06) 2645–2649www.elsevier.com/locate/matlet

Materials Letters 60 (20

Photoluminescence properties of Y0.95− xMxBO3:Eu3+ (M=Ca, Sr, Ba, Zn,

Al, 0≤x≤0.1) in 100–450 nm regions

Yuhua Wang ⁎, Lingli Wang

Department of Material Science, School of Physics Science and Technology, Lanzhou University, Lanzhou 730000, PR China

Received 17 November 2005; accepted 21 January 2006Available online 10 February 2006

Abstract

The single phases of Y0.95− xMxBO3:5%Eu3+ (M=Ca, Sr, Ba, Zn, Al, 0≤x≤0.1) were synthesized successfully by solid-state reaction. Theirluminescent properties were studied under UV and VUV excitation. The results indicated that with the incorporation of Ca2+, Sr2+, Ba2+, Zn2+ orAl3+ into the host lattice of YBO3:Eu

3+, the high symmetry around Eu3+ was destroyed and the ratio of red emission(5D0–7F2) to orange one

(5D0–7F1) increased, leading to a better chromaticity. Furthermore, the co-doping ions such as Ca2+, Zn2+ and Al3+ were beneficial to enhance the

luminescent intensity of Eu3+. These phenomena were evaluated, and possible explanations were proposed.© 2006 Elsevier B.V. All rights reserved.

Keywords: Phosphors; YBO3:Eu3+; Incorporation; Optical materials and properties

1. Introduction

Over the last few years, more and more attention has beenfocused on plasma display panels (PDPs) and Hg-free lamps[1–3]. The phosphors as an important part of PDP device emitvisible light under vacuum ultraviolet (VUV) excitation of 147nm from Xe/He gas plasma, whose progress is key to thedevelopment of PDP. Tricolor inorganic luminescent materialsare used in PDP to emit red, green and blue light. As far as redphosphor is concerned, (Y,Gd)BO3:Eu

3+ is widely used today,but the intensity of orange emission at about 593 nm is muchhigher than that of the red one at about 610 nm, resulting in apoor chromaticity. In order to get satisfied VUV red phosphor,the search for improving current phosphors or developing newred PDP phosphors is urgently required.

The pure red emission, which can be obtained from5D0→

7F2 transition of Eu3+ is hypersensitive to the latticesymmetry of the host crystal and will be relatively strong if thesymmetry of the crystal is low [4]. In the red phosphor of YBO3:Eu3+, the Y3+ site has an inversion symmetry for which Eu3+

substitutes, so the transition of 5D0→7F1 (593 nm) is

⁎ Corresponding author. Tel.: +86 931 8912772; fax: +86 931 8913554.E-mail address: wyh@lzu.edu.cn (Y. Wang).

0167-577X/$ - see front matter © 2006 Elsevier B.V. All rights reserved.doi:10.1016/j.matlet.2006.01.056

predominated [5]. Introducing other atoms into the ortho-boratehost lattice may be an effective method to reduce its crystalsymmetry. Elements such as Ca, Sr, Ba, Zn, Al were selected asco-doping ions because of their wide application in luminescenthost materials. Since their different ionic radii from that of Y3+,

Fig. 1. XRD patterns of Y0.95− xMxBO3:Eu3+ (M=Ca, Sr, Ba, Zn, Al, x=0.1)

prepared by solid-state reaction.

2646 Y. Wang, L. Wang / Materials Letters 60 (2006) 2645–2649

the crystal lattice would be distorted, and the high symmetryaround Eu3+ site can be destroyed. Due to the lower sitesymmetry of Eu3+ in the synthesized samples, the red emissionincreased, and a better color purity was achieved.

In this paper, we investigated the luminescent properties ofY0.95− xMxBO3:Eu

3+ (M=Ca, Sr, Ba, Zn, Al, 0≤x≤0.1) underUV and VUV region and then studied the effects of theincorporation ions on the luminescent properties of Eu3+.

2. Experimental

The starting materials are Y2O3, Eu2O3(99.99%), Al2O3

(99.5%), ZnO, CaCO3, SrCO3, BaCO3(99.0%), Li2CO3(97.0%)and H3BO3(99.5%). Powder samples of Y0.95− xMxBO3:Eu

3+

(M=Ca, Sr, Ba, Zn, Al, 0≤x≤0.1) were prepared by heatingthe mixture of the starting materials in appropriate ratio, Li+ wasintroduced to equalize the electric charge when Y3+ wassubstituted by bivalent cation. The mixture was heated at 500 °Cfor 2.5 h, reground, and again heated at 1100 °C for 3 h.

All the samples were characterized by powder X-raydiffraction (XRD) using a Rigaku diffractometer with Ni-filtered CuKá radiation at room temperature. The photo-luminescence spectra under ultraviolet (UV) region weremeasured by a Xe lamp (FLS-920T). All of the luminescencespectra in VUV region were measured by an ARC model VM-504-type vacuum monochromator, and calibrated by sodiumsalicylate. A deuterium lamp was used as radiation light. Thestandard VUV spectrometer was set up as described in Ref. [6].Its performance was comparable to synchrotron radiation. Allthe luminescent spectra were recorded at room temperature.

3. Results and discussion

All the samples doped with 5% Eu3+ were recognized as singlephase through the X-ray powder diffraction measurement. As anexample, Fig. 1 showed the XRD patterns of Y0.85M0.1BO3:Eu

3+

(M=Ca, Sr, Ba, Zn, Al). The peak positions agreed well with those ofthe standard pattern JCPDF(16-0277) for YBO3:Eu

3+, and there was nosecond phase observed.

The samples were indexed to a hexagonal system of vaterite typeand their lattice parameters were calculated by the least square method

Fig. 2. a and c parameters of the unit cell of Y0.85Ba0.1BO3:Eu3+, Y0.85Sr0.1BO3:Eu

3+

Eu3+.

with Si as the inner standard. The parameters of the unit cell ofY0.85Ba0.1BO3:Eu

3+, Y0.85Sr0.1BO3:Eu3+, YBO3:Eu

3+, Y0.85Ca0.1BO3:Eu3+, Y0.85 Zn0.1BO3:Eu

3+ and Y0.85Al0.1BO3:Eu3+ were shown in

Fig. 2.Great changes of a and c parameters were observed when the lattice

site of Y3+ was substituted by the co-doping ions because of thedifferent ionic radii of Ba2+ (1.43 Å), Sr2+ (1.12 Å), Ca2+ (0.99 Å),Zn2+ (0.74 Å) and Al3+ (0.54 Å) from that of Y3+ (0.90 Å).Therefore, the crystal lattice was distorted and the high symmetry ofthe crystal lattice site around Eu3+ was destroyed.

Fig. 3a) exhibited the emission spectra of YBO3:5%Eu3+ andY0.85M0.1BO3:Eu

3+ (M=Ca, Sr, Ba, Zn, Al) under 254 nm irradiation.Generally, the emission spectra consisted of three sharp lines at about593, 610 and 627 nm, which were assigned to the transitions from theexcited 5D0 level to

7FJ (J=1, 2) levels of Eu3+ activators [7,8]. As can

be seen, the ratio of the red emission at 610 nm to the orange one at 593nm (R/O value) of the co-doped samples was much higher than that ofYBO3:Eu

3+ because of the low symmetry of the crystal lattice achievedwith the incorporation of co-doping ions. The R/O values for differentincorporations were given in Fig. 3b) and higher R/O value wasobtained with the decrease of the radii of the co-doping ionsðgBa2þNgSr2þNgCa2þNgZn2þNgAl3þÞ. The best chromaticity wasobtained when Al3+ was introduced into the host lattice. Thiscould be explained by the maximal difference between the radii ofAl3+ and Y3+, which corresponded to the furthest level of disorder ofthe crystal lattice.

Fig. 4 showed the luminescent intensity changes of YBO3:5%Eu3+

with different incorporation ions. Generally, the luminescent efficien-cy of phosphors will be lowered if some co-doped ions wereintroduced into the host lattice. It was true for Sr2+ and Ba2+, and thiscould be attributed to various pathways of nonradiative relaxationcaused by the incorporation. While for Ca2+, Zn2+ and Al3+ whoseradii were smaller than that of Y3+, the luminescent intensity of Eu3+

was enhanced. Similar phenomenon was observed in other systems[9]. Higher emission is obtained with the incorporation of smaller ionsinto the host lattice which can help enhance the charge transfer (CT)excitation band of Eu3+ and the absorption band of the host lattice atthe same time.

The samples with the incorporation of Al3+ exhibited the bestluminescent properties. Fig. 5a) displayed the emission spectra ofY0.95− xAlxBO3:5%Eu3+ under 254 nm excitation with Al3+ concen-trations of 2%, 4%, 6%, 8% and 10%. The luminescent intensity wasimproved with the increase of the concentration of Al3+. As we couldsee from Fig. 5b), the R/O value was also enhanced, indicating that a

, YBO3:Eu3+ (⁎), Y0.85Ca0.1BO3:Eu

3+, Y0.85Zn0.1BO3:Eu3+ and Y0.85Al0.1BO3:

Fig. 4. The luminescence intensity change of YBO3:Eu3+ caused by different

incorporation ions(λex=254 nm).

Fig. 3. (a) Emission spectra and (b) R/O values of Y0.95− xMxBO3:Eu3+ (M=Ca,

Sr, Ba, Zn, Al, x=0.1) under 254 nm excitation and that of YBO3:Eu3+.

2647Y. Wang, L. Wang / Materials Letters 60 (2006) 2645–2649

higher concentration of Al3+ was favorable to achieve superiorchromaticity. The strongest emission as well as the maximum R/Ovalue was obtained when 10% Al3+ was co-doped. The colorcoordinate of Y0.85Al0.1BO3:Eu

3+ was x=0.654, y=0.348 comparedwith that of YBO3:5%Eu3+ (x=0.65, y=0.35).

Fig. 6 showed the emission spectra of the synthesized samples andthat of YBO3:5%Eu3+ under the excitation of 147 nm. The spectra hada little difference from that under 254 nm excitation, the emission atabout 627 nm was much higher than that at 610 nm, so the R/O valuewas defined as the ratio of the red emission at 627 nm to the orange oneat 593 nm. As shown in Fig. 6(a), the R/O values for YBO3:Eu

3+ andthat of Y0.85M0.1BO3:Eu

3+ (M=Ca, Sr, Ba, Zn, Al) were 0.91, 0.89,0.90, 0.91, 0.95 and 1.2, respectively. The smaller radii of the co-doping ions were favorable to obtain better chromaticity. Fig. 6(b)showed the emission spectra and R/O values of Y0.95− xAlxBO3:Eu

3+

(0≤x≤0.1) under 147 nm irradiation. Same to that under 254 nmexcitation, the phosphors with higher concentration of Al3+ exhibitedhigher R/O value, and the best chromaticity was achieved when 10%Al3+ was co-doped into the host lattice.

The excitation spectra of all the samples measured in VUV regionwere similar. As an example, the excitation spectra of Y0.85Al0.1BO3:Eu3+ and that of YBO3:5%Eu3+ exhibited in Fig. 7. Two main broadbands with maxima at 160 and 230 nm were observed in the excitationspectra. According to Ref. [10], the broad band at about 230 nm was

Fig. 5. (a) Emission spectra and (b) R/O values of Y0.95− xAlxBO3:Eu3+ under

254 nm irradiation for samples with different aluminum ion concentrations.

Fig. 7. Excitation spectra of the luminescence of YBO3:Eu3+ and Y0.85Al0.1BO3:

Eu3+.

Fig. 6. (a) Emission spectra and R/O values of Y0.95− xMxBO3:Eu3+ (M=Ca, Sr, Ba, Zn, Al, x=0.1) and YBO3:Eu

3+ under 147 nm irradiation. (b) Emission spectra andR/O values of Y0.95− xAlxBO3:Eu

3+ under 147 nm irradiation.

2648 Y. Wang, L. Wang / Materials Letters 60 (2006) 2645–2649

assigned to the CT band of Eu3+–O2− resulting from an electrontransfer from the ligand O2−(2p6) orbitals to the empty states of 4f6 forEu3+ configuration. The band at about 160 nm was assigned to theabsorption of the host lattice [7]. The intensity of the excitation bandsof Y0.85Al0.1BO3:Eu

3+ was much higher than that of YBO3:Eu3+ which

corresponds to the high luminescent intensity of Y0.85Al0.1BO3:Eu3+.

4. Conclusion

Y0.95− xMxBO3:5%Eu3+ (M=Ca, Sr, Ba, Zn, Al, 0≤x≤0.1)were synthesized and investigated for PDP application. Theemission spectra showed that the pure red emission from5D0→

7F2 transition of Eu3+ was improved with the incorpo-ration of Ca2+, Sr2+, Ba2+, Zn2+, Al3+ into the host lattice ofYBO3:Eu

3+, leading to a better color purity. The luminescentintensity was also increased when Ca2+, Zn2+, Al3+ wereintroduced. Y0.85Al0.1BO3:5%Eu3+ showed the best chroma-ticity and highest luminescent intensity, it would be the mostpromising red component in PDPs and Hg-free lamps.

2649Y. Wang, L. Wang / Materials Letters 60 (2006) 2645–2649

Acknowledgements

We thank the Ministry of Science and Technology of China863 program (2003AA324020), the NSFC (50272026) andEYTP (Excellent Young Teachers Program of M0E, P.R.C.) forthe financial support.

References

[1] Tsutae Shinoda, ASID'04, Proceedings of The 8th Asian Symposium onInformation Display, 2004, p. 1.

[2] C. Okazaki, M. Shiiki, T. Suzuki, K. Suzuki, J. Lumin. 87–89 (2000) 1280.[3] T. Justel, J.-C. Krupa, D.U. Wiechert, J. Lumin. 93 (2001) 179.

[4] S. Shionoya, W.M. Yen, Phosphor Handbook, CRC Press, Boca Raton,1999, p. 190.

[5] Yuhua Wang, Tadashi Eudo, Ling He, Chunfang Wu, J. Cryst. Growth 268(2004) 568–574.

[6] Thomas Jüstel, Jean-Claude Krupa, Detlef U. Wiechert, J. Lumin. 93(2003) 179–189.

[7] Yuhua Wang, Xuan Guo, Tadashi Endo, Yukio Murakami, MizumotoUshirozaw, J. Solid State Chem. 177 (2004) 2242–2248.

[8] C. Feldmann, T. Jüstel, C.R. Ronda, D.U. Wiechert, J. Lumin. 92 (2001)245.

[9] Lianhua Tian, Sun-il Mho, Byung-Yong Yu, Chong-Hong Pyun, J. Soc.Inf. Disp. 12 (2004) 517.

[10] G. Blasse, A. Brill, Philips Res. Rep. 22 (1966) 2356.