Photoluminescence and energy transfer properties of Sm3+ doped CFB glasses

Photoluminescence and energy transfer properties of Sm3+ doped CFB glasses

Solid State Sciences 13 (2011) 1548e1553 Contents lists available at ScienceDirect Solid State Sciences journal homepage: www.elsevier.com/locate/ss...

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Solid State Sciences 13 (2011) 1548e1553

Contents lists available at ScienceDirect

Solid State Sciences journal homepage: www.elsevier.com/locate/ssscie

Photoluminescence and energy transfer properties of Sm3þ doped CFB glasses J. Suresh Kumar a, K. Pavani a, T. Sasikala a, A. Sreenivasa Rao b, Neeraj Kumar Giri c, S.B. Rai c, L. Rama Moorthy a, * a

Department of Physics, Sri Venkateswara University, Tirupati 517 502, India Department of Physics, Koneru Lakshmaiah University, Vaddeswaram 522 502, India c Laser and spectroscopy Laboratory, Department of Physics, Banaras Hindu University, Varanasi 221005, India b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 9 December 2010 Received in revised form 16 March 2011 Accepted 24 May 2011 Available online 30 May 2011

The present paper describes the optical absorption, photoluminescence and lifetime measurements of trivalent samarium doped calcium fluoroborate (CFB) glasses. From the observed energy levels, the free-ion energy level parameters for the 4f5 electronic configuration of Sm3þ ion have been evaluated using HFI model. The experimental oscillator strengths of absorption bands have been analyzed to determine the JuddeOfelt (JeO) parameters. From the evaluated JeO parameters and luminescence data, the radiative parameters such as AR, sR and se values were obtained from the excited 4G5/2 level to different lower energy levels. The decay curves of 4G5/2 / 6H7/2 transition were analyzed by the InokutieHirayama model assuming dipoleedipole interaction between the dopant ions. The decrease of fluorescence intensities as well as measured lifetimes at higher concentrations has been explained based on energy transfer process through cross-relaxation between Sm3þ ions. Ó 2011 Elsevier Masson SAS. All rights reserved.

Keywords: Glass Melt quenching JeO theory Luminescence quenching Energy transfer

1. Introduction In recent years, considerable attention has been paid towards the search of new host materials for doping of rare-earth ions [1]. Inorganic glasses are the promising hosts for several transition/rareearth metal ions in laser excited spectroscopy [2]. Glasses doped with rare-earth ions have attracted significantly because they can be fabricated easily in several forms [3]. Due to the sharpness of rareearth ions absorption bands in glasses, it is easy to evaluate the intensities of the spectral lines and also to predict different radiative properties. Among various glass hosts, oxide glasses doped with rare-earths are more stable to obtain efficient luminescence. On the other hand, borate glasses have more advantages than any other oxide glasses due to lower melting temperature and accommodation of high rare-earth concentrations [4]. Generally, rare-earth elements exists in trivalent state and the 4f electrons are shielded by the 5s outer shells. However, among the RE3þ ions, there are less number of studies available on spectroscopic properties of Sm3þ doped materials. Also, glasses doped with trivalent samarium ions have not been considered as promising

* Corresponding author. Tel.: þ91 877 2289472 (office), þ91 877 2242766 (residence), fax: þ91 877 2225211. E-mail address: [email protected] (L. Rama Moorthy). 1293-2558/$ e see front matter Ó 2011 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.solidstatesciences.2011.05.020

laser active materials for a long time. Thus, a little practical interest as well as interpretation difficulties of experimental data due to high density states of Sm3þ ions, causes a violation of the intermediate coupling scheme and also limits the studies on spectroscopic investigations of Sm3þ ions [5e8]. Recently spectral studies of Sm3þ ions in different hosts have been carried out in order to know the various processes involved in its luminescence quenching due to its cumbersome energy level structure [9e11]. In the process of characterization of different rare-earth doped borate glasses, we have already reported the upconversion fluorescence and NIR emission properties of Nd3þ doped calcium fluoroborate (CFB) glasses [12]. In the present study the effect of Sm3þ ions concentration on luminescence quenching and decay times has been discussed. 2. Experimental procedure Calcium fluoroborate (CFB) glasses with the chemical composition of (42  x) B2O3 þ 20 CaF2 þ 15 CaO þ 15 BaO þ 8 Al2O3 þ x SmF3 (x ¼ 0.05, 0.1, 0.5, 1.0, 2.0 and 4.0) were prepared with high purity chemicals to ensure good optical quality. The raw materials include H3BO3, CaF2, CaO, Al2O3, BaCO3 and SmF3. The mixture was crushed to fine powder until a homogeneous mixture was obtained. This homogeneous mixture taken into a platinum crucible was then heated upto 1050  C in an electric furnace for 1 h.

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The melt was then quenched on a pre-heated brass mould. The samples thus prepared were kept for annealing at 420  C to remove thermal strains and to obtain mechanical stability. The prepared glasses were polished to get square shaped samples. Different physical properties like density (3.265 gm/cc), refractive index (1.609 at 589.3 nm wavelength), thickness (0.447 cm) and the concentration of Sm3þ ions (2.44  1020 ions/cc) were measured for 1.0 mol% SmF3 doped CFB (CFBSm10) glass. FTIR spectrum was recorded using Thermo Nicolet IR200 FTIR spectrophotometer in the region of 400e4000 cm1. The absorption spectrum of CFBSm10 glass was recorded in the spectral region 300e2500 nm using PerkineElmer Lambda 950 double beam spectrophotometer with a step width of 0.2 nm. Third harmonic of Nd:YAG laser (355 nm) was used to get the photoluminescence spectra of CFBSm glasses using a computer controlled charge coupled device (CCD). Decay curves were recorded using Jobin YVON Fluorolog 3 spectrofluorometer by exciting with 35 w pulsed xenon lamp. All the measurements were made at room temperature. 3. Results and discussion 3.1. FTIR spectral measurements Fig. 1 shows the transmission spectrum of CFBSm10 glass in the region 400e4000 cm1. The observed peaks in the spectrum correspond to different groups/bonds present in the glass. The vibrational bands and their corresponding molecular bonds responsible for the absorption are very clearly shown in Fig. 1. The different vibrational bands corresponding to molecular bonds present in the infrared spectrum include metaleoxygen bond vibrations of CaeO, AleO and BaeO (400e650 cm1) [13,14], bending and stretching vibrations of BO4 group (714 and 940e1060 cm1) and stretching vibrations of BO3 group (1380 cm1) [15]. The peaks around 2370 cm1 and 2700e3500 cm1 correspond to CO2 and OH bond vibrations [16]. The intensities of transmission peaks below 1500 cm1 are low and hence the absorption in this region is high. Thus, the highest energy peak around 1350 cm1 is the phonon energy of CFBSm glasses. Fig. 2. (a) Optical absorption spectrum of CFBSm10 glass in 330e600 nm region. (b) Optical absorption spectrum of CFBSm10 glass in 900e2100 nm region.

3.2. Optical absorption spectra and energy level analysis Fig. 2(a) and (b) show the optical absorption spectra of CFBSm10 glass in the UVeVis and NIR regions respectively. Totally twenty

one absorption bands are observed at 344.0, 360.8, 374.4, 390.2, 402.2, 416.0, 421.4, 437.8, 461.4, 476.8, 501.0, 526.6, 562.6, 943.8, 1076.6, 1225.0, 1370.2, 1474.2, 1520.0, 1583.0 and 2019.8 nm corresponding to6H5/2 / 4D7/2, 4D3/2, 6P7/2, 4L15/2, 6P3/2, 4L13/2, 6P5/2, 4 G9/2, 4I13/2, 4I11/2, 4G7/2, 4F3/2, 4G5/2, 6F11/2, 6F9/2, 6F7/2, 6F5/2, 6F3/2, 6 H15/2, 6F1/2 and 6H13/2 transitions respectively. The assignment of absorption transitions has been done based on earlier studies [17,18]. The absorption bands designated as 4L13/2; 6P5/2 and 4I13/2; 4 I11/2 around 420 and 470 nm respectively are convoluted together and hence they are de-convoluted to obtain their individual absorbance. The three peaks in visible region at 501.0, 526.6, 562.6 nm and one peak in NIR region at 2019.8 nm are very weak. All these peaks are rarely realized in literature for the Sm3þ doped glasses. In order to visualize clearly, the absorbance data of the three low intense bands in the visible region is multiplied by a factor of five and plotted separately. The free-ion Hamiltonian (HFI) model used to analyze the energy level structure of Sm3þ ions in CFB glasses can be written as [19]

^ ¼E H FI AVG þ þ Fig. 1. FTIR spectrum of CFBSm10 glass in the region 400e4000 Cm

1.

X i

X k

T i^t i þ

^ þ a^ ^ ^ F k ^f k þ x4f A Lð^ Lþ1Þþ bGðG SO 2 Þþ gGðR7 Þ

X k

^k þ Pkp

X j

^j Mj m

(1)

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where EAVG represents the kinetic energy of f electrons, their coulomb interaction with the nucleus and spin-independent spherically symmetric terms. It shifts only the barycentre of the whole 4f n configuration. Fk (k ¼ 2, 4 and 6) are the Slater integrals which describe the Coulomb interaction between 4f electrons and x4f is the spineorbit coupling constant which splits the term into the 2Sþ1LJ states. a, b and g are the two-body electrostatic Tree’s parameters. Ti (i ¼ 2, 3, 4, 6, 7 and 8) are the Judd parameters, which describe the corresponding threeebody interactions and Mj (j ¼ 0, 2 and 4) are the spin-other-orbit interaction parameters and Pk (k ¼ 2, 4 and 6) represents the electrostatically correlated spineorbit interactions. Among these interactions, the second and the third terms representing the inter-electronic repulsion and the spineorbit interaction are the main contributions which give rise to the 2Sþ1LJ levels and the rest only make correction to the energies of these levels without removing their degeneracy. Table 1 shows the experimental (Eexp) and calculated (Ecal) energy positions of CFBSm10 glass along with the assignment of energy levels and the best fit free-ion parameters. The parametric analysis between the experimental and calculated energy levels has been carried out using the standard least square fit method with minimum root mean square deviation (srms), which is considered as a figure of merit for describing the quality of the fit. The srms is defined as

srms

2 31=2 2P i i Eexp  Ecal 5 ¼ 4 P

(2)

i i where Eexp and Ecal are the experimental and calculated energies respectively for level i, and P denotes the total number of levels used in the fitting procedure. The srms value calculated for CFBSm10 glass is 119 cm1. During the parametric fit the free-ion parameters namely FK and x are varied and only eight out of the twenty free-ion parameters defined in Eq. (1), have been used as independent fitting variables [20]. The calculated energy values

Table 1 Experimental (Eexp) and calculated (Ecal) energies (cm1) of absorption bands and the best fit free-ion parameters for CFBSm glasses. Level

6

H5/2 H7/2 H9/2 6 H11/2 6 H13/2 6 F1/2 6 H15/2 6 F3/2 6 F5/2 6 F7/2 6 F9/2 6 F11/2 4 G5/2 4 F3/2 4 G7/2 4 I11/2 4 I13/2 4 G9/2 4 P5/2 4 L13/2 6 P3/2 4 L15/2 6 P7/2 4 D3/2 4 D7/2 6 6

srms

Energy Eexp (10)

Ecal (10)

0 1083 2295 3765 5094 6319 6581 6783 7293 8163 9289 10596 17775 18990 19984 21000 21691 22779 23719 24039 24863 25628 26709 27716 29070 119

129 1159 2361 3683 5083 6450 6529 6696 7193 8055 9231 10622 17853 18756 19967 21024 21552 22666 23990 24095 24954 25628 26712 27580 29044

Parameter

Value (cm1)

EAVG F2 F4 F6

47554 79788 55912 39982 21.55 717 1668 283 30 100 231 265 319 1165 2.34 315

a b g T2 T3 T4 T6 T7 T8

x MTOT PTOT

obtained by diagonalizing the Sm3þ ð4f 5 Þ energy matrix using the evaluated free-ion parameters are also given in Table 1. 3.3. JuddeOfelt theory According to JuddeOfelt (JeO) theory [21,22], the calculated oscillator strength of an induced electricedipole transition from the ground state JJ, to an excited state j0 J 0 is given by

fcal ¼

2   2 X 8p2 mcy n2 þ 2 Ul jJkU l kj0 J 0 3hð2J þ 1Þ 9n

(3)

l ¼ 2;4;6

where n is refractive index of the medium, y is the energy of the transition in cm1, (n2 þ 2)2/9n is the Lorentz local field correction for dipoleedipole transition, J is the total angular momentum of the ground state and Ul (l ¼ 2, 4, 6) are JeO intensity parameters and kU ðlÞ k2 are the doubly reduced matrix elements of the unit tensor operator evaluated from the intermediate coupling scheme for a transition jJ/j0 J 0 [23]. The experimental oscillator strength of an absorption band (fexp) is directly proportional to the area under the absorption curve which can be expressed as [24,25]

fexp ¼

2303mc2 N pe2

Z

eðyÞdy ¼ 4:32  109

Z

eðyÞdy

(4)

where m and e are mass and charge of an electron, c is the velocity of light, N is the Avogadro’s number, eðyÞ is the molar absorptivity of a band at wavenumber y (cm1). A least squares fit method is then used for Eqs. (3) and (4) to determine JeO intensity parameters (Ul), which gives the best fit between the experimental (fexp) and calculated (fcal) oscillator strengths by the relation

drms ¼

vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi uP 2 u fexp  fcal t

(5)

P

Table 2 presents the experimental and calculated oscillator strengths along with small drms of 0.47  106 indicating the good fit between experimental and calculated values. From the experimental oscillator strengths it is noticed that the 6H5/2 / 6P3/2 Table 2 Experimental (fexp) and calculated (fcal) oscillator strengths (106) of CFBSm10 glass. Transition 6H5/2 /

Oscillator strengths fcal

fexp 6

H13/2 6 F1/2 6 H15/2 6 F3/2 6 F5/2 6 F7/2 6 F9/2 6 F11/2 4 G5/2 4 F7/2 4 G7/2 4 I11/2 4 I13/2 4 G9/2 6 P5/2 4 L13/2 6 P3/2 4 L15/2 6 P7/2 4 D3/2 4 D7/2

drms

0.01 0.46 1.29 2.30 3.43 5.07 3.32 0.48 0.04 0.04 0.03 1.64 0.50 0.32 0.27 0.59 7.18 0.19 1.66 1.34 1.17

                    

0.00 0.01 0.03 0.05 0.08 0.11 0.07 0.01 0.00 0.00 0.00 0.04 0.01 0.01 0.01 0.01 0.16 0.00 0.04 0.03 0.03

0.30 0.62 0.03 2.03 3.57 5.12 3.18 0.50 0.02 w0.00 0.11 0.20 0.55 0.11 1.04 0.53 6.97 0.14 1.88 1.16 1.01 0.47  106

                    

0.00 0.01 0.00 0.05 0.08 0.13 0.08 0.01 0.00 0.00 0.01 0.04 0.01 0.00 0.02 0.01 0.19 0.00 0.04 0.03 0.01

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Table 3 Comparison of JeO intensity parameters (Ul, 1020 cm2) of CFBSm10 glass with different hosts matrices. Host matrix

U2

U4

U6

CFBSm10 PKFBA [17] ZPCS [26] ZBLAN [27] LBTAF [28]

1.92  0.04 1.50 2.18 2.15 0.27

6.35  0.15 3.75 3.80 3.05 2.52

3.44  0.06 1.89 2.15 1.56 2.47

transition in the visible and the 6H5/2 / 6F7/2 transition in the NIR region possess highest fexp values. The best set of JeO intensity parameters evaluated from the least square fit method are U2 ¼ 1.92  1020, U4 ¼ 6.35  1020 and U6 ¼ 3.44  1020 cm2. In general, JeO parameters provide information about the local structure and bonding prevailing near RE ion embedded in any matrix. The high magnitude of U6 indicates low rigidity of the present CFB glass host compared to various other hosts as presented in Table 3 [17,26e28]. 3.4. Emission spectra and radiative properties Fig. 3 Shows the emission spectra of Sm3þ:CFB glasses recorded in the spectral range of 500e1000 nm under excitation at 355 nm. Six out of seven observed emission peaks are due to the 4G5/2 / 6HJ (J ¼ 5/2, 7/2, 9/2, 11/2, 13/2 and 15/2) multiplet. The other one is due to 4G5/2 / 6F3/2 transition. The intensity of all the emission peaks increases upto 0.5 mol% of Sm3þ ion concentration and then quenches. The data is normalized to the highest peak intensity of 4 G5/2 / 6H7/2 transition for 0.5 mol% concentration of Sm3þ ions in CFB glasses. Since, the 4G5/2 state is separated from the lower lying states 6F11/2 by about 7000 cm1, five phonons with the highest energy of 1350 cm1 are essential to bridge this energy gap. This makes the multi-phonon relaxation negligible. Therefore, the radiative transitions and energy transfer through cross-relaxations are the main processes which de-populate the excited 4G5/2 state as depicted in Fig. 5. The JeO parameters obtained by least square fit are used to predict the radiative properties of 4G5/2 excited states of Sm3þ ion. The radiative transition probabilities (AR) for a transition jJ/j0 J 0 can be calculated from the following equation [24]

Fig. 4. Decay curves of the 4G5/2 level of Sm3þ ions in CFB glasses at varies concentrations of SmF3.



0 0

AR jJ; j J

"  # 2 64p4 y3 n n2 þ 2 3 ¼ Sed þ n Smd 3hð2J þ 1Þ 9

(6)

where Sed and Smd are the electric and magnetic-dipole line strengths. Table 4 presents different radiative parameters calculated for Sm3þ:CFB glasses. The considerably high radiative transition rates (AR) of 4G5/2 / 6H5/2, 6H7/2, 6H9/2 and 6H11/2 transitions indicate the intense emissions at 561, 598, 644 and 706 nm respectively compared to other three radiative transition rates of 4G5/2 / 6H13/2, 6H15/2 and 6F5/2 transitions. By summing up all the radiative transition probabilities of the excited level, the total radiative transition probability (AT) and hence the radiative lifetime (sR) can be found using the equation

sR ðjJÞ ¼

1 AT

(7)

The total radiative transition rate, AT of the 4G5/2 level is 424 s1 and hence sR of this level is 2358 ms. The branching ratios (bR) corresponding to the emissions from an excited level jJ to its lower levels j0 J 0 are given by



0 0

bR jJ; j J

  AR jJ; j0 J 0  ¼ P  AR jJ; j0 J 0

(8)

j0 J 0

The branching ratio, bR is also a measure of the relative intensities of all emission lines originating from a given excited level. The measured branching ratios (bmes) are determined from the relative areas under the emission lines. The measured (bmes) as well as the calculated branching ratios (bR) of all the transitions originating form 4G5/2 level are given in Table 4. From the magnitude of branching ratios it is evident that the 4G5/2 / 6H7/2 and 6H9/2 transitions are more intense compared to other transitions. The peak stimulated emission cross-section (s(lp)), of an emission band having a probability AR ðjJ; j0 J 0 Þ can be expressed as



s lp

Fig. 3. Photoluminescence spectra for different concentrations of SmF3 doped in CFB glasses in the region of 500e1000 nm.

 

jJ; j0 J 0



¼

l4p   AR jJ; j0 J 0 2 8pcn Dleff

(9)

where lp is the peak wavelength and Dleff is its effective line width found by dividing the area of the emission band by its average height. The values of s(lp) presented in Table 4 indicate that the

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Fig. 5. Energy level diagram showing different emission transitions and cross-relaxation channels in CFBSm glasses.

transitions 4G5/2 / 6H7/2 and 6H9/2 possess maximum emission cross-sections. 3.5. Decay curve analysis and InokutieHirayama model Fig. 4 shows the decay curves of 4G5/2 excited level measured for different concentrations of Sm3þ ions doped CFB glasses. The measured lifetimes (smes) of 4G5/2 emitting level could be determined by taking first e-folding times of the decay curves. Theoretically smes can also be expressed as [29]

1

smes

¼

1

s

þ WNR

(10)

where smes and sR are the measured and calculated lifetimes obtained from decay curves and JeO parameters respectively. WNR is the non-radiative relaxation rate which includes multi-phonon

Table 4 Emission peak wavelengths (lp, nm), effective bandwidths (Dleff, nm), radiative transition probabilities (AR, s1), total radiative transition rate (AT, s1), peak stimulated emission cross-sections (s(lp), 1022 cm2), radiative lifetime(sR, ms) experimental branching ratios (bmes) and calculated branching ratios (bR) of 4G5/2 level in CFBSm10 glass. Transition 4G5/2/

lp

Dleff

AR

Branching ratios

bR

bmes

6

561 598 644 706 785 e 906 950 e e e e

14.63 17.31 18.37 24.96 36.95 e 37.77 29.56 e e e e

27 190 117 51 6 1 w0 7 17 6 2 w0 AT ¼ 424 s1

0.06 0.45 0.28 0.12 0.02 w0.00 w0.00 0.02 0.04 0.01 w0.00 w0.00 sR ¼ 2358 ms

0.10 0.48 0.32 0.08 0.01 0.00 0.01 0.01 e e e e

H5/2 6 H7/2 6 H9/2 6 H11/2 6 H13/2 6 F1/2 6 H15/2 6 F3/2 6 F5/2 6 F7/2 6 F9/2 6 F11/2

s(lp) 0.94 72.39 56.49 26.18 3.18 e e 9.95 e e e e

relaxation (WMP), energy transfer through cross-relaxation (WCR) and several other non-radiative processes. The quantum efficiency (h) for the 4G5/2 excited state is given by equation



smes sR

(11)

For all the concentrations except for 0.05 mol%, the decay curves are found to be non-exponential in nature. Inokuti and Hirayama [30] developed a theory to account for the energy transfer between 4f n energy levels of triply ionized lanthanides. They described the effect of energy transfer on luminescence decay by the formula

( IðtÞ ¼ I0 exp



t

s0

Q

 3=S ) t

s0

(12)

where t is the time after excitation, s0 is the intrinsic decay time of isolated donors in the absence of acceptors. Q is the energy transfer parameter which is defined as

Q ¼

  4p 3 G 1  NA R30 3 S

(13)

The magnitude of Q depends on S and the gamma function (G), which is equal to 1.77 for dipoleedipole (S ¼ 6), 1.43 for dipolequadrupole (S ¼ 8) and 1.30 for quadrupoleequadrupole (S ¼ 10) interactions respectively. NA is the acceptor concentration, which is almost equal to total concentration of Sm3þ ions and R0 is the critical transfer distance defined as the donoreacceptor separation for which the rate of energy transfer between donors and acceptors is equal to the rate of intrinsic decay s1 0 . The dipoleedipole interaction parameter, CDA describing elementary energy transfer of direct donoreacceptor interaction between Sm3þ ions at the distance R0 is given by

CDA ¼

R60

s0

(14)

J. Suresh Kumar et al. / Solid State Sciences 13 (2011) 1548e1553 Table 5 Variation of measured lifetimes (smes, ms), energy transfer parameters (Q), critical transfer distances (R0, A0), donoreacceptor interaction parameters (CDA, 1042 cm6/s), cross-relaxation rate (WCR, s1) and quantum efficiencies (h, %) with concentration (mol%) of SmF3 in CFBSm glasses. Concentration

smes

Q

0.05 0.1 0.5 1.0 2.0 4.0

2185 2183 2139 1945 1627 1308

e w0.00 0.03 0.15 0.41 0.82

R0

CDA

WCR

h

e 1.79 3.09 4.39 4.85 4.92

e 0.01 0.37 3.02 5.51 5.99

33 34 43 90 190 340

93 93 91 83 69 56

Table 5 presents the measured lifetimes (smes ) along with the energy transfer parameters (Q), critical transfer distances (R0), donoreacceptor interaction parameters (CDA), quantum efficiencies (h) and non-radiative cross-relaxation rates (WCR) for 4G5/2 level as a function of Sm3þ ion concentration in CFB glasses. Dipoleedipole interaction is prevailing among the Sm3þ ions in the nonexponential decay of the excited atoms. The energy transfer parameter (Q) increases from w0 to 0.82 with increasing of concentration from 0.1 to 4.0 mol% of Sm3þ ions. The same trend is observed in the case of critical transfer distances (R0 ¼ 1.72e4.92 and the donoreacceptor interaction parameters A0) (CDA ¼ 0.01e5.99  1042 cm6/s). The non-exponential nature of the decay curves with the increase of Sm3þ ions concentration has been attributed to the energy transfer through cross-relaxation channels among the Sm3þ ions. Fig. 5 shows the partial energy level diagram depicting the non-radiative relaxation and radiative emission from the luminescent level 4G5/2 to different terminal levels. In addition the four different cross-relaxation channels responsible for the quenching of emission intensity and also the non-exponential nature of the decay curves at higher concentrations are clearly shown in Fig. 5. The cross e relaxation channels for luminescence quenching are: (4G5/2,6H5/2) / (6F11/2,6F5/2), (6F9/2,6F7/2), (6F7/2,6F9/2) and (6F5/2,6F11/2). In every channel the two Sm3þ ions one in the ground state and another in the emitting state interchange their energies to reach some intermediate states and then relaxes non-radiatively loosing all the acquired energies. These are the only four major cross-relaxation channels which are responsible for the quenching of emission intensity. Among the four cross-relaxation channels, the (4G5/2,6H5/2) / (6F5/2,6F11/2) is

Fig. 6. Variation of experimental lifetimes (smes) and energy transfer parameter (Q) with concentration of SmF3 doped in CFB glasses.

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the resonant energy transfer channel, because both emission 4 G5/2 / 6F5/2 and absorption 6H5/2 / 6F11/2 band energies are equal. Fig. 6 shows the variation of measured lifetime (smes) and the energy transfer parameter (Q) with Sm3þ ions concentration variation. 4. Conclusions Calcium fluoroborate glasses doped with trivalent samarium were prepared. The magnitude of U2 parameters obtained from JeO theory reveals weak covalent bond among the Sm3þ ions and the host matrix. The emission spectra recorded in the region of 500e1000 nm for various concentrations of Sm3þ ions indicate the quenching of emission intensity beyond 0.5 mol%. The decay curves of 4G5/2 / 6H7/2 transition exhibit single-exponential at 0.05 mol% of Sm3þ ions and becomes non-exponential for higher concentrations (>0.1 mol%). The decrease in fluorescence intensities as well as lifetimes at higher concentrations has been attributed to the energy transfer through cross-relaxation channels between the Sm3þ ions. Acknowledgement The authors JSK and LRM are thankful for the financial support by University Grants Commission (UGC), New Delhi for sanctioning of Research Fellowship for Meritorious Students (RFSMS) and sanction of Major Research Project (MRP) No. 33-22/2007 respectively. References [1] A.G. Sowza Filho, J. Mendes Filho, F.E.A. Melo, M.C.C. Cusodio, R. Lebullenger, A.C. Hernandes, J. Phys. Chem. Solids 61 (2000) 1535e1542. [2] M.J. Weber, in: W.M. Yen, P.M. Selzer (Eds.), Second ed., Laser Spectroscopy of Solids Berlin Heidelberg, Springer-Verlag New York, 1986, pp. 189e240. [3] M. Nogami, G. Kawamura, G.J. Park, H. Yu, T. Hayakawa, J. Lumin. 114 (2005) 178e186. [4] Y. Zhang, C. Lu, L. Sun, Z. Xu, Y. Ni, Mat. Res. Bull. 44 (2009) 179e183. [5] G. Dominiak-Dzik, J. Alloy. Compd. 391 (2005) 26e32. [6] V.P. Dotsenko, N.P. Efryushina, Phys. Stat. Solidi 130 (1992) 199. [7] S. Duffy, J.P.R. Wells, H.G. Gallagher, T.P.J. Han, J. Cryst. Growth 203 (1999) 405. [8] J.-P.R. Wells, A. Sugiyama, T.P.J. Han, H.G. Gallagher, J. Lumin. 85 (1999) 91. [9] L. Boehm, R. Reisfeld, W. Sector, J. Solid State Chem. 28 (1979) 75. [10] A. Herrmann, D. Ehrt, J. Non Cryst. Solids 354 (2008) 916. [11] V.K. Rai, C.B. de Araujo, Spectro Chem. Acta A 69 (2008) 509. [12] J. Suresh Kumar, A. Mohan Babu, T. Sasikala, L. Rama Moorthy, Chem. Phys. Lett. 484 (2010) 207. [13] Y.H. Zhou, J. Lin, S.B. Wang, H.J. Zhang, Opt. Mater. 20 (2002) 13. [14] D. Boyer, G. Bertrand-Chadeyron, R. Mahiou, Opt. Mater. 26 (2004) 101. [15] S.G. Motke, S.P. Yawale, S.S. Yawale, Bull. Mater. Sci. 25 (2002) 75. [16] F.F. Sene, J.R. Martinelli, L. Gomes, J. Non Cryst. Solids 348 (2004) 63. [17] T. Suhasini, J. Suresh Kumar, T. Sasikala, K. Jang, H.S. Lee, M. Jayasimhadri, J.H. Jeong, S.S. Yi, L. Rama Moorthy, Opt. Mater. 31 (2009) 1167. [18] C.K. Jayasankar, P. Babu, J. Alloy Compd. 307 (2000) 82. [19] B.G. Wybourne, Spectroscopic Properties of Rare-Earth Ions in Crystals. Wiley, New York, 1965, pp. 35. [20] C.K. Jorgensen, Prog. Inorg. Chem. 4 (1962) 73. [21] B.R. Judd, Phys. Rev. 127 (1962) 750. [22] G.S. Ofelt, J. Chem. Phys. 37 (1962) 511. [23] W.T. Carnall, H. Crosswhite, H.M. Crosswhite, Energy Level Structure and Transition Probabilities of the Trivalent Lanthanides in LaF3 Argonne National Laboratory Report (1977). [24] R.D. Peacock, Struct. Bond. 22 (1975) 83. [25] R. Reisfeld, Struct. Bond. 22 (1975) 123. [26] K. Binnemans, R. Van Deun, C. Görller-Walrand, J.L. Adam, J. Non Cryst. Solids 238 (1998) 11. [27] R. Van Deun, K. Binnemans, C. Görller-Walrand, J.L. Adam, SPIE 3622 (1999) 175. [28] B.C. Jamalaiah, J. Suresh Kumar, A. Mohan Babu, T. Suhasini, L. Rama Moorthy, J. Lumin. 129 (2009) 363. [29] C. Gorller-Walrand, K. Binnemanns, in: K.A. GschneidnerJr., L. Eyring (Eds.), Handbook on the Physics and Chemistry of Rare Earths, vol. 25, Elsevier, North-Holland, Amsterdam, 1998 (chapter 167). [30] M. Inokuti, F. Hirayama, J. Chem. Phys. 43 (1965) 1978.