Luminescence of Eu2+ In Some Chlorides

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Physics Procedia 29 (2012) 42 – 45

Conference Title

Luminescence of Eu2+ In Some Chlorides D.H. Gahanea* N.S. Kokodea, B.M.Bahirwarb and S.V. Moharilc a

Nevjabai Hitkarini College, Bramhapuri, Distt. - Chandrapur,441206 India b Gurunanak College of Science, Balharshah, Distt. - Chandrapur, India c Department of Physics, Nagpur University, Nagpur, 440010, India Received 25 July 2011; accepted 25 August 2011

Abstract Efficient luminescence is reported for the first time in Eu2+ activated double Chlorides ABCl3. A simple wet-chemical preparation is described. Emission intensities are comparable to that of the commercial phosphor. Excitation covers near UV region. © 2012 Published by Elsevier B.V. Selection and/or peer-review under responsibility of the organizing committee © 2011 Published by C. Elsevier Ltd. and/or peer-review under responsibility of [name organizer] represented by Stephen Rand and theSelection guest editors. Keywords: chlorides; luminescence

1. Introduction Eu2+ activated phosphors find use in many applications. e.g. BaMgAl10O17:Eu and Sr5(PO4)3Cl:Eu are efficient tri-colour lamp phosphors[1]. BaFBr:Eu is used as intensifier for x-ray imaging films[2,3] as well as for x-ray imaging using photo-stimulated luminescence (PSL)[4], CaSO4:Eu is near UV emitting phosphor for photoluminescent liquid crystal displays (PLLCD)[5]. Eu2+ emission arises from the lowest band of 4f65d1 configuration to 8S7/2 state of 4f7 configuration. The excitation arises from the transition from 8S7/2 state of 4f7 configuration to the states belonging to 4f65d1 configuration. The ground state electronic configuration of Eu2+ is 4f7. This results in a 8S7/2 level for the ground state. The next f7 manifold (6Pj) lies approximately 28000 cm-1 higher. The lowest lying 4f65d levels begin near 34000 cm-1 and are labeled 8HJ for the free ion. The 4f65d levels experience much more crystal field splitting than the levels of 4f7 configuration due to the increased spatial extent of the 5d orbitals and often are the metastable state, or the lowest excited state, when the Eu2+ ion is incorporated in a crystalline host. The effect of the crystal field of octahedral symmetry on the 5d electron is to split the 5d orbitals into two components t2g and eg. For lesser symmetries, the splitting can be as much as five fold. The isotropic part of the exchange interaction between 5d and 4f electrons results in an exchange

1875-3892 © 2012 Published by Elsevier B.V. Selection and/or peer-review under responsibility of the organizing committee represented by Stephen C. Rand and the guest editors. doi:10.1016/j.phpro.2012.03.689

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splitting into states with total spins of S=7/2 and 5/2. Thus for the absorption spectra of Eu 2+ in the solids, the lowest energy band arises from the state described by the notation |4f 6(7FJ)eg, S=7/2> [6]. The lowest energy configuration corresponds to the situation where 7FJ(4f6) state couples to the 5d eg orbital such that all spins are parallel. Spectral positions of these bands vary a great deal from lattice to lattice. The most commonly observed emission is the dipole and spin allowed d-f-emission starting from the relaxed 4f6(7F0)5d1 level. Due to allowed nature of the transition, d-f emission is intense. In some cases especially in certain fluorides, position of the band corresponding to f-d transition lies above f-f levels. Line emission corresponding to 6Pj→ 8S7/2 transitions of 4f 7 configuration is then observed [7,8]. A third type of emission involving the Eu2+ ions is often characterized by a very large Stokes (5000 –10000 cm–1) shift, very broad (>4000 cm–1) emission bands, and deviating temperature behavior. This “anomalous” emission has been attributed to auto-ionization of the 5d electron to conduction band level. The electron is localized on the cations around the hole that stays behind on Eu2+; and an impurity trapped exciton state is created. The “anomalous” emission is the radiative transfer of the electron back to the ground state of Eu2+. Auto-ionization can also be a cause for absence of Eu2+ luminescence. Efficient emission of Eu2+ luminescence has been observed in alkaline earth chlorides [9]. Recently, use of CaCl2:Eu2+ phosphor for phototherapy lamps has been proposed [10]. Several solid solutions of alkaline earth halides are known [11,12,13,14]. There is no information on Eu2+ luminescence in these compounds. We observed very intense Eu2+ luminescence in these compounds. These results are reported in this paper. 2. Experimental details Samples were prepared by dissolving desired quantities of metal carbonates and Eu2O3 in HCl. Excess acid was then boiled off and the solutions were evaporated to dryness. The resulting mass was dried at 475 K for 2 h in air, crushed to fine powders (<72mm) and then annealed for 1 h at 925 K in a reducing atmosphere provided by burning charcoal so as to reduce the activator to divalent state. The annealed powders were quickly sandwiched between quartz plates and transferred to photoluminescence (PL) cell. Of about 300mg powder was used every time. Photoluminescence spectra were recorded at the room temperature in the spectral range 220–700nm on Hitachi F-4000 spectro-fluorimeter with spectral slit widths of 1.5 nm. Precision of the method was adequate. The variations in PL intensities for several trials for the same phosphor were within 5%. X-ray diffraction patterns were recorded on Philips PANalytical X’pert Pro diffractometer. 3. Results and discussion Data on PL of KMgCl3, KCaCl3 and KSrCl3 are presented in Fig 1. Weak Eu2+ emission was observed around 397 nm with the excitation at 320 nm / 268 nm (Fig 1, curves a and b). The excitation spectrum has a maximum around 268 nm and shoulder at 320 nm. Intense PL emission is observed for both KCaCl 3 and KSrCl3. For KCaCl3, maximum intensity was observed for the sample containing 1 mole% Eu and quenched from 925 K. Upon excitation with 370 nm light intense emission is observed with a maximum around 430 nm (Fig. 1, curve c). We did not find any literature results for comparison and these are probably the first observations of luminescence of Eu2+ in the ABCl3 type host. The excitation spectrum consists of several overlapping bands around 251, 352 and 370 nm (Fig. 1, curve d). Fig 1 (curves e and f) shows PL spectra of Eu 2+ activated KSrCl3. Maximum intensity was observed for the sample containing 1 mol % Eu2+ and quenched from 900 K. For 365 nm excitation, intense emission was observed with a maximum around 423 nm. The excitation curve contains several unresolved bands around 290 nm, 340 nm and 364 nm. In both KCaCl 3 and KSrCl3 excitation in near UV region is much

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more prominent. Emission intensities are comparable. To get an idea of PL intensity, PL spectra for the commercial BAM (Sylvania 2466) are also compared. (Fig.1, curves g and h). Peak height for both is more than that of BAM.

Fig. 1 :PL spectra for KMgCl3, KCaCl3 and KSrCl3. (b) KMgCl3 excitation for 397 nm emission, (a) KMgCl3 Emission for 320 nm excitation, (c) KCaCl3 Emission for 370 nm excitation,, (d) KCaCl3 excitation for 429 nm emission, (f) KSrCl3 excitation for 423 nm emission, (e) KSrCl3 Emission for 365 nm excitation, For comparison PL spectra for commercial lamp phosphor (BAM, sylvania 2466 blue) are included. (g) BAM Emission for 365 nm excitation, (h) BAM excitation for 450 nm emission,

Fig. 2

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PL spectra for CsMgCl3 CsCaCl3 and CsSrCl3.

(a) CsMgCl3 Emission for 365 nm excitation, (b) CsMgCl3 excitation for 430 nm emission, (d) CsCaCl3 excitation for 436 nm emission, (c) CsCaCl3 Emission for 380 nm excitation, (f) CsSrCl3 excitation for 429 nm emission, (e) CsSrCl3 Emission for 355 nm excitation, For comparison PL spectra for commercial lamp phosphor (BAM, sylvania 2466 blue) are included. (g) BAM Emission for 365 nm excitation, (h) BAM excitation for 450 nm emission,

CsMgCl3 has perovskite structure with hexagonal lattice, space symmetry group identical to the better known crystal CsNiCl3 [15] and has the following parameters a = b = 7.269 Ao, c = 6.187 Ao [16]. Data on PL of CsMgCl3, CsCaCl3, CsSrCl3 are presented in Fig.2. CsMgCl3 containing 0.5 mol% Eu2+, and

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quenched from 900 K shows strong violet blue emission with maximum intensity at 430 nm with shoulder at 463 nm. (Fig.2, curve a). The excitation spectrum is broad consisting of peaks at 352, 280 nm and shoulder at 258 nm. (Fig.2 curve b). CsCaCl3 has ideal perovskite structure [17,18]. Fig.2 (curves c and d) shows PL spectra of CsCaCl 3 quenched at 900K. For Eu concentration 0.5 mol% prominent blue emission is observed at 436 nm. There is a weak emission around 548 nm. The corresponding excitation spectrum is shown in the Fig. 2 (curve d). Excitation spectrum shows several unresolved bands with prominent peaks at 340, 355, 380 nm and shoulder at 254 nm. CsSrCl3 has perovskite type structure with monoclinic lattice. CsSrCl3 is isomorphous to the well known CsPbCl3. CsSrCl3 undergoes structural phase transformations at 113, 108 and 89oC [19,20]. Similar to almost all ABCl3 type chlorides, CsSrCl3 shows most intense violet-blue emission at 429 nm (Fig. 2 curve e). Excitation spectrum is similar to that for CsCaCl 3 with prominent peaks at 260, 350 and 379 nm (Fig.2, cur. f). To get an idea of the PL intensity, PL spectra for the commercial BAM (sylvania 2466 blue, BaMgAl10O17:Eu2+) are also displayed (Fig, 2, curves g and h). Peak heights for all ABCl 3 type chlorides are comparable. Emission intensities in KCaCl3, KSrCl3, CsCaCl3 and CsSrCl3 are double or more to BAM. All figures are expressed in the same relative unit. 4. Conclusions:Eu2+ emission in all the ABCl3 type chlorides reported here, except KMgCl3 is very intense. As compared to KBCl3:Eu2+ type phosphor CsBCl3 type phosphors show more intensity. Also emission intensity varies in order AMgCl3 < ACaCl3 < ASrCl3. 5. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20]

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