Optical spectroscopy of Eu3+ ions in lithium borate and lithium fluoroborate glasses

Physica B 279 (2000) 262}281

Optical spectroscopy of Eu3` ions in lithium borate and lithium #uoroborate glasses P. Babu, C.K. Jayasankar* Department of Physics, Sri Venkateswara University, Tirupati-517502, India Received 21 June 1999; received in revised form 20 September 1999

Abstract Optical absorption, excitation and emission spectra of Eu3` ions in lithium borate and lithium #uoroborate glasses have been investigated. Judd}Ofelt intensity parameters are derived from the integrated absorption spectra and also from the emission spectra under various constraints. The relative merits of thermal correction to the oscillator strengths of the transitions originating from the ground state (7F ) of Eu3` ion observed in the absorption spectra have been discussed. 0 The Judd}Ofelt parameters obtained from the emission spectra have been used to calculate the total spontaneous emission probabilities (A), lifetimes (q ) and branching ratios (b ) for eight excited levels that include, 5K , 3P , 5H , 5L , R R 6 0 3 7 5D , 5D , 5D and 5D . The predicted values of q and b are compared with the measured values for 5D level. The 3 2 1 0 R R 0 stimulated emission cross-sections (p (j )) are also evaluated for the 5D P7F and 5D P7F transitions. The variation 1 0 J 1 J of optical properties with compositional changes of lithium oxide and lithium #uoride contents in the glasses are discussed and compared with similar results. ( 2000 Elsevier Science B.V. All rights reserved. PACS: 42.62.Fi; 76.30.Kg; 78.20; 78.40.Pg Keywords: Rare earth ions; Optical properties; Borate glasses; Judd}Ofelt analysis; Laser spectroscopy

1. Introduction Optical properties of rare earth ions in glasses vary in a wide range that depends on the chemical composition of glass former and modi"er. Study of the optical properties of the rare earth ions in glasses provides fundamental data that includes transition positions and cross-sections, transition probabilities, radiative and non-radiative decay

* Corresponding author. Tel.: #91-8574-50666; fax: #918574-27499. E-mail address: [email protected] (C.K. Jayasankar)

rates, branching ratios, etc. for the excited states. This data is essential to estimate/design optical devices such as lasers, colour displays, upconverters, "bre ampli"ers and so on. In this direction, in order to identify new optical devices, devices for speci"c utility, or devices with enhanced performance, active work is being carried out by selecting appropriate new hosts (chemical compositions) doped with rare earth ions [1]. The optical properties of rare earth ions in several compounds were described and estimated quantitatively by using Judd}Ofelt (JO) theory [2,3] based on absorption and emission spectra of f}f transitions. Eu3` : glasses are extensively

0921-4526/00/$ - see front matter ( 2000 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 9 9 ) 0 0 8 7 6 - 5

P. Babu, C.K. Jayasankar / Physica B 279 (2000) 262}281

investigated by optical spectroscopy to tailor materials for good optical devices [4}17]. The Eu3` ion is also used as a probe for "nding the local structure around the rare earth ion in a crystal or a glass due to relative simplicity of its energy level structure with non-degenerate ground 7F and 0 emitting 5D states [18}22]. 0 However, it has been observed that the JO theory has not been applied uniformly to characterise the optical properties of Eu3` : glasses. For example, Dejneka et al. [4] used thermal correction to the oscillator strengths of the transitions of absorption spectra before using JO theory as 7F and 0 7F levels are lying very close together ( 250 cm~1). 1 Peng and Izumitani [6] used 5D P7F , 7F and 0 2 4 7F emission levels and determined X , X and X , 6 2 4 6 respectively. Van Deun et al. [17] used individual transitions of 7F P5D , 5D and 5L from the 0 2 4 6 absorption spectra and determined X , X and X , 2 4 6 respectively, ignoring thermal correction. Fermi et al. [9] and Capobianco et al. [18] have considered both absorption as well as emission data and derived X (j"2, 4 and 6) parameters. j Also, the exact matrix elements of DD;jDD2 and DD¸#2SDD2 required for JO analysis and inturn to predict radiative properties have not been taken uniformly, though they are independent of host matrix. For example, the DD;jDD2 values for 7F to 0,1 excited levels are from aquo-ion [23], 5D to 0,1,2 lower levels are from Eu3` : LaF [24] and in some 3 other cases [9,25] the DD;jDD2 values have been calculated with the wave functions reported by Ofelt [26]. On the other hand, DD;jDD2 values for excited levels such as 5H were not available and 3 this restricts the prediction of radiative properties to compare with experimental results for Eu3` : ZBLA [4] glass. The aim of the present study is (1) to examine the variation of optical properties in lithium borate glasses with changes in composition of lithium oxide and lithium #uoride contents, (2) to systemise the JO parameters for Eu3` ion, (3) to determine the radiative properties for signi"cant levels, (4) to compute the necessary matrix elements, DD;jDD2 and DD¸#2SDD2, using uniform wave functions, (5) to compare the experimental and predicted radiative properties for 5D level, and (6) to compare and 0 discuss the optical properties of Eu3` : glasses.

263

2. Experimental The molar compositions of europium-doped lithium borate (LnBE, n"4, 5, 6 and 7) and lithium #uoroborate (LxFBE, x"2, 5) glasses investigated in the present work are as follows : L4BE : 59.5Li CO #39.5H BO #1Eu O , 2 3 3 3 2 3 L5BE : 49.5Li CO #49.5H BO #1Eu O , 2 3 3 3 2 3 L6BE : 39.5Li CO #59.5H BO #1Eu O , 2 3 3 3 2 3 L7BE : 29.5Li CO #69.5H BO #1Eu O , 2 3 3 3 2 3 L2FBE : 24.75Li CO #24.75LiF 2 3 #49.5H BO #1Eu O , 3 3 2 3 L5FBE : 49.5LiF#49.5H BO #1Eu O . 3 3 2 3 About 5 g batches of the above 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 in the temperature range 900}9503C for about 30 min. 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 3503C for 5 h in an oven to remove strains and then polished to measure their optical properties. Absorption spectra in the near infrared, visible and ultraviolet regions are measured using a Hitachi U}3400 spectrophotometer. Excitaton and emission spectra are recorded on a Hitachi 650} 10 s #uorescence spectrophotometer. Lifetime measurements are made by exciting the sample with Ar` laser at 395 nm. All these measurements are made at room temperature. The refractive indices, n, are measured using an Abbe refractometer at sodium wavelength (589.3 nm) with 1-Bromonaphthalin (C H Br) as 10 7 contact liquid. Densities are measured by the Archimedes method using xylene as an immersion liquid.

3. Theoretical 3.1. Oscillator strengths } Judd}Ofelt analysis The experimental oscillator strengths ( f ) of the %91 absorption bands are determined by using the

264

P. Babu, C.K. Jayasankar / Physica B 279 (2000) 262}281

formula [11,27]

P

f "4.318]10~9 e(l) dl, %91

(1)

where e(l) is the molar absorptivity at a wave number l. According to JO theory [2,3], the calculated oscillator strength for an induced electric dipole transition from the ground state to an excited state is f"[8p2mcl/3h(2J#1)][(n2#2)2/9n] ]+ X (WJDD;jDDW@J@)2, (2) j/2,4,6 j where n is refractive index of the medium, J is the total angular momentum of the ground state, l is the wave number of the transition, X (j"2, 4, 6) j are JO intensity parameters and DD;jDD2 are the doubly reduced matrix elements evaluated in the intermediate coupling approximation for a transition WJPW@J@. Oscillator strengths of various observed transitions are evaluated using Eq. (1) and are used in Eq. (2). A least-squares "tting approach is then used for Eq. (2) to determine X parameters which j gives the best "t between experimental and calculated oscillator strengths [1,4]. The theoretical oscillator strengths, f , are then calculated using #!Eq. (2) and X . j 3.1.1. Judd}Ofelt analysis for Eu3` (4F6) ion ¹hermal correction: For Eu3` ion the ground term 7F is very close to the next excited 7F level 0 1 and therefore both levels are populated at room temperature and absorption takes place from both the levels to excited states. The fractional thermal population C /C of any excited level 2s`1¸ is J 0 J given by [4,28] C /C "[g /g ]exp((!E !E )/k¹), (3) J 0 J 0 J 0 where C , g and E refer to the population, degen0 0 0 eracy and energy, respectively, of the ground state. The subscript `Ja refers to the excited level. Expression (3) corrects the thermalisation e!ect and allows one to estimate the exact oscillator strengths from the ground state to excited states. Therefore, JO analyses have been carried out after thermal correction for the oscillator strengths of

Eu3` : glasses [4,7,28]. Due to linear relationship among f , DD;jDD2 and X (Eq. (2)) parameters, %91 j thermalisation e!ect may be considered by correcting any one of f [4], DD;jDD2 [7] or X [28]. %91 j Non-zero DD;jDD values: As seen from Eq. (2), the transition intensity ( f ) depends on the DD;jDD2 %91 values between WJ and W@J@ manifold. Due to selection rules and the unique nature of transition intensities for Eu3` ion, any one of the DD;jDD2 parameters decide the intensities of the transitions since the remaining two are zero. For example, X , X and 2 4 X can be determined from the absorption bands of 6 7F P5D , 7F P5D and 7F P5L , respective0 2 0 4 0 6 ly [17]. Similarly, X , X and X can also be 2 4 6 evaluated independently from the emission transitions of 5D P7F , 5D P7F and 5D P 0 2 0 4 0 7F , respectively [6]. This simplicity is mainly due 6 to D;4DD2 and DD;6DD2 are zero for 7F P5D and 0 2 5D P7F , DD;2DD2 and DD;6DD2 are zero for 0 2 7F P5D and 5D P7F and DD;2DD2 and DD;4DD2 0 4 0 4 are zero for 7F P5L and 5D P7F transitions. 0 6 0 6 This method has been used and X values estij mated from the individual levels without leastsquares "t for Eu3` : glasses [6,7]. Electric and magnetic dipole transition probabilities: Careful examination of emission for 5D P7F transitions reveals that the 5D P7F 0 J 0 1 transition is allowed by magnetic dipole and 5D P7F transitions are allowed by induced 0 2,4,6 electric dipole mechanisms. It is well known that the magnetic dipole radiative transition probability (A ) for 5D P7F transition is independent of 1 0 1 host matrix. Similarly, the electric dipole radiative transition probability (A ) for 5D P7F (J"2, 4, J 0 J 6) transitions depend solely on X , X and X , 2 4 6 respectively. This characteristic trend helps one to determine X from the ratios of intensities of the j 5D P7F transitions to the intensity of the 0 2,4,6 5D P7F transition as follows: 0 1

P NP I dl J

I dl"A /A "[e2/S ][l3/l3 ] 1 J 1 .$,1 J 1 ][n (n2 #2)2/9n3 ]) SDD;JDDT2, 1 J J J (4)

where S refers to the magnetic dipole line .$,1 strength due to 5D P7F transition. 0 1

P. Babu, C.K. Jayasankar / Physica B 279 (2000) 262}281

This method has been adopted and X values j obtained for Eu3` : phosphate glasses [7]. 3.2. Radiative properties The electric (S ) and magnetic (S ) dipole lines%$ .$ trengths are calculated using the formulae [1,4,6,7,10,27] S "e2 + X (xWJDD;jDDW@J@)2, %$ j j/2,4,6 S "(e2h2/16p2m2c2)(WJDD¸#2SDDW@J@)2. (5) .$ With the help of Eq. (5), electric (A ) and mag%$ netic (A ) dipole radiative transition probabilities .$ are evaluated from the following expressions A "[64p4l3/3h(2J#1)][n(n2#2)2/9]S , %$ %$ A "[64p4l3/3h(2J#1)]n3S . (6) .$ .$ The sum of A and A gives the radiative %$ .$ transition probability (A) for a transition WJPW@J@ as A(WJ,W@J@)"A #A . (7) %$ .$ The total radiative transition probability (A ) for T an excited state is given as the sum of the A(WJ,W@J@) terms calculated over all the terminal states. A (WJ)" + A(WJ, W@J@). (8) T W {J{ As an excited state WJ is relaxed to several lower-lying states W@J@, the radiative branching ratio (b ) is de"ned as R b (WJ,W@J@)"A(WJ,W@J@)/A (WJ). (9) R T The branching ratios can be used to predict the relative intensities of all emission lines originating from a given excited state. The experimental branching ratios can be found from the relative areas of the emission lines. The rate of depopulation of an excited state is given by the radiative lifetime, q (WJ): R q (WJ)"[A (WJ)]~1. (10) R T The peak-stimulated emission cross-section, p(j )(WJ,W@J@), between the states WJ and W@J@ 1

265

having a probability of A(WJ,W@J@) is given by [4] p(j )(WJ, W@J@)"[j4 /8pcn2*j ]A(WJ, W@J@), (11) 1 %&& 1 where j is the wavelength of peak emission (in nm) 1 and *j is the e!ective line width of the transition %&& (in nm) found by dividing the integrated area of the emission band by its average height. All the above relations, Eqs (1)}(11), are used to compute various spectroscopic properties reported in this study.

4. Results 4.1. Absorption, emission and excitation spectra The absorption spectra of Eu3` : LBE (LnBE, n"4, 5, 6 and 7 and LxFBE, x"2 and 5) glasses for UV-VIS and NIR regions are shown in Fig. 1. The assignment of the bands in the absorption spectra, originating from 7F and 7F , is also 0 1 shown in Fig. 1. The band positions along with assignments for Eu3` : LBE glasses are shown in Table 1. The band positions for some reported systems of Eu3` : ZBLA [4], phosphate (PHOS) [8], lead meta phosphate (PbPO) [18], lead silicate (PbSi) [9] and aquo-ion [23] are also given in Table 1 for comparison. Excitation and emission spectra of Eu3` : L5FBE glass is shown in Figs. 2 and 3, respectively. The transitions of excitation spectra are assigned (originating from 7F and 7F ) based on free-ion 0 1 energy level structure and by taking into account both the electric and magnetic dipole contributions. Emission is mainly observed for 5D P7F 0 J transitions and a weak emission is noticed for 5D P7F transitions which are assigned in 1 0,1,2 Fig. 3. 4.2. Oscillator strengths and Judd}Ofelt analysis The oscillator strengths for the observed absorption bands of Eu3` : LBE glasses are calculated from Eq. (1) by measuring their integrated areas and are tabulated in Table 2. These oscillator strengths are also corrected for thermal population [8,9,18] at room temperature (253C). In Table 3, the oscillator strengths corrected for thermalisation

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P. Babu, C.K. Jayasankar / Physica B 279 (2000) 262}281

Fig. 1. Room temperature absorption spectra of Eu3` : LBE glasses: (a) L4BE; (b) L5BE; (c) L6BE; (d) L7BE; (e) L2FBE; (f ) L5FBE.

e!ect for Eu3` : L6BE glass is shown along with reported Eu3` : glasses. The reported systems which were compared include: zinc (ZnPO), lead (PbPO) and barium (BaPO) meta phosphate [18], PbSi [9] and PHOS [8] glasses. Table 4 shows JO parameters determined under various constraints for Eu3` : L4BE and L6BE glasses along with predicted lifetimes.

The matrix elements DD;jDD2 used for the present work are calculated by us from intermediate coupling approximation [23}27] and are collected in Table 5. These matrix elements have been calculated with the free-ion wave functions computed with the following free-ion parameters (cm~1) : E "63945, F2"83220, F4"59112, F6" AVG 43369, a"21.16, b"!503, c"1290, ¹2"370,

P. Babu, C.K. Jayasankar / Physica B 279 (2000) 262}281

267

Table 1 Observed band positions (cm~1) and bonding parameters (bM and d) for Eu3` : LBE glasses and reported Eu3` : systems! Energy level

L4BE

L5BE

L6BE

L7BE

L2FBE

L5FBE

ZBLA [4]

PHOS [8]

PbPO [18]

PBSi [9]

AQUO [23]

7F P7F 1 6 7F P7F 0 6 7F P5D 1 0 7F P5D 0 0 7F P5D 1 1 7F P5D 0 1 7F P5D 1 2 7F P5D 0 2 7F P5D 1 3 7F P5L 1 6 7F P5L 0 6 7F P5G 0 2 7F P5G 0 4 7F P5D 0 4 bM d

4549 4808 16998 17241 18 727 19 011 21 180 21 455 24 159 * 25 361 * * * 0.996 0.402

4533 4781 16 949 17 211 18 709 18 948 21 142 21 482 24 131 * 25 316 * * * 0.993 0.735

4541 4792 17 010 17 274 18 734 18 993 21 172 21 482 24 137 24 981 25 361 26 212 26 575 27 678 0.996 0.371

4538 4783 16 906 17 277 18 789 19 001 21 105 21 445 24 125 24 888 25 355 26 212 26 525 27 624 0.995 0.533

4546 4790 17 047 17 292 18 744 19 008 21 160 21 468 24 160 24 984 25 361 26 212 26 575 * 0.995 0.513

4547 4781 17053 17 259 18 762 18 993 21 146 21 501 24 155 24 919 25 400 * * * 0.996 0.442

4550 4835 17 007 17 310 18 770 19 055 * 21 550 24 120 * 25 422 26 357 26 674 27 670 0.998 0.200

* * 16 771 16 319 18 700 18 993 * 21 493 * * 25 380 * * 27 567 0.998 0.241

* * * * 18762 19011 * 21505 * 25062 25413 * 26596 27624 1.000 !0.040

* 4833 17 045 17 307 18 772 19 030 * 21 552 24 284 25 013 25 426 26 392 26 645 27 601 1.000 !0.009

4630 4980 16 920 17 277 18 691 19 028 21 164 21 519 24 038 25 000 25 400 26 300 26 620 27 670

!The band positions for other Eu3` : systems are compared to those levels that are observed for Eu3` : LBE glasses

¹3"40, ¹4"40, ¹6"!330, ¹7"380, ¹8"370, f"1334, M0"2.38, M2"0.56M0, M4"0.38M0, P2"245, P4"0.75P2, P6" 0.50P4. These free-ion energy parameters have been determined in such a way that yields improved (best-"t) calculated-versus-experimental energy levels not only for Eu3` : LBE glasses but also for Eu3` : #uorozirconate glass [20] for which relatively more experimental energy level data are available. The notation and de"nitions for these parameters follow the usual conventions [29}31]. The set of free-ion parameters for Ln3` : host are determined using precisely the same methodologies and strategies employed in the earlier work [29}31]. To minimise the space the matrix elements in Table 5 are given only for those levels for which the radiative properties are predicted. However, the reduced matrix elements and magnetic dipole linestrengths have been computed for all the levels that are lying up to 50,000 cm~1 and are available on request from the authors.

the excited states of Eu3` : LBE glasses that are lying up to 45,000 cm~1 have been predicted. The detailed results of the predicted radiative proerties are presented in Table 5 for the excited states, 5K , 6 3P , 5H , 5L , 5D , 5D 5D and 5D of 0 3 7 3 2 1 0 Eu3` : L5FBE glasses. Table 6 gives the predicted radiative lifetimes for the above excited levels of Eu3`: LBE glasses along with other reported Eu3` : glass [4,9,10]. The JO parameters and refractive indices used to predict radiative proerties are also shown in Table 6. The peak-stimulated emission cross-sections (p(j )), calculated using Eq. (11), and the experi1 mental b for the emission peaks from the 5D R 0 and 5D levels are shown in Table 7 1 along with measured lifetimes for Eu3` : L5FBE glass. The experimental b and lifetimes are comR pared with those calculated from JO analysis for Eu3` : L5FBE glass and for Eu3` : sodium #uorophosphate (NaFPO) [12] glass. Table 8 represents intensity ratios (R) of 5D P7F to 5D P7F 0 2 0 1 transition for Eu3` : glasses [5,10,11,13,18,32,33].

4.3. Radiative properties

4.4. Nephelauxetic ewect } bonding parameter

Using Eqs. (2), (5)}(10), the electric (A ) and mag%$ netic (A ) dipole radiative transition probabilities, .$ radiative lifetimes (q ) and branching ratios (b ) for R R

The bonding parameter (d) [34,35] is de"ned as d"[(1!bM )/bM ]]100,

(12)

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P. Babu, C.K. Jayasankar / Physica B 279 (2000) 262}281

Fig. 2. Room temperature excitation spectrum (j "612 nm) of Eu3` : L5FBE glass. %.

where bM "+ b/N and b (the Nephelauxetic raN tio)"l /l . l and l are the energies of the corre# ! # ! sponding transitions in the complex [4,8,9,18] and aquo-ion [23], respectively, and N refers to the number of levels used to compute bM values. Depending upon the environmental "eld, d may be positive or negative indicating covalent or ionic bonding, respectively. bM and d values are collected for Eu3` : systems [4,8,9,18] in Table 1.

5. Discussion 5.1. Electronic transitions The absorption spectra recorded at room temperature for Eu3` : LBE glasses shown in Fig. 1 are

similar to those reported for Eu3` : glasses [4,10,18]. The transitions of the bands are also assigned in Fig. 1. In order to locate and assign quite conveniently the spectral characteristics of Eu3` : LBE glasses, the spectra recorded between 650 and 350 nm for Eu3` : L6BE glass have been magni"ed by a factor of 20 and shown at the top of Fig. 1. As seen from the absorption bands shown in Fig. 1 for Eu3` : LBE glasses, the 7F , 5D , 5D 6 0 1 and 5D transitions are associated with a shoulder 2 at low-energy side. This reveals that the absorption takes place from two thermally populated ground and next excited states, 7F and 7F , as they are 0 1 closely spaced of the order of k¹, where k is Boltzmann's constant and ¹ is the absolute temperature. The fractional populations of 7F (J"0, 1, J 2, 3) levels are calculated for the present Eu3` : LBE

P. Babu, C.K. Jayasankar / Physica B 279 (2000) 262}281

Fig. 3. Room temperature emission spectrum (j

glasses at 253C using Eq. (3). For example, C /+C (J"0, 1, 2, 3) for Eu3` : L4BE glass are J J 7F : 0.6619; 7F : 0.3036; 7F : 0.0342 and 0 1 2 7F : 0.0003. Similarly, the values of C for 3 0 Eu3` : L4BE, L5BE, L6BE, L7BE, L2FBE and L5FBE are 0.6619, 0.6596, 0.6690, 0.6689, 0.6702 and 0.6640, respectively. Thus, the fractional populations obtained for 7F are found to be more or 0 less similar in all the Eu3` : LBE glasses. Therefore, to consider the thermalisation e!ect, the observed oscillator strengths have to be multiplied by a factor of around 1.5 to get the actual oscillator strengths for those transitions originating from 7F . 0 Absorption studies yield a maximum of 14 transitions, which allows one to identify 9 excited states 7F , 5D , 5D , 5D , 5D , 5L , 5G , 5G and 6 0 1 2 3 6 2 4 5D for Eu3` : L6BE and L7BE glasses. Only 10 4 transitions are observed for Eu3` : L4BE and

%9%

269

"395 nm) of Eu3` : L5FBE glass.

L5BE glasses. Out of six glasses of LBE compositions, more levels have been identi"ed in those compositions for which Li CO and LiF contents 2 3 are less. In all the compositions the 7F P7F and 0 6 7F P7F levels lying in the IR region are well 1 6 resolved. As seen in Fig. 1, the Li CO and LiF 2 3 contents may restrict the observation of the absorption bands below 360 nm. The absorption band assigned as 7F P5L is found to be more intense 0 6 than the other transitions, though it is forbidden by the *S and *¸ selection rules but allowed by the *J selection rule. A relatively weak intensity has been noticed for 7F P5D transition as it is for0 0 bidden by the selection rules. The intensity of 7F P5D magnetic dipole allowed transition is 0 1 relatively weaker than that of 7F P5D induced 0 2 electric dipole allowed transition for Eu3` : LBE glasses. However, due to hypersensitive nature of 7F P5D , its intensity may vary over a wide 0 2

270

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Table 2 Experimental and calculated oscillator strengths (]10~6) for Eu3` : LBE glasses! Energy level

7F P7F 1 6 7F P7F 0 6 7F P5D 1 0 7F P5D 0 0 7F P5D 1 1 7F P5D 0 1 7F P5D 1 2 7F P5D 0 2 7F P5D 1 3 7F P5L 1 6 7F P5L 0 6 7F P5G 0 2 7F P5G 0 4 7F P5D 0 4 N p"

L4BE

L5BE

L6BE

L7BE

L2FBE

L5FBE

f %91

f #!-

f %91

f #!-

f %91

f #!-

f %91

f #!-

f %91

f #!-

f %91

f #!-

0.808 1.172 * * 0.129 0.048 0.072 0.317 0.038 * 1.693 * * * 7 $0. 44

0.830 1.011 * * 0.146 * 0.009 0.158 0.038 * 0.568 * * *

0.432 0.742 0.010 0.007 0.155 0.047 0.056 0.275 0.043 * 1.414 * * * 7 $0.42

0.457 0.556 * * 0.166 * 0.011 0.179 0.043 * 0.313 * * *

0.591 0.824 0.017 0.011 0.134 0.052 0.074 0.223 0.090 0.412 2.233 0.350 0.246 0.327 11 $0.56

0.607 0.739 * * 0.150 * 0.010 0.162 0.118 0.081 0.416 0.134 0.145 0.282

0.518 0.882 0.010 0.010 0.078 0.049 0.084 0.332 0.084 0.284 2.207 0.490 0.350 0.372 11 $0.58

0.549 0.667 * * 0.015 * 0.007 0.124 0.125 0.073 0.376 0.102 0.168 0.328

0.481 0.695 0.015 0.007 0.129 0.050 0.073 0.297 0.049 0.322 1.930 0.162 0.204 * 10 $0.49

0.497 0.604 * * 0.144 * 0.009 0.156 0.071 0.066 0.340 0.128 0.067 *

0.392 0.495 0.013 0.010 0.065 0.028 0.050 0.175 0.027 0.157 1.157 * * * 8 $0.32

0.396 0.480 * * 0.076 * 0.005 0.082 0.027 0.052 0.272 * * *

!The f values are presented ignoring thermalisation e!ect. f are calculated by using the X parameters determined from all the %91 #!j observed levels of absorptoin spectra (SET A of Table 4). "The root mean square deviation, p, of the experimental and calculated values gives a "gure of merit to describe the quality of the "t de"ned as: p"([+N ( f !f )2]N~1)1@2. i/1 %91 #!-

Table 3 Experimental oscillator strengths (]10~6) corrected to the thermal population of the 7F level for Eu3`:glasses! 1 f %91 Transition

7F P7F 1 6 7F P7F 0 6 7F P5D 1 0 7F P5D 0 0 7F P5D 1 1 7F P5D 0 1 7F P5D 1 2 7F P5D 0 2 7F P5D 1 3 7F P5L 1 6 7F P5L 0 6 7F P5G 0 2 7F P5G 0 4 7F P5D 0 4

Spectral region (nm)

Present

Reported

L6BE

ZnPO [18]

PbPO [18]

BaPO [18]

PbSi [9]

PHOS [8]

2202 2087 588 579 534 527 472 466 414 400 394 382 376 361

1.983 1.232 0.057 0.016 0.450 0.078 0.248 0.333 0.302 1.383 3.338 0.523 0.368 0.489

* * * * * 0.018 * 0.219 * * 0.79 * * *

* * * * * 0.021 * 0.136 * * 1.33 * 0.092 0.212

* * * * * 0.022 * 0.219 * * 1.110 * * *

* 0.861 0.025 0.009 0.203 0.022 * 0.303 * * 1.11 * * 0.393

* * 0.009 0.001 0.051 0.015 * 0.125 * * 0.898 * * 0.193

!Source of data as well as title of the system, see Section 4.2. The f for other Eu3` : glasses are compared to those levels that are %91 observed for Eu3` : LBE glasses.

P. Babu, C.K. Jayasankar / Physica B 279 (2000) 262}281

271

Table 4 Judd}Ofelt parameters (]10~20 cm2) and the respective predicted lifetimes (q ms) for 5D level of Eu3` : L4BE and L6BE glasses #!0 under various constraints Constraint

SET

L4BE X

All absorption levels without thermal correction All absorption levels with thermal correction Only emission levels SET A and SET C levels SET B and SET C levels 7F P5D , 5D and 5L levels 0 2 4 6 without thermal correction SET F levels with thermal correction

2

L6BE X

4

X

6

N

p

q #!-

X

2

X

4

6

N

p

q #!-

0.69

11

$0.56

2.70

A

5.79

0.38

0.95

7

$0.44

3.70

5.92

B

17.20

1.92

2.86

7

$0.59

1.41

17.96

11.92 2.13

11

$0.78

1.09

C D E F

6.34 15.97 20.82 11.62

4.97 8.25 9.07 *

5.10 0.95 2.86 2.82

3 10 10 3

* $0.43 $0.50 *

2.65 1.29 1.03 2.11

5.68 8.80 14.11 8.15

5.13 5.79 8.59 6.52

5.34 0.69 2.13 3.71

3 14 14 3

* $0.50 $0.70 *

2.79 2.10 1.39 2.13

G

17.56

*

4.26

3

*

1.45

12.18

9.75

5.55

3

*

1.48

range in such a way that in some cases its intensity becomes lower than that of 7F P5D , as seen for 0 1 Eu3` : aquo-ion [28]. On the other hand, 7F P5D transition has not been observed as it is 0 3 forbidden by a selection rule (*J"3). Below 360 nm (28,000 cm~1), the absorption edge of the host glass climbs rapidly and therefore the levels lying above 28,000 cm~1 cannot be resolved. However, some of the levels lying above 360 nm can be extracted from excitation studies besides con"rming the levels obtained form absorption studies. Fig. 2 shows the excitation spectra for Eu3` : L5FBE glass at RT monitored for 5D P7F (612 nm) transition. From excitation 0 2 spectra, the high-energy transitions such as 7F P5H '5F and 7F P 5L , 5D , 5H and 0 4 4 1 7 4 3 5F have been located and assigned. Assignments 3 were made by taking into account the electric and magnetic dipole linestrengths based on JO analysis. For all the Eu3` : LBE glasses strong emission for 5D P7F (J"0, 1, 2, 3, 4, 5 and 6) transitions 0 J and very weak (less by a factor of 200) emission for 5D P7F (J"0, 1 and 2) transitions have been 1 J observed. Fig. 3 shows the emission spectra for Eu3` : L5FBE glass. The emission spectra have been recorded by exciting the 5L level of Eu3` ion 6 with 395 nm laser line of Ar` laser. As seen in Fig. 3, the red line emission of 5D P7F is relatively 0 2 stronger than the other emission channels of Eu3` : LBE glasses as observed for Eu3` : glasses

5.62

X

[4,10,18]. The splitting of 5D P7F transition 0 1 into three components (Fig. 3), though the highenergy component has not been well resolved, shows that the Eu3` ions in LBE glasses occupy lowest symmetry * orthorhombic, monoclinic or triclinic. As seen from Table 1, it is found that there is no systematic variation in the absorption band positions with change in glass composition for Eu3` ions in LBE glasses. Examination of band positions found for 7F P7F and 7F P7F reveal that the 1 6 0 6 separation between 7F and 7F is found to be 0 1 &250 cm~1 for Eu3` : LBE glasses which is smaller than the separation of 285 cm~1 found for Eu3` : ZBLA glasses [4]. As seen from Table 1, Eu3` : LBE glasses exhibit covalent bonding character which is similar to those found for ZBLA [4] and phosphate [8] glasses. However, Eu3` ions exhibit ionic character in lead phosphate [18] and lead silicate [9] glasses. Out of 12 systems that are compared in Table 1, it is found that the Eu3` ions posses more covalent nature in LBE glasses than those found in ZBLA and phosphate glasses whereas lead phosphates [18] are found to be more ionic than those found for lead silicate [9] glasses. 5.2. Oscillator strengths and Judd}Ofelt analysis The experimental oscillator strengths ( f ) for %91 the observed absorption bands of Eu3` : LBE

272

P. Babu, C.K. Jayasankar / Physica B 279 (2000) 262}281

Table 5 Energies (l, cm~1), squared reduced matrix elements (S¸JDD;jDDS@¸@J@)2, radiative transition probabilities (A , A , A and A ), radiative %$ .$ T lifetimes (q ), branching ratios (b ) for excited levels of Eu3` : L5FBE glass R R S¸J 5K 6

S@¸@J@

l

5K 1156 5 5I 1997 7 5I 2449 6 5I 2464 8 5I 3006 5 5I 3351 4 5F 3414 5 5F 3921 4 5F 4074 1 5F 4343 3 5F 4382 2 3P 4477 0 5H 5970 5 5H 5973 6 5H 6201 4 5H 6439 7 5H 6641 3 5L 9031 10 5L 9524 9 5D 9848 4 5L 10268 8 5G 10787 5 5G 10793 6 5G 10805 4 5G 10902 3 5G 11104 2 5L 11179 7 5L 12225 6 5D 13107 3 5D 15998 2 5D 18462 1 5D 20192 0 7F 32535 6 7F 33615 5 7F 34666 4 7F 35626 3 7F 36475 2 7F 37136 1 7F 37513 0 A (5K )"6912 s~1, q (5K )"144 ls T 6 R 6 3P 5H 1493 0 5 5H 1496 6 5H 1725 4 5H 1963 7 5H 2165 3 5L 4554 10 5L 5047 9 5D 5371 4 5L 5792 8 5G 6310 5 5G 6316 6

DD;2DD2

DD;4DD2

DD;6DD2

A

0.0693 0.0012 0.0132 0.0000 0.3470 0.0001 0.0028 0.0025 0 0 0 0 0.0006 0.0306 0.0770 0.0001 0 0 0 0.0001 0.0011 0.0001 0.0148 0.0001 0 0 0.2634 0.1065 0 0 0 0 0.0000 0.0001 0.0001 0 0 0 0

0.0961 0.0884 0.1269 0.0183 0.1642 0.1672 0.0070 0.0481 0 0.1300 0.4122 0 0.0037 0.0143 0.0344 0.0208 0.1218 0.0000 0.0083 0.0005 0.0053 0.0004 0.0104 0.0631 0.1132 0.0419 0.0712 0.0001 0.0071 0.0001 0 0 0.0001 0.0000 0.0012 0.0046 0.0024 0 0

0.1168 0.1098 0.0125 0.0752 0.2492 0.0796 0.0004 0.0017 0.0045 0.0001 0.0007 0.0033 0.0126 0.0001 0.0037 0.0925 0.0916 0.0064 0.0181 0.1279 0.0104 0.1695 0.1116 0.0056 0.0506 0.0034 0.0848 0.3866 0.1140 0.0225 0.0042 0.0142 0.0084 0.0041 0.0002 0.0031 0.0006 0.0004 0.0009

0.41 1.43 1.88 1.32 19.86 8.00 0.36 2.59 0.29 8.60 28.02 0.29 3.38 9.16 26.28 28.65 54.97 4.60 21.05 119.77 16.82 208.32 166.68 71.24 182.37 50.90 2004.99 889.42 263.71 90.43 25.81 113.84 285.79 152.65 52.14 301.74 123.96 18.84 46.71

0 0 0 0 0 0 0 0 0 0 0

0 0 0.0120 0 0 0 0 0.0051 0 0 0

0 0.0003 0 0 0 0 0 0 0 0 0.0037

0 0.01 0.64 0 0 0 0 8.31 0 0 11.79

%$

(s~1)

A

.$

(s~1)

0.33 0.01 0.35 0 0.01 0 0.01 0 0 0 0 0 0.11 1.90 0 0.07 0 0 0 0 0 0.03 7.23 0 0 0 0.36 0.54 0 0 0 0 0.17 1493.45 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

A (s~1)

b R

0.74 1.44 2.23 1.32 19.88 8.00 0.36 2.59 0.29 8.60 28.02 0.29 3.49 11.06 26.28 28.72 54.97 4.60 21.05 119.77 16.82 208.35 173.91 71.24 182.37 50.90 2005.35 889.95 263.71 90.43 25.81 113.84 285.95 1646.09 52.14 301.74 123.96 18.84 46.71

0.0001 0.0002 0.0003 0.0002 0.0029 0.0012 0.0001 0.0004 0.0000 0.0012 0.0041 0.0000 0.0005 0.0016 0.0038 0.0042 0.0080 0.0007 0.0030 0.0173 0.0024 0.0301 0.0252 0.0103 0.0264 0.0074 0.2901 0.1288 0.0382 0.0131 0.0037 0.0165 0.0414 0.2382 0.0075 0.0437 0.0179 0.0027 0.0068

0 0.01 0.64 0 0 0 0 8.31 0 0 11.79

0 0.0001 0.0028 0 0 0 0 0.0359 0 0 0.0509

P. Babu, C.K. Jayasankar / Physica B 279 (2000) 262}281

273

Table 5 (continued) S¸J 3P 0

S@¸@J@

l

5G 6329 4 5G 6425 3 5G 6627 2 5L 6703 7 5L 7748 6 5D 8631 3 5D 11522 2 5D 13985 1 5D 15716 0 7F 28059 6 7F 29138 5 7F 30179 4 7F 31149 3 7F 31999 2 7F 32659 1 7F 33036 0 A (3P )"232 s~1, q (3P )"4318 ls T 0 R 0 5H 5L 2389 3 10 5L 2883 9 5D 3207 4 5L 3627 8 5G 4145 5 5G 4151 6 5G 4164 4 5G 4261 3 5G 4463 2 5L 4538 7 5L 5583 6 5D 6466 3 5D 9357 2 5D 11821 1 5D 13551 0 7F 25894 6 7F 26973 5 7F 28015 4 7F 28984 3 7F 29834 2 7F 30494 1 7F 30871 0 A (5H )"710 s~1, q (5H )"1409 ls T 3 R 3 5L 5L 1045 7 6 5D 1928 3 5D 4819 2 5D 7283 1 5D 9013 0 7F 21356 6 7F 22435 5 7F 23477 4 7F 24447 3 7F 25296 2 7F 25957 1 7F 26334 0

DD;2DD2

DD;4DD2

DD;6DD2

0 0 0.0003 0 0 0 0.0001 0 0 0 0 0 0 0.0002 0 0

0.0003 0 0 0 0 0 0 0 0 0 0 0.0000 0 0 0 0

0 0 0 0 0.0054 0 0 0 0 0.0000 0 0 0 0 0 0

0.84 0 1.25 0 31.76 0 2.50 0 0 9.11 0 4.94 0 72.59 0 0

0 0 0 0 0 0 0 87.50 0 0 0 0 0 0 0.34 0

0.84 0 1.25 0 31.76 0 2.50 87.50 0 9.11 0 4.94 0 72.59 0.34 0

0.0036 0 0.0054 0 0.1371 0 0.0108 0.3778 0 0.0393 0 0.0213 0 0.3135 0.0015 0

0 0 0.0001 0 0.0001 0 0.0001 0.0095 0.0059 0 0 0.0007 0.0007 0.0000 0 0 0.0000 0.0000 0.0002 0.0007 0.0004 0

0 0 0.0104 0 0.0057 0.0055 0.0127 0.0349 0.0082 0.0303 0.4356 0.0171 0.0283 0.0200 0 0.0000 0.0000 0.0000 0.0000 0.0002 0.0004 0

0 0.0122 0.2246 0.0112 0.1948 0.1595 0.0589 0.0104 0 0.0405 0.1437 0.0641 0 0 0 0.0018 0.0004 0.0017 0.0021 0 0 0

0 0.53 13.96 0.97 25.78 21.29 9.10 6.90 2.09 11.11 158.86 38.74 35.89 49.53 0 57.97 13.54 66.56 107.57 46.94 35.84 0

0 0 0.00 0 0 0 0.07 1.64 2.42 0 0 0.00 0.00 0.00 0 0.00 0 0.59 1.11 0.55 0 0

0 0.53 13.96 0.97 25.78 21.29 9.18 8.55 4.50 11.11 158.86 38.74 35.89 49.53 0 57.97 13.54 67.14 108.68 47.48 35.84 0

0 0.0008 0.0197 0.0014 0.0363 0.0300 0.0129 0.0120 0.0063 0.0157 0.2239 0.0546 0.0506 0.0698 0 0.0817 0.0191 0.0946 0.1532 0.0669 0.0505 0

0.0152 0 0 0 0 0.0000 0.0000 0 0 0 0 0

0.3293 0.0074 0 0 0 0.0001 0.0002 0.0001 0.0000 0 0 0

0.1464 0.1366 0.2446 0.2004 0 0.0013 0.0074 0.0081 0.0003 0.0110 0.0183 0

0.42 0.87 23.17 65.54 0 11.71 72.39 89.33 3.31 150.67 270.58 0

0.23 0 0 0 0 0.00 0 0 0 0 0 0

0.65 0.87 23.17 65.54 0 11.71 72.39 89.33 3.31 150.67 270.58 0

0.0009 0.0013 0.0337 0.0952 0 0.0170 0.1052 0.1298 0.0048 0.2189 0.3932 0

A

%$

(s~1)

A

.$

(s~1)

A (s~1)

b R

274

P. Babu, C.K. Jayasankar / Physica B 279 (2000) 262}281

Table 5 (continued) S¸J

S@¸@J@

l

A (5L )"688 s~1, q (5L )"1453 ls T 7 R 7 5D 5D 2891 3 2 5D 5355 1 5D 7085 0 7F 19428 6 7F 20507 5 7F 21549 4 7F 22518 3 7F 23368 2 7F 24028 1 7F 24405 0 A (5D )"262 s~1, q (5D )"3821 ls T 3 R 3 5D 5D 2464 2 1 5D 4194 0 7F 16537 6 7F 17617 5 7F 18658 4 7F 19628 3 7F 20477 2 7F 21138 1 7F 21515 0 A (5D )"295 s~1, q (5D )"3389 ls T 2 R 2 5D 5D 1730 1 0 7F 14073 6 7F 15153 5 7F 16194 4 7F 17164 3 7F 18013 2 7F 18674 1 7F 19051 0 A (5D )"322 s~1, q (5D )"3106 ls T 1 R 1 5D 7F 12343 0 6 7F 13422 5 7F 14464 4 7F 15433 3 7F 16283 2 7F 16944 1 7F 17320 0 A (5D )"321 s~1, q (5D )"3118 ls T 0 R 0

DD;2DD2

DD;4DD2

DD;6DD2

0.0337 0.0175 0 0 0.0001 0.0040 0.0011 0.0002 0.0004 0

0.0149 0.0069 0 0.0000 0.0014 0.0003 0.0005 0.0020 0.0012 0

0 0 0 0.0000 0.0000 0.0001 0.0001 0 0 0

2.09 6.72 0 0.47 19.93 80.79 34.29 43.92 37.42 0

0.0118 0.0138 0 0 0.0021 0.0024 0.0018 0.0001 0.0008

0 0 0.0000 0.0016 0.0004 0.0026 0.0015 0 0

0 0 0.0001 0.0000 0.0000 0 0 0 0

0 0 0 0 0.0039 0.0008 0.0026 0

0 0 0.0007 0.0028 0.0019 0 0 0

0 0 0 0 0.0033 0 0

0 0 0.0023 0 0 0 0

glasses shown in Fig. 1 are measured using Eq. (1) and are collected in Table 2. As 7F level is very 1 close (&250 cm~1) to the ground state 7F , about 0 30% of the total atoms are present in 7F level at 1 253C. Therefore, the experimental oscillator strengths must be corrected for thermal population by dividing them by their respective initial level fractional populations [4]. In the present case, the

A (s~1)

b R

4.03 0 0 0 0 31.67 0.03 0.38 0 0

6.12 6.72 0 0.47 19.93 112.46 34.32 44.30 37.42 0

0.0234 0.0257 0 0.0018 0.0761 0.4297 0.1311 0.1692 0.1430 0

0.47 2.70 1.22 18.41 41.00 89.97 69.38 3.75 22.00

2.99 0 0 0 0 41.30 0.04 1.77 0

3.46 2.70 1.22 18.41 41.00 131.27 69.42 5.53 22.00

0.0117 0.0092 0.0041 0.0624 0.1390 0.4450 0.2353 0.0187 0.0746

0 0.0003 0 0 0 0 0 0

0 4.07 9.06 41.51 120.68 20.37 76.38 0

0.95 0 0 0 0 44.18 0.01 4.74

0.95 4.07 9.06 41.51 120.68 64.55 76.39 4.74

0.0029 0.0126 0.0281 0.1289 0.3749 0.2005 0.2373 0.0147

0.0003 0 0 0 0 0 0

6.05 0 74.24 0 188.43 0 0

0 0 0 0 0 51.90 0

6.05 0 74.24 0 188.43 51.90 0

0.0189 0 0.2316 0 0.5877 0.1619 0

A

%$

(s~1)

A

.$

(s~1)

corrected oscillator strengths for Eu3` : L6BE glass are presented in Table 3 along with similar corrections made and reported for Eu3` : glasses. Due to changes in glass composition for Eu3` : LBE glasses, the oscillator strengths can be reduced or increased by 0.5 times. For example, the oscillator strengths for 7F P5L transition varied 0 6 from 2.233]10~6 (Eu3` : L6BE) to 1.157]10~6

P. Babu, C.K. Jayasankar / Physica B 279 (2000) 262}281

275

Table 6 Judd}Ofelt parameters (X ,]10~20 cm2), refractive index (n) and predicted radiative lifetimes (q , ls) for excited states of Eu3` : glasses j #!Present work L4BE

Reported

L5BE

L6BE

L7BE

L2FBE

L5FBE

ZBLA [4]

PbSi [9]

KMgSi [10]

JO parameters X 6.34 2 X 4.97 4 X 5.10 6

6.14 5.04 6.30

5.68 5.13 5.34

5.74 4.35 5.97

6.06 4.57 5.58

5.64 4.44 5.38

0.64 4.87 2.84

6.44 4.13 1.44

6.36 3.94 0.51

Refractive index n 1.572 States q #!-

1.573

1.575

1.577

1.543

1.539

1.522

1.760

1.520

5K 6 3P 0 5H 3 5L 7 5D 3 5D 2 5D 1 5D 0

122 3745 1144 1156 3257 2903 2664 2679

132 3977 1246 1359 3348 2994 2765 2788

128 3885 1235 1210 3526 3126 2856 2864

138 4138 1346 1388 3619 3206 2932 2942

145 4318 1409 1453 3821 3390 3106 3119

264 7368 2129 2833 6565 6244 6276 6561

124 3223 1528 3442 2387 2088 1907 1914

219 5413 3033 15189 3937 3430 3132 3143

131 3899 1284 1432 3238 2875 2637 2652

Table 7 Emission band position (j , nm), e!ective linewidth (*j , nm), radiative transition probability (A, s~1), peak stimulated emission 1 %&& cross-section (p(j ),]10~22 cm2), experimental and calculated branching ratios (b ) and ifetimes (q , ms) for 5D level of Eu3`: glasses 1 R R 0 b R

L5FBE

L5FBE

5D P7F 0 0 7F 1 7F 2 7F 3 7F 4 7F 5 7F 6 5D P7F 1 0 7F 1 7F 2 q of 5D R 0

NaFPO [12]

j p

*j %&&

A

p(j ) 1

Exp

Cal

Exp

Cal

578 592 612 652 700 748 813 526 536 553

3 11 12 8 11 18 36 65 11 10

0.00 59.90 188.43 0.00 74.24 0.00 6.05 64.55 76.39 4.74

0.000 3.745 12.335 0.000 9.074 0.000 0.411 0.426 3.210 0.248

0.010 0.156 0.575 0.020 0.216 0.005 0.018 0.032! 0.524 0.444 2.24

0.000 0.162 0.588 0.000 0.232 0.000 0.019 0.033! 0.524 0.443 3.12

0.011 0.290 0.559 0.017 0.123 * * * * *

0.000 0.169 0.533 0.000 0.287 0.000 0.011 * * *

!Branching ratios are normalised to unity for three transitions, 5D P 7F . 1 0,1,2

(Eu3` : L5FBE). Thermal correction yields similar trends but with increased magnitude of oscillator strengths. The Judd}Ofelt analysis has been carried out by "tting the observed oscillator strengths for all the

absorption bands as carried out for Eu3` : ZBLA glass [4] without thermal correction. The total number of transitions considered are 7, 7, 11, 11, 10 and 8 for L4BE, L5BE, L6BE, L7BE, L2FBE and L5FBE, respectively. Good agreement between

276

P. Babu, C.K. Jayasankar / Physica B 279 (2000) 262}281

Table 8 The #uorescence intensity ratio (R) of 5D P7F to 5D P7F transitions of Eu3` : glasses 0 2 0 1 Glass composition

R

Ref.

49AlF }20BaF }30CaF }1EuF 3 2 2 3 48InF }24PbF }24BaF }3LaF }1EuF 3 2 2 3 3 60ZrF }33BaF }6LaF }1EuF 4 2 3 3 97.5Pb(PO ) }2.5Eu(PO ) 32 33 89.5B O }10Li O}0.5Eu O 2 3 2 2 3 98Na(PO ) }2Eu O 32 2 3 97.5Ba(PO ) }2.5Eu(PO ) 32 33 39.5Li O}59.5SiO }1Eu O 2 2 2 3 39.5BaO}59.5SiO }1Eu O 2 2 3 19.5Li O}20SrO}59.5SiO }1Eu O 2 2 2 3 66P O }32BaO}2Eu O 2 5 2 3 59.5B O }40Li O}0.5Eu O 2 3 2 2 3 49P O }49BaO}2Eu O 2 5 2 3 74.5B O }25Li O}0.5Eu O 2 3 2 2 3 64SiO }16K O}16BaO}4Eu O 2 2 2 3 0.5GeO }63.5SiO }16K O}16BaO}4Eu O 2 2 2 2 3 19.5Li O}20CaO}59.5SiO }1Eu O 2 2 2 3 4GeO }60SiO }16K O}16BaO}4Eu O 2 2 2 2 3 19.5Li O}20BaO}59.5SiO }1Eu O 2 2 2 3 12GeO }52SiO }16K O}16BaO}4Eu O 2 2 2 2 3 39.5SrO}59.5SiO }1Eu O 2 2 3 19.5Na O}20BaO}59.5SiO }1Eu O 2 2 2 3 39.5Na O}59.5SiO }1Eu O 2 2 2 3 49.5LiF}49.5B O }1Eu O 2 3 2 3 57GeO }40K O}3Eu O 2 2 2 3 39.5K O}59.5SiO }1Eu O 2 2 2 3 19.5Li O}20MgO}59.5SiO }1Eu O 2 2 2 3 39.5Li O}59.5B O }1Eu O 2 2 3 2 3 49P O }49CaO}2Eu O 2 5 2 3 29.5Li O}69.5B O }1Eu O 2 2 3 2 3 49P O }49CaO}2Eu O 2 5 2 3 49P O }49SrO}2Eu O 2 5 2 3 32GeO }32SiO }16K O}16BaO}4Eu O 2 2 2 2 3 97.5Zn(PO ) }2.5Eu(PO ) 32 33 66P O }32CaO}2Eu O 2 5 2 3 85.5P O }12.5CaO}2Eu O 2 5 2 3 19.5K O}20BaO}59.5SiO }1Eu O 2 2 2 3 63.7GeO }33.3K O}3Eu O 2 2 2 3 19.5Rb O}20BaO}59.5SiO }1Eu O 2 2 2 3 73P O }25CaO}2Eu O 2 5 2 3 24.75Li O }24.75LiF}49.5B O }1Eu O 2 3 2 3 2 3 68GeO }24K O}5Na O}3Eu O 2 2 2 2 3 19.5Na O}20SrO}59.5SiO }1Eu O 2 2 2 3 19.5Na O}20CaO}59.5SiO }1Eu O 2 2 2 3 52GeO }12SiO }16K O}16BaO}4Eu O 2 2 2 2 3 39.5Cao}59.5SiO }1Eu O 2 2 3 67GeO }30K O}3Eu O 2 2 2 3 49.5Li O}49.5B O }1Eu O 2 2 3 2 3 68GeO }29K O}3Eu O 2 2 2 3 68GeO }4K O}25Na O}3Eu O 2 2 2 2 3 60GeO }4SiO }16K O}16BaO}4Eu O 2 2 2 2 3 64GeO }16K O}16BaO}4Eu O 2 2 2 3

0.90 1.00 1.30 2.08 2.41 3.00 3.16 3.20 3.23 3.23 3.24 3.32 3.39 3.40 3.42 3.46 3.50 3.50 3.56 3.61 3.62 3.64 3.67 3.69 3.70 3.72 3.72 3.73 3.77 3.77 3.81 3.81 3.82 3.88 3.88 3.88 3.91 3.91 3.93 3.95 3.97 3.98 3.99 4.00 4.02 4.03 4.05 4.05 4.05 4.06 4.07 4.07

[32] [32] [32] [18] [5] [13] [18] [10] [10] [10] [5] [5] [5] [5] [5] [5] [10] [5] [10] [5] [10] [10] [10] Present [5] [10] [10] Present [5] Present [5] [5] [5] [18] [5] [5] [10] [5] [10] [5] Present [5] [10] [10] [5] [10] [5] Present [5] [5] [5] [5]

work

work work

work

work

P. Babu, C.K. Jayasankar / Physica B 279 (2000) 262}281

277

Table 8 (continued) Glass composition

R

Ref.

68GeO }14K O}15Na O}3Eu O 2 2 2 2 3 68GeO }29Na O}3Eu O 2 2 2 3 66P O }32CaO}2Eu O 2 5 2 3 49P O }49MgO}2Eu O 2 5 2 3 72GeO }25K O}3Eu O 2 2 2 3 59.5Li O}39.5B O }1Eu O 2 2 3 2 3 66P O }32SrO}2Eu O 2 5 2 3 19.5K O}20SrO}59.5SiO }1Eu O 2 2 2 3 19.5Rb O}20SrO}59.5SiO }1Eu O 2 2 2 3 66P O }32MgO}2Eu O 2 5 2 3 19.5Na O}20MgO}59.5SiO }1Eu O 2 2 2 3 81GeO }16K O}3Eu O 2 2 2 3 19.5K O}20CaO}59.5SiO }1Eu O 2 2 2 3 84.5GeO }12.5K O}3Eu O 2 2 2 3 19.5K O}20MgO}59.5SiO }1Eu O 2 2 2 3 72}74.5SiO }5BaO}15Na O}5ZnO}0.25}3.Eu O 2 2 2 3 19.5Rb O}20CaO}59.5SiO }1Eu O 2 2 2 3 49.5BaO}49.5P O }1Eu O 2 5 2 3 19.5Rb O}20MgO}59.5SiO }1Eu O 2 2 2 3 49.5SrO}49.5P O }1Eu O 2 5 2 3 49.5CaO}49.5P O }1Eu O 2 5 2 3 49.5MgO}49.5P O }1Eu O 2 5 2 3

4.07 4.08 4.08 4.10 4.13 4.18 4.19 4.19 4.26 4.33 4.43 4.45 4.57 4.60 4.85 4.85 4.90 5.28 5.48 5.69 6.30 7.04

[5] [5] [5] [5] [5] Present work [5] [10] [10] [5] [10] [5] [10] [5] [10] [33] [10] [11] [10] [11] [11] [11]

f and f is noticed for 7F P7F , 7F P7F , %91 #!1 6 0 6 7F P5D and 7F P5D transitions whereas no1 1 1 3 ticeable di!erences exist between f and f for %91 #!7F P5D , 7F P5D , 7F P5L and 7F P5G 1 2 0 2 0 6 0 4 transitions. The rms deviations of $0.44, $0.42, $0.56, $0.58, $0.49 and $0.32 are obtained for Eu3` : L4BE, L5BE, L6BE, L7BE, L2FBE and L5FBE glasses, respectively. The JO analysis for Eu3` ions yields good agreement between f and %91 f for some levels and disagreement for some other #!levels. This may be due to most of the zero and few of the non-zero values for DD;jDD2 of WJ%W@J@ transitions of Eu3` ion. Also, the non-zero values of DD;jDD2 are not according to the f values, since %91 f should be proportional to the sum of the prod%91 ucts of X and DD;jDD2 (Eq. (2)). For example, for j 7F P7F , the f "0.808]10~6 and DD;6DD2" 1 6 %91 0.37739 and for 7F P5L , the f "1.693 and 0 6 %91 DD;6DD2"0.01543. Because of few non-zero values of DD;jDD2 and transitions from ground (7F ) as well as next excit0 ed state (7F ), the JO analysis has been carried out 1 in di!erent ways for f}f transitions of Eu3` ions. Therefore, in the present work the JO analysis for

Eu3` : LBE glasses is carried out using all possible methods. The results are shown in Table 4 for Eu3` : L4BE and L6BE glasses. The f values are corrected for thermal popula%91 tion of the 7F level which are presented in Table 1 3 for Eu3` : L6BE glass along with corrected and reported Eu3` : glasses [8,9,18]. As seen from Table 3, the f for Eu3` : LBE glasses are found to %91 be relatively higher than those observed for phosphate [8,18] and lead silicate [9] glasses. In the second method, the JO analysis has been carried out after thermal correction and the results are shown as SET B in Table 4. The magnitudes of JO parameters as well as the rms deviation have been increased compared to the parameters obtained without thermal correction (SET A). The JO parameters, X , X and X have been 2 4 6 calculated from emission intensities of 5D P7F , 0 2 7F and 7F , respectively, and are shown as SET C. 4 6 The SET D (without thermal correction) and SET E (with thermal correction) refer the JO analysis for the levels that include both absorption as well as emission transitions. The SET F refers to those values of X , X and X , obtained from absorption 2 4 6

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bands of 7F P5D , 5D and 5L , respectively. 0 2 4 6 The SET G refers to the same as SET F but corrected to thermalisation e!ect. As seen from Table 4, there are seven di!erent sets of JO parameters which are signi"cantly di!erent from one another. For example, SET A is di!erent from SET B, SET C di!ers from SET F and SET G. However, to test the validity of these seven di!erent sets of parameters, they are used to predict lifetimes (q ) for 5D level and are compared with #!0 experimental values (q ). For Eu3` : L4BE glass, %91 the q for 5D level is found to be 1.84 ms which is %91 0 comparable to q of 2.65 ms and 2.11 ms for SET #!C and SET F, respectively, as q should be less %91 than or equal to q . Similarly, for Eu3` : L6BE, #!the q (5D )"1.92 ms which is comparable to %91 0 q of 2.70, 2.79, 2.10 and 2.13 ms for SET A, SET #!C, SET D and SET F, respectively. This reveals that the JO parameters obtained from the individual levels of absorption without thermal correction (SET F) and only from emission data (SET C) may be more reliable than the other sets. For Eu3` : L4BE, L5BE, L2FBE and L5FBE systems, the 7F P5D transition from absorption is miss0 4 ing which restricts the calculation of X for SET 4 F and SET G parameters. In such cases (ignoring emission data) radiative properties have been predicted by assuming X "0. Examination of SET 4 F for Eu3` : L4BE and Eu3` : L6BE helps one to understand the importance of X , as in some cases 4 7F P5D level is missing due to either weak 0 4 intensity or strong absorption of the host. For Eu3` : L6BE glass, the q is found to increase #!from 2.13 ms (X "6.52]10~20 cm2) to 2.83 ms 4 (X "0) and from 1.48 ms (X "9.75]10~20 cm2) 4 4 to 2.00 ms (X "0) for SET F and SET G, respec4 tively. This shows that when X is not included 4 while evaluating q , it may yield higher value than #!the normal value. In our further analysis, the JO parameters obtained from emission data (SET C) have been used and radiative properties predicted for Eu3` : LBE glasses. The above discussion clearly implies that care should be taken when we compare JO parameters of one Eu3` : system to those of another Eu3` : system as the JO parameters depend strongly on the constraint under which they have been determined.

5.3. Radiative properties The radiative properties for eight excited states of Eu3` : LBE glasses that are lying up to 45,000 cm~1 have been predicted using JO parameters obtained from emission spectra as carried out for Eu3` : glasses [6,7]. These eight states are of an order of well above 1,000 cm~1 from the lower state and emission from these levels are expected when the phonon energy of a host is small. For Eu3` : glasses, emission has already been observed experimentally from 5D (J"0, 1, 2 and 3) J consisting of visible transitions and from 5H level, 3 emitting UV radiation [4]. All the predicted radiative properties for one glass, Eu3` : L5FBE, are presented in Table 5. The spontaneous electric dipole emission depends on DD;jDD2 and X values, whereas the magnetic dipole j emission depends on the wave functions generated for the Eu3` ion from intermediate coupling approximation. As seen from the emission properties of 5K level, 5K P5L at 11,179 cm~1 is domin6 6 7 ated by A contribution due to larger value of %$ DD;jDD2 and 5K P7F at 33,615 cm~1 is dominated 6 5 by A due to wave function components coma.$ pred to other transitions. Also these transitions possess relatively larger branching ratio and therefore 5K P5L and 7F transitions may emit in 6 7 5 suitable hosts. From the predicted radiative properties for 5H level, 5H P5L and 5H P7F possess rela3 3 6 3 3 tively higher branching ratios than those found for 5H P7F transition, though experimentally the 3 1 emission has been observed for 5H P7F transi3 1 tin [4]. Dejneka et al. [4] were unable to predict radiative properties for this level due to lack of DD;jDD2 values. Therefore, the predicted results will compliment the experimental results for 5H level 3 of Eu3` : ZBLA glass [4]. For the other transitions shown in Table 5, the most intense emission channels that possess relatively higher branching ratios are : 3P P5D and 0 1 7F ; 5L P7F and 7F ; 5D P7F and 7F ; 2 7 2 1 3 4 2 5D P7F and 7F ; 5D P7F and 7F and 2 3 2 1 3 1 5D P7F and 7F . 0 4 2 The predicted radiative lifetimes (q ) for all the #!Eu3` : LBE glasses are collected in Table 6. The radiative properties for Eu3` : ZBLA [4], lead

P. Babu, C.K. Jayasankar / Physica B 279 (2000) 262}281

silicate [9] and 30K O}10MgO}60SiO (KMgSi) 2 2 [10] glasses have also been predicted (q ) and #!presented in Table 6. Out of nine Eu3` : systems compared in Table 6, ZBLA host possess relatively higher q values. Within 5D multiplet 5D and #!J 1 5D have more or less equal with shorter 0 q whereas 5D has larger q . Out of eight levels, #!3 #!q is found to be minimum for 5K level and #!6 maximum for 3P level except for PbSi and KMgSi 0 glasses. The 5L level possess, longer q in PbSi 7 #!and KMgSi glasses. This is mainly due to the smaller values of X for these glasses and the reduced 6 matrix elements from 5L to lower levels play a sig7 ni"cant role. As shown in Table 5, from 5L to 7 lower levels, the DD;2DD2 values are almost zero and DD;6DD2 values exist for many transitions. Therefore, maximum variation of q has been noticed for #!5L level. 7 The q (q ) in ms for 5D level of Eu3` : L4BE, %91 #!0 L5BE, L6BE, L7BE, L2FBE and L5FBE are found to be 1.84 (2.65), 1.89 (2.68), 1.97 (2.79), 2.00 (2.86), 2.11 (2.94) and 2.24 (3.12), respectively. Wide di!erences between q and q have also been reported %91 #!for Eu3` : phosphate glasses [7] when q have #!been predicted with X parameters obtained from j 5D P7F emission transitions (SET C of 0 2,4,6 Table 4). Both experimental and predicted lifetimes increase with decrease in lithium oxide content and also with the addition of lithium #uoride. As the energy gap between 5D and 7F levels is 0 J very large (&12,300 cm~1), non-radiative decay due to multiphonon relaxation is negligible. The di!erence between predicted and experimental lifetimes may be due to Eu3` ion}Eu3` ion interaction. The non-radiative decay rates in the present Eu3` : LBE glasses are estimated using the following expression [6] = "[1/q ]!(A ) . (13) /3 %91 3!$ JO The non-radiative decay rates (s~1) for 5D of 0 Eu3` : L4BE, L5BE, L6BE, L7BE, L2FBE and L5FBE are 167, 156, 149, 150, 134 and 126, respectively. From these results, it is clear that non-radiative decay rate is maximum in Eu3` : L4BE glass. It decreases with decrease in lithium oxide content up to Eu3` : L6BE and then increases slightly. When lithium oxide is replaced by lithium #uo-

279

ride the non-radiative decay rate decreases from 156 s~1 (Eu3` : L5BE) to 126 s~1 (Eu3` : L5FBE). The emission spectra of Eu3` : LBE glasses show two groups of transitions coming from 5D and 0 5D levels as shown in Fig. 3. All the transitions 1 corresponding to 5D P7F (J"0, 1, 2, 3, 4, 5 and 0 J 6) group have been identi"ed whereas only three transitions corresponding to 5D P7F (J"0, 1 J 1 and 2) group have been identi"ed. As usual, 5D P7F group is found to be more intense than 0 J 5D P7F group. Within 5D P7F group, 1 J 0 J 5D P7F and 7F transitions are weaker and 0 5 6 found to be less by a factor of 20. The relative areas known as experimental branching ratios for 5D P7F transitions of Eu3` : LBE glasses are 0 J found to be of the order of 5D P7F ' 0 2 7F '7F '7F '7F '7F '7F . The experi4 1 3 6 0 5 mental branching ratios are compared with those predicted using JO theory for 5D P7F and 0 J 5D P7F transitions of Eu3` : LBE glasses but 1 J the results are presented only for Eu3` : L5FBE glass in Table 7 and compared with Eu3` : sodium #uorophosphate (NaFPO) [12] glass. The experimental b are more comparable with R the predicted values for Eu3` : L5FBE glass than those of Eu3` : NaFPO glass. The b are found to R be maximum for 5D P7F transition. The other 0 2 important radiative properties such as e!ective line width (*j ) and peak-stimulated emission cross%&& section (p(j )) have been determined for 1 Eu3` : LBE glasses and the results are presented in Table 7 for Eu3` : L5FBE glass. For Eu3` : LBE glasses, p(j ) is found to be maximum for 1 5D P7F transition. The value of p(j )] 0 2 1 10~22 cm2 for 5D P7F transition of Eu3` : L4BE, 0 2 L5BE, L6BE, L7BE, L2FBE and L5FBE are found to be 12.19, 12.12, 10.94, 12.68, 13.30 and 12.34, respectively. The emission intensity of 5D P7F transition is 0 2 subjected to local symmetry as this transition is allowed only by electric dipole nature. Similarly, 5D P7F transition is allowed by magnetic0 1 dipole and therefore its intensity is independent of local symmetry [36,37]. Therefore, the relative intensity of the 5D P7F to 5D P7F 0 2 0 1 transition, represented by #uorescence intensity ratio (R), allows one to estimate the deviation from the site symmetries of Eu3` ions [36,37]. The

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observed intensity ratios (R) of the Eu3` : LBE glasses are presented in Table 8 along with reported Eu3` : glasses [5,10,11,13,18,32,33]. The values of R, for Eu3` : glasses, are presented in Table 8 in the increasing order. The ratio, R, is found to have minima of 0.9, 1.0 and 1.3 for alumina, indium and zirconium-based fuoride glasses [32], respectively, and maxima of 5.28, 5.69, 6.30 and 7.04 for BaO}P O , SrO}P O , CaO}P O and MgO} 2 5 2 5 2 5 P O [11], respectively. However, Oomen and 2 5 Dongen [5] reported the R values of 3.39, 3.81, 3.81 and 4.10 for BaO}P2O , SrO}P O , CaO}P O 5 2 5 2 5 and MgO}P O , respectively. These di!erences 2 5 between Reisfeld et al. [11] and Oomen and Donger [5] results may be due to Eu3` ion concentration as the former work contains 1% of Eu3` ions and the latter work contains 2% of Eu3` ions. As seen from Table 8, most of the Eu3` : glasses exhibit the R value in the range of 3.5}4.2. For Eu3` : LBE glasses, the R value changes from 3.69 (Eu3` : L5FBE) to 4.18 (Eu3` : L4BE). At "rst, the ratio R is found to decrease with decrease in lithium oxide content (Eu3` : L4BE to L6BE) and then increases with decrease in lithium oxide content (Eu3` : L7BE) whereas the R value decreases with increase in lithium #uoride content. The intensity ratio for 5D P7F to 5D P7F 0 4 0 1 transition is found to be 1.56, 1.57, 1.60, 1.35, 1.42 and 1.39 for Eu3` : L4BE, L5BE, L6BE, L7BE, L2FBE and L5FBE, respectively. These ratios are comparable to the values reported for oxide glasses [5,14]. The variations of R and X are of the same trend 2 provided X is solely determined from 5D P7F 2 0 2 transition. The increase of R and the bonding parameter (d) is due to increase of the covalency between the Eu3` ion and the ligands such as oxygen coordination in LnBE glasses (except L7BE) and oxygen/#uorine coordination in LxFBE glasses [38].

cantly from those obtained either by introduction of thermalisation e!ect or from individual levels of emission/absorption studies. The JO parameters obtained from emission levels are found to be relatively more reliable as these parameters predict radiative properties for 5D level that agree with 0 experimental values. The reduced matrix elements and magnetic dipole linestrengths have been computed for all the excited states of Eu3` (4f 6) ion that are lying up to 45,000 cm~1. The radiative properties are reported for the excited states of 5K , 3P , 3H , 5L and 6 0 3 7 5D . J Due to most of the zero as well as negligibly small values of reduced matrix elements for Eu3` ion, a greater change in the JO parameters produces wide range in predicted radiative properties for some excited levels such as 5L level situated at 7 &26,300 cm~1. Either the relative intensity ratio of 5D P7F 0 2 to 5D P7F transition or the bonding parameter 0 1 (d) is helpful to estimate the relative strength of covalent/ionic bonding between the Eu3` ion and the ligands. The relative intensity ratio of 5D P7F to 0 2 5D P7F transition is found to decrease with 0 1 decrease in lithium oxide content and then increases with decrease in lithium oxide content. But the intensity ratio of 5D P7F to 5D P7F 0 4 0 1 transition is found to increase with decrease in lithium oxide content and then decreases. However, the intensity ratio of 5D P7F to 5D P7F and 0 2 0 1 5D P7F to 5D P7F decreases with increase 0 4 0 1 in lithium #uoride content. The optical properties of Eu3` : lithium borate glasses can be improved with the addition of #uorides. The optical properties of Eu3` : LBE glasses of the present study are characteristic of the these compositions and are comparable with those of oxide, #uoride and phosphate glasses.

6. Conclusions

Acknowledgements

The optical behaviour of Eu3` ions is studied in lithium borate and lithium #uoroborate glasses. The phenomenological JO parameters determined by the usual "tting procedure di!er signi"-

One of the authors (PB) is grateful to University Grants Commission (UGC), New Delhi for awarding the Teacher fellowship under the Faculty Improvement Programme.

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References [1] C. Gorller-Walrand, K. Binnemans, Spectral Intensities of f}f transitions, in: K.A. Gschneidner Jr., L. Eyring (Eds.), Handbook on the Physics and Chemistry of Rare Earths, 25, Elsevier Amsterdam, North-Holland, 1998, pp. 101}264. [2] B.R. Judd, Phys. Rev. 127 (1962) 750. [3] G.S. Ofelt, J. Chem. Phys. 37 (1962) 511. [4] M. Dejneka, E. Snitzer, R.E. Riman, J. Lumin. 65 (1995) 227. [5] E.W.J.L. Oomen, A.M.A. van Dongen, J. Non-Cryst. Solids 111 (1989) 205. [6] B. Peng, T. Izumitani, Rev. Laser Eng. 22 (1994) 16. [7] H. Ebendor!-Heidepriem, D. Ehrt, J. Non-Cryst. Solids 208 (1996) 205. [8] R. Reisfeld, L. Boehm, J. Solid State Chem. 4 (1972) 417. [9] F. Fermi, L. Tellini, G. Ingletto, A. Vinattieri, M. Bettinelli, Inorg. Chim. Acta 150 (1988) 141. [10] Y. Nageno, H. Takebe, K. Morinaga, T. Izumitani, J. Non-Cryst. Solids 169 (1994) 288. [11] R. Reisfeld, L. Boehm, M. Ish-Shalom, R. Fischer, Phys. Chem. Glasses 15 (1974) 76. [12] K. Binnemans, R. Van Deun, C. Gorller-Walrand, J.L. Adam, J. Non-Cryst. Solids. 238 (1998) 11. [13] R. Reisfeld, R.A. Velapoldi, L. Boehm, M. Ish-Shalom, J. Phys. Chem. 75 (1971) 3980. [14] R. Reisfeld, R.A. Velapoldi, L. Boehm, J. Phys. Chem. 76 (1992) 1293. [15] G.J. Quarles, A. Suchocki, R.C. Powell, J. Appl. Phys. 63 (1988) 861. [16] M. Takahashi, R. Kanno, Y. Kawamoto, S. Tanabe, K. Hirao, J. Non-Cryst. Solids 168 (1994) 137. [17] R. Van Deun, K. Binnemans, C. Gorller-Walrand, J.L. Adam, J. Alloys Compounds 283 (1999) 59. [18] J.A. Capobianco, P.P. Proulx, M. Bettinelli, F. Negrisolo, Phys. Rev. B 42 (1990) 5936. [19] S. Todoroki, K. Hirao, N. Soga, J. Appl. Phys. 72 (1992) 5853.

281

[20] J.L. Adam, V. Poncon, J. Lucas, G. Boulon, J. Non-Cryst. Solids 91 (1987) 191. [21] V. Lavin, V.D. Rodriguez, I.R. Martin, U.R. RodriguezMendoza, J. Lumin. 72}74 (1997) 437. [22] R. Balda, J. Fernandez, J.L. Adam, M.A. Arriandiaga, Phys. Rev. B 54 (1996) 12076. [23] W.T. Carnall, P.R. Fields, K. Rajnak, J. Chem. Phys. 49 (1968) 4450. [24] W. T. Carnall, H. Crosswhite, H. M. Crosswhite, Energy level structure and transition probabilities of the trivalent lanthanides in LaF , Argonne National Laboratory, Ar3 gonne, USA, 1977. [25] M.J. Weber, Relaxation processes for excited states of Eu3` in LaF3, in: H.M. Crosswhite, H.W. Moos (Eds.), Optical Properties of Ions in Crystals, Wiley, NewYork, 1967, pp. 467}484. [26] G.S. Ofelt, J. Chem. Phys. 38 (1963) 2171. [27] W.T. Carnall, P.R. Fields, B.G. Wybourne, J. Chem. Phys. 42 (1965) 3797. [28] W.T. Carnall, P.R. Fields, K. Rajnak, J. Chem. Phys. 49 (1968) 4412. [29] P.A. Tanner, V.V. Ravi Kanth Kumar, C.K. Jayasankar, M.F. Reid, J. Alloys Compounds 215 (1994) 349. [30] E. Rukmini, A. Renuka Devi, C.K. Jayasankar, Physica B 193 (1994) 166. [31] C.K. Jayasankar, E. Rukmini, Opt. Mater. 8 (1997) 193. [32] S. Todoroki, K. Hirao, N. Soga, J. Non-Cryst. Solids 143 (1992) 46. [33] R.A. Velapoldi, R. Reisfeld, L. Boehm, Phys. Chem. Glasses 14 (1973) 101. [34] C.K. Jorgensen, Orbitals in Atoms and Molecules, Academic Press, London, 1962. [35] S.P. Sinha, Complexes of the Rare Earths, Pergamon Press, Oxford, 1966. [36] P.K. Gallagher, C.R. Kurkjian, P.M. Bridenbaugh, Phys. Chem. Glasses 6 (1965) 95. [37] G. Blasse, A. Bril, W.C. Nieuwpoort, J. Phys. Chem. Solids 27 (1966) 1587. [38] H. Inoue, K. Soga, A. Makishima, J. Non-Cryst. Solids 222 (1997) 212.