Red-emitting enhancement by inducing lower crystal field symmetry of Eu3+ site in CaWO4:Eu3+ phosphor for n-UV w-LEDs

Article abstract of DOI:10.1016/j.jallcom.2017.12.242

A series of enhanced red-emitting phosphors Ca_1-x-yMg_xSr_yWO₄:zEu³⁺ (x = 0.00–0.30, y = 0.00–0.12, z = 0.00–0.16) were successfully synthesized via a high temperature solid-state reaction method. The phase structure, photoluminescence properties and thermal stability were investigated detailedly. All as-prepared samples, especially Ca_0.65Mg_0.20Sr_0.05WO₄:0.10Eu³⁺ phosphors exhibit an extremely enhanced red emission peaking at 616 nm originating from ⁵D₀―⁷F₂ transition of Eu³⁺ under the 392 nm n-UV excitation. Mg²⁺ and Sr²⁺ were introduced into CaWO₄:0.10Eu³⁺ to induce crystallographic parameters and crystal symmetry changes of Eu³⁺ ions in CaWO₄ host. Enhanced red emission, broadband excitation spectra (from 200 to 600 nm), strong ultraviolet absorption matched well with n-UV LEDs chips (350–420 nm) and high color purity (the R/O value is up to 7.52) indicate that as-prepared Ca_0.65Mg_0.20Sr_0.05WO₄:0.10Eu³⁺ sample can be a potential candidate red phosphor for n-UV white light-emitting diodes.

Full text of DOI:10.1016/j.jallcom.2017.12.242

Accepted Manuscript
3+
Red-emitting enhancement by inducing lower crystal field symmetry of Eu site in
3+
CaWO :Eu phosphor for n-UV w-LEDs
4
Minfang Xia, Zhenghua Ju, Huan Yang, Zhenbin Wang, Xiuping Gao, Fuxing Pan,
Weisheng Liu
PII:
S0925-8388(17)34438-9
10.1016/j.jallcom.2017.12.242
JALCOM 44328
DOI:
Reference:
To appear in: Journal of Alloys and Compounds
Received Date: 9 October 2017
Revised Date: 21 December 2017
Accepted Date: 22 December 2017
Please cite this article as: M. Xia, Z. Ju, H. Yang, Z. Wang, X. Gao, F. Pan, W. Liu, Red-emitting
3+
3+
enhancement by inducing lower crystal field symmetry of Eu site in CaWO :Eu phosphor for n-UV
4
w-LEDs, Journal of Alloys and Compounds (2018), doi: 10.1016/j.jallcom.2017.12.242.
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to
our customers we are providing this early version of the manuscript. The manuscript will undergo
copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please
note that during the production process errors may be discovered which could affect the content, and all
legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT
Red-emitting enhancement by inducing lower crystal field symmetry
3+
3+
of Eu site in CaWO :Eu phosphor for n-UV w-LEDs
4
a
a
a
a
b
Minfang Xia , Zhenghua Ju , Huan Yang , Zhenbin Wang , Xiuping Gao , Fuxing
a
a,
Pan and Weisheng Liu *
a
Key Laboratory of Nonferrous Metal Chemistry and Resources Utilization of Gansu
Province and State Key Laboratory of Applied Organic Chemistry, College of
Chemistry and Chemical Engineering, Lanzhou University, Lanzhou, 730000,China.
b
School of physical Sciences and Technology, Lanzhou University, Lanzhou, 730000,
China.
Corresponding author: liuws@lzu.edu.cn (W. S. Liu).
Tel: +86 931 8915151; Fax: +86 931 8912582
Abstract:
3
+
A series of enhanced red-emitting phosphors Ca1-x-yMg Sr WO :zEu (x =
x
y
4
0
.00―0.30, y = 0.00―0.12, z = 0.00―0.16) were successfully synthesized via a high
temperature solid-state reaction method. The phase structure, photoluminescence
properties and thermal stability were investigated detailedly. All as-prepared samples,
3+
especially Ca0.65Mg0.20Sr0.05WO :0.10Eu phosphors exhibit an extremely enhanced
4
5
7
3+
red emission peaking at 616nm originating from D ― F transition of Eu under the
0
2
2
+
2+
3+
3
92 nm n-UV excitation. Mg and Sr were introduced into CaWO :0.10Eu to
4

ACCEPTED MANUSCRIPT
3
+
induce crystallographic parameters and crystal symmetry changes of Eu ions in
CaWO host. Enhanced red emission, broadband excitation spectra (from 200 to 600
4
nm), strong ultraviolet absorption matched well with n-UV LEDs chips (350—420
nm) and high color purity (the R/O value is up to 7.52) indicate that as-prepared
3
+
Ca0.65Mg0.20Sr0.05WO :0.10Eu sample can be a potential candidate red phosphor for
4
n-UV white light-emitting diodes.
3+
Keywords: n-UV w-LEDs, Enhanced red emission, Ca0.65Mg0.20Sr0.05WO :0.10Eu
4
phosphor
1
. Introduction
In recent years, white light-emitting diodes (w-LEDs) have attracted quite a
lot of attention because of their indispensable applications in solid-state lighting
field, display devices, monochrome signaling, etc. Furthermore, on account of
these superior advantages such as energy saving, high luminous efficacy, long
operation time, tunable optical performance and environmental friendliness,
w-LEDs have been regarded as the next generation light source [1-5].
3+
Currently, combining blue InGaN chips with yellow-emitting Y Al O :Ce
3
5
12
(
YAG: Ce) phosphors is the most common approach to prepare w-LEDs.
However, it is a pity that above-mentioned combination has some inevitable
disadvantages, such as poor color rendering index (CRI) and high correlated
color temperature (CCT) due to the deficiency of weak emission in the red
spectral region [2-7]. As an alternative method, a near ultraviolet (n-UV) LED

ACCEPTED MANUSCRIPT
chip coupled with RGB phosphors (red, green, and blue phosphors) has been
another promising way to fabricate w-LEDs [4, 8-11]. In this regard, it is very
significant to explore high efficient RGB phosphors which can be excited by
n-UV LED chips. In comparison with commercialized green and blue
phosphors, red phosphors with lower reliability and poorer luminous efficiency
become the major challenge for n-UV light-emitting diodes [7, 12, 13].
Although nitride phosphor with high stability has attracted much attention as a
red phosphor, it is limited by expensive cost and harsh synthesis conditions
demanding high temperatures and high nitrogen pressure [14, 15]. Therefore,
searching for high quality red phosphors has been a hot issue in recent years.
With excellent thermal stability, favorable luminescence properties, and
unique structural, metal tungstates have emerged as an important family of
luminescent materials. Among them, calcium tungstate (CaWO ) with a
4
2-
scheelite structure containing WO4 tetrahedral groups is a self-activating
phosphor that can emit high-efficiency blue emission under the ultraviolet light
excitation [7, 16-20]. Introduction of rare earth ions into the CaWO host lattice
4
3+
can improve its luminescence properties. In particular, the incorporation of Eu
ions into calcium tungstate generates the red emission in luminescence spectra
3+
excited at UV light [21-24]. The trivalent europium ions (Eu ) exhibit an
5
7
intense red emission originating from the transition of D ― F (J=0, 1, 2, 3, 4).
0
J
3+
Additionally, the crystal sites Eu ions situated in host can determine the
3+
emission characteristics. When Eu ions occupy the sites without inversion

ACCEPTED MANUSCRIPT
symmetry in the host lattice, pure and high quality red emission at around 615
5
7
nm due to the transition of D ― F can be obtained [5, 7, 25]. Therefore, the
0
2
3+
Eu ions luminescence can be improved by doping and altering the crystal
structure.
3
+
The reason why Eu doped CaWO phosphors have been numerous studied is that
4
the phosphors can absorb the 395 nm n-UV light and emit strong red emission [17]. In
3
+
order to enhance the red emission in CaWO :Eu , the effort has been made to reduce
4
3
+
Eu ions symmetry in the host lattices. Therefore, a series of red emission
3+
Ca1-x-yMg Sr WO :zEu (x = 0.00―0.30, y = 0.00―0.12, z = 0.00―0.16) phosphors
x
y
4
2
+
2+
3+
were prepared through doping Mg ions and Sr ions to alter Eu ions lattice
parameters and crystal symmetry in CaWO host so that enhanced the red emission of
4
3
+
CaWO :Eu phosphors.
4
2
2
. Experimental section
.1 Materials and synthesis
3
+
A series of enhanced red-emitting concentration gradient samples, including Eu
3+
2+
3+
2+
2+
ions single doped , Eu , Mg co-doped and Eu , Mg Sr tri-doped phosphors
were prepared through a simple high temperature solid-state reaction route with
CaCO (A.R.), WO (A.R.), MgO (A.R.), SrCO (A.R.), Eu O (99.99%). The raw
3
3
3
2
3
materials were weighted according to the certain stoichiometric ratio. All of the raw
materials were thoroughly mixed in an agate mortar and finely ground with an
appropriate amount of ethanol (A.R.) for 30 minutes firstly. After that, the powder
mixtures were put into the alumina crucibles and transferred to the box furnace to
sinter at 1000℃ in air condition for 3 hours. Finally, the as-prepared samples were
cooled down to room temperature naturally and reground for subsequent

ACCEPTED MANUSCRIPT
measurements.
.2 Measurements and characterization
The X-ray diffraction (XRD) data of samples were collected from a Rigaku
2
D/max-2400 X-ray Diffractometer with Cu Kα radiation (λ = 0.154187 nm) over a 2θ
range from 10° to 90° at scanning step of 0.02°, while the Rietveld refinement data
performed on a D2 Focus diffractometer with graphite monochromatized Cu Kα
-1
radiation (1.54056 Å) at a scanning step rate of 2° 2θ min . The photoluminescence
emission (PL), photoluminescence excitation (PLE) spectra and temperature
dependent PL spectra were recorded on a Hitachi F—7000 spectrophotometer
equipped with a 150 W xenon lamp as the excitation source. The luminescence decay
curves and quantum yields were measured using an Edinburgh Instruments
spectrofluorometer FLS920. The diffuse reflection spectra (DRS) were measured on
PE lambda 950 UV-vis spectrophotometer using BaSO white powder as reference.
4
The morphology of enhanced red-emitting samples was characterized using a MIRA3
TESCAN scanning electron microscope (SEM). These measurements were performed
at room temperature, except for the temperature dependent PL spectra and lifetime
decays at varied temperatures.
3
3
. Results and discussion
.1 Structural analysis
Fig. 1 (a-b) show the Rietveld structural refinements of the powder diffraction
3
+
patterns of CaWO host and Ca0.90WO :0.10Eu using the general structure analysis
4
4
system (GSAS) program. The crosses and red solid lines are experimental data and
calculated patterns, respectively. The magenta short vertical lines stand for the
position of Bragg reflections of the calculated pattern, and the cyan lines at the bottom

ACCEPTED MANUSCRIPT
plot the difference between experimental data and calculated patterns. The observed
peaks meet the reflection condition and the structural refinements of the host and
3
+
Ca0.90WO :0.10Eu were performed by a tetragonal space group of I41/a. The
4
residual factors and refined unit cell parameters are summarized in Tab. 1. For the
2
host CaWO , the refinement finally converge to R =12.86%，R =8.72%，χ =2.992.
4
wp
p
As exhibited in Fig. 1 (c), the CaWO crystallizes in a tetragonal unit cell with lattice
4
6+
2-
parameters as a = b = 5.2224 Å，c =11.3344 Å. W ion is located at [WO4]
2
+
2-
tetrahedron center while Ca ion is coordinated by eight O ions with S point
4
3
+
2+
2+
symmetry. Eu (r = 1.066 Å for CN = 8), Mg (r = 0.89 Å for CN = 8), Sr (r = 1.26
2
+
Å for CN = 8) are deduced to occupy Ca sites because of their similar ionic radius
2
+
3+
of Ca (r = 1.12 Å for CN = 8). The crystal data of Ca WO :0.10Eu from the
0
.90
4
3
+
Rietveld refinement indicate that Eu ions are successfully incorporated into the host
lattice. Tab. S1 shows the lattice constants of standard XRD patterns of CaWO4,
3+
MgWO , SrWO and as-prepared Ca
Mg Sr WO :0.10Eu samples. It is
x y 4
4
4
0.90-x-y
obvious that greater changes happened in the c constants with the increasing doping
2
+
2+
concentration of Mg and Sr , which indicates the symmetry of the host lattice has
changed a lot. Compared carefully with standard cards of CaWO (ICSD: 18135),
4
MgWO (ICSD: 22357) and SrWO (ICSD: 43219) in Fig. 1 (d), it is perceived that
4
4
all as-prepared phosphors matched well with standard CaWO data and little
4
significant changes were produced in the XRD patterns.
3
.2 Surface morphology analysis
Fig. 2 (a) demonstrates the scanning electron microscopy (SEM) image of the
prepared CaWO host. The grains exhibit obvious agglomeration with relatively
4
regular polyhedron shape morphology and the size is about 1—2 ꢀm. When

ACCEPTED MANUSCRIPT
3
+
2+
2+
introduced Eu , Mg , Sr ions into the host, it is exhibited no compositional shape
3
+
and size dependence as shown in Fig. 2 (b - d). Ca0.90WO : 0.10Eu particles take on
4
3
+
closely aggregated and irregular polyhedrons shape, while Ca0.70Mg0.20WO : 0.10Eu
4
grains show thin and long polyhedrons morphology with smaller size. In addition,
3
+
Ca0.65Mg0.20Sr0.05WO : 0.10Eu
phosphor presents inhomogenous size and
4
morphology. Although dopants made marked differences in size and shape, there
appeared aggregated polyhedrons in morphology as measured. And the results
correspond to the XRD analysis. This observation reveals the inherent characteristics
of the adopted high temperature solid state method.
3
.3 Photoluminescene properties
3+
Fig. 3 shows the changing emission spectra of Ca0.90-x-yMg Sr WO :0.10Eu
x
y
4
(
x=0, 0.20 and y=0, 0.02, 0.05, 0.08, 0.10) phosphors under 392 nm excitation.
The spectra contains some sharp lines ranging from 500 nm to 750 nm, which
5
7
are correlated with the transition from the excited state D — F (J=0, 1, 2, 3, 4)
0
J
3+
levels of Eu . The weaker emission peaks 580 nm, 655 nm, 703 nm are
5
7
5
7
5
7
attributed to D — F , D — F , and D — F transition, respectively. The
0
0
0
3
0
4
5
7
magnetic transition D - F is observed at 592 nm, which is insensitive to the
0
1
crystal field environment. Meanwhile, the hypersensitive electric dipole
5
7
transition D — F is located at 616 nm. The dominant reddish orange emission
0
2
3+
will be obtained if Eu occupies an inversion symmetry site, while the
electronic transition is predominant in a site without inversion symmetry
5
7
5
7
[
26-28]. Therefore, the intensity ratio of the transition D — F to D — F
0 2 0 1

ACCEPTED MANUSCRIPT
3+
(
R/O) becomes a measure for the symmetry of Eu . The lower crystal field
3+
symmetry of Eu site will lead to a higher R/O value. In this case, the increase
3+
3+
R/O values calculated for Ca WO :0.10Eu , Ca Mg WO :0.10Eu and
0.90
4
0.70
0.20
4
3+
Ca Mg Sr WO :0.10Eu were 7.01, 7.11 and 7.52, respectively, which is
0.65
0.20 0.05
4
3+
much larger than that of common phosphors Y O :Eu (3.8) [29]. The
2
3
2+
2+
incremental R/O values of above samples indicate that the doped Mg and Sr
3+
induced the local symmetry around Eu to non-centrosymmetric, so that these
phosphors exhibit high color purity and bright red emission. The optimal
2+
doping content of Sr is found out to be 0.05, as shown in Fig. 3. The CIE
3+
chromaticity coordinates of the red emission phosphors Ca WO :0.10Eu ,
0.90
4
3+
3+
Ca Mg WO :0.10Eu
and Ca Mg Sr WO :0.10Eu
are (0.634,
0
.70
0.20
4
0.65
0.20 0.05
4
0
.363), (0.639, 0.358), and (0.642, 0.356), which get closer and closer to the
NTSC system standard red chromaticity (0.67, 0.33). And the quantum
efficiency is measured with 392 nm excitation to be 50.1%, 58.3%, 37.1%,
respectively.
3+
The decay curves of the CaWO4 host, Ca WO :0.10Eu ,
0.90
4
3+
Ca Mg WO :0.10Eu and Ca Mg Sr WO phosphors excited at 392
0.70
0.20
4
0.65
0.20 0.05
4
nm and monitored at 616 nm are depicted in Fig. 4. The host decay curve can
be fitted well with the first-order exponential decay mode by eq 1 [30, 31]:
I
(
t
) = I0
+
A
exp (-t/τ),
(1)
3+
2+
2+
while the Eu , Mg and Sr doped phosphors complied with a second-order
exponential decay as the following biexponential eq 2 [7, 32, 33]:

ACCEPTED MANUSCRIPT
) = A1 exp (-t/τ1) + A2 exp (-t/τ2),
where ) and I0 are the luminescence intensity at time t and 0; A, A and A
are constants;
can be calculated by eq 3 [34]:
I
(
t
(2)
I (t
1
2
τ, τ1 and τ2 are the decay time. The average decay time (τav) also
2
2
τav = (A τ
1 1
+ A τ
2 2
)/( A τ
1 1
+ A τ )
(3)
2 2
On the basis of above equation and the obtained decay curves, the average
3+
lifetime values of as-measured the CaWO4 host, Ca WO :0.10Eu ,
0.90
4
3+
Ca Mg WO :0.10Eu and Ca Mg Sr WO phosphors are marked in
0.70
0.20
4
0.65
0.20 0.05
4
2+
2+
3+
detail in Fig. 4. Although Mg ions and Sr ions were co-doped with Eu into
3+
the host, the average lifetime values of Ca0.90WO :0.10Eu ,
4
3+
Ca Mg WO :0.10Eu
and Ca Mg Sr WO phosphors deviated
0.65 0.20 0.05 4
0
.70
0.20
4
slightly.
The diffuse reflection spectra (DRS) of un-doped, 0.10Eu single doped,
3+
3+
2+
3+
2+
2+
0
.10Eu , 0.20Mg co-doped and 0.10Eu , 0.20Mg , 0.05Sr tri-doped
CaWO and the photoluminescence excitation spectra (PLE) were measured
4
3+
2+
2+
and the spectra of 0.10Eu , 0.20Mg , 0.05Sr tri-doped as a typical example
to investigate in Fig. 5. The excitation spectra monitored at 616 nm consists of
a broad band ranging from 200 nm to 310 nm with the maximum at 274 nm and
several sharp peaks from 310 nm to 600 nm. The former corresponds to the
3+
2-
6+
2-
charge transfer bands (CTB) of the Eu —O and W —O transitions, and the
sharp excitation peaks are associated with the characteristic 4f—4f transitions
3+
7
5
7
5
of Eu . In addition, these peaks can be attributed to the F — D , F — L ,
0
4
0
7

ACCEPTED MANUSCRIPT
7
5
7
5
7
5
7
5
F — L , F — D , F — D , and F — D [35]. It is obvious that the
0
6
0
3
0
2
0
1
3+
UV-visible DRS of un-doped and Eu doped CaWO are approximately the
4
same, while the latter has the enhanced absorption in n-UV region because of
3+
the 4f—4f transitions of Eu . Furthermore, the peaks at 392 nm and 465 nm
3+
can be observed in the 0.10Eu doped CaWO DRS, which are attributed to the
4
7
5
7
5
3+
F — L and F — D transitions of Eu and are consistent with the PLE
0
6
0
2
2+
2+
spectrum. With the doping of Mg , Sr , the absorption in UV region is
3+
3+
stronger and stronger. The DRS of un-doped, 0.10Eu doped and 0.10Eu ,
2+
0
.20Mg , co-doped CaWO and their PLE are shown in Fig. S1.
4
Simultaneously, the energy gap estimated according to the DRS spectrum of the
host is 4.58eV and the direct band gap calculated by density function theory
(
DFT) [36] is 4.15eV. The results are detailedly depicted in Fig. S2. Both the
PLE and DRS spectra exhibit strong absorption in n-UV region, which matches
well with the n-UV LED chips (350 nm—420 nm), indicating that the
Ca Mg Sr WO sample can be a candidate red-emitting phosphor for
0.65
0.20 0.05
4
n-UV LEDs [4].
3
.3 Thermal quenching properties
The thermal quenching property of phosphors is an indispensable parameter for the
high power LEDs application. According to numerous investigation of thermal
quenching behavior, the activation energy of non-radiation relaxation and the rate of
temperature induced direct tunneling are prevailing factors, which prevents the
different activator ions’ emissive transition between excited state and ground state in

ACCEPTED MANUSCRIPT
the host [37-39]. The temperature dependent emission spectra of the
3
+
3+
3+
Ca0.90WO :0.10Eu , Ca Mg WO :0.10Eu , and Ca0.65Mg0.20Sr0.05WO :0.10Eu
4
0.70
20
4
4
excited at 392 nm are illustrated in Fig. 6 (a - c), respectively. The thermal quenching
data were collected from 298 K to 573 K. As shown in Fig. 6 (d), it is clear that the
5
7
normalized emission intensities of D — F transition gradually decrease with the
0
2
3
+
temperature increasing. The emission intensities of Ca0.90WO :0.10Eu
and
4
3+
Ca0.70Mg WO :0.10Eu at 573 K are 16.81 % and 10.62% of their initial intensities
20
4
2+
at 298 K. When Sr is co-doped into the host, the thermal properties decline rapidly.
In order to further investigate thermal quenching mechanism, the lifetime decays of
3
+
3+
3+
Ca0.90WO :0.10Eu , Ca Mg WO :0.10Eu and Ca0.65Mg0.20Sr0.05WO :0.10Eu at
4
0.70
20
4
4
different temperatures were measured at the 392 nm excitation by 616 nm emission
3
+
monitoring. Additionally, the decay curves of Ca0.90WO :0.10Eu sample is
4
3
+
illustrated in Fig.
7
and those of Ca0.70Mg WO :0.10Eu
and
20
4
3
+
Ca0.65Mg0.20Sr0.05WO :0.10Eu samples are shown in Fig. S3. Obviously, the decay
4
curves measured at various temperatures are with high similarity to each other and
they all fitted well to the biexponential function as eq 2. Besides, the short lifetime τ1
and long lifetime τ of all samples decreased with the increasing temperature and
2
3+
listed in Table S2. Especially, the decay tendency of Ca0.65Mg0.20Sr0.05WO :0.10Eu
4
sample is greater, which indicates that the sample temperatures have no considerable
3
+
2+
2+
impact on the energy transfer between Eu ions and the dopant Mg and Sr cause
relatively greater decays.

ACCEPTED MANUSCRIPT
3+
According to many studies on Eu doped phosphors, it is validated that the
crossover process is responsible for the thermal quenching mechanism of the
5
7
3+
D — F emission of Eu [14, 40]. Moreover, the crossover process can be
0
2
5
depicted as the populations at the D level overcome the energy barrier and
0
2-
3+
7
migrate to the O —Eu CTB; finally, they relax to F level. The crossover
J
process is described in Fig. 8 as process
to the 616 nm emission as . In addition, the thermal activation process can
be expressed by the Arrehenius-type activation model as [40, 41]
① and the other electrons contributed
②
-
ꢀE/kT -1
I
(
T
) = I0 [1+
Ce
]
(4)
where I is the initial intensity, I(T) is the intensity at given temperature T, k is
0
5
Boltzmann’s constant, C is a constant, and ꢀE is the activation energy for the D
0
level to the CTB. In addition, ꢀE can be regarded as a constant for nonsignificant
changes in the thermal quenching emission curve. The experimental data show a good
linear fit as displayed in Fig. S4, which indicated that the thermal quenching process
3+
complies well with eq (4). The activation energy of the Ca0.90WO :0.10Eu ,
4
3+
3+
Ca0.70Mg WO :0.10Eu
and Ca0.65Mg0.20Sr0.05WO :0.10Eu
phosphors were
20
4
4
calculated to be 0.36eV, 0.38eV, 0.36eV, respectively. The higher position of
2
-
3+
O —Eu CTB locates, the higher activation energy is, the harder the crossover
process occurs and the more stable the phosphor is.
4
. Conclusions
In summary, a series of enhanced and bright red-emitting Eu , Mg , Sr ions
3
+
2+
2+
3
+
doped tungstate phosphors were designed by changing the symmetry of Eu in the

ACCEPTED MANUSCRIPT
host lattices and successfully fabricated using a simple solid-state method. The
structure and characteristic photoluminescence properties were investigated in detail.
3
+
Especially, the Ca0.65Mg0.20Sr0.05WO :0.10Eu phosphors exhibit stronger ultraviolet
4
absorption which conforms to the n-UV LED chips and higher red emission peaking
5
7
3+
at 616 nm due to the D — F transition of Eu . The thermal quenching property was
0
2
discussed in a simplified configurational coordination diagram and shown excellent
thermal stability. On the basis of above mentioned experimental results, the
3
+
Ca0.65Mg0.20Sr0.05WO :0.10Eu sample can be a potential candidate red phosphor in
4
n-UV LEDs application.
Acknowledgements
This work is supported by the National Nature Science Foundation of China
(
Grant nos. 21431002, 21701072 and 21701070). The Fundamental Research
Funds for the Central Universities (No. lzujbky-2017-183). F. Pan gratefully
acknowledges the support by the Fundamental Research Funds for the Central
Universities (223000/862678).
References
[
1] P. Singh, C.M. Tan, Degradation Physics of High Power LEDs in Outdoor
Environment and the Role of Phosphor in the degradation process, 6 (2016)
4052.
2] E.F. Schubert, J.K. Kim, Solid-State Light Sources Getting Smart, Science,
08 (2005) 1274-1278.
2
[
3

ACCEPTED MANUSCRIPT
[
3] G. Li, Y. Tian, Y. Zhao, J. Lin, Recent progress in luminescence tuning of
3+
2+
Ce and Eu -activated phosphors for pc-WLEDs, Chemical Society Reviews,
4 (2015) 8688-8713.
4] Z. Xia, Z. Xu, M. Chen, Q. Liu, Recent developments in the new inorganic
solid-state LED phosphors, Dalton Transactions, 45 (2016) 11214-11232.
5] J. Zhong, D. Chen, H. Xu, W. Zhao, J. Sun, Z. Ji, Red-emitting
4
[
[
3+
CaLa (SiO ) O:Eu phosphor with superior thermal stability and high quantum
4
4 3
efficiency for warm w-LEDs, Journal of Alloys and Compounds, 695 (2017)
11-318.
6] M. Peng, X. Yin, P.A. Tanner, M.G. Brik, P. Li, Site Occupancy Preference,
3
[
4+
Enhancement Mechanism, and Thermal Resistance of Mn Red Luminescence
4+
in Sr Al O : Mn for Warm WLEDs, Chemistry of Materials, 27 (2015)
4
14 25
2
938-2945.
[
7] Y. Zhang, W. Gong, J. Yu, Z. Cheng, G. Ning, Multi-color luminescence
3+
3+
properties and energy transfer behaviour in host-sensitized CaWO :Tb ,Eu
4
phosphors, RSC Advances, 6 (2016) 30886-30894.
[
8] L. Li, Y. Pan, Y. Huang, S. Huang, M. Wu, Dual-emissions with energy
3+
3+
transfer from the phosphor Ca Al Zn O :Bi ,Eu
for application in
14
10
6
35
agricultural lighting, Journal of Alloys and Compounds, 724 (2017) 735-743.
3+
[
9] S. Kaur, M. Jayasimhadri, A.S. Rao, A novel red emitting Eu doped
calcium aluminozincate phosphor for applications in w-LEDs, Journal of
Alloys and Compounds, 697 (2017) 367-373.

ACCEPTED MANUSCRIPT
[
10] H. Guo, J. Zhang, L. Ma, J.L. Chavez, L. Yin, H. Gao, Z. Tang, W. Chen,
A Non-rare-Earth Ions Self-Activated White Emitting Phosphor under Single
Excitation, Advanced Functional Materials, 25 (2015) 6833-6838.
[
11] Y.-T. Tsai, H.-D. Nguyen, A. Lazarowska, S. Mahlik, M. Grinberg, R.-S.
Liu, Improvement of the Water Resistance of a Narrow-Band Red-Emitting
2+
SrLiAl N :Eu Phosphor Synthesized under High Isostatic Pressure through
3
4
Coating with an Organosilica Layer, Angewandte Chemie International Edition,
5 (2016) 9652-9656.
12] K. Li, S. Liang, M. Shang, H. Lian, J. Lin, Photoluminescence and Energy
5
[
3+
3+
Transfer Properties with Y+SiO Substituting Ba+PO in Ba Y(PO ) :Ce /Tb ,
4
4
3
4 3
3+
3+
Tb /Eu Phosphors for w-LEDs, Inorganic Chemistry, 55 (2016) 7593-7604.
[
13] A.B. Ellis, K.J. Nordell, T.F. Kuech, S.A. Stockman, G.C. Lisensky, S.M.
Condren, LEDs: New Lamps for Old and a Paradigm for Ongoing Curriculum
Modernization, Journal of Chemical Education, 78 (2001) 1033-1040.
[
14] G. Zhu, Z. Ci, Y. Shi, M. Que, Q. Wang, Y. Wang, Synthesis, crystal
structure and luminescence characteristics of a novel red phosphor
3+
Ca Mg (PO ) :Eu for light emitting diodes and field emission displays,
19
2
4 14
Journal of Materials Chemistry C, 1 (2013) 5960-5969.
15] H. Zhu, C.C. Lin, W. Luo, S. Shu, Z. Liu, Y. Liu, J. Kong, E. Ma, Y. Cao,
[
R.-S. Liu, X. Chen, Highly efficient non-rare-earth red emitting phosphor for
warm white light-emitting diodes, 5 (2014) 4312.

ACCEPTED MANUSCRIPT
[
16] S. Vidya, S. Solomon, J. Thomas, Synthesis of Nanocrystalline CaWO as
4
Low-Temperature Co-fired Ceramic Material: Processing, Structural and
Physical Properties, Journal of electronic materials, 42 (2013) 129-137.
[
17] B. Ding, H. Qian, C. Han, J. Zhang, S.-E. Lindquist, B. Wei, Z. Tang,
Oxygen Vacancy Effect on Photoluminescence Properties of Self-Activated
Yttrium Tungstate, The Journal of Physical Chemistry C, 118 (2014)
2
5633-25642.
[
18] Y. Zhang, W. Gong, J. Yu, H. Pang, Q. Song, G. Ning, A new
3+
single-phase white-light-emitting CaWO :Dy
phosphor: synthesis,
4
luminescence and energy transfer, RSC Advances, 5 (2015) 62527-62533.
3
+
3+
[
19] P. Du, L. Krishna Bharat, J.S. Yu, Strong red emission in Eu /Bi ions codoped
CaWO4 phosphors for white light-emitting diode and field-emission display
applications, Journal of Alloys and Compounds, 633 (2015) 37-41.
[
20] P. Du, L.K. Bharat, X.-Y. Guan, J.S. Yu, Synthesis and luminescence properties
3
+
of color-tunable Dy -activated CaWO phosphors, Journal of Applied Physics, 117
4
(2015) 083112.
[
21] L. He, X. Zou, T. Wang, Q. Zheng, J. Liao, C. Xu, Y. Liu, D. Lin,
Cation-Induced Variation of Micromorphology and Luminescence Properties of
Tungstate Phosphors by a Hydrothermal Method, Inorganic Chemistry, 55
(
2016) 12944-12952.

ACCEPTED MANUSCRIPT
[
22] M. Kubus, A.M. Kłonkowski, A. Lotnyk, L. Kienle, Luminescence
3+
enhancement in composite material: CaWO :Tb nanocrystals incorporated
4
into silica xerogel, Materials Chemistry and Physics, 149 (2015) 424-429.
3+
[
23] X. Lou, D. Chen, Synthesis of CaWO :Eu phosphor powders via a
4
combustion process and its optical properties, Materials Letters, 62 (2008)
681-1684.
24] S.P. Culver, R.L. Brutchey, Lanthanide-activated scheelite nanocrystal
1
[
phosphors prepared by the low-temperature vapor diffusion sol-gel method,
Dalton Transactions, 45 (2016) 18069-18073.
[
25] A.M. Kaczmarek, R. Van Deun, Rare earth tungstate and molybdate
compounds - from 0D to 3D architectures, Chemical Society Reviews, 42
(
[
2013) 8835-8848.
26] Y. Su, L. Li, G. Li, Synthesis and Optimum Luminescence of
3+
+
CaWO -Based Red Phosphors with Codoping of Eu and Na , Chemistry of
4
Materials, 20 (2008) 6060-6067.
[
27] Z. Qianghui, L. Laihui, L. Weiping, W. Feifei, An effective method to detect the
3
+
3+
Curie transition of Er /Yb
photoluminescence intensity ratio, Journal of Physics D: Applied Physics, 49 (2016)
65303.
28] J. Wang, L.H. Luo, The NbO octahedral distortion and phase structural
co-doped BaTiO3 ceramics by up-conversion
2
[
6
3
+
transition of Eu -doped K Na NbO -xLiNbO ferroelectric ceramics, Journal of
0
.5
0.5
3
3
Alloys and Compounds, 101 (2018) 400-407.

ACCEPTED MANUSCRIPT
[
29] H. Song, B. Chen, B. Sun, J. Zhang, S. Lu, Ultraviolet light-induced
3+
spectral change in cubic nanocrystalline Y O :Eu , Chemical Physics Letters,
2
3
3
72 (2003) 368-372.
[
30] Z. Xia, Y. Zhang, M.S. Molokeev, V.V. Atuchin, Structural and
2+
2+
Luminescence Properties of Yellow-Emitting NaScSi O :Eu Phosphors: Eu
2
6
2+
Site Preference Analysis and Generation of Red Emission by Codoping Mn
for White-Light-Emitting Diode Applications, The Journal of Physical
Chemistry C, 117 (2013) 20847-20854.
[
31] M. Shang, G. Li, X. Kang, D. Yang, D. Geng, J. Lin, Tunable
3+
Luminescence and Energy Transfer properties of Sr AlO F:RE (RE = Tm/Tb,
3
4
Eu, Ce) Phosphors, ACS Applied Materials & Interfaces, 3 (2011) 2738-2746.
3+
3+
3+
[
32] J. Zhou, Z. Xia, Luminescence color tuning of Ce , Tb and Eu
codoped and tri-doped BaY Si O phosphors via energy transfer, Journal of
2
3
10
Materials Chemistry C, 3 (2015) 7552-7560.
33] J. Wang, H. Zhang, B. Lei, Z. Xia, H. Dong, Y. Liu, M. Zheng, Y. Xiao,
Enhanced photoluminescence and phosphorescence properties of red
[
2+
CaAlSiN :Eu phosphor via simultaneous UV-NIR stimulation, Journal of
3
Materials Chemistry C, 3 (2015) 4445-4451.
[
34] P. Du, J.S. Yu, Energy transfer mechanism and color controllable
3
+
3+
luminescence in Dy /Eu -codoped NaLa(MoO ) phosphors, Journal of
4
2
Alloys and Compounds, 653 (2015) 468-473.

ACCEPTED MANUSCRIPT
[
35] Y. Tian, B. Chen, R. Hua, N. Yu, B. Liu, J. Sun, L. Cheng, H. Zhong, X.
Li, J. Zhang, B. Tian, H. Zhong, Self-assembled 3D flower-shaped
3+
NaY(WO ) :Eu
microarchitectures: Microwave-assisted hydrothermal
4
2
synthesis, growth mechanism and luminescent properties, CrystEngComm, 14
(
[
2012) 1760-1769.
36] L. Dong, R. Jia, B. Xin, B. Peng, Y. Zhang, Effects of oxygen vacancies on the
structural and optical properties of β-Ga O , Scientific Reports, 7 (2017) 40160.
2
3
[
37] C.C. Lin, Z.R. Xiao, G.-Y. Guo, T.-S. Chan, R.-S. Liu, Versatile
Phosphate Phosphors ABPO in White Light-Emitting Diodes: Collocated
4
Characteristic Analysis and Theoretical Calculations, Journal of the American
Chemical Society, 132 (2010) 3020-3028.
[
38] Y.Q. Li, N. Hirosaki, R.J. Xie, T. Takeda, M. Mitomo,
3+
Yellow-Orange-Emitting
CaAlSiN :Ce
Phosphor:
Structure,
3
Photoluminescence, and Application in White LEDs, Chemistry of Materials,
0 (2008) 6704-6714.
39] X. Piao, T. Horikawa, H. Hanzawa, K.-i. Machida, Characterization and
2
[
2+
luminescence properties of Sr Si N : Eu
phosphor for white
2
5
8
light-emitting-diode illumination, Applied Physics Letters, 88 (2006) 161908.
40] B. Tian, B. Chen, Y. Tian, X. Li, J. Zhang, J. Sun, H. Zhong, L. Cheng, S.
Fu, H. Zhong, Excitation pathway and temperature dependent luminescence in
[
3+
color tunable Ba Gd Zn O : Eu phosphors, Journal of Materials Chemistry C,
5
8
4
21
1
(2013) 2338-2344.

ACCEPTED MANUSCRIPT
+
3
5
[
41] W. Fonger, C. Struck, Eu
D Resonance Quenching to the
Charge
‐Transfer States in Y O S, La O S, and LaOCl, The Journal of Chemical
2
2
2
2
Physics, 52 (1970) 6364-6372.
Figures captions
Fig. 1 Experimental (crosses) and calculated (red solid line) results of XRD
3
+
refinement of (a) the host and (b) Ca0.90WO : 0.10Eu phosphors; (c) the crystal
4
3+
structure of CaWO ; (d) The representative XRD patterns of Ca WO :zEu
4
1-z
4
3
+
3+
(
z=0.04—0.14), Ca0.70Mg0.20WO : 0.10Eu and Ca0.65Mg0.20Sr0.05WO : 0.10Eu
4
4
phosphors.
3
+
Fig. 2 The SEM micrographs of (a) the CaWO host, (b) Ca0.90WO : 0.10Eu ), (c)
4
4
3
+
3+
Ca0.70Mg0.20WO :0.10Eu , and (d) Ca0.65Mg0.20Sr0.05WO : 0.10Eu phosphors.
4
4
3+
Fig. 3 The PL emission spectra of the Ca0.90-x-yMg Sr WO :0.10Eu (x = 0,
x
y
4
0
.20 and y = 0, 0.02, 0.05, 0.08, 0.10) phosphors excited at 392 nm. The inset
2+
5
7
shows the Sr contents dependent D — F intensity and the magnified parts
0
2
5
7
5
7
5
reveal the transitions from D — F , D — F and higher D energy
.
0
3
0
4
1
3+
Fig. 4 The decay curves of (a) the CaWO host, (b) Ca WO :0.10Eu , (c)
4
0.90
4
3+
3+
Ca Mg WO :0.10Eu and (d) Ca Mg Sr WO :0.10Eu phosphors
0.70
0.20
4
0.65
0.20 0.05
4
excited at 392 nm.
Fig. 5 UV-visible DRS of un-doped CaWO host, Ca WO :0.10Eu ,
3+
4
0.90
4
3+
3+
Ca Mg WO :0.10Eu
and Ca Mg Sr WO :0.10Eu
phosphors
0
.70
0.20
4
0.65
0.20 0.05
4
measured
at
room
temperature
and
the
PLE
spectra of
3+
Ca Mg Sr WO :0.10Eu phosphors monitored at 616 nm.
0.65
0.20 0.05
4

ACCEPTED MANUSCRIPT
Fig. 6 The temperature dependent emission spectra of (a) Ca WO :0.10Eu ,
3+
0.90
4
3+
3+
(
b) Ca Mg WO :0.10Eu , (c) Ca Mg Sr WO :0.10Eu excited at 392
0.70
20
4
0.65
0.20 0.05
4
nm, and (d) their thermal stability comparisons.
3+
Fig. 7 Decay curves of Ca WO :0.10Eu sample measured at various
0.90
4
temperatures.
Fig. 8 Schematic illustration of a configurational coordinate diagram of the
3+
thermal quenching process in Eu doped CaWO .
4

ACCEPTED MANUSCRIPT
3
+
Table 1. Results of the structural refinement of CaWO and Ca0.90WO :0.10Eu
4
4
3
+
Formula
CaWO4
Ca0.90WO :0.10Eu
4
Crystal system
Tetragonal
Tetragonal
Space group
I41/
a
I41/
a
Lattice constants
a
= 5.2224
，
c
=11.3344
a
= 5.2248，c =11.3359
Profile range
10-80°
10-80°
Step-scan increment
0.02°
0.02°
2
2
R
factors
Rwp=12.86
，
Rp=8.72%
，
χ =2.992
Rwp=11.99，Rp=7.70%，χ =3.260

ACCEPTED MANUSCRIPT
Figures
Fig.1
Fig. 2

ACCEPTED MANUSCRIPT
Fig. 3
Fig. 4

ACCEPTED MANUSCRIPT
Fig. 5
Fig. 6

ACCEPTED MANUSCRIPT
Fig. 7
Fig. 8

ACCEPTED MANUSCRIPT
Highlights:
3
+
ꢀ
An enhanced red-emitting phosphor Ca0.65Mg0.20Sr0.05WO :0.10Eu
is
4
developed.
2
+
2+
2+
3+
ꢀ
ꢀ
ꢀ
Mg and Sr replace Ca sites to reduce the crystal field symmetry of Eu .
The phosphor matches well with the n-UV chips and the R/O value is up to 7.52.
The phosphor exhibits an excellent thermal stability.