Structural, thermal and optical investigations of Dy3+ ions doped lead containing lithium fluoroborate glasses for simulation of white light

Optical Materials 35 (2013) 1385–1394

Contents lists available at SciVerse ScienceDirect

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

Structural, thermal and optical investigations of Dy3+ ions doped lead containing lithium fluoroborate glasses for simulation of white light Sd. Zulfiqar Ali Ahamed a, C. Madhukar Reddy a, B. Deva Prasad Raju b,a,⇑ a b

Department of Physics, Sri Venkateswara University, Tirupati 517 502, India Department of Future Studies, Sri Venkateswara University, Tirupati 517 502, India

a r t i c l e

i n f o

Article history: Received 4 October 2012 Received in revised form 24 January 2013 Accepted 8 February 2013 Available online 18 March 2013 Keywords: Glasses Raman spectra Absorption Color coordinates Energy transfer Lifetime

a b s t r a c t Lead containing barium zinc lithium fluoroborate (LBZLFB) glasses doped with different concentrations of trivalent dysprosium ions were synthesized by conventional melt quenching method and characterized through the XRD, DSC, FTIR, FT-Raman, optical absorption, photoluminescence and decay curve analysis. X-ray diffraction studies revealed amorphous nature of the studied glass matrices. The thermal behavior has been reported by recording DSC thermograms. Coexistence of trigonal BO3 and tetrahedral BO4 units was evidenced by IR and Raman spectroscopy. Judd–Ofelt intensity parameters have been evaluated for 1.0 mol% Dy3+ ions doped LBZLFB glass. The measuring branching ratios are reasonably high for transitions 4F9/2 ? 6H15/2 and 6H13/2 suggesting that the emission at 486 and 577 nm, respectively can give rise to lasing action in the visible region. From the visible emission spectra, the yellow to blue (Y/B) intensity ratios and chromaticity color coordinates were estimated. A combination of blue and yellow emissions has emerged in the glasses, which allows the observation of white light when the glasses are excited by the ultraviolet/blue light. These Dy3+ doped glasses are studied for their utility for white light generation under 454 nm excitation and the present LBZLFB glass is more suitable for generation of white light for blue LED chips. Ó 2013 Elsevier B.V. All rights reserved.

1. Introduction In modern lighting and display technology fields the lanthanide ions (Ln3+) have vigorous research activities because of the abundant emission colors based on their 4f–4f or 4f–5d transitions. The study on Ln3+ ions doped materials gained significant importance due to their potential applications in the field of photonics as optical storage, display monitors, X-ray imaging, sensors, lasers, up conversion and amplifiers for fiber optic communications [1]. Glasses doped with Ln3+ ions are good laser materials as they emit intense radiation in the visible, infrared and near-infrared regions under suitable excitation conditions and the studies on the optical properties of the Ln3+ ions in glasses is essential to design optical devices such as laser, color displays, upconverters and fiber amplifiers. Advancement in telecommunications industry also demands a rapid growth in the development of suitable photonic materials for the optical fiber technology [2,3]. In recent years white light emitting diodes (WLEDs) have attracted great attention as potential candidates for the replacement of conventional incandescent fluorescent lamps. In comparison with incandescent fluorescent

⇑ Corresponding author at: Department of Future Studies, Sri Venkateswara University, Tirupati 517 502, India. Tel.: +91 94402 81769. E-mail address: [email protected] (B. Deva Prasad Raju). 0925-3467/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.optmat.2013.02.006

lamps the WLEDs have longer lifetime, higher efficiency, better reliability and eco friendly. Although commercial availability of WLEDs is currently from phosphors excited by the blue LED chip, a new trend is to realize the white light emission in glasses. The versatility of glasses regarding the possibility of a wide doping concentration, lower production cost, simple preparation procedure, halo effect and the narrow lines emission spectra of the lanthanide ions could be considered as a promising alternative approach than phosphors since the first simulation of white light in borate glass [4]. In order to identify the new optical devices for specific utility or the devices with enhanced performance active work is being carried out by selecting appropriate new hosts doped with Ln3+ ions. Dy3+-doped glasses and crystals have been considered as promising laser active materials and the 6H11/2 (6H9/2) ? 6H15/2 transition of Dy3+ around 1.3 lm is found to be useful for optical fiber communication [5]. Little attention has been paid to the visible emission originating from the 4F9/2 state situated at about 21, 000 cm1 [6–8] and crystal field analysis. The complicated electronic structure of the 4f9 configuration of Dy3+ ion and the large number of energy levels lying close to each other makes the crystal field analysis more cumbersome. However, tremendous progress in the development of laser diodes is going to overcome this disadvantage. Commercialization of blue laser diodes opens new possibilities of optical pumping; therefore potential laser transition in

1386

Sd. Zulfiqar Ali Ahamed et al. / Optical Materials 35 (2013) 1385–1394

the visible region will be the topic of research for future interest. Among the lanthanide ions trivalent dysprosium (Dy3+) doped glasses have been considered as promising materials for white light emission. Since Dy3+ ions possesses intense emission at blue and yellow regions, which are associated with the 4F9/2 ? 6H15/2 and 4F9/2 ? 6H13/2 transitions respectively. Further the later one is a hypersensitive transition which is strongly influenced by the environment. At a suitable yellow to blue (Y/B) intensity ratio, Dy3+ ions will emit white light. Thus the luminescent materials doped/co-doped with Dy3+ ions are usually used to the generation of white light both in glasses and phosphors [9–13]. Pure white light could be produced by adjusting the yellow to blue integral intensity values. The yellow to blue (Y/B) luminescence intensity ratio can be modulated by varying the composition of glass, Dy3+ concentration, excitation wavelength for the generation of white light [14]. A proper selection of the host will facilitate the extraction of primary colors, yellow from the 4F9/2 ? 6H13/2 transition and blue from the 4F9/2 ? 6H15/2 transition of Dy3+ ions. The luminescence phenomenon is the process of absorbing incident energy and converting it into visible light. Among all the classical network formers, boric oxide (B2O3) is one of the significant glass former and flux material, having wide applications in phosphors, solar energy converters and in the fabrication of number of optoelectronic devices. The luminescence studies on borate compounds have been started since 1967 [15]. Considerable studies have to focus on the borate glasses to improve and enhance its emission by photoluminescence. Consequently, these enhancements make the host network more stable and acquire specific properties, which have significant impact on medical and industrial disciplines. It is well known that the ability of boron atom to joint with either three or four oxygen atoms to generate a variety of atom groups. Alkali/alkaline earth oxides were frequently applied as modifiers, consequently, these oxides shift up the boroxol rings, and the active groups in the mixture, to form tri- and tetra-bond on the host [16]. The borate glasses offer good heat stability and lower melting temperature compared with other glasses [17]. However, it is difficult to realize efficient infrared to visible upconversion emission in borate based glasses due to high vibrational energy. The introduction of heavy metal compounds such as PbO and PbF2, in conventional glasses like silicate and borate, is of interest for realization of more efficient laser systems as their presence improves the effective fluorescence. The addition of lead oxide (PbO) decreases the host phonon energy and there by suppress the non-radiative losses [18,19]. The presence of structurally different groups such as BO3 and BO4 in the PbO–B2O3 glass network gives rise to a variety of spectroscopic properties to be investigated. The lead borate glasses are optically transparent from visible to NIR region. The spectroscopic and luminescence properties of RE ions are strongly influenced by the presence of highly polarizable Pb2+ ions due to the strong and direct nature of Pb–O bond [20,21]. Because of a dual role such as network modifier (when Pb–O is ionic) and network former (when Pb–O is covalent with PbO3 and PbO4 structural units) of Pb2+ in the glass structure, the PbO can form stable glasses and make them more moisture resistant [22]. Because of above facts, lead borate glasses are significant in the field of solid state lasers. Though different spectroscopic characterization have been significantly studied by the modification of chemical composition or ion concentration to improve the performance of laser hosts, still there is a demand for new host materials with high efficiency. On the basis of the above mentioned considerations, the authors have designed, synthesized and characterized the LBZLFB glasses to meet the needs of present photonic devices and was reported by doping Sm3+ ions for visible solid state lasers [23]. In continuation of spectroscopic studies of LBZLFB glasses, the authors are extended the work by doping with Dy3+ ions to produce solid state

lasers suitable for visible region and for simulation of white light at 454 nm excitation i.e., for blue LED chip. 2. Experimental methods 2.1. Glass preparation The glass samples were prepared with chemical composition of 20PbO + 5BaO + 5ZnO + 10LiF + (60  x)B2O3 + xDy2O3, (where x = 0.1, 0.5, 1.0, 1.5 and 2.0 mol%). Approximately 10 g batches of homogeneous mixture of reagent grade Pb3O4, BaCO3, ZnO, LiF, H3BO3 and Dy2O3 were mixed and grinded in required proportions in an agate mortar and melted in an electric furnace at 950 °C in porcelain crucible for about 1 h. The melt was poured into preheated brass molds and annealed at 300 °C for 5 h to remove thermal strains. The glass samples were slowly cooled to the room temperature, shaped and polished to measure their physical and optical properties. 2.2. Physical properties For concentration determination, density measurements were made by the Archimedes’s method using distilled water as the immersion liquid. Refractive index was measured with an Abbe’s refractometer with sodium vapor lamp using 1-bromonapthalene as the contact liquid and the thickness (optical path length) was measured by a screw gauge. From the measured values of refractive index (1.581), sample thickness (0.3 cm) and density (5.84 g/cc), the rare earth ion concentration (1.499  1020 ions/cc) was estimated for 1.0 mol% of Dy3+-doped LBZLFB glasses. The various measured and calculated physical properties of the 1.0 mol% Dy3+-doped LBZLFB glasses are presented in Table 1. 2.3. Spectral measurements The X-ray diffraction (XRD) profile was recorded using Seifert Xray diffractometer. Differential scanning calorimeter (DSC) profile on a Netzsch DSC 204 differential scanning calorimeter in the temperature range of 0–500 °C, at the rate of 10 °C/min, under N2 gas atmosphere was recorded. FT-IR spectrum in the range 450– 4000 cm1 using Perkin–Elmer Spectrum One FTIR spectrophotometer by the standard KBr pellet technique was recorded. FT-Raman spectra in the range 50–5000 cm1 with Bruker RFS 27 stand alone Raman spectrometer using 1064 nm light from Nd:YAG laser on glass powders were recorded in back scattering geometry with a resolution of 2 cm1. The absorption spectra were recorded using a double beam Perkin Elmer Lambda 950 spectrophotometer in the wavelength range 250–2500 nm. The excitation,

Table 1 Measured and calculated physical properties for 1.0 mol% Dy3+-doped LBZLFB glass. Physical quantities

LBZLFB

Sample thickness (cm) Refractive index (n) Density (g/cc) Concentration (mol/l) Concentration (ions cm3  1020) Average molecular weight (g) Dielectric constant (e) Molar volume Vm (cm3/mol) Glass molar refractivity (cm3) Electronic polarizability ae (1024 cm3) Reflection losses R (%) Polaron radius rp (Å) Inter ionic distance ri (Å) Field strength F (1014 cm2)

0.300 1.581 4.840 0.249 1.499 193.850 2.500 40.000 13.350 5.290 5.070 7.500 18.824 5.300

Sd. Zulfiqar Ali Ahamed et al. / Optical Materials 35 (2013) 1385–1394

1387

Fig. 2. DSC profile of 1.0 mol% Dy3+-doped LBZLFB glass. Fig. 1. XRD profile of 1.0 mol% Dy3+-doped LBZLFB glass.

emission and lifetime measurements were carried out using Jobin Yvon Fluorolog-3 spectrofluorometer with xenon flash lamp as source. All the measurements were carried out at room temperature. 3. Results and discussion 3.1. X-ray diffraction (XRD) studies The XRD spectrum of all glass samples shows the similar diffraction patterns and confirms the amorphous nature of the glasses. As a reference the X-ray diffraction pattern of the 1.0 mol% Dy3+-doped LBZLFB glass is shown in Fig. 1. It could be observed that the broad diffuse scattering at lower angles (10° 6 h 6 80°), which is the characteristic feature of structural disorder that confirms the amorphous nature of the prepared glass under investigation. 3.2. Thermal studies Fig. 2 represents the DSC profile of 1.0 mol% Dy3+-doped LBZLFB glass. The DSC trace provides the glass transition temperature (Tg) and the crystallization temperature (Tc), which are found to be 221 °C and 381 °C, respectively. The difference between glass transition temperature and the crystallization temperature DT = Tc  Tg is a measure of glass thermal stability. The larger value of glass thermal stability (DT) gives a larger working range during operations for fiber drawing. If DT > 100 °C, the glass exhibits relatively good thermal stability and could be useful for fiber drawing [24]. The thermal stability (DT) for the present 1.0 mol% Dy3+-doped LBZLFB glass is found to be 160 °C. The glass having a high thermal stability is the choice for rod/fiber fabrication. The lower value of Tg implies that glass formation is easier in LBZLFB glasses. Due to the high value of thermal stability (DT) for the LBZLFB glass indicates the reasonably good glass forming tendency and the stability of the 1.0 mol% Dy3+-doped LBZLFB glass system and hence can be used for the development of laser glass systems.

characteristic groups of atoms in the network are independent of vibrations of other groups. Such an empirical treatment can provide significant information on the arrangement of atoms in glasses. The characteristic IR absorption bands are assigned as follows: the absorption band at 3430 cm1 is attributed to the fundamental vibrations of OH group and the two weak IR bands at 2920 cm1 and 2853 cm1 originates from hydrogen bonding. The IR absorption at 2334 cm1 frequency is also assigned to moisture present in the glass structure. The IR bands at 1650 cm1 and 1753 cm1 may be due to bending modes of water molecule. The IR absorption band at about 1268 cm1 is attributed to B–O stretching vibrations of BO3 units in pyroborate groups. The band at 907 cm1 could be due to the stretching vibrations in BO4 units. The IR band at 709 cm1 is assigned to B–O–B bending vibrations in BO4 group and the band at 506 cm1 may be due to the O–B–O bending vibrations and covalent Pb–O vibrations. The assignment of the IR bands is based on the previous literature [25–29]. It can be seen from Fig. 3 that the percentage of absorption of the radiation is found to be maximum between 700 cm1 and 1500 cm1 indicating the transmittance is low in this region. It is observed that the intensity of IR absorption band at 1268 cm1 is found to be very high. The energy corresponding to this band could be treated as the phonon energy of the 1.0 mol% Dy3+-doped LBZLFB glass and it is found lower than the other fluoroborate glasses [30,31].

3.3. Vibrational studies 3.3.1. Infrared spectroscopy The FT-IR spectrum of 1.0 mol% Dy3+-doped LBZLFB glass host in the range 400–4000 cm1 is shown in Fig. 3. It is known that the

Fig. 3. FT-IR spectrum of 1.0 mol% Dy3+-doped LBZLFB glass.

1388

Sd. Zulfiqar Ali Ahamed et al. / Optical Materials 35 (2013) 1385–1394

3.3.2. Raman spectroscopy The FT-Raman spectrum of 1.0 mol% Dy3+-doped LBZLFB glass in the region 0–2000 cm1 is shown in Fig. 4. In the present work, Raman spectrum shows different peaks at 91, 482, 630, 728, 824, 980, 1059, 1170, 1280, 1481, 1595 cm1. From the spectrum it is observed that one high intensity band at 91 cm1, which is related to the heavy metal (Pb) ion vibrations such as symmetric Pb-O stretching in the PbO4 pyramid configuration. The band at 482 cm1 is assigned to a ring angle bending (B–O–B), which is observed at 470 cm-1 for pure B2O3. The bands located at 630 cm1, 728 cm1 are due to symmetric vibrations of metaborate rings. The band at 824 cm1 is owing to symmetric stretching vibrations of B–O–B bridges in pyroborate units. The occurrence of band at around 980 cm1 and 1059 cm1 is an indication of the presence of pentaborate and orthoborate groups in the borate glasses. At 1170 cm1, the spectrum consists of a band due to the symmetric stretching vibration of terminal B-O bonds in diborate groups. The band centered at 1280 cm1 is assigned to pyroborate groups. The band at 1481 cm1 is assigned to the B–O stretching vibrations in metaborate rings and chains. The band at 1595 cm1 is due to bending mode of water molecule (O–H groups). The assignment of the Raman bands is based on the earlier literature [25–29,32]. It is known that the phonon energy of the host can be defined as the highest vibrational energy of the host which can be measured from Raman spectrum. From Fig. 4 it is found that the maximum phonon energy of 1280 cm1 is characteristic of studied glass, which plays a major role in the optical properties of the luminescent ions. From the FT-IR spectrum also it is evident that the maximum phonon energy is 1268 cm1.

Fig. 5. VIS absorption spectrum of 1.0 mol% Dy3+-doped LBZLFB glass.

3.4. Absorption spectrum Optical absorption spectra of 1.0 mol% Dy3+-doped LBZLFB glasses at room temperature in the visible and near infrared regions are shown in Figs. 5 and 6, respectively. Each absorption peak corresponds to the transition from the 6H15/2 ground state to various excited states of Dy3+ ion. The spectra consists of a distinct and inhomogeneous broad absorption bands at 430, 453, 472, 753, 801, 899, 1095, 1277, 1676 nm corresponding to the 6H15/2 ? 4G11/2, 4 I15/2, 4F9/2, 6F3/2, 6F5/2, 6F7/2, 6F9/2 + 6H7/2, 6F11/2 + 6H9/2, and 6H11/2 transitions respectively. The assignments of transitions have been done according to Carnall et al. [33] as presented in Table 2. Slight variation in peak intensities can be observed with change in the composition. The transition from the next excited state 6H13/2 is ruled out due to thermalization effect, as the energy difference

Fig. 6. NIR absorption spectrum of 1.0 mol% Dy3+-doped LBZLFB glass.

Table 2 Experimental and calculated oscillator strengths (106) for 1.0 mol% Dy3+-doped LBZLFB glass. Transition (6H15/2?)

Energy (cm1)

fexp

fcal

6

5959 7849 9149 11136 12469 13,351 drms = ± 0.38  106

2.57 15.39 6.28 5.77 2.90 0.40

2.97 15.33 6.47 5.13 2.40 0.45

H11/2 F11/2 + 6H9/2 F9/2 + 6H7/2 6 F7/2 6 F5/2 6 F3/2 6 6

Fig. 4. FT-Raman spectrum of 1.0 mol% Dy3+-doped LBZLFB glass.

between 6H15/2 and 6H13/2 is around 3000 cm1. The electronic configuration of the Dy3+ ion is 4f9, which gives 6H as the ground state multiplet. The most intense transitions of Dy3+ ions are found in the near infrared region and the assignment of free-ion levels in the UV–Vis regions are not easy because of the overlap of the different 2S+1LJ levels. The transition from the ground 6H15/2 state to 6 H and 6F terms are spin allowed (DS = 0). Moreover, the transitions within the 6H term are also allowed by the orbital angular momentum selection rule, DL = 0 and hence, these transitions lying in the NIR region are intense. The transitions that lie above 4G11/2 are not identified as glass absorption predominating above 23,300 cm1. The presence of Pb2+ ions in LBZLFB glass may be responsible for the disappearance of some of the absorption

1389

Sd. Zulfiqar Ali Ahamed et al. / Optical Materials 35 (2013) 1385–1394

transitions in the UV region. The strong absorption of host glass causes the absence of visible absorption bands [18]. The position and spectral intensities of certain transitions of Ln3+ ions are found to be sensitive to the environment of the Ln3+ ion. They follow the selection rules DJ = 0, DL = 0 and DS = 0 and such transitions are known as hypersensitive transitions and they are associated with large values of spectral intensities as well as reduced matrix elements U k2 . For Dy3+ ions, the transition 6H15/2 ? 6 F11/2 is identified to be a hypersensitive transition and is found to be more intense than the other transitions. Analyzing the trends of energy positions of Dy3+ ions, the order of magnitude is found to be glasses > acqo ion > crystals. Transition energies associated with Dy3+ in the present glass host is relatively higher than that represented in crystals, and it indicates that the 4f orbitals of Dy3+ ions interact relatively stronger with the glass than  dÞ have been calthe crystals. Dy3+-ligand bonding parameters ðb; culated from the nephelauxetic ratio (b) using the equation b = mc/ma, where mc is the wavenumber (in cm1) of a particular transition for an ion in the host under investigation and ma is the wavenumber (in cm1) of the same transition for the aqua ion.  the bonding parameter d is given From the average values of bðbÞ,  b.  The bonding will be covalent or ionic depending by d ¼ ð1  bÞ= upon the positive or negative sign of d. For the present glass the  and d are 1.004 and 0.004. The negative sign of d for value of b the 1.0 mol% Dy3+-doped LBZLFB glass indicating the ionic nature of the Dy–O bond in the present glasses. 3.5. Oscillator strengths and Judd–Ofelt intensity parameters The absorption spectra of lanthanides arise due to intra configurational f–f transitions. The majority of transitions are induced electric dipole in nature. The intensities of the absorption bands can be expressed in terms of measured oscillator strengths (fexp) by the area method using the formula [34]

fexp ¼ 4:318  109

Z

eðv Þdv

ð1Þ

where e(m) is the molar absorptivity at energy m cm1 and can be obtained using Beer–Lambert’s law. The Judd–Ofelt (J–O) model [35,36] gives the theoretical estimation of the intensities of intra configurational f–f transitions of Ln3+ ions. According to this model, the electric dipole transition probabilities between electronic states of Ln3+ ions can be expressed and calculated in terms of a small number of phenomenological intensity parameters characteristic of the ion–host system. This theory has been extensively and effectively used in the optical characterization of Ln3+ doped single crystals, polycrystalline, glasses and solutions since it is of value for interpreting and predicting intensities of crystal field induced electric–dipole transitions in absorption and emission spectra of Ln3+ ions. Generally, the three J–O intensity parameters are determined empirically from the room temperature absorption spectrum by minimizing the difference between calculated and experimental oscillator strengths of a series of excited multiplets by standard least squares or chi squares methods. The calculated oscillator strengths (fcal) of the transitions from the initial state WJ to the final state W0 J0 is given by

fcal

8p2 mcm ðn2 þ 2Þ2 X ¼ Xk ðWJkU k W0 J0 Þ2 3hð2J þ 1Þ 9n k¼2;4;6

ð2Þ

where m is the electron mass, c is the speed of light, h is the Planck’s constant and (2J + 1) is the degeneracy of the ground state. The fac2 þ2Þ2 tor ðn 9n is the Lorentz local field correction factor which indicates that the Dy3+ ion is not in vacuum but in a dielectric medium of refractive index n. kU k k2 are the doubly reduced squared matrix elements evaluated in the intermediate coupling approximation for

the state WJ to W0 J0 which are almost independent of the host matrix. The measured oscillator strengths (fexp) of various observed absorption bands were evaluated using Eq. (1). The Judd–Ofelt analysis has been performed for the measured oscillator strengths by using Eq. (2) in order to obtain the calculated oscillator strengths (fcal) and hence the J–O intensity parameters. The measured and calculated oscillator strengths of various absorption bands of 1.0 mol% Dy3+-doped LBZLFB glass along with the root mean square deviation (drms) are given in Table 2. The small drms of ±0.3  106 between the experimental and the calculated spectral intensities indicated well fit between the two values and also the validity, accuracy and applicability of J–O theory. Judd–Ofelt analysis leads to different set of magnitude of J–O intensity parameters since it depends on the nature of levels used for the analysis. Therefore in the present study, the J–O intensity parameters are derived from six energy levels and the results are presented and compared with other glass hosts in Table 3. It is observed that the value of J–O intensity parameter, X2 is higher compared to other two intensity parameters X4 and X6 and follows the trend as X2 > X6 > X4. From the magnitudes of Xk values, one can conclude that the trends of the intensity parameters mainly depend on the ligand environment around the Ln3+ ion. According to Jorgensen and Reisfeld [37], the magnitude of the X2 parameter depends on the covalence of metal ligand bond and also on the asymmetry of ion sites in the neighborhood of Ln3+ ion while the magnitudes of X4 and X6 parameters related to the bulk properties such as rigidity and viscosity of the medium in which they are present and also they are vibronic dependent. The lower value of X2k parameter indicates that higher degree of symmetry around the Ln3+ ion and stronger (weaker) ionic (covalent) bond between Ln3+ ion–oxygen ligand. The value of X2 is relatively larger for oxide glasses, smaller for fluoride glasses, while intermediate for oxyfluoride glasses. This implies that the Dy–O covalency decreases when pure oxide glasses are modified with fluoride content, which is also reflected from the bonding parameter values. From the relative magnitudes of Xk , it can be concluded that the coordination structure and symmetry are higher in LBZLFB glass. The higher magnitude of X6 (5.71  1020 cm2) indicates the higher rigidity of LBZLFB glass than those of other reported systems [6,7,22,38,39]. Jacob and Weber [40] reported that the X4/X6 ratio is called as spectroscopic quality factor and is used to characterize the quality of the prepared glasses. Based on the magnitude of spectroscopic quality factor in the present work (0.94), Dy3+-doped LBZLFB glass appears to be a better optical glass and the larger spectroscopic quality factor predicts higher stimulated emission cross section in the 1.0 mol% Dy3+-doped LBZLFB glass. According to J–O theory, the transitions that show a strong dependence on X2 parameters are called hypersensitive transitions. These absorption transitions of each Ln3+ ion are very sensitive to the host environment and ion concentration due to the inhomogeneity of the ligand environment and obeying the selection rules kDS = 0k, kDL 6 2k and kDJ 6 2k. The hypersensitive transitions are associated with large values of the reduced matrix elements kU k k2 and hence the hypersensitivity. The relative variation of X2 parameter of a Ln3+ ion in different environments gives the Table 3 Comparison of J–O intensity parameters (1020 cm2), their trends and spectroscopic quality factors (X = X4/X6) for Dy3+ ions in LBZLFB glass with different glass hosts. Glass system

X2

X4

X6

Trend

4 X¼X X6

LBZLFB[Present glass] Lead borate [6] L5FBD [7] PKBAD [38] PKBFAD [38] LCZSFB [39]

14.44 4.90 9.01 9.72 10.41 11.25

5.23 0.94 2.56 3.08 2.29 2.45

5.71 2.07 3.52 1.66 2.07 5.16

X2 > X6 > X4 X2 > X6 > X4 X2 > X6 > X4 X2 > X4 > X6 X2 > X4 > X6 X2 > X6 > X4

0.92 0.46 0.72 1.86 1.11 0.48

1390

Sd. Zulfiqar Ali Ahamed et al. / Optical Materials 35 (2013) 1385–1394

measure of degree of hypersensitivity exhibited by that ion. In case of Dy3+ ion, the 6H15/2 ? 6F11/2 transition is the hypersensitive transition. In general, the intensity of the hypersensitive transition is higher when compared to the other transitions in any host medium. For hypersensitive transitions the magnitude of experimental oscillator strength values are also high, which will be noticed from Table 2, for the present glass. 3.6. Excitation and emission spectra In order to investigate the luminescence properties as a function of Dy3+ ion concentration, the excitation spectra were recorded in the spectral region 300–500 nm by monitoring the emission at 577 nm (4F9/2 ? 6H13/2). Fig. 7 shows the excitation spectrum of 1.0 mol% Dy3+-doped LBZLFB glass along with the assignment of band positions. The excitation bands centered at 353, 367, 389, 428, 454 and 474 nm corresponding to 6H15/2 ? 6 P7/2, 4P3/2, 4I13/2, 4G11/2, 4I15/2 and 4F9/2, transitions, respectively. It is a well known fact that the wavelength corresponding to the prominent excitation band can give intense emission. In the present investigation, the excitation band centered at 454 nm is found to be more intense. Thus, the luminescence spectra were carried out by exciting the samples with 454 nm wavelength. Fig. 8 shows the normalized luminescence spectra of 1.0 mol% Dy3+-doped for different concentrations of LBZLFB glasses recorded in the spectral region 450–700 nm. The Dy3+ ions show a strong luminescence of 486 nm and 577 nm corresponding to the 4F9/2 ? 6 H15/2 (blue) and 4F9/2 ? 6H13/2 (yellow) transitions respectively. The weak luminescence at 668 nm is attributed to the 4F9/2 ? 6 H11/2 (red) transition. The shape of the luminescence spectra is similar in all the glasses except in intensity of the emission transitions. The variation of intensity of the 4F9/2 ? 6H13/2 (577 nm) and the 4F9/2 ? 6H15/2 (486 nm) transitions with Dy3+ ion concentration is described in Fig. 9. It is found that the intensity of Dy3+ increases with increase of the concentration of Dy3+ ion, reaching a maximum value at 1.0 mol% of Dy3+ and then decreases when the concentration of Dy3+ is larger than 1.0 mol% of Dy3+. This is because of a well known phenomenon of concentration quenching in Dy3+ ions doped systems due to mutual Dy3+–Dy3+ interactions at higher concentrations and due to the increase of non-radiative energy transfer through cross relaxation and resonant energy channels. To obtain visible emission the photo excitation at a particular wavelength followed by luminescence at a shorter wavelength is being explored in different materials. The upconversion luminescence of rare earth ions has aroused much attention for its potential use in visible laser. It is well known that upconversion

mechanism involves excited state absorption, energy transfer, and also photon avalanche process. The upconversion luminescence of Dy3+ here may arise from one of these following mechanisms. However, due to high vibrational energy of Dy3+ ions doped borate based glasses, the NIR to visible upconversion has been reported rarely [30,41–43]. In this way it is evident that if the Dy3+ ions doped LBZLFB glasses are excited with radiant energy at 850 nm [41–43], the conversion of incident energy into visible light may be observed. 3.7. Simulation of white light The 4F9/2 ? 6H13/2 (577 nm) transition is a hypersensitive electric dipole (ED) transition and its emission intensity is strongly influenced by the coordination environment in comparison to less sensitive magnetic dipole (MD) transition 4F9/2 ? 6H15/2 (486 nm). In fact the intensity of an emission transition depends on the site symmetry i.e., the higher symmetry lowers the luminescence intensity. Generally, the intensity ratio of ED and MD transitions has been used to measure the symmetry of the local environment of the Ln3+ ions. In the present work the ED transition of Dy3+ ions is less intense than MD transition; it results in different Y/B luminescence intensity ratios. Fig. 9 represents the variation of Y/B intensity ratios with Dy3+ ion concentration and is attributed to the changes around the coordination environment of Dy3+ ion. Table 4 presents the Y/B values of Dy3+ ions doped LBZLFB glasses. For all the glasses the values of Y/B intensity ratios are less than one, which indicate the lower degree of covalence. The evaluated Y/B intensity ratios reveal that Dy-O covalence increases with increase of Dy3+ ion concentration in LBZLFB glasses. As can be seen, the Y/B intensity ratios of visible emissions vary from 0.54 to 0.71 with the evaluated Dy3+ concentration over a wide range indicating the feasibility of generation of white light in the present glasses. The abundant excitation peaks in the 350–480 nm range implies that the Dy3+ ions can be efficiently excited by the popular blue (440–470 nm) and the near UV (350–420 nm) LEDs, it is of significance for the white light emission in practical applications. In the present work the emission spectra was recorded with excitation wavelength of 454 nm. Based on that we calculated the Y/B intensity ratios and CIE color coordinates as shown in Table 4. All these coordinates lie within the white light region of CIE 1931 (Commission International de I’Eclairage) chromaticity diagram as shown in Fig. 10, though they are not very far away from the ideal equal energy white light illumination (0.33, 0.33), there exists a tendency to move towards white light region with increase in Dy3+ ion concentration. So the Y/B ratio can be adjusted by changing RE3+ ion concentration as well as glass composition to enhance the red emission part in order to get white light. Since the chromaticity color coordinates at 454 nm excitation are closer to the equal energy point (0.33, 0.33), the present glass is may be more suitable for simulation of white light for the blue LED chip. By co-doping and triple-doping of RE3+ ions and also up-conversion mechanisms, there may be possibility to enhance the red emission part to get white light. Gouveia-Neto et al. [9], da Silva et al. [10], Amorim et al. [11], Chen et al. [12] and Giri et al. [13] carried out the research to generate white light in glasses by co-doping and tripledoping of RE3+ ions and also up-conversion mechanisms, by enhancing the red emission. 3.8. Radiative and laser properties The radiative properties can be calculated using the values of

Xk , since the spontaneous transition probability is expressed as Fig. 7. Excitation spectrum of 1.0 mol% Dy3+-doped LBZLFB glass.

AR ðWJ;W0 J0 Þ ¼ Aed þ Amd

ð3Þ

Sd. Zulfiqar Ali Ahamed et al. / Optical Materials 35 (2013) 1385–1394

1391

Fig. 8. Fluorescence spectra for different concentrations of Dy3+-doped LBZLFB glasses. Inset figure represents fluorescence intensity as a function of Dy3+ concentration.

where n(n2 + 2)2/9 is the local field correction for the electric dipole transitions and n3 for magnetic dipole transitions and Sed and Smd are the electric and magnetic dipole line strengths, respectively, calculated from the following equations

Sed ¼ e2

X

Xk ðWJkU k kW0 J0 Þ2

ð6Þ

k¼2;4;6 2

Smd ¼

e2 h ðWJkL þ 2SkW0 J0 Þ2 16p2 m2 c2

ð7Þ

The sum of AR (WJ, W0 J0 ) for the states involved gives the total radiative probability (AT),

AT ðWJÞ ¼

X AR ðWJ;W0 J0 Þ

ð8Þ

W0 J0

Fig. 9. Variation of Y/B intensity ratio with concentration of Dy3+-doped LBZLFB glasses.

where the sum is extended over all the states at energy lower than W0 J0 . The radiative lifetime of an emitting state is related to the total spontaneous emission probability for all transitions from this state by

sR ðWJÞ ¼ where Aed and Amd are the electric and magnetic dipole contributions, respectively, which are calculated from

" # 64p4 m3 nðn2 þ 2Þ2 Aed ¼ Sed 3hð2J þ 1Þ 9 Amd

ð4Þ

64p4 m3 ¼ ½n3 Smd  3hð2J þ 1Þ

ð5Þ

Table 4 Y/B ratios, chromaticity color coordinates of all concentrations of Dy3+-doped LBZLFB glasses. Concentration

0.1 0.5 1.0 1.5 2.0

Y/B ratio

0.537 0.566 0.600 0.645 0.710

Color coordinates x

y

0.281 0.284 0.286 0.291 0.297

0.348 0.344 0.341 0.339 0.336

1 AT ðWJÞ

ð9Þ

Another important radiative property, the fluorescent branching ratio (bR) for the different emissions with the same initial level is calculated from

bR ðWJ;W0 J0 Þ ¼

AR ðWJ;W0 J0 Þ AT ðWJÞ

ð10Þ

and is used to predict the relative intensity of lines originating from a given excited state. The experimental branching ratios can be found from the relative areas of the emission bands. The peak stimulated emission cross-section, re (WJ, W0 J0 ), between the emission levels WJ and W0 J0 having a probability of AR(WJ, W0 J0 ) can be expressed as

re ðWJ;W0 J0 Þ ¼

k4p AR ðWJ;W0 J0 Þ 8pcn2 Dkp

ð11Þ

where kp is the transition peak wavelength and Dkp is its effective linewidth found by dividing the area of the emission band by its average height.

1392

Sd. Zulfiqar Ali Ahamed et al. / Optical Materials 35 (2013) 1385–1394

0.90 0.80 0.70 0.60

y

0.50 0.40 0.30 0.20 0.10 0.00 0.00

0.10

0.20

0.30

0.40

x

0.50

0.60

0.70

0.80

0.90

Fig. 10. CIE chromaticity diagram for Dy3+-doped LBZLFB glasses.

Once the J–O intensity parameters are obtained, the spontaneous emission probabilities and fluorescence branching ratios between J-multiplities of Ln3+ ions can be determined. In addition, non-radiative decay rates and quantum efficiencies can also be readily estimated by combining calculated radiative decay probabilities with measurements of excited state lifetimes with the Eqs. (3)–(11). Spontaneous emission probability (AR), radiative lifetime (sR) and radiative branching ratio (bR) of the 4F9/2 excited state for emission transitions of Dy3+ in 1.0 mol% Dy3+ doped LBZLFB glass are presented in Table 5. The magnitude of branching ratio characterizes the lasing power of a transition and it has been well established that an emission transition having luminescence branching ratio (bR) greater than 0.50 has more potential for laser emission. The radiative (0.73) and measured (0.34) values of branching ratios obtained for the 4F9/2 ? 6H13/2 transition indicate the potentiality of studied glasses as 577 nm laser materials. The variation in radiative and experimental branching ratio is attributed to the non-radiative contributions from the 4F9/2 level of Dy3+ ion in Dy3+-doped LBZLFB glass. From the emission spectra, peak emission wavelength ðkp Þ, effective linewidth ðDkp Þ and peak stimulated emission cross sections (re) for the observed emission transitions, 4F9/2 ? 6HJ (J = 15/2, 13/2 and 11/2) are obtained and are presented in Table 5. The value of re has been used to identify the potential laser transitions of Ln3+ ions in a host medium. A good laser transition can have a large stimulated emission cross section. The re of 54.35  1022 cm2 obtained for the 4F9/2 ? 6H13/2 transition of present glass is compared with the other 1.0 mol% Dy3+doped glass hosts [6,44–49]. The gain band width ðre  Dkp Þ and

Table 5 Radiative properties of 4F9/2 ? 6HJ transitions in 1.0 mol% Dy3+-doped LBZLFB glass. Radiative properties

Peak emission wavelength (kp , nm) Effective linewidth (Dkp , nm) Radiative transition probability (AR, s1) Radiative branching ratio (bR) Experimental branching ratio (bm) Stimulated emission cross section (re  1022 cm2) Gain band width (re  Dkp , 1028 cm3) Optical gain parameter (re  sm, 1025 cm2 s) Radiative lifetime (sR, ls)

4

F9/2?

6

H15/2

486 18 427 0.21 0.65 7.03 12.65 2.25

6

6

577 16 1478 0.73 0.34 54.35 86.96 17.40

668 15.58 140 0.07 0.01 9.60 14.76 3.07 489

H13/2

H11/2

optical gain (re  sm) parameters are critical to predict the amplification of the medium in which the Ln3+ ions are situated. A good optical amplifier should have higher values of ðre  Dkp Þ and (re  sm). The relatively higher values of bR or bm, re, re  Dkp and re  sm suggest that the 1.0 mol% Dy3+-doped LBZLFB glass is a suitable candidate to accomplish laser action and for optical amplifiers.

3.9. Decay curve analysis The decay profiles of 4F9/2 emission level of Dy3+ ions in LBZLFB glass containing different concentrations of Dy3+ ions were recorded under excitation at 454 nm and emission at 577 nm. It is observed that, the decay profiles are found to be single exponential for lower concentrations, i.e., for 0.1 and 0.5 mol% and for higher concentrations (1.0, 1.5 and 2.0 mol%) of Dy3+ ions the decay curves deviate towards non-exponential nature. In general, the non –exponential behavior of decay curves of Ln3+ ions doped materials arises from the ion–ion interactions which are governed mainly by the dopant concentration. From the decay curves, lifetime (sm) of the 4F9/2 level has been determined by taking the first e-folding time of the decay intensity. The measured decay time (sm) of the 4F9/2 emission state is found to be 445, 400, 320, 230 and 190 ls for 0.1, 0.5, 1.0, 1.5 and 2.0 mol% Dy3+-doped LBZLFB glasses, respectively. The fluorescence lifetime at lower concentrations is close to the radiative lifetime (sR); however as the concentration increases, the lifetime decreases which indicates the presence of non-radiative energy transfer processes from excited state to neighboring unexcited state of Dy3+ ions. The discrepancy between the measured and calculated lifetimes is mainly due to energy transfer through cross relaxation or multi phonon relaxation or both. The measured lifetime (sm) of an emitting state is related with the radiative lifetime (sR) and non-radiative decay rates (WNR) as s1m ¼ s1R þ W NR . The non-radiative decay rates (WNR) play an important role on the quenching of lifetime of an excited state and it can be written as WNR = WMRP + WET + WCR + WOH, where WMPR + WET + WCR + WOH denote the non-radiative decay rates corresponding to the multi-phonon relaxation, energy transfer between donor to donor or donor to acceptor and hydroxyl (OH) groups respectively. The increase of non-radiative relaxation rates and decrease in experimental lifetimes (sm) with increasing Dy2O3 concentration may be due to the increase in energy transfer through cross-relaxation and resonant energy transfer channels. As shown in Fig. 11,

Sd. Zulfiqar Ali Ahamed et al. / Optical Materials 35 (2013) 1385–1394

1393

Fig. 11. Energy level diagram showing emission mechanism and cross-relaxation channels for Dy3+-doped LBZLFB glass system.

the energy gap between 4F9/2 and 6F11/2 levels matches well with that of 6F3/2 and 6H15/2 levels. Also energy gap between the levels 4 F9/2 and 6F3/2 matches with 6H15/2 and 6F11/2 levels. Cross relaxation is due to energy transfer from the emitted 4F9/2 emission level to the nearby Dy3+ ions in the ground level 6H15/2. This transfer of cross relaxation occurs through 4F9/2 ? 6F11/2 transition on one ion and 6H15/2 ? 6F3/2 transition on the other i.e., (4F9/2, 6H15/2) ? (6F11/2, 6 F3/2). In the present work for Dy3+ ions, the cross relaxation may take place between the two transitions, (4F9/2, 6H15/2) ? (6F11/2, 6 F3/2) and (4F9/2, 6H15/2) ? (6F3/2, 6F11/2). Similarly resonant energy transfer may takes place between the levels 4F9/2 and 6H15/2 of two Dy3+ ions. Fig. 11 shows the energy level diagram and resonant energy transfer and cross relaxation channels in Dy3+ ions of present LBZLFB glasses.

The luminescence quantum efficiency (g) is defined as the ratio of the number of photons emitted to the number of photons absorbed. For Ln3+ ion systems, it is equal to the fluorescence lifetime to the radiative lifetime for respective levels given by g = sm/sR. The decay profiles of Dy3+-doped LBZLFB glasses are shown in Fig. 12. The inset of Fig. 12 shows the variation of lifetime (sm) and energy transfer (WET) parameter with concentration. Table 6 represents measured lifetime (sm), quantum efficiencies (g) and non-radiative relaxation rates (WNR) for 4F9/2 level of Dy3+ for different concentrations of Dy3+ ions in LBZLFB glass matrix. It is observed that the non-radiative relaxation rates (WNR) arte increased from 202 to 3218 s1 and quantum efficiencies (g) decreased from 91% to 39% with increase of Dy3+ concentration.

Fig. 12. The decay profiles of 4F9/2 ? 6H13/2 level for different concentrations of Dy3+ ions in LBZLFB glasses. Inset figure represents variation of measured lifetimes and nonradiative energy transfer rate with Dy3+ concentration in LBZLFB glasses.

1394

Sd. Zulfiqar Ali Ahamed et al. / Optical Materials 35 (2013) 1385–1394

Table 6 Variation of lifetime (sm, ls), quantum efficiency (g, %) and energy transfer rate (WET, s1) with respect to concentration (mol%) of Dy3+ ions in LBZLFB glasses. Concentration

Lifetime (ls)

g

WNR

0.1 0.5 1.0 1.5 2.0

445 400 320 230 190

91 82 66 47 39

202 455 1080 2303 3218

4. Conclusions LBZLFB glasses doped with different Dy3+ ion concentrations have been prepared and characterized through XRD, DSC, FTIR, FT-Raman, optical absorption, fluorescence and decay measurements at room temperature. The detailed analysis of emission spectra reveals that most of the luminescent emissions are from the 4F9/2 energy level. The quenching of emission intensity with concentration has been observed. The luminescence decay curves from the 4F9/2 energy level and yellow to blue emission intensity ratios (Y/B) have been analyzed as a function of Dy3+ ion concentration. For lower Dy3+ ion concentrations, the fluorescence decay curves show nearly single exponential nature and at higher concentrations become non-exponential indicating non-radiative energy transfer. Based on the magnitudes of high bR, rðkp Þ, and AR, it is predicted that the LBZLFB glasses containing 1.0 mol% Dy3+ ions are promising materials for developing solid state lasers for visible region. The magnitude of evaluated chromaticity coordinates for the emission spectra of Dy3+:LBZLFB glasses have been found to be in the white light region. With the elevated Dy3+ ions concentration, the Y/B intensity ratios of visible emissions increases over a wide range indicate the feasibility of simulation of with-light in LBZLFB glass for the blue LED chip. Acknowledgements The authors are highly grateful to Prof. C.K. Jayasankar, Department of Physics, Sri Venkateswara University, Tirupati for utilizing their lab facilities and also acknowledging the Sophisticated Analytical Instrument Facility (SAIF), Indian Institute of Technology, Chennai for extending instrumental facilities. References [1] Y. Gao, Q.H. Nie, T.F. Xu, X. Shen, Spectrochim. Acta A 61 (2005) 2822–2826. [2] T. Catunda, M.L. Bacsso, Y. Menaddeeq, M.A. Aegester, J. Non-Cryst. Solids 213 (1997) 225–232. [3] M.R. Ozalp, G. Ozwn, A. Sennaroghu, A. Kurt, Opt. Commun. 217 (2003) 281– 289. [4] Z.J. Chao, C. Parent, G.L. Flem, P. Hagenmuller, J. Solid State Chem. 93 (1991) 17–29. [5] Y.G. Choi, J. Heo, J. Non-Cryst. Solids 217 (1997) 199–207. [6] P. Babu, C.K. Jayasankar, Opt. Mater. 15 (2000) 65–79. [7] P. Nachimuthu, R. Jagannathan, V. Nirmal Kumar, D. Narayana Rao, J. NonCryst. Solids 217 (1997) 215–223.

[8] C.K. Jayasankar, V. Venkatramu, S. Surendra Babu, P. Babu, J. Alloys Compd. 374 (2004) 22–26. [9] A.S. Gouveia-Neto, L.A. Bueno, R.F. do Nascimento, E.A. da Silva Jr., E.B. da Costa, Appl. Phys. Lett. 91 (2007) 091114. [10] J.E.C. da Silva, G.F. de Sa, P.A. Santa-Cruz, J. Alloys Compd. 344 (2002) 260–263. [11] H.T. Amorim, M.V.D. Vermelho, A.S. Gouveia-Neto, F.C. Cassanjes, S.J.L. Ribeiro, Y. Messaddeq, J. Solid State Chem. 171 (2003) 278–281. [12] G.Y. Chen, Y. Liu, Y.G. Zhang, G. Somesfalean, Z.G. Zhang, Q. Sun, F.P. Wang, Appl. Phys. Lett. 91 (2007) 133103. [13] N.K. Giri, D.K. Rai, S.B. Rai, J. Appl. Phys. 104 (2008) 113107. [14] M. Jayasimhadri, K.W. Jang, H.S. Lee, B. Chen, S.S. Yi, J.H. Jeong, J. Appl. Phys. 106 (2009) 013105. [15] V.A. Kazanskaya, V.V. Kuzmin, E.E. Minaeva, A.D. Sokolov, in: Proceedings of the Fourth International Conference on Luminescence Dosimetry, Krakow, Poland, 1974. [16] C.K. Jorgensen, B.R. Judd, Mol. Phys. 8 (1964) 281. [17] S.R. Anishia, M.T. Jose, O. Annalakshmi, V. Ramasamy, J. Lumin. 131 (2011) 2492–2498. [18] P. Srivastava, S.B. Rai, D.K. Rai, Spectrochim. Acta A 59 (2003) 3303–3311. [19] M.B. Saisudha, J. Ramakrishna, Phys. Rev. B 53 (1996) 6186. [20] J.A. Capobianco, G. Prevost, P.P. Proul, P. Kabro, M. Bettinelli, Opt. Mater. 6 (1996) 175–184. [21] M. Zahir, R. Olazcuaga, C. Parent, G. Le flem, P. Hagenmuller, J. Non-Cryst. Solids 69 (1985) 221–229. [22] J. Pisarska, J. Phys. Condens. Matter 21 (2009) 285101. [23] Sd. Zulfiqar Ali Ahamed, C. Madhukar Reddy, B. Deva Prasad Raju, Spectrochim. Acta A 103 (2013) 246–254. [24] H. Chen, Y.H. Lin, Y.F. Zhou, Z.H. Jiang, J. Alloys Compd. 397 (2005) 286–290. [25] J. Coelho, C. Freire, N.S. Hussain, Spectrochim. Acta A 86 (2012) 392–398. [26] J. Hanuza, M. Maczka, J. Lorenc, A.A. Kaminskii, L. Bohaty, P. Becker, Spectrochim. Acta A 71 (2008) 68–72. [27] J. Hanuza, M. Maczka, J. Lorenc, A.A. Kaminskii, L. Bohaty, P. Becker, J. Raman Spectrosc. 39 (2008) 409–414. [28] M. Maczka, A. Waskowska, A. Majchrowski, J. Kisielewski, W. Szyrski, J. Hanuza, J. Solid State Chem. 181 (2008) 3211–3216. [29] M. Maczka, A. Waskowska, A. Majchrowski, J. Zmija, J. Hanuza, G.A. Peterson, D.A. Keszler, J. Solid State Chem. 180 (2007) 410–419. [30] J. Suresh Kumar, K. Pavani, A. Mohan Babu, N.K. Giri, S.B. Rai, L. Rama Moorthy, J. Lumin. 130 (2010) 1916–1923. [31] S.S. Sundari, K. Marimuthu, M. Sivaraman, S. Surendra Babu, J. Lumin. 130 (2010) 1313–1319. [32] A.G. Souza Filho, J. Mendes Filho, F.E.A. Melo, M.C.C. Custodio, R. Lebullenger, A.C. Hernandes, J. Phys. Chem. Solids 61 (2000) 1535–1542. [33] W.T. Carnall, P.R. Fields, K. Rajak, J. Chem. Phys. 49 (1968) 4424–4442. [34] C. Gorller-Walrand, K. Binnemans, in: K.A. Gschneidner Jr., L. Eyring (Eds.), Handbook on the Physics and Chemistry of Rare Earths, vol. 25, North-Holland, Amsterdam, 1998, pp. 101–264 (Chapter 167). [35] B.R. Judd, Phys. Rev. 127 (1962) 750–761. [36] G.S. Ofelt, J. Chem. Phys. 37 (1962) 511–519. [37] C.K. Jorgensen, R. Reisfeld, J. Less – Common Met. 93 (1983) 107–112. [38] S.S. Babu, P. Babu, C.K. Jayasankar, Th. Troster, W. Sievers, G. Wortmann, Opt. Mater. 31 (2009) 624–631. [39] C. Madhukar Reddy, G.R. Dillip, B. Deva Prasad Raju, J. Phys. Chem. Solids 72 (2011) 1436–1441. [40] R.R. Jacobs, M.J. Weber, IEEE J. Quantum Electron. 12 (1976) 102–111. [41] V.K. Rai, S.B. Rai, D.K. Rai, Opt. Commun. 257 (2006) 112–119. [42] G. Tripathi, V.K. Rai, S.B. Rai, Spectrochim. Acta A 62 (2005) 1120–1124. [43] V.K. Rai, S.B. Rai, Solid State Commun. 132 (2004) 647–652. [44] L.A. Diaz-Torres, E. De la Rosa, P. Sala, V.H. Romero, C. Angeles-Chavez, J. Solid State Chem. 181 (2008) 75–80. [45] Ch. Basava Poornima, C.K. Jayasankar, Th. Troster, W. Sievers, G. Wortmann, Opt. Mater. 31 (2009) 624–631. [46] Y. Dwivedi, S.B. Rai, Opt. Mater. 31 (2009) 1472–1477. [47] J. Pisarska, W.A. Pisarski, W. Ryba-Romanowski, Opt. Laser Technol. 42 (2010) 805–809. [48] P. Abdul Azeem, S. Balaji, R.R. Reddy, Spectrochim. Acta A 69 (2008) 183–188. [49] R. Praveena, R. Vijaya, C.K. Jayasankar, Spectrochim. Acta A 70 (2008) 577–586.