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Advance Physics Letter
________________________________________________________________________________
ISSN (Print) : 2349-1094, ISSN (Online) : 2349-1108, Vol_1, Issue_1, 2014
67
Effect of Tb3+
and Gd3+
on Photoluminescence behavior of Eu3+
doped
La 0.6Y 0.4PO4 Phosphor
1
P. Indira, 2
S. Kondala Rao, 3
M Srinivas and 4
K.V.R. Murthy
1
Department of Physics, Sri ABR Govt. Degree College, Repalle 522265, A.P., India
2
Department of Physics, QIS engineering College, Ongole 523002, A.P. India
2
Display Materials Laboratory, Applied Physics Department, Faculty of Technology & Engineering, M.S University of
Baroda, Vadodara – 390 001, India.
3
Luminescent Materials Laboratory, Physics Department, Faculty of Science, The M. S. University of Baroda,
Vadodara-390002, India;
Email: polineni1997@yahoo.com
Abstract -The present paper reports, synthesis and
photoluminescence (PL) studies of Eu3+
(0.5%) doped La 0.6
Y0.4 PO4 phosphor codoped with Tb3+
(0.5%) and
Gd3+
(0.5%) respectively. The phosphors (La 0.6 Y0.4 PO4 : Eu3+
(0.5%) : Tb3+
(0.5%) and La 0.6 Y0.4 PO4 : Eu3+
(0.5%) : Gd3+
(0.5%)) were prepared by solid state diffusion reaction
method, which is the suitable method for large-scale
production. The received phosphor samples were
characterized using XRD, SEM, PL, &CIE techniques. With
dopent (Eu3+)
and with codopents (Tb3+
& Gd3+
) the PL
emission was observed in the range of 530-630 nm under 254
nm excitation. The ion radius distinction between La3+
and
Y3+
is so large therefore, they show the pure monoclinic
phase. From the XRD data, using the Scherer’s formula, the
calculated average crystallite size of La 0.6 Y0.4 PO4: Eu3+
(0.5%) , La 0.6 Y0.4 PO4 : Eu3+
(0.5%) : Tb3+
(0.5%) and La 0.6
Y0.4 PO4: Eu3+
(0.5%) : Gd3+
(0.5%) is around 45.6nm,
41nm,and39nm respectively.The present phosphor can act as
a single host for Red light emission which is used in display
devices,lamps and bio-medical imaging applications.
Keywords: Photoluminescence (PL), Eu3+
doped phosphor,
codopents, CIE coordinates
INTRODUCTION:
Red light emission from (600-630nm) Eu3+
doped
phosphors are extensively used in lamp and display
applications and many Eu3+
doped materials are being
examined for use in new flat panel display technologies.
Among them phosphate phosphors have proved to be
potential candidates for such applications. [1]. The
luminescence of the rare earth ions in inorganic hosts
has been extensively investigated during the last few
decades. In particular, rare earth ortho phosphates
[REPO4] are a very interesting class of host lattices for
activator ion due to their physical-chemical inercy (High
insolubility, stability against high temperature or against
high energy excitations), thus providing durable
phosphor [2]. Moreover, these materials can present
high excitabilities in the ultraviolet region, which
enables them, applicability in plasma display panels
(PDPS) and in new generation fluorescent lamps
(mercury free) [3,4]. The luminescent properties of rare
earth phosphates can be conferred by the presence of
lanthanide (III) ions as activators due to their intense and
narrow emission bands arising from f-f transitions,
which are proper for the generation of individual colors
in multi phosphor devices [5 – 7]. Mixed
orthophosphates composed of two rare earth elements
have also been investigated, indicating that these
phosphates can be used as host lattices for spectroscopic
investigations [8-14].
For REPO4 Phosphor small RE3+
with larger ion
radius, the monoclinic crystal phase structure is
preferred. For REPO4 phosphor of middle RE3+
with
intermediate radius, a partly formation of hydrated
hexagonal structure is favorable. For REPO4 phosphor
of heavy RE3+
with a small radius, a tetragonal crystal
phase is adopted. Therefore, it is very interesting that
what will happen when rare earth ion with different radii
are introduced into one REPO4 systems with PO4
3-
. In
this paper, we have investigated a new crystal phase,
structures, microstructures of the mixed ortho
phosphates REPO4 (RE=La, Y) prepared by solid state
diffusion reaction method. Because of the difference in
ionic radii between these rare earth ions, the crystal
phase microstructures of the products show obvious
differences. At the same time Eu3+
ions and some
codopents (Tb3+
, Gd3+
) have been doped in the mixed
rare earth phosphates. In order to examine the influence
of the codopents on the luminescence of Eu3+
, whose
Photoluminescent behaviors are studied in detail.
SYNTHESIS OF MIXED PHOSPHATES
The starting materials Lanthanum oxide (La2O3), yttrium
oxide (Y2O3), (NH4) H2PO4 , Europium
Advance Physics Letter
________________________________________________________________________________
ISSN (Print) : 2349-1094, ISSN (Online) : 2349-1108, Vol_1, Issue_1, 2014
68
oxide(Eu2O3),Gd2O3and Terbium oxide (Tb4 O7) of high
purity (99.9%) chemicals were used as starting materials
to prepare La 0.6Y 0.4PO4 and Eu,Gd, Tb doped
phosphor. Lanthanum oxide (La 2O3), Yttrium oxide in
stoichiometric proportions is weighed and ground into
a fine powder using agate mortar and pestle. The
grounded sample was placed in an alumina crucible and
fired at 1200°C for 3 hours in a muffle furnace with a
heating rate of 5°C /min. The sample is allowed to cool
to room temperature in the same furnace for about 30
hours. Rare earth ions Eu,Gd, Tb were doped 0.5 molar
percentages. The samples were found out to be white in
color and studied photoluminescence behavior.
PHYSICAL CHARTERIZATION
The X-ray powder diffraction (XRD) pattern of samples
is performed on a Rigaku-D/max 2500 using Cu Kα
radiation. The surface morphology of the samples was
studied using scanning electron microscopy (SEM) (XL
30 CP Philips). The particle size distribution histogram
recorded and particle size was measured using laser
based system Malvern instrument U.K.
Spectrofluorophotometer (SHIMADZU, RF-5301 PC)
was used for PL studies. All the PL spectra were
recorded at room temperature.
RESULTS AND DISCUSSIONS
4.1 Crystal phase and microstructure of mixed rare
earth phosphate:
The present study focuses on the XRD pattern of La3+
to Y3+
of 3:2 molar ratio, i.e. La0.6Y0.4PO4 doped with
Eu3+
and with codopents is shown in figure 1. From the
XRD pattern, it was found that the prominent phase
formed in La 0.6Y 0.4PO4, after diffraction peaks were
well indexed based on JCPDS no. 96-900-1648
indicating the monoclinic phase of monozite structure.
The main peaks were founded around
29.218°,29.290°&29.343° corresponding to a d-value of
about 3.10A° followed by other less intense peaks
corresponds to the monoclinic system of crystal
structures of Lanthanum Yttrium phosphate. All
diffraction patterns were obtained using Cu Kα radiation
(λ= 1.54060 A°). Measurements were made from 2θ=0°
to 60° with steps of 0.008356°. Li et.al, have studied the
crystal phase structure of the mixed rare earth
phosphates indicating the pure LaPO 4and YPO4
crystallize in monoclinic phase and tetragonal phase
respectively, while the mixed phosphate La0.5Y0.5 PO4
belong to the hexagonal phase [15]. Bing Yan et.al
studied with La3+
to Y3+
of 9:1 molar ratio, the product
shows the pure monoclinic phase, just like pure LaPO4
[15].
The crystallite size was calculated using the Scherrer
equation D=k λ/β cos θ. Where the constant k= (0.9), λ
the wavelength of X-rays (1.54060 A°), β the full-width
at half maxima (FWHM) and θ the Bragg angle of the
XRD peak. The calculated average crystallite size of
La 0.6Y 0.4PO4:Eu 3+
(0.5%) is ~ 45.6, La 0.6Y 0.4PO4:Eu
3+
(0.5%) codoped withTb3+
is~ 41nm and when codoped
with Gd average crystallite size is ~39 nm. Fig. 1 is the.
XRD pattern of La 0.6Y 0.4PO4: Eu (0.5%.) along with
codopents.
20 30 40 50 60
0
500
60
0
350
60
0
400
21.162
29.172
33.762 41.952
48.522
57.612
La0.6
Y0.4
PO4
:Eu
29.218
Intensity(arb.u)
2Theta
La0.6
Y0.4
PO4
:Eu codoped with Gd
La0.6
Y0.4
PO4
:Eu codoped with Tb
29.290
29.343
Figure :1 XRD Pattern of La 0.6Y 0.4PO4: Eu(0.5%)
phosphor
4.2. SEM STUDY:
Characterization of particles, surface morphology and
size of nano crystals is done routinely using scanning
electron microscope. The main advantage of SEM is that
they can be used to study the morphology of prepared
nano particles and nano composites. Direct size
measurements obtained from images are often used in
conjunction with other measurements such as powder,
X-ray diffraction (XRD). Figure 2shows the SEM
micrographs of La0.6Y 0.4PO4:Eu (0.5%), La 0.6Y
0.4PO4:Eu (0.5%): Tb (0.5%) and La 0.6Y 0.4PO4:Eu
(0.5%): Gd (0.5%) phosphors. Direct size
measurements obtained from images and the average
particle diameter is observed. From the Scanning
Electron Micrographs of La 0.6Y 0.4PO4: Eu (0.5%)
phosphors, it is found the particles are irregular in
shape with various sizes from of submicron to few
micros and also clusters are found.
Advance Physics Letter
________________________________________________________________________________
ISSN (Print) : 2349-1094, ISSN (Online) : 2349-1108, Vol_1, Issue_1, 2014
69
Fig 2. SEM images of La 0.6Y 0.4PO4: Eu (0.5%), La 0.6Y 0.4PO4: Eu (0.5%):Tb(0.5%)& La 0.6Y 0.4PO4: Eu
(0.5%):Gd(0.5%)
4.3 Photoluminescence Study (PL)
The Photoluminescence Properties under 254 nm
excitation of obtained mixed ortho phosphates are
presented in figure 3. The excitation spectrum La 0.6 Y0.4
PO4 :Eu3+
[figure 3a] presents a broad, intense band
maximum at ~ 254 nm related to a ligand-metal charge
transfer between PO4
3-
groups and RE3+
ions. The strong
Eu3+
intra-configurational f-f transitions are also
observed in the excitation spectrum.
The emission spectrum of La 0.6 Y0.4 PO4: Eu3+
(figure
3b) display the characteristic 5
D0 → 7
Fj (j=1, 2,3,4)
transition of Eu3+.
In general, when the Eu3+
ion is
located at crystallographic site without inversion
symmetry, its hypersensitive forced electric – dipole
transition 5
D0 → 7
F2 red emission dominates in the
emission spectrum. If the Eu3+
site possesses an
inversion center 5
D0 → 7
F1 orange emission is dominant
[16]. Emission spectrum consists of emission peaks in
the range 530 – 630 nm which results from 5
Dj→ 7
Fj (J=
1,2,3) transition of Eu3+
ion under 254 nm excitation.
The Emission spectrum of La 0.6 Y0.4 PO4: Eu3+
codoped
with Tb3+
consist of sharp emission peaks at 544 nm,
589 nm, 594 nm and 612 nm. The transition at 544 nm
originates from the allowed transition 5
D4 → 7
F5 of Tb3+
which is having less intensity. The maximum intensity
peak observed at 612 nm . The intensity of the 612 nm
peak in Eu:Tb doped phosphor increased by two times
when compared with the intensity of the 612 nm peak of
La 0.6 Y0.4 PO4: Eu3+
(0.5%). The distinct emission line
lying between 580 and 630 nm are observed due to
transition from excited 5
D0→7
FJ (J=0,1,2,3) levels of
Eu3+
ions. The origin of these transitions (electric dipole
and magnetic dipole) from emitting levels to transition
levels depending upon of the location of Eu3+
ions in La
0.6 Y0.4 PO4: Eu3+
lattice and type of transition is
determined by selection rule [17]. The most intense peak
at 612 and hump at 623 nm correspond to the
hypersensitive transition between 5
D0 → 7
F2 levels due
to the forced electric dipole transition mechanism. The
less intense peak in the vicinity of 594 nm is ascribed to
the magnetic dipole transition of 5
D0 → 7
F1 levels. The
increase of intensity in La 0.6 Y0.4
PO4: Eu3+
codoped with Tb3+
is due to cross relaxation
energy transfer (CRET). To develop an efficient
quantum cutting phosphor is to utilize a pair of ions that
can share the initial excitation energy [18]. CRET can
occur by multiple – multiple interactions or by exchange
interactions. [19].The Emission spectrum of La 0.6 Y0.4
PO4: Eu3+
codoped with Gd3+
consist of sharp emission
peaks at 535 nm, 589 nm, 594 nm and 612 nm The
emission at 535 nm originates from allowing transition
5
D1→ 7
F1 of Gd3+
which having less intensity. In this
case also maximum intensity peak observed at 612 nm.
The intensity of 612 nm peak increased by two and half
times when compared with La 0.6 Y0.4 PO4:Eu3+
. The
specific case of CRET process for the Gd3+
, Eu3+
pair
[20] is shown in figure3(f). The excited Eu3+
is
responsible for the first visible photon. Resonant energy
migration among excited Gd3+
ion occurs from within
the 6
P state until the energy resides nearby another Eu3+
ion to which it can transfer its energy. The curved arrow,
labeled 2, indicates this. The second excited Eu3+
ion
produces the second visible photon, achieving quantum
splitting. The results indicate that La 0.6 Y0.4 PO4: Eu3+:
Gd3+
can select as a potential candidate for FL
(fluorescent lamp) and compact fluorescent lamps (CFL)
(Ex 254 nm)
Advance Physics Letter
________________________________________________________________________________
ISSN (Print) : 2349-1094, ISSN (Online) : 2349-1108, Vol_1, Issue_1, 2014
70
220 240 260 280 300 320
20
40
60
80
100
120
140
160
1
Excitation spectrum (1)
254 nm
O
2-
Eu
3+
Charge transfer band
PLIntensity(arbu)
Wavelength (nm)
560 580 600 620 640
0
20
40
60
80
100
120
623 nm
612 nm
594 nm
589 nm
PLIntensity(arbu)
Wavelength (nm)
B Emission spectrum
3(a) 3(b)
Fig:3(a):Excitation spectrum of La0.6Y0.4PO4:Eu3+
(0.5%) ; Fig:3(b): Emission spectrum of La0.6Y0.4PO4: Eu3+
(0.5%)
200 250 300 350 400 450 500 550 600 650
0
20
40
60
80
100
120
140
160
180
Fig:3(c):Excitation spectrum monitoring at 611 nm
and Emission spectum of
LaYPO4
:Tb(0.5%)&Eu(0.5%)under 254 nm Ex.
623nm
612 nm
594nm
589nm
544 nm
469 nm
452 nm
398 nm
364 nm254 nm
Excitation spectrum (1)
Emission spectrum (2)
2
1
PLIntensity(arbu)
Wavelength (nm)
200 250 300 350 400 450 500 550 600 650
0
20
40
60
80
100
120
140
160
180
200
220
240
260
623nm
612 nm
594nm
589nm
535 nm
467 nm
254 nm
Fig:3(d):Excitation spectrum monitoring at 611 nm
and Emission spectrum of
LaYPO4
:Gd(0.5%)&Eu(0.5%)under 254 nm Ex.
2
1
PLIntensity(arbu)
Wavelength (nm)
Excitation spectrum (1)
Emission spectrum (2)
3(c) 3(d)
Fig 3(e): Energy level diagrams for all possible transitions of Eu
3+
ion
623nm
612nm
594nm
7Fj
4F
7
2P
1
5D2
5D1
5D0
J=3
J=2
J=1
J=0
Energy(eV)
Transitions
5
D0
7
F2
5
D0
7
F1
Emission wavelength(nm)
612 nm and 621 nm
594 nm
Various transitions of Eu
3+
ion in LaYPO4
host
Fig 3(e):Energy level diagram for all possible transitions of Eu3+
ion
Advance Physics Letter
________________________________________________________________________________
ISSN (Print) : 2349-1094, ISSN (Online) : 2349-1108, Vol_1, Issue_1, 2014
71
0
10
20
30
40
50
Eu
3+
2
1
23
4
5
7
F6
0
1
2
5
D3
5
HJ
0
1
1
6
GJ
6
DJ
6
IJ
6
PJ
5
HJ
8
S7/2
Gd
3+
Eu
3+
5
D3
2
1
0
7
F6
5
4
3 2
1
0
E
(10
3
cm
-1
)
Fig:3(f):Energy level diagrams of Eu3+
and Gd3+
showing the cross-relaxation energy transfer process that leads to
quantum splitting. The dotted arrows labeled 1 identify the resonant CRET which leave the first Eu3+
ion in its 5
D0 state
from which the first photon is produced. The curved arrow labeled 2 describes the subsequent energy transfer from Gd3+
to a second Eu3+
,which produces the second photon.
4.4 CIE Co-ordinates
The CIE co-ordinates calculated by the
Spectrophotometric method using the spectral energy
distribution of the La0.6 Y0.4PO4:Eu(0.5%) sample is
shown in figure 4.The color co-ordinates for the La0.6
Y0.4PO4:Eu(0.5%) sample is x=0.667 and y=0.322
(these co-ordinates are very near to standard values of
Red). Hence this phosphor having excellent color
tunability to Red.
Fig. 4: C.I.E 1931 chart of Eu3+
doped La0.6Y0.4PO4
CONCLUSIONS:
La0.6Y0.4PO4 phosphor doped with the Eu and with co
dopents (Tb, Gd) were synthesized via high temperature
solid state diffusion reaction method. The crystal phases
and microstructures of the products can be favorable for
the formation of phosphors in pure phase La0.6Y0.4PO4
presents the pure monoclinic phase and nano size
crystalline material. The La0.6Y0.4PO4 : Eu (0.5%) and
with codepents phosphor materials present very
attractive luminescent property for the generation of the
red color. Cross relaxation energy transfer process
(CRET) observed when La0.6Y0.4PO4 : Eu(0.5%)
codoped with Gd3+
as well as co doped with Tb3+
. The
maximum PL intensity observed when La0.6Y0.4PO4:Eu
codoped with Gd3+
. From the PL studies, it is concluded
that Eu(0.5%) : Gd (0.5%) doped La0.6Y0.4PO4 phosphor
under 254 nm excitation can act as a single host for
producing Red light with good intensity for all practical
display devices in particular compact fluorescent lamps.
REFERENCES.
[1] William M, Yen Shigeo. Shionoya, Hajime
Yamamoto Phosphor Handbook, CRC Press,
2006 p.m. 462
[2] C.Fledman, T.Jüstel, C.Ronda, P.Schmidt, Adv.
Funct.Mater.13 (2003) 511
Advance Physics Letter
________________________________________________________________________________
ISSN (Print) : 2349-1094, ISSN (Online) : 2349-1108, Vol_1, Issue_1, 2014
72
[3] L. Ninistoo;M. Leskelain;K.A. Gscheidner Jr.. L.
Eyring [Eds] Handbook on the Physics and
Chemistry of rare Earths, Elsevier, Amsterdam,
1987 (chapter59)
[4] D.C
Tuan,R.Olazcuaga,F.Guillen,A.Garcia,B.Moine,
C.Fouassier.J.Physics 1v 123 (2005) 259
[5] G. Blasse, B. C. Grabmaier, Luminescent
materials springer, Berlin 1994 (Chapter 2, 6)
[6] P. Porcher, in: R. Saez, P. A. Caro (Eds), Rare
Earths, Editorial Complutense, Madrid, 1999
(chapter 3)
[7] J-C. G. Bunzli, in: J- C. G. Bunzli, G. R. Choppin
(Eds) Lanthanide probes in life, Chemical and
Earth science, Elsevier, Amsterdam, 1989
(chapter7)
[8] Riwotzki K,Messamy H,Kornowski,Hasse
M:Angew.chem.Int.Ed..2001 40:573
[9] Riwotzki K,Messamy H,Kornowski,Hasse
J.Phys.chemB.2000,104:2824
[10] Nishihama S, Hirai T, Komasawa I:
J.Mater.Chem..2002, 12:1053
[11] Schuetz P,Caruso F Chem.Mater 2002,14:4509
[12] Mullica DF,Sappenfield EL,Boatner LA:
Inorg.Chem.Acta.1996 244:247
[13] Ito H, Fujishiro Y,Sato T,Okuwaki A:
Br.Ceram.Trans..1995,94:146
[14] Yan RX,Sun XM,Wang X,Peng Q, Li YD:
Chem.Eur.J..2005,11:2183
[15] Bing Yan and Xiuzhen Xiao Nano Research
Letters 2010,5:1962
[16] Lee Kyong-Gue, Yu Byung-Yong, Pyun Chong-
Hong, Mho Sun-II, Vacuum ultraviolet excitation
and photoluminescence characteristics of
(Y,Gd)Al3(BO3)4/Eu3+
solidstate
commun.122(2002)485
[17] Jagjeet Kaur,N.S.Suryanarayana,Dubey Vikas,
Rajput Neha.Res.J.Chem.sci.(6)(2011)
[18] Dexter, D.L., J. Chem. Phys 21,836,1953
[19] Wegh, R.T, Donker, H, Oskam, K., and
Meijerink, A, J. Lumin.82, 93,1999
[20] William M, Yen Shigeo. Shionoya, Hajime
Yamamoto Phosphor Handbook, CRC Press,
2006 p.m. 519


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17

  • 1. Advance Physics Letter ________________________________________________________________________________ ISSN (Print) : 2349-1094, ISSN (Online) : 2349-1108, Vol_1, Issue_1, 2014 67 Effect of Tb3+ and Gd3+ on Photoluminescence behavior of Eu3+ doped La 0.6Y 0.4PO4 Phosphor 1 P. Indira, 2 S. Kondala Rao, 3 M Srinivas and 4 K.V.R. Murthy 1 Department of Physics, Sri ABR Govt. Degree College, Repalle 522265, A.P., India 2 Department of Physics, QIS engineering College, Ongole 523002, A.P. India 2 Display Materials Laboratory, Applied Physics Department, Faculty of Technology & Engineering, M.S University of Baroda, Vadodara – 390 001, India. 3 Luminescent Materials Laboratory, Physics Department, Faculty of Science, The M. S. University of Baroda, Vadodara-390002, India; Email: polineni1997@yahoo.com Abstract -The present paper reports, synthesis and photoluminescence (PL) studies of Eu3+ (0.5%) doped La 0.6 Y0.4 PO4 phosphor codoped with Tb3+ (0.5%) and Gd3+ (0.5%) respectively. The phosphors (La 0.6 Y0.4 PO4 : Eu3+ (0.5%) : Tb3+ (0.5%) and La 0.6 Y0.4 PO4 : Eu3+ (0.5%) : Gd3+ (0.5%)) were prepared by solid state diffusion reaction method, which is the suitable method for large-scale production. The received phosphor samples were characterized using XRD, SEM, PL, &CIE techniques. With dopent (Eu3+) and with codopents (Tb3+ & Gd3+ ) the PL emission was observed in the range of 530-630 nm under 254 nm excitation. The ion radius distinction between La3+ and Y3+ is so large therefore, they show the pure monoclinic phase. From the XRD data, using the Scherer’s formula, the calculated average crystallite size of La 0.6 Y0.4 PO4: Eu3+ (0.5%) , La 0.6 Y0.4 PO4 : Eu3+ (0.5%) : Tb3+ (0.5%) and La 0.6 Y0.4 PO4: Eu3+ (0.5%) : Gd3+ (0.5%) is around 45.6nm, 41nm,and39nm respectively.The present phosphor can act as a single host for Red light emission which is used in display devices,lamps and bio-medical imaging applications. Keywords: Photoluminescence (PL), Eu3+ doped phosphor, codopents, CIE coordinates INTRODUCTION: Red light emission from (600-630nm) Eu3+ doped phosphors are extensively used in lamp and display applications and many Eu3+ doped materials are being examined for use in new flat panel display technologies. Among them phosphate phosphors have proved to be potential candidates for such applications. [1]. The luminescence of the rare earth ions in inorganic hosts has been extensively investigated during the last few decades. In particular, rare earth ortho phosphates [REPO4] are a very interesting class of host lattices for activator ion due to their physical-chemical inercy (High insolubility, stability against high temperature or against high energy excitations), thus providing durable phosphor [2]. Moreover, these materials can present high excitabilities in the ultraviolet region, which enables them, applicability in plasma display panels (PDPS) and in new generation fluorescent lamps (mercury free) [3,4]. The luminescent properties of rare earth phosphates can be conferred by the presence of lanthanide (III) ions as activators due to their intense and narrow emission bands arising from f-f transitions, which are proper for the generation of individual colors in multi phosphor devices [5 – 7]. Mixed orthophosphates composed of two rare earth elements have also been investigated, indicating that these phosphates can be used as host lattices for spectroscopic investigations [8-14]. For REPO4 Phosphor small RE3+ with larger ion radius, the monoclinic crystal phase structure is preferred. For REPO4 phosphor of middle RE3+ with intermediate radius, a partly formation of hydrated hexagonal structure is favorable. For REPO4 phosphor of heavy RE3+ with a small radius, a tetragonal crystal phase is adopted. Therefore, it is very interesting that what will happen when rare earth ion with different radii are introduced into one REPO4 systems with PO4 3- . In this paper, we have investigated a new crystal phase, structures, microstructures of the mixed ortho phosphates REPO4 (RE=La, Y) prepared by solid state diffusion reaction method. Because of the difference in ionic radii between these rare earth ions, the crystal phase microstructures of the products show obvious differences. At the same time Eu3+ ions and some codopents (Tb3+ , Gd3+ ) have been doped in the mixed rare earth phosphates. In order to examine the influence of the codopents on the luminescence of Eu3+ , whose Photoluminescent behaviors are studied in detail. SYNTHESIS OF MIXED PHOSPHATES The starting materials Lanthanum oxide (La2O3), yttrium oxide (Y2O3), (NH4) H2PO4 , Europium
  • 2. Advance Physics Letter ________________________________________________________________________________ ISSN (Print) : 2349-1094, ISSN (Online) : 2349-1108, Vol_1, Issue_1, 2014 68 oxide(Eu2O3),Gd2O3and Terbium oxide (Tb4 O7) of high purity (99.9%) chemicals were used as starting materials to prepare La 0.6Y 0.4PO4 and Eu,Gd, Tb doped phosphor. Lanthanum oxide (La 2O3), Yttrium oxide in stoichiometric proportions is weighed and ground into a fine powder using agate mortar and pestle. The grounded sample was placed in an alumina crucible and fired at 1200°C for 3 hours in a muffle furnace with a heating rate of 5°C /min. The sample is allowed to cool to room temperature in the same furnace for about 30 hours. Rare earth ions Eu,Gd, Tb were doped 0.5 molar percentages. The samples were found out to be white in color and studied photoluminescence behavior. PHYSICAL CHARTERIZATION The X-ray powder diffraction (XRD) pattern of samples is performed on a Rigaku-D/max 2500 using Cu Kα radiation. The surface morphology of the samples was studied using scanning electron microscopy (SEM) (XL 30 CP Philips). The particle size distribution histogram recorded and particle size was measured using laser based system Malvern instrument U.K. Spectrofluorophotometer (SHIMADZU, RF-5301 PC) was used for PL studies. All the PL spectra were recorded at room temperature. RESULTS AND DISCUSSIONS 4.1 Crystal phase and microstructure of mixed rare earth phosphate: The present study focuses on the XRD pattern of La3+ to Y3+ of 3:2 molar ratio, i.e. La0.6Y0.4PO4 doped with Eu3+ and with codopents is shown in figure 1. From the XRD pattern, it was found that the prominent phase formed in La 0.6Y 0.4PO4, after diffraction peaks were well indexed based on JCPDS no. 96-900-1648 indicating the monoclinic phase of monozite structure. The main peaks were founded around 29.218°,29.290°&29.343° corresponding to a d-value of about 3.10A° followed by other less intense peaks corresponds to the monoclinic system of crystal structures of Lanthanum Yttrium phosphate. All diffraction patterns were obtained using Cu Kα radiation (λ= 1.54060 A°). Measurements were made from 2θ=0° to 60° with steps of 0.008356°. Li et.al, have studied the crystal phase structure of the mixed rare earth phosphates indicating the pure LaPO 4and YPO4 crystallize in monoclinic phase and tetragonal phase respectively, while the mixed phosphate La0.5Y0.5 PO4 belong to the hexagonal phase [15]. Bing Yan et.al studied with La3+ to Y3+ of 9:1 molar ratio, the product shows the pure monoclinic phase, just like pure LaPO4 [15]. The crystallite size was calculated using the Scherrer equation D=k λ/β cos θ. Where the constant k= (0.9), λ the wavelength of X-rays (1.54060 A°), β the full-width at half maxima (FWHM) and θ the Bragg angle of the XRD peak. The calculated average crystallite size of La 0.6Y 0.4PO4:Eu 3+ (0.5%) is ~ 45.6, La 0.6Y 0.4PO4:Eu 3+ (0.5%) codoped withTb3+ is~ 41nm and when codoped with Gd average crystallite size is ~39 nm. Fig. 1 is the. XRD pattern of La 0.6Y 0.4PO4: Eu (0.5%.) along with codopents. 20 30 40 50 60 0 500 60 0 350 60 0 400 21.162 29.172 33.762 41.952 48.522 57.612 La0.6 Y0.4 PO4 :Eu 29.218 Intensity(arb.u) 2Theta La0.6 Y0.4 PO4 :Eu codoped with Gd La0.6 Y0.4 PO4 :Eu codoped with Tb 29.290 29.343 Figure :1 XRD Pattern of La 0.6Y 0.4PO4: Eu(0.5%) phosphor 4.2. SEM STUDY: Characterization of particles, surface morphology and size of nano crystals is done routinely using scanning electron microscope. The main advantage of SEM is that they can be used to study the morphology of prepared nano particles and nano composites. Direct size measurements obtained from images are often used in conjunction with other measurements such as powder, X-ray diffraction (XRD). Figure 2shows the SEM micrographs of La0.6Y 0.4PO4:Eu (0.5%), La 0.6Y 0.4PO4:Eu (0.5%): Tb (0.5%) and La 0.6Y 0.4PO4:Eu (0.5%): Gd (0.5%) phosphors. Direct size measurements obtained from images and the average particle diameter is observed. From the Scanning Electron Micrographs of La 0.6Y 0.4PO4: Eu (0.5%) phosphors, it is found the particles are irregular in shape with various sizes from of submicron to few micros and also clusters are found.
  • 3. Advance Physics Letter ________________________________________________________________________________ ISSN (Print) : 2349-1094, ISSN (Online) : 2349-1108, Vol_1, Issue_1, 2014 69 Fig 2. SEM images of La 0.6Y 0.4PO4: Eu (0.5%), La 0.6Y 0.4PO4: Eu (0.5%):Tb(0.5%)& La 0.6Y 0.4PO4: Eu (0.5%):Gd(0.5%) 4.3 Photoluminescence Study (PL) The Photoluminescence Properties under 254 nm excitation of obtained mixed ortho phosphates are presented in figure 3. The excitation spectrum La 0.6 Y0.4 PO4 :Eu3+ [figure 3a] presents a broad, intense band maximum at ~ 254 nm related to a ligand-metal charge transfer between PO4 3- groups and RE3+ ions. The strong Eu3+ intra-configurational f-f transitions are also observed in the excitation spectrum. The emission spectrum of La 0.6 Y0.4 PO4: Eu3+ (figure 3b) display the characteristic 5 D0 → 7 Fj (j=1, 2,3,4) transition of Eu3+. In general, when the Eu3+ ion is located at crystallographic site without inversion symmetry, its hypersensitive forced electric – dipole transition 5 D0 → 7 F2 red emission dominates in the emission spectrum. If the Eu3+ site possesses an inversion center 5 D0 → 7 F1 orange emission is dominant [16]. Emission spectrum consists of emission peaks in the range 530 – 630 nm which results from 5 Dj→ 7 Fj (J= 1,2,3) transition of Eu3+ ion under 254 nm excitation. The Emission spectrum of La 0.6 Y0.4 PO4: Eu3+ codoped with Tb3+ consist of sharp emission peaks at 544 nm, 589 nm, 594 nm and 612 nm. The transition at 544 nm originates from the allowed transition 5 D4 → 7 F5 of Tb3+ which is having less intensity. The maximum intensity peak observed at 612 nm . The intensity of the 612 nm peak in Eu:Tb doped phosphor increased by two times when compared with the intensity of the 612 nm peak of La 0.6 Y0.4 PO4: Eu3+ (0.5%). The distinct emission line lying between 580 and 630 nm are observed due to transition from excited 5 D0→7 FJ (J=0,1,2,3) levels of Eu3+ ions. The origin of these transitions (electric dipole and magnetic dipole) from emitting levels to transition levels depending upon of the location of Eu3+ ions in La 0.6 Y0.4 PO4: Eu3+ lattice and type of transition is determined by selection rule [17]. The most intense peak at 612 and hump at 623 nm correspond to the hypersensitive transition between 5 D0 → 7 F2 levels due to the forced electric dipole transition mechanism. The less intense peak in the vicinity of 594 nm is ascribed to the magnetic dipole transition of 5 D0 → 7 F1 levels. The increase of intensity in La 0.6 Y0.4 PO4: Eu3+ codoped with Tb3+ is due to cross relaxation energy transfer (CRET). To develop an efficient quantum cutting phosphor is to utilize a pair of ions that can share the initial excitation energy [18]. CRET can occur by multiple – multiple interactions or by exchange interactions. [19].The Emission spectrum of La 0.6 Y0.4 PO4: Eu3+ codoped with Gd3+ consist of sharp emission peaks at 535 nm, 589 nm, 594 nm and 612 nm The emission at 535 nm originates from allowing transition 5 D1→ 7 F1 of Gd3+ which having less intensity. In this case also maximum intensity peak observed at 612 nm. The intensity of 612 nm peak increased by two and half times when compared with La 0.6 Y0.4 PO4:Eu3+ . The specific case of CRET process for the Gd3+ , Eu3+ pair [20] is shown in figure3(f). The excited Eu3+ is responsible for the first visible photon. Resonant energy migration among excited Gd3+ ion occurs from within the 6 P state until the energy resides nearby another Eu3+ ion to which it can transfer its energy. The curved arrow, labeled 2, indicates this. The second excited Eu3+ ion produces the second visible photon, achieving quantum splitting. The results indicate that La 0.6 Y0.4 PO4: Eu3+: Gd3+ can select as a potential candidate for FL (fluorescent lamp) and compact fluorescent lamps (CFL) (Ex 254 nm)
  • 4. Advance Physics Letter ________________________________________________________________________________ ISSN (Print) : 2349-1094, ISSN (Online) : 2349-1108, Vol_1, Issue_1, 2014 70 220 240 260 280 300 320 20 40 60 80 100 120 140 160 1 Excitation spectrum (1) 254 nm O 2- Eu 3+ Charge transfer band PLIntensity(arbu) Wavelength (nm) 560 580 600 620 640 0 20 40 60 80 100 120 623 nm 612 nm 594 nm 589 nm PLIntensity(arbu) Wavelength (nm) B Emission spectrum 3(a) 3(b) Fig:3(a):Excitation spectrum of La0.6Y0.4PO4:Eu3+ (0.5%) ; Fig:3(b): Emission spectrum of La0.6Y0.4PO4: Eu3+ (0.5%) 200 250 300 350 400 450 500 550 600 650 0 20 40 60 80 100 120 140 160 180 Fig:3(c):Excitation spectrum monitoring at 611 nm and Emission spectum of LaYPO4 :Tb(0.5%)&Eu(0.5%)under 254 nm Ex. 623nm 612 nm 594nm 589nm 544 nm 469 nm 452 nm 398 nm 364 nm254 nm Excitation spectrum (1) Emission spectrum (2) 2 1 PLIntensity(arbu) Wavelength (nm) 200 250 300 350 400 450 500 550 600 650 0 20 40 60 80 100 120 140 160 180 200 220 240 260 623nm 612 nm 594nm 589nm 535 nm 467 nm 254 nm Fig:3(d):Excitation spectrum monitoring at 611 nm and Emission spectrum of LaYPO4 :Gd(0.5%)&Eu(0.5%)under 254 nm Ex. 2 1 PLIntensity(arbu) Wavelength (nm) Excitation spectrum (1) Emission spectrum (2) 3(c) 3(d) Fig 3(e): Energy level diagrams for all possible transitions of Eu 3+ ion 623nm 612nm 594nm 7Fj 4F 7 2P 1 5D2 5D1 5D0 J=3 J=2 J=1 J=0 Energy(eV) Transitions 5 D0 7 F2 5 D0 7 F1 Emission wavelength(nm) 612 nm and 621 nm 594 nm Various transitions of Eu 3+ ion in LaYPO4 host Fig 3(e):Energy level diagram for all possible transitions of Eu3+ ion
  • 5. Advance Physics Letter ________________________________________________________________________________ ISSN (Print) : 2349-1094, ISSN (Online) : 2349-1108, Vol_1, Issue_1, 2014 71 0 10 20 30 40 50 Eu 3+ 2 1 23 4 5 7 F6 0 1 2 5 D3 5 HJ 0 1 1 6 GJ 6 DJ 6 IJ 6 PJ 5 HJ 8 S7/2 Gd 3+ Eu 3+ 5 D3 2 1 0 7 F6 5 4 3 2 1 0 E (10 3 cm -1 ) Fig:3(f):Energy level diagrams of Eu3+ and Gd3+ showing the cross-relaxation energy transfer process that leads to quantum splitting. The dotted arrows labeled 1 identify the resonant CRET which leave the first Eu3+ ion in its 5 D0 state from which the first photon is produced. The curved arrow labeled 2 describes the subsequent energy transfer from Gd3+ to a second Eu3+ ,which produces the second photon. 4.4 CIE Co-ordinates The CIE co-ordinates calculated by the Spectrophotometric method using the spectral energy distribution of the La0.6 Y0.4PO4:Eu(0.5%) sample is shown in figure 4.The color co-ordinates for the La0.6 Y0.4PO4:Eu(0.5%) sample is x=0.667 and y=0.322 (these co-ordinates are very near to standard values of Red). Hence this phosphor having excellent color tunability to Red. Fig. 4: C.I.E 1931 chart of Eu3+ doped La0.6Y0.4PO4 CONCLUSIONS: La0.6Y0.4PO4 phosphor doped with the Eu and with co dopents (Tb, Gd) were synthesized via high temperature solid state diffusion reaction method. The crystal phases and microstructures of the products can be favorable for the formation of phosphors in pure phase La0.6Y0.4PO4 presents the pure monoclinic phase and nano size crystalline material. The La0.6Y0.4PO4 : Eu (0.5%) and with codepents phosphor materials present very attractive luminescent property for the generation of the red color. Cross relaxation energy transfer process (CRET) observed when La0.6Y0.4PO4 : Eu(0.5%) codoped with Gd3+ as well as co doped with Tb3+ . The maximum PL intensity observed when La0.6Y0.4PO4:Eu codoped with Gd3+ . From the PL studies, it is concluded that Eu(0.5%) : Gd (0.5%) doped La0.6Y0.4PO4 phosphor under 254 nm excitation can act as a single host for producing Red light with good intensity for all practical display devices in particular compact fluorescent lamps. REFERENCES. [1] William M, Yen Shigeo. Shionoya, Hajime Yamamoto Phosphor Handbook, CRC Press, 2006 p.m. 462 [2] C.Fledman, T.Jüstel, C.Ronda, P.Schmidt, Adv. Funct.Mater.13 (2003) 511
  • 6. Advance Physics Letter ________________________________________________________________________________ ISSN (Print) : 2349-1094, ISSN (Online) : 2349-1108, Vol_1, Issue_1, 2014 72 [3] L. Ninistoo;M. Leskelain;K.A. Gscheidner Jr.. L. Eyring [Eds] Handbook on the Physics and Chemistry of rare Earths, Elsevier, Amsterdam, 1987 (chapter59) [4] D.C Tuan,R.Olazcuaga,F.Guillen,A.Garcia,B.Moine, C.Fouassier.J.Physics 1v 123 (2005) 259 [5] G. Blasse, B. C. Grabmaier, Luminescent materials springer, Berlin 1994 (Chapter 2, 6) [6] P. Porcher, in: R. Saez, P. A. Caro (Eds), Rare Earths, Editorial Complutense, Madrid, 1999 (chapter 3) [7] J-C. G. Bunzli, in: J- C. G. Bunzli, G. R. Choppin (Eds) Lanthanide probes in life, Chemical and Earth science, Elsevier, Amsterdam, 1989 (chapter7) [8] Riwotzki K,Messamy H,Kornowski,Hasse M:Angew.chem.Int.Ed..2001 40:573 [9] Riwotzki K,Messamy H,Kornowski,Hasse J.Phys.chemB.2000,104:2824 [10] Nishihama S, Hirai T, Komasawa I: J.Mater.Chem..2002, 12:1053 [11] Schuetz P,Caruso F Chem.Mater 2002,14:4509 [12] Mullica DF,Sappenfield EL,Boatner LA: Inorg.Chem.Acta.1996 244:247 [13] Ito H, Fujishiro Y,Sato T,Okuwaki A: Br.Ceram.Trans..1995,94:146 [14] Yan RX,Sun XM,Wang X,Peng Q, Li YD: Chem.Eur.J..2005,11:2183 [15] Bing Yan and Xiuzhen Xiao Nano Research Letters 2010,5:1962 [16] Lee Kyong-Gue, Yu Byung-Yong, Pyun Chong- Hong, Mho Sun-II, Vacuum ultraviolet excitation and photoluminescence characteristics of (Y,Gd)Al3(BO3)4/Eu3+ solidstate commun.122(2002)485 [17] Jagjeet Kaur,N.S.Suryanarayana,Dubey Vikas, Rajput Neha.Res.J.Chem.sci.(6)(2011) [18] Dexter, D.L., J. Chem. Phys 21,836,1953 [19] Wegh, R.T, Donker, H, Oskam, K., and Meijerink, A, J. Lumin.82, 93,1999 [20] William M, Yen Shigeo. Shionoya, Hajime Yamamoto Phosphor Handbook, CRC Press, 2006 p.m. 519 