Local Site Symmetry Determination of Scheelite-Type Structures by

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J. Phys. Chem. C 2010, 114, 17905–17913

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Local Site Symmetry Determination of Scheelite-Type Structures by Eu3+ Spectroscopy Guohua Jia,*,†,‡ Cuifang Wang,† and Shiqing Xu*,† College of Materials Science and Engineering, China Jiliang UniVersity, Hangzhou 310018, P. R. China, and Department of Biology and Chemistry, City UniVersity of Hong Kong, Tat Chee AVenue, Kowloon, Hong Kong S.A.R., P. R. China ReceiVed: August 14, 2010; ReVised Manuscript ReceiVed: September 3, 2010

SrWO4:x%Eu3+,Na+ (x ) 0, 0.05, 0.5, and 5) have been prepared by the solid-state reaction method. Low temperature, high resolution emission and absorption spectroscopies have been employed to probe the local site symmetry of Eu3+ ions occupied the Sr2+ site in SrWO4 with a doping concentration up to 5% using Na+ as the charge compensation and, hence, to disapprove the formation of multicenters in the title compound containing trivalent lanthanide ions. The assignment of features in the 10 K visible emission spectra from 5 D0,1 f 7FJ (J ) 0-4) multiplet terms and absorption spectrum from the ground-state level 7F0 f 5DJ (J ) 0-2) of SrWO4:Eu3+,Na+ is consistent with the predicted number of bands for certain transitions based on the point group selection rules for the S4 site symmetry. The incorporation of Na+ together with Eu3+ into the SrWO4 lattice makes the vibronic bands associated with the 5D0 f 7FJ (J ) 0, 2, and 3) transitions shift to lower energy. The detailed spectroscopic interpretation conducted in this work serves as one of the few examples for Eu3+ accommodating the S4 site symmetry, which have only been rarely performed previously in the host of LiYF4, and also gives insight into the assignment of lanthanide spectroscopy interpreted by forced electric and magnetic dipole selection rules. Introduction The scheelite-type structured orthotungstates and orthomolybdates with a general formula ABO4 (A ) Ca, Sr, Ba, Pb, Cd; B ) W, Mo) have been extensively investigated1-10 due to their wide potential industrial applications such as scintillators,11-13 phosphors,14-21 photocatalysts,22,23 batteries,24 and solid-state lasers.25-38 These compounds crystallize in an I41/a (C14h) space group with four molecules in each crystallographic cell at ambient atmosphere (Figure 1a).39-41 The divalent A2+ and hexavalent B6+ atoms coordinate with eight and four O atoms, respectively, and both of the above sites form an S4 site symmetry.39 Figure 1b and 1c show the nearest-neighbor coordination environments of the A2+ ions. Trivalent lanthanide ions (Ln3+) provide a particularly favorable situation for substitution in A2+ sites with favorable isostructural replacement, and the same is true for Na+ and Nb5+ in the A2+ and B6+ sites, respectively.42 The ionic radii of Eu3+ in VIII coordination and Nb5+ in IV coordination differ from those of eightfold coordinated Sr2+ and fourfold coordinated W6+ by 15.4% and 12.5%,43 respectively, so that Eu3+ is expected to substitute predominantly at the Sr2+ sites in the presence of Na+. Various charge compensation mechanisms such as interstitial ions, vacancies, electron trapping, coupled substitutions, pairing of Ln3+, etc., have been proposed and discussed in the literatures to explain the substitution of Ln3+ in the presence or in the absence of charge compensation of Na+ or Nb5+ when Ln3+ incorporates into various scheelite structured hosts. Nassau and Lolacono42 inferred the formation of cationic vacancy in CaWO4:Nd3+,Na+ by paramagnetic resonance results. Nassau44 developed a theory for the coupled substitution of Nd3+ and Na+ in Ca2+ sites in CaWO4 scheelite * To whom correspondence should be addressed. E-mail: ghjia@ hotmail.com (G.J.); [email protected] (S.X.). † China Jiliang University. ‡ City University of Hong Kong.

lattice and concluded that it is possible to balance the substitution of one Na+ for each Nd3+ and therefore minimize the vacancies in the host lattice. Kedzie and Kestigian45 concluded that Nd3+ may also occupy the W6+ site in CaWO4 with or without adding Na+ for charge compensation. More recently, a new type of mechanism, namely, the interstitial oxygen ions, has been proposed.26,46-48 The incorporation of lanthanide ions into the scheelite structured hosts most probably leads to the presence of multicenters, which has been schematically investigated by both electron paramagnetic resonance spectra42,45 and optical spectroscopy.26,47,49 Although the charge compensation mechanism in Ln3+ incorporated scheelite structured hosts has been extensively investigated, there are still ambiguities concerning the influence of charge compensators on the optical spectroscopy of the multicenter of Ln3+. The spectroscopic investigations are generally conducted by doping Nd3+ either at room temperature or with a low spectral resolution. It is much more simple and convenient to combine Eu3+ optical spectroscopy with the scheelite structure hosts to probe whether the multicenters exist since Nd3+ has a 4f3 configuration, and, hence, the corresponding electronic spectra are very complicated.50,51 By combining the spectroscopy of Eu3+ ions with the selection rules for induced electric dipole (ED) and magnetic dipole (MD) transitions,52 it is possible to determine the local site symmetry of lanthanide ions in crystal lattices and the point groups of the molecules, which may serve as an alternative to the crystallographic method.53 As far as the S4 point group of Ln3+ sites in crystalline host matrices is concerned, there are only a few hosts, e.g., LiYF4, LiLuF4, and ABO4 (A ) Ca, Sr, Ba, Pb, Cd; B ) W, Mo) scheelite structures.54 Spectroscopic assignments to the energy levels of LiYF4:Eu3+ are made on the basis of the observed energies, polarization characteristics of the spectral lines, and electric and magnetic dipole selection rules relevant to the S4 site symmetry, and the simulations of the crystal field energy levels have been previously attempted.55-58

10.1021/jp1077054  2010 American Chemical Society Published on Web 09/22/2010

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Figure 1. (a) Unit cell of SrWO4. (b) Local coordination geometry environment of Sr2+ (S4 axis is marked, and the effect of a 90° rotation about this axis is evident). (c) Top-view of (b).

More recently, we have parametrized the second rank of crystal field strengths of Eu3+ accommodated on a distorted S4 site in europium nicotinate and isonicotinate complexes with some approximations.59 To the best of our knowledge, low temperature, high resolution spectroscopy of Eu3+ accommodating the A2+ site in scheelite tungstate or molybdate host matrices has not been investigated, and the electric and magnetic dipole selection rules relevant to this site symmetry have not been verified. This paper has three aims. The first one is to employ low temperature, high resolution site selective spectroscopy to clarify where multicenters of Eu3+ in SrWO4:Eu3+,Na+ exist or not. The second aim is to verify the electric and magnetic dipole selection rules relevant to the S4 site symmetry and to determine the relative energy levels of multiplets of Eu3+ in this host. The last one is to combine the Eu3+ spectroscopy and selection rules to tell the site symmetry of a certain ion in crystal lattice and, hence, to elucidate the crystal structure of the hosts. The low temperature, high resolution visible emission and absorption spectra of SrWO4:Eu3+,Na+ have therefore been recorded, and the selection rules relevant to the S4 site symmetry have been employed to interpret the spectra. The partial energy level scheme of Eu3+ has been derived from the luminescence and absorption spectra. The room temperature diffuse reflection, emission, and excitation spectra of neat and different concentrations (0.05%, 0.5%, and 5%) of Eu3+ and Na+ codoped SrWO4 together with the lifetime of 5D0 of Eu3+ have been measured and compared. Experimental Section The powder samples, SrWO4:x%Eu3+ (x ) 0, 0.05, 0.5, 5) with Na+ as the charge compensation, have been prepared using the solid-state reaction method. SrCl2 (99.99%, Aldrich) and/ or Eu2O3 (99.999%, Aldrich) were/was dissolved in concentrated HCl (Riedel-de Hae¨n, 37%, analytical reagent) with heating and stirring in a beaker. After complete dissolution, a Na2WO4 · 2H2O solution was added into the above solution and a white precipitate was obtained. The precipitate was filtered, washed

with deionized water, and finally dried at 1200 °C for 12 h. The nominal dopant ion molar ratios of Eu3+ and Na+ to Sr2+ employed in the experiment were 0%, 0.05%, 0.5%, and 5%. The X-ray powder diffraction (XRD) pattern was obtained by a Siemens D500 diffractometer using Cu KR radiation (λ ) 1.540 56 Å) with a scanning rate of 1.2° min-1 in the angle range from 10° to 80°. The room temperature Raman spectrum was recorded by a Perkin-Elmer Spectrum 2000 spectrometer using a resolution of 4 cm-1. The diffuse reflection spectrum in pellet form was recorded between 180 and 600 nm at a resolution of 4 cm-1 at room temperature using a Perkin-Elmer PE 1600 spectrometer. High resolution emission spectra were recorded by using a tunable Panther OPO system pumped by the third harmonic of a Surelite Nd:YAG pulsed laser. The signal was collected at 90° with an Acton 0.5 m monochromator, having gratings with 1800, 1200, or 600 groove mm-1, blazed at 250, 500, or 750 nm, respectively, and a back-illuminated SpectruMM CCD detector. The powder sample was housed in an Oxford Instruments closed cycle cryostat, with base temperature 10 K, for all the measurements. The setup of the absorption spectral measurements using compressed discs was the same as that employed for high resolution emission, except that a tungsten lamp was employed as the light source. The samples did not exhibit any characteristics of a phase transition on cooling to 10 K so that the Eu3+ site symmetry is taken as unchanged. Room temperature emission and excitation spectra were measured by a Horiba Jobin Yvon Fluorolog spectrofluorometer using a xenon lamp as the light source and a TBX-04-A single-photon detection module. For lifetime measurements, the emission was analyzed with a 0.25 m Jobin-Yvon monochromator and the signal detected by a Hamamatsu R636 photomultiplier. Results and Discussion XRD. Figure 2 shows the XRD pattern of neat SrWO4 powder sample together with the standard pattern as the reference. The peaks in the neat powder sample are in good agreement with those in the standard pattern. No extra peak is observed. It is

Site Symmetry Determination by Eu3+ Spectroscopy

J. Phys. Chem. C, Vol. 114, No. 41, 2010 17907 TABLE 2: Number of Bands in Transitions from 5D0,1 to 7 FJ (J ) 0-4) and from the Ground State 7F0 to 5DJ (J ) 0-2) for S4 Site Symmetries of Eu3+ a number of bands in the 5D0,1 f 7FJ transition original state 5 5

D1

D0(Γ1) Γ1 Γ3,4

J)0

J)1

J)2

J)3

J)4

0 (ED) 1 (MD) 1 (MD)

2 (MD) 1 (ED) 2 (ED)

3 (ED) 3 (ED) 4 (ED)

4 (ED) 4 (ED) 5 (ED)

4 (ED) 4 (ED) 7 (ED)

number of bands in the 7F0 f 5DJ transition ground state 7

F0(Γ1)

J)0

J)1

J)2

0 (ED)

2 (MD)

3 (ED)

a

Figure 2. XRD pattern of neat SrWO4 powder sample with standard file. The standard data of SrWO4 (JCPDS 89-2568) is given for reference.

TABLE 1: Forced ED and MD Selection Rules for S4 Site Symmetry in Koster’s Notation62,a S4

Γ1(A)

Γ2(B)

Γ3,4(E)

Γ1(A) Γ2(B) Γ3,4(E)

MD ED ED + MD

ED MD ED + MD

ED + MD ED + MD ED + MD

a

ED, electric dipole transition; MD, magnetic dipole transition.

conclusive from our X-ray powder pattern that the X-ray pattern corresponds to the I41/a space group. Selection Rules for Transition. A sophisticated theory for the selection rules and parametrization of electric dipole and vibronic intensity using group theory and the local-field effect on transition intensities has been presented by Duan and co-workers.52,60,61 For this study we are more concerned with the detailed case of Eu3+ with S4 site symmetry. Go¨rller-Walrand and co-workers55,57 and Bihari58 have worked out the selection rules of electric and magnetic dipole transitions relevant to the S4 site symmetry in bulk crystal LiYF4:Eu3+. The assignments in the above literature are based on the polarization characteristics (R, σ, and π spectra) of the transitions since the size of the crystal is big enough to be oriented. In the present work, the measurements of polarized emission and absorption spectra of the powder SrWO4:Eu3+,Na+ samples are not possible so that the selection rules summarized in Table 1 only describe the characteristics of the transitions whether they are ED/MD allowed, without considering the characteristics of polarization. It enables the prediction of the nature of the transitions in Ln3+ doped samples when the samples are not good and/or large enough to perform polarized spectroscopic measurements. To easily understand the emission and absorption spectra of Eu3+ in SrWO4:Eu3+,Na+, the numbers of spectral features expected for some emission transitions from different original states, e.g., 5 D1 and 5D0, to 7FJ multiplet terms and those for some absorption transitions from the ground-state level 7F0 to 5DJ (J ) 0-2) are described in Table 2 for Eu3+ ions accommodated on the S4 site symmetry. Herein Koster’s62 notation was employed to describe the Stark sublevels of SLJ of Eu3+ according to J value. The 5D0 (J ) 0) multiplet term is nondegenerate so that the corresponding multiplet term can only be Γ1. The stark splitting of the 5D1 (J ) 1) level occurs at Γ1 and Γ3,4 in S4. The relative energy for Γ1 and Γ3,4 of 5D1 (J ) 1) multiplet terms can be determined by counting the numbers of the bands for the

ED, electric dipole transition; MD, magnetic dipole transition. Note that only ED allowed transition is considered when a certain transition is both ED and MD allowed since the intensity of ED allowed transition is generally three magnitudes stronger that of MD.

transitions to 7FJ multiplet terms by referring to Table 2, without performing the polarized spectral measurements. 10 K High Resolution Emission Spectra of SWO4: Eu3+,Na+. The 10 K survey emission spectra of SrWO4:5%Eu3+ with Na+ as the charge compensation under 465 nm excitation are shown in Figure 3a and Figure 3b. The emission spectra are predominated by the transitions from 5D1,0, and no transition from 5D2 is observed due to the fast nonradiative relaxation process from 5D2. The integrated intensity of transitions from 5 D1 is at least two magnitudes weaker than that of transitions from 5D0. The strongest red emission bands centered at 16 341 cm-1 (611.96 nm) and 16 246 cm-1 (615.54 nm) can be unambiguously attributed to the 5D0 f 7F2 electric dipole allowed transitions since the site symmetry of Sr2+ (S4) in SrWO4:Eu3+,Na+ lacks a center of inversion. The peak 1 at 18 998 cm-1 corresponds to the magnetic dipole transition 5D1 (Γ1 or Γ3,4) f 7F0 (Γ1). It is not conclusive whether peak 1 is associated with Γ1 f Γ1 or Γ3,4 f Γ1 since both of them are magnetic dipole allowed and the final state 7F0 (Γ1) is nondegenerate. However, the numbers of bands of the emission transitions from the same initial state to the degenerated final states, e.g., 7F1, 7F2, 7F3, and 7F4, may help us to ascertain which is the initial state of 5D1 (Γ1 or Γ3,4) based on the forced electric and magnetic selection rules summarized in Table 2. As shown in Figure 3, the transitions from 5D1 to 7F1, 7F2, 7F3, and 7F4 contain two (peak 2 at 18 671 cm-1, peak 3 at 18 538 cm-1), four (peak 4 at 18 092 cm-1, peak 5 at 17 997 cm-1, peak 6 at 17 871 cm-1, peak 7 at 17 817 cm-1), five (peak 9 at 17 145 cm-1, peak 10 at 17 122 cm-1, peak 11 at 17 078 cm-1, peak 12 at 17 044 cm-1, peak 13 at 16 979 cm-1) and seven (peak 20 at 16 326 cm-1, peak 21 at 16 160 cm-1, peak 22 at 16 154 cm-1, peak 23 at 16 124 cm-1, peak 24 at 16 037 cm-1, peak 25 at 16 019 cm-1, peak 26 at 15 952 cm-1) bands, respectively. With referring to Table 2, the number of bands experimentally observed is in good agreement with that which has been predicted in the 5D1 (Γ3,4) f 7FJ (J ) 1 - 4) transitions, and no extra band is detected. This result enables the conclusion that the energy of 5D1 (Γ3,4) is lower than that of 5D1 (Γ1) since the latter is not populated at 10 K, and the forced electric and the magnetic dipole selections for Eu3+ occupying an S4 site symmetry in SrWO4:Eu3+,Na+ are strictly obeyed. Now turning to the emission spectra from 5D0 to 7FJ (J ) 1-4): only one zero-phonon line of 5D0 f 7F0 located at 17 249 cm-1 (peak 8) is observed under the 465 nm excitation so that the emission in Figure 3 originates from the Eu3+ ions accommodating a unique site only. The transitions from 5D0 to

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Figure 3. Survey 10 K emission spectra of SrWO4:5%Eu3+,Na+ using 465 nm excitation (a) from 520 to 710 nm and (b) in difference spectral ranges. The inset shows an intensity scale expansion. Refer to the text for band numbering. 7

F1 (two bands, peak 14 and peak 15), 7F3 (four bands, peak 31, peak 32, peak 33, and peak 34), and 7F4 (four bands, peak 37, peak 38, peak 39, and peak 40) do not overlap other transitions and vibronic bands, and therefore, they can be easily assigned. The Stark splitting of the 7F2 multiplet results in Γ1, 2Γ2, and Γ3,4 in S4. Only transition from 5D0 (Γ1) to 2Γ2 and Γ3,4 are electric dipole allowed whereas the transition to Γ1 is forbidden. Two bands (peak 18 and peak 19) can be unambiguously attributed to the 5D0 (Γ1) to Γ2 and Γ3,4 transitions. However, it is difficult to assign the third band since it overlapped or superposed on the transitions of 5D1 f 7F4 multiplet terms. In this case, the 5D1 f 7F2 (aΓ2, Γ3,4, bΓ2) help

us to assign the remaining peak providing their energy differences from 5D0 (Γ1), which are 908, 1002, and 1125 cm-1, respectively, are known. Detained assignments of the above transitions are summarized in Table 3. The results obtained from the emission of 5D0 to 7FJ (J ) 1-4) verified the conclusions derived from the emission of 5D1 to 7FJ (J ) 1-4). Vibronic Spectra Associated with Electric Dipole Transitions. The energies of phonons in SrWO4 can be obtained from the measurements of the infrared spectra and Raman scattering spectra.10,63 The measured Raman scattering spectrum of neat SrWO4 powder sample in Figure 4a is in good agreement with those published in the above literature. Figures 4 b-e show

Site Symmetry Determination by Eu3+ Spectroscopy

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TABLE 3: Assignments for Observed Emission from Eu3+ in SrWO4:5%Eu3+ in Figure 2a peak no. energy (cm-1) 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23

18998 18671 18538 18092 17997 17871 17817 17249 17145 17122 17078 17044 16979 16921 16786 16494 16477 16341 16247 16326 16160 16154 16124

24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40

16037 16019 15952 15589 15569 15492 15474 15395 15373 15296 15232 14474 14462 14422 14372 14286 14252

assignment

∆E (cm-1)

D1(Γ3,4) f 7F0(Γ1) D1(Γ3,4) f 7F1(Γ3,4) 5 D1(Γ3,4) f 7F1(Γ1) 5 D1(Γ3,4) f 7F2(aΓ2) 5 D1(Γ3,4) f 7F2(Γ3,4) 5 D1(Γ3,4) f 7F2(bΓ2) 5 D1(Γ3,4) f 7F2(Γ1) 5 D0(Γ1) f 7F0(Γ1) 5 D1(Γ3,4) f 7F3(aΓ2) 5 D1(Γ3,4) f 7F3(aΓ3,4) 5 D1(Γ3,4) f 7F3(Γ1) 5 D1(Γ3,4) f 7F3(bΓ2) 5 D1(Γ3,4) f 7F3(bΓ3,4) 5 D0(Γ1) f 7F1(Γ3,4) 5 D0(Γ1) f 7F1(Γ1) 5 D0(Γ1) f 7F0(Γ1) + vib. 5 D0(Γ1) f 7F0(Γ1) + vib. 5 D0(Γ1) f 7F2(aΓ2) 5 D0(Γ1) f 7F2(Γ3,4) 5 D1(Γ3,4) f 7F4(aΓ1) 5 D1(Γ3,4) f 7F4(aΓ3,4) 5 D1(Γ3,4) f 7F4(bΓ1) 5 D0(Γ1) f 7F2(bΓ2) 5 D1(Γ3,4) f 7F4(aΓ2) 5 D1(Γ3,4) f 7F4(bΓ2) 5 D1(Γ3,4) f 7F4(bΓ3,4) 5 D1(Γ3,4) f 7F4(cΓ1) 5 D0(Γ1) f 7F2(aΓ2) + vib. 5 D0(Γ1) f 7F2(aΓ2) + vib. 5 D0(Γ1) f 7F2(Γ3,4) + vib. 5 D0(Γ1) f 7F2(Γ3,4) + vib. 5 D0(Γ1) f 7F3(aΓ2) 5 D0(Γ1) f 7F3(aΓ3,4) 5 D0(Γ1) f 7F3(bΓ2) 5 D0(Γ1) f 7F3(bΓ3,4) 5 D0(Γ1) f 7F3(bΓ3,4) + vib. 5 D0(Γ1) f 7F3(bΓ3,4) + vib. 5 D0(Γ1) f 7F4(aΓ3,4) 5 D0(Γ1) f 7F4(aΓ2) 5 D0(Γ1) f 7F4(bΓ2) 5 D0(Γ1) f 7F4(bΓ3,4)

0 327 460 906 1001 1127 1181 0 1853 1876 1920 1954 2019 332 463 755 772 908 1002 2672 2838 2844 1125 2874 2961 2979 3046 752 772 755 773 1854 1876 1953 2017 758 770 2827 2877 2963 2997

5 5

a See Figure 2 for numbering of peaks. ∆E is the energy of final multiplet except for the electronic + vibronic transition.

the vibronic spectra associated with the electric transitions from 5 D0 to 7FJ (J ) 0, 2, and 3) multiplets of Eu3+ in SrWO4: Eu3+,Na+ observed in the emission spectra at 10 K. Vibronic transitions with fine structures associated with 7F0 f 5D0,1,2 of Eu3+ in CaWO4:Eu3+,Na+ were carefully investigated by Yamada and Shionoya.64 They found that the intensity of the vibronic transition (magnetic dipole allowed) associated with the 7F0 f 5D1 transition is weaker by 1 order of magnitude than those (electric dipole allowed) associated with the 7F0 f 5 D0,2 transitions, and the results are well interpreted by the use of Judd and Ofelt’s theory.65,66 In our experiment, the vibronic spectra associated with the transitions from 5D0 to 7F1 are not observed due to their low intensity of the magnetic dipole allowed transitions while those electric dipole allowed vibronic transitions associated with the 5D0 f 7FJ (J ) 0, 2, and 3) transitions are detected: bands 16 and 17 are associated with 5 D0 (Γ1) f 7F0 (Γ1), bands 27 and 28 are associated with 5D0 (Γ1) f 7F2 (aΓ2), bands 29 and 30 are associated with 5D0 (Γ1) f 7F2 (Γ3,4), and bands 35 and 36 are associated with 5D0 (Γ1) f 7F3 (bΓ3,4). The vibronic spectra associated with the electric transitions and Raman scattering spectrum are compared in Figure 4.

Figure 4. Vibronic spectra associated with the (b) 5D0 (Γ1) f 5D0 (Γ1), (c) 5D0 (Γ1) f 5D2 (aΓ2), (d) 5D0 (Γ1) f 5D2 (Γ3,4), and (e) 5D0 (Γ1) f 5D2 (bΓ3,4) transitions of Eu3+ in SrWO4:Eu3+,Na+. The Raman spectrum of a neat SrWO4 (a) powder sample at the bottom is given for comparison. The dashed lines denote the vibronic bands originating from the 5D0 f5DJ (J ) 0, 2, 3) electronic transitions at 755 and 772 cm-1, respectively.

One can see from Figure 4 that the observed energies of the vibronic phonons derived from the emission spectra do not coincide with the energies of phonons in neat SrWO4. This is rather strange since the derived vibronic bands shift to lower energies at 755 and 772 cm-1 (marked by the dashed line in Figure 4). The shift of vibronic bands to lower energies might be ascribed to the pseudolocalized vibrations, which is similar to those found in the vibronic bands associated with the 5D0 f 7 F0 transition of Sm2+ in KBr67 and also of Eu3+ in CaWO4: Eu3+,Na+.64 It is well-known that tungsten atoms strongly couple with oxygen atoms in the WO42- group in crystals.68 The energy shifts of the vibronic spectra associated with the 5D0 f 7FJ (J ) 0, 2, 3) transitions in the present case can be attributed to the lattice distortion induced by the substitution of one Eu3+-Na+ pair of two Sr2+ ions in SrWO4:Eu3+,Na+.64 Excitation Wavelength and Concentration Dependence of Emission Spectra of SrWO4:Eu3+,Na+ Some previous reports concluded that multicenters probably present in Ln3+ incorporated scheelite structured hosts.26,42,45,47,49 In another report, symmetry reduction of Eu3+ in SrWO4 has been observed by Rivera-Lopez and co-worker69 due to the phase transition induced by pressure. More recently, Su et al.19 have investigated the site symmetry of Eu3+ in CaWO4:Eu3+,Na+, but the luminescence spectra were only measured at room temperature with a low resolution. In view of the distortion of the local site symmetry of the Sr2+ site substituted by Eu3+ and Na+, samples with various concentrations of Eu3+ and Na+ (0.05%, 0.5%, and 5%) were prepared and subjected to spectroscopic investigations. 10 K high resolution spectra were recorded, and different excitation lines were employed to verify whether the multicenters of Eu3+ ions form in SrWO4:Eu3+,Na+ with the Eu3+ concentration up to 5% (Figure 5). One can tell how many multicenters (Eu3+ sites) are present by counting the number of the 5D0 f 7F0 zero-phonon line. The 10 K high resolution emission spectra of SrWO4:5%Eu3+,Na+ and SrWO4: 0.05%Eu3+,Na+ excited by different laser lines are compared in Figure 5. First, we focus upon the emission spectra of SrWO4: 5%Eu3+,Na+ in Figure 5a. The 465, 464.8, and 464.7, and 526.2 nm are expected to excite into 5D2 and 5D1 multiplets,

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Figure 5. Excitation wavelength and concentration dependence of 10 K high resolution emission spectra of (a) SrWO4:5%Eu3+,Na+ and (b) SrWO4:0.05%Eu3+,Na+. The excitation wavelengths in nm (1, 454.7; 2, 464.8; 3, 465; 4, 526.2) are marked on each spectrum and on the absorption spectra in Figure 6b and c.

respectively. All the spectra are predominated by the transitions from 5D0 to 7FJ (J ) 1-4). The location of the 5D0 f 7F0 zerophonon lines (17 249 cm-1, 579.74 nm) are identical for samples with different Eu3+ concentrations (5% and 0.05%) and identical also when using different excitation lines. The independence of the 5D0 f 7F0 zero-phonon line enables us to conclude that Eu3+ ions only occupy one distinct site in SrWO4:Eu3+,Na+ even when the concentration of Eu3+ has been increased to 5%, and therefore, no multicenters have been found in this host codoped with Eu3+ and Na+. The emission features in Figure 5a obtained under 465, 464.7, and 526.2 nm excitation are similar to each other. However, the 464.8 nm excitation leads to extra bands in the 5D0 f 7F1,3,4 for the sample of SrWO4:5%Eu3+,Na+. This is may be due to the minor decrease of S4 site symmetry induced by the distortion of Eu-O polyhedra in SrWO4:5%Eu3+,Na+. It should be noted that the S4 site symmetry of Sr2+ in SrWO4 substituted by Eu-O polyhedra is only subjected to minor distortion even when the Eu3+ concentration is increased to 5%. In order to further elucidate the excitation wavelength dependence of emission spectra of SrWO4:Eu3+,Na+, a dilute

Eu3+ doped sample with a nominal concentration of 0.05% was prepared and subjected to spectral investigations, as shown in Figure 5b. Compared with the spectra of SrWO4:5%Eu3+,Na+ in Figure 5a, no extra bands were observed in the spectra of SrWO4:0.05%Eu3+,Na+, and the emission spectra obtained by different excitation laser lines are identical. Combining this result with the observed bands for the 5D0,1 f 7FJ transitions discussed in the previous sections, it is fairly reasonable to conclude that Eu3+ ions accommodate the Sr2+ site with a unique ideal S4 local site symmetry in dilute SrWO4:Eu3+,Na+ and no multicenters were observed. 10 K High Resolution Absorption Spectra of SWO4: Eu3+,Na+. To strengthen the interpretation given above, the 10 K and room temperature visible electronic absorption spectra of SrWO4:5%Eu3+,Na+ was recorded (Figure 6) and consistently interpreted. Trace impurities or defect sites exhibit only minor intensity since the absorption spectrum monitors the bulk sample.51 Absorption takes place from the ground-state level of 7F0 (Γ1) and for the transitions to 5DJ (J ) 0-2) and 5L6 at 10 K (Figure 6). The Stark splittings of the 5D1 and 5D2 multiplet

Site Symmetry Determination by Eu3+ Spectroscopy

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Figure 6. Ten kelvin and room temperature electronic absorption spectra of Eu3+ from 7F0 (Γ1) to (a) 5L6, (b) 5D2 (aΓ2, Γ3,4, bΓ2,), (c) 5D1 (Γ1, Γ3,4), and 5D0 (Γ1) multiplets of a powder sample SrWO4:5%Eu3+. The ordinate is absorption (arbitrary unit). The excitation wavelengths utilized in Figure 5 (labeled 1-4) are marked in Figure 6b and c.

Figure 7. Emission, excitation spectra of (b) neat SrWO4, (c) SrWO4:0.05%Eu3+,Na+, (d) SrWO4:0.5%Eu3+,Na+, (e) SrWO4:5%Eu3+,Na+, and (f) lifetime decay curve of SrWO4:x%Eu3+,Na+ (x ) 0, 0.05%, 0.5%, and 5%) powder samples at room temperature. The room temperature diffuse reflection spectrum of a neat SrWO4 powder sample (a) is given for comparison. The excitation spectra of neat and Eu3+ doped SrWO4 were measured by monitoring 417 and 615 nm, respectively, whereas the emission spectra were measured under 258 and 464 nm excitation, respectively. Lifetime decay curves were obtained by monitoring the 615 nm emission under 464 nm excitation.

terms result in Γ1, Γ3,4, and Γ1, 2 × Γ2, Γ3,4, respectively, in S4 site symmetry. Absorption transitions from the ground-state level 7 F0 (Γ1) to 5D1 (Γ1, Γ3,4) are magnetic dipole allowed transitions, while those to 5D2 are electric dipole allowed transitions. Note that the transition from 7F0 (Γ1) to 5D0 (Γ1) is both electric and magnetic dipole forbidden, which is not expected to be observed in the absorption spectrum. These considerations are exemplified by the observation of two and three bands for 7F0 (Γ1) f 5D1 (Γ1, Γ3,4) (Figure 6c) and 7F0 (Γ1) f 5D2 (aΓ2, bΓ2 Γ3,4) (Figure 6b), respectively, whereas only one weak band can be inferred

in the transition 7F0 (Γ1) f 5D0 (Γ1) (both electric and magnetic dipole forbidden) in Figure 7d (as marked in the figure). The observed numbers of bands for each transition are in good agreement with the predicted number of bands for the absorption 7 F0 f 5DJ transition in Table 2. The experimental energy levels of Eu3+ in SrWO4:Eu3+,Na+ derived from the interpretation of the emission and absorption spectra are summarized in Table 4. Comparison of Diffuse Reflection, Emission, and Excitation Spectra. The room temperature emission and excitation spectra of neat and Eu3+ doped (0.05%, 0.5%, and 5%) SrWO4

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TABLE 4: Experimental Energy Levels of Eu3+ in SrWO4 energy (cm-1)

multiplet 7 7

F0 F1

7

F2

7

F3

7

F4

5 5

D0 D1

5

D2

Γ1 Γ3,4 Γ1 aΓ2 Γ3,4 bΓ2 Γ1 aΓ2 aΓ3,4 Γ1 bΓ2 bΓ3,4 aΓ1 aΓ3,4 bΓ1 aΓ2 bΓ2 bΓ3,4 cΓ1 Γ1 Γ3,4 Γ1 aΓ2 Γ3,4 bΓ2 Γ1

0 332 463 908 1002 1125 1181 1854 1876 1920 1953 2017 2672 2827 2844 2877 2963 2977 3046 17251 19002 19030 21447 21506 21525 -

using Na+ as charge compensation powder samples together with the diffuse reflection spectrum of neat SrWO4 are shown in Figure 7. The absorption in the diffuse reflection spectrum of neat SrWO4 powder sample corresponds to the absorption of the WO42- group and that of the host. Under 258 nm excitation, the neat SrWO4 sample presents a broad blue emission in the range from 330 to 580 nm with the maximum at 417 nm, which is associated with WO42- luminescence.70 The WO42- luminescence was almost completely quenched with the incorporation of Eu3+, and the doped samples present the characteristics of Eu3+ luminescence with the red emission 5D0 f 7F2 being dominant. The energies of the broad absorption band in the excitation spectra of Eu3+ doped samples are much lower than that of the neat sample, and there is only a small overlap between them. Besides the absorption bands of the WO42- group corresponding to energy transfer from the WO42group to Eu3+, the Eu-O charge transfer also contributes to the broad absorption bands of the excitation spectra of Eu3+ doped samples, which make the maximum absorption shift to lower energy. The emission intensity of sample containing 5% Eu3+ is about two magnitudes stronger than that of sample containing 0.05% Eu3+ with the same experimental setup, which has provided evidence on the increase of the dopant concentrations of Eu3+ of these samples. The lifetimes of 5D0 of Eu3+ with different concentrations (0.05%, 0.5%, and 5%) compensated by Na+ are perfectly monoexponential and invariant with the 454 nm excitation, which serves to indicate the concentration independence of the lifetime of 5D0 of Eu3+ in the samples. The measured room temperature lifetime values in milliseconds are 0.05%Eu3+ 0.6653 ( 0.0008, 0.5%Eu3+ 0.6595 ( 0.0008, and 5%Eu3+ 0.6583 ( 0.0008. This shows that the quenching concentration for Eu3+ in this host is much higher than 5%.71 Conclusions There are several investigations on the formation of muliticenters of Ln3+, e.g., Nd3+, in scheelite structured tungsten previously conducted by both electron paramagnetic resonance

Jia et al. and optical spectroscopies.26,42,45,47,49 Compared with Nd3+, the Eu3+ spectroscopy is more determinative on the structure probe,72 and therefore it was employed to determine the local site symmetry of Eu3+ accommodated on the Sr2+ site in SrWO4: Eu3+,Na+, and accordingly to discriminate whether the muliticenters exist in scheelite structured tungstates containing Ln3+. The invariability of the location of the 5D0 f 7F0 zero phonon line under different excitation wavelengths has proved that only a single site of Eu3+ is present in the title compound charge compensated by Na+. In view of the distortion of the local site symmetry of the Sr2+ site substituted by Eu3+ and Na+, samples containing different concentrations of Eu3+ and Na+ (0.05%, 0.5%, and 5%) were prepared and subjected to spectroscopic investigations. A comparison of the excitation wavelength and concentration dependence of the spectroscopy of above samples clearly indicates that the local site symmetry of Eu3+ in SrWO4 compensated by Na+ with a concentration of 5% is subjected to minor distortion, whereas Eu3+ occupies a Sr2+ site with the perfect S4 site symmetry in the sample containing 0.05% Eu3+. The numbers of bands observed for the emission transitions from 5 D0,1 to 7FJ (J ) 0-4) and those for the absorption transitions from 7F0 to 5DJ (J ) 0-2) are in accordance with those worked out by the point selection rules for the S4 site symmetry, and therefore, the forced electric and magnetic dipole selection rules are strictly obeyed. Acknowledgment. This research is supported by the Project of the National Nature Science Foundation of China (Grant Nos.11004177, 51072190, and 50772102), Program for New Century Excellent Talents in University (Grant No. NCET-070786), and the Nature Science Foundation of Zhejiang Province (Grant No. Z4100030). G.J. is indebted to Prof. Peter A. Tanner for allowing the use of equipment at City University of Hong Kong. References and Notes (1) Chen, D.; Ye, J. H. AdV. Funct. Mater. 2008, 18, 1922. (2) Daturi, M.; Busca, G.; Borel, M. M.; Leclaire, A.; Piaggio, P. J. Phys. Chem. B 1997, 101, 4358. (3) Fujita, M.; Itoh, M.; Katagiri, T.; Iri, D.; Kitaura, M.; Mikhailik, V. B. Phys. ReV. B 2008, 77, 155118. (4) Itoh, M.; Fujita, M. Phys. ReV. B 2000, 62, 12825. (5) Abraham, Y.; Holzwarth, N. A. W.; Williams, R. T. Phys. ReV. B 2000, 62, 1733. (6) Fujita, M.; Itoh, M.; Horimoto, M.; Yokata, H. Phys. ReV. B 2002, 65, 195105. (7) Stoltzfus, M. W.; Woodward, P. M.; Seshadri, R.; Klepeis, J.-H.; Bursten, B. Inorg. Chem. 2007, 46, 3839. (8) Kuti, L. M.; Bhella, S. S.; Thangadurai, V. Inorg. Chem. 2009, 48, 6804. (9) Sun, L. N.; Guo, Q. R.; Wu, X. L.; Luo, S. J.; Pan, W. L.; Huang, K. L.; Lu, J. F.; Ren, L.; Cao, M. H.; Hu, C. W. J. Phys. Chem. C 2007, 111, 532. (10) Ling, Z. C.; Xia, H. R.; Ran, D. G.; Liu, F. Q.; Sun, S. Q.; Fan, J. D.; Zhang, H. J.; Wang, J. Y.; Yu, L. L. Chem. Phys. Lett. 2006, 426, 85. (11) Errandonea, D.; Martı´nez-Garcı´a, D.; Lacomba-Perales, R.; RuizFuertes, J.; Segura, A. Appl. Phys. Lett. 2006, 89, 091913. (12) Minowa, M.; Itakura, K.; Moriyama, S.; Ootani, W. Nucl. Instrum. Methods Phys. Res., Sect. A 1992, 320, 500. (13) Angloher, G.; Bucci, C.; Cozzini, C.; von Feilitzsch, F.; Frank, T.; Hauff, D.; Henry, S.; Jagemann, Th.; Jochum, J.; Kraus, H.; Majorovits, B.; Ninkovic, J.; Petricca, F.; Pro¨bst, F.; Ramachers, Y.; Rau, W.; Seidel, W.; Stark, M.; Uchaikin, S.; Stodolsky, L.; Wulandari, H. Nucl. Intrum. Methods Phys. Res., Sect. A 2004, 520, 108. (14) Zhou, Z. Y.; Li, C. X.; Yang, J.; Lian, H. Z.; Yang, P. P.; Chai, R. T.; Cheng, Z. Y.; Lin, J. J. Mater. Chem. 2009, 19, 2737. (15) Jia, R. P.; Zhang, G. X.; Wu, Q. S.; Ding, Y. P. Appl. Phys. Lett. 2006, 89, 043112.

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