Spectroscopic properties of Pr3+ and Er3+ ions in lead-free borate glasses modified by BaF2

Spectroscopic properties of Pr3+ and Er3+ ions in lead-free borate glasses modified by BaF2

Optical Materials 47 (2015) 548–554 Contents lists available at ScienceDirect Optical Materials journal homepage: www.elsevier.com/locate/optmat Sp...

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Optical Materials 47 (2015) 548–554

Contents lists available at ScienceDirect

Optical Materials journal homepage: www.elsevier.com/locate/optmat

Spectroscopic properties of Pr3+ and Er3+ ions in lead-free borate glasses modified by BaF2 Joanna Pisarska a,⇑, Wojciech A. Pisarski a, Dominik Dorosz b, Jan Dorosz b a b

University of Silesia, Institute of Chemistry, Szkolna 9, 40-007 Katowice, Poland Bialystok University of Technology, Faculty of Electrical Engineering, Wiejska 45D, 15-351 Bialystok, Poland

a r t i c l e

i n f o

Article history: Received 16 May 2015 Received in revised form 15 June 2015 Accepted 17 June 2015 Available online 21 June 2015 Keywords: Glasses Rare earth ions Absorption Luminescence Spectroscopic properties

a b s t r a c t Lead-free oxyfluoride borate glasses singly doped with Pr3+ and Er3+ were prepared and next investigated using absorption and luminescence spectroscopy. In the studied glass system, barium oxide was substituted by BaF2. Two luminescence bands of Pr3+ located at visible spectral region are observed, which correspond to 3P0–3H4 (blue) and 1D2–3H4 (reddish orange) transitions, respectively. The luminescence bands due to 1D2–3H4 transition of Pr3+ are shifted to shorter wavelengths, when BaO was substituted by BaF2. Near-infrared luminescence spectra of Er3+ ions in lead-free borate glasses modified by BaF2 correspond to 4I13/2–4I15/2 transition. Their spectral linewidths increase with increasing BaF2 concentration. The changes in measured lifetimes of rare earth ions are well correlated with the bonding parameters calculated from the optical absorption spectra. Ó 2015 Elsevier B.V. All rights reserved.

1. Introduction In recent years PbO/PbF2 and/or CdO/CdF2 are often used as a heavy metal glass components, but they are rather prohibited to use in manufacturing due to environment protecting. They have been designated as toxic substances, and consequently they are being eliminated from various practical applications due to their hazardous effect on health and environment. Therefore, rare earth doped bismuthate [1] and germanate [2] glasses are very useful for exploiting as lead- or cadmium-free glasses for optical applications. Especially, lead-free oxyfluoride germanate glasses modified by BaF2 singly doped with Tm3+ [3] and doubly doped with Tm3+ and Yb3+ [4] have been investigated for application as near-infrared laser materials at 1800 nm. Further studies suggest that Tm3+-doped germanate glass fibers with a large core diameter has proved to be promising infrared optical and high-power level laser materials [5]. Also, alkali borate and fluoroborate glasses belonging to lead-free glass family are the most suitable one for the rare earth doping (for example Pr3+ and Er3+) due to their hardness, transparency, resistance toward moisture and chemical durability. An interesting characteristic feature of the borate and fluoroborate glasses containing Pr3+ and Er3+ ions is the appearance of variations

⇑ Corresponding author. E-mail address: [email protected] (J. Pisarska). http://dx.doi.org/10.1016/j.optmat.2015.06.037 0925-3467/Ó 2015 Elsevier B.V. All rights reserved.

in its structural and spectroscopic properties, when alkali or alkaline earth cations are introduced [6–10]. Generally, the Pr3+ doped glasses are known as reddish-orange emitting materials due to the 3 P0–3H6 and/or 1D2–3H4 transitions, depending on host compositions [11,12], whereas glasses with Er3+ ions find wide applications, especially in telecommunication field as broadband amplifier operating at near-infrared region [13]. The previously published work indicates that Er3+ doped fluoroborate glasses with MF (where M = Li, Na, and K) are promising for near-infrared broadband amplifiers [14]. Fluoroborate glasses containing monovalent and/or divalent fluoride components and rare earth ions have been usually studied as a function of activator concentration [15–17]. The experimental results for Sm3+ [18], Eu3+ [19] and Ho3+ [20] ions in fluoroborate glasses with different modifier oxides were also presented and discussed in details. To the best of our knowledge, the spectroscopic investigations of rare earth doped fluoroborate glasses with fluoride content are rather less documented in the literature. Previous investigations clearly suggest that an addition of fluoride compounds improves the broad luminescence and lifetime of rare earths compared to pure oxide glasses [21]. Jayasankar and Babu [6] inform that the predicted lifetimes for fluorescent states of Pr3+ increase with increasing LiF content in borate glass composition. The spectral intensities of most of the bands of Dy3+ increased with the increase sodium fluoride content in fluoroborate glass [22]. Therefore, it is very essential to optimize the relationship between fluoride content in glass composition and the

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spectroscopic parameters for rare earth ions, which is important from the optical point of view. Among inorganic glasses, only a few works is devoted to rare earth doped borate glasses modified by BaF2 and their crystallization behavior. Dwivedi and Rai [23] suggest that an addition of Bi2O3 during Eu3+-doped oxyfluoroborate glass formation, with a fixed concentration of BaF2 content, increases the glass stability and heat treatment of such a glass leads to increase in its luminescence intensity. Komatsu et al. [24] proposed a new oxyfluoride glass BaF2–Al2O3–B2O3 with a large fraction of fluorine in order to fabricate glass–ceramics containing highly oriented BaAlBO3F2 nonlinear optical crystals. The introduction of barium oxyfluoride network into the B2O3–TeO2–BaO–BaF2–Er2O3 system enhances the optical and mechanical properties of the glasses and acts as a good modifier [25]. These facts motivated us to carry out the spectroscopic studies of rare earth doped lead-free fluoroborate glasses with BaF2 content. Here, we present spectroscopic results obtained for rare earth ions in lead-free borate glasses containing barium fluoride. The optically active ions were limited to trivalent Ln3+ from the beginning (Pr3+) and the end (Er3+) of lanthanide series. The spectroscopic properties of Pr3+ and Er3+ ions in oxyfluoride borate glasses have been examined with BaF2 content. 2. Experimental Series of samples: xBaF2–(30  x)BaO–60B2O3–9.5Ga2O3– 0.5Ln2O3 (x = 0, 5, 20, 30 mol%, Ln = Pr, Er) were prepared by mixing and melting appropriate amounts of metal anhydrous oxides and fluorides of high purity (99.99%, Aldrich Chemical Co.) as starting materials. In order to prepare samples, appropriate amounts of all components were mixed homogeneously together. Due to the hygroscopicity of the fluorides and, in order to minimize the adsorbed water content, all glass components were weighted and stored in glove box, in a protective atmosphere of dried argon. Then, they were melted in Pt crucibles at 1200 °C for 45 min. Transparent glassy plates of 10  10 mm dimension were obtained. Each glass sample of 2 mm in thickness was polished for optical measurements. The nature of the studied samples was identified using the X-ray diffraction analysis (X’Pert X-ray diffractometer). The samples were characterized by a Perkin–Elmer differential scanning calorimeter (DSC). The DSC curves were acquired with heating rate of 10 °C/min. The glass transition temperature Tg was determined with accuracy of 0.5 °C. Absorption spectra were recorded using a Varian 5000 UV–VIS–NIR spectrophotometer. The luminescence spectra and their decays were performed on a PTI QuantaMaster QM40 coupled with tunable pulsed optical parametric oscillator (OPO), pumped by a third harmonic of a Nd:YAG laser (Opotek Opolette 355 LD). The luminescence was dispersed by double 200 mm monochromators. The luminescence spectra were recorded using a multimode UVVIS PMT (R928) and Hamamatsu H10330B-75 detectors controlled by a computer. The spectral measurements were carried out with a resolution of 0.1 nm. Luminescence lifetimes were determined with accuracy of 1 ls. All measurements were carried out at room temperature. 3. Results and discussion 3.1. X-ray diffraction and glass transition temperature Lead-free borate glasses modified by BaO and/or BaF2 were prepared. In order to prove amorphous or semi-crystalline state of the obtained samples the phase analysis was done with use of the X-ray diffraction. Fig. 1 shows typical X-ray diffraction patterns

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Fig. 1. Typical X-ray diffraction patterns for lead-free borate glasses.

for the studied samples with BaF2 content (5 mol%). The X-ray diffraction patterns reveal only two broad peaks, which are typical for glassy state. Independently on rare earth dopants (Pr3+ or Er3+) and BaF2 concentration, all the studied samples are fully amorphous. Fig. 2 presents DSC curves for lead-free borate glass samples recorded under standard heating rate (10 °C/min). From DSC curves glass transition temperatures, Tg were evaluated. Fig. 2 (on left) shows variation of glass transition temperature with BaF2 content. The values of Tg are also given in Table 1. Generally, the glass transition temperatures are not dependent on rare earth ion dopants (Pr3+ or Er3+), but their values are reduced with increasing BaF2 concentration in glass composition. Similar effects were observed earlier by us for lead borate glasses, where the values Tg decreased with increasing PbF2 concentration [26]. 3.2. Absorption The optical absorption spectra of Pr3+ (Fig. 3) and Er3+ (Fig. 4) ions in lead-free borate glasses recorded at room temperature in the UV–visible and near-infrared wavelength region are presented. The spectra consist of several inhomogeneously broadened absorption lines characteristic for transitions of trivalent rare earth ions within 4f2 (Pr3+) and 4f11 (Er3+) electronic configurations, respectively [27]. The absorption bands of Pr3+ ions correspond to transitions originating from the 3H4 ground state to the 3H6, 3F2, 3F3, 3F4 (near-infrared region) and 1D2, 3P0, 3P1, 3P2 (visible region) excited states. The absorption bands of Er3+ are related to electronic transitions from the 4I15/2 ground state to the 4I13/2, 4I11/2 (near-infrared region) and 4F9/2, 4S3/2, 2H11/2, 4F7/2, 4F3/2, 4F5/2, 2G9/2, 4G11/2, 2K15/2, 4 G9/2, 2G7/2 (UV–visible region) excited states. From optical absorption spectra bonding parameters (b and d) were calculated using the following relation [28]:

d ¼ ½ð1  bÞ=b  100 where b = RN = b*/N and b* = mc/ma, b is the shift of energy level position (Nephelauxetic effect), mc and ma are energies of the corresponding transitions in the investigated complex and free-ion [29], respectively, and N denotes the number of levels used for b values calculation. Positive or negative sign for the d value indicates covalent or ionic bonding between the rare earth ions and surrounding ligands. The results are given in Table 2. The bonding parameters d

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Fig. 2. DSC curves for lead-free borate glasses with BaF2 content.

Table 1 Glass composition. Glass sample

Chemical composition (mol%)

Tg (°C)

0% BaF2 5% BaF2 20% BaF2 30% BaF2

60B2O3–30BaO–9.5Ga2O3–0.5Ln2O3 60B2O3–25BaO–5BaF2–9.5Ga2O3–0.5Ln2O3 60B2O3–10BaO–20BaF2–9.5Ga2O3–0.5Ln2O3 60B2O3–30BaF2–9.5Ga2O3–0.5Ln2O3

566 552 542 537

Ln = Pr or Er.

for both Pr3+ and Er3+ doped oxide glass samples with BaO are close to 0.675 and 0.823, respectively. It clearly indicates that bonding between the optically active ions and the nearest surroundings is ionic. There is in a good agreement with recent electronic studies of alkaline earth metal oxides using density functional theory (DFT) calculations, where the charge density plots confirm the ionic nature of metal oxides MO (M = Mg, Ca, Sr, Ca, Ba) and the ionic bonding is the strongest in BaO. Cinthia et al. [30] concluded that in BaO, the charge density in the interstitial region is very low due to the localization of the electrons, caused by the complete transfer of the valence electrons to the oxygen atom, featuring the ionic nature of the bond between Ba ion and oxygen ion. It is interesting to see that the ionic bonding increases slightly to 0.774 (Pr3+-doped sample) and 0.843 (Er3+-doped sample), when BaO was partially substituted by BaF2 in glass composition. However, further investigations clearly indicate that the bonding parameters are increased with increasing BaF2 concentration (Table 2), suggesting less ionic bonding between the rare earths and the nearest surroundings. Based on absorption spectra and calculations of bonding parameters, we can conclude that the presence of BaF2 promotes a modification of the borate glass structure as well as the number of non-bridging B–O groups contribute to the chemical bonds between Ln3+ (Ln = Pr or Er) and surrounding ligands (O2, F).

3.3. Excitation and luminescence Fig. 5 presents excitation spectra for trivalent Pr3+ ions in lead-free borate glasses modified by BaF2. The spectra were monitored at 640 nm emission wavelength. Similarly to UV–visible absorption spectra (Fig. 3), the excitation spectra consists of four characteristic bands, which correspond to transitions originating

from the 3H4 state to the higher-lying 3P2, 3P1, 3P0 and 1D2 states of Pr3+. The excitation spectra recorded for Pr3+ ions indicate that the energy gaps between 3P2, 3P1 and 3P0 excited states are very small. Thus, the excitation energy transfers very fast from the 3P2 state via 3P1 state to the 3P0 state by nonradiative relaxation. Energy gap between 3P0 state and the next lower lying 1D2 state is found to be 3900 cm1. The phonon energy of the borate based glass host is close to about ⁄m = 1300 cm1, and only three phonons are needed to bridge 3P0–1D2 energy gap [31]. Thus, the 1D2 state is populated quite efficiently by 3P0 state of Pr3+ and consequently visible luminescence from both 3P0 and 1D2 excited states can be successfully observed. Luminescence spectra for Pr3+ ions in lead-free borate glasses with different BaF2 content are presented in Fig. 6. The glass samples were excited at 445 nm (3P2 state of Pr3+). Luminescence spectra of Pr3+ ions in lead-free borate glasses modified by BaF2 consist of two less-intense and higher-intense bands located at about 490 nm and 600 nm, which correspond to 3 P0–3H4 (blue) and 1D2–3H4 (reddish orange) transitions of Pr3+. The similar phenomena were observed for Pr3+ in lead borate glass [32]. From the exponential dependence of multiphonon relaxation rates for rare earths in various glasses on the energy gap to the next-lower level, it is clearly seen that Wnr value for borate glass is approximately 103 times larger than that of fluoride system [33]. Thus, the 1D2 state is populated more efficiently in lead-free oxyfluoride borate glass than fluoroindate glass [34]. At consequence, reddish orange luminescence corresponding to 1D2–3H4 transition of Pr3+ ions is quite well observed. The emission intensity is significantly higher for the 1D2–3H4 transition (reddish orange band) than 3P0–3H4 transition (blue band). In contrast to fluoroindate glass modified by P2O5 [34], the luminescence band due to 1D2–3H4 transition of Pr3+ is shifted to shorter wavelengths, when BaO was substituted by BaF2 in borate glass. Fig. 7 presents excitation spectra for Er3+ ions in lead-free borate glasses with different BaF2 content. The spectra were monitored at 545 nm emission wavelength. The characteristic bands of Er3+ from the 4I15/2 state to the 4F7/2, 4F5/2, 4F3/2, 2G9/2, 4G11/2, 4G9/2 states are observed in the 350–500 nm spectral region. Due to the relatively low energy gaps between excited state of Er3+ ions and relatively high phonon energy of the host, borate glass usually eliminates luminescence from the higher lying states. Thus, the excitation energy transfers nonradiatively very fast from the 4F7/2 state to the 4I13/2 state and near-infrared luminescence corresponding to 4I13/2–4I15/2 transition of Er3+ can be detected.

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excited at 488 nm. The luminescence band located at about 1550 nm corresponds to main 4I13/2–4I15/2 laser transition of Er3+. In our case, linewidth defined as the full width at half maximum (FWHM) is changed significantly with substitution BaO by BaF2. Spectral linewidth increases from 98 nm (sample without BaF2) to 116 nm (5% BaF2), 124 nm (20% BaF2) and 133 nm (30% BaF2) with increasing BaF2 content (Fig. 9). The linewidths for oxyfluoride glass samples with BaF2 are considerably higher than values obtained for lead borate glass singly doped with Er3+ (Dk = 100.5 nm) [40] and doubly doped with Er3+–Yb3+ (Dk = 110 nm) [41]. Also, the previously published results for barium borate glass confirm broadband NIR luminescence properties of Nd3+ [42]. 3.4. Luminescence decay analysis Luminescence decays from the 1D2 (Pr3+) and 4I13/2 (Er3+) excited states of rare earth ions in lead-free borate glasses modified by BaF2 were also measured. Luminescence decay curves for

Fig. 3. UV–visible (top) and near-infrared (bottom) absorption spectra for Pr3+ ions in lead-free borate glasses modified by BaF2.

Generally, trivalent erbium ions are the most popular rare earths emitting light in the near-infrared ranges. Several Er3+-doped systems were applied as a solid-state lasers and optical amplifiers. Especially, glasses or glass fibers containing Er3+ can operate as optical amplifiers in the standard telecommunication window. Some spectroscopic parameters of Er3+ ions, for example luminescence linewidths and lifetimes, are necessary to characterize material for solid-state laser active media or broadband optical amplifiers [35–39]. It is well established that infrared radiative transition occurs from the 4I13/2 excited state of Er3+ at about 1.5 lm, which is the low loss optical window of the waveguide. Active Er3+-doped optical fiber can be quite easily obtained from the precursor borate glass. However, Er3+-doped borate glasses are rather inefficient fluorescent materials. The high frequencies of the B–O vibrations can quench the Er3+ emission at 1.5 lm and degrade the laser performance. For that reason, luminescence has not been observed for various borate based glass systems. Fig. 8 shows near-infrared emission spectra for Er3+ ions in lead-free borate glasses modified by BaF2. The glass samples were

Fig. 4. UV–visible (top) and near-infrared (bottom) absorption spectra for Er3+ ions in lead-free borate glasses modified by BaF2.

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Table 2 Bonding parameters d for Pr3+ and Er3+ in lead-free borate glasses modified by BaF2. Ln3+

Transition

Pr3+

3

Er3+

H4?

4

I15/2?

Band position (cm1) 0% BaF2

5% BaF2

20% BaF2

30% BaF2

3

H6 3 F2 3 F3 3 F4 1 D2 3 P0 3 P1 3 P2

4489 5281 6623 7108 17,081 20,780 21,327 22,680

4521 5294 6610 7100 17,104 20,790 21,322 22,676

4532 5187 6625 7084 17,107 20,789 21,321 22,687

4535 5220 6627 7133 17,048 20,782 21,322 22,653

b d

1.0068 0.675

1.0078 0.774

1.0067 0.666

1.0055 0.547

4

I13/2 I11/2 4 F9/2 4 S3/2 2 H11/2 4 F7/2 4 F5/2 4 F3/2 (2G,4F)9/2 4 G11/2 4 G9/2, 2K15/2 2 G7/2

6549 10,252 15,373 18,441 19,257 20,549 22,254 22,674 24,667 26,542 27,490 28,133

6543 10,266 15,396 18,450 19,255 20,562 22,263 22,684 24,651 26,549 27,510 28,116

6544 10,258 15,380 18,445 19,245 20,546 22,232 22,678 24,658 26,514 27,488 28,120

6536 10,245 15,388 18,445 19,251 20,553 22,240 22,687 24,648 26,526 27,488 28,120

b d

1.0083 0.823

1.0085 0.843

1.0079 0.784

1.0078 0.774

4

Fig. 6. Luminescence spectra of Pr3+ ions in lead-free borate glasses with BaF2 content.

Pr3+ (Fig. 10) and Er3+ (Fig. 11) were fitted to single exponential function because of the low activator concentration and the lack of energy transfer processes between rare earth ions. Based on decays, luminescence lifetimes for the 1D2 (Pr3+) and 4I13/2 (Er3+) excited states of rare earth ions were determined. Generally, the values of luminescence lifetimes are changed, when BaO was substituted by BaF2. The 1D2 (Pr3+) measured lifetime is slightly reduced from 14.9 ls (0% BaF2) to 14.3 ls (5% BaF2) and next starts to increase up to 15.4 ls (20% BaF2) and 16 ls (30% BaF2), respectively. These effects were obtained previously for Pr3+ ions in lead borate glass, where the increase of the PbF2 concentration changes the 1D2 luminescence lifetime from 13.5 to 16.5 ms [43]. For Er3+-doped glass samples, the 4I13/2 measured lifetime is reduced from 424 ls (0% BaF2) to 384 ls (5% BaF2) and next increases up

Fig. 7. Excitation spectra of Er3+ ions in lead-free borate glasses modified by BaF2.

to 420 ls (20% BaF2) and 469 ls (30% BaF2) with increasing BaF2 content. However, the most interesting phenomenon is that both measured lifetimes for the 1D2 (Pr3+) and 4I13/2 (Er3+) excited states of rare earth ions are well correlated with bonding parameters calculated from the optical absorption spectra. Fig. 12 shows the influence of BaF2 content on bonding parameters and measured lifetimes for excited states of trivalent rare earths. The trend of both parameters is similar and confirms spectroscopically that BaF2 give important contribution to borate glass structure and luminescence characteristics of rare earth ions.

4. Summary

Fig. 5. Excitation spectra of Pr3+ ions in lead-free borate glasses modified by BaF2.

Rare earth doped lead-free borate glasses modified by BaF2 were prepared and next studied using absorption and luminescence spectroscopy. The spectroscopic results obtained for Pr3+ and Er3+ ions in oxyfluoride borate glasses lead to the following conclusions:

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Fig. 8. NIR luminescence spectra of Er3+ ions in lead-free borate glasses with BaF2 content.

Fig. 10. Luminescence decays from the 1D2 state of Pr3+ ions in lead-free borate glasses modified by BaF2.

Fig. 9. Influence of BaF2 content on spectral linewidth of 4I13/2–4I15/2 transition of Er3+.

1. The bonding parameters d were calculated from the optical absorption spectra. All glass samples have negative sign for the d values indicating ionic bonding between the rare earths and surrounding ligands. Their values increased with increasing BaF2 concentration. 2. Luminescence of Pr3+ and Er3+ was examined with BaF2 content. Visible luminescence spectra consist of two bands located at 490 nm and 600 nm, which correspond to 3 P0–3H4 (blue) and 1D2–3H4 (reddish orange) transitions of Pr3+. The bands due to 1D2–3H4 transition of Pr3+ are shifted to shorter wavelengths, when BaO was substituted by BaF2. Near-infrared luminescence spectra correspond to 4 I13/2–4I15/2 transition of Er3+. The spectral linewidths for 4 I13/2–4I15/2 transition of Er3+ in lead-free borate glasses modified by BaF2 are relatively large (above 100 nm). Their values increased with increasing BaF2 content.

Fig. 11. Luminescence decays from the 4I13/2 state of Er3+ ions in lead-free borate glasses modified by BaF2.

3. Luminescence decays from the 1D2 (Pr3+) and 4I13/2 (Er3+) excited states of rare earths were examined. The measured lifetimes increased with increasing BaF2 concentration. The changes in luminescence lifetimes of trivalent rare earths (Pr3+, Er3+) with BaF2 content are well correlated with the bonding parameters calculated from the optical absorption spectra.

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Fig. 12. Influence of BaF2 content on bonding parameters and luminescence lifetimes for excited states of rare earth ions.

[33] [34] [35] [36]

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