopi, Anns George, Cyriac Joseph, N.V. Unnikrishnan, P.R. Bijju, J. Lumin. 187 (2017) 113–120. [43] Lakshmi Mukhopadhyay, Vineet Kumar Rai, Renuka Bakalia, K. Sreenivas, J. Lumin. 187 (2017) 368–377. [44] Foud El-Diasty, M. Abdel-Baki, J. Sol. State Chem. 184 (2011) 2762–2769. [45] M. Shek-Bahae, A.A. Said, T.H. Wei, D.J. Hagam, E.W. Vanstryland, IEEE. J. Quantum Electron. 26 (1990) 760. [46] F. Kaizar, J. Messier, J. Appl. Phys. 60 (1986) 3040. [47] M. Abdel-Baki, F.A. Abdel-Wahab, A. Radi, Foud El-Diasty, J. Phys-Chem. Solids. 68 (2007) 1457. [48] R. Rolli, M. Mountaga, S. Chaussedent, A. Monteil, V.K. Tikhomitrov, M. Ferrari, Opt. Mater. 21 (2003) 743–748.

[49] C. Narayana Reddy, R.P. Sreekanth Chakradhar, J. Mater. Res. Bull. 47 (2007) 1337–1347. [50] B.K. Chethana, C. Narayana Reddy, K.J. Rao, J. Mater. Research Bull. 47 (2012) 1810–1820. [51] Y. Cheng, H. Xiao, W. Guo, Thermochim. Acta. 444 (2006) 173–178. [52] P. Pascuta, L. Culea, Mater. Lett.62 (20080) 4127–4129. [53] E. Culea, L. Pop, S. Simon, M. Culea, J. Magn. Magn. Mater. 290-291 (2005) 1465–1468. [54] G.N. Rocha, I.F.L. Melo, M.C. Castro, Mater. Chem. Phys. 139 (2013) 494. [55] H. Fan, I. Hu, K. Yang, Y. Fang, J. Non-Cryst. Solids. 356 (2010) 1814–1818. [56] C. Narayana Reddy, V.C. Veeranna Gowda, R.P.S. Chakradhar, J. Non-Cryst. Solids 354 (2008) 32–40. [57] M. Subadra, P. Kistaiah, Vib. Spectrosc. 62 (2012) 23–27. [58] L.R.P. Kassab, L.C. Courrol, R. Seragioli, N.U. Wetter, S.H. Tatmi, L. Gomes, J. NonCryst. Solids 348 (2004) 94–97. [59] Y.C. Fang, S.-.Y. Chu, P.-.C. Kao, Y.-.M. Chuang, Zeng Z-L, J. Electrochem. Soc. 158 (2) (2011) J1–J5. [60] K. Subrahmanya, M. Salagram, Opt. Mater.15 (200) 181–186.

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