Structural and spectroscopic characteristics of Eu3+-doped tungsten phosphate glasses

Structural and spectroscopic characteristics of Eu3+-doped tungsten phosphate glasses

Optical Materials xxx (2015) xxx–xxx Contents lists available at ScienceDirect Optical Materials journal homepage: S...

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Optical Materials xxx (2015) xxx–xxx

Contents lists available at ScienceDirect

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Structural and spectroscopic characteristics of Eu3+-doped tungsten phosphate glasses M. Reza Dousti a,⇑, Gael Yves Poirier b, Andrea Simone Stucchi de Camargo a,⇑ a b

Instituto de Física de São Carlos, Universidade de São Paulo, São Carlos, SP 13566-590, Brazil Instituto de Ciências e Tecnologia, Universidade Federal de Alfenas, Poços de Caldas, MG 37715-400, Brazil

a r t i c l e

i n f o

Article history: Received 11 February 2015 Received in revised form 11 March 2015 Accepted 11 March 2015 Available online xxxx Keywords: Tungsten phosphate glasses Eu3+ UV–VIS spectroscopy Judd–Ofelt parameters

a b s t r a c t Tungsten based phosphate glasses are interesting non-crystalline materials, commonly known for photochromic and electrochromic effects, but also promising hosts for luminescent trivalent rare earth ions. Despite very few reports in the literature, association of the host´s functionalities with the efficient emissions of the dopant ions in the visible and near-infrared spectra could lead to novel applications. This work reports the preparation and characterization of glasses with the new composition 4(Sb2O3)96x(50WO3 50NaPO3)xEu2O3 where x = 0, 0.1, 0.25, 0.5 and 1.0 mol%, obtained by the melt quenching technique. The glasses present large density (4.6 g cm3), high glass transition temperature (480 °C) and high thermal stability against crystallization. Upon excitation at 464 nm, the characteristic emissions of Eu3+ ions in the red spectral region are observed with high intensity. The Judd–Ofelt intensity parameters X2 = 6.86  1020, X4 = 3.22  1020 and X6 = 8.2  1020 cm2 were calculated from the emission spectra and found to be higher than those reported for other phosphate glass compositions. An average excited state lifetime value of 1.2 ms, was determined by fitting the luminescence decay curves with single exponential functions and it is comparable or higher than those of other oxide glasses. Ó 2015 Elsevier B.V. All rights reserved.

1. Introduction The design of new glasses and glass ceramic compositions remains a very active field of research due to their exciting application possibilities and the ability to produce them in large scale at relatively low cost [1–4]. Among oxide glasses, phosphate based compositions are one of the most versatile ones. Besides presenting characteristic small liquidus viscosity and softening temperatures, these glasses exhibit high UV–VIS transparence window and high solubility for glass modifiers or intermediates such as alkaline, rare earth (RE) and transition metal ions [5–7]. Currently, they are key materials for the development of laser and optoelectronic devices [6] but further important applications in glass-to-metal seals and as fast ion conductors [7,8] have been reported. The structure of phosphate glasses strongly depends on the amount of modifier species around the fundamental glass forming groups. Thus, the glassy state of NaPO3 or P2O5 glass formers can change from Q3 groups, with three bridging oxygens, to Q2, Q1 and Q0 groups presenting two, three and four non-bridging oxygens, respectively [1]. ⇑ Corresponding authors. E-mail addresses: [email protected] (M.R. Dousti), [email protected] (A.S.S. de Camargo).

Tungsten-sodium-phosphate glasses based on the binary system NaPO3–WO3 are known for their very good forming ability and very high thermal stability against devitrification due to the intermediate behavior of tungsten octahedra inside the metaphosphate network [9–12]. In addition, the incorporation of WO3 in high enough concentrations (>10 mol%) decreases the maximum phonon energy of the phosphate network by formation of WO6 clusters, leading to interesting effects such as nonlinear optical absorption [13], photochromism [14,15] and improved luminescent properties of RE ions [16]. Although at high WO3 concentration these glasses are not fully-transparent in the visible region, compromising RE emission, this drawback can be overcome by addition of Sb2O3 reagent to prevent the reduction of W6+ to W5+ and W4+ species according to [15]: 5þ


þ W6þ þ W5þ

þ 2W6þ :

þ W5þ þ W4þ ! Sb 5þ


þ 2W

! Sb

The spectroscopic behavior of trivalent RE ions are mainly associated to forced electric-dipole transitions that can be somewhat affected by the ion´s vicinity. Therefore, understanding the local environment of RE ions in glassy system is an important topic that allows optimization of glass compositions. There are very few 0925-3467/Ó 2015 Elsevier B.V. All rights reserved.

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reports on the optical properties of RE-doped tungsten phosphate glasses. Frequency upconversion in Tm3+-doped NaPO3–BaF2–WO3 tungsten fluorophosphate glasses were studied by Poirier et al. [14] and Subbalakshmi et al. reported on Ho3+-doped MO–WO3–P2O5 (MO = PbO, ZnO and CaO) glasses [16]. Thanks to its relatively simple energy level diagram, the Eu3+ ions can expose the local structure around the RE ions in various host materials and it is commonly used as a structural probe [17–20]. When excited at 464 nm, for instance, Eu3+-doped materials present an intense red-to-orange emission at 610 nm (5D0 ? 7F2 transition), which is largely explored in phosphors [21] and also suggested for laser applications [22–25]. The intensity of this electric dipole, hypersensitive, transition depends on the chemical environment where Eu3+ is located, whereas the intensity of the magnetic dipole 5 D0 ? 7F1 transition, at 590 nm, is independent of the local ligand field. Thus, by normalization of the emission intensity of the latter, the intensity ratio R = I(5D0 ? 7F2)610nm/I(5D0 ? 7F1)590nm is often employed as an indicator of the symmetry of the Eu3+ site in different materials, that is, a lower symmetry and stronger Eu–O bonding [26,27] results in higher intensity at 610 nm and higher R. The aim of this work is to present the structural and spectroscopic characterization of new Eu3+-doped glasses with composition Sb2O3–WO3–NaPO3. To the best of our knowledge, this is the first time that such glass composition is studied from the structural and spectroscopic points of view. Although there are several reports of other Eu3+-doped glasses, we highlight that those studied in this work are of superior physical properties when compared to more commonly studied oxide compositions, and the possibility of combining Eu3+ emissions with the tungsten phosphate host´s photochromism could lead to potential, increased functionality. The samples were prepared by the conventional melt quenching technique and characterized by thermal analysis, X-ray diffraction and UV–VIS photoluminescence spectroscopy.

2. Experimental The Eu3+-doped tungsten phosphate glasses with composition 4(Sb2O3)96x(50WO3 50NaPO3)xEu2O3 with x = 0, 0.1, 0.25, 0.5 and 1.0 mol% were prepared by conventional melt-quenching technique. The precursors WO3 (Aldrich 99.9%), Sb2O3 (Aldrich 99.9%) and Eu2O3 (Aldrich 99.9%) were used as received, and the Graham salt (NaPO3) was freshly prepared by heating commercial H2NaPO4 (Nuclear 98.0%) according to: heat

H2 NaPO4 ! NaPO3 þ H2 O:

samples were allowed to slowly cool down to room temperature, cut and optically polished. The glass labels and corresponding compositions are listed in Table 1. The X-ray diffraction (XRD) patterns were obtained in a Rigaku diffractometer model Ultima IV using the Cu Ka radiation (40 kV, 30 mA). The measurements were performed in the range 10–90° with a resolution of 0.02°. Differential scanning calorimetry (DSC) measurements were done in a Netzsch calorimeter model STA Jupiter 449 F3 in order to determine the glass transition temperatures (Tg) in both powder and bulk forms. The temperature range was 200–1000 °C with a heating rate of 10 °C/min. The Tg values were determined by the method of intersecting tangent curves. The densities of the glasses were calculated by Archimedes’ method, using water and xylene ethanol as suspension liquids. The UV–VIS–NIR absorption spectra were recorded using a Perkin-Elmer Lambda 1050 dispersive spectrophotometer in the range 200–3000 nm with a resolution of 1 nm. The PL excitation and emission spectra of the Eu3+-doped samples were collected in a HORIBA Jobin Yvon spectrofluorimeter model Fluorolog FL3-221, equipped with CW xenon flash lamp and a photomultiplier detector (HORIBA PPD-850). The excited state lifetime values of Eu3+ (5D0 state) were determined by exponentially fitting the PL decay curves, which were measured in the same spectrofluorimeter, using a pulsed flash lamp. The CIE chromaticity coordinates were calculated according to Ref. [29].

3. Results and discussion The tungsten phosphate glasses were obtained with very good optical quality and thermal stability. The photograph of representative samples is presented in the inset of Fig. 1. The XRD analysis (Fig. S1, Supplemental Material) confirmed the amorphous nature of all the undoped and doped samples. As seen in Table 1, their average experimental density value, measured in water and xylene ethanol, is 4.7 g cm3 varying within a 10% margin. This result is not surprising considering the low Eu3+ doping concentrations employed. The DSC profiles of the undoped and Eu3+-doped glasses, from which the glass transition temperature Tg values were determined, are also presented in the Supplemental Material. As depicted in Table 1 and seen in Fig. S2, there is a slight increase in Tg values when going from 0 to 1.0 mol% Eu3+-doping. In the temperature range 300–1000 °C there is no sharp crystallization (Tp) or melting (Tm) temperature peaks. However, the powder samples present a big hump of crystallization and a larger endothermic melting point. The higher crystallization tendency

Batches of eight grams of reagent powder mixtures were grained and pre-heated at 120 °C for about 20–40 min before melting at 1050 °C for 1 h in a platinum crucible. The melts were regularly stirred to achieve a homogenous viscous liquid and then poured between two brass molds. Annealing continued for 1 h at 480 °C near the glass transition temperature [28]. Finally, the glass

Table 1 Nominal compositions and densities of Eu3+-doped tungsten phosphate glasses.

a b

Glass label

[WO3– NaPO3] (mol%)

Sb2O3 (mol%)

Eu2O3 (mol%)

Density ±0.01 (g cm3)a

Density ±0.01 (g cm3)b

Tg ±1 (°C)

GEu0 GEu01 GEu025 GEu05 GEu1

96 95.9 95.75 95.50 95

4 4 4 4 4

0 0.1 0.25 0.50 1.00

4.55 4.29 4.60 4.61 4.63

4.76 4.63 4.75 4.61 4.66

479 481 482 485 485

Xylene ethanol. Water.

Fig. 1. Room temperature UV–Vis (a), and mid-infrared (b) absorption spectra of Eu3+-doped tungsten phosphate glasses.

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Fig. 2. Representative excitation spectrum of the GEu05 sample monitoring emission at 610 nm. The inset presents the vibronic bands observed in the blue region of the spectrum.

λexc =464 nm 5


1.0 0.8




0.6 0.4




393 nm 464 nm 531 nm

Normalized intensity


Luminescence intensity (arb. unit)

of powder samples when compared to bulk glasses is often attributed to a dominant surface nucleation of the considered crystalline phase. Anyway, the lack of crystallization peaks for bulk samples point out the very high thermal stability against crystallization for these glass compositions. Fig. 1 presents the UV–VIS–NIR absorption spectra of GEux glass samples. The undoped and GEu01 glass samples present a broad absorption band around 650 nm attributed to electronic transitions associated to remaining traces of the reduced states of tungsten, W5+ and W4+, in this glass [15]. Such broad absorption band is not detected for the other glass samples because of the higher overall absorption coefficient value in this wavelength range. Due to the low concentrations of Eu3+ ions explored in this work, the absorption bands of the ion are not seen with significant intensities. Only the electronic transition 7F0 ? 5D1 around 540 nm is detected. These results support the previous experimental observations of high transparency in the visible and the efficiency of antimony oxide as an oxidizing agent. Fig. 2 presents the representative excitation spectrum of the 1.0 mol% doped sample where the bands at 393, 464, 532 and around 580 nm correspond to transitions from the lower lying energy states 7F0 and 7F1 to the various excited states, as indicated in the figure. Two phonon sidebands (PSB) are observed at 445 nm (22,472 cm1) and 441 (22,676 cm1) nm which, by comparison to the purely electronic band centered at 464 nm (21,552 cm1) indicate phonon modes of 920 cm1 and 1124 cm1. This observation is in conformity with the observed vibrational modes of ([email protected]) and (WAO) in the 910–960 cm1 region and PO2 symmetric vibrations mode at around 1150 cm1 in (100x)NaPO3xWO3 glasses, with high and low WO3 contents, respectively [28]. The emission spectra of the Eu3+-doped phosphate glass samples, measured with excitation at 464 nm, are presented in Fig. 3. The bands around 577, 590, 610, 651, 700, 745 and 800 nm are attributed to the radiative transitions from the excited state 5D0 to the lower energy lying states 7FJ, where J = 0, 1, 2, 3, 4, 5 and 6, respectively. The inset of the figure presents the emission spectra of the 0.5 mol% doped sample measured by excitation with different wavelengths (kexc = 393, 464, 531 nm), and normalized by the emission band at 610 nm. As it can be seen, there is an evident dependence of intensities with the kexc, being 464 and 531 nm the preferable ones to yield higher emission intensities. The trend in intensity is corroborated by the excited state lifetime (s) measurements obtained by monitoring the 5D0 ? 7F2 transition at 610 nm, upon excitation with the same wavelengths. Fig. 4(a) presents the decay curves of GEu05 from which the

0.0 550








0 550







GEu01 GEu025 GEu05 GEu1











Wavelength (nm) Fig. 3. Emission spectra of all the GEux glasses upon excitation at 434 nm. The inset presents the excitation wavelength dependence of the emission spectra of representative GEu05.

Fig. 4. Intensity decay profiles of: (a) GEu05 sample upon different excitation (393, 464, 531 nm); (b) GEux samples upon excitation at 464 nm. The decay curves were acquired by monitoring the emission at 610 nm (5D0 ? 7F2 transition).

lifetime values of 1.22 (kexc = 531 nm), 1.17 (kexc = 464 nm) and 1.36 ms (kexc = 393 nm) were determined by single exponential fitting. Although such apparent increase in lifetime value is often observed for Eu3+-doped systems when the excitation is done at higher energies, out of resonance with the emitting level 5D0 (see diagram in Fig. 5(a)), the intrinsic value of the excited state lifetime is not expected to vary and it is taken to be 1.2 ms on average. Fig. 4(b) presents the decay curves for all GEux samples excited at 464 nm, and the lifetime values obtained by fitting the curves with single exponential functions are listed in Table 2. As the Eu3+ concentration increases from 0.1 to 1.0 mol%, there is a slight decrease (15%) in s values that could be related to some extent of concentration quenching, already at low doping levels, but to obtain further insight in this matter, samples with higher Eu3+ concentrations would have to be tested. In Table 2, it is noted that the s values of our tungsten phosphate glasses are comparable or slightly lower than those of other phosphates, tellurite, borates and silicates glasses and glass ceramics commonly studied for efficient red emission. The values of the symmetry factor R are also listed in Table 2. The decrease of R-values is associated to the decrease of covalence between the Eu3+ ions and the surrounding ligands [26,27]. On the other hand, the high R value of the tungsten phosphate GEux glasses indicates highly asymmetric environment around Eu3+ ions, with high Eu–O covalency. These results are also in agreement

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Fig. 5. (a) Energy level diagram of Eu3+ ions under 393, 464 and 531 nm excitation wavelengths, depicting radiative and non-radiative (NR) decays. (b) CIE chromaticity diagram of GEu05 glass and the location given by its color coordinates. The inset presents a photograph of the sample excited at 464 nm.

Table 2 Excitation wavelength, intensity ratio (R), and lifetime (s) of 5D0 excited state in different glasses doped with Eu3+ ions. Glass sample

kexc (nm) ±1

R ±0.01

s (ms)

GEu01 GEu025 GEu05

464 464 393 532 464

7.09 7.16 5.56 6.54 6.81

1.35 1.22 1.36 1.22 1.17

GEu1 Telluro-fluoroborate [17] Tellurite [19] Tellurite ceramics [20] Borophosphate [22] Fluorosilicate [18] Silicate [30] Fluorosilicate glass ceramics [31] Phosphate [32] Aluminophosphate [33]

464 469 464 465 394 394 395 394 – 392

7.01 4.54 2.78 1.89 2.15 4.90 3.19 – 3.64 –

1.19 1.601 0.793 – 2.30 0.91 1.71–2.19 0.236 2.34–2.47 2.93


with previous structural studies performed on glasses in which it has been shown that tungsten phosphate glasses with high WO3 contents exhibit highly distorted WO6 clusters crosslinking the phosphate chains [28,34,35]. In tungsten phosphate glasses, WO3 and P2O5 are both glass formers and the rare earth can occupy sites with at least two different types of chemical environments considering first and second coordination sphere. Thus, it is not excluded that Eu3+ ions can be at least partially inserted in WO4 or WO6 tungsten clusters [36] experiencing high distortion in the first oxygen bonds. In any case, the observation of the intense red emission related with the Eu3+ low site symmetry suggests that these glasses as promising candidates for laser generation and lighting applications. These assumptions are also supported by the CIE chromaticity coordinates (x = 0.65 and y = 0.35) in Fig. 5 for the representative GEu05 sample under 464 nm excitation. A strong red emission is observed at naked eye (see inset). The Judd–Ofelt [37,38] theory (J.O.) is a semi-empirical method largely used to evaluate the spectroscopic quality of rare earth doped materials. This theory provides approaches to calculate the radiative rates, the intrinsic lifetimes and the branching ratios of the excited states of rare earth ions, based on the determination of the phenomenological intensity parameters Xt (t = 2, 4, 6 in this case). In general, the procedure [39] is based on equaling the

experimental oscillator strengths fexp – associated to transitions between the ground state 7F0 and the observed excited states, to their calculated oscillator strengths fcalc which is written in terms of the Xts parameters and the reduced matrix elements ||U(t)|| [40] for each transition. This way, a system of as many equations as observed transitions is obtained and solved by least square fitting, so as to result in the optimized Xt values. Therefore, the larger the number of transitions accounted for, the lower will be the error involved in the J.O. calculations. In the case of Eu3+, the bands observed in the UV–VIS absorption spectra are not adequate to allow for a good fit using this procedure, due to several null values of the reduced matrix elements. Consequently, the calculations result in large and unordinary X2 and X6, and undetermined X4. Alternatively, we used the procedure presented by Li et al. [41], which employs the emission spectrum instead of the absorption one. The bands observed in the emission spectrum of the representative sample GEu05 (Fig. 3) correspond to transitions from the upper energy state 5D0 to lower lying 7FJ states, with J = 0 to 6. The sum of the integrated emission intensities of all these bands is proportional to the radiative transition rate as:



AJ!J0 ¼ aA;


J 0 ¼0;1;2;3;4;5;6

where AJ ? J0 is the radiative rate for each 5D0 ? 7FJ0 transition and a is a constant. The total radiative transition rate (A) is equal to the inverse of the intrinsic radiative lifetime value of 5D0 state. Because the energy gap between 5D0 and the first lower lying state (7F6) corresponds to 12,000 cm1, and also because the ionic concentration of the studied sample is quite low (0.5 mol%), nonradiative decays via multiphonon relaxation and ion-ion energy transfer are negligible for this sample. Therefore, the intrinsic lifetime value is taken as the average experimental value (s = 1.2 ms) from which a can be obtained. The individual rates AJ?J0 for each transition are related to the total radiative transition rate A through the luminescence branching ratios b as:

AJ!J0 : A


The b values can be determined from the emission spectra and are often used as a parameter to evaluate lasing potential, that is, an emission transition with branching ratio larger than 50% can potentially yield laser action [42]. For the GEu05 sample under

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M.R. Dousti et al. / Optical Materials xxx (2015) xxx–xxx Table 3 Wavelength, reduced matrix elements, and experimentally derived radiative emission probabilities and branching ratios of different transitions of Eu3+ ions. Transitions 5 D0 ?

Wavelength ±1 (nm)



577 590 610 651 700 745 800

0 0 0.0032 0 0 0 0

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


0 0 0 0 0.0023 0 0


0 0 0 0 0 0 0.0002

AJ?J0 ±0.01 (s1)

b ±0.01 (%)

8.06 96.30 599.41 23.69 120.18 5.40 8.16

0.94 11.18 69.60 2.75 13.95 0.63 0.95

Glass and reference




Current study Fluorosilicate [18] Borophosphate [22] Telluro-fluoroborate [17] Tellurite ceramics [20] Lead–zinc phosphate [32] Sodium phosphate [45] Pentaphosphate crystal [45]

6.86 7.44 2.98 6.92 2.95 6.52 4.12 3.66

3.22 0.72 0.42 3.43 0.39 2.29 4.69 1.43

8.20 – – 0 – 0.98 1.83 3.07

464 nm excitation, the maximum experimental branching ratio value b  70% was determined for the 5D0 ? 7F2 transition at 610 nm, as presented in Table 3 along with the AJ?J´ rates. This b value is higher than the one calculated for germanate (60%, s = 1.7 ms [43]), boro-tellurite glasses (60%, s = 1.4 ms [43]), ZBLA (10% [44]), pentaphosphate glass (61% [45]) and zirconium fluoride glasses (24.2% [45]). The 5D0 ? 7F1 transition of Eu3+ ions is a magnetic dipole transition, whose line strength Smd = 7.83  1042 [42] is independent from the host composition or the ionic concentration. Therefore, one can estimate the refractive index of the host by

64p n

4 3

3hð2J þ 1Þk3



where e, m, c, h and k have their common physical definitions. The refractive index for the GEu05 glass is found to be 2.1, which is comparable to values reported for similar compositions [10,46]. The 5D0 ? 7FJ0 transitions are forbidden for J0 = 0, 3 and 5, and are allowed for J0 = 2, 4, and 6. In particular, the radiative transition rates AJ?J0 can be calculated for the allowed transitions by

Aed ¼ 5 D !7 F 0 2;4;6

64p4 e2 3hð2J þ 1Þk3 

2 nðn2 þ 2Þ X Xt jhðS; LÞJkU ðtÞ kðS0 ; L0 ÞJ0 ij2 ; 9 t¼2;4;6

phosphate [32] glasses. The values of X6 are rarely reported due to calculation exertions based on the method that uses the absorption spectrum. 4. Conclusion

Table 4 Judd–Ofelt intensity parameters (1020 cm2) of Eu3+ ions in different hosts.

Amd 5 D !7 F ¼ 0 1



where the squared reduced matrix elements ||U(t)||2 (t = 2, 4 and 6) are invariable from host to host [47,48] and are listed in Table 3. By employing the calculated rates AJ?J0 , the refractive index n and the squared reduced matrix elements, the J.O. intensity parameters X2, X4 and X6 were determined using Eq. (4) and their values are listed in Table 4, in comparison to other glasses. The interpretation of the meaning of these parameters is controversial but it is generally accepted that X2 is related to the covalence of rare earth bonds, being usually higher when the ion occupies highly polarized and asymmetric sites. For X4 and X6 the interpretation is less specific but their dimension is often directly related to the rigidity of the medium [49]. As observed in Table 4 the X2 and X4 values are similar to those reported for telluro-fluoroborate [17] and lead–zinc

Tungsten-phosphate glasses with composition 4(Sb2O3)96x (50WO3 50NaPO3)xEu2O3 where x = 0, 0.1, 0.25, 0.5 and 1.0 mol% were prepared and investigated as potential optical materials in the visible spectral region. The introduction of Eu3+ ions in the glasses slightly increases the glass transition temperature and results an average density of 4.7 g cm3. The glasses present high thermal and chemical stability and they can be fabricated at low cost in a variety of sizes and shapes. Analysis by Judd–Ofelt theory indicated high values of relative intensity ratio (R), intensity parameters (Xk), and experimental branching ratio (b) at 610 nm. The excited state lifetime values (1.2 ms) are comparable to those of other glassy systems such as germanate and tellurite. All these characteristics suggest these glasses can become promising new materials for lasing and lighting applications in the visible spectral region. Acknowledgements This research was supported by FAPESP - Fundação de Amparo à Pesquisa do Estado de São Paulo (CEPID Project No. 2013/07793-6 and Post-doctoral fellowship Project No. 2013/24064-8) and CNPq - Conselho Nacional de Desenvolvimento Científico e Tecnológico (Universal Project No. 479672/2012-1). Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at 033. References [1] R.K. Brow, J. Nanophotonics 263&264 (2000) 1–28. [2] A. Zakery, S.R. Elliott, J. Non Cryst. Sol. 330 (2003) 1–12. [3] C.M. Baldwin, R.M. Almeida, J.D. Mackenzie, J. Non. Cryst. Sol. 43 (1981) 309– 344. [4] J.C. Mauro, E.D. Zanotto, Int. J. Appl. Glas. Sci. 15 (2014) 313–327. [5] B.C. Sales, Mater. Res. Soc. Bull. 12 (1987) 32. [6] L. Bih, N. Allali, A. Yacoubi, A. Nadiri, D. Boudlich, M. Haddad, A. Levasseur, J. Phys. Chem. Glasses 40 (1999) 229–237. [7] R.K. Brow, D.R. Tallant, J. Non Cryst. Sol. 222 (1997) 396–406. [8] S.W. Lee, J.H. Lee, J. Phys. Chem. Glasses 36 (1995) 127–130. [9] P.A. Tick, N.F. Borrelli, L.K. Cornelius, M.A. Newhouse, J. Appl. Phys. 78 (1995) 6367–6374. [10] G. Poirier, M. Poulain, Y. Messaddeq, J. Non-Cryst. Sol. 351 (2005) 293–298. [11] S.H. Santagneli, C.C. Araujo, H. Eckert, G. Poirier, S.J.L. Ribeiro, Y. Messaddeq, J. Phys. Chem. B 111 (2007) 10109–10117. [12] I. Rosslerova, L. Kouldelka, Z. Csernosedk, P. Mosner, L. Benes, J. Non-Cryst. Sol. 384 (2014) 41–46. [13] G. Poirier, C.B. de Araujo, Y. Messaddeq, S.J.L. Ribeiro, J. Appl. Phys. 91 (12) (2002) 10221–10223. [14] G. Poirier, M. Nalin, L. Cescato, S.J.L. Ribeiro, Y. Messaddeq, J. Chem. Phys. 125 (2006) 161101. [15] G. Poirier, F.S. Ottoboni, F.C. Cassanjes, A. Remonte, Y. Messaddeq, S.J.L. Ribeiro, J. Phys. Chem. B 112 (2008) 4481–4487. [16] P. Subbalakshmi, B.V. Raghavaiah, R. Balaji Rao, N. Veeraiah, P. Babu, C.K. Jayasankar, Eur. Phys. J. Appl. Phys. 26 (2004) 169–176. [17] R. Vijayakumar, K. Maheshvaran, V. Sudarsan, K. Marimuthu, J. Lumin. 154 (2014) 160–167. [18] D. Ramachari, L. Rama Moorthy, C.K. Jayasankar, J. Lumin. 143 (2013) 674–679. [19] A.M. Babu, B.C. Jamalaiah, T. Suhasini, T.S. Rao, L.R. Moorthy, Solid State Sci. 13 (2011) 574–578. [20] B.J. Chen, E.Y.B. Pun, H. Lin, J. Alloys Compd. 479 (2009) 352–356. [21] S. Neeraj, N. kijima, A.K. Cheetham, Chem. Phys. Lett. 387 (2004) 2–6. [22] N. Kiran, J. Mol. Struct. 1065–1066 (2014) 93–98. [23] C. Kodaira, H. Brito, O. Malta, O. Serra, J. Lumin. 101 (2003) 11–21. [24] C.R. Kesavulu, K.K. Kumar, N. Vijaya, K.-S. Lim, C.K. Jayasankar, Mater. Chem. Phys. 141 (2013) 903–911.

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