Optical Materials 34 (2012) 1251–1260
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Structural and optical investigations of Eu3+ ions in lead containing alkali fluoroborate glasses B. Deva Prasad Raju a,⇑, C. Madhukar Reddy b a b
Department of Future Studies, Sri Venkateswara University, Tirupati 517 502, India Department of Physics, Sri Venkateswara University, Tirupati 517 502, India
a r t i c l e
i n f o
Article history: Received 23 October 2011 Received in revised form 5 January 2012 Accepted 19 January 2012 Available online 26 February 2012 Keywords: Glasses FT-Raman Oscillator strengths J–O parameters Fluorescence Lifetimes
a b s t r a c t Lead containing alkali fluoroborate glasses (LAFB) with molar composition of 20PbO + 5CaO + 5ZnO + 10AF + 59B2O3 + 1Eu2O3 (where A = Li, Na and K) were prepared and investigated by the TG-DTA, FT-Raman, optical absorption, fluorescence and decay curve analysis. The influence of alkali content on the structure of borate glasses was investigated by FT-Raman spectroscopy. The thermal properties of the glasses have been studied by TG-DTA analysis. Judd–Ofelt intensity parameters are derived from the absorption spectra and also from the emission spectra under various constraints. The effect of thermalization on the oscillator strengths of the absorption transitions originating from the ground (7F0) and the first excited (7F1) states of Eu3+ ions have been discussed. The J–O intensity parameters obtained by applying thermal correction to 7F0 ? 5D2 and 7F6 absorption oscillator strengths were used to calculate the various spectroscopic properties. The predicted values of radiative lifetime (sR) and luminescence intensity branching ratio (bR) are compared with the measured values for 5D0 level. The decay profiles were found to be single exponential in all the three glasses. The spectroscopic properties confirm the potentiality of present LAFB glasses doped with Eu3+ ions as laser host materials to produce an intense red luminescence at 612 nm corresponding to 5D0 ? 7F2 emission level and have significant importance in the development of emission rich optical systems. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction The inventions in photonics are basis for the telecommunications revolution of the late 20th century and it is also alternate technology for the future. The unique applications of photonics continue to emerge in the fields of quantum optics, optomechanics, electrooptics, optoelectronics and quantum electronics. Till now in the area of photonics much attention has been devoted towards the preparation and optimization of novel optical materials and also they have become more significant because of their function in various photonic devices. In order to identify new optical devices for specific utility, or devices with enhanced performance, active research is being carried out by choosing appropriate new hosts doped with RE ions. Rare earth ions doped solid-state materials like crystals and glasses are very striking in emergent phosphors, infrared to visible up converters, compact lasers, broad band amplifiers and white light emitting devices [1–3]. Due to their wide inhomogeneous line widths, glasses are approving hosts for RE ions compared to crystals [4]. Interest has also been paid to glasses for their relatively easy manufacture, shaping, and flexibility of active ion concentrations and the possibility of easily obtaining bulk samples ⇑ Corresponding author. Tel.: +91 94402 81769. E-mail address:
[email protected] (B. Deva Prasad Raju). 0925-3467/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2012.01.027
when compared to single crystalline matrices. To examine the influence of chemical environment on the optical properties of the rare earth ions, glasses are the promising hosts. The increasing importance of glasses doped with rare earth ions has witnessed the tremendous progress in the development of laser level engineering. The absorption and luminescence properties of RE ions in glasses vary in wide range that depends on the chemical composition, structure and nature of bonds of host glass [5]. Oxide glasses are attracting hosts for obtaining efficient luminescence in rare earth ions. Among them, borate glass is a suitable optical material for rare earth ions with high transparency, low melting point, high thermal stability, good rare earth ion solubility and show more clear relationship between glass structure and physical properties [6]. The addition of fluoride content to borate glasses decreases the phonon energy and increases the moisture resistance and transparency in the visible region, which in turn contribute to the reduction in the non-radiative losses. The fast anion conducting behavior with fluorine ions, the electrical conductivity considerably increases due to the fluorine migration, which is useful in the development of advanced batteries and electrochemical devices [7]. A large glass forming region is also exists in the oxyfluoride systems with good and easy glass formation. The large mass, low field strength and high polarisability of lead oxide give some special significance with a good ability to form stable
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glasses over a wide range of concentrations due to dual role as glass modifier and glass former [8]. In a consequence the presence of PbO in borate glass enhanced significantly luminescence of rare earth ions. B2O3 is a glass forming oxide, PbO is a conditional glass former and with these two chemicals in the glass matrix a low rate of crystallization, moisture resistance, stable and transparent glasses have been achieved. Thus the B2O3–PbO glasses have achieved significant structural and optical properties. These advantages are useful for structural and optical investigations in oxyfluoride lead borate glasses [9,10]. Research is focused on RE ions doped glasses is not limited to infrared optical devices, but also an increasing interest in visible optical devices [11,12]. With the growing demand of a variety of visible lasers and light sources, the trivalent europium (Eu3+ (4f6)) ions doped glasses show great interest in the orange-red region because Eu3+ ions emit narrow band and almost monochromatic light and have long lifetime of the excited states. Eu2O3-doped phosphors exhibit higher luminescence efficiency compared with other luminescence materials [13–17], so these are commonly used in the field emission technology and LEDs. The Eu3+ ion is well known spectroscopic probe to estimate the local environment around RE ions in various crystals and glass matrices due to their simple energy level structures, the ground 7F0 state and the excited 5D0 state of Eu3+ ions are singlet and non-degenerate under any symmetry and usually acts as a powerful emitting center. The information regarding the local environment around Eu3+ ion depends only on the splitting of the 5D0 ? 7FJ emission spectra. Depending on the number of stark components into which 5D0 ? 7FJ emission transitions split, the symmetry at the Eu3+ ion site can be predicted by the crystal field parameterization [6,18]. Due to the technological importance of europium ion and the advantages of above research, the Eu2O3-doped PbO + CaO + ZnO + AF + B2O3 (A = Li, Na and K) glasses have been prepared and investigated. The plan of the present study is (1) to study the variation of structural and optical properties in LAFB glasses, (2) to determine the J–O intensity parameters for trivalent europium ions, (3) to determine the radiative properties for significant levels, (4) to compare the experimental and predicted radiative properties for 5D0 level, (5) to measure and analyze the luminescence decay curves of the 5D0 level of Eu3+ ions in LAFB glasses and (6) the results obtained in the present work are compared with those reported Eu2O3-doped glass systems. 2. Experimental methods The molar composition of europium-doped lead containing alkali fluoroborate (LAFB) glasses investigated in the present work is as follows.
LLiFB : 20PbO þ 5CaO þ 5ZnO þ 10LiF þ 59B2 O3 þ 1Eu2 O3 LNaFB : 20PbO þ 5CaO þ 5ZnO þ 10NaF þ 59B2 O3 þ 1Eu2 O3 LKFB : 20PbO þ 5CaO þ 5ZnO þ 10KF þ 59B2 O3 þ 1Eu2 O3 The above glasses were prepared by conventional melt quenching method, with the reagent grade chemicals. About 10 g batches of the above chemical compositions are thoroughly crushed in an agate mortar in order to mix the chemicals homogeneously. The mixture is then taken in a porcelain crucible and melted in an electric furnace at 950 °C for about an hour. The melt is then air quenched by pouring it on a thick brass plate and pressed against another brass plate. The glasses are annealed at 360 °C (above which the sample may lose its glassy nature) for about 8 h in an oven to remove thermal strains and then polished to measure their physical and optical properties.
The refractive indices (n) are measured using an Abbe’s refractometer at sodium wave length of 589.3 nm with 1-bromonapthlin (C10H7Br) as contact liquid. The densities are measured by the Archimedes method using distilled water as an immersion liquid. The various physical properties of the Eu2O3-doped LAFB glasses are presented in Table 1. TG-DTA profiles were recorded on a Netzsch STA 409 thermal analyzer in the temperature range of 30–1400 °C, at the rate of 10 °C/min, under N2 gas atmosphere. FT-Raman spectra in the range of 50–5000 cm1 with Bruker RFS 27 stand alone Raman spectrometer using 1064 nm light from Nd: YAG laser on glass powder was recorded in black scattering geometry with a resolution of 2 cm1. Absorption spectra in the ultraviolet, visible and near infrared regions are measured using a Varian Cary 5E UV–vis–NIR spectrophotometer. Excitation, emission and lifetime measurements are carried out by using Jobin Yvon Fluorolog-3 fluorimeter using xenon flash lamp as excitation source. All these measurements were carried out at room temperature.
3. Results and discussion 3.1. Thermal (TG-DTA) studies The thermal behavior (TG-DTA) profiles of LAFB glasses are shown in Fig. 1. From these profiles, the values of glass transition (Tg), crystallization (Tc) and melting temperatures (Tm) have been identified and from them, glass stability factor (S) and also related Hruby’s parameter (Kgl) were computed [19] and the obtained results were given in Table 1. The stability of the glasses can be known by using the glass stability factor; where as, the stability of the glass against devitrification can be known by Hruby’s parameter. The thermal stability factor (DT) has been used frequently to understand the glass stability. To achieve a large working range of temperature in sample fiber drawing, it is advantageous for a glass to have DT as large as possible. The exothermic peak at around 250 °C in all the studied glasses is related to the loss of OH and the decomposition of hydroxide. The glass transition temperature (Tg) is related to the density of covalent cross-linking, the number and strength of the coordinate links formed between oxygen atoms and the cation, and the oxygen density of the network [20]. The TG curve shows only a small weight loss of about 1% in LNaFB glass and about 4% in LLiFB and LKFB glasses in the complete range of investigation i.e., from 30 °C to 1100 °C.
Table 1 Measured and calculated physical properties for Eu3+ ions in LAFB glasses. Physical quantities
LLiFB
LNaFB
LKFB
Sample thickness (cm) Refractive index (n) Density (g/cc) Concentration (mol/liter) Concentration (ions cm3 1020) Average molecular weight (g) Dielectric constant (e) Molar volume Vm (cm3/mol) Glass molar refractivity (cm3) Electronic polarisability ae (1024 cm3) Reflection losses R (%) Polaron radius rp (A°) Inter ionic distance ri (A°) Field strength F (1014 cm2) Glass transition temperature, Tg (°C) Crystallization temperature, Tc (°C) Temperature of melting, Tm (°C) Glass stability factor, S = Tc Tg (°C) Hruby’s parameter, Kgl = ((Tc Tg)/(Tm Tc))
0.280 1.586 4.780 0.253 1.524 188.780 2.515 39.494 13.252 5.255 5.135 7.540 18.720 5.270 468 742 943 274 1.363
0.290 1.587 4.820 0.253 1.526 190.385 2.518 39.499 13.271 5.263 5.148 7.540 18.710 5.270 460 730 930 270 1.350
0.300 1.588 4.880 0.254 1.528 191.996 2.522 39.343 13.242 5.251 5.162 7.536 18.700 5.280 457 735 943 278 1.336
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Fig. 2. FT-Raman spectra of Eu3+-doped LAFB glasses. Fig. 1. TG and DTA profiles of Eu3+-doped LAFB glasses.
3.2. FT-Raman spectroscopy The FT-Raman spectroscopy is an effective tool to investigate the structure of the glasses. B2O3-based glasses have been widely studied over the years due to their interesting structural particularities as the occurrence of boron anomaly [21,22]. Generally in most of the borate glasses, the change of boron coordination from three to four along with the formation of BO4 tetrahedra and also several other structural groups were observed. Addition of various alkali/alkaline earth oxide/fluoride modifiers A2O/AF (A = Li, Na, K, Mg, Ca, Sr, Ba) to the borate glasses brings drastic changes in the structural units. The structure of glassy alkali borates in a complex three dimensional network of boron and oxygen composed of larger structural units. The band assignment and the interpretation of the FT-Raman spectra is rather difficult due to the disordered nature of the vitreous materials and the more number of possible borate structural units like boroxol rings, penta-, tri-, di- and meta borate groups with bridging and non bridging oxygen (NBO) ions and also with various extensively overlapping bands [7]. FT-Raman spectra of Eu3+ ions doped alkali fluoroborate glasses are presented in Fig. 2. The Raman band at around 89 cm1 is associated to the symmetric Pb-O stretching in the PbO4 pyramid configuration [10,23]. The band at around 475 cm1 is assigned to a ring angle bending (B–O–B), which is observed at 470 cm1 for pure B2O3 [24]. The bands located at 635 and 730 cm1 are related to the chain and ring type meta borate groups [7,23,25]. The occurrence of band at around 905 cm1 is an indication of the presence of pentaborate groups in the borate glasses [7,23]. The band centered at around 1265 cm1 is assigned to pyroborate groups [23–26]. The bands in the region 1300–1600 cm1 is similar to that in the Raman spectra of large number of modifier borate glasses. These bands have been assigned to the stretching of B–O stretching in metaborate rings and chains [24–26]. In the present alkali fluoroborate glasses it is observed that in all the studied three glasses the positions of FT-Raman bands are different from one
another indicating the structure of alkali fluoroborate glasses depends strongly on mass and size of the added alkali ion. The broadening of bands after 500 cm1 in the Raman spectrum of LLiFB glass is caused may be due to the anharmonicity of the interaction of the ions with the BO4 groups in the borate rings [27,28]. In the LLiFB glass a relatively larger number of borate rings with BO4 groups are formed than LNaFB and LKFB glasses. Therefore the impact of the anharmonicity on the vibrations of the borate rings will be stronger for lighter alkali ions i.e., for Li+, and decrease as we go to K+. In the case of lithium ions, a stronger network structure is formed due to the formation of BO4 units that depends strongly on the mass of the alkali ion. The formation rate of BO4 units decreases as the mass of the alkali ion is increased. In the present glasses it is observed that the formation rate of non-bridging oxygens increases as the radius of the cation is increased. The process of transformation of a boron atom from three to four co-ordinations depends strongly on the mass of the alkali ion. This process is less effective at heavier cations due to the enhanced formation of non-bridging oxygens. 3.3. Optical absorption spectra Room temperature optical absorption spectra of Eu3+ ions in LAFB glasses in the visible and near infrared regions are shown in Figs. 3 and 4, respectively. The absorption spectrum presents three weak bands at 464, 526 and 534 nm corresponding to 7 F0 ? 5D2, 7F0 ? 5D1 and 7F1 ? 5D1 transitions in the visible region and two intense NIR bands at 2090 and 2210 nm corresponding to 7 F0 ? 7F6 and 7F1 ? 7F6 transitions respectively. The assignment of the absorption bands was made according to Carnall et al. [29]. The 7 FJ M 5DJ absorption and emission bands are spin forbidden and hence they are very weak [30]. The close examination of band positions (7F0 ? 7F6, 7F1 ? 7F6); (7F0 ? 5D1, 7F1 ? 5D1) reveals that the energy gap between 7F0 and 7F1 levels is 260 cm1 which is comparable to other Eu3+-doped glasses. This small energy gap results in the population of 7F1 first excited state in addition to 7F0 ground state. In lanthanide ions the observed absorption bands in the
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Fig. 3. VIS absorption spectra of Eu3+-doped LAFB glasses.
Fig. 5. Absorption cross-section spectra of Eu3+ in the VIS region for LAFB glasses.
Fig. 4. NIR absorption spectra of Eu3+-doped LAFB glasses.
absorption spectra are due to intra-configurational f–f transitions. The majority of the transitions are induced electric dipole transitions. However, a few transitions are magnetic dipole in nature and their contribution is not significant. In Eu3+ ion the absorption bands are correspond to the f–f optical excitations from the ground 7 F0 state and thermally populated 7F1 state to the various excited states of the Eu3+ ion. The absorption band positions of all the transitions occur almost at the same wavelength for all the three glasses. Although the spectra of these glasses have similar absorption spectral profiles, their spectral intensities are significantly different with the content of alkali metals (A = Li, Na and K) around the Eu3+ ion. Absorption cross-section is a measure for the probability of an absorption process, and is used to quantify the probability of a certain particle–particle interaction. The absorption cross-section is proportional to the intensity of the absorption between the two levels involved. The absorption of light is in general governed by the Beer–Lambert law, for optically thin films and Beer–Lambert law forms the basis for the determination of absorption crossI 2:3026log 0I section [31,32] using the equation rabs ¼ , where rabs lN is the absorption cross-section of the ground state absorption, log II0 is the absorbance, l is the path length (thickness of the glass sample in cm) and N is the rare earth ion concentration per cm3 in the glass. Figs. 5 and 6 represent the absorption cross-section of Eu3+-doped LAFB glasses in the visible and near infrared regions,
Fig. 6. Absorption cross-section spectra of Eu3+ in the NIR region for LAFB glasses.
respectively. The values of absorption cross-section of Eu3+ at 464 nm are 4.55 1020 cm2 (LLiFB), 6.40 1020 cm2 (LNaFB) and 6.8 1020 cm2 (LKFB), at 2090 nm are 3.96 1020 cm2 (LLiFB), 4.38 1020 cm2 (LNaFB) and 5.63 1020 cm2 (LKFB) and at 2210 nm are 3.82 1020 cm2 (LLiFB), 4.20 1020 cm2 (LNaFB) and 5.33 1020 cm2 (LKFB). 3.4. Thermal correction All the RE3+ ions possess single populated ground states except Eu ion. This is due to that the first excited free-ion level 7F1 of Eu3+ is only about 260 cm1 above the 7F0 ground state and the energy difference is of the order of KBT, where KB is the Boltzmann’s constant and T is the absolute temperature [33]. At room temperature (300 K), the magnitude of KBT is about 208 cm1. At room temperature, about 65% of the ions populate 7F0 ground level and about 35% is the 7F1 first excited level, so we cannot neglect the fractional thermal population of the 7F1 first excited level. Hence, in the absorption spectrum of a Eu3+-doped glass matrix at room temperature, due to the overlapping of these transitions, one can observe the absorption spectrum not only from the transitions originating from the 7F0 level, but also from 7F1 level. Thus, at room temperature the classical J–O analysis has become very cumbersome. To overcome this difficulty, it is necessary either to record 3+
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the absorption spectra at lower temperature (77 K) to avoid the population in the excited free-ion levels [34] or the effect of thermalization (thermal correction) on the population of energy levels has been taken into account to correct the oscillator strengths of the absorption spectra obtained at room temperature. For any excited level (2S+1)LJ the fractional thermal population is given by n o g CJ E E ¼ g J exp KJ B T0 , where C0 and CJ are the thermal correcC0 0
tion factors for the ground and excited levels, respectively. gJ = (2J + 1) is the degeneracy and EJ is the energy of the excited level. E0 and g0 are the energy and degeneracy of the ground level. The above expression corrects the thermalization effect and allows one to estimate the exact oscillator strengths from 7F0 ground state to the different excited states. In the present LAFB glass system, the energy difference between the ground 7F0 and the first excited 7F1 level is 260 cm1 only. In the present study, the thermal correction factors corresponding to the 7F0 ground state and the first excited 7F1 state at 20 °C are C0 = 0.6329 and C1 = 0.3386, respectively. Now to get the 100% population of the ions from the 7F0 ground level, the measured oscillator strengths (fexp) have been corrected for thermal population by dividing them with their respective initial level fractional populations [33].
3.5. Oscillator strengths The intensities of an absorption bands are expressed in terms of their oscillator strengths. The experimentally measured oscillator strengths (fexp) of absorption bands have been obtained from the R expression fexp = 4.318 109 e(v)dv, by measuring the relative areas under the absorption bands, where e(v) is the molar extinction coefficient which can be calculated using Beer–Lambert law. Theoretically calculated oscillator strengths, fcal of absorption transitions between ground state WJ and excited state W0 J0 can be determined by using the J–O theory [35,36] and using the formula
fcal ¼
8p2 mcv ðn2 þ 2Þ2 Rk¼2;4;6 Xk ðWJkU k kW0 J0 Þ2 3hð2J þ 1Þ 9n
ð1Þ
In this work we have used the reduced matrix elements reported by Carnall et al. [29]. For all the observed transitions, kU 4 k2 values are zero and hence, the least square fit procedure is not possible to evaluate J–O intensity parameters (Xk) as well as calculated oscillator strengths (fcal). The experimental band energies and oscillator strengths, fexp with thermal correction and with out thermal correction are presented in Table 2 along with their assignments. These results clearly indicate that the dependence of the value of fexp of various absorption bands on glass composition and the intensity of the spin forbidden transition 7F1 ? 5D1 is higher than the magnetic dipole transition. Due to thermal correction we observe the similar trends in the Xk parameters but with increased magnitude of oscillator strengths and Xk parameters. Table 2 Absorption transitions, energies and experimental oscillator strengths (106) for Eu3+ ions in LAFB glasses. Transition
7F0 ? 5D2 7 F0 ? 5D1 7 F1 ? 5D1 7 F0 ? 7D6 7 F1 ? 7D6 a b
Energy (m cm1)
21,552 19,012 18,727 4785 4525
Without thermal correction. With thermal correction.
LLiFB
LNaFB
3.6. Judd–Ofelt analysis In the case of Eu3+ ions doped materials; we obtain the J–O intensity parameters by two different methods [15,17,34]. One way is to find the Xk parameters from the optical absorption spectra by considering the selected transitions at room temperature and the second way is from the analysis of emission spectra at room temperature. 3.6.1. Evaluation of J–O parameters from the absorption spectra For all the observed absorption transitions identified in the studied glasses, the magnitude of kU 4 k2 matrix element is zero. Therefore the J–O intensity parameters are determined by applying an alternative method. Van Deun et al. [34] adopted an alternate method to determine the Xk parameters by using the selected absorption transitions measured at room temperature by ignoring thermal correction. Dejneka et al. [33] evaluated Xk parameters by using thermally corrected oscillator strengths obtained from the room temperature absorption bands. In the present investigation, we have evaluated the J–O intensity parameters with thermal correction and without thermal correction using the experimental oscillator strengths of 7F0 ? 5D2 and 7F0 ? 7F6 absorption bands because for these two transitions, the kU 2 k2 and kU 6 k2 , respectively are the only non-zero matrix elements and do not overlap with the 7 F1 ? 5D2 and 7F1 ? 7F6 transitions. As kU 4 k2 ¼ 0 for all the observed transitions, the contribution of X4 parameter becomes zero on the bulk properties. The evaluated J–O parameters presented in Table 3 are compared with other reported values [17,34,37–41]. When the thermal correction is applied to evaluate the oscillator strengths there is an increase in value of X2 parameter is observed. This indicates the structural change in the surrounding area of Eu3+ ions. The symmetry as well as structural details around Eu3+ ions can be explained with the X2 parameter, because X2 parameter is very sensitive to the ligand environment compared to X4 and X6 parameters. The larger the value of X2, the stronger the covalency and the lower is the symmetry [4]. In the present LAFB glasses the higher magnitude of X2 suggests that the ligands around the Eu3+ ions possess higher distortion as well as high covalent nature. 3.6.2. Evaluation of J–O parameters from the emission spectra The J–O intensity parameters have been determined based on the analysis of emission spectra at room temperature. The transition 5D0 ? 7F1 is not dependent on the host matrix and is magnetic dipole allowed, among 5D0 ? 7FJ (J = 0–4) transitions. But 5D0 ? 7FJ (J = 2, 4 and 6) transitions are induced electric dipole allowed and strongly depend on the host matrix. For a particular transition, the emission intensity (I) can be considered as proportional to area under the emission curve for that transition. The intensity will also proportional to the radiative decay rate of the transition (AR). I ¼ hcv AR N a (area under the curve), where hcv is the energy separation in the initial and final levels, N is the population of the 5D0 level (in case of Eu3+). The 5D0 ? 7FJ (J = 2, 4, and 6) transitions are electric dipole allowed whose radiative transition rates AR can be represented by the equation
Aed
LKFB
fexpa
fexpb
fexpa
fexpb
fexpa
fexpb
0.37 0.04 0.06 1.67 0.69
0.58 0.07 0.10 2.63 1.11
0.38 0.04 0.07 1.48 0.58
0.6 0.06 0.11 2.35 0.92
0.37 0.02 0.08 2.37 0.88
0.58 0.03 0.13 3.75 1.39
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" # 64p4 v 3 nðn2 þ 2Þ2 2 k 0 0 2 ¼ e Rk Xk ðWJkU kW J Þ 3hð2J þ 1Þ 9
ð2Þ
The transition rate of the magnetic dipole allowed 5D0 ? 7F1 transition is represented by the equation
Amd ¼
64p4 v 3 ½n3 Smd 3hð2J þ 1Þ
ð3Þ
where Smd is the magnetic-dipolar transition line strength and is independent of host matrix. Thus, X2 can be calculated from the
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Table 3 Comparison of J–O intensity parameters (1020 cm2) for Eu3+ ions in LAFB glasses with different glass hosts under various constraints. Host matrix
Absorption (all 5 transitions)
Absorption (selected transitions)
Emission
Without thermal correction
With thermal correction
Without thermal correction
With thermal correction
X2
X4
X6
X2
X4
X6
X2
X4
X6
X2
X4
X6
X2
X4
X6
Present work LLiFB LNaFB LKFB
3.75 3.77 4.25
– – –
0.72 0.75 1.15
6.02 5.95 6.71
– – –
1.15 1.19 1.82
13.77 14.27 13.84
– – –
1.55 1.38 2.14
21.75 22.56 21.86
– – –
2.45 2.18 3.50
3.62 3.62 3.81
1.19 1.43 1.42
– – –
Reported PKBAEu [17] PKBFAEu [17] Fluorophosphate[34] Oxyfluoroborate[37] Zinc borate [38] Fluoridephosphate [39] Sodiumfluoroborate [40] Tellurite [41]
8.07 6.15 – – – – 2.34 –
6.30 3.66 – – – – 9.25 –
0.45 0.45 – – – – 0.36 –
21.77 16.30 – – – – – –
13.19 7.04 – – – – – –
1.28 1.25 – – – – – –
11.59 10.98 – 11.08 9.50 9.00 – 11.06
6.24 4.03 – 5.76 7.00 5.30 – 4.58
0.58 1.42 – 3.32 3.60 2.30 – 0.96
18.02 17.06 6.50 – – – – –
9.72 6.28 5.40 – – – – –
0.90 2.21 3.90 – – – – –
6.91 7.13 – – – 4.1 – –
5.01 5.18 – – – 1.2 – –
5.76 5.78 – – – – – –
ratio of the intensity of the 5D0 ? 7FJ (J = 2, 4, and 6) transitions R R IJ dv , to the intensity of the 5D0 ? 7F1 transition I1 dv .
R I dv AJ e2 R J ¼ ¼ I1 dv A1 Smd;1
v 3J v 31
" # nðn2 þ 2Þ2 Xk ðWJkU k kW0 J 0 Þ2 9n3
ð4Þ
where Smd,1 refers to the magnetic dipole line strength due to 5 D0 ? 7F1 transition. For the present system, its value was taken as 9.6 1042 esu2 cm2 as reported by [42,43]. The values for the reduced square matrix elements kU J k2 for the 7F2 and 7F4 transitions were taken as 0.0032 and 0.0023, respectively. Since the emission transition at around 810 nm has not been observed from the emission spectra, the authors could not determine the X6 parameter. The evaluated Xk parameters from the absorption spectra (with thermal correction and without thermal correction) and emission spectra are presented in the Table 3. The current trend in the J–O parameters (X2 > X4) has also been observed for various other glass matrices [17,34,37–41]. This confirms the covalency existing between the Eu3+ ions and surrounding ligands.
transitions 5D0 ? 7FJ with J = 5 and 6 are not observed as transition probabilities of these transitions are very weak and the 5D0 ? 7FJ (J = 2, 4 and 6) transitions are electric dipole (ED) transitions. The observed narrow emission bands in the emission spectra of LAFB glasses is due to the shielding effect of 4f6 electrons by 5s and 5p electrons in outer shells in the Eu3+ ion. Among these observed five emission bands, the transition 5D0 ? 7F2 (612 nm) has shown a strong red emission which is considered as a hypersensitive transition following the selection rules DJ = 2. Another transition 5 D0 ? 7F1 (591 nm) with DJ = 1 could be found as a magnetic dipole transition [44] and is independent of the crystal field strength around Eu3+ ion. This transition could be used for the estimation of transition probabilities of various excited levels [45]. The emission mechanism (Energy level diagram) of Eu3+ ions in the LAFB glasses is represented in Fig. 9. The transitions 5D0 ? 7FJ with J = 5 and 6 are not observed as transition probabilities are very weak. Due to the presence of high energy phonons in the glasses, the emissions starting from the excited levels 5DJ (J = 1, 2 and 3) are suppressed i.e., there is a fast non-radiative relaxation takes place at 5D0 level when the Eu3+ ions are excited to any level above the 5D0 level.
3.7. Excitation and emission spectra 3.8. Strength of Eu–O bond covalency and Eu3+ site symmetry In order to analyze the luminescence properties, it is necessary to know the correct excitation wavelength of the ion. Fig. 7 shows the excitation spectrum recorded for Eu3+-doped LLiFB glass by monitoring the emission at 612 nm. Totally six excitation bands are observed at 360, 365, 375, 380, 392 and 423 nm corresponding to the 7F0 ? 5D4, 7F1 ? 5D4, 7F0 ? 5G2, 7F1 ? 5L7, 7F0 ? 5L6 and 7 F1 ? 5D3 transitions, respectively. From these excitation transitions, since the excitation band corresponding to the 7F0 ? 5L6 transition at 392 nm is more prominent, and hence all the LAFB glasses have been excited with 392 nm to record the photoluminescence spectra. Fig. 8 shows the room temperature photoluminescence spectra obtained upon excitation with 392 nm corresponding to the 7 F0 ? 5L6 transition for LAFB glasses in the 550–750 nm spectral range. After excitation into the 5L6 excited level, appreciable radiative emissions takes place from the 5D0 meta stable state to 7FJ (J = 0, 1, 2, 3 and 4) states in the range of 550–750 nm. When any other levels above the 5D0 are excited, there is quick non-radiative relaxation to this excited fluorescent level due to the small energy gaps between them and consequently the same emission spectrum is obtained. From Fig. 8, it is noticed that the fluorescence spectra of all the three glasses are found to be almost identical and shown five emission lines at 579, 591, 612, 655 and 702 nm corresponding to the 5D0 ? 7FJ (J = 0, 1, 2, 3 and 4) transitions, respectively. The
The ratio of the emission intensity of the 5D0 ? 7F2 transition to that of the 5D0 ? 7F1 transition is defined as fluorescence intensity ratio R = (5D0 ? 7F2)/(5D0 ? 7F1) can be used to investigate the relative strength of covalent/ionic bonding between Eu3+ ions and the surrounding ligands and to establish the degree of asymmetry in the vicinity of Eu3+ ions. The emission intensities of the corresponding transitions can be estimated by measuring the area under various 5D0 ? 7FJ emission bands. The fluorescence intensity ratio R value is also depends on the J–O intensity parameter X2, which is used to explain the covalency and/or structural changes in the vicinity of the Eu3+ ion (short range effects). With the help of fluorescence intensity ratio R, it is possible to estimate the deviation from the site symmetries of the Eu3+ ions. In the present glasses, the trend of R value is in the order LLiFB (2.32) < LNaFB (2.38) < LKFB (2.44). Higher value of R is usually recognized to higher local asymmetry for Eu3+ ions. The increase in R value from LLiFB to LKFB is due to decreasing local symmetry for Eu3+ ions. Hence the symmetry in the present glasses decreases in the same order. These R values are found to be comparable and smaller than the other borate and phosphate glasses (Table 4) [17,38,41,46,47] indicates in the present glasses the higher the symmetry around the Eu3+ ions and the higher the Eu–O covalence. The high intensity ratio of 2.44 observed for LKFB glass indicates the formation of
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strong Eu–O bond in the glass. The absorption and emission studies also reveal that the LKFB glass exhibit greater degree of Eu–O bond covalency with relatively higher fluorescence intensity for 5 D0 ? 7F2. 3.9. Radiative and fluorescence properties Using the phenomenological J–O intensity parameters (Xk) obtained from the thermal correction of 7F0 ? 5D2 and 7F6 absorption oscillator strengths, can be used to calculate the spontaneous emission probability (AR) from an initial state WJ to a final state W0 J0 can be calculated using the following equation
" # 64p4 v 3 nðn2 þ 2Þ2 3 AR ðWJ; W J Þ ¼ Sed þ n Smd 3hð2J þ 1Þ 9 0 0
ð5Þ
where Sed and Smd are electric and magnetic dipole line strengths, respectively. The required tensor operators kU k k2 necessary for the calculation of these parameters are taken from Carnall et al. [48]. From these values it is possible to calculate the radiative branching ratio (bR) and radiative lifetime (sR) from an initial state WJ to a final state W0 J0 can be calculated using the following equations.
bR ðWJ; W0 J 0 Þ ¼
sR ðWJÞ ¼
AR ðWJ; W0 J 0 Þ RW0 J0 AR ðWJ; W0 J0 Þ 1
RW0 J0 AR ðWJ; W0 J0 Þ
Fig. 8. Visible fluorescence spectra of Eu3+-doped LAFB glasses.
ð6Þ
ð7Þ
In addition the integrated absorption cross-section (ra) for stimulated emission is estimated from the equation
ra ¼
Z
rv dv ¼
AR ðWJ; W0 J 0 Þ 8pcv 2 n2
ð8Þ
The stimulated emission cross-section (re) between WJ and W0 J0 levels is an important parameter and its value signifies the rate of energy extraction from the optical material is represented as
re ðWJ; W0 J0 Þ ¼
k4p AR ðWJ; W0 J 0 Þ 8pcn2 DkP
ð9Þ
Fig. 9. Energy level diagram showing emission transitions of Eu3+-doped in LAFB glasses.
where kp is the peak wavelength and DkP is the effective linewidth, found by dividing the area of the emission band by its average height. Table 4 Comparison of intensity ratio R = (5D0 ? 7F2)/ (5D0 ? 7F1) of Eu3+ ions in LAFB glasses with different hosts.
Fig. 7. Excitation spectrum of Eu3+-doped LAFB (LLiFB) glasses.
Host matrix
Intensity ratio (R)
Present work LLiFB LNaFB LKFB
2.320 3.323 2.434
Reported PKBAEu [17] PKBFAEu [17] Zinc borate [38] Tellurite [41] ZBS2 [46] BLEu [47]
4.550 4.690 3.940 4.280 2.690 2.410
The calculated radiative transition probabilities (AR), total transition probabilities (AT), radiative branching ratios (bR), stimulated absorption cross-section (ra), radiative lifetimes (sR) from the 5D0
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Table 5 Radiative properties such as electric dipole line strengths (Sed, 1040 esu2 cm2), radiative transition probabilities (AR, s1), total radiative transition probabilities (AT, s1), radiative lifetimes (sR, ms), radiative branching ratios (bR) and stimulated absorption cross-sections (ra 1020 cm) of Eu3+ ions in LAFB glasses. Level
5
D0 ? 7F1 5 D0 ? 7F2
LLiFB
LNaFB
LKFB
v
Sed
AR
bR
ra
Sed
AR
bR
ra
Sed
AR
bR
ra
16,920 16,313
0 1.05
55 515 AT = 570 sR = 1.75
0.10 0.89
10.14 101.50
0 1.71
55 534 AT = 589 sR = 1.70
0.10 0.90
10.12 105.30
0 1.66
55 518 AT = 573 sR = 1.74
0.10 0.89
10.10 102.00
Table 6 Emission properties such as peak emission wavelength (kp), effective linewidth (DkP, nm), radiative transition probabilities (AR, s1), stimulated emission cross-section (re 1022 cm2), experimental branching ratios (bm) and gain band width parameters ((re DkP) 1025 cm3) for Eu3+ ions in LAFB glasses. Level
D0 ? 7F1 5 D0 ? 7F2
5
LLiFB
LNaFB
LKFB
kp
DkP
re
AR
bm
re DkP
Dk P
re
AR
bm
re DkP
Dk P
re
AR
bm
re DkP
591 613
12.9 13.5
2.7 28.3
55 515
0.26 0.59
35.3 381.4
12.3 12.2
2.9 32.5
55 534
0.25 0.58
35.2 395.7
12.5 12.2
2.8 31.5
55 518
0.25 0.60
35.3 383.7
excited level to all its lower lying levels are presented in Table 5. The Xk parameters which are determined by applying thermal correction to 7F0 ? 5D2 and 7F6 oscillator strengths have found maximum values of the electric dipole line strengths (Sed) and the spontaneous emission probability rates (AR). The radiative transition probability (AR) is found to be considerably larger in magnitude for the 5D0 ? 7F2 transition with a value of 515, 534 and 518 s1 for LLiFB, LNaFB and LKFB glasses, respectively. For laser action, the higher values of measured branching ratios (bm) which are obtained from the areas under the emission bands are more important. The measured branching ratios of 59%, 58% and 60% for LLiFB, LNaFB and LKFB glasses, respectively suggest that 5 D0 ? 7F2 transition is most intense and there by suggesting that Eu3+-doped LAFB glasses could be used for laser emission at 612 nm. Table 6 presents the comparison of gain cross sections (AR), branching ratios (bm), effective line widths (DkP), stimulated emission cross sections (re) and optical gain band widths (re DkP) for 5D0 ? 7F1 and 5D0 ? 7F2 transitions in all three glasses. High gain band width parameter and large stimulated emission cross section for intense stimulated emission are the characteristics of good laser transitions. The higher magnitudes of spontaneous emission probability (AR), stimulated emission cross section (re) and measured branching ratio (bm) indicates that the 5D0 ? 7F2 emission line of Eu3+ in LAFB glass exhibit intense red luminescence at 612 nm.
relaxation rates are 175, 142 and 160 s1 for LLiFB, LNaFB and LKFB glasses, respectively. The small magnitudes of non –radiative relaxation rates are mainly due to the small deviations between the experimental and calculated lifetimes. To obtain highly efficient and stable laser active materials, the optical gain parameter (re sm) values are very important [49]. The optical gain parameters of 5D0 ? 7F2 fluorescent level are 37.855, 43.11 and 4.79 (1025 cm2 s) for LLiFB, LNaFB and LKFB glasses, respectively. The quantum efficiency which is the ratio of measured lifetime to the radiative lifetime (g ¼ ssmR 100%) is also an important parameter to predict the laser host material. The quantum efficiencies (g) of an emission state depends on emission cross sections, radiative transition probabilities, lifetimes of the meta stable states, concentration of the RE ions and the ligand field effect. Table 7 presents the comparison of measured lifetimes (sm), radiative lifetimes (sR), quantum efficiencies (g), non-radiative relaxation rates (WNR) and optical gain parameters (re sm) of 5D0 ? 7F2 transition of Eu3+ ions in LAFB glasses with different glass hosts. For LAFB glasses the quantum efficiencies are 77%, 80% and 78% for LLiFB, LNaFB and LKFB glasses, respectively. In the present investigation, the values of g for all the three LAFB glasses are found to be less than unity, which is clearly due to manifestation of the non-radiative process. The values of g for the 5D0 level are found to be nearly 80% for all the three glasses which is due to the absence of major non-radiative channels such as multi phonon relaxation, cross-relaxation etc., from the 5D0 level. The fluorescence intensity emitted by rare earth ions doped
3.10. Fluorescence decay curve analysis In order to get additional information about the fluorescence properties of Eu3+-doped LAFB glasses for the 5D0 ? 7F2 transition, fluorescence decay curves at room temperature are measured by exciting the samples at 392 nm and monitoring the emission at 612 nm and were shown in Fig. 10. Lifetimes of the 5D0 level of Eu3+ in all the glass systems are obtained by taking the first e-folding times of the emission intensities. These decay curves for all glass samples are found to be almost single exponential nature with lifetimes of 1.34, 1.37 and 1.36 ms for LLiFB, LNaFB and LKFB glasses, respectively. The measured lifetimes (sm) are found comparable to those reported in literature for 5D0 excited state [17,37–41]. The radiative lifetimes (sR) obtained by J–O theory are 1.94, 1.87 and 1.93 ms, respectively. The non-radiative relaxation rates from the 5 D0 excited state to its lower lying states may be reason for the observed deviation between sm and sR values for all these glasses. The non-radiative relaxation rates (WNR) are calculated according to the equation W NR ¼ s1m s1R s1 . In the present study, the non-radiative
Fig. 10. Fluorescence decay profiles of 5D0 level of Eu3+-doped LAFB glasses.
B. Deva Prasad Raju, C. Madhukar Reddy / Optical Materials 34 (2012) 1251–1260 Table 7 Comparison of measured lifetimes (sm), radiative lifetimes (sR), quantum efficiencies (g), non-radiative relaxation rates (WNR) and optical gain parameters (re sm) of 5 D0 ? 7F2 transition of Eu3+ ions in LAFB glasses with different glasses. Host matrix LLiFB [present work] LNaFB [present work] LKFB [present work] PKBAEu [17] PKBFAEu [17] Oxyfluoroborate[37] Zinc borate [38] Fluoridephosphate [39] Sodiumfluoroborate [40] Tellurite [41]
sm
sR
re sm
g
(ms)
(ms)
(1025 cm2 s)
(%)
WNR (s1)
1.34 1.37 1.36 2.51 2.52 1.17 1.62 2.30 2.95 0.43
1.75 1.70 1.74 2.64 2.60 – – 3.90 3.64 0.51
37.85 43.11 42.79 – – – – – – –
77 80 78 95 97 – – 60 81 84
175 142 160 17 12 – – 178 64 365
glasses varies with the concentration of the rare earth ions and the composition of the glass host. Suraj Hussain et al. [50] and Kumar et al. [41] Mahato et al. [37] reported that if Eu3+ ion concentration increases beyond 1.0 mol%, then the decrease in luminescence intensity (concentration quenching) is observed. But Jamalaiah et al. [51] reported that the fluorescence intensity increases up to 2.0 mol%, i.e., no concentration quenching is observed beyond 1.0 mol%. Also Babu et al., [52] and Pisarski et al. [9] showed the compositional dependence of the emitted fluorescence intensity in the Eu3+ ions doped glasses. Therefore the observed fluorescence intensity is different in different Eu3+ ions doped glass hosts and the fluorescence intensity depends strongly on Eu3+ ion concentration, also the composition of the glass hosts and it in turn affects the lifetimes. 4. Conclusions In summary, it could be concluded that we have successfully developed and analyzed good optical quality, moisture resistant and more stable Eu3+ ions doped lead containing alkali fluoroborate glasses. Thermal properties of present glasses have been understood from the measurement of TG-DTA profiles. The FT-Raman data indicates the presence of BO3 and BO4 structural units in the glasses, the network structure being mainly built by: di-, tri-, tetra-, penta- and ortho-borate groups. FT-Raman results show lighter cations favour a stronger B–O network in which three – dimensional BO4 groups are attached to a large borate ring. The phenomenological J–O intensity parameters determined by the usual fitting procedure differ significantly from those obtained either by introduction of the thermal correction or from individual levels of absorption or emission studies. Due to the good agreement of predicted radiative properties for 5D0 level with experimental values, it is concluded that J–O parameters obtained by applying thermal correction are found to be more reliable than without thermal correction to the absorption oscillator strengths of 7F0 ? 5D2 and 7F6, individual levels. The relative strength of covalent/ionic bond between the Eu3+ ion and the ligands is estimated with the help of the relative fluorescence intensity ratio of 5 D0 ? 7F2 and 5D0 ? 7F1 transitions. The variation of optical properties with compositional changes of alkali contents in the glasses are discussed and compared with similar results. Due to the negligible non-radiative energy transfer process, the measured lifetimes of 5D0 level are found to be nearly equal to one another. Large stimulated emission cross-section, high branching ratios and quantum efficiencies are the usual characteristics of the good material for laser action. Relatively higher magnitudes of experimentally determined radiative and fluorescence properties confirm that the present glasses with Eu3+ ions emits intense red luminescence
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at 612 nm corresponding to 5D0 ? 7F2 transition by suggesting LAFB glasses are the most potential laser host materials and have promising potential use in optoelectronic luminescent display devices. Acknowledgments The authors are highly grateful to Prof. C.K. Jayasankar, Department of Physics, Sri Venkateswara University, Tirupati for permitting us to prepare the glass samples and also acknowledging the Sophisticated Analytical Instrument Facility (SAIF), Indian Institute of Technology, Chennai for extending instrumental facilities. References [1] S. Jiang, M. Myers, N. Peyghambarian, J. Non-Cryst, Solids 239 (1998) 143–148. [2] S. Shen, A. Jha, S.J. Wilson, E. Zhang, J. Non-Cryst, Solids 326–327 (2003) 510– 514. [3] H. Lin, K. Liu, E.Y.B. Pun, T.C. Ma, X. Peng, Q.D. An, J.Y. Yu, S.B. Jiang, Chem. Phys. Lett. 398 (2004) 146–150. [4] J. Wang, H. Song, X. Kong, H. Peng, B. Sun, B. Chen, J. Zhang, W. Xu, H. Xia, J. Appl. Phys. 93 (2003) 1482–1486. [5] M.J. Weber, J. Non-Cryst. Solids 123 (1990) 208–222. [6] V. Venkatramu, P. Babu, C.K. Jayasankar, Spectrochim. Acta A 63 (2006) 276– 281. [7] W.A. Pisarski, J. Pisarska, M. Maczka, W. Ryba-Romonwski, J. Mol. Struct. 792– 793 (2006) 207–211. [8] V. Venkatramu, D. Navarro-Urrios, P. Babu, C.K. Jayasankar, V. Lavin, J. NonCryst. Solids 351 (2005) 929–935. [9] W.A. Pisarski, J. Pisarska, G. Dominiak-Dzik, M. Maczka, W. Ryba-Romanowski, J. Phys. Chem. Solids 67 (2006) 2452–2457. [10] W.A. Pisarski, J. Pisarska, M. Maczka, R. Lisiecki, L. Grobelny, T. Goryczka, G. Dominiak-Dzik, W. Ryba-Romanowski, Spectrochim. Acta A 79 (2011) 696– 700. [11] J. Hao, J. Gao, M. Cocivera, Appl. Phys. Lett. 82 (2003) 2224–2226. [12] T. Tsuboi, Eur. Phys. J. Appl. Phys. 26 (2004) 95–101. [13] G. Fuxi, J. Yasi, J. Fusong, J. Non-Cryst. Solids 52 (1982) 263–273. [14] A. Herrmann, S. Fibikar, D. Ehrt, J. Non-Cryst. Solids 355 (2009) 2093–2101. [15] G. Tripathi, V.K. Rai, S.B. Rai, Opt. Commun. 264 (2006) 116–122. [16] A. Ivankov, J. Seekamp, W. Bauhofer, J. Lumin. 121 (2006) 123–131. [17] S. Surendra Babu, P. Babu, C.K. Jayasankar, W. Sievers, Th. Troster, G. Wortmann, J. Lumin. 126 (2007) 109–120. [18] C. Brecher, J. Chem. Phys. 61 (1974) 2297–2315. [19] C.H. Kam, S. Buddhudu, J. Quant. Spectrosc. Radiat. Trans. 87 (2004) 325–337. [20] G. Lakshminarayana, J. Qiu, M.G. Brik, I.V. Kityk, J. Phys.: Condens. Matter. 20 (2008) 335106. [21] A.A. Alemi, H. Sedghi, A.R. Mirmohseni, V. Golsanamlu, Bull. Mater. Sci. 29 (2006) 55–58. [22] R.P.S. Chakradhar, A. Murali, J.L. Rao, J. Alloys Compd. 265 (1998) 29–37. [23] A.G. Souza Filho, J. Mendis Filho, F.E.A. Melo, M.C.C. Custodio, R. Lebullenger, A.C. Hernandes, J. Phys. Chem. Solids 61 (2000) 1535–1542. [24] D. Maniu, T. Iliescu, I. Aredelen, S. Cinta-Pinzaru, N. Tarcea, W. Kiefer, J. Mol. Struct. 651–653 (2003) 485–488. [25] C.N. Santos, D.D.S. Meneses, P. Echegut, D.R. Neuville, A.C. Hernandes, A. Lbanez, Appl. Phys. Lett. 94 (2009) 15190–151903. [26] R.K. Brow, D.R. Tallant, J. Am. Ceram. Soc. 80 (1997) 1239–1244. [27] W. Soppe, J. Kleerebezem, H.W. den Hartog, J. Non-Cryst. Solids 93 (1987) 142– 154. [28] B.P. Dwivedi, B.N. Khanna, J. Phys. Chem. Solids 56 (1995) 39–49. [29] W.T. Carnall, P.R. Fields, K. Rajnak, J. Chem. Phys. 49 (1968) 4450–4455. [30] R. Reisfeld, E. Greenberg, R.N. Brown, M.G. Drexhage, C.K. Jorgensen, J. Chem. Phys. Lett. 95 (1983) 91–95. [31] X. Shen, Q. Nie, T.F. Xu, Y. Gao, Spectrochim. Acta A 61 (2005) 2189–2193. [32] P.L. Pernas, E. Cantelar, Phys. Scripta. T118 (2005) 93–97. [33] M. Dejneka, E. Snitzer, R.E. Riman, J. Lumin. 65 (1995) 227–245. [34] R. Van Deun, K. Binnemans, C. Gorller-Walrand, J.L. Adam, J. Phys. Chem. Matter. 10 (1998) 7231–7241. [35] B.R. Judd, Phys. Rev. B 127 (1962) 750–761. [36] G.S. Ofelt, J. Chem. Phys. 37 (1962) 511–520. [37] K.K. Mahato, S.B. Rai, A. Rai, Spectrochim. Acta A 60 (2004) 979–985. [38] M. Bettinelli, A. Speghini, M. Ferrari, M. Montagna, J. Non-Cryst. Solids 201 (1996) 211–221. [39] H.E. Heidepriem, D. Ehrt, J. Non-Cryst. Solids 208 (1996) 205–216. [40] S. Balaji, P. Abdul Azeem, R.R. Reddy, Physica B 394 (2007) 62–68. [41] A. Kumar, D.K. Rai, S.B. Rai, Spectrochim. Acta A 58 (2002) 2115–2125. [42] M.H.V. Werts, R.T.F. Jukes, J.W. Verhoeven, Phys. Chem. Chem. Phys. 4 (2002) 1542–1548. [43] M.J. Weber, T.E. Varitimos, B.H. Matsinger, Phys. Rev. B 8 (1973) 47–53. [44] L. Chen, Y. Liu, Y. Li, J. Alloys Compd. 381 (2004) 266–271. [45] J.L. Adam, V. Poncon, J. Lucas, G. Boulon, J. Non-Cryst. Solids 91 (1987) 191– 202.
1260
B. Deva Prasad Raju, C. Madhukar Reddy / Optical Materials 34 (2012) 1251–1260
[46] W.T. Carnall, P.R. Fields, B.G. Wybourne, J. Chem. Phys. 42 (1965) 3797–3806. [47] E.W.J.L. Oomen, A.M.A. Van Dongen, J. Non-Cryst. Solids 111 (1989) 205–213. [48] W.T. Carnall, H. Crosswhite, H.M. Crosswhite, Energy Level Structure and Trasnsition Probabilities of the Trivalent Lanthanides in LaF3, Aragonne National Laboratory, Illinoise, 1977. [49] M. Liao, Z. Duan, L. Hu, Y. Fang, L. Wen, J. Lumin. 126 (2007) 139–144.
[50] N. Sooraj Hussain, Y. Prabhakara Reddy, S. Buddhudu, Mater. Res. Bull. 36 (2001) 1813–1821. [51] B.C. Jamalaiah, J. Suresh Kumar, A. Mohan Babu, L. Rama Moorthy, J. Alloys Compd. 478 (2009) 63–67. [52] P. Babu, C.K. Jayasankar, Physica B 279 (2000) 262–281.