Enhancement of photoluminescence and color purity of CaTiO3: Eu phosphor by Li doping

Article abstract of DOI:10.1111/j.1551-2916.2011.04967.x

High color purity red phosphors of Ca_1-3/2xEu _xTiO₃ and Ca_1-2xEu_xLi _xTiO₃ (0 < x ≤ 0.3) are synthesized via a solid-state reaction method. The red emission photoluminescence intensity and color purity are enhanced by the incorporation of Li⁺ into CaTiO ₃:Eu³⁺. The Li⁺ doping increases the emission probability from ⁵D₀ state, increases photoluminescence intensity by 1.6 times, increases color purity to 92.1%, and shortens the decay time. With increasing Eu³⁺ and Li⁺ content, the color coordinates approach the ideal red chromaticity values, coming closer than commercial Y₂O₂S:Eu³⁺ red phosphor.

Full text of DOI:10.1111/j.1551-2916.2011.04967.x

J. Am. Ceram. Soc., 1–7 (2011)
DOI: 10.1111/j.1551-2916.2011.04967.x
© 2011 The American Ceramic Society
ournal
J
Enhancement of Photoluminescence and Color Purity of
CaTiO3:Eu Phosphor by Li Doping
Yun-Fang Wu,‡ Yung-Tang Nien,§ Yi-Jhang Wang,‡ and In-Gann Chen‡,§,†
‡Department of Materials Science and Engineering, National Cheng Kung University, Tainan 70101, Taiwan
§Center for Micro/Nano Science and Technology, National Cheng Kung University, Tainan 70101, Taiwan
High color purity red phosphors of Ca1ꢀ3/2xEuxTiO3 and
Ca1ꢀ2xEuxLixTiO3 (0 < x ꢁ 0.3) are synthesized via a solid-
state reaction method. The red emission photoluminescence
intensity and color purity are enhanced by the incorporation of
Li+ into CaTiO3:Eu3+. The Li+ doping increases the emission
probability from 5D0 state, increases photoluminescence inten-
sity by 1.6 times, increases color purity to 92.1%, and shortens
the decay time. With increasing Eu3+ and Li+ content, the
color coordinates approach the ideal red chromaticity values,
coming closer than commercial Y2O2S:Eu3+ red phosphor.
replacing Pr3+ with Eu3+ in CaTiO3:Pr3+, which can be
excited by UV as well as blue light to increase the usability of
the phosphor.10 According to previous studies,11,12 photolumi-
nescence can be improved by doping Li+ into the host. Since
few titanate-related phosphor studies have been conducted,
the present study focuses on the synthesis of CaTiO3 red phos-
phor doped with both Eu3+ and Li+. The characteristics and
performance of the phosphor are investigated and discussed.
II. Experimental Procedure
The synthesis of CaTiO3 phosphors doped with trivalent
europium (Eu3+) and monovalent lithium (Li+) was carried
out via a solid-state reaction method. The raw materials,
CaCO3 (Mallinckrodt, Hazelwood, MO, 99.99%), TiO2
(Showa, Tokyo, Japan, 99.9%), Eu2O3 (Alfa Aesar, Ward
Hill, MA, 99.99%), and Li2CO3 (Mallinckrodt, 99%), were
well mixed with alcohol (99%) stoichiometrically, and then
dried in the oven at 80°C. The dried mixture was allowed to
react in the furnace at 1300°C for 8 h in an air atmosphere
with a heating rate of 5°C/min.
The crystal structure of samples was confirmed using
X-ray diffractometry (XRD, Rigaku D/max VIII, Tokyo,
Japan) with Cu-Ka (k = 1.54056 A) radiation operated at
30 kV and 20 mA. Photoluminescence (PL) spectra and exci-
tation (PLE) spectra were acquired using a fluorescence spec-
trophotometer (Hitachi F-7000, Tokyo, Japan) equipped
with a 150-W xenon lamp as the excitation source. The
quantum efficiency was measured using an integrating sphere
coated with BaSO4 installed in the fluorescence spectropho-
tometer. The photoluminescence decay time was recorded
I. Introduction
HE development of flat panel displays (FPDs) has seen
great growth in the last decade. White-light-emitting
T
diodes (WLEDs) have attracted a lot of attention in recent
years due to their numerous applications, such as backlights
in liquid crystal displays (LCDs), automotive headlights, and
general illumination. The development of phosphor materials
that can enhance the luminescence and efficiency of WLEDs
is essential for the FPD industry. The most commonly used
commercial WLEDs, which use a blue LED chip with
Y3Al5O12:Ce3+ yellow phosphor powder,1,2 have a low color
rendering index (CRI) and thus require a red phosphor. Fur-
thermore, blue- and UV-chip-excited RGB color phosphors
also require a high color-purity red phosphor.
Titanate (TiO32ꢀ)-based ceramics systems have unique opti-
cal, thermal, electrical, and mechanical properties,3 which can
be exploited to fabricate promising phosphor materials. The
idiosyncratic deformation of the perovskite structure influ-
ences the luminescence behavior of titanate.4 According to
Kyomen,5 the f–f transition would be increased due to the low-
ering of the point symmetry at the alkaline-earth site in (Ca,
Sr, Ba) TiO3:Pr3+. The component CaTiO3:Pr3+ with an
orthorhombic structure has better photoluminescence intensity
than that of the other two components (BaTiO3 and SrTiO3
with tetragonal and cubic structures, respectively). The Ca-
TiO3 was thus chosen as the host in the present study. Table I
summarizes recent studies related to titanate phosphors.
using the fluorescence spectrophotometer with
a pulsed
xenon lamp. Fourier transform infrared (FTIR) spectra were
obtained using a Jasco FTIR-200E spectrometer with the
KBr pellet technique. All results were measured at room tem-
perature and atmospheric pressure.
III. Results and Discussion
A promising phosphor material, CaTiO3:Pr3+ red phos-
phor used for field emission displays (FEDs) was first reported
in 1994.6 The CaTiO3:Pr3+ with high color purity and a low
voltage excitation compared to those of commercial Y2O3:Eu3+
phosphor was later developed.7 However, the excitation spec-
trum of CaTiO3:Pr3+ has a narrow peak near 335 nm,8,9
which limits its application in the field of LEDs. Therefore,
the present study improves the excitation characteristics by
(1) XRD Characteristics
The perovskite CaTiO3 formed during sintering at 1300°C
for 8 h belongs to the orthorhombic structure. In this struc-
ture, a Ca2+ ion is at the center of a cubic unit cell sur-
rounded by twelve O2ꢀ ions, and Ti4+ ions are at the
corners of the cubic unit cell surrounded by six O2ꢀ ions in
the cube-octahedral coordination.13 The XRD patterns for
the Ca1ꢀ3/2xEuxTiO3 (CET) and Ca1ꢀ2xEuxLixTiO3 (CELT)
series are shown in Fig. 1. The results are consistent with
JCPDS Card No. 22-0153 (cell parameters: a = 5.440 A,
b = 7.643 A, and c = 5.381 A). An impurity phase of
Eu2Ti2O7 appears when the Eu3+ content is greater than
10 mol% (x = 0.1) in the CET series [see Fig. 1(a)]. The
impurity phase disappears when a commensurate amount of
Li+ is added [see Fig. 1(b)].
J. McKittrick—contributing editor
Manuscript No. 29871. Received June 14, 2011; approved October 18, 2011.
†Author to whom correspondence should be addressed. e-mail: ingann@mail.ncku.
edu.tw
1

2
Journal of the American Ceramic Society—Wu et al.
Table I. Summary of Titanate-Related Research
Host
Activator
Process
Excitation/emission
Year
(Ba,Sr)TiO3 Eu3+
Solid-state reaction (1200°C for 2 h)
Solid-state reaction (600°C–1000°C for 2 h)
Solid-state reaction (1300°C for2 h)
465 nm/617, 667, and 695 nm
330 nm/614 nm
360 and 300 nm/600 and 616 nm
200425
200726
200827
200928
CaTiO3
BaTiO3
CaTiO3
CaTiO3
Pr3+
Pr3+
Eu3+
Solid-state reaction (1300°C–1600°C for 4 h) 400 nm/618 nm
Eu3+, Li+ Solid-state reaction (1300°C for 8 h)
397 and 466 nm/590, 594, and 616 nm This study
maximum content of Eu3+ was around 15 mol% in CaTiO3
under sintering at 1300°C for 8 h. The parameter values
decrease with increasing x (equal amounts of Eu3+ and Li+)
in the CELT series [see Fig. 2(b)]. Since Eu3+ (0.947 A) and
Ca2+ (0.99 A) have similar atomic radii, Eu3+ can substitute
the Ca2+ site smoothly. However, the Li+ radius (0.76 A) is
smaller than that of Ca2+, and thus co-doped Li+ decreases
the cell volume. Li+ is not only a charge compensator, but it
also plays the role of flux, which can enhance the crystallinity
and increase the partial solid solubility of Eu3+ in the Ca-
TiO3 host.
(a)
Figure 3 shows the SEM images of CET and CELT
(x = 0.15) synthesized via the solid-state reaction method.
Comparing the surface morphology of these two samples, it
can be seen that the particles of CET are fine and spherical-
like, whereas the particles of CELT are large and irregular.
The average particle sizes of CET and CELT are around 3
and 5 lm, respectively. This reveals that the particle size
increased after Li+ was doped into CaTiO3: Eu. Thus, Li+
doping enhances the crystallinity and the growth of CaTiO3
particles, as shown in the XRD and SEM results.
(b)
(2) Photoluminescence and Quantum Efficiency
Figure 4 shows the PL and PLE spectra of the CET and
CELT series (x = 0.15) for 397 and 466 nm excitation wave-
lengths and 594 and 616 nm emission wavelengths. The PL/
PLE spectra of CEL and CELT are similar. There was no
change in the emission peak position after doping with Li+
in CET. The schematic energy level of Eu3+ in CaTiO3 illus-
trating the emissions of Eu3+ is shown in Fig. 5. From the
PLE spectra in Fig. 4(a), the excitation bands of CEL and
CELT samples centered at 361, 386, 397, 416, and 466 nm
5
5
5
are due to the 7F05? D4, 7F0 ? L7, 7F0 ? L6,
7F0 ? D3, and 7F0 ? D2 electron transitions of Eu3+
,
5
respectively. In the PL spectra [shown in Fig. 4(b)], the emis-
sion peaks at 590, 594, and 616 nm can be attributed to the
7
energy relaxation of 5D0 ? F1 (590, 594 nm) and
Fig. 1. XRD patterns of (a) Ca1ꢀ3/2xEuxTiO3 and (b)
Ca1ꢀ2xEuxLixTiO3 sintered at 1300°C for 8 h.
5D0 ? F2.14 The narrow band was attributed to the charac-
7
teristic electron shielding effect in trivalent rare-earth ions
(Eu3+). From the PL/PLE spectra, the optimal excitation
and the strongest emission wavelengths are 397 and 616 nm,
respectively. The PLE spectra reveal that the as-prepared
phosphors are suitable for being excited by near-UV and
blue light. Furthermore, the PL spectra show that regardless
of the excitation source, the energy transfer process is always
The unit-cell parameters of CET and CELT were deter-
mined using Program Unit Cell software (T.J.B. Holland,
S.A.T. Redfern, Department of Earth Sciences, Cambridge,
U.K., 1995); the calculation results are shown in Table II
and Fig. 2. In the CET series [see Fig. 2(a)], the values of
these parameters slightly decrease with increasing x; the low-
est values were obtained for x = 0.15. This indicates that the
5
from the higher energy level to the D0 energy level through
Table II. Unit-Cell Parameters of Ca1ꢀ3/2xEuxTiO3 and Ca1ꢀ2xEuxLixTiO3 for various x values
Ca1ꢀ3/2xEuxTiO3
Ca1ꢀ2xEuxLixTiO3
x
a (A)
b (A)
c (A)
Volume (A3)
a (A)
b (A)
c (A)
Volume (A3)
0
5.4402
5.4417
5.4408
5.4381
5.4315
5.4327
5.4309
7.6454
7.645
5.3704
5.3694
5.3685
5.3659
5.3669
5.3656
5.3639
223.3650
223.3777
223.2676
223.0038
222.7788
222.8546
222.7339
5.4402
5.4392
5.4427
5.438
5.4242
5.4219
5.4233
7.6454
7.6437
7.6424
7.6426
7.639
5.3704
5.3695
5.3655
5.3658
5.3404
5.3261
5.3179
223.3650
223.2385
223.1819
223.0035
221.2777
220.1905
219.6024
0.01
0.05
0.1
0.15
0.2
0.3
7.6438
7.6423
7.6424
7.6452
7.6461
7.6249
7.6144

Photoluminescence and Color Purity of CaTiO3:Eu Phosphor by Li Doping
3
(a)
(b)
Fig. 2. Unit-cell parameters of (a) Ca1ꢀ3/2xEuxTiO3 and (b) Ca1ꢀ2xEuxLixTiO3 for various x values.
7
around the activator. In contrast, the 5D0 ? F1 transition
belongs to the magnetic dipole transition, which is not
influenced by the environment. According to the calcula-
tion results of Eq. (1), the asymmetry ratio of the Eu3+
site is about 2 (R;2), which means that the electric dipole
transition is twice as strong as the magnetic dipole transi-
tion. With increased doping content, the symmetry of the
local environment of Eu3+ decreases, which strengthens
the transition from energy level 5D0 to energy level 7F1,
and thus leads to stronger red emission and higher color
purity.
(a)
Figure 6 shows the integrated emission intensity for CET
and CELT under 397 and 466 nm excitations. The intensities
indicate that concentration quenching occurs as x approaches
0.15. For both excitation wavelengths, the CELT series has a
much higher photoluminescence emission intensity than that
of the CET series. This is due to the charge imbalance caused
by Eu3+ substituting the Ca2+ sites; Ca vacancies are
required for charge neutrality. However, an excessive number
of vacancies makes the structure unstable and reduces the
photoluminescence intensity. According to Liu,16 Ca2+
vacancies are not favorable for the emission of the Eu3+
activator because of the fact that the energy transfer from
Eu3+ to the vacancies is more efficient, and hence a certain
number of vacancies will affect the photoluminescence inten-
sity. In the CET system, for maintaining charge neutrality,
the doped Eu3+ induces Ca vacancies, which can be
expressed as:
(b)
Fig. 3. SEM images of (a)
Ca1ꢀ2xEuxLixTiO3 (x = 0.15).
Ca1ꢀ3/2xEuxTiO3
and (b)
3CaꢂCa þ 2EuꢂEu ! 2EuCꢃ a þ V
(2)
00
Ca
To eliminate the Ca vacancies and the impurity phase,17
the charge compensation method is used here to improve
the multi-phonon relaxation process, and then they start to
emit lights.
7
It has been reported that the ratio of 5D0 ? F2 to
crystallinity. A possible approach for compensating the
valance and decreasing the number of Ca vacancies is doping
7
5D0 ? F1 can represent the asymmetry ratio (R) around
Eu3+, and this asymmetry structure site can benefit photolu-
minescence. The asymmetry ratio is expressed as15:
Li+. The process can be expressed as:
2CaꢂCa þ EuEꢂu þ LiꢂLi ! EuCꢃ a þ Li0Ca
(3)
5
7
Ið D0 ꢀ F2Þ
R ¼
:
(1)
5
7
Ið D0 ꢀ F Þ
1
Based on the above results, we propose that the concentra-
tion quenching which occurs in CET is caused by the redun-
dant defects (including Ca vacancies and the impurity phase),
which lead to a large energy loss between Eu3+ and vacan-
The 5D0 ? F2 transition belongs to the electric dipole
transition, which can be influenced by the asymmetry
7

4
Journal of the American Ceramic Society—Wu et al.
(a)
(b)
Fig. 4. (a) PLE and (b) PL spectra of Ca1ꢀ3/2xEuxTiO3 and Ca1ꢀ2xEuxLixTiO3 (x = 0.15) for various excitation and emission wavelengths.
cies. In CELT, Li+ doping increases the partial solid solubil-
: (Multi-phonon relaxation process)
ity of Eu3+ in CaTiO3. With increasing Eu3+ content, the
distance between Eu3+ ions decreases. Thus, the concentra-
tion quenching occurred due to the energy loss from non-
5L6
5D3
5D2
5D1
5D0
radiation between Eu3+
.
The absorption coefficient (A) and quantum efficiency (g)
were estimated using the following equations18 (details of the
calculation method can be found elsewhere19):
R
k½EðkÞ ꢀ RðkÞꢄdk
R
A ¼
g ¼
(4)
kEðkÞdðkÞ
J=6
J=5
J=4
J=3
7FJ
R
kPðkÞdk
R
(5)
J=2
J=1
J=0
k½EðkÞꢀRðkÞꢄdk
Fig. 5. Schematic energy levels of Eu3+ in the CaTiO3 host.
where E(k), R(k), and P(k) are the light intensity per unit
wavelength in the spectra of excitation, reflectance, and
emission of the phosphor, respectively. The calculation
results of A and g with excitation at 397 and 466 nm for
the CET and CELT series are shown in Table III (the A
and g values of commercial YAG:Ce yellow phosphor mea-
sured using our instrument are 84.7% and 73.2%, respec-
tively). The results suggest that A increases with increasing
Eu3+ and Li+ content. Furthermore,
g increases with
increasing Eu3+ and Li+ content up to x = 0.15 (quenching
concentration), and then starts to decrease. A and g are
higher for both the CET and CELT series under 397 nm
excitation than under 466 nm excitation. In the as-prepared
phosphors, the optimal value of g is 19.9%, which in the
CELT series was obtained at x = 0.15. The optimal value
of A is 59.6%, which in the CELT series was obtained at
x = 0.30. The value of g in this system is not satisfactory, it
may be derived from the value of A which is not high
enough. However, this could be further improved through
the modification technique. According to the literature,20,21
modification of the surface of particles, enhancement unifor-
mity, and doping with other elements (such as: Sm3+, etc.)
Fig. 6. Integrated emission intensity of Ca1ꢀ3/2xEuxTiO3 and
Ca1ꢀ2xEuxLixTiO3 for various x values (0 < x ꢁ 0.3).

Photoluminescence and Color Purity of CaTiO3:Eu Phosphor by Li Doping
5
Table III. Properties of Ca1ꢀ3/2xEuxTiO3 and Ca1ꢀ2xEuxLixTiO3 for Various x Values
Ca1ꢀ3/2xEuxTiO3
at 466 nm
Ca1ꢀ2xEuxLixTiO3
at 466 nm
at 397 nm
CIE
at 397 nm
CIE
C.P.
(%)
C.P.
(%)
x
A (%)
g (%)
A (%)
g (%)
τ (ms)
x
y
A (%)
g (%)
A (%)
g (%)
τ (ms)
x
y
0.01 28.7
0.05 32.3
1.9
17.2
1.0
0.79 0.615 0.385
0.78 0.642 0.358
0.76 0.646 0.353
0.75 0.648 0.351
0.71 0.648 0.351
0.61 0.648 0.352
36.5
42.5
49.2
51.7
52.4
59.6
2.0
21.2
29.8
34.1
35.0
35.5
41.4
1.3
0.78 0.629 0.371 86.8
0.77 0.643 0.356 89.9
0.76 0.649 0.351 91.4
0.70 0.651 0.349 91.8
0.63 0.652 0.348 92.1
0.41 0.652 0.348 92.1
83.8
89.7
90.6
91.1
91.1
91.2
6.8
10.6
14.1
12.7
7.3
21.5
23.1
23.7
25.6
26.6
2.5
3.8
6.8
6.3
4.2
8.3
13.8
19.9
18.5
11.2
3.0
5.9
9.4
8.9
6.8
0.1
37.3
0.15 39.6
0.2
0.3
44.1
47.1
C.P., color purity; τ, decay time; A, absorption coefficient; g, quantum efficiency.
e0
1 mol%
(a)
x
t
(ms)
0.79
0.78
0.76
0.75
0.71
0.61
5 mol%
10 mol%
15 mol%
20 mol%
30 mol%
1 mol%
5 mol%
10 mol%
15 mol%
20 mol%
30 mol%
(a)
e-1
(b)
e-2
0.0
0.5
1.0
1.5
2.0
Time (ms)
e0
1 mol%
x
t (ms)
0.78
0.77
0.76
0.7
(b)
5 mol%
10 mol%
15 mol%
20 mol%
30 mol%
1 mol%
5 mol%
10 mol%
15 mol%
20 mol%
30 mol%
Fig. 7. FTIR spectra of Ca1ꢀ3/2xEuxTiO3 and Ca1ꢀ2xEuxLixTiO3
(x = 0.15).
0.63
0.41
e-1
can be used to enhance the photoluminescence intensity and
efficiency.
e-2
(3) FTIR Spectra and Multi-Phonon Process
To further investigate the enhanced luminescent intensity due
to Li+ doping, the FTIR spectra of CET and CELT
(x = 0.15) are presented in Fig. 7. The FTIR spectra were
used to obtain the phonon energy of vibrational modes in
the host. The phonon energies at 577 and 442 cmꢀ1 are
attributed to the characteristics of alkaline titanates and the
Ti–O bond vibration of (TiO3)2ꢀ groups, respectively.22 After
0.0
0.5
1.0
1.5
2.0
Time (ms)
Fig. 8. Natural logarithm of the normalized emission intensity of
5D0 7F2 (616 nm) versus the decay time under an excitation of
397 nm for (a) Ca1ꢀ3/2xEuxTiO3 and (b) Ca1ꢀ2xEuxLixTiO3.
?
Li+ doping, higher energy phonons appear at 716 cmꢀ1
.
From the PL results [shown in Fig. 4(b)], regardless of the
excitation to 5L6 or 5D2 of Eu3+, the energy in 5L6 or 5D2
can be relaxed to 5D0 of Eu3+, and then the phonons start
to emit light. The energy relaxation process from the Eu3+
excited state to the Eu3+ 5D0 state is called multi-phonon
relaxation process, which is also a kind of non-radiative tran-
sition. The non-radiative transition rate of the multi-phonon
relaxation process as a function of temperature can be
expressed by16:
p
WðTÞ ¼ Wð0Þðn þ 1Þ
(6)
where W(T) is the rate at temperature T, n = [exp (hm/kT)
ꢀ 1]ꢀ1, p = DE/hm is the number of phonons, DE is the
energy difference between the levels involved, and hm is the
phonon energy. W(0) is temperature-independent and large

6
Journal of the American Ceramic Society—Wu et al.
Fig. 9. CIE chromaticity coordinates of Ca1ꢀ3/2xEuxTiO3 and Ca1ꢀ2xEuxLixTiO3, and photographs of as-prepared samples excited by various
light sources.
for high phonon energy or low p. W(0) can be expressed
Figure 8 shows the natural logarithm of the normalized
7
by:16
emission intensity of 5D0 ? F2 (616 nm) versus the decay
time under an excitation of 397 nm for CET and CELT. The
decay time results can be well fitted with the following single
exponential decay function23:
Wð0Þ ¼ b exp½ꢀðDE ꢀ 2hmÞaꢄ
(7)
where a and b are constants, and m is the highest available
vibration frequency of the surroundings of the rare-earth
ions.
ꢀt
I ¼ Io expð
Þ
(8)
s
where Io is the initial emission intensity at t = 0 and τ is the
decay time. The decay times of CET and CELT were evalu-
ated using Eq. (8); the results are shown in Table III. The
decay times decrease with increasing x. The photolumines-
cence decay time of the phosphor (τ) is affected by the radia-
tive decay time, non-radiative rate, and energy transfer rate,
which can be expressed by14:
According to the above formula, the multi-phonon relax-
ation rate is related to the highest available vibration fre-
quency of the host. The multi-phonon relaxation rate
increases with higher phonon energy of vibrational mode in
the host. To calculate the number of phonons that partici-
pated in the process of multi-phonon relaxation, the energy
difference between the levels involved was divided by the
phonon energy of the highest available vibration fre-
quency.14 The rate of multi-phonon relaxation from the Eu3
ð1=sÞ ¼ Ar þ Anr þ Pt
(9)
+
excited state to the Eu3+ 5D0 state step by step (as shown
where Ar is the radiative transition rate (Ar = (1/τ0)), Anr is
the non-radiative transition rate due to multi-phonon relaxa-
tion, and Pt is the transfer rate due to energy transfer.
in Fig. 5) is correlated with the highest available vibration
frequency of the host (716 cmꢀ1). Thus, with a higher avail-
able vibration frequency of the host, fewer phonons partici-
pated in the process of multi-phonon relaxation from the
Eu3+ excited state to the Eu3+ 5D0 state. This increased the
multi-phonon relaxation rate, increasing the emission proba-
bility as well as PL intensity from energy level 5D0 to the
ground state.
The Li+ doping leads to fewer phonons participating in
the multi-phonon relaxation process from the Eu3+ excited
state to the Eu3+ 5D0 state, leading to a higher multi-phonon
relaxation rate, a higher emission probability, and intensity
5
from energy level D0, and a higher radiative transition rate.
According to Eq. (9), the luminescence decay time of the
phosphor is affected by the radiative decay time, non-radia-
tive rate, and energy transfer rate. The non-radiative rate of
multi-phonon relaxation and the radiative transition rate are
inversely proportional to the radiative decay time of the
donor ions. The luminescence decay time decreases with
increasing Li+ content.
In this study, a higher vibration frequency of the host
appears at 716 cmꢀ1 after Li+ doping. This makes fewer
phonons participate in the multi-phonon relaxation process
from the Eu3+ excited state to the Eu3+ 5D0 state, leading
to a higher multi-phonon relaxation rate, a higher emission
probability from 5D0 state, and thus a stronger intensity
5
from D0 state, as shown in Fig. 5.

Photoluminescence and Color Purity of CaTiO3:Eu Phosphor by Li Doping
7
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(4) Color Purity
Figure 9 shows the Commission Internationale de l’Eclairage
(CIE) color coordinates of the CET and CELT specimens,
and photographs of as-prepared samples excited by various
light sources. From the CIE results, with increasing Eu3+
and Li+ content, the color coordinates of CET and CELT
approach the ideal red chromaticity (0.67, 0.33) for the
National Television Standard Committee (NTSC) system (as
shown in the inset of Fig. 9). When the co-doping content of
Eu3+ and Li+ is over 15 mol%, the CIE color coordinates
are closer to the ideal red chromaticity than those of the
commercial Y2O2S:Eu3+ red phosphor. To investigate the
effect of the incorporation of Li+ on color purity, the color
purity was calculated using24:
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qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
2
2
ðxs ꢀ xiÞ þ ðys ꢀ yiÞ
10G. Blasse and B. Grabmaier, Luminescent Materials. Springer, Berlin, Ger-
man, 1994.
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
Color purity ¼
ꢂ 100%
(10)
2
2
ðxd ꢀ xiÞ þ ðyd ꢀ yiÞ
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where (xd, yd) are the coordinates of the dominant wave-
length, (xs, ys) are the coordinates of a sample point, and (xi,
yi) are the coordinates of the illuminant point. In this study,
(xd, yd) = (0.68, 0.32) and (xi, yi) = (0.3101, 0.3162) for the
dominant wavelength at 616 nm.
The calculation results shown in Table III indicate that
Li+ affects the color purity, which can be improved to
92.1%. These results show that Ca1ꢀ2xEuxLixTiO3 red phos-
phors with high color purity have promising application in
solid-state lighting devices fabricated with UV (InGaN) or
blue (GaN) chips.
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IV. Conclusions
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Using the solid-state reaction method, Ca1ꢀ3/2xEuxTiO3 and
Ca1ꢀ2xEuxLixTiO3 red phosphors with bright red emission
and high color purity were synthesized. The Ca1ꢀ3/2xEuxTiO3
powder phosphors exhibited a main red emission peak cen-
tered at 616 nm under 397 nm UV light excitation. The pho-
toluminescence intensity was increased by 1.6 times and the
color purity was increased to 92.1% with Li+ doping. These
improvements can be attributed to two effects: (i) the
compensation of charges, which decreases the number of
Ca vacancies and (ii) leads to a higher emission probability
from 5D0 state and stronger red emission. Thus,
Ca1ꢀ2xEuxLixTiO3 powder phosphors can be regarded as a
promising red phosphor material which is applicable to solid-
state lighting devices when coupled with UV or blue chips.
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Acknowledgment
The authors thank the Bureau of Energy, Ministry of Economic Affairs, R.O.
C., for financially supporting this research under grant 100-D0204-6 and the
LED Lighting and Research Center, NCKU, for their assistance regarding
LED measurement.
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h