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Materials Science and Engineering B 133 (2006) 218–221

Short communication

Spectroscope properties of KCaY(PO4)2:Eu3+

in vacuum ultraviolet region

Dan Wang, Yuhua Wang ∗Department of Material Science, Lanzhou University, Lanzhou 730000, China

Received 22 December 2005; received in revised form 18 May 2006; accepted 28 May 2006

bstract

In order to investigate the energy migration mechanisms of Eu3+ doped hexagonal KCaY(PO4)2 in vacuum ultraviolet (VUV) region,CaY1−x(PO4)2:Eux

3+ (0 � x < 0.1) are prepared and their photoluminescence (PL) properties are evaluated under 147 nm excitation. Monitoredy 616 nm emission, the weak band around 136–162 nm in the excitation spectra of KCaY0.95(PO4)20.05:Eu3+ could be assigned to the absorptionf PO 3−. The sharp peak at 213 nm which overlaps with the charge transition band (CTB) of Eu3+–O2− at about 177–280 nm should be assigned

4

o the transition to 3H4 levels of Eu3+. Under 147 nm excitation the strongest emission peak is located at about 616 nm. When x reaches 0.05 inCaY1−x(PO4)2:Eux

3+(0 < x < 0.1), the optimum emission intensity is obtained. A main reason for the low quenching concentration of Eu3+ is thatu3+ occupies not only the Y3+ position but also the K+ and Ca2+ sites, so the energy migration among Eu3+ ions could be in three-dimensional.2006 Elsevier B.V. All rights reserved.

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eywords: VUV; KCaY(PO4)2; Eu3+; Concentration quenching

. Introduction

In recent years, rare earth doped phosphates have attracteduch attention due to their efficient luminescence under

ltraviolet (UV) and vacuum ultraviolet (VUV) excitation.CaY(PO4)2 has the same structure with hexagonal LaPO4

xcept that K+ occupies the large empty tunnels of the latticend La3+ position is statistically occupied by Y3+ or Ca2+. It isade up of –(Y3+ or Ca2+)–PO4–(Y3+ or Ca2+)– chains along

he c axis, linked to similar chains by corner shared with four PO4etrahedrons located in the (0 0 1) plane [1,2]. The characteristicf this compound is that Eu3+ might occupy not only the Y3+

osition but also the K+ and Ca2+ positions, so the absorbednergy would migrate in three-dimensions and the quenchingoncentration would be low. In addition, the absorption band ofO4

3− in VUV region has been observed in some phosphates,

uch as 151 nm in (La,Gd)PO4:RE3+ (RE = Eu, Tb) [3], 149 nmn YPO4:Tb3+ [4], 157 nm in Sr3(PO4)2:RE3+ (RE = Ce, Sm,u, Tb) [5], 152, 159, 160 and 145 nm in RPO4:Eu3+ (R = Y, La,d, Lu) [6]. Therefore, KCaY(PO4)2 could be a suitable host

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

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921-5107/$ – see front matter © 2006 Elsevier B.V. All rights reserved.oi:10.1016/j.mseb.2006.05.008

or fundamental investigation, and application in Hg-free lampsnd plasma display panels (PDPs).

In this paper, hexagonal KCaY1−x(PO4)2:Eux3+ (0 � x < 0.1)

re prepared and their photoluminescence (PL) properties arenvestigated under 147 nm excitation. A possible mechanism ofhe energy transfer from the host to Eu3+ in VUV region isroposed.

. Experiment

K2CO3 (99%), CaCO3 (99%), Y2O3 (99.99%), Eu2O399.99%) and (NH4)2HPO4 (98.5%) are used as the startingaterials. Firstly, the starting materials are mixed together in

toichiometric amounts and dissolved completely in the nitriccid. Then the solution is to be dried. The resulting mixture iseated at 900 ◦C for several hours and cooled to the room tem-erature.

The phase of the samples is checked by X-ray diffractionXRD) using Rigaku diffractometer with Ni-filtered Cu K� radi-tion. The VUV excitation and emission spectra are measured byLS-920T fluorescence spectrophotometer with a VM-504-type

acuum monochromator using a deuterium lamp as the lightingource. The excitation spectrum is corrected by sodium salicy-ate, whose quantum efficiency is almost constant in this region.ll the measurements are performed at room temperature.

D. Wang, Y. Wang / Materials Science and Engineering B 133 (2006) 218–221 219

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astassigned to the 5D0 → 7F1 and 5D0 → 7F3 transitions of Eu3+,

Fig. 1. The XRD pattern of KCaY(PO4)2.

. Results and discussion

All the samples are checked as a single phase. As an exam-le, Fig. 1 shows the XRD pattern of KCaY(PO4)2 and it cane well indexed with the lattice parameters a = b = 0.6896 nm,= 0.6347 nm. The space group is P6222. The crystal vol-me of KCaY1−x(PO4)2:Eux

3+ (0 � x < 0.1) is shown in Fig. 2.hen the concentration of Eu3+ in KCaY1−x(PO4)2:Eux

3+

ncreases, the crystal volume decreases. Since the ion radii:+ > Ca2+ > Eu3+ > Y3+, Eu3+ will prefer to occupy the positionf K+ than that of other ions.

Fig. 3 shows the excitation spectrum of KCaY0.95(PO4)2:u0.05

3+ monitored by the 616 nm emission. The broad bandselow 177 nm and at about 177–280 nm are assigned to the hostbsorption and the charge transition band (CTB) of Eu3+–O2−,espectively. Considered the VUV reflection spectra of KH2PO4

nd NH4H2PO4 [7] and the excitation spectra of rare earthhosphates in VUV region [3–6], and the anion groups of allhe phosphates are composed of P–O tetrahedron, so the band

ig. 2. Concentration dependence of unit cell parameter of volume of KCaY1−x

PO4)2:Eux3+ (0 � x < 0.1).

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ig. 3. The excitation spectrum of KCaY0.95(PO4)2: 0.05Eu3+, λem = 616 nm.

t about 136–162 nm could be assigned to the absorption ofO4

3−. According to the report about the CTB of Y3+–O2−n (Gd,Y)BO3:Eu3+ and YAlO3:Eu3+ [8], the band at about62–177 nm in KCaY0.95(PO4)2:Eu0.05

3+ should include theTB of Y3−–O2−. The CTB of Eu3+ at about 177–280 nmith the maximum at about 220 nm has split into two parts.main possible reason is that Eu3+ occupies not only the Y3+

r Ca2+ position but also the K+ position in this compound.his is in agreement with the conclusion of the powder XRD.sharp peak at 213 nm is observed in the excitation spectrum

f KCaY0.95(PO4)2:Eu0.053+, and it could be assigned to the

ransition to 3H4 levels of Eu3+ [9].The emission spectra of KCaY1−x(PO4)2:Eux

3+ (0 < x < 0.1)re shown in Fig. 4. Under 147 nm excitation the strongest emis-ion peak locates at about 616 nm assigned to the 5D0 → 7F2ransition of Eu3+. The peaks at about 590 and 651 nm are

espectively. The optimum emission intensity is obtained whenhe Eu3+ concentration reaches 0.05 in KCaY1−x(PO4)2:Eux

3+

0 < x < 0.1). A factor which would influence the optimum emis-

ig. 4. The emission spectra of Y0.95PO4:Eu0.053+ (a) and KCaY1−x(PO4)2:

ux3+ (0 < x < 0.1) (b), λex = 147 nm.

220 D. Wang, Y. Wang / Materials Science and Engineering B 133 (2006) 218–221

Table 1Comparison of the cell parameters for KCaY(PO4)2 and KCaNd(PO4)2

Composition Structure a = b (nm) c (nm) V (10−3 nm3)

KCaY(PO4)2 Hexagonal 0.6896 0.6347 261.43 This workKCaNd(PO4)2 Hexagonal 0.7026 0.6398 273.52 JCPDF 34-0112

Table 2Interatomic distance for KCaNd(PO4)2

P–O (Nd,Ca)–(Nd,Ca) (Nd,Ca)–K K–K (Nd,Ca)–O K–O

Distance (nm) 0.1555 (6) 0.4112 (2) 0.3674 (2) 0.4264 (2) 0.2580 (6) 0.2894 (6)

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ef. [10].

ion intensity of rare earth ions is the crystal structure ofhe host. For KCaY(PO4)2:Eu3+, which has the same struc-ure with KCaNd(PO4)2 (Table 1). The distances of P–O,Nd,Ca)–(Nd,Ca), (Nd,Ca)–K and K–K in KCaNd(PO4)2 arehown in Table 2. Because the radius of Y3+ is smaller than thatf Nd3+ and the cell parameter of KCaY(PO4)2 is less than that inCaNd(PO4)2 (Table 1), the distances of P–O, (Y,Ca)–(Y,Ca),

Y,Ca)–K and K–K in KCaY(PO4)2 are shorter than that inCaNd(PO4)2. The cation–cation distance in KCaY(PO4) is

hown as follows: K–K > (Y,Ca)–(Y,Ca) > (Y,Ca)–K. Due to theistance of activators should influence the quenching concentra-ion [11], so a main reason which leads to the low concentrationuenching in KCaY1−x(PO4)2:Eux

3+ (0 < x < 0.1) is that Eu3+

referably occupies the K+ position. When the concentration ofu3+ is increased, Eu3+ should occupy Ca2+ or Y3+ position.s mentioned above, the distance of (Y3+, Ca2+)–K+ is shortest

n KCaY(PO4)2, which could result in the absorbed energy notnly migrating in the chains, but also in the three-dimensionalmong Eu3+. So the intensity of the emission peaks is

ecreased.

Under 147 nm excitation the intensity of the emission peaksn KCaY1−x(PO4)2:Eux

3+ (0 < x < 0.1) is weaker than that in

Fig. 5. Projection of KCaNd(PO4) structure on the (0 0 1) plane.

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0.2363 (6) 0.2988 (6)

0.95PO4:Eu0.053+ as shown in Fig. 4. A possible reason is that

he distance of K–O is larger than that of Y–O (Table 2; Fig. 5).hen Eu3+ occupies the K+ position, the CTB of Eu3+–O2−ill shift to the long wavelength. The overlapping of the CTBf Eu3+–O2−, the CTB of Y3+–O2− and the absorption band ofO4

3− is less, so the energy absorbed by the host could not befficiently transferred to the Eu3+, which leads to a low emissionfficiency in VUV region.

. Conclusions

The PL properties of KCaY1−x(PO4)2:Eux3+ (0 < x < 0.1) in

UV region are evaluated. The band at about 136–162 nmould be assigned to the absorption of PO4

3−. The opti-um emission intensity is obtained when x reaches 0.05 inCaY1−x(PO4)2:Eux

3+ (0 < x < 0.1). A main reason is that theu3+ occupies not only the K+ position but also the Ca2+ and Y3+

ositions, therefore, the energy migration is in three-dimensionalnd the intensity of the emission peaks is decreased. The smallverlapping of the CTB of Eu3+, the CTB of Y3+–O2− andhe absorption band of PO4

3− leads to the low energy trans-er efficiency from the host to the CTB of Eu3+–O2− and theow emission efficiency.

cknowledgements

This work was partially supported by the National Naturalcience Foundation of China (50272026), Program for Newentury Excellent Talents in University (NCET, 04-0978) andpecialized Research Fund for the Doctoral Program of Higherducation (SRFDP, 20040730019).

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