Thin Solid Films 518 (2010) e72–e74

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Synthesis and luminescence of YNbO4:Eu3+, Tb3+ by a flux method

Eun Young Lee, Young Jin Kim ⁎

Department of Materials Science & Engineering, Kyonggi University, Suwon 443-760, Republic of Korea

⁎ Corresponding author. Tel.: +82 31 249 9766; fax:E-mail address: yjkim@kyonggi.ac.kr (Y.J. Kim).

0040-6090/$ – see front matter © 2010 Elsevier B.V. Adoi:10.1016/j.tsf.2010.03.121

a b s t r a c t

a r t i c l e i n f o

Available online 2 April 2010

Keywords:LuminescenceYttriumNiobium oxideTerbiumEuropium oxide

YNbO4:Eu3+ powders were prepared using LiCl as a flux. With increasing the LiCl content up to 7 wt.%,YNbO4:Eu3+ powders with the high crystallinity and the uniform size could be obtained, leading to theenhancement of the red emission intensity at 613 nm. The excitation spectra consisted of the charge transfer(CT) band and f–f transitions of Eu3+. The excitation intensity ratio of f–f transition to CT band also increasedwith the LiCl content. The emission spectra under 315 nm excitation of Y0.85Nb0.9Ta0.1O4:Eu0.15− yTbyrevealed that the absorbed energy by Tb3+ ions could be obviously transferred to the nearest Eu3+ ions.

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1. Introduction

ABO4 type, namely, niobates, tantalates, and tungstates were widelyinvestigated as x-ray phosphors. Brixner et al. reported that the emissionmechanismwas attributed to the energy transition from excited states of[NbO4]3−, [TaO4]3−, and [WO4]2−, respectively [1,2]. And also these ABO4

typematerials canconvert ultraviolet (UV) tovisible lightsbydoping rare-earth (RE) ions. For YNbO4:Eu3+, the absorbed energy of self-activated[NbO4]3− under UV, which allows the blue emission by itself, is non-radiatively transferred to Eu3+ ions by the charge transfer (CT)mechanism, leading to red emissions due to the 5D0→

7FJ (J=0, 1, 2...)transition of Eu3+ [3]. As the concentration of an activator (Eu3+)increases, the blue emission due to a sensitizer ([NbO4]3−) is surpassed bythe red emission of Eu3+ ions in photoluminescence (PL) intensity. In theexcitation spectra, intense f–f transitions of Eu3+ around near UV as wellas CT band are observed, which strongly depend on the preparationconditions [4]. To control PL emission properties of YNbO4 based-phosphors, various dopants such as Bi3+, Eu3+, Ce3+, Dy3+, and Tb3+

have been reported [5–9], while the mixed compounds of niobates andtantalates cause the shift of CTband [10].However, the effects of afluxandEu3+–Tb3+ co-doping on the luminescent properties in YNbO4 have notbeen reported in detail.

In this work, the effects of LiCl on CT and f–f transitions in YNbO4:Eu3+ were investigated. Furthermore the luminescent properties ofEu3+ and Tb3+ co-doped mixed compounds of niobates and tantalite,YNb1− xTaxO4, were characterized first.

2. Experiment

YNbO4 powders were synthesized by using Y2O3 (99.99%), Nb2O5

(99.99%), Ta2O5 (99.99%), Tb4O7 (99.999%), and Eu2O3 (99.99%)powders. A flux, LiCl, was added to the stoichiometric mixtures topromote the solid-state reactions and the luminescent properties. Themixtures were ball-milled for 24 h, and then fired at 1300°C for 12 hunder N2 atmosphere in an electric tube furnace. The crystal structurewas measured by XRD (X-ray diffractometer, SIEMENS D5005) usingCuKα radiation (λ=1.5406 Å) at the scanning step of 5°/min. PLspectra were measured by PL (PSI Darsa 5000) system at the roomtemperature. The excitation source of PL was a xenon lamp andvarious excitation wavelengths were refracted by a monochromator.

3. Results and discussion

It has been reported that a flux contributed to particle shapes, thedecrease in synthesizing temperatures, and the uniform distributionof dopants [11,12]. XRD patterns of YNbO4:Eu3+ (5 mol%) prepared at1300°C as a function of LiCl are shown in Fig. 1. YNbO4 with amonoclinic structure alone appeared at the entire LiCl content, 0–10 wt.%. There were no significant distinctions in XRD patterns, butthe microstructure such as the crystallinity was affected by the LiClcontent. The variation of FWHM (full width of half maximum) valuesof (021) peaks at 2θ=28.5° as a function of the LiCl content is shownin Fig. 2. With increasing the LiCl content up to 7 wt.%, FWHMcontinuously decreased, indicating that the crystallinity was en-hanced. However, it increased again at 10 wt.% due to the excess LiCl.According to SEM micrographs, at 10 wt.% the particle size wassignificantly larger than those at 5 and 7 wt.%. This abrupt growingcaused the inferior crystallinity as shown in Fig. 2.

As shown in Fig. 3, the excitation spectra of YNbO4:Eu3+ (5 mol%)exhibited not only the broad band around 270 nm due to the chargetransfer (CT) between [NbO4]3− and Eu3+ ions, but also sharp peaks

Fig. 1. XRD patterns of YNbO4:Eu3+ (5 mol%) as a function of the LiCl content. (a) 0 wt.%,(b) 5 wt.%, (c) 7 wt.%, and (d) 10 wt.%.

Fig. 2. FWHM of XRD (021) peak of YNbO4:Eu3+ as a function of the LiCl content.

Fig. 4. The variation of the excitation intensity ratio of 7F0→5L6 (R397/CT) to CT bandwiththe LiCl content.

e73E.Y. Lee, Y.J. Kim / Thin Solid Films 518 (2010) e72–e74

(290–475 nm) assigned to f–f transitions of Eu3+ 4f6 configuration[3,4]. 7F0→5L6 at 397 nm among f–f transitions appeared as thestrongest one followed by 7F0→5L7 at 385 nm. The intensity of the CTbands and f–f transitions peaked at 7 wt.% of the LiCl content, whichwas in accordance with the variation of FWHM values and the particle

Fig. 3. The excitation spectra of YNbO4:Eu3+ as a function of the LiCl content. (a) 0 wt.%,(b) 5 wt.%, (c) 7 wt.%, and (d) 10 wt.%.

size. This demonstrated that the luminescent properties were closelycorrelated with the micro-structural properties that were attributedto the LiCl content. The wavelengths of CT band at various LiCl contentshowed the weak shift, but those of f–f transitions including 7F0→5L6did not move at all. This arose from the reason that CT is stronglysensitive to the crystal field, but not f–f transitions. Accordingly thehigh crystallinity at 7 wt.% of a flux caused the strong crystal field,leading to the red-shift of CT band. The excitation intensity ratio of7F0→5L6 at 397 nm to CT band (R397/CT) varied with the LiCl content asshown in Fig. 4. The maximum R397/CT was achieved at 7 wt.%. Thisrevealed that with the increase in the LiCl content f–f transitions weremore enhanced than CT. PL emission spectra under CT band excitationare shown in Fig. 5. The emissions around 420 nm due to self-activated [NbO4]3−were absolutely quenched, and only red emissionsin which 5D0→

7F2 at 613 nm were the most intense, generated bytransferring the absorbed energy of [NbO4]3− to Eu3+ ions by CTmechanism. The inset in Fig. 5 exhibits a variation of PL intensity as afunction of the LiCl content under CT band and f–f (7F0→5L6, 397 nm)excitation, respectively. 397 nm excitation caused the steeperincrease up to 7 wt.% than CT band excitation, coinciding with thevariation of R397/CT in Fig. 4.

It was reported that CT band could be modified by partly replacingNb with Ta, namely, YNb1− xTaxO4:Eu3+ system [10,13–14], and alsothe PL spectra of YNbO4:Tb3+ exhibited emission lines that consistedof 5D4→

7FJ (J=3–6) of Tb3+ ions [8]. In this work the spectralproperties of Eu3+ and Tb3+ co-doped YNb0.9Ta0.1O4 were

Fig. 5. PL spectra of YNbO4:Eu3+ as a function of the LiCl content.

Fig. 6. PL spectra of Y0.85Nb0.9Ta0.1O4:Eu0.15− yTby powders.

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investigated first. The excitation spectra of Y0.85Nb0.9Ta0.1O4:Tb0.15exhibited the peaked intensity around 315 nm, henceY0.85Nb0.9Ta0.1O4:Eu0.15− yTby was excited by 315 nm as shown inFig. 6. At y=0, the red emission at 613 nm due to Eu3+ aloneappeared, while at y=15 only the green-yellow emission around545 nm generated from the transition of Tb3+ ions. At y=0.01–0.14,the emission peaks due to Eu3+ and Tb3+ ions were observedsimultaneously. According to the excitation spectra of YNbO4:Eu3+ inFig. 3. the excitation intensity was insignificant around 315 nm,consequently a weak red emission peak was achieved at 613 nm dueto 5D0→

7F2 transition of Eu3+ ions, while more intense green-yellowemission around 545 nm generated from 5D4→

7F5 transition of Tb3+

ions. However it was interesting that red emission peaks at 613 nmassigned to Eu3+ ions gradually increased with the increase of theconcentration of Tb3+ ions. That is to say, despite the decrease in Eu3+

concentration (the increase in Tb3+ ions) this emission peak intensityat 613 nm continuously increased. This revealed that the absorbedenergy by Tb3+ ions was partly transferred to the nearest Eu3+ ions.At the high concentration of y=0.14, this red emission intensitysignificantly reduced due to the concentration quenching effect. A

similar work was carried out by Zhang et al. [15]. They reported thatfor Eu2+ and Tb3+ co-doped Ca5(PO4)3Cl system the process of energytransfer from Eu2+ to Tb3+ was observed, which was attributed to abalance of non-radiative resonant energy transfer caused by theexchange interaction and the phonon-assisted non-radiative process.

4. Conclusion

For YNbO4:Eu3+ powders, the increase in the LiCl content led tothe enhancement of the red emission intensity due to the highcrystallinity and the uniform powder size, and also caused theincrease of the excitation intensity ratio of CT band to 7F0→5L6 f–ftransition of Eu3+. However, the enlarged particle size and the poorcrystallinity at 10 wt.% of LiCl reduced the red emission. ForY0.85Nb0.9Ta0.1O4:Eu0.15−yTby, the increase of the red emission dueto Eu3+ ions with the increase of the concentration of Tb3+ ions (y)demonstrated that the energy transfer from the absorbed energy byTb3+ ions to the nearest Eu3+ ions could proceed.

Acknowledgment

This work was supported by a grant from Korea Ministry ofKnowledge Economy (10033221).

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