Spectroscopic properties of Sm3+ ions doped

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 185 (2017) 139–148

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Spectroscopic properties of Sm3 + ions doped Alkaliborate glasses for photonics applications R. Nagaraj a, P. Suthanthirakumar a, R. Vijayakumar b, K. Marimuthu a,⁎ a b

Department of Physics, Gandhigram Rural University, Gandhigram, 624 302, India Department of Physics, Sasurie College of Engineering, Vijayamangalam, 638 056, India

a r t i c l e

i n f o

Article history: Received 19 November 2016 Received in revised form 3 May 2017 Accepted 23 May 2017 Available online 24 May 2017 Keywords: Absorption Bonding parameter Judd-Ofelt parameters Luminescence Stimulated emission cross-section

a b s t r a c t A new series of Sm3+ doped alkaliborate glasses have been prepared by melt quenching technique and their structural and spectroscopic properties were analysed employing XRD, FTIR, optical absorption, photoluminescence and decay spectral measurements in order to explore their suitability for photonic applications. The amorphous nature have been confirmed through XRD analysis and the FTIR spectra reveal the presence of fundamental stretching and bending vibrations of the borate networks in the prepared glasses. From the absorption peak positions, bonding parameter (δ) values were calculated to examine the nature of the metal-ligand bond. The optical band gap (Eopt) corresponds to the direct and indirect allowed transitions and the Urbach energies (ΔE) were calculated from the absorption spectra to understand the electronic band structure of the studied glasses. The Judd-Ofelt (JO) intensity parameters Ωλ (λ = 2, 4 and 6) were determined to explore the symmetry of the ligand environment around the Sm3+ ions in the studied glasses. The luminescence spectra exhibit four emission bands in the visible region due to the 4G5/2 → 6H5/2, 6H7/2, 6H9/2 and 6H11/2 transitions. The radiative parameters such as transition probability (A), stimulated emission cross-section (σEP), branching ratios (βR) and radiative lifetime (τR) have been determined from the luminescence spectra using JO theory to ensure the suitability of the studied glasses for optoelectronic applications. The luminescence spectra were characterized through CIE 1931 chromaticity diagram to examine the dominant emission color of the studied glasses. The lifetime values of the Sm3+ doped studied glasses pertaining to the 4G5/2 excited level have been determined through decay curve measurements and the non-exponential decay curves were fitted to the Inokuti-Hirayama model to analyze the energy transfer mechanism between the nearby Sm3+ ions. The obtained results were discussed and compared with the similar reported glasses. © 2017 Elsevier B.V. All rights reserved.

1. Introduction Over the past few decades, optical properties of trivalent rare earth (RE3 +) ions doped materials have been extensively studied due to their potential applications in the field of optoelectronics such as lasers, optical amplifiers, fluorescent display devices, optical fibers and optical detectors etc., [1–4]. Glasses doped with RE3+ ions exhibit broader absorption and emission spectra when compared to the crystalline hosts which make them quite interesting materials in spectroscopic as well as technological point of view. Further, investigations on absorption and emission spectra of RE3+ ions in glass materials provide valuable information about radiative properties like transition probability, energy level structure, optical gain and stimulated emission cross-section. These findings play a key role in the development of new optoelectronic devices or to improve performance of the existing devices. Furthermore,

⁎ Corresponding author. E-mail address: [email protected] (K. Marimuthu).

http://dx.doi.org/10.1016/j.saa.2017.05.048 1386-1425/© 2017 Elsevier B.V. All rights reserved.

these radiative properties exhibit considerable dependency on the symmetry and structure of the ligand environment around the RE3+ ions and the phonon energy of the glass host [5,6]. Hence, radiative properties of the RE3 + ions doped glasses can be optimized by varying the glass composition, pumping wavelength and RE3+ ion concentration. The materials doped with Sm3+ ions are most interesting to analyze the fluorescence properties when compared to the other RE3+ ions because of their practical importance in the fields of color displays, solid state lasers, high-density memory devices and undersea communications etc., [7]. Further, Sm3+ ions exhibit four different emission bands in the visible region corresponding to the 4G5/2 → 6H5/2, 6H7/2, 6H9/2 and 6H11/2 transitions. Furthermore, emission corresponding to the 4 G5/2 level of the Sm3+ ions exhibit higher quantum efficiency due to the fact that phonon energy of the borate glasses (1300–1600 cm−1) is very much lower than the energy gap (~ 7000 cm−1) between the 4 G5/2 emission level and the next lower level of Sm3+ ions [7] hence multi-phonon non-radiative decay is negligible in the case of Sm3 + ions. It is important to find suitable host material for RE3+ ion doping because of the fact that luminescence properties exhibit considerable

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R. Nagaraj et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 185 (2017) 139–148

dependency on the nature of the ligand environment and the phonon energy of the host matrix. Among the several glass hosts, borate glasses are appropriate for the doping of RE3+ ions due to their peculiar properties like high bond strength, moisture resistance, low melting point, high refractive index, high transparency, coordination geometry, high thermal stability and high rare earth ion solubility [8]. Addition of lithium enhances the mechanical stability there by creating vacancies in the borate network and reducing its hygroscopic nature [9]. Further, incorporation of Nb2O5 into the borate glass improves their thermal stability, vitrification and optical non-linearity. Furthermore, addition of ZnO into the borate network possess some unique properties like non-hygroscopic nature, direct wide band gap, non-toxicity and intrinsic emitting properties which make them promising materials for non-linear and photonic applications [10]. PbO mixed borate glasses are advantageous due to their peculiar properties such as excellent infrared transmission capacity, high non-linear optical susceptibility, high polarizability and moisture resistant capability [11,12]. Sailaja et al. [13] reported the physical, structural and spectroscopic investigations on Sm3+ doped ZnO mixed alkaliborate glass. The role of the network modifier PbO in Sm3+ doped borate glasses has been studied by Krause et al. [14]. The spectroscopic properties of Sm3+ doped germanotellurite glasses were studied and reported by Wang et al. [15]. Swapna et al. [14] studied the optical properties of Sm3 + ions doped zinc alumino bismuth borate glasses. Ramteke et al. [16] have investigated the concentration effect of Sm3+ ions on structural and luminescence properties of lithium borate glasses. Spectroscopic and laser properties of Sm3+ ions doped lithium fluoroborate glasses have been studied by Aamed et al. [17]. Still, the luminescence properties of Sm3+ doped alkaliborate glasses have to be improved for the design and development of new efficient optoelectronic devices. The aim of the present study is to (i) synthesis Sm3+ doped alkaliborate glasses, (ii) explore the presence of various functional groups through FTIR spectral analysis, (iii) determine the bonding parameters (βand δ) to claim covalent/ionic nature of the metal-ligand bond in the prepared glasses, (iv) determine the optical band gap energy, band tailing parameter and Urbach's energy and the fundamental absorption edge, (v) evaluate the Judd-Ofelt (JO) parameters Ωλ (λ = 2, 4 and 6) using JO theory and to compare the trends of Ωλ with respect to other reported Sm3+ doped glasses, (vi) determine radiative properties for the significant energy levels and to compare the results with similar Sm3+ systems and finally (vii) to study the excited state dynamics of 4G5/2 energy level of the Sm3+ ions in the studied glasses through lifetime analysis and to compare the results with similar studies.

2. Experimental The Sm3 + doped alkaliborate glasses with the composition (65– x)B2O3 + 15Li2O + 12K2O + 8Nb2O5 + xSm2O3 (where, x = 0.1, 0.25, 0.5, 0.75 and 1 wt%) have been synthesized by melt quenching technique following the procedure reported in literature [18]. The prepared glass samples were labeled as BNL0.1Sm, BNL0.25Sm, BNL0.5Sm, BNL0.75Sm and BNL1Sm depending upon the Sm3+ ion concentration. The refractive indices were measured at sodium wavelength (5893 A°) using Abbe refractometer keeping mono-bromonapthaline as a contact liquid. The density of the studied glasses has been measured employing Archimedes principle, where xylene was used as an immersion liquid. The selection of the immersion liquid depends on host glass chemical durability and water resistivity. Certain compositions react with particular solutions easily even at room temperature. Borate glasses are not highly durable that may absorb water as soon as immersed. It will drastically change the structural and optical properties of the borate glass samples. Since they are amorphous in nature, it is easy to leach out ions from the glasses. Hence, the immersion liquid should not react with the sample. As Xylene evaporates easily, thus most of it goes into the air and gets broken by sunlight into the other

chemicals. Therefore, Xylene was used as immersion liquid to measure density of prepared glass samples. X-ray diffraction measurements were carried out using JEOL 8030C X-ray diffractometer using CuKα radiation to confirm the amorphous nature. Perkin-Elmer paragon 500 FTIR spectrometer has been employed to record the infrared spectra in the spectral region of 500– 4000 cm−1. The absorption spectral measurements were made in the wavelength region 375–1800 nm using CARY 500 UV–Vis-NIR spectrometer with a spectral resolution of ± 0.1 nm. Perkin-Elmer LS55 spectrophotometer was used to record the emission spectra with a spectral resolution of ±0.1 nm. The decay measurements were made employing Sciencetech modular spectrometer using xenon flash lamp as an excitation source. The physical properties of the prepared glasses were determined/calculated and the results are presented in Table 1. It is observed from Table 1 that the density increases upto 0.25 wt% of Sm3+ ion concentration and then decreases for 0.5 and 0.75 wt% of Sm3+ ion. The addition of small amount of Sm2O3 into borate glass network initially may control the creation of non-bridging oxygens (NBO's) and hence the density increases. Further increase in Sm2O3 concentration leads to the creation of NBO's resulting decrease in glass density [19]. At higher Sm3+ ion concentration (1 wt%), the molar volume suddenly falls to a lower value due to the decrease in the inter-atomic distance leads to the structural change in the glass network. This decreasing inter-atomic distance may increase stretching force constant of the bonds within the glass network resulting in high density glass [20]. Also, it is observed from Table 1 that the molar volume varies inversely with the density as expected. It is observed from Table 1 that, while increasing the concentration of the Sm3 + ions in the studied glasses, the polaron radius, interionic distance decreases whereas the field strength is found to increase linearly from 0.338 to 1.772. 3. Results and discussion 3.1. Structural analysis Fig. 1 shows the XRD pattern of one of the prepared glasses (BNL0.5Sm) as a representative case. It is observed from the figure that a broad hump at lower angles authenticates the presence of short range ordering in the studied glasses which is the characteristics of non-crystalline nature. The FTIR spectra of the studied glasses recorded in the region 500–4000 cm−1 were shown in Fig. 2. The observed band positions and their band assignments are presented in Table 2. The structural network of the borate glasses consists of both BO3 and BO4 units and their proportion is influenced by the modifier content (Sm3 + ions) [21]. The band observed around ~ 716 cm−1 is ascribed to the B–O–B bending vibrations in BO4 groups and its intensity increases with the increase in Sm3 + ion concentration which results lack in the formation of B\\O rings in the prepared glasses [17]. The band centered at ~1024 cm−1 is attributed to the B\\O bond stretching in BO4 units in the studied glasses [22]. The stretching vibrations of B\\O bonds in BO3 units have been confirmed through the band observed around ~ 1380 cm−1 [23]. It is observed from Fig. 2 that the intensity of the bands around 1024 cm− 1 and 1380 cm− 1 corresponds to the bending and stretching vibrations of B\\O bonds in BO4 and the BO3 units decreases due to the reduction of B2O3 content in the prepared glasses which is already confirmed through the band observed around ~ 716 cm−1. The two weak bands observed around ~ 2858 cm− 1 and ~2927 cm−1 originate from hydrogen bonding and the broad band observed around ~3420 cm−1 is attributed to fundamental stretching of O\\H bond in the prepared glasses [22]. 3.2. Optical analysis 3.2.1. Absorption spectra and bonding parameters The absorption spectrum of the BNL1Sm is shown in Fig. 3 as a representative case and the spectrum consists of eleven inhomogeneously


141

Table 1 Physical properties of the Sm3+ doped alkali borate glasses. Physical Properties

BNL0.1Sm

BNL0.25Sm

BNL0.5Sm

BNL0.75Sm

BNL1Sm

Density ρ (g/cm3) Refractive index nd (589.3 nm) RE ion concentration N (1020 ions/cm3) Polaron radius rp (A°) Inter ionic distance ri (A°) Field strength F (1014 cm−2) Electronic polarizability αe (10−22 cm3) Molar refractivity Rm (cm3) Dielectric constant (ε) Reflection losses R (%) Molar volume V (cm3)

2.809 1.617 0.378 12.00 29.79 0.338 22.120 1.869 2.615 5.559 31.831

3.041 1.621 1.019 8.62 21.96 0.655 8.244 1.735 2.628 5.614 29.544

3.005 1.623 1.998 6.89 17.10 1.025 4.216 1.760 2.634 5.641 30.136

2.792 1.626 2.763 6.18 15.35 1.273 3.060 1.902 2.644 5.683 32.692

3.467 1.630 4.539 5.24 13.01 1.772 1.872 1.540 2.657 5.738 26.534

broadened absorption bands centered at 403, 420, 440, 473, 947, 1084, 1231, 1375, 1481, 1588 and 1959 nm corresponding to the transitions from the 6H5/2 ground state to the various excited states 4 L 15/2 + 4 L 13/2 + 4F7/2 + 6P3/2, 4M19/2 + 6P5/2, 4G11/2 + 4 M17/2 + 4 F5/2, 4I13/2 + 4I11/2 + 4 M15/2 + 4I9/2, 6F11/2, 6F9/2, 6F7/2, 6F5/2, 6F3/2 + 6 H15/2, 6F1/2 and 6H13/2 respectively. The transitions from the 6H5/2 ground state to the 6H and 6F terms are spin allowed transitions (|ΔS| = 0) [24]. It is observed from the absorption spectrum that the combined transition 6 H5/2 → 4 L15/2 + 4 L13/2 + 4F7/2 + 6P3/2 is found to be more intense when compared to all other transitions. Among all the transitions, intensity of the 6H5/2 → 6F1/2, 6F3/2, 6F7/2, 6F9/2 transitions are found to be sensitive to the nature of the ligand field environment around the RE ion site thus called as hypersensitive transitions because they obey the selection rules |ΔS| = 0, |ΔL| ≤ 2 and |ΔJ| ≤ 2. When RE3+ ions are doped into the glass matrices, there is a shift in their energy level positions due to the overlapping of the 4f electronic orbitals of the RE ions with the oxygen orbitals thus changes the RE ion energy level positions and provide information about the nature of the metal-ligand bond. To explore the bonding nature of the RE3+ ion and the oxygen ligands in the studied glasses, bonding parameter (δ) values were calculated from the absorption spectra using the below given expression [25], δ¼

1−β 100 β

ð1Þ

the metal-ligand will be ionic/covalent depending upon the negative or positive sign of δ. The δ values of the studied glasses are calculated and presented in Table 3 and the negative sign of the δ values indicates the ionic nature of the Sm\\O bond in the studied glasses. The ionicity is found to gradually increase with the increase in Sm3+ ion concentration in the studied glasses. The addition of rare earth ions into the glass matrix affects the ligand network significantly and as a result there is a decrease in number of bridging oxygen and increase in non-bridging oxygen (NBO's) which is primarily responsible for changes in the bonding parameter values [26,27]. The increase in ionicity with the addition of Sm3+ ion content is due to the increasing bonding defect and non-bridging oxygen (NBO's) in the studied glass network. 3.2.2. Optical band gap and Urbach's energy studies The analysis of optical absorption spectra over a wide range of photon energy is one of the beneficial tools for understanding the electronic band structure of crystalline as well as amorphous materials. In the present work, absence of sharply defined absorption edges in the absorption spectra illustrates the glassy nature. The dependency of absorption coefficient (α(ν)) on photon energy (hν) can be expressed according to Mott and Davis theory [28] as, n αhν ¼ B hν−Eopt

ð2Þ

where, β is the average value of Nephelauxetic ratios (β). The Nephelauxetic ratio (β = νc/νa) can be referred as the ratio of wave number of a particular RE ion transition (νc) in the glass matrix to the corresponding aquo-ion transition (νa). The nature of the bonding between

where, Eopt is the band gap energy, B is the band tailing parameter and n is the index number that can have the values n = 2 for direct allowed transitions and n = 1/2 for indirect allowed transitions. The band gap values of the studied glasses were obtained by extrapolating the linear region of Tauc's curve [29] plotted between (αhναhν)n and hν as

Fig. 1. XRD pattern of the BNL0.5Sm glass.

Fig. 2. FTIR spectra of the Sm3+ doped alkali borate glasses.

142


Table 2 FTIR band positions and their band assignments of the Sm3+ doped alkali borate glasses. Sl. No.

BNL 0.1Sm

BNL 0.25Sm

BNL 0.5Sm

BNL 0.75Sm

BNL 1Sm

Assignments

1 2 3 4 5 6

3420 2925 2858 1387 1030 718

3431 2927 2858 1380 1024 716

3410 2927 2849 1380 1024 716

3420 2927 2858 1380 1015 714

3420 2927 2849 1380 1024 713

Fundamental stretching of O\ \H bond Hydrogen bonding

shown Fig. 4. The direct and indirect band gap values calculated for the studied glasses are given in Table 4 along with the band tailing parameters. It is observed from the table that both direct and indirect Eopt values decreases with the increase in Sm3+ ion content in the prepared glasses showing the tendency towards semiconducting behavior. The excess presence of Sm3+ ions in the studied glasses act as modifiers, hence leads to decrease the band gap values by increasing the concentration of non-bridging oxygens (NBO's) [30]. The NBO's are quite attributed to the valence band maximum (VBM) and they shift the VBM towards the conduction band. This decrease in Eopt values would also account for the decrease in the average bond energy of the system which in turn decreases the energy of the conduction band minimum. The information about disorder effects in the amorphous materials can be obtained from the shape of the absorption edge in the exponential (Urbach) region. The lack of long-range periodicity in glass materials is related to the tailing of density of states. At lower absorption region (α b 104), absorption coefficient (α) exhibits exponential dependency on the photon energy and it can be expressed by Urbach's rule as [31], hν α ðνÞ ¼ α0 exp ΔE

ð3Þ

where, α0 is a constant and ΔE is the Urbach's energy which is determined by taking the reciprocal of the slope of the curves drawn between lnα and the photon energy. The obtained Urbach's energy values are presented in Table 4. It is observed from the table that the ΔE values of the studied glasses increases with the addition of Sm3+ ion content into the borate glass and it varies inversely with the band gap values. The results show that with Sm3+ concentration higher is the ΔE, consequently more defects into glassy structure decreasing the band gap. 3.2.3. Oscillator strengths and Judd-Ofelt intensity parameters The intensity of the absorption transitions originate from the ground state of the Sm3+ ions in the borate glass matrix can be expressed in

Fig. 3. UV–Vis-NIR absorption spectrum of the BNL1Sm glass.

Stretching vibrations of B\ \O bonds in BO3 units B\ \O bond stretching in BO4 units B\ \O\ \B bending vibrations in BO4 groups

terms of oscillator strengths. The experimental oscillator strength (ƒexp) values can be measured by integrating the areas under each absorption band of the absorption spectrum using the below given expression [32], f¼

2:303mc2 ∫εðν Þdν ¼ 4:318 10−9 ∫εðνÞdν Nπe2

ð4Þ

where, N is the Avogadro's number, m is the mass of an electron, c is the velocity of light, ε(ν) is the molar absorptivity of the band at a wave number ν (cm−1) and e is the charge of an electron respectively. The JO theory [33,34] has been applied to obtain the calculated oscillator strength (ƒcal) of the electric dipole transitions from the ground state (ΨJ) to the excited state (Ψ'J') within the 4f configuration and the same can be expressed as, 2 # " 2 0 0 2 n þ2 8π2 mcν : ∑λ¼2;4;6 Ωλ ΨJU λ Ψ J 3hð2J þ 1Þ 9n

f cal ¼

ð5Þ

where J is the total angular momentum of the ground state, h is the planck's constant, n is the refractive index of the prepared glasses, Ωλ (λ = 2, 4 and 6) are the JO intensity parameters and ║U2║2, ║U4║2and ║U6║2are the doubly reduced square matrix elements and their values were taken from the literature reported by Carnall et al. [35]. The least square fitting approximation has been applied between the experimental and calculated oscillator strengths to obtain JO intensity parameters [33,34]. The ƒexp and ƒcal values are shown in Table 5 along with the root mean square (σrms) deviation. The lower σrms values indicate the validity of the JO theory and the goodness of the fit between ƒexp and ƒcal values. The Ω2 intensity parameter gives information pertaining to the symmetry of the ligand environment in which RE3+ ions are situated and the covalency of the RE\\O bond. The bulk properties of the studied glasses such as viscosity and rigidity can be inferred from the Ω4 and Ω6 intensity parameters respectively [6]. The Table 6 shows the JO parameter values calculated for all the studied glasses with the reported Sm3 + glasses. The JO intensity parameters follow the trend as Ω4 N Ω6 N Ω2 for all the studied glasses. It is observed from the table that the Ω2 values of the studied glasses are found to be lower than the reported TWSm10 [36], PNABZSm [37], ZBBSm [38], PKBAS [39] and ZBNALSm [40] glasses. The lower Ω2 values indicate the fact that the Sm3+ ions are located at the higher symmetrical ligand environment and the Sm\\O bonds possess less covalency (more ionic) in nature. The optical basicity of the glass matrix influences the covalency of the Sm\\O bond significantly. The addition of Sm3+ ions will affect the basicity of the host glass matrix by two ways. i) Sm3+ ions modify the basicity of the glass matrix by themselves ii) addition of Sm3 + ions changes the structure of the glass network. The addition of Sm3+ ions into the borate glasses leads to structural change from BO4 units to BO3 units containing more number of non-bridging oxygens and the conversion is more at higher concentration. The group basicity of BO3 structural units is smaller than the BO4 units and the optical basicity of the glass matrix may also decreases significantly [41,42]. Hence, the replacement of B2O3 by Sm2O3 content and the conversion of structural units from BO4 to BO3 will be more at 1 wt% of Sm3+ ion concentration and the same may be the reason for the sudden decrease in covalency of


143

Table 3 Observed band positions (in cm−1) and bonding parameters (βand δ) of the Sm3+ doped alkali borate glasses. Transition

BNL 0.1Sm

BNL 0.25Sm

BNL 0.5Sm

BNL 0.75Sm

BNL 1Sm

Aqua-ion [18]

6

6286 6749 7262 8120 9228 10,564 21,290 22,702 – 24,812 1.0055

6297 6752 7278 8121 9248 10,573 21,106 22,714 23,852 24,798 1.0057

6295 6756 7271 8126 9263 10,546 21,101 22,781 23,787 24,725 1.0057

6298 6756 7269 8118 9217 10,552 21,175 22,723 23,795 24,742 1.0060

6290 6746 7271 8122 9225 10,535 21,410 – 23,807 24,759 1.0080

6400 6600 7100 8000 9200 10,500 21,100 22,700 24,050 24,950 –

−0.551

−0.562

−0.567

−0.591

−0.796

–

F1/2 F3/2 + 6H15/2 F5/2 6 F7/2 6 F9/2 6 F11/2 4 I13/2 + 4I11/2 + 4 M15/2 + 4I9/2 4 G11/2 + 4 M17/2 + 4F5/2 6 P5/2 + 4 M19/2 4 L15/2 + 4 L13/2 + 4F7/2 + 6P3/2 6 6

β δ

the Sm\\O bonds (or Ω2 values) which is already inferred from the bonding parameter values. It is observed from Table 6 that the Ω4 and Ω6 values of the RE doped glasses are initially increases with the addition of Sm3+ ion concentration and then decrease for higher concentration of Sm3+ ions. This indicate the fact that the viscosity and rigidity of the present glasses initially increases may be due to the increasing ionic packing ratio (which is related to rigidity) of the glass up to particular concentration of doping ions and then decreases as a result of vibronic transitions of the RE ions bound to the ligand atoms [43,44]. 3.2.4. Luminescence spectra and radiative properties Fig. 5 shows the excitation spectrum of one of the studied glasses BNL0.5Sm recorded in the wavelength region 325–550 nm by fixing the emission wavelength at 601 nm. The spectrum exhibit eight excitation transitions originate from the 6H5/2 ground state to the various excited states such as 4H9/2, 4D3/2, 4D1/2, 4F7/2, 6P5/2, 4G9/2, 4I13/2 + 4I11/2 and 4 F3/2 centered at 345, 363, 376, 404, 419, 439, 474 and 528 nm respectively. Among all the excitation bands, the band centered at 404 nm corresponding to the 6H5/2 → 4F7/2 transition is found to be higher in intensity and the same is used as a pumping wavelength to record the emission spectra of the studied glasses. In order to obtain the radiative properties of Sm 3 + ions doped alkaliborate glasses, luminescence spectra have been recorded in the wavelength range 530–730 nm by keeping excitation at 404 nm and the same is shown in Fig. 6. The spectra exhibit four emission bands at around 565, 601, 647 and 709 nm attributed to

the 4 G 5/2 → 6 H5/2 , 4 G 5/2 → 6 H7/2 , 4 G 5/2 → 6 H 9/2 and 4 G 5/2 → 6 H11/2 transitions respectively. The reddish-orange emission observed at 601 nm correspond to the 4 G5/2 → 6 H7/2 band is magnetic dipole allowed transition because it obeys the selection rule | ΔJ | = ± 1 but the contribution of electric dipole transition is predominant and it possesses maximum intensity compared to the other emission bands. The 4G5/2 → 6H5/2 and 4G5/2 → 6H9/2 transitions are magnetic and electric dipole transitions respectively. It is observed from the luminescence spectra that the luminescence intensity increases with the increase in Sm3 + ion content up to 0.5 wt% beyond that it decreases due to luminescence quenching resulting from the energy transfer process taking place between the nearby Sm3 + ions in the studied glasses. The intensity ratio of the 4G5/2 → 6H9/2 electric dipole transition to that of the 4G5/2 → 6H5/2 magnetic dipole transition (Orange/Red intensity ratio) gives information pertaining to the symmetry of the ligand environment around the Sm 3 + ion site. The obtained O/R intensity ratio values are found to be 1.549, 1.624, 1.753, 1.581 and 1.286 corresponding to the BNL0.1Sm, BNL0.25Sm, BNL0.5Sm, BNL0.75Sm and BNL1Sm glasses respectively. The lower intensity ratio values indicate the higher symmetry ligand field around the vicinity of the Sm3 + ion site. The Judd-Ofelt intensity parameters along with the refractive index values have been used to determine the radiative properties such as transition probability (AR), branching ratios (βR), effective line width (Δ λeff), and stimulated emission cross-section (σEP) of the studied glasses using the expressions reported in literature [44]. Such radiative parameters calculated for all the 4G5/2 → 6H5/2, 6H7/2, 6H9/2 and 6H11/2 emission transitions of the studied glasses are presented in Table 7. The branching ratio (βR) gives the probability of obtaining laser action from a particular transition and is used to characterize the distribution of emissive transitions in the emission spectra. When a transition possesses the value of βR N 0.5, then most of the emission radiation will concentrate on the corresponding transition and it can be utilized for laser generation. In the present study, for all the studied glasses 4G5/2 → 6H7/2 transition exhibit highest βR value centered at 601 nm thus indicate that efficient laser emission can be obtained at 601 nm. The energy extraction efficiency can be obtained from the emission spectra through the evaluation of stimulated emission cross-section (σEP) using JO theory. It Table 4 Direct and indirect band gap energies (Eg), band tailing parameter (B) and Urbach's energies of the Sm3+ doped alkali borate glasses. Sample code

Fig. 4. Tauc's plot for direct allowed transitions of the Sm3+ doped alkali borate glasses [Inset shows the Tauc's plot for indirect allowed transitions].

BNL0.1Sm BNL0.25Sm BNL0.5Sm BNL0.75Sm BNL1Sm

Direct allowed transitions

Indirect allowed transitions

Eg (eV)

B (cm−2 eV)

Eg (eV)

B (cm−2 eV)

3.43 3.42 3.41 3.29 3.26

1600.76 1632.78 1939.83 1885.19 2708.85

3.22 3.20 3.19 3.09 3.08

23.60 23.18 23.03 31.82 27.82

Urbach's energy ΔE (eV)

0.3932 0.4000 0.4120 0.4225 0.4242

144


Table 5 Experimental and calculated oscillator strengths (×10−6), number of absorption bands (N) and root mean square deviation (σrms) of the Sm3+ doped alkali borate glasses. Transitions

BNL 0.1Sm

6

F1/2 F3/2 + 6H15/2 F5/2 6 F7/2 6 F9/2 6 F11/2 4 I13/2 + 4I11/2 + 4 M15/2 + 4I9/2 4 G11/2 + 4 M17/2 + 4F5/2 6 P5/2 + 4 M19/2 4 L15/2 + 4 L13/2 + 4F7/2 + 6P3/2 N σrms 6 6

BNL 0.25Sm

BNL 0.5Sm

BNL 0.75Sm

BNL 1Sm

fexp

fcal

fexp

fcal

fexp

fcal

fexp

fcal

fexp

fcal

0.048 1.816 1.902 3.593 2.334 2.678 0.679 1.502 − − 8 ±0.602

0.508 1.174 1.858 3.557 2.469 0.775 0.172 3.740 − −

0.0526 1.905 1.844 3.773 2.377 0.249 1.568 0.230 0.254 2.473 10 ±0.504

0.510 1.226 1.976 3.667 2.528 0.415 0.787 0.177 0.000 3.964

0.073 1.852 1.850 3.692 2.466 0.361 1.698 0.119 0.263 2.847 10 ±0.431

0.495 1.221 1.986 3.669 2.526 0.413 0.776 0.176 0.000 3.985

0.046 1.690 1.508 3.139 2.297 0.318 1.349 0.088 0.187 1.961 10 ±0.415

0.482 1.045 1.606 3.186 2.238 0.370 0.699 0.156 0.000 3.213

0.032 0.705 0.705 1.572 0.923 0.146 0.575 0.112 0.434 − 9 ±0.255

0.209 0.454 0.701 1.476 1.055 0.175 0.333 0.178 1.410 −

is observed from the Table 7 that among the prepared glasses, BNL0.5Sm glass exhibit higher σEP value (7.26 × 10− 22 cm2) for the 4 G5/2 → 6H7/2 transition and is higher than the reported lithium fluoroborate (5.744 × 10− 22 cm2) [45], zinc borate (3.310 × 10−22 cm2) [24], calcium phosphate (5.080 × 10−22 cm2) [46] and bismuth borate (6.950 × 10−22 cm2) [47] glasses. The radiative (τcal) and experimental (τexp) lifetimes and the quantum efficiency (η) of the Sm3+ doped alkali borate glasses are presented in Table 8. For an efficient optical amplifier, the magnitude of the two essential parameters such as optical gain (σEP × τexp) and gain bandwidth (σEP × Δλeff) should be on the higher side [6]. The (σEP × τexp) and (σEP × Δλeff) values were calculated for the 4G5/2 → 6H7/2 transition of the BNLxSm glasses due to their versatility and the values are found to be 12.07, 10.49, 10.37, 8.81, 4.14 (× 10− 25 cm− 2 s) and 83.17, 84.70, 86.84, 73.20, 38.87 (× 10−22 nmcm2) corresponding to the BNL0.1Sm, BNL0.25Sm, BNL0.5Sm, BNL0.75Sm, BNL1Sm glasses respectively. Among all the studied glasses, the important radiative parameters such as A, σEP, βR and gain bandwidth (σEP × Δλp) are found to be higher for the BNL0.5Sm glass pertaining to the 4G5/2 → 6H7/2 transition and it can be recommended as a suitable candidate for the fabrication of visible solid state lasers as well as optical amplifiers.

simulation required to match the color of the given spectral power density (P(λ)) can be obtained using the below given three functions, X ¼ ∫xðλÞP ðλÞdλ

ð6Þ

Y ¼ ∫yðλÞP ðλÞdλ

ð7Þ

Z ¼ ∫zðλÞP ðλÞdλ

ð8Þ

where X, Y and Z are the tristimulus values called artificial colors give the simulation for each one of the three primary colors (blue, green and red) to match the color of given spectral power density (P(λ)). The x, y chromaticity coordinates of the studied glasses can be estimated from the tristimulus values using the below given expression [48], x¼

X XþYþZ

ð9Þ

y¼

Y XþYþZ

ð10Þ

The calculated chromaticity coordinates (x,y) are found to be (0.566, 0.406), (0.575, 0.405), (0.573, 0.405), (0.528, 0.416) and (0.508, 0.421) corresponding to the BNL0.1Sm, BNL0.25Sm, BNL0.5Sm, BNL0.75Sm

3.3. CIE chromaticity diagram The color of any visible light emission can be illustrated with the combination three primary colors using the CIE (Commission International d'Eclairage) 1931 chromaticity diagram. The standard equal energy point (x = 0.33, y = 0.33) attributed to the white light emission is always located at the center of the CIE 1931 diagram. To examine the color of any light source, three dimensionless quantities called color matching functions ( xðλÞ , yðλÞ , zðλÞ ) are required. The degree of

Table 6 Judd-Ofelt intensity parameters (Ωλ × 10−20 cm2) and its trends of Sm3+ doped alkali borate glasses. Glass code

Ω2

Ω4

Ω6

Trends of Ωλ

References

BNL0.1Sm BNL0.25Sm BNL0. 5Sm BNL0.75Sm BNL1Sm TWSm10 PNABZSm ZBBSm PKBASm ZBNALSm

1.514 1.512 1.464 1.427 0.614 2.01 2.61 1.93 4.49 2.06

2.209 3.411 3.431 2.741 1.192 4.38 3.08 1.87 7.36 2.55

2.772 2.814 2.801 2.507 1.182 1.56 1,65 1.79 3.87 1.63

Ω4˃ Ω6˃ Ω2 Ω4˃ Ω6˃ Ω2 Ω4˃ Ω6˃ Ω2 Ω4˃ Ω6˃ Ω2 Ω4˃ Ω6˃ Ω2 Ω4˃ Ω6˃ Ω2 Ω4˃ Ω6˃ Ω2 Ω4˃ Ω6˃ Ω2 Ω4˃ Ω6˃ Ω2 Ω4˃ Ω6˃ Ω2

Present Present Present Present Present [36] [37] [38] [39] [40] Fig. 5. Excitation spectrum of the Sm3+ doped BNL0.5Sm glass.


145

Fig. 6. Luminescence spectra of the Sm3+ doped alkali borate glasses.

and BNL1Sm glasses respectively. The emission spectra were converted into the CIE 1931 chromaticity diagram to examine the dominant emission color of the present Sm3+ doped alkaliborate glasses and the same are shown in Fig. 7. The color coordinates of the Sm3+ doped glass materials exhibit considerable dependency on the O/R intensity ratio values. It is observed from Fig. 7 that the x,y coordinates of the BNL0.1Sm glass sample are found to lie in the reddish-orange region and shift towards red region with the increase of Sm3+ ions concentration up to 0.5 wt% along with the O/R intensity ratio values. Finally, color coordinates were back to reddish-orange region for higher Sm3+ ion

concentration due to the fall in O/R intensity ratio values. The color coordinates of the present glasses vary similar to the reported literature [49]. Hence, the present Sm3+ doped alkaliborate glasses can be used as promising materials for the fabrication of visible light emitting devices. 3.4. Luminescence decay and energy transfer mechanism in Sm3+ ions Fig. 8 shows the decay curves of the luminescence originates from the 4G5/2 level of Sm3+ ions recorded by monitoring an excitation at

Table 7 Peak wavelength (λp, nm), effective line width (Δλeff, nm), radiative transition probability (A, s−1), peak stimulated emission cross-section (σEP ×10−22 cm2), experimental and calculated branching ratios (βR) of the Sm3+ doped alkali borate glasses. Transitions 4G5/2 →

Parameters

BNL0.1Sm

BNL0.25Sm

BNL0.5Sm

BNL0.75Sm

BNL1Sm

6

H5/2

6

H7/2

6

H9/2

6

H11/2

λp Δλeff Α σEP βR(Exp) βR(Cal) λp Δλeff Α σEP βR(Exp) βR(Cal) λp Δλeff Α σEP βR(Exp) βR(Cal) λp Δλeff Α σEP βR(Exp) βR(Cal)

565 11.74 22.25 0.98 0.1714 0.0810 601 11.63 125.35 7.15 0.5040 0.4561 647 10.18 68.76 6.01 0.2997 0.2502 709 11.25 29.39 3.35 0.0249 0.1069

565 11.69 22.50 0.99 0.1884 0.0795 601 11.79 128.50 7.18 0.5067 0.4540 647 10.46 71.93 6.09 0.2812 0.2541 709 10.79 29.91 3.40 0.0238 0.1057

565 10.69 22.35 1.07 0.1687 0.0791 601 11.97 131.96 7.26 0.5138 0.4534 647 10.45 72.18 6.11 0.2956 0.2556 709 10.79 29.10 3.44 0.0220 0.1030

565 10.66 21.53 1.03 0.1869 0.0880 601 11.62 111.56 6.30 0.4991 0.4560 647 10.32 59.43 5.07 0.2845 0.2429 709 9.83 24.84 3.21 0.02958 0.1015

565 10.56 19.47 0.94 0.1807 0.1301 601 11.66 59.60 3.33 0.5142 0.3983 647 10.01 37.43 3.27 0.2855 0.2501 709 7.70 11.58 1.90 0.0196 0.0774

146


Table 8 The calculated and experimental lifetimes (ms), quantum efficiency η (%), energy transfer parameter (Q), critical transfer distance R0 (×10−8 m) and donor-acceptor interaction parameter CDA (×10−44 cm6/s) of the Sm3+ doped alkali borate glasses. Parameters

BNL0.1Sm

BNL0.25Sm

BNL0.5Sm

BNL0.75Sm

BNL1Sm

τcal (ms) τexp(ms) η Q Ro CDA

3.55 1.69 47% – – –

3.41 1.46 43% 0.531 8.894 28.89

3.40 1.43 42% 0.582 8.682 24.98

3.39 1.39 41% 0.717 7.049 7.16

3.16 1.24 39% 0.779 6.143 3.14

404 nm and emission at 601 nm. It is clearly observed from the figure that the luminescence of the 0.1 wt% Sm3+ ions containing glass decays exponentially with a lifetime of 1.69 ms and it becomes non-exponential for higher Sm3 + ions concentration (N0.1 wt%) because of the contribution of ion-ion interaction. The effective decay time can be determined by subjecting the decay curves of the studied glasses to exponential and non-exponential curve fitting methods using the expressions given below [50], It ¼ I0 e−t =τ ðexponentialÞ

ð11Þ

I¼ A1 expð‐t=τ1 Þ þ A2 expð‐t=τ2 Þ ðnon‐exponentialÞ

ð12Þ

where I0 and It are the emission intensities at time t = ‘0’ and at ‘t’ respectively, τ is the lifetime of the excited level. A1 and A2 are the decay constants, τ1 and τ2 are the lifetimes of the two channels involved in the decay processes. The experimental lifetime (τexp) values of the non-exponential decay curves can be estimated using the below given equation,

τexp ¼

A1 τ21 þ A2 τ 22 ðA1 τ 1 þ A2 τ2 Þ

ð13Þ

The obtained τexp values of the BNLxSm glasses are presented in Table 8 along with the theoretically calculated lifetime values. The

Fig. 8. Exponential and non-exponential fitted decay curves of the Sm3+ doped alkali borate glasses [Inset shows the IH fit for the BNL1Sm glass].

τ exp values of the studied glasses are found to decrease gradually with the increase in Sm3 + ion content due to the energy transfer process takes place through cross-relaxation and multi-phonon non-radiative decay [17]. The energy gap between the 4G5/2 level and the next lower lying level is ~ 7250 cm− 1 which is very much higher than the phonon energy of the borate glass (~ 1300 cm− 1) and needs approximately six phonons together to absorb that much energy. Hence, the contribution of multi-phonon decay is less to the energy transfer process and the fall in τexp values is mainly due to the energy transfer process takes place through cross-relaxation between excited (donor) and non-excited (acceptor) Sm 3 + ions. The possible cross-relaxation channels are found to be A: (4G5/2 → 6 F5/2: 6H5/2 → 6F11/2), B: (4G5/2 → 6F7/2 : 6H5/2 → 6F9/2), C: (4G5/2 → 6F9/2: 6H5/2 → 6F7/2) and D: (4G5/2 → 6F11/2: 6H5/2 → 6F5/2) respectively and the same are represented in the energy level diagram

Fig. 7. CIE 1931diagram of the Sm3+ doped alkali borate glasses.


147

(τ−1 o ) of the donors. The donor-acceptor interaction parameter (CDA) of the studied glasses can be obtained from the following expression, CDA ¼

Fig. 9. Partial energy level diagram of the Sm3+ doped alkali borate glasses along with the possible cross-relaxation channels.

of Sm3 + ions and is shown in Fig. 9. During the cross-relaxation process, donor ion gives half of its energy to the acceptor ion and then both of them reach the metastable state. Finally, donor and acceptor ions in the meta-stable state attain the ground state through non-radiative decay [32]. For fibre lasers and planar waveguide applications, the quantum efficiency (η) of the emission transition is an essential factor that determines the performance of a material. The η values of the studied glasses can be obtained as the ratio of the experimental lifetime to the radiative lifetimes of the 4G5/2 excited state and can be written as, η¼

τexp 100 τcal

ð14Þ

The η values calculated for all the studied glasses are presented in Table 8 and it is observed from the table that the η values found to decrease when the Sm3+ ions concentration increases due to the energy transfer process takes place between the donor and acceptor Sm3 + ions in the studied glasses. In the present work, the non-exponential decay curves were fitted into IH model (Inokuti-Hirayama model) [51] to explore the energy transfer process takes place between the donor and acceptor ions and it can be expressed as, (

3=S ) t t Iðt Þ ¼ I 0 exp − −Q τ0 τ0

ð15Þ

where τ0 is the intrinsic decay time of the donors without acceptor ions, t is the time after excitation and S can have the values as 6 for dipole-dipole, 8 for dipole-quadrupole and 10 for quadrupole-quadrupole interactions respectively. The Q is the energy transfer parameter can be represented as,

Q¼

4π 3 Γ 1− N0 R30 3 S

ð16Þ

where, the Γ(x) function is equal to 1.77, 1.43 and 1.3 corresponding to the dipole-dipole, dipole-quadrupole and quadrupole-quadrupole interactions respectively, N0 is the acceptor ion concentration per cubic centimeter which is approximately equal to the RE ion concentration and R0 is the critical transfer distance equal to the intrinsic decay rate

RS0 τo

ð17Þ

The non-exponential decay curves of the present BNLxSm glasses exhibit best fit to the IH model for S = 6 which illustrate the fact that the energy transfer process takes place between the nearby Sm3+ ions is due to dipole-dipole interaction in the studied glasses. The intrinsic decay time (τo) value has been obtained by fitting the decay curve of 0.05 wt% Sm3 + ions doped glass following the IH fitting procedure and the τo value is found to be 1.71 ms. The Q, R0 and CDA values determined from the IH fitted curves are presented in Table 8. It is observed from the table that the Q values are found to be higher for glasses containing larger Sm3+ ions concentration thus indicates the occurrence of efficient energy transfer process takes place between the nearby Sm3+ ions in the studied glasses and are similar to the reported Sm3+ doped glasses [52–56]. It is observed that for IH fitting model, s = 6 is well fitted with the non-exponential decay curves than the other two, which confirms that the energy transfer between the Sm3 + ions is due to dipole–dipole interaction in the present work. Further, the donor-acceptor interaction parameter (CDA) and R0 values are found to decrease with the increase in Sm3+ ions concentration. 3.5. Conclusion Alkaliborate glasses doped with varying Sm3+ ions concentration have been synthesized by melt quenching technique. The broad hump observed at the lower angles of the XRD pattern confirms the amorphous nature and the FTIR spectra reveal the presence of stretching and bending vibrations of B\\O bonds in BO3 and BO4 units. The negative sign of the δ values indicates the fact that the Sm\\O bond in the present glasses is of ionic in nature and the ionicity increases due to the increasing bonding defects and non-bridging oxygens in the studied glasses. The band gap values found to decrease as the Sm3+ ion concentration increases due to the formation of non-bridging oxygens and it varies inversely with the Urbach's energy. The lower Ω2 values suggest that the Sm3+ ions are surrounded by higher symmetrical ligand environment and the higher magnitude of Ω4 values confirms the higher rigidity of the studied glasses. The x, y coordinates of the studied glasses passes through the reddish-orange region in the CIE 1931 color chromaticity diagram suggest its suitability for orange light emitting devices. The 4G5/2 → 6H7/2 transition pertaining to the BNL0.5Sm glass exhibit higher A, βR, σEP, gain bandwidth values among all the studied glasses and the same can be suggested as a promising base material for lasers, optical amplifiers and luminescence devices operating in the reddish-orange spectral region. The nonexponential decay curves are well fitted to the IH model for S = 6 thus indicates the fact that the energy transfer process takes place between the Sm3 + ions is due to dipole-dipole interaction thus in turn leads to the luminescence quenching. References [1] Ch. Basavapoornima, C.K. Jayasankar, J. Lumin. 153 (2014) 233–241. [2] B.J. Chen, L.F. Shen, E.Y.B. Pun, H. Lin, Spectrochim. Acta A 60 (2004) 637–642. [3] Sd. Zulfiqar Ali Ahamed, C. Madhukar Reddy, B. Deva Prasad Raju, Spectrochim. Acta Part A 103 (2013) 246–254. [4] D. Umamaheswari, B.C. Jamalaiah, T. Sasikala, L. Il-Gon Kim, Rama Moorthy, J. NonCryst. Solids 358 (2012) 782–787. [5] Fangfang Fu, Baojie Chen, Lifan Shen, Edwin Yue Bun Pun, Hai Lin, J. Alloys Compd. 582 (2014) 265–272. [6] K. Swapna, Sk. Mahamuda, A. Srinivasa Rao, T. Sasikala, L. Rama Moorthy, J. Lumin. 146 (2014) 288–294. [7] Phan Van Do, Vu Phi Tuyen, Vu Xuan Quang, Le Xuan Hung, Luong Duy Thanh, Tran Ngoc, Ngo Van Tam, Bui The Huy, Opt. Mater. 55 (2016) 62–67. [8] F. Zaman, J. Kaewkhao, G. Rooh, N. Srisittipokakun, H.J. Kim, J. Alloys Compd. 676 (2016) 275–285.

148


[9] S.A. Azizan, S. Hashim, N.A. Razak, M.H.A. Mhareb, Y.S.M. Alajerami, N. Tamchek, J. Mol. Struct. 1076 (2014) 20–25. [10] S. Ibrahim, M. Abdel-Baki, Fouad El-Diasty, Opt. Eng. 51 (2012) 093401–093407. [11] Sandhya Rani Pelluri, Rajender Singh, J. Magn. Magn. Mater. 418 (2016) 206–212. [12] Y. Gandhi, P. Rajanikanth, M. Sundara Rao, V. Ravi Kumar, N. Veeraiah, M. Piasecki, Opt. Mater. 57 (2016) 39–44. [13] B. Sailaja, R. Joyce Stella, G. Thirumala Rao, B. Jaya Raja, V. Pushpa Manjari, R.V.S.S.N. Ravikumar, J. Mol. Struct. 1096 (2015) 129–135. [14] S. Krause, C. Pfau, M. Dyrba, P.T. Miclea, S. Schweizer, J. Lumin. 151 (2014) 29–33. [15] F. Wang, B.J. Chen, H. Lin, E.Y.B. Pun, J. Quant. Spectrosc. Radiat. Transf. 147 (2014) 63–70. [16] D.D. Ramteke, V.Y. Ganvir, S.R. Munishwar, R.S. Gedam, Phys. Procedia 76 (2015) 25–30. [17] Sd. Zulfiqar Ali Ahamed, C. Madhukar Reddy, B. Deva Prasad Raju, Spectrochim. Acta, Part A 103 (2013) 246–254. [18] S. Selvi, K. Marimuthu, G. Muralidharan, J. Lumin. 159 (2015) 207–218. [19] B. Eraiah, Bull. Mater. Sci. 29 (2006) 375–378. [20] Awang Asmahani, S.K. Ghoshal, M.R. Sahar, M. Reza Dousti, Raja J. Amjad, Fakhra Nawaz, Curr. Appl. Phys. 13 (2013) 1813–1818. [21] S. Kabi, A. Ghosh, J. Phys. Chem. C 115 (2011) 9760–9766. [22] S. Sailaja, C. Nageswara Raju, C. Adinarayana Reddy, B. Deva Prasad Raju, YoungDahl Jho, B. Sudhakar Reddy, J. Mol. Struct. 1038 (2013) 29–34. [23] K. Swapna, Sk. Mahamuda, A. Srinivasa Rao, S. Shakya, T. Sasikala, D. Haranath, G. Vijaya Prakash, Spectrochim. Acta, Part A 125 (2014) 53–60. [24] Kirti Nanda, R.S. Kundu, Inder Pal, R. Punia, N. Kishore, J. Alloys Compd. 676 (2016) 521–526. [25] S.P. Sinha, Complexes of the Rare Earths, Pergamon, Oxford, 1966. [26] Nawaz Fakhra, M.R. Sahar, S.K. Ghoshal, Asmahani Awang, R.J. Amjad, J. Lumin. 147 (2014) 90–96. [27] Suwat Rakpanic, Jakrapong Kaewkhao, Kittipun Boonin, Jeongmin Park, Hong Joo Kim, Pichet Limsuwan, New J. Glass and Ceramics 3 (2013) 6–10. [28] N.F. Mott, E.A. Davis, Electronic Processes in Non-Crystalline Materials, second ed. Clarendon Press, Oxford, 1979. [29] J. Tauc, Amorphous and Liquid Semiconductors, Plenum Press, New York, 1974 159–220. [30] Fouad El-Diasty, Fathy A. Abdel Wahab, Manal Abdel-Baki, J. Appl. Phys. 100 (2006) 093511-1–093511-7. [31] F. Urbach, Phys. Rev. 92 (1953) 1324. [32] Sk. Mahamuda, K. Swapna, M. Venkateswarlu, A. Srinivasa Rao, Suman Latha Shakya, G. Vijaya Prakash, J. Lumin. 154 (2014) 410–424.

[33] [34] [35] [36] [37] [38] [39] [40] [41] [42] [43] [44] [45] [46] [47] [48] [49] [50] [51] [52] [53] [54] [55] [56]

B.R. Judd, Phys. Rev. 127 (1962) 750–761. G.S. Ofelt, J. Chem. Phys. 37 (1962) 511–520. W.T. Carnall, P.R. Fields, K. Rajnak, J. Chem. Phys. 49 (1968) 4424–4442. G. Venkataiah, C.K. Jayasankar, K. Venkata Krishnaiah, P. Dharmaiah, N. Vijaya, Opt. Mater. 40 (2015) 26–35. Y.K. Sharma, S.S.L. Surana, R.P. Dubedi, V. Joshi, Mater. Sci. Eng. B 119 (2005) 131–135. R.V. Deun, K. Binnemans, C. Gorller-Walrand, J.L. Adam, SPIE Proc. 3622 (1999) 175–181. A. Agarwal, I. Pal, S. Sanghi, M.P. Aggarwal, Opt. Mater. 32 (2009) 339–344. V. Venkatramu, P. Babu, C.K. Jayasankar, Th. Tröster, W. Sievers, G. Wortmann, Opt. Mater. 29 (2007) 1429–1439. R.S. Kirti Nanda, Sarita Sharma Kundu, Devendra Mohan, R. Punia, N. Kishore, Solid State Sci. 45 (2015) 15–22. R.S. Kirti Nanda, Inder Pal Kundu, R. Punia, N. Kishore, J. Alloys Compd. 676 (2016) 521–526. Fakhra Nawaz, Md. Rahim Sahar, S.K. Ghoshal, Asmahani Awang, Ishaq Ahmed, Physica B 433 (2014) 89–95. T. Sasikala, L. Rama Moorthy, A. Mohan Babu, Spectrochim. Acta Part A 104 (2013) 445–450. C.K. Jayasankar, P. Babu, J. Alloys Compd. 307 (2000) 82–95. M. Seshadri, K. Venkata Rao, J.L. Raob, Y.C. Ratnakaram, J. Alloys Compd. 476 (2009) 263–270. F. Wang, B.J. Chen, H. Lin, E.Y.B. Pun, J. Quant. Spectrosc. Radiat. Transf. 147 (2014) 63–70. B. Shanmugavelu, V.V. Ravi Kanth Kumar, J. Lumin. 146 (2014) 358–363. D.R.N. Brito, M.N. Queiroz, M.J. Barboza, A. Steimacher, F. Pedrochi, Opt. Mater. 64 (2017) 114–120. M. Chandra Shekhar Reddy, B. Appa Rao, M.G. Brik, A. Prabhakar Reddy, P. Raghava Rao, C.K. Jayasankar, N. Veeraiah, Appl. Phys. B Lasers Opt. 108 (2012) 455–461. M. Inokuti, F. Hirayama, J. Chem. Phys. 43 (1965) 1978–1989. K. Brahmachary, D. Rajesh, Y.C. Ratnakaram, J. Lumin. 161 (2015) 202–208. Phan Van Do, Vu Phi Tuyen, Vu Xuan Quang, Le Xuan Hung, Luong Duy Thanh, Tran Ngoc, Ngo Van Tam, Bui The Huy, Opt. Mater. 55 (2016) 62–67. K. Kiran Kumar, C.K. Jayasankar, J. Mol. Struct. 1074 (2014) 496–502. J. Suresh Kumar, K. Pavani, T. Sasikala, A. Sreenivasa Rao, Neeraj Kumar Giri, S.B. Rai, L. Rama Moorthy, Solid State Sci. 13 (2011) 1548–1553. K. Linganna, Ch. Basavapoornima, C.K. Jayasankar, Opt. Commun. 344 (2015) 100–105.