www.elsevier.com/locate/optmat

Optical Materials 27 (2005) 1587–1592

Synthesis and luminescent performances of some europiumactivated yttrium oxide based systems

Michael Nazarov a,*, Jong Hyuk Kang a, Duk Young Jeon a,Sergey Bukesov a, Tatiana Akmaeva b

a Department of Materials Science and Engineering, KAIST, 373-1 Guseong-dong, Yuseong-gu, Daejeon 305-701, Koreab Saratov State University, Saratov, Russia

Received 20 July 2004; accepted 25 October 2004

Available online 15 December 2004

Abstract

Phosphors based on solid solutions of Y2O3–La2O3–Gd2O3 and Y2O2S–La2O2S–Gd2O2S and YVO4–GdVO4 doped by Eu3+

were synthesized by different methods. The influence of a matrix composition and an activator concentration on phosphor�slight-emitting performances is described in order to propose the optimized synthesis conditions. Europium-doped yttrium, lantha-

num and gadolinium oxysulphides phosphors show higher CL-efficiency at low applied voltages than oxides ones. The presented

results confirm that europium doped solid solutions of Y–Gd–La oxides, oxysulphides and vanadates are perspective for PL and

CL application in the display manufacture.

� 2004 Elsevier B.V. All rights reserved.

PACS: 78.55.�m; 78.55.HxKeywords: Phosphor; Y2O3–La2O3–Gd2O3; Y2O2S–La2O2S–Gd2O2S; YVO4–GdVO4; Optical properties; Luminescence

1. Introduction

Phosphors based on Eu3+ doped rare-earth (RE) ele-

ments such as yttrium, lanthanum and gadolinium are

widely used to provide red light emission for moderndisplay devices. The quality of phosphors is of great

importance for flat panel displays (FPDs) performances

because they directly influence their brightness and life-

time [1–6].

In this paper, the main attention is paid to the perfor-

mances of different red-emitting phosphors based on

Eu3+ doped solid solutions of Y2O3–La2O3–Gd2O3and Y2O2S–La2O2S–Gd2O2S and YVO4–GdVO4 that

0925-3467/$ - see front matter � 2004 Elsevier B.V. All rights reserved.doi:10.1016/j.optmat.2004.10.013

* Corresponding author. Tel.: +82 42 864 2154; fax: +82 42 869

3310.

E-mail address: nazarov@kaist.ac.kr (M. Nazarov).

were synthesized by both solid state reaction and copre-

cipitation methods. Because the concentration of the

activator ions is relatively low, the energy of excitation

is absorbed mainly by the phosphor matrix. Therefore,

the requirements for the matrix composition and crystal-line structure become very important. RE oxides such as

Y2O3, La2O3 and Lu2O3, are suitable for phosphor

matrices because their cations have no unpaired elec-

trons in their electronic configurations and their total

orbital and spin moments are equal to zero. These oxi-

des are transparent for visible light, have no low-energy

levels and the interaction between the RE activator ions

and the matrix is minimal. Gd2O3 is close to the afore-mentioned oxides, according to their optical properties.

Other promising matrices for phosphor application are

RE oxysulphides, such as Y2O2S, La2O2S, Gd2O2S,

and Lu2O2S [7,8]. They form isostructural series crystal-

lizing in the trigonal singony which is commonly repre-

sented in the hexagonal aspect with a local symmetry of

1588 M. Nazarov et al. / Optical Materials 27 (2005) 1587–1592

the C3v lanthanide site [9]. Red-emitting YVO4:Eu is a

well-known phosphor and nowadays this composition

is widely used as the red component in CRTs. Due to

its good colour purity, this phosphor is also considered

as a promising candidate for PDPs [10].

The aim of the present work is to reveal influence ofsynthesis conditions and composition of the matrix in

the some complex europium activated yttrium oxide

based systems on their light-emitting performances, to

investigate their physical and chemical properties and

optimize the synthesis conditions and parameters, to de-

sign and prepare high efficient phosphor materials under

electron and UV excitation.

2. Experimental

2.1. Chemical synthesis

2.1.1. Synthesis of europium doped yttrium–gadolinium–

lanthanum oxide phosphor

Appropriate amounts of Y2O3 and/or Gd2O3 and/orLa2O3 were dissolved in nitric acid of volumic density

q = 1.2g/cm3. The activator was introduced as europiumnitrate, in amounts ranging from 2.5 to 10mol%, taken

as europium oxide. The pH of the solution was brought

to 2.0 by adding ammonia solution. Then a saturated

solution of the oxalic acid was added till complete copre-

cipitation of Y, Gd, and La oxalates. The precipitates

were filtered, washed with distilled water, and dried inair at about 80–100 �C. The blends were placed intoquartz crucibles and annealed in air at 1100 �C during12h.

Alternatively, relatively large amount of sodium car-

bonate was added to the oxalates blend in order to de-

crease the firing temperature; Na2CO3 was introduced

in fivefold concentration of the blend mass. The homo-

geneous mixture was annealed at 900 �C for 6h andquenched in air. All samples were grinded, washed in

distilled water, and dried at 80–100 �C.

2.1.2. Synthesis of europium doped yttrium–gadolinium–

lanthanum oxysulphide phosphors

Solid solutions of yttrium, gadolinium and lantha-

num oxysulphides were synthesized by coprecipitation

of oxalates with their subsequent thermal decomposi-tion, in presence of sodium carbonate, sodium phos-

phate and sulphur. For the blend synthesis, solutions

of RE oxides in nitric acid (q = 1.2g/cm3) and saturatedoxalic acid were used. The RE oxalate precursors were

filtered and dried in air, at 80–100 �C. Stoichiometricamounts of sodium carbonate, sodium phosphate

(·12H2O), and a threefold excess of sulphur were addedto the oxalate precursors. The blend was wetted withethyl alcohol to gruel consistency and dried under per-

manent mixing till full free-flowing. The dry blend was

placed into quartz ampoules for annealing in a reducing

atmosphere or vacuum (<5 · 10�3Torr). The annealingwas carried out within 800–1100 �C during 1–8h. Afterthat, the phosphors were cooled, powdered, washed

and filtered. The phosphors were dried in air at 80–100 �C during 2h.Four series of two- and three-component systems of

Y, Gd, and La oxysulphides were prepared according

to the general formula: a Y2O2S: b Gd2O2S: c La2O2S

where a + b + c = 1

Series A, system Y2O2S–La2O2S: a = 0–0.90; b = 0

and c = 0–0.90;

Series B, system Y2O2S–Gd2O2S: a = 0–0.90; b = 0–0.90 and c = 0;

Series C, Gd2O2S–La2O2S: a = 0; b = 0.1–0.50;

c = 0.50–0.90;

Series D, Y2O2S–Gd2O2S–La2O2S: a = 0.34–0.80;

b = 0.10–0.33; c = 0.10–0.33.

2.1.3. Synthesis of (YX,Gd1�X)VO4:Eu phosphor by

coprecipitation and solid state reaction route

Nanoparticles of Y0.95Eu0.05VO4 phosphor were pre-

pared by the coprecipitation (CP) method. For this aim,

three aqueous solutions were prepared as follows.

Appropriate amounts of Y2O3 and Eu2O3 were dis-

solved in hydrochloric acid (HCl 30%) to form solution

A. V2O5 was dissolved in hydrochloric acid (30%) to

generate solution B containing vanadil VO2+ ions. Solu-

tion C is a mixture of NH4OH (25%) and H2O2 (30%)taken in 3:1 ratio. Finally, a diluted solution A was

added to a diluted solution B and pH of the resulting

mixture was brought to 8.0 by adding the solution C.

The final mixture was stirred and heated to about

80 �C until the precipitation was over. The precipitateswere washed with distilled water, dried in trays with a

large exposed surface area, and fired in air during 2h

at 1100–1400 �C. Some (YX,Gd1�X)VO4:Eu (X = 0–1.0)phosphor samples were also prepared by solid state

reaction (SSR) route and compared with the samples

prepared by CP method.

2.2. Structural and luminescent characterization

Phosphor samples were characterized by crystalline

structure and luminescent properties. Particle sizesand morphologies of the investigated phosphors were

determined by scanning electron microscope (SEM) Phi-

lips-XL30SFEG. Crystalline structure of samples was

determined by X-ray diffraction, using a X-ray diffrac-

tometer D/max-RC, Rigaku, Japan with CuKa radiation(k = 1.54056A). Photoluminescence (PL) characteristicswere estimated on the basis of emission and excitation

spectra registered at room temperature (Xe 500W lamp)with DARSA PRO 5100 PL System (Professional Scien-

tific Instrument Co, Korea).

Fig. 2. PL intensity of (YX,La1�X)2O3:Eu phosphor vs. activator

concentration.

Fig. 3. PL excitation spectra of Y2O3:Eu (1) and (Y1�XLaX)2O3:Eu

(2).

M. Nazarov et al. / Optical Materials 27 (2005) 1587–1592 1589

3. Results and discussion

3.1. Phosphors based on europium doped Y2O3–La2O3–

Gd2O3

Y2O3, Gd2O3, and Eu2O3 crystallize in cubic (C)modification and form continuous series of solid solu-

tions. Lanthanum oxide crystallizes in hexagonal (H)

modification but passes into the monoclinic (M) one

after dissolution in yttrium and gadolinium oxides. The

ionic radius of the La3+ (0.117nm) differs from those of

the Y3+, Eu3+, and Gd3+ (0.104, 0.108, and 0.109nm,

respectively). The difference being under 15%, yttrium,

gadolinium and lanthanum oxide could form, in cer-tain conditions, solid solutions that could efficiently

accommodate europium ions to form red emitting

phosphors.

The solid solutions of Y2O3–La2O3–Gd2O3 can be

formed over the whole composition triangle and crystal-

lize in the cubic, monoclinic, and hexagonal modifica-

tions depending on the composition, Fig. 1.

The ternary solid solutions in the explored areas ofthe composition triangle crystallize predominantly in

the cubic structure. Hexagonal and monoclinic phases

of solid solutions are formed only in La2O3-rich blends

and their fractions increase with the concentration of

lanthanum oxide. A pure hexagonal phase without

impurities was obtained only for the solid solutions con-

taining more than 90mol% of La2O3. Pure monoclinic

phase was not isolated in our synthesis conditions. Theintroduction of Eu3+ activator does not influence the

crystalline structure of the phosphor matrix. Maximum

PL intensity (under k = 394nm excitation) for Y2O3:Euand Gd2O3:Eu phosphors was achieved at 10mol% of

europium oxide, Fig. 2.

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0 0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

Ga2 O

3

C +M

C+M

C

C

H+MH

La2O 3

Y2O3

Fig. 1. Ternary diagram in the system Y2O3–La2O3–Gd2O3 (C, H, M

are the cubic, hexagonal, and monoclinic phases, respectively).

PL intensity of the solid solutions is increased by

introducing of La2O3 into Y2O3:Eu and Gd2O3:Eu

phosphors increases, due to a significant changes in

the PL excitation spectra (Fig. 3).

The (Y1�XLaX)2O3:Eu phosphor show enhanced PL

intensity as compared to the pure Y2O3:Eu, at lowerenergies of exciting photons (<4.7eV). The use of so-

dium carbonate as mineralising agent (flux) the synthesis

temperature to be reduced, but with negative effects on

phosphor quality. The output is diminished and the

product contamination is higher in this case.

3.2. Phosphors based on europium doped Y2O2S–

La2O2S–Gd2O2S

Depending on the thermal treatment regime, Y2O2S,

La2O2S and Gd2O2S show either limited or unlimited

solubility. Y2O2S–La2O2S solid solutions are formed at

1590 M. Nazarov et al. / Optical Materials 27 (2005) 1587–1592

900 �C, when the content of Y2O2S does not exceed20mol%. When the synthesis temperature is raised up

to 1000 �C, the solubility is increased up to 40mol%.The same amount of Gd2O2S can be dissolved in

La2O2S at a synthesis temperature of 1000 �C. Unlimitedsolubility is shown in Y2O2S–Gd2O2S system, which canbe explained by the close values of the cationic radii. As

a result, almost no shift of the photoluminescence bands

over the whole composition range could be observed in

this system. The introduction of 20mol% of La2O3 into

Y2O2S:Eu phosphor leads to the increase PL and CL

brightness with about 20–25%. This variation is accom-

panied by the shift of the main emission peak of the

luminescent spectra towards shorter wavelength. Someternary compositions Y2O2S–La2O2S–Gd2O2S with

equal amounts of La2O2S and Gd2O2S and variable

amount Y2O2S were synthesised at 900 �C temperature.PL characteristics and crystalline structure of samples

(Y0.700Gd0.150La0.150)2O2S, (Y0.600Gd0.200La0.200)2O2S

and (Y0.650Gd0.175La0.175)2O2S, doped with 10mol%

Eu2O3 were determined. The investigated composi-

tion and their characteristics are summarized in theTable 1.

XRD spectra showed that the crystalline structure

of (Y0.700Gd0.150La0.150)2O2S:Eu(10%), (Y0.600Gd0.200-

La0.200)2O2S:Eu(10%) and (Y0.650Gd0.175La0.175)2O2-

S:Eu(10%) is mainly cubic, mixed hexagonal and cubic

and mixed hexagonal and monoclinic, respectively. At

1000 �C, the decrease of Y2O2S amount from 0.700 to

0.600mol/mol phosphor seems to be in the favour ofthe formation of the hexagonal structure. The same

effect was observed for the decrease of the europium

oxide concentration. Sample (Y0.650Gd0.175La0.175)2-

O2S:Eu(2%) possesses a well formed hexagonal crystal-

line structure. It seems that europium trends to stabilize

the monoclinic structure of the matrix. PL spectra illus-

trate that for (Y,Gd,La)2O2S:Eu(10%), the strongest

emission is at about 611nm for Y-rich sample withcubic structure and at about 630nm for sample with

medium Y-content, showing mixed hexagonal-mono-

clinic structure.

Table 1

Synthesis conditions and PL characteristics of phosphors based on europium

Phosphor composition C

Matrix composition Eu2O3 mol/100mol matrix

0.200Y2O2S Æ 0.800La2O2S 10 H

0.600Y2O2S Æ 0.200Gd2O2S Æ 0.200La2O2S 10 C

o

0.650Y2O2S Æ 0.175Gd2O2S Æ 0.175La2O2S 10 H

0.650Y2O2S Æ 0.175Gd2O2S Æ 0.175La2O2S 2 H

0.700Y2O2S Æ 0.150Gd2O2S Æ 0.150La2O2S 10 C

3.3. Phosphors based on europium doped YVO4–GdVO4

system

Fig. 4 shows morphologies and particle sizes of

YVO4:Eu phosphor powders prepared by SSR and CP

methods. The particles of YVO4:Eu prepared by CPmethod are about 200nm, show narrow size distribu-

tion, spherical shape and have a small degree of

agglomeration.

XRD of nanosized YVO4 phosphor obtained just

after simple CP process without heat-treatment is pre-

sented in Fig. 5. A single and partly crystallized phase

was recorded in this case. The introduction of Eu3+ acti-

vator does not influence on the crystalline structure ofthe phosphor matrix, as it was confirmed by XRD anal-

ysis. After subsequent annealing of the samples at

1200 �C during 2h, the crystalline phases of YVO4:Euand Y8V2O17 were obtained (Fig. 5b).

The phosphors of YVO4:Eu and (YX,Gd1�X)VO4:Eu

(X = 0–1.0) synthesized by SSR have tetragonal crystal-

line structure with small deviations in lattice constants

(Fig. 5(c and d)). The values of the lattice constants ofYVO4:Eu and (Y0.4,Gd0.6)VO4:Eu were estimated as

a = b = 7.112, c = 6.29 and a = b = 7.178 and c = 6.329,

respectively. Because ionic radius of Gd is larger than

that one of Y, all three crystalline axes were elongated

by the introduction of gadolinium ions into YVO4 crys-

tal structure. As a result, crystal symmetry of YVO4 was

distorted.

Fig. 6 shows PL emission spectra of YVO4:Eu3+ and

(Y0.4,Gd0Æ6)VO4:Eu3+ phosphor. When Eu replaces Y

in non-centrosymmetric site, the very sharp peaks in

red region are appeared instead of broad band blue

emission of self-activated YVO4. The emission spectra

of YVO4:Eu3+ are mainly determined by the transitions

from the 5D0 level to the7FJ (J = 0,1,2,3,4) in Eu

3+

ion. The well known electric dipole transitions are5D0! 7F2, which occur when the Eu

3+ is located innon-centrosymmetric site of the matrix. The aforemen-

tioned results confirm that as prepared europium

doped solid solutions of Y–Ga–La oxides, oxysulphides

doped Y2O2S–La2O2S–Gd2O2S

rystalline structure PL characteristics

(kexc = 394nm)

kmax Irel

ex–(Y0.2La0.8)O2S 611 45

ub-(Y,Gd,La)O2S and hex La2O2S

r unreacted La2O3

611 59

ex+Mcl 630 74

ex 615 38

ub-(Y0.700Gd0.150La0.150)O2S 611 45

Fig. 5. Powder XRD patterns of (a) As-synthesized YVO4:Eu (PM),

(b) YVO4:Eu obtained after heat treatment (CP), (c) YVO4:Eu (SSR),

and (d) (Y,Gd)VO4:Eu (SSR).

Fig. 6. PL emission spectra of YVO4:Eu and (Y,Gd)VO4:Eu

phosphor.

Fig. 4. SEM micrographs of YVO4:Eu prepared by (a) coprecipitation method and (b) solid-state reaction.

M. Nazarov et al. / Optical Materials 27 (2005) 1587–1592 1591

and vanadates are perspectives for PL and CL appli-

cation.

4. Conclusions

The investigated phosphors effectively convert excita-

tion energy into visual light and show high chemical and

radiation stability.

The solid solutions Y2O3–La2O3–Gd2O3 and Y2O2S–La2O2S–Gd2O2S with different content of Y2O3 and

Y2O2S were synthesised. The ternary diagram in the sys-

tem Y2O3–La2O3–Gd2O3 was constructed and ex-

plained. The obtained samples possess well formed

hexagonal crystalline structure. The activator introduc-

tion does not affect the phosphor matrix structure thus

confirming the substitution of RE of the phosphor ma-

trix by europium ions.Europium-doped Y, La, Gd oxysulphides phosphors

show higher CL efficiency at low applied voltages than

oxides ones. The introduction of 20mol% of La2O3 into

Y2O2S:Eu phosphor leads to the increase PL and CL

brightness with about 20–25%.

Optimized (Y0.4,Gd0.6)VO4:Eu3+ phosphors also

show luminescence intensity higher with about 10% in

comparison with similar commercial sample.The obtained results confirmed that europium doped

solid solutions of Y–Ga–La oxides, oxysulphides and

vanadates are perspectives for PL and CL application

in the display manufacture.

Acknowledgment

This research was supported by a grant (M1-02-KR-

01-0001-02-K18-01-025-1-3) from Information Display

R&D Center, one of the 21st Century Frontier R&D

Program funded by the Ministry of Science and Tech-

nology of Korean government.

References

[1] R. Ranson, E. Evangelou, C. Thomas, J. Appl. Phys. 72 (1998)

2663.

[2] B. Bihari, H. Eilers, B. Tissue, J. Lumin. 75 (1997) 1.

1592 M. Nazarov et al. / Optical Materials 27 (2005) 1587–1592

[3] R. Schmechel, M. Kennedy, H. von Seggem, H. Winkler, M.

Kolbe, R. Fisher, L. Xaomao, A. Benker, M. Winterer, H. Hahn,

J. Appl. Phys. 89 (2001) 1679.

[4] P. Majewski, M. Rozumek, H. Schluckwerder, F. Aldinger, Int.

J. Inorganic Mater. 3 (2001) 1343.

[5] Garcia-Murillo, C. Luyer, C. Garapon, C. Dujardin, E. Bernstein,

C. Pedrini, J. Mugnier, Opt. Mater. 19 (2002) 161.

[6] W. Jungowska, J. Thermal Anal. Calorimetry 60 (2000) 193.

[7] Feldmann, T. Justel, C. Ronda, P. Schmidt, Adv. Funct. Mater.

13 (7) (2003) 511.

[8] Y. Nakanishi, in: Proc. of the III Int. meeting on Information

Display, IMID, Daegu, Korea, 2003, p. 203.

[9] V.L. Levshin, M.A. Konstantinova, E.A. Trapeznikova, On the

application of rare-earth elements in the chemistry of phosphors,

in: Rare-earth Elements, USSR AN Publishing, Moscow, 1959,

p. 314.

[10] T. Hisamune, Technical trend of phosphors for plasma display

panels, in: Proceedings of the 9th International Display Work-

shops, 2002, p. 685.