Electrochemical and Solid-State Letters, 8 ~1! H6-H8 ~2005!1099-0062/2004/8~1!/H6/3/$7.00 © The Electrochemical Society, Inc.

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Luminescence Properties of Eu2¿-Activated CaAl2Si2O8by Photoluminescence SpectraJoung Kyu Park,z Jae Myung Kim, Eun Suk Oh, and Chang Hae Kim

Advanced Materials Division, Korea Research Institute of Chemical Technology, Daejon 305-600, Korea

We studied the luminescence properties of Eu21 in CaAl2Si2O8 . In CaAl2Si2O8 :Eu21 phosphor, two emission bands wereobserved. Because the two emission bands of CaAl2Si2O8 :Eu21 phosphor are quite different, it can be presumed that Eu21 ionsoccupy two types of sites, namely, Eu1 and Eu2 . The energy transfer between the Eu21 ions are elucidated from the criticalconcentration quenching data based on the electric multipolar interaction of the Eu1 and Eu2 sites, respectively. In addition, thedominant multipolar interaction character of the Eu2-site emission center was investigated from the relationship between theemission intensity per activator concentration and the activator concentration.© 2004 The Electrochemical Society.@DOI: 10.1149/1.1836113# All rights reserved.

Manuscript submitted June 16, 2004; revised manuscript received July 22, 2004. Available electronically November 22, 2004.

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It has been found that when Eu21 is incorporated into a hosdifferent kinds of emission centers can be formed. This occuthere are several kinds of cations replaceable by Eu21 in the hosmatrix or if there exist several different sites for a specific catio1-3

The fluorescence of Eu21-activated compounds usually broadeand depended on the surrounding of the Eu21 ions. The Eu21 ionshave complex energy levels. The first excited 4f65d configurationlies close to the lowest excited 4f7 levels.4 The transitions betweethe first excited 4f65d configuration and the 4f7 ground state ardipole allowed.5 Moreover, the absorption and emission due totransition between 4f7 and 4f65d states of Eu21 strongly depend othe crystal field acting on Eu21.6 Consequently, the choice of homaterials is a critical parameter for determining the optical proties of the Eu21 ions.

Morris et al. reported the nature of the sites occupied by E21

ions in CaAl2Si2O8 structure by using electron paramagnetic renance~EPR! measurements.7-9 However, the optical and luminecent properties of Eu21 doped CaAl2Si2O8 are not yet well undestood. In this study, we report on interesting luminescent propeof Eu21-activated CaAl2Si2O8 .

Experimental

CaCO3 , a-Al2O3 , SiO2 , and Eu2O3 were employed as the staing materials~these materials are all analytical grade!. Preweighedpowders were mixed thoroughly in acetone in an agate mortadried at 130°C for;8 h to drive off the solvent and successivheat-treated at different temperature and duration times, followan additional grinding and firing in reduction atmosphere. Thesulting powders were identified using an X-ray diffraction~XRD!system with Cu Karadiation~Ni filter!. The excitation and emissiospectra of the fired samples were measured using a Perkin-LS-50 luminescence spectrometer with a xenon flash lamp (Dv1/2

5 10ms).

Results and Discussion

The lattice parameters of CaAl2Si2O8 samples, estimated froXRD data, in terms of Eu21 concentration are shown in Fig. 1. Ta, b, andc axis triclinic lattice parameters slightly decrease withEu21 concentration, and then increase again. The approximateof the solubility of Eu21 in CaAl2Si2O8 could be carried out. It warecognized that a slight difference in the ionic radius between21

ions resulted in a slight difference in the lattice parameter oCaAl2Si2O8 being only induced. The crystal symmetry, howedecreased with increased Eu21 concentration. Generally, the mosymmetrical structures give the least complicated diffraction

z E-mail: parkjk@krict.re.kr

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terns as the number of constituents in the crystal increasesindicated that phosphors, which are crystallized in the systemsthe highest degree of symmetry, generally have the highest lumcence efficiency. The diffraction patterns are more complicatedas formation of solid solution and symmetry as the concentratiEu21 increases. This indicates that the phosphor formulas havelimited significance with respect to the constitutions of phosphMoreover, compounds such as CaAl2Si2O8 have both layered anframework structure. In the layered structure, the Ca21 ions arelocated in the interlayers of the double tetrahedral layer consisticorner-shared SiO4 and AlO4 tetrahedra10 and are expected to echange readily through the interlayers. In other words, the Eu21 ionsin this structure can be expected to be loosely bound when21

ions are incorporated in the Ca21 sites. The immediate implicatioof this weak coupling to the lattice is easy oxidation (E21

→ Eu31) and reduction (Eu21 → Eu31), which is indeed observeexperimentally. This process would have been difficult and irreible if the Eu21 ions were to strongly coupled to the latticeoccupying the regular lattice sites. Morris reported that the E21

ions are incorporated substitutionally at the Ca21 structural siteswhich observed in the EPR spectra of CaAl2Si2O8 :Eu21.7 Thebroad-band character of the emissions, as shown in Fig. 2 aprovide evidence that europium is present in the divalent statethere is no indication that the fluorescence from the Eu31 ions, in-dicating that the Eu31 ions are completely reduced into Eu21 in thereduction atmosphere. Megawet al.,11 reported that the unusualarge positional disorder of the large cation~in this case, Ca21 ofCaAl2Si2O8 :Gd31) in feldspar structures. From the EPR dataMegawet al., to a valence of31 indicates that the strain in somethe tetrahedral angles near the Gd31 ions would be in the order8-11°. The difference in strain between incorporating Eu21 andEu31 in feldspars contributes to the observed preferential incoration of Eu in plagioclase feldspar. The Eu21 ion, being;20%larger than Ca21, presumably would not be able to assume asa range of positions at the Ca21 sites in CaAl2Si2O8 . However, thestrain in the tetrahedral bonds is absent 1° and the Ca-Olength ranges from 2.49 to 2.54° in the crystal structureCaAl2Si2O8 .11,12Consequently, CaAl2Si2O8 does not have a highstrained crystal structure and neither is positional disorder indiat the Ca21 site because of the short Ca–O bond.

The emission spectra of the CaAl2Si2O8 samples in terms oEu21 concentrations are shown in Fig. 2. As can be seen fromfigure, the emission band at;428 nm shifts slightly to a longwavelength with an increase in Eu21 concentration. Qiuet al.,13

reported that the probability of an energy transfer among Eu21 ionsincreases when the Eu21 concentration increases. A nonradiaenergy transfer from one Eu21 ion to another Eu21 ion usually occurs as a result of an exchange interaction or multipole-mult

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Electrochemical and Solid-State Letters, 8 ~1! H6-H8 ~2005! H7

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interaction. In the case of the Eu21 ion, the 4f7 → 4f65d1 transitionis allowed, while the exchange interaction is responsible forenergy transfer of forbidden transitions and typical critical distawhich is about 5 Å14. This indicates that the exchange interacplays no role in the energy transfer in the present case and willonly as a result of an electric multipolar interaction. As an increof Eu21 concentration, the distance between Eu21 ions becomeless, and the probability of energy transfer among Eu21 ions in-creases. In other words, the probability of Eu21 ions at higher levelof 5d, which make energy transfer to the lower 5d levels of E21

ions, increases with an increase of Eu21 concentration. This makespossible to shift the emission peak to the longer wavelength wiincrease of Eu21 concentration.

As Eu21 concentration increased, two emission bands wereserved at 440 and 485 nm, as shown in Fig. 3. This figure showrescaled spectra of 0.1 mol Eu21 and 0.2 mol Eu21 samples separated into two components for convenience using Gaussian funwhich enables one to know more precise peak positions otwo emission bands. Because the two emission spectrCaAl2Si2O8 :Eu21 sample is quite different, it can be presumedEu21 ions occupy two types of sites in the CaAl2Si2O8 lattice separately and form two corresponding emission center, namely,1-and Eu2-site, which peak at 440 and 485 nm, respectively. As

Figure 1. The compositional variations in thea, b, and c axis tricliniclattice parameters with respect to Eu21 concentrations in CaAl2Si2O8 .

Figure 2. The shift of the emission band to longer wavelength as increEu21 concentration under the excitation light wavelength of 365 nm.

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creased Eu21 concentration more and more, these emission palso, slightly shift to longer wavelength as shown in inset of Fi

Consequently, there is a need to know which critical distvalue originates from the following equation for energy tranbased on the critical concentration quenching15

RC 5 2F 3V

4pxCNG1/3

@1#

wherexC is critical concentration,N is the number of Z ions in uncell, andV is the volume of the unit cell~in this case, the volume1338.75 Å3). The emission intensities for Eu1-site and Eu2-site withconcentration of Eu21 are shown in Fig. 4, respectively. When Eu21

concentration is low, the Eu1-site emission prevails over the Eu2-siteemission. With increasing the concentration of Eu21, however, th

Figure 3. Two emission sites of CaAl2Si2O8 :Eu21 phosphor for variouEu21 concentration under the excitation light wavelength of 365 nm.~a! 0. 1and ~b! 0.2 mol Eu21. These spectra separated into two componentconvenience using Gaussian function.~dash-dotted line: 0.1 mol Eu21, dot-ted line: 0.2 mol Eu21) The inset of this figure shows the two bands ofmol ~solid line! and 0.4 mol~dashed-dot line! Eu21.

Figure 4. The Eu21 emission intensities as a function of Eu21 concentrationunder the excitation wavelength of 365 nm. It is indicated that the relaship between the emission intensities of Eu- and Eu-site emission.

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Electrochemical and Solid-State Letters, 8 ~1! H6-H8 ~2005!H8

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Eu2-site emission intensity constantly increases and after a cvalue~in this case, 0.1 mol!it decreases again. This value is resptively higher than the Eu1-site emission. Using the above equatthe critical distance is determined to be about 11 Å for Eu1-sitecenter and 7 Å for Eu2-site center.

As already mentioned, the energy transfer in the present casoccur by an electric multipolar interaction because the7

→ 4f65d1 transition of Eu21 ion is allowed. Consequently, therea need to know which multipolar interaction type originates fromfollowing equation for energy transfer by assuming that the nodiative losses are attributable to multipolar transfer16-18

I

x5 K@1 1 b~x!u/3#21 @2#

where,I /x is the emission intensity (I ) per activator concentratio(x), K andb is a constant for each interaction, andu is an indicationof electric multipolar character;u 5 6, 8, and 10 for dipole-dipo~d-d!, dipole-quadrupole~d-q!, and quadrupole-quadrupole~q-q! in-teractions, respectively. When we assume thatx is larger than thcritical quenching concentrationxC and the nonradiative lossesattributable to multipolar transfer andb(x)u/3 @ 1, the simplifiedequation

I

x5 K8/b~x!u/3 @3#

Figure 5. A plot of the experimental data in terms of Eu21 concentration foEu2-site in CaAl2Si2O8 according to Eq. 3.

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is obtained, whereK8 is a constant. To obtain a correctu value forEu2-site in CaAl2Si2O8 :Eu21, the logarithm of the emission intesity (I ) per activator concentration (xEu2

) is plottedvs.the logarithm

of the activator concentration (xEu2). This plot should yield

straight line with a slope equal tou/3 . The fitting result of ln(I/xEu2)

with respect to the ln(xEu2) is shown in Fig. 5. As can be seen fr

Fig. 5, the u value is determined at;6. This indicates that thdominant multipolar interaction character of the Eu2-site emissioncenter is d-d coupling in the present case.

Conclusion

We have synthesized Eu21-activated CaAl2Si2O8 phosphor aninvestigated some fundamental luminescent properties by usinUV source. The luminescence process of the CaAl2Si2O8 :Eu21

phosphor is characterized by the two corresponding emission cnamely, Eu1- and Eu2-site, which peak at 440 and 485 nm, resptively. The critical distance value of energy transfer between E21

ions was obtained experimentally using the concentration quendata. The electric multipolar character of CaAl2Si2O8 :Eu21 phos-phor obtained from the fitting results is nearly coincident withconventional value of d-d interaction.

The Korea Research Institute of Chemical Technology assisted in mthe publication costs of this article.

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