Spectroscopic properties and intense red-light emission of (Ca, Eu, M)WO4 (M = Mg, Zn, Li)

Article abstract of DOI:10.1016/j.saa.2007.04.012

The red-emitting phosphors of (Ca, Eu, M)WO₄ (M = Mg, Zn, Li) were prepared through solid-state reactions, and their spectroscopic properties were studied. After the addition of a small amount of Mg²⁺, Zn²⁺ or Li⁺ in (Ca, Eu)WO₄, the red-light emission intensity of Eu³⁺ increases obviously. In the luminescence spectra of the phosphors, the predominant transition emission is ⁵D₀ → ⁷F₂ (616 nm), whereas the other emissions are very weak. The excitation spectra are composed of interweaved ligand-to-metal charge-transfer bands (CTB) of W⁶⁺-O^2- and Eu³⁺-O^2-, and a few 4f excitation transitions of Eu³⁺. Among the 4f excitation transitions of Eu³⁺, there are three strong excitation lines corresponding to ⁷F₀ → ⁵L₆, ⁷F₀ → ⁵D₂ and ⁷F₀ → ⁵D₁ transitions_, whose relative excitation intensity ratio is seriously affected when Li⁺ doped in the host. The new phosphors may be applied as red-emitting phosphors for white light emitting diodes.

Full text of DOI:10.1016/j.saa.2007.04.012

Available online at www.sciencedirect.com
Spectrochimica Acta Part A 69 (2008) 396–399
Spectroscopic properties and intense red-light emission of
(Ca, Eu, M)WO4 (M = Mg, Zn, Li)
Shikao Shia,∗, Xingren Liub, Jing Gaoa, Ji Zhouc
a College of Chemistry and Materials Science, Hebei Normal University, Shijiazhuang 050016, PR China
b Changchun Institute of Optics, Fine Mechanics and Physics, Chinese Academy of Sciences,
Changchun 130021, PR China
c Department of Materials Science & Engineering, Tsinghua University, Beijing 100084, PR China
Received 20 November 2006; received in revised form 16 April 2007; accepted 17 April 2007
Abstract
The red-emitting phosphors of (Ca, Eu, M)WO4 (M = Mg, Zn, Li) were prepared through solid-state reactions, and their spectroscopic properties
were studied. After the addition of a small amount of Mg2+, Zn2+ or Li+ in (Ca, Eu)WO4, the red-light emission intensity of Eu3+ increases obviously.
In the luminescence spectra of the phosphors, the predominant transition emission is 5D0 → F2 (616 nm), whereas the other emissions are very
7
weak. The excitation spectra are composed of interweaved ligand-to-metal charge-transfer bands (CTB) of W6+–O2− and Eu3+–O2−, and a few
5
4f excitation transitions of Eu3+. Among the 4f excitation transitions of Eu3+, there are three strong excitation lines corresponding to 7F0 → L6,
5
5
7F0 → D2 and 7F0 → D1 transitions, whose relative excitation intensity ratio is seriously affected when Li+ doped in the host. The new phosphors
may be applied as red-emitting phosphors for white light emitting diodes.
© 2007 Elsevier B.V. All rights reserved.
Keywords: Spectroscopic properties; Eu3+ ion; CaWO4; f–f transitions
1. Introduction
emitting phosphors coated on the near-UV LED chip, which is
a focus in material and luminescence research nowadays. The
commercialized products on LED with near-UV (390–410 nm)
excitation are available. Nevertheless, the red-emitting phosphor
that can be effectively excited with the near-UV light is still
lacking. The luminescent efficiency of Y2O2S:Eu3+ phosphor
used in color television is very low under near-UV irradiation.
In consequence, much attention has been paid to make superior
red phosphor for white LED.
Recently, the trivalent europium ions activated tungstates and
molybdates are shown to have efficient red-light emission in
the near-UV irradiation. Neeraj et al. [4] reported the emission
intensityofthephosphorNaY0.95Eu0.05(WO4)2−x(MoO4)x with
scheelite structure is 7.28 times of Y2O2S:Eu, Sm excited at
395 nm, while the emission intensity of NaY0.95Eu0.05(WO4)2
is just 3.25 times of Y2O2S:Eu, Sm excited at 395 nm. Similar
red phosphor AgLa0.95Eu0.05(WO4)2−x(MoO4)x was reported
by Sivakumar and Varadaraju [5], and NaLa1−xRx(MoO4)2
(R = Eu, Sm) was also prepared by Wang et al. [6]. In addition,
the phosphor Ca1−2xEuxLixMoO4 prepared by Jing and co-
workers [7] has shown superior luminescence efficiency compa-
red with that of (Ca, Eu)MoO4 reported in the earlier paper [8].
Due to the rapid rises of new semiconductor solid-state ligh-
ting devices—white light emitting diodes (LEDs) since the end
of the last century, tremendous achievements have been made
in the development of key materials and technology including
semiconductor photoelectron, lighting project and new lumines-
cent materials. White LED is a remarkable resource because it
is environment-friendly and energy-saving in the 21st century.
Therefore, it is promising for the market applications [1]. One
of the most common methods to assemble white LED is to com-
bine an InGaN blue LED chip with a yellow-emitting phosphor
YAG:Ce3+, which has been commercialized [2,3]. However,
there exist at least two drawbacks in this combination. Firstly,
the overall efficiency is decreased rapidly when the correlated
color temperature of the device is low. Secondly, the output light
is deficient in the red region of the visible light spectrum. The
other promising white LED is to use red, green and blue light-
∗
Corresponding author. Tel.: +86 311 86268748.
E-mail address: ssk02@mails.tsinghua.edu.cn (S. Shi).
1386-1425/$ – see front matter © 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2007.04.012

S. Shi et al. / Spectrochimica Acta Part A 69 (2008) 396–399
397
Table 1
Composition and relative intensities of (Ca, Eu, M)WO4 (M = Li, Mg, Zn)
Sample no.
Composition
I616
I616/I592
01
02
03
04
05
CaWO4
Ca0.76Eu0.24WO4
Ca0.70Eu0.24Mg0.06WO4
Ca0.70Eu0.24Zn0.06WO4
Ca0.52Eu0.24Li0.24WO4
–
100
145
151
147
–
1:0.138
1:0.116
1:0.105
1:0.117
CaWO4 with the scheelite structure was first used as X-
ray luminescent materials as soon as it was found in 1896
[9], which has been used all the same in the following whole
century. Typical self-activated CaWO4 is a high-efficiency mate-
rial emitting blue light under X-ray, electron beam and UV
irradiation [10,11]. Between the 1970s and 1980s, Powell and
co-workers [11,12] had studied the luminescent properties and
energy transfer of CaWO4 and CaWO4:Eu3+ systems. Neverthe-
less, the change of Eu3+ 4f energy transition excitation spectra
has been neglected for a long time. To date, it is still neces-
sary to make a further cognition about the excitation spectra of
Eu3+ in tungstates and molybdates. In this paper, the spectrosco-
pic properties of Eu3+ were studied in CaWO4 doped with Li+,
Mg2+ or Zn2+ under UV–visible light irradiation, especially the
influence of the Eu3+ 4f excitation spectra (monitored at 616 nm)
at 290–550 nm on the luminescence intensity of Eu3+.
Fig. 1. XRD patterns of CaWO4 (01), (Ca,Eu)WO4 (02) and (Ca,Eu,Zn)WO4
(04).
Ca2+ is eight-coordinated with O2−, forming a distorted cube.
The dopant Eu3+ or other metal ions occupy the Ca2+ site. With
the increase of the dopant concentration, the characteristic dif-
fraction intensity decreases obviously, but there is no shift in the
peak location, indicating that the doped Eu3+ or other metal ions
have little influence on the host structure.
3.2. Emission spectra of Eu3+
2. Experiments
Fig. 2 shows the emission spectra of four samples (No. 02,
03, 04 and 05) excited by near-UV (395 nm) at ambient tem-
perature. Their emission spectra are similar, corresponding to
typical 4f levels specific transitions of Eu3+. The characteristic
emission of WO42− in CaWO4:Eu3+ is quenched absolutely and
only red-light emission of Eu3+ appears. The strong emission
peak around 616 nm is due to the electric dipole energy transi-
Undoped and doped CaWO4 samples were prepared by
the solid-state reaction. The starting materials WO3 (99.9%),
CaCO3 (G.R.), Eu2O3 (99.99%), Li2CO3 (A.R.), MgO (A.R.)
and ZnO (A.R.) were blended in appropriate stoichiometric ratio
(Table 1). These powders were ground thoroughly in an agate
mortar, and then the heterogeneous mixture was put into an alu-
mina crucible and calcined in a muffle furnace at 1070–1100 ◦C
for 3 h in air. When more Li2CO3 was added, the calcination
temperature should be decreased properly. The final samples
are white powders.
7
tion of 5D0 → F2. The weak emission in the vicinity of 592 nm
is ascribed to the magnetic dipole transition of 5D0 → F1. The
7
ratio of the two emission intensities (I616/I592) and the relative
The crystal structure of the sample was identified by X-ray
diffraction (XRD), which was recorded on a Rigaku D/max-rb
X-ray diffraction running Cu K␣ radiation at 40 kV and 250 mA.
A VARIAN Cary-Eclipse fluorescence spectrometer was used
to detect the excitation and emission spectra of the samples at
ambient temperature.
3. Results and discussion
3.1. Structure of the phosphors
Fig. 1 gives the XRD patterns of CaWO4 (01), (Ca, Eu)WO4
(02) and (Ca, Eu, Zn)WO4 (04). The XRD patterns of (Ca, Eu,
Mg)WO4 (03) and (Ca, Eu, Li)WO4 (05) are not shown, due to
their similarity with (Ca, Eu, Zn)WO4 (04). All the five com-
pounds are isostructural and belong to the tetragonal system of
Scheelite type. In this structure, W6+ occupies the tetrahedral
sites constructed with O2−, composing WO42− anion complex.
Fig. 2. Emission spectra of (Ca, Eu)WO4 (02), (Ca, Eu, Mg)WO4 (03), (Ca Eu,
Zn) WO4 (04) and (Ca Eu, Li)WO4 (05) excited at 395 nm.

398
S. Shi et al. / Spectrochimica Acta Part A 69 (2008) 396–399
Table 2
Transition energies of Eu3+ in the excitation spectra of (Ca, Eu, M)WO4
(M = Mg, Zn, Li) at ambient temperature
Transitions
Excitation maximum (nm)
5
7F0 → F4
298
304
319
327
363a, 367
377, 382a, 385
395a, 400
416
465
474
5
7F0 → F2
5
7F0 → H3
5
7F2 → H4
5
7F0 → D4
5
7F0 → L7
5
7F0 → L6
5
7F0 → D3
5
7F0 → D2
5
7F1 → D2
5
7F2 → D2
486
527, 536a
7F0 → D1
5
a
Represents the predominant line in the group.
Fig. 3. Excitation spectra of CaWO4 (01), (Ca, Eu)WO4 (02), (Ca, Eu, Mg)WO4
(03), (Ca Eu, Zn) WO4 (04) and (Ca Eu, Li)WO4 (05).
under short-wavelength UV match well with that of CTB(W6+).
With the introduction of Mg2+ and Zn2+ into CaWO4, the
excitation peaks (λmax ∼ 235 nm) of CTB(W6+) shift to the
short-wavelength side in a small amount. However, the same
result is not observed in the (Ca, Eu, Li)WO4 (05). In the exci-
tation spectra of (Ca, Eu)WO4 and (Ca, Eu, M)WO4 (M = Mg,
Zn, Li), the relative intensities of CTB(W6+) are much weaker
than that of pure CaWO4. The relative weak excitation bands
at the UV region (∼285 nm) correspond to the CTB(Eu3+) of
Eu3+–O2−. This phenomenon is similar to CTB (260 nm) of
W6+–O2− and CTB (310 nm) of Eu3+–O2− in Eu2(WO4)3 [13].
In luminescent materials, there exist two kinds of cou-
plings between luminescent center and crystal lattice. One is
emission intensities of the four samples at 616 nm (I616) are pre-
7
sented in Table 1, respectively. It is evidently that the 5D0 → F2
transition emission intensity of Eu3+ increases significantly with
incorporation of Mg2+(03), Zn2+(04), Li+(05) ions into CaWO4.
7
The 5D0 → F2 transition is hypersensitive to the local environ-
ment of the Eu3+. It is generally a forced electric dipole transition
whose intensity depends on coupling with either odd terms in the
crystal field or odd parity lattice vibration modes. In the tetrago-
nal system CaWO4, Eu3+ ions substitute partially for Ca2+ ions
and have S4 site symmetry. Nevertheless, the emission spec-
5
7
tra are dominated by the electric dipole transition D0 → F2
of Eu3+, which manifests that Eu3+ ions occupy the asymme-
try center site [11]. The doping with Mg2+, Zn2+ or Li+ into
CaWO4 results in the enhancement of I616/I592 (see Table 1).
This implies that the crystal field environment of Eu3+ is modi-
fied and more Eu3+ ions occupy the asymmetry center sites. As
2−
2−
the strong coupling (such as WO4
and MoO4
groups)
with high Huang-Rhys factor. The other belongs to the weak
coupling (such as Eu3+and Tb3+ ions) with relatively low
Huang-Rhys factor [14,15]. In CaWO4:Eu3+, the strong cou-
pling of CTB(W6+) is predominant, whereas the weak coupling
of CTB(Eu3+) is subordinate. When CTB(W6+) is excited, the
energy absorbed from charge-transfer state is efficiently trans-
ferred to Eu3+ ion by a non-radiative mechanism, and generate
5
7
a result, the D0 → F2 transition emission intensity increases
dramatically and the color purity of the red light is improved.
In addition, other radiation transition emissions of Eu3+ 4f high
energy (such as 5D1,2) are very weak.
red-light emission of the 5D0 → FJ transition of Eu3+. Simul-
7
3.3. Charge-transfer bands (CTB)
taneously, the blue-light emission of the self-activated CaWO4
is quenched.
Fig. 3 shows the excitation spectra of the samples at ambient
temperature. The excitation spectrum (monitored at 418 nm) of
pure CaWO4 (01) shows a strong broadband at short-wavelength
UV region. The excitation curve has peaks at about 240 nm,
which is ascribed to a charge-transfer band of W6+–O2− within
the WO42− group (abbreviated as CTB(W6+)).
3.4. The 4f excitation transitions of Eu3+
Chronically, the main study on the excitation spectra of Eu3+
is the charge-transfer band of Eu3+–O2−, while the excitation
spectra variation of Eu3+ 4f energy transition (290–550 nm) is
neglected. Here, we try to make a brief analysis on this aspect.
There are groups of sharp excitation lines of Eu3+ 4f levels in
the wide range of excitation spectra from 290 nm to 550 nm,
which are listed in Table 2. Among these excitation transi-
The Eu3+ excitation spectra (monitored at 616 nm)
of the samples (02, 03, 04 and 05) are quite simi-
lar, consisting of two interweaved broad excitation bands
in the vicinity of 200–340 nm and sharp 4f transition
excitation lines of Eu3+, which cover the ranges from long-
wavelength UV to visible green-light region (300–550 nm).
The excitation band (λmax = 235–240 nm) at short-wavelength
UV region (200–280 nm) overlaps with another excita-
tion band (λmax ∼ 285 nm) at middle-wavelength UV region
(250–340 nm). The profile and peaks of this excitation spectra
5
5
5
tions, 7F0 → L6 (395 nm), 7F0 → D2 (465 nm) and 7F0 → D1
(536 nm) lines are the most intense ones.
The excitation peaks of these samples (02, 03, 04 and 05)
are practically identical, coinciding with other tungstates [13],
because the f–f transitions are in little measure affected in

S. Shi et al. / Spectrochimica Acta Part A 69 (2008) 396–399
399
the crystal field environment. However, the intensity diffe-
rences of the excitation spectra can be clearly observed from
Fig. 3. The relative excitation intensities of Eu3+ monitored
(536 nm) transitions, which transcend the charge-transfer bands
of W6+–O2− and Eu3+–O2−. The relative excitation intensities
of the three lines depend on the dopant and dopant concentration.
Particularly when Li+ doped in the host, their relative excita-
tion intensity ratio is dramatically changed and the excitation
transitions of the blue and green lines are enhanced obviously.
The efficient phosphors (Ca, Eu, M)WO4 may be promising
candidates as red phosphors for white LED.
7
at 616 nm (5D0 → F2) vary with different excitation wave-
7
5
length. Especially, the intensities of F0 → D2 (465 nm) and
7F0 → D1 (536 nm) compared with F0 → L6 (395 nm) are
enhanced in the sequence of 02 < 03(Mg) < 04(Zn) < 05(Li). For
example, in Ca0.76Eu0.24WO4 (02), the ratios of relative exci-
tation intensities (I395:I465:I536) of Eu3+ (λem = 616 nm) are
1:0.76:0.55. Whereas, the ratios of relative excitation intensities
in Ca0.70Eu0.24Mg0.06WO4 are changed to 1:0.87:0.59. When
Li+ doped in the host, the ratios of relative excitation intensities
are 1:0.93:0.91, which means that the excitation transition of
the blue (465 nm) and green (536 nm) lines are greatly enhan-
ced. Another noticeable feature is that, in (Ca, Eu, M)WO4
(M = Mg, Zn, Li) systems, the relative intensities of Eu3+ 4f exci-
5
7
5
Acknowledgments
This work was supported by the National 973-project (Grant
No. 2002CB61306 and 2001CB6104) and National Natural
Science Foundation (Grant No. 50472020 and 50425204) of the
People’s Republic of China.
5
5
7
5
tation lines (7F0 → L6,7F0 → D2 and F0 → D1) transcend
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4. Conclusions
The incorporation of Mg2+, Zn2+ or Li+ ions into the CaWO4
crystalline lattice makes the red-light emission intensity of Eu3+
increases obviously. In the emission spectra, the strongest emis-
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(616 nm), while the magnetic dipole transition orange emis-
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7
5
7
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to F0 → L6 (395 nm), F0 → D2 (465 nm) and F0 → D1