Photoluminescent properties of phosphors in the system Ca xCd1-xMoO4:Eu3+, Li+

Article abstract of DOI:10.1149/1.1923708

The luminescent properties of the new red phosphors in the solid solution system Ca_xCd_1-xMoO₄:Eu³⁺, Li ⁺ are reported. Their dominating emission peaks are at 615 nm, which satisfies color purity. Under the excitation of ～ 320 nm UV light, some selected samples have luminescent intensity 30% higher than that of the commercial red phosphor Y₂O₂S:Eu³⁺. Therefore, it is a promising material for N₂ plasma display application.

Full text of DOI:10.1149/1.1923708

G534
Journal of The Electrochemical Society, 152 ͑7͒ G534-G536 ͑2005͒
0
013-4651/2005/152͑7͒/G534/3/$7.00 © The Electrochemical Society, Inc.
Photoluminescent Properties of Phosphors in the System
3
+
+
Ca Cd MoO :Eu , Li
x
1−x
4
a,b
b,z
b
b
b
Jiaguo Wang, Xiping Jing, Chunhua Yan, Jianhua Lin, and Fuhui Liao
a
College of Chemistry and Materials Science, Wenzhou Normal University, Wenzhou 325027,
China
b
The State Key Laboratory of Rare Earth Materials Application and Chemistry, College of Chemistry
and Molecular Engineering, Peking University, Beijing 100871, China
The luminescent properties of the new red phosphors in the solid solution system Ca Cd1−xMoO :Eu3+, Li+ are reported. Their
x
4
dominating emission peaks are at 615 nm, which satisfies color purity. Under the excitation of ϳ320 nm UV light, some selected
3
+
samples have luminescent intensity 30% higher than that of the commercial red phosphor Y O S:Eu . Therefore, it is a promising
2
2
material for N plasma display application.
2
©
2005 The Electrochemical Society. ͓DOI: 10.1149/1.1923708͔ All rights reserved.
Manuscript submitted September 30, 2004; revised manuscript received January 17, 2005. Available electronically June 3, 2005.
1
2−
Choi et al. have put forward a plasma display panel ͑PDP͒ with
O , which forms dodecahedrons; the dodecahedrons connect to
2
−
6+
N and Ne mixture as discharge gas, which emits UV light in the
each other by sharing edges. Each O bonds one Mo and two
bivalent cations. When doped with Eu , an equal amount of Li
was codoped for the charge balance. Phase analysis indicated that all
samples, doped and undoped, were phase pure, which suggested that
the doped samples formed solid solutions. The doping mechanism
2
3
+
+
wavelength range 300-400 nm. Compared to conventional PDP
devices, which emit vacuum UV with the wavelengths 147 and 173
nm from Xe, the longer emitting wavelength in the new device,
named N -PDP, is beneficial to energy efficiency, because in this
device, the phosphors have smaller Stokes shift. Moreover, due to
excitation by lower energy radiation, the phosphors may have better
2
3
+
+
2+
2+
3+
should be: Eu + Li ↔ 2Ca ͑Cd ͒. Eu ͑106.6 pm͒ has similar
2+
2+
5
radius to Ca ͑112 pm͒ and Cd ͑110 pm͒; thus, Scheelite struc-
duration in this device. N -PDP requires phosphors having effective
excitations in the wavelength range 300-400 nm. Due to the
ture was kept when doping Eu and Li+ to both CaMoO4 and
3+
2
2+
2+
3+
+
CdMoO , even if all Ca or Cd was replaced by Eu and Li ,
though Li has a small radius. In our experimental results,
Eu0.5Li0.5MoO had a similar unit cell to CaMoO , and with the unit
cell parameters a = 0.5201 nm, c = 1.1332 nm, the values were sig-
4
2
+
excitation spectra of blue phosphors Sr ͑PO ͒ Cl :Eu
and
+
10
4 6
2
2
+
MgBaAl O :Eu extending over 400 nm, they can be employed
10
17
4
4
as the blue components for the N -PDP devices. The green phos-
2
2
+
2+
phor, MgBaAl O :Mn , Eu , also has an excitation band in the
1
0
17
nificantly smaller than those of CaMoO . The unit cell parameters of
4
range 300-400 nm; thus, it may be utilized as the green component.
Because both YVO :Eu and Y O S:Eu show their excitation
peaks at around 330 nm, they may be considered as red components.
Our group patented molybdate as red phosphor matrices previ-
ously. In the present work, we report the detailed luminescent prop-
erties of the phosphors in the system Ca Cd MoO :Eu , Li .
They emit saturated red light with good color purity. Some of them
show better efficiency than the commercial phosphors YVO :Eu
and Y O S:Eu .
Eu0.5Li0.5MoO4 were considerably larger than those of CdMoO4.
This may be due to the particular structure features of this bivalent
cation wholly replaced sample. Actually, the dodecahedron located
by the bivalent cation is too large for the small cation Li ͑92 pm͒
and it is an unstable structure for Li to place at this site. Thus Li
tends to deviate from the center and shift to one side of the dodeca-
hedrons. This structural feature may be related to the unit cell in-
crease. A similar structural feature was also found in another molyb-
3
+
3+
4
2
2
2
,3
+
+
+
3
+
+
x
1−x
4
3
+
4
3
+
6
2
2
date, Li05625Li0.3125MoO . The variation of unit cell volume with
4
doping content in the system Cd1−xEu0.5xLi0.5xMoO4 is shown in
Experimental
Raw materials Eu O ͑99.99%͒, MoO ͑99.5%͒, CdO ͑analytical
3
+
Fig. 1. The Eu -doped CaMoO was discussed in our previous
4
7
2+
2+
2
3
3
paper. Furthermore, Ca and Cd have almost the same radii;
hence, the system Ca Cd MoO formed solid solutions over the
reagent͒, CaCO ͑Ͼ99%͒, and Li CO ͑analytical reagent͒ were
3
2
3
x
1−x
4
2
+
used for the sample preparation. They were weighed and ground by
using an agate mortar and pestle. A small amount of ethanol was
added during the grindings in order to obtain homogenous mixtures.
The samples were heated in a muffle furnace at 550°C for 10 h and
then at 800°C for another 2 h. Finally, the samples were ground into
powders for characterization.
whole range. The variation of the unit cell volume with Cd content
is also represented in Fig. 1.
Reference 8 shows that undoped CaMoO emits faint green light
4
with an emission peak at 520 nm under excitation of ϳ295 nm UV.
Similarly, CdMoO also emits green light at the same wavelength
4
with the excitation peak at ϳ335 nm, but its intensity is even
weaker ͑see Fig. 2͒. The red shifts for the excitation peaks from
CaMoO to CdMoO are attributed to the difference of the elec-
Phase purities were analyzed by using a Rigaku D/max2000
X-ray diffractometer. Photoluminescences were measured using a
Hitachi F4500 fluorescent spectrophotometer and the luminescent
decay curves were recorded by using a lab-established yttrium alu-
minum garnet ͑YAG͒:Nd laser system with an excitation wavelength
of 266 nm.
4
4
tronegativity between Ca ͑1.0͒ and Cd ͑1.7͒; smaller electronegativ-
ity difference between the bivalent cation elements and oxygen
make the electrons in the lattice more delocalized and the excitation
energy lower. Groenink et al. and Abraham et al. reported the in-
9
,10
trinsic optical properties of CaMoO to CdMoO , respectively.
In
4
4
Results and Discussion
2
−
both hosts, the complex ions MoO are considered as the lumines-
4
4
Both CaMoO4 and CdMoO4 belong to Scheelite structure,
cent centers. Their excitation and emission are due to the charge-
which has a tetragonal unit cell with space group I4 /a, Z = 4, the
6+
2−
1
transfer transitions of Mo -O . As a potential laser material,
unit cell parameters a = 0.5226 nm, c = 1.1430 nm for the former,
and a = 0.5170 nm, c = 1.1190 nm for the latter. In this structure,
3+
3+
CaMoO4 doped with other rare earth ions ͑Nd , Dy ͒ was re-
11,12
ported in the past few years.
6
+
2−
2−
4
Mo occupies the tetrahedral sites constructed by O , and MoO
Figure 2 also represents the excitation and emission spectra of
tetrahedrons are isolated. Bivalent cations are eight-coordinated by
the Eu and Li+ codoped sample Cd0.96Eu0.02Li0.02MoO , which
3+
4
3
+
shows typical Eu red emissions with color coordinates x = 0.665
and y = 0.335. The excitation spectrum may be divided into two
regions. The strong and wide band below 360 nm may be assigned
z
E-mail: xpjing@pku.edu.cn
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Journal of The Electrochemical Society, 152 ͑7͒ G534-G536 ͑2005͒
G535
Figure 1. Variation of unit cell volume with Eu3+ and Li or Ca content in
+
2+
Figure 3. Luminescent decay curves of Eu3+ in Cd0.98Eu0.01Li0.01MoO and
MoO4 in CaMoO4.
4
the systems Cd1−xEu0.5xLi0.5xMoO or Ca Cd0.9−xEu0.1Li0.1MoO4.
4
x
2−
3
+
2−
to the combination of the charge-transfer transitions of Eu -O
6
+
2−
2−
and Mo -O . Compared with the profile of the excitation spectrum
of the undoped sample, it can be seen that the excitation transition of
Figure 2 again shows that the emission from the MoO group
4
3
+
overlaps the f-f excitation of Eu . Therefore, it may be deduced that
when the codoped sample is excited by UV light in the range of the
wide excitation band, first the energy is mainly absorbed by the
MoO group and then transferred to Eu . This process is similar to
that in the phosphor YVO :Eu ͑Ref. 14͒. Decay time ͑␶͒ was
measured, which fit well with the single exponential decay function
3
+
the Eu -doped sample is mainly due to the charge-transfer transi-
6
+
2−
3+
tion of Mo -O and Eu doping making the band slightly wider
to the short wavelength side. The weak and narrow peaks above 360
2
4
−
3+
3
+
nm may be assigned to the f-f transitions of Eu . The emission
spectrum of the codoped samples are dominated by the red peaks at
3+
4
5
7
6
13 and 615 nm due to the electric dipole transition D → F ,
2−
0
2
͑see Fig. 3͒. MoO4 emission in CaMoO ͑the emission of CdMoO4
is too weak for the decay time measurement͒ has a decay time of
4
3
+
which indicates that Eu
occupies a site lacking inversion
symmetry. Though in the host matrix the sites of bivalent cations
have inversion symmetry, the introduction of trivalent Eu makes
the inversion symmetry disappear. The orange peak at 592 nm due
13
3+
␶
= 0.17 ms. The decay time measurements for Eu emissions were
3
+
3+
conducted on the system Cd1−xEu0.5xLi0.5xMoO with the Eu con-
4
tent x Ͻ 0.15. The measurements showed that the decay time ␶ =
ϳ 0.45 ms for the samples was not obviously influenced by the
5
7
to the magnetic dipole transition D → F also appears with weak
0
1
intensity. The intensity ratio of 615 nm emission to 592 nm emission
normally named as the ratio of red/orange͒ is 10. To be a red
component of the tricolor phosphors for display devices, the
3+
Eu content.
͑
3+
The variations of the luminescent properties with Eu content
were investigated ͑Fig. 4͒. The peak position of the band excitation
3
+
color purity of this phosphor is better than those of YVO :Eu
3+
4
shifts toward shorter wavelength with the increase of Eu content
3
+
and Y O S:Eu . Excited by the optimized wavelengths of
2
2
͑from 335 nm for Cd0.98Eu0.01Li0.01MoO4 to 285 nm for
Eu0.5Li0.5MoO ͒; this also can be understood by the electronegativ-
ity differences between cation elements ͑Eu, ϳ1.2, Li, 1.0͒ and
oxygen. The optimized Eu content for the luminescent intensity is
3
+
+
CdMoO :Eu , Li
͑325 nm͒ and commercial phosphor
nm͒, the emission intensity for
4
4
3
+
Y O S:Eu
͑320
2
2
Cd0.7Eu0.15Li0.15MoO is about 85% of that of the latter.
3+
4
about 0.075.
Figure 2. Excitation and emission spectra of Cd0.96Eu0.02Li0.02MoO ͑solid
Figure 4. Variations of the luminescent intensity and the excitation peak
wavelength with Eu3+ content x in the system Cd1−xEu0.5xLi0.5xMoO4.
4
line, labeled P͒ and CdMoO ͑normalized, dash line, labeled H͒.
4
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G536
Journal of The Electrochemical Society, 152 ͑7͒ G534-G536 ͑2005͒
emission intensity of the sample Ca0.3Cd0.6Eu0.05Li0.05MoO4 was
about 30% higher. The excitation peak wavelength has blue shift
2
+
from 330 to 285 nm with the increase of Ca content.
Conclusion
3
+
+
The phosphors in the system Cd1−xCa MoO :Eu , Li have in-
x
4
tense red emissions with good color purities. The phosphors may be
considered as a red component for N -PDP.
2
Acknowledgments
We thank Professor Chunhui Huang and her student Daobo Nie
for the luminescent lifetime measurements. This work was finan-
cially supported by the State Key Program for Basic Research of
China ͑G1998061306͒, the National Science Foundation of China
͑
͑
20221101͒, and the High Tech Research of Zhejiang Province
2003C31029͒.
Peking University assisted in meeting the publication costs of this article.
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Figure 5. Variations of the luminescent intensities with Ca2+ content x in the
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+
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+
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+
1
cent intensities. For the system Ca Cd0.9−xEu0.05Li0.05MoO , com-
x
4
pared to the end member Cd0.9Eu Li0.05MoO , the intensity was
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4
2
+
1
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x
4
1
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͑
1999͒.
3
+
Cd0.8Eu0.1Li0.1MoO . Compared with commercial Y O S:Eu , the
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4
2
2
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