Structural and spectroscopic properties of Eu3+-doped zinc fluorophosphate glasses

Structural and spectroscopic properties of Eu3+-doped zinc fluorophosphate glasses

Journal of Molecular Structure 1036 (2012) 42–50 Contents lists available at SciVerse ScienceDirect Journal of Molecular Structure journal homepage:...

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Journal of Molecular Structure 1036 (2012) 42–50

Contents lists available at SciVerse ScienceDirect

Journal of Molecular Structure journal homepage: www.elsevier.com/locate/molstruc

Structural and spectroscopic properties of Eu3+-doped zinc fluorophosphate glasses N. Vijaya, C.K. Jayasankar ⇑ Department of Physics, Sri Venkateswara University, Tirupati 517 502, India

h i g h l i g h t s 3+

" Eu -doped zinc fluorophosphate glasses were prepared by melt quenching method. " Structural and optical properties of the glasses have been investigated. 5

" The glasses show efficient luminescence and longer lifetimes for the D0 level. 3+

" Eu :zinc fluorophosphate glasses could be useful to develop visible red lasers.

a r t i c l e

i n f o

Article history: Received 1 July 2012 Received in revised form 10 September 2012 Accepted 13 September 2012 Available online 20 September 2012 Keywords: Glasses Eu3+ ions Optical properties Judd–Ofelt analysis Radiative properties

a b s t r a c t Zinc fluorophosphate (ZFPEu: P2O5–K2O–Al2O3–ZnF2–LiF–Eu2O3) glasses doped with different Eu3+ ion concentrations have been prepared and characterized through DTA, Raman, absorption, luminescence, excitation, phonon side band spectra and decay measurements at room temperature. An intense red luminescence has been observed due to 5D0 ? 7F2 transition of Eu3+ ions in these glasses. Raman and phonon side band spectroscopic techniques have been used to investigate the local structure around Eu3+ ions and phonon energy of the host, respectively. The analysis of optical intensities based on absorption and luminescence spectra has been performed under different constraints using Judd–Ofelt (JO) theory. The JO intensity parameters have been used to predict the radiative properties such as radiative lifetime, branching ratio, and stimulated emission cross-section for the 5D0 ? 7FJ (J = 0–6) transitions. Decay rates for the 5D0 level of Eu3+ ions has been found to be single exponential for all concentrations. Luminescence properties of the 5D0 ? 7F2 transition of Eu3+ ion revealed that the present ZFPEu glasses may be useful for developing visible red lasers as well as optical display devices at around 611 nm. Ó 2012 Elsevier B.V. All rights reserved.

1. Introduction In the new millennium, extensive research work has been carried out for developing optical devices based on fluorescent material doped with rare earth (RE3+) ions. Indeed, there have been increasing demands for these optical devices including solid-state lasers, fiber amplifiers, infrared to visible up-converters, phosphors, field emission displays, biosensors, solar cells, etc. for variety of applications [1–3]. Due to prominent features of RE3+ ions such as many excited levels suitable for optical pumping, sharp absorption and emission bands from ultra violet (UV) to infrared (IR) spectral range, and longer lifetimes have been well suited for these applications. Good host of glass matrix is very important factor for the development of efficient optical devices doped with RE3+ ions. Among many potential host materials, fluorophosphate glasses are suitable for RE3+ ions owing to their optical properties as they ⇑ Corresponding author. Tel.: +91 877 2248033; fax: +91 877 2289472. E-mail address: [email protected] (C.K. Jayasankar). 0022-2860/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.molstruc.2012.09.037

show combined advantages of fluoride and phosphate glasses such as high transparency, low phonon energy, good moisture resistance, physical and chemical stability, low nonlinear refractive index and high transparency from near UV to mid IR spectral range besides low large-scale production cost [4,5]. The host materials having low phonon energies have been effectively used to increase the efficiency of the radiative emissions [6]. Among the RE3+ ions, Eu3+ ion is of special interest from spectroscopic point of view because of the following reasons: (1) Eu3+ ion is most preferable choice to estimate the local structure of RE3+ ions in glasses due to its relatively simple energy level structure and high sensitivity of its fluorescence on the environment [7]; (2) Since the ground 7F0 state and the fluorescent 5D0 state of Eu3+ ions are non-degenerate under any symmetry, information regarding the local environment around the Eu3+ ion strongly depends only on the splitting of the 5D0 ? 7FJ (J = 0–6) transitions in emission spectra [8]; (3) persistent spectral hole burning can be performed for the 7F0 ? 5D0 transition of Eu3+ ion at room temperature which has potential use in high-density optical data storage [9]; (4) the measurement of the phonon side band

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associated with the 7F0 ? 5D2 transition reveals information on the local structure of Eu3+ ion sites which affect the non-radiative decay and (5) electron–phonon coupling strength in a host can also be evaluated from the ratio of the integrated intensities of the phonon side band (PSB) to that of the pure electronic band (PEB) [10,11]. The Eu3+ ions doped glasses deserve extensive research due to their potential applications which exhibit narrow emission band (red light, 5D0 ? 7F2) from the Eu3+ ions arising from the intra 4f–4f parity forbidden transition. Hence, in this work we presented a systematic study on Eu3+-doped zinc fluorophosphate glasses using differential thermal analysis (DTA), Raman, absorption, excitation, phonon side band and luminescence spectra as well as decay rate analysis. 2. Experimental procedure

Table 1 Physical properties of the Eu3+:ZFPEu10 glass. Properties

ZFPEu10

Refractive index, n Density, d (g cm3) Concentration, C (1020 ions cm3) Optical path length, l (cm) Molar volume, Vm (cm3/mol) Glass molar refraction, Rm (cm3) Electronic polarisability, ae (1024 cm3) Polaron radius, rp (A°) Inter ionic distance, ri (A°) Field strength, F (1014 cm2) Dielectric constant, (e) Optical dielectric constant, (e  1) Glass transition temperature, Tg (°C) Crystallization temperature, Tx (°C) Melting temperature, Tm (°C) Glass stability factor, DT = Tx  Tg (°C)

1.540 2.73 2.77 0.32 5.51 1.73 0.69 6.18 15.34 7.86 2.37 1.37 418 562 862 144

2.1. Glass preparation Eu3+-doped zinc fluorophosphate (ZFPEu) glasses with the molar composition of 41P2O5–17K2O–8Al2O3–(24  x)ZnF2–10LiF–x Eu2O3, where x = 0.05, 0.1, 0.5, 1.0, 2.0 and 4.0 mol% referred as ZFPEu005, ZFPEu01, ZFPEu05, ZFPEu10, ZFPEu20 and ZFPEu40, respectively, were prepared by the conventional melt quenching technique. The raw materials of Al(PO3)3, KH2PO4, ZnF2, LiF and Eu2O3 having more than 99.9% purity have been used as starting materials. Batches were well mixed and then taken in the platinum crucible and melted at 1250 °C for 2 h in an electric furnace. The melts were then air quenched and quickly poured onto a preheated brass mold. To obtain thermal and structural stability, the glass samples were annealed at a temperature of 395 °C for 12 h and the samples were cooled slowly to the room temperature. Finally, these glass samples were polished to achieve good optical transparency and smooth surfaces for measuring their optical properties. 2.2. Characterization methods Fig. 1. DTA curve of the Eu3+:ZFPEu10 glass.

For the ZFPEu10 glass, optical path length and refractive index were measured by digital vernier calipers and Abbe refractometer at wavelength of 589.3 nm, respectively. The densities of glasses were determined by Archimedes method using distilled water as an immersion liquid. The glass transition (Tg), onset crystallization (Tx) and melting (Tm) temperatures were determined by DTA at a heating rate of 10 °C/min using SDT Q600 V8.3 Build 101 differential thermal analyzer. The structural properties of the glass were studied by using Raman spectroscopy. The Raman spectrum was recorded in the range of 50–1400 cm1 using a Lab Ram HR800 spectrometer equipped with 514 nm line of Ar+ laser and CCD detector in a back scattering geometry. The optical absorption spectrum was measured using a Perkin Elmer Lambda-950 UV–visible–NIR spectrophotometer in the wavelength range of 340–2400 nm with a spectral resolution of 1 nm. The excitation, phonon side band and luminescence spectra and decay curves of the 5D0 level of Eu3+ ions in these glasses were measured by JOBIN YVON Fluorolog-3 spectrofluorimeter using xenon arc lamp as radiation source. All these measurements were carried out at room temperature. 3. Results and discussion 3.1. Physical and thermal properties Various physical properties have been calculated for the ZFPEu10 glass and are presented in Table 1. Fig. 1 shows the DTA curve for the

Eu3+:ZFPEu10 glass. From DTA, the values of Tg, Tx and Tm are determined and found to be 418, 562 and 862 °C, respectively. The Tg, estimated from the point of intersection of the tangents drawn at the slope change is also indicated in Fig. 1. Generally, the difference in temperature DT = Tx  Tg has been frequently used as a rough measure of glass stability factor. To achieve large working range during operation such as fiber drawing, the value of DT is preferred to be as large as possible. The DT for ZFPEu10 glass is found to be 144 °C, which suggests that it is stable against devitrification and may be suitable for fiber drawing. 3.2. Raman analysis The interpretation of Raman spectrum has been carried out according to the assumption that the glass structure consists of P–O bonds existing in amorphous P2O5. The phosphate network generally built up from the corner sharing of PO4 tetrahedral units. Depending on the number of bridging and non-bridging oxygens the tetrahedral sites are classified according to their connectivity (Qn) [12], where n is the number of bridging oxygens per PO4 tetrahedron. Consequently, the Raman response of phosphate network can be divided into three spectral regions related to the activity of: (i) non-bridging oxygen modes (900–1400 cm1), (ii) bridging oxygen modes (700–900 cm1) and (iii) deformation (500 cm1) modes.

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Fig. 3. Tauc’s plots of the (ahm)1/2 and (ahm)2 as a function of hm for the Eu3+:ZFPEu10 glass.

Fig. 2. Raman spectrum of the Eu3+:ZFPEu10 glass.

The Raman features observed for the ZFPEu10 glass and their assignments are shown in Fig. 2. Totally eight bands are observed at about 343, 528, 629, 747, 938, 1079, 1130 and 1279 cm1. The band at 343 cm1 is attributed to the skeletal deformation vibrations of various phosphate chains and PO3 deformation vibrations of phosphate polyhedra [13,14]. The band at 528 cm1 corresponds to bending vibrations of O–P–O units and the PO2 modes [15]. The weak band at 629 cm1 belongs to Al2O3 vibrations [16,17]. The band at 747 cm1 was ascribed to the vibrations of P–F of fluoride chain formation [17–19]. The weak Raman band at 938 cm1 might be due to the (PO4) symmetric stretching vibrations of the Q0 species, which represents an isolated ðPO4 Þ3 tetrahedron with no bridging oxygens to neighboring tetrahedral; ðPO4 Þ3 groups are also known as orthophosphate units [20,21]. The Raman line at 1079 cm1 is related to the asymmetric stretching mode of P–O–P groups linked with small metaphosphate groups [15,22,23]. Finally, two bands at 1130 and 1279 cm1, corresponds to the symmetric and asymmetric stretching vibrations of the non-bridging oxygen atoms bonded to phosphorous atoms (O–P–O) in the Q2 phosphate tetrahedron, respectively [13–15,21–23]. The phonon energy of the host can be defined as highest vibrational energy measured from the Raman spectrum and it is found to be 1130 cm1 for ZFPEu10 glass, which plays major role on the optical properties (particularly multiphonon relaxation) of the optically active ions. 3.3. Optical band gap The optical band gap is one of the important parameter to describe a solid state laser material. The optical absorption edges are not sharply defined which characterize the glassy nature of samples. In amorphous materials the relation between the absorption coefficient a(hm) and photon energy (hm) for direct and indirect optical transitions can be expressed as [24,25]:



aðhmÞ ¼ A

ðhm  Eg Þn hm



aðhmÞ ¼ 0 for hm < Eg

for hm > Eg

ð1Þ ð2Þ

where the exponent n = 1/2 is for allowed direct transition, while n = 2 is for allowed indirect transition, A is constant, hm is the photon energy and Eg is the optical energy gap. Plotting (ahm)1/2 and (ahm)2 against photon energy (hm) gives a straight line with intercept equal to the optical energy band gap for indirect and direct transitions, respectively. Fig. 3 shows the Tacu’s plot of the (ahm)1/2 and (ahm)2 as a function of photon energy (hm) used to

determine the optical band gap of ZFPEu10 glass and Eg is found to be 3.57 eV and 3.74 eV for indirect and direct transitions, respectively. 3.4. Absorption spectra and energy level analysis Fig. 4a and b shows the absorption spectra of Eu3+ ions doped ZFPEu10 glass for UV, visible and near infrared (NIR) regions. The majority of transitions in the visible absorption spectrum of Eu3+ ion are very weak. The energy level structure of Eu3+ ion can be deduced partially both from the absorption and emission spectra. The absorption spectra (Fig. 4) consist of absorption bands originated from both the 7F0 (ground) and 7F1 (closely spaced first excited) levels. Transitions from 7F0 ground state will take place only at low temperature (77 K and lower). But at room temperature (RT), it is also possible to observe transitions starting from the thermally populated 7F1 level which is located at nearly 380 cm1 above the 7F0 ground level. The characteristic absorption bands of Eu3+ ions originating from the 7F0 ground state were centered at 363 nm (5D4), 366 nm (5L8), 372 nm (5G5), 376 nm (5G3), 382 nm (5G2), 393 nm (5L6), 464 nm (5D2), 526 nm (5D1) and 2083 nm (7F6), whereas the bands originated from 7F1 level are centered at 400 (5L6), 415 nm (5D3), 533 nm (5D1) and 2207 nm (7F6). Among all the absorption bands, 7F0 ? 5L6 transition is found to be more intense than any other transitions, even though it is forbidden by the S and L selection rules but allowed for the J selection rule. The intensity of 7F0 ? 5D1 magnetic-dipole allowed transition is relatively weaker than that of 7F0 ? 5D2 (hypersensitive transition) induced electric-dipole allowed transition. The experimental energy positions for all the transitions from the ground 7F0 state to various excited states of Eu3+ ions in ZFPEu10 glass are measured from the absorption spectrum and are presented in Table 2. In Table 2, the blank spaces indicate that these transitions are not observed, which may be due to the fact that they are relatively very weak, overlapped by stronger band or forbidden by selection rules. The Eu3+ ion has 4f6 configuration and characterized by 119 SLJ (free-ion levels) manifolds which split in the presence of crystal– field interactions into a total of 3003 sublevels (Stark levels). The observed energy levels have been well analyzed using parameterized free-ion Hamiltonian model [26,27] that can be expressed as:

^ FI ¼ EAVG þ H

X k ^ SO þ a^Lð^L þ 1Þ þ bGðG ^ 2 Þ þ cGðR ^ 7Þ F ^f k þ n4f A k

X i X k X j ^k þ ^j þ T ^ti þ P p Mm i

k

j

ð3Þ

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Table 2 (a) Experimental (Eexp) and calculated (Ecal) energies (cm1) and (b) free-ion parameters for Eu3+:galsses. 2S+1

LJ

ZFPEu10 Eexp

(a) Free-ion levels 7 F0 0 7 F1 379 7 F2 904 7 F3 1957 7 F4 2646 7 F5 – 7 F6 4801 5 D0 17,271 5 D1 19,011 5 D2 21,551 5 D3 – 5 L6 25,445 5 G2 26,178 5 L7 – 5 G3 26,596 5 G4 – 5 G5 26,881 5 G6 – 5 L8 27,322 5 D4 27,548 a r (N)

PKMAEu10 [28]

PKFSAEu10 [28]

Ecal

Eexp

Ecal

Eexp

Ecal

31 338 987 1823 2776 3796 4844 17,275 19,055 21,519 24,351 25,410 26,206 26,451 26,549 26,782 26,905 26,918 27,347 27,488 ±65 (15)

0 371 906 1952 3017 – 4789 17,269 19,007 21,520 – 25,430 26,205 – – 26,575 – – – 27,621

3 373 1030 1876 2839 3867 4922 17,239 19,052 21,545 24,399 25,338 26,155 26,398 25,502 26,740 26,857 26,867 27,306 27,550 ±95(13) (13)

0 379 1045 1949 3016 – 4797 17,269 19,013 21,551 – 25,443 26,176 – – 26,597 – – – 27,625

22 398 1056 1092 2865 3892 4945 17,242 19,069 21,567 24,415 25,364 26,130 26,428 26,490 26,742 26,870 26,882 27,338 27,552 ±82 (13)

(b) Free-ion parametersb Eavg 65,533 F2 86,172 F4 65,571 F6 38,992 F2/F4 1.32 F2/F6 2.22 4 6 F /F 1.68 n 1304 k RF 190,735

65,137 85,044 65,491 38,824 1.30 2.19 1.69 1316 18,9359

65,196 84,981 66,365 38,323 1.28 2.22 1.71 1315 189,669

a ‘r’ indicates root mean square deviation (cm1) between experimental and calculated values and N represents the number of levels used in the fitting. b The other parameters (cm1): a = 16.82; b = 630; c = 1750; T2 = 370; T3 = 40; T4 = 40; T6 = 330; T7 = 380; T8 = 370; M0 = 2.38; P2 = 245 were fixed to the values of Eu3+:LaCl3 [27]

Fig. 4. Optical absorption spectrum of Eu3+:ZFPEu10 glass in: (a) UV–visible and (b) NIR regions.

3.5. Evaluation of Judd–Ofelt intensity parameters where the symbols represent their usual physical parameters and operators [26,27]. In the present systematic energy level analysis, all the initial parameters correspond to Eu3+:LaCl3 which are taken from Jayasankar et al. [27], since the free-ion parameters are expected to be nearly independent of the host. Among the various interactions that contribute to the total free-ion Hamiltonian, the major contribution comes from the inter-electronic (Fk) and the spin–orbit (n) interactions which govern the 2S+1LJ levels. The rest of the terms will only give small modifications to the energy of these levels without removing their degeneracy. Hence, during the fitting process, after observing the trends and consistency, out of 20 free-ion parameters, the only parameters that are allowed to vary are Fk and n. Table 2 presents the observed and calculated free-ion band positions and free-ion parameter (Fk and n) values for Eu3+ ions in the present glass along with other reported fluorophosphate glasses such as PKFMAEu10 [28] and PKFSAEu10 [28]. The net electrostatic effect (RFk) experienced by the Eu3+ ion in ZFPEu10 glass is found to be 190,735 cm1 which is higher than PKFMAEu10 [28] and PKFSAEu10 [28] glasses. The hydrogenic ratios, F2/F4, F2/F6 and F4/F6, are found to be 1.32, 2.22 and 1.68, respectively, for the ZFPEu10 glass which are more or less similar for all the glass systems that are presented in Table 2 which indicate the radial integral part of the f-orbitals of the Eu3+ ions remains unchanged even though the glass compositions are changed. This behavior may be due to strong shielding of the 4f shell by the 5s2 and 5p6 orbitals.

In the case of Eu3+ ion, 7FJ (J = 0, 1 and 2) levels are populated thermally even at RT. The fractional thermal populations of the 7 F0, 7F1 and 7F2 levels are found to be 65%, 31% and 4%, respectively, and the population of 7F1 level cannot be neglected at RT. Therefore, the absorption transitions starting not only from 7F0 state but also from 7F1 state can be observed and some of these transitions often overlap making Judd–Ofelt (JO) analysis [29,30] rather cumbersome at RT. Using the absorption spectrum of ZFPEu10 glass, the experimental oscillator strengths of various absorption lines have been evaluated. As the absorption originates from the ground 7F0 as well as thermally populated 7F1 states, oscillator strengths are corrected for thermalization effect at room temperature by dividing with respective population of 7F0,1 levels. These results have been used to calculate the JO parameters (Xk, (k = 2, 4 and 6)) using standard least squares fitting approach [5,28–31]. The experimental (fexp) and calculated (fcal) oscillator strengths and doubly reduced squared matrix elements (||Uk||2) along with rms deviation (r) for Eu3+:ZFPEu10 glass without and with thermal correction are presented in Table 3. In the present fitting procedure the doubly reduced squared matrix elements of the unit tensor operator are taken from Ref. [31], given for the Eu3+ ions in lithium fluoroborate glass. We can use these matrix elements, because the matrix elements are virtually host independent. The JO intensity parameter without and with thermal correction are presented in Table 4 as SET A and SET B, respectively. From the intensity

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Table 3 Experimental energies (m, cm1), reduced matrix elements (||Uk||2, k = 2,4 and 6), experimental (fexp) and calculated (fcal) oscillator strengths (106) without and with thermal correction for the Eu3+:ZFPEu10 glass. Transition

Energy (m)

Reduced matrix elements

Oscillator strengths Without thermal correction

7

7

F1 ? F6 F0 ? 7F6 7 F1 ? 5D1 7 F0 ? 5D1 7 F0 ? 5D2 7 F1 ? 5D3 7 F1 ? 5L6 7 F0 ? 5L6 7 F0 ? 5G2 7 F0 ? 5G3 7 F0 ? 5G5 7 F0 ? 5L8 7 F0 ? 5D4 r(N)c 7

a b c

4531 4801 18,762 19,011 21,551 24,096 25,000 25,445 26,178 26,596 26,881 27,322 27,548

||U2||2

||U4||2

||U6||2

0 0 0.0026 0 0.0008 0.0004 0 0 0.0006 0 0 0 0

0 0 0 0 0 0.0012 0 0 0 0 0 0 0.0011

0.3773 0.1450 0 0 0 0 0.0090 0.0155 0 0 0 0 0

fexp 0.60 2.47 0.11 0.03 0.37 0.09 0.26 1.92 0.52 0.40 0.08 0.10 0.33 ±0.59 (13)

fcala 0.79 0.97 0.15 0.0 0.16 0.11 0.11 0.55 0.14 0.0 0.0 0.0 0.26

With thermal correction fexp 1.94 3.82 0.35 0.05 0.57 0.30 0.82 2.97 0.80 0.61 0.12 0.16 0.51 ±0.61(13)

fcalb 2.10 2.56 0.40 0.0 0.42 0.27 0.28 1.45 0.38 0.0 0.0 0.0 0.62

Obtained with SET A of Table 4. Obtained with SET B of Table 4. See foot note ‘a’ of Table 2.

parameters of SET A and SET B, one can observe that the magnitude of intensity parameters as well as rms deviation increased considerably compared to those obtained without thermal correction. An alternative approach to determine the JO intensity parameters at RT takes the advantage of the fact that ||U2||2, ||U4||2 and ||U6||2 are the only non-zero matrix elements for the 7F0 ? 5D2, 7 F0 ? 5D4 and 7F0 ? 5L6 transitions, respectively (Table 3). From the oscillator strengths of these three transitions, the respective three Xk (k = 2, 4 and 6) parameters have been determined. The parameters obtained in this way are: X2 = 14.25  1020 cm2, X4 = 7.43  1020 cm2 and X6 = 3.26  1020 cm2 (SET C of Table 4). By making thermal corrections for the oscillator strength of these three transitions, the JO parameters are found to be X2 = 22.02  1020 cm2, X4 = 13.60  1020 cm2 and X6 = 5.04  1020 cm2 (SET D of Table 4). Following this approach also reveals that the magnitude of JO parameters increases considerably compared to those obtained without thermal correction. Since to determine these three JO parameters only three transitions are used, a one-to-one agreement is found between calculated and experimental oscillator strengths, but errors could not be determined on these parameters. The JO parameters have also been determined from the emission spectra (Fig. 5) by evaluating the luminescence intensity ratio of the 5D0 ? 7FJ (J = 2, 4 and 6) transitions to the intensity of the 5 D0 ? 7F1 magnetic dipole transition [31]. As most of the matrix elements for transitions starting from the 5D0 level are zero, except for 5D0?7FJ (J = 2, 4 and 6) transitions. The luminescence intensity for 5D0?7F6 transition is taken as zero since it is not observed. The

JO parameters obtained using this method is also presented in Table 4 as SET E. In all cases, the JO parameters for ZFPEu10 glass follow the same trend as X2 > X4 > X6. Among the three JO parameters, X2 parameter is sensitive to the local environment around the RE3+ site and strongly affected by covalency between RE3+ ions and ligand anions, where as X4 and X6 are related to the viscosity and rigidity of the host medium in which the ions are situated. Higher the value of X2, stronger the covalency and lower the symmetry [32].

3.6. Luminescence spectra Eu3+ ions in ZFPEu glasses emit red color under 393 nm excitation. By this excitation, the 5L6 level is well populated. All the transitions observed in the luminescence spectrum start from the 5D0 level, which is populated by radiationless depopulation of the 5L6 level. Fig. 5 shows luminescence spectra of Eu3+:ZFPEu glasses and consists of narrow emission bands in the range of 560– 720 nm, which may be due to the shielding effect of 4f6 electrons by 5s and 5p electrons in outer shells in the Eu3+ ion. The emission

Table 4 The Judd–Ofelt parameters (Xk, 1020 cm2) of the Eu3+:ZFPEu10 glass under various constraints. Seta

X2

X4

X6

A B C D E

6.06 16.16 14.25 22.02 3.62

5.77 13.59 7.43 13.60 0.41

0.93 2.46 3.26 5.04 0

a All absorption levels (shown in column 1 of Table 3) without (Set A) and with (Set B) thermal correction, 7F0 ? 5D2, 5D4 and 5L6 levels for X2, X4 and X6, respectively, without (Set C) and with (Set D) thermal correction and only emission (5D0 ? 7F2,4,6 levels) levels (Set E).

Fig. 5. Luminescence spectra for different concentrations of the Eu3+:ZFPEu glasses.

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N. Vijaya, C.K. Jayasankar / Journal of Molecular Structure 1036 (2012) 42–50 Table 5 Calculated radiative properties of the Eu3+:ZFPEu10 glass under different constraints. Seta

Transition

AR (s1)

bcal

A

5

D0 ? F 6 7 F5 7 F4 7 F3 7 F2 7 F1 7 F0

1 0 97 0 203 52 0

0 0 0.27 0 0.58 0.15 0

353

2.83

B

5

D0 ? 7F6 7 F5 7 F4 7 F3 7 F2 7 F1 7 F0

3 0 228 0 541 52 0

0 0 0.28 0 0.66 0.063 0

824

1.21

C

5

D0 ? 7F6 7 F5 7 F4 7 F3 7 F2 7 F1 7 F0

4 0 122 0 477 52 0

0.01 0 0.19 0 0.73 0.08 0

655

1.53

D

5

D0 ? 7F6 7 F5 7 F4 7 F3 7 F2 7 F1 7 F0

6 0 228 0 737 52 0

0.01 0 0.22 0 0.72 0.05 0

1023

0.98

E

5

D0 ? 7F6 7 F5 7 F4 7 F3 7 F2 7 F1 7 F0

0 0 7 0 121 52 0

0 0 0.04 0 0.67 0.29 0

180

5.55

a

7

AT (s1)

sR (ms)

around 592 nm is independent of the host matrix [33]. When a Eu3+ ion occupies an inversion symmetry site in the host matrix, the orange emission will be the dominated emission. The obtained spectra have been normalized with respect to maximum intense transition, 5D0 ? 7F2. From Fig. 5, it is observed that there is no significant change in the position of emission bands with increasing Eu3+ ion concentration. Also, the 5D0 ? 7F1 transition splits into three components due to crystal-field which indicate a total removal of the crystal-field degeneracy. This reveals that the symmetry at Eu3+ ion site is low: orthorhombic, monoclinic or triclinic. The highest possible symmetry is C2v [34]. The hypersensitive ratio R (Ratio between integrated intensities of 5D0 ? 7F2 and 5D0 ? 7F1 transitions), known as the asymmetric ratio, allows one to estimate the covalent nature and polarization of the surrounding of the Eu3+ ions by short range effects and centrosymmetry distortion of Eu3+ ion site. Higher the value of R, lower the symmetry around the Eu3+ ions and higher the Eu–O covalence, and vice versa [35,36]. Hence, X2 and the hypersensitive ratio R strongly reveal similar physical significance of the asymmetry and covalent bonding nature between Eu3+ ion and the surrounding ligand. The R for the present glass is found to be 2.48 and is lower than PKFMAEu10 (3.03) [28], PKFSAEu10 (2.85) [28] and PKFBAEu10 (3.75) [28] glasses which suggests the presence of higher symmetry around the Eu3+ ions in the present glass. The appearance of the non-degenerate 5D0 ? 7F0 transition indicates that the Eu3+ ion is in an environment of low symmetry [36] in title glass system. Moreover, it is observed that, R value depends only on host material but it is independent of Eu3+ ion concentration. 3.7. Radiative properties

See the footnote of Table 4.

spectra of ZFPEu glasses exhibit five typical emission bands corresponding to 5D0 ? 7F0 (579 nm), 5D0 ? 7F1 (591 nm), 5 D0 ? 7F2 (611 nm), 5D0 ? 7F3 (653 nm) and 5D0 ? 7F4 (701 nm) transitions. Among these five transitions, the three transitions corresponding to 5D0 ? 7F1,2,4 are intense and the other two transitions, 5D0 ? 7F0,3, are weaker. Only transitions for which DJ = 2, 4 and 6 are allowed by induced electric-dipole mechanism if the luminescence starts from a level for which J = 0. The emission peak corresponding to the hypersensitive 5D0 ?7F2 transition in the red region around 611 nm is the most intense among all emission transitions and it is very sensitive to the site symmetry of Eu3+ions. Whereas the 5D0 ? 7F1 transition is magnetic-dipole allowed transition, appears in the moderate bright orange region

By using the JO intensity parameters and the refractive index, various radiative properties such as radiative transition probability (AR), radiative lifetime (sR) and branching ratios (bR) for the 5 D0 ? 7FJ (J = 0–4) transitions of Eu3+ ion have been evaluated under different constraints and are presented in Table 5. From Table 5, it is observed that the AR of the 5D0 ?7F1 magnetic-dipole transition is independent of magnitude of JO parameters. For the 5 D0 fluorescent level, the total radiative transition probability (AT) increased and sR decreased considerably compared to those obtained without thermal correction. From Tables 4 and 5, it is also inferred that sR is inversely proportional to X2, which indicates sR in the present ZFPEu10 glass decreases with increase in covalency among Eu3+ ions. In order to know the probable lasing transition in the emission spectrum of Eu3+ ion, the effective bandwidth (Dkeff), experimental branching ratio (bexp) and peak stimulated emission cross-sections (re(kp)) for the 5D0 ? 7FJ (J = 0–4) transitions have been determined and are collected in Table 6. As can be seen from Table 6, it is found that the re(kp) for the 5D0 ? 7F2 and 5D0 ? 7F4 transitions strongly depend on JO intensity parameters whereas for the 5D0 ? 7F1 transition the emission cross-section is independent of JO

Table 6 Emission band positions (kp, nm), effective bandwidths (Dkeff, nm), experimental branching ratio (bexp) and peak stimulated emission cross-sections (re(kp), 1021 cm2) for the Eu3+:ZFPEu10 glass. Transiotion 5D0 ?

7

F0 F1 F2 7 F3 7 F4 7 7

a

See the footnote of Table 4.

kp

579 592 611 653 701

Dkeff

2.0 11.4 10.0 7.0 15.0

bexp

0.01 0.28 0.66 0.01 0.04

re(kp)a Set A

Set B

Set C

Set D

Set E

0 0.31 1.58 0 0.87

0 0.31 4.21 0 2.05

0 0.31 3.72 0 1.10

0 0.31 5.75 0 2.05

0 0.31 0.94 0 0.06

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N. Vijaya, C.K. Jayasankar / Journal of Molecular Structure 1036 (2012) 42–50

parameters. It is also observed that whatever may be the constraint the value of re(kp) is found to be higher for the hypersensitive (5D0 ? 7F2) transition. The large stimulated emission cross-section is one attractive feature for the design and development of low-threshold and high gain laser applications. The luminescence branching ratio is another important parameter that characterizes the lasing power of a transition and it is well established that an emission transition having bR greater than 50% is considered to be more potential for laser emission [37]. The value of bexp obtained for 5D0 ? 7F2 hypersensitive transition in the ZFPEu10 (0.66) glass is comparable with those of PKFMAEu10 (0.72) [28], PKFSAEu10 (0.72) [28], PKFBAEu10 (0.70) [28], PTBEu10 (0.65) [38], LiTFP (0.68) [39], NaTFP (0.68) [39] and KTFP10 (0.67) [39] glasses. Moreover, from Table 6, it is concluded that 5D0 ? 7F2 transition of ZFPEu10 glass has the highest value of r(kp) besides bR > 50%, which suggest that it may be useful for the development of visible red laser as well as optical display devices at around 611 nm. Fig. 7. Phonon side band spectrum of the Eu3+:ZFPEu10 glass.

3.8. Excitation and phonon side band spectra The excitation spectrum of Eu3+:ZFPEu10 glass obtained by monitoring the emission at 611 nm is shown in Fig. 6. From the excitation spectrum, high-energy transitions have been located and assigned. There are eight excitation peaks such as 7F0 ? 5D4 (362 nm), 7F0 ? 5G3 (376 nm), 7F1 ? 5G2 (381 nm), 7F0 ? 5L6 (393 nm), 7F1 ? 5D3 (415 nm), 7F0 ? 5D2 (466 nm), 7F0 ? 5D1 (525 nm) and 7F1 ? 5D1 (533 nm) in the 340–570 nm spectral range. Among these, the 7F0 ? 5L6 (393 nm) transition is found to be more prominent and it was used to record the emission spectra of ZFPEu glasses in the 560–720 nm spectral region. Phonon side band spectroscopy is one of the useful techniques to investigate the local structure surrounding the RE3+ ions in any host matrix [11]. It is also used to determine the phonon energy, electron–phonon coupling strength of the host matrix and the phonon mode coupled to the RE3+ ions. These parameters of the host strongly affect the multiphonon relaxation processes. In the case of Eu3+ ion, the phonon side band (PSB) associated with the 7F0 ? 5D2 zero phonon (pure electronic) transition is clearly observed in the excitation spectrum, in the wavelength range of 430–480 nm, by monitoring the 5D0 ? 7F2 emission at 611 nm. Fig. 7 shows the phonon side band spectrum of Eu3+ ion in ZFPEu10 glass. The intense band due to the pure electronic transition (PET), 7 F0 ? 5D2, is located at 464 nm (21,552 cm1), while the PSB

coupled to the 7F0 ? 5D2 transition of Eu3+ ions is observed in the higher-energy range at 441 nm (22,676 cm1). The difference between the position of the PSB and PET gives the phonon energy ( hx) of the host [11], which is found to be 1124 cm1 in the present glass. It is in good agreement with the phonon energy (1130 cm1) observed from the Raman spectrum. This phonon mode corresponds to the symmetric vibration of the non-bridging oxygen atoms bonded to phosphorous atoms (O–P–O) in the Q2 phosphate tetrahedron [14,21]. Also the comparison of Raman spectrum with phonon side band spectrum reveals that the intense phonon side band was caused by the vibrations with the greatest phonon energy. Thus, the phonons due to PO2 groups with the highest energy are mainly considered to contribute to the multiphonon relaxation of Eu3+ ions in the present glass. The electron–phonon coupling strength (g) is the intensity ratio R R Þ to the PET ð IPET dm Þ defined as [11]: of the PSB ð IPSB dm

R  IPSB dm g¼R  IPET dm

ð4Þ

The value of ‘g’ in the present glass is found to be 24  103. Table 7 presents the comparison of h  x and ‘g’ of ZFPEu10 glass with other reported Eu3+:glasses [40,41]. Among all other glasses, fluoride glasses have lower phonon energy, whereas phosphate and borate glasses have higher phonon energy. From Table 7, it is observed that ‘ hx ’ and g values of present ZFPEu10 glass lies between fluoride and phosphate glasses. This indicates that the phonon energy of the phosphate glasses decreases due to addition of fluoride content. 3.9. Decay rate analysis Fig. 8 presents the decay rates for the 5D0 level of Eu3+:ZFPEu glasses obtained by monitoring the 5D0 ? 7F2 transition. The

Table 7 Comparison of phonon energy (hx) and coupling strength (g) of Eu3+:glasses.

Fig. 6. Excitation spectrum of the Eu3+:ZFPEu10 glass.

Glass

hx (cm1) 

g (103)

Fluoride [40,41] Tellurite [40,41] Germanate [41] ZFPEu10 [present glass] Phosphate [41] Borate [41]

500–600 600–850 800–975 1130 1200–1350 1340–1480

15–35 – – 24 11.8 18

N. Vijaya, C.K. Jayasankar / Journal of Molecular Structure 1036 (2012) 42–50

49

nature of levels used (besides thermalization effect) for the analysis of f–f transitions of Eu3+:ZFPEu10 glass. Both the values of X2 parameter and hypersensitive ratio R (5D0 ? 7F2/5D0 ? 7F1) strongly disclose the similar physical significance of the asymmetry and covalent bonding nature between Eu3+ ions and the surrounding ligand. The phonon energy obtained by Raman and phonon side band spectra is in good agreement. The phonons due to PO2 groups having the energy 1130 cm1 are responsible for multiphonon relaxation of Eu3+ ions in the present host matrix. Lifetime for the 5D0 level of Eu3+ ions in ZFPEu glasses is independent of Eu3+ ion concentration but depends on host material and the probability of non-radiative energy transfer between Eu3+ ions which is negligible. The r(kp) and bR values for the 5D0 ? 7F2 transition of ZFPEu10 glass revealed that it could be used for the development of visible red laser as well as optical display devices at visible region (611 nm). Acknowledgements Fig. 8. Decay rates for the 5D0 level of Eu3+:ZFPEu glasses for different concentrations of Eu3+ ions.

Table 8 Comparison of experimental lifetimes (sexp, ms) for the 5D0 level of Eu3+ ions in Eu3+:systems along with concentration (C, mol%) of Eu3+ ions. System

C

sexp

ZFPEu [present work]

0.01 0.05 0.1 0.5 1.0 2.0 4.0 1.0 1.0 1.0 1.0 0.06 0.06 2.0 1.0

2.53 2.30 2.28 2.27 2.47 2.50 2.55 2.20 2.50 2.40 2.50 1.89 3.08 2.20 1.27

PTBEu10 [38] KTFP [39] NaTFP [39] LiTFP [39] La2O3–3B2O3 [42] La(BO2)3 [42] P2O5–Al2O3–CaO–SrO–BaO–Eu2O3 [43] ZnF2–WO3–TeO2–Eu2O3 [44]

experimental lifetime (sexp) of the 5D0 level of Eu3+ ion in all the glass systems are obtained by taking the first e-folding times of the emission intensities. As can be seen from Fig. 8, it is clear that the decay profiles are exhibiting single exponential nature for all concentrations of Eu3+ ions. The sexp obtained for different concentration of Eu3+ ions in ZFPEu glasses are comparable with other reported Eu3+-doped PTBEu10 [38], LiTFP [39], NaTFP [39], KTFP [39], La2O3–3B2O3 [42], La(BO2)3 [42], P2O5–Al2O3–CaO–SrO–BaO–Eu2O3 [43] and ZnF2–WO3–TeO2–Eu2O3 [44] systems which are collected in Table 8. From Table 8, it can be seen that there is no considerable variation in lifetime of the 5D0 level with Eu3+ ion concentration which indicates that the probability of non-radiative energy transfer between Eu3+ ions in ZFPEu glasses is negligible and is also independent of Eu3+ ion concentration but depends only on host material.

4. Conclusions The structural and optical properties of Eu3+:ZFPEu glasses have been reported. The detailed and systematic analysis of optical intensities has been performed from absorption and emission spectra using Judd–Ofelt (JO) theory under different constraints. It has been found that the JO theory depends on the number and

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