Investigation of the structure and photoluminescence properties of Ln 3+(Eu3+, Dy3+, Sm3+) ion-doped NaY(MoO4)2

Article abstract of DOI:10.1149/2.012112jes

Novel phosphors of NaY(MoO₄)₂ activated with the trivalent rare-earth Ln³⁺ (Eu³⁺, Dy³⁺, Sm ³⁺) ions were synthesized by facile hydrothermal method with further calcinations. The ultraviolet-visible and photoluminescence spectra of there compounds were characterized to study the optical properties. The electronic structure and orbital population of NaY(MoO₄)₂ were determined by means of density functional theory calculation. The NaY(MoO ₄)₂:Ln³⁺ (Eu³⁺, Dy³⁺, Sm³⁺) phosphors show strong light emissions with different colors coming from different Ln³⁺ ions under blue excitation, which can be used as a potential candidate for white-light-emitting diode and other display devices.

Full text of DOI:10.1149/2.012112jes

Journal of The Electrochemical Society, 158 (12) J387-J393 (2011)
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0013-4651/2011/158(12)/J387/7/$28.00 © The Electrochemical Society
Investigation of the Structure and Photoluminescence Properties
of Ln3+(Eu3+, Dy3+, Sm3+) Ion-Doped NaY(MoO4)2
Qi Wu,a,b Huaiyong Li,d Wanwan Xia,a,b Xihong Fu,e Zuoling Fu,a,b,z Shihong Zhou,c
Siyuan Zhang,c and Jung Hyun Jeongd,z
aState Key Laboratory of Superhard Materials, College of Physics, Jilin University, Changchun 130012, China
bKey Lab of Coherent Light, Atomic and Molecular Spectroscopy, Ministry of Education, Changchun 130021, China
cState Key Laboratory of Rare Earth Resources Utilization, Changchun Institute of Applied Chemistry,
Chinese Academy of Sciences, Changchun 130022, China
dDepartment of Physics, Pukyong National University, Busan 608-737, South Korea
eChangchun Institute of Optics, Fine Mechanics and Physics, Chinese Academy of Sciences, Changchun 130033, China
Novel phosphors of NaY(MoO4)2 activated with the trivalent rare-earth Ln3+ (Eu3+, Dy3+, Sm3+) ions were synthesized by facile
hydrothermal method with further calcinations. The ultraviolet-visible and photoluminescence spectra of there compounds were
characterized to study the optical properties. The electronic structure and orbital population of NaY(MoO4)2 were determined
by means of density functional theory calculation. The NaY(MoO4)2:Ln3+ (Eu3+, Dy3+, Sm3+) phosphors show strong light
emissions with different colors coming from different Ln3+ ions under blue excitation, which can be used as a potential candidate
for white-light-emitting diode and other display devices.
© 2011 The Electrochemical Society. [DOI: 10.1149/2.012112jes] All rights reserved.
Manuscript submitted May 25, 2011; revised manuscript received August 31, 2011. Published November 1, 2011.
Rare earth (RE) ion doped phosphors have attracted great inter-
est during the past several decades due to their unique physical and
chemical properties.1–3 Recently, the development of flat panel dis-
plays, such as field emission displays (FEDs), plasma display panels
(PDPs) and thin film electro-luminescent devices (TFEL), or white
light-emitting diodes (w-LEDs), have emerged as the principal moti-
vation for research into rare-earth luminescence, and the present ar-
ticle therefore concentrates on the variety of different ways in which
rare-earth luminescence has been exploited in this field.4–7 The rare
earth-molybdenum (VI) oxides system constitutes a very rich family
within which a great variety of compounds can be synthesized which
differ in stoichiometry and structure. Recently, many works focused
on luminescence properties research of molybdates doped with rare
earth ions also have been carried out8–11 because molybdates have
been widely used as host candidates for white LEDs. MLn(MoO4)2
(M = alkali metal, Ln = rare earth) molybdate crystals are attrac-
tive host materials, not only because of their large lanthanide ad-
mittance, but especially owing to their good optical properties.12–16
The MLn(MoO4)2 single crystals, which is with a scheelite structure,
can be used as self-doubling solid-state laser host materials.12,17,18
In the fields of phosphor materials, MLn(MoO4)2 materials doping
Eu or Tb also present excellent luminescent properties.18,19 In Wang
et al.’s19 reports, it is interesting that no concentration quenching
of Eu can be observed in the samples of NaLn1−Eux(MoO4)2 and
LiEu(MoO4)2 systems, both of which exhibit the strongest red emis-
sion under 395 nm light excitation and appropriate CIE chromaticity
coordinates (0.66, 0.34) close to the NTSC standard values. To our
knowledge, luminescence in Dy3+, Sm3+-doped NaY(MoO4)2 pow-
ders has not been reported. In this paper, Dy3+, Sm3+ ions activated
NaY(MoO4)2 were prepared by hydrothermal method besides Eu3+
ions, and the phases, morphologies, and photoluminescent properties
were then studied. In addition, it is known that the physical properties
of materials are closely related to their chemical composition, crystal
structure and electronic structure, and density functional theory (DFT)
calculations can offer us detailed information about atom and orbital
arrangement, which can help us to understand the structure-property
relationship better. Therefore DFT calculations were also carried out
in this work to determine the crystal structure and electronic structure
of the host NaY(MoO4)2.
Experimental
Synthesis of the samples.— Materials.— Lanthanide nitrate solu-
tions were obtained by dissolving lanthanide oxide (99.99%) in dilute
nitric acid solution under heating with agitation. All the other chem-
icals were of analytical grade and used as received without further
treatment. For the hydrothermal treatment, we used 60 mL Teflon
cups.
Synthesis.— In a typical procedure of preparing NaY(MoO4)2:
Eu3+, 20 ml 0.1 mol/L Y(NO3)3 and 1 mL of 0.8 mol/L Eu(NO3)3
were first added into test tube. Subsequently, 15 mL of aqueous solu-
tion containing 0.57 mmol of (NH4)6Mo7O24 · 4H2O was added into
the above solution with strong magnetic stirring at room tempera-
ture for 10 minutes to form a homogeneous solution. Next, the same
solutions were adjusted to pH = 4.5, 5, 5.5, 6, 7, 7.5 by adding
MoO3
Na2MoO4
(h) pH=7.5
(g) pH=7
(f) pH=6.5
(e) pH=6
(d) pH=5.5
(c) pH=5
(b) pH=4.5
(a) JCPDS# 52-1802
10
20
30
40
50
60
2θ/Degree
Figure 1. XRD powder patterns of NaY(MoO4)2:Eu3+ samples prepared by
hydrothermal method at 180◦C for 12 h at (b) pH = 4.5; (c) pH = 5; (d) pH
= 5.5; (e) pH = 6; (f) pH = 6.5; (g) pH = 7; (h) pH = 7.5 with further
calcination at 800 ◦C for 4 h.
z E-mail: zlfu@jlu.edu.cn; jhjeong@pknu.ac.kr

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Journal of The Electrochemical Society, 158 (12) J387-J393 (2011)
dropwise amount of NaOH (5M) into the above solution under vigor-
ous stirring before hydrothermal treatment. The amount of NaOH was
controlled by the showing of pH value on pH-meter(the Micro-Digit
PH/mv Meter with high accuracy of 0.01pH), still assisting with the
diluted HNO3 solution by adding dropwise when it was necessary.
Then, NaOH immediately reacted with the molybdenum source and
rare earth nitrate solutions, and a slurry-like white precipitate was
formed. The mixture was stirred again for 1 h. Finally, the mixture
was transferred in a Teflon bottle held in a stainless steel autoclave
sealed and maintained at 180◦C for 12 h. As the autoclave was cooled
to room temperature naturally, the precipitate was filtered and washed
with alcohol and deionized water several times. The precipitates were
dried at 60◦C for 12 h on air. The as-prepared products were retrieved
through a heat treatment at 800◦C in air for 4 h and then slowly cooled
to room temperature. Finally, we can obtain NaY(MoO4)2:Eu3+ pow-
ders. For NaY(MoO4)2:Dy3+ and NaY(MoO4)2:Sm3+, the detailed
hydrothermal procedure is similar to Eu3+-doped NaY(MoO4)2. In
0.45
0.40
0.35
0.30
0.25
0.20
0.15
0.10
300
400
500
600
700
800
addition, it should be mentioned that the activators content (Eu3+
,
Wavelength/nm
Dy3+ and Sm3+) was maintained at 4 mol % for the prepared
samples.
Figure 2. Diffused reflectance spectrum of NaY(MoO4)2 powders.
Characterization.— The structure of the final products was ex-
amined by Powder X-ray diffraction (XRD) using Cu Kα radia-
tion (λ = 0.15405 nm) on a Rigaku-Dmax 2500 diffractometer. A
field emission-scanning electron microscope (FE-SEM, JSM-6700F,
JEOL) was used to characterize the size and morphology of the syn-
thesized NaY(MoO4)2 phosphors. For the optical investigation, the
diffused reflectance spectra of the samples over a range for 190-
700 nm was recorded by using a Uarian Cary 100 scan UV-Visible
spectrophotometer equipped with a Labsphere diffuse reflectance ac-
cessory. The ultraviolet-visible photoluminescence (PL) excitation
and emission spectra were recorded with a Hitachi F-7000 spec-
trophotometer equipped with Xe-lamp as an excitation source. All
the measurements were performed at room temperature.
Energy gap, Band structure, and Density of State (DOS).— The
diffused reflectance spectrum of NaY(MoO4)2 was measured and
shown in Figure 2. The optical absorption can be used for estimat-
ing the band gap by using the Kubelka-Munk equation.26 Kubelka-
Munk’s equation was described as follows: F(R) = (1 − R)2/2
= K/S, where R, K and S were the reflectivity, the absorption, and
the scattering coefficients, respectively. For NaY(MoO4)2, the absorp-
tion edge was around 378 nm (3.28 eV), which was similar to the other
result of Na5Gd(MoO4)4.27
To understand the chemical bonding properties and electronic ori-
gin of optical transition, the band structure and density of states were
performed by the DFT method. The calculated band structures along
the high symmetry points of the first Brillouin zone for NaY(MoO4)2
were shown in Figure 3a. We can see that the lowest energy of con-
duction bands (CBs) was localized at the G point, while the highest
energy of valence bands (VBs) was also localized at the G point.
Hence, NaY(MoO4)2 was a direct bandgap insulator. What’s more,
the gap between the lowest energy of the conduction band and the
highest energy of the valence band was about 3.42 eV, which was
comparable to the experimental value (3.28 eV). Figure 3b and 3c
presented the density of states for the tetragonal NaY(MoO4)2 since
the partial density of states (PDOS) was a useful tool to analyze the
chemical bonding in solids. The upper valence band between −3.0 eV
and the Fermi level (0.0 eV) was dominated by the O-2p, mixing with
very small amount of Na-3s2p, Y-4d and Mo-4d5s states. The con-
duction band just above the Fermi level was dominated by Mo-4d
states, with a slight contributions of the Y-4d and O-2p. Therefore,
their optical absorptions at low energy region can be mainly ascribed
to the charge transitions from O-2p to Mo-4d states.
Calculations.— DFT calculations were performed on
NaY(MoO4)2 by using CASTEP code.20 For the properties
calculation, the Vanderbilt ultrasoft pseudopotential,21 which de-
scribes the interaction of valence electrons with ions, was used with
the same cutoff energy of 340 eV. The 3 × 3 × 4 k-points were
generated using the Monkhorst-Pack scheme.22 The exchange and
correlation functions were treated by the local density approximation
(LDA) in the formulation of CA-PZ.23,24 Valence electrons were de-
scribed by using Vanderbilt-type nonlocal ultrasoft pseudopotentials
while tightly bound core electrons were ignored.25
Results and Discussion
Crystal structure and morphologies.— Figure 1 shows the results
of the XRD analysis of the hydrothermal reaction products obtained
at 180◦C for 12 h under different pH values with further calcina-
tion at 800◦C for 4 h. Suitable pH value for the synthesis of sin-
gle phase crystalline NaY(MoO4)2:Eu3+ powders was investigated
by varying the base (NaOH) concentration used in the reaction sys-
tem. When the value of pH < 5, some of Na2MoO4 and MoO3 ap-
peared as impurity(Figure 1b pH = 4.5), whereas beginning with
pH = 5, no impurity peaks were detected in this experimental range
(Figure 1c). All of the diffraction peaks can be indexed to the scheelite-
type tetragonal structure (JCPDS no.52-1802) with I41/a lattice sym-
metry. The lattice constants are calculated to be a = b = 5.189 Å and c
= 11.313 Å. In addition, when the pH value varied from 5.5 to 7, we
also can obtain the pure phase (Figure 1d-1g). When the value of pH
= 7.5, trace MoO3 appeared as impurity (Figure 1h). The above-stated
results indicated that the pH value was very important for preparing
the pure phase NaY(MoO4)2. We have also performed the FE-SEM
measurement on the NaY(MoO4)2 powders, as shown in the inset of
Figure 1. It was about 200 nm with regular distribution when the value
of pH = 7(Sample (g)).
Photoluminescent properties of NaY(MoO4)2:Eu3+.— It is well
known that the Eu3+ ion is a widely used luminescence activator with
orange or red light emission, as well as a promising structural probe.
The Eu3+ ion energy levels arise from the 4f n configuration, and the
transitions from excited 5D0 level to 7FJ (J = 0–6) levels result in its
emission.
At the same level of Eu3+, the phosphors NaY(MoO4)2 were pre-
pared at various pH values and the evolution of the emission and
excitation spectra as the increasing of pH values were presented in
Figure 4. All spectra are similar in the shapes and locations exclud-
ing relative intensities. With the change of pH value from 5.5 to 7,
the excitation and emission intensities increase gradually and reach
maximum at pH = 7, which is consistent with the results of XRD
and FE-SEM. The morphology and photoluminescence are optimal
when pH = 7. The excitation spectra(Figure 4, left) are monitored at

Journal of The Electrochemical Society, 158 (12) J387-J393 (2011)
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Figure 3. The calculated energy band structure(a); density of states (b) and partial DOS (c) of NaY(MoO4)2 near the Fermi energy level. The Fermi energy is the
zero of the energy scale.

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Journal of The Electrochemical Society, 158 (12) J387-J393 (2011)
λex=466nm
λem=617nm
(a)
6
10
pH=5
4
pH=5.5
1
9
pH=6
pH=6.5
pH=7
7
2 3 5
8
200
250
300
350
400
450
500
550
600
650
700
750
800
Wavelength/nm
(b)
CTS
35
30
25
20
15
10
5
(5I,5H)5
5F4
3P0
5H3
5L10
5D4
5L6
5D3
5G2(5L7,5G3,5G4)
5D2
5D1
5D0
7F6
7F5
7F4
7F3
7F2
77F
F01
0
1 2
3
4
5
6 7 8 9 10
Figure 4. (a) the excitation(left) and emission(right) spectra of NaY(MoO4)2:Eu3+ with different pH values; (b) the transitions depicted from 1 to 10 of
Eu3+
.
active Eu3+ ion can produce a modification in the number of observed
5D0–7FJ transitions, their relative intensity and the magnitude of the
crystal field splitting for each 7FJ state. When Eu3+ ions are embed-
7
an emission wavelength of 617 nm for 5D0 → F2 transition. We can
see that the excitation spectrum shows a broad band in the range of
200-350 nm with the maximum centered around 286 nm, which can
5
ded in sites with inversion symmetry, the D0–7F1 magnetic dipole
2−
be ascribed to the host absorption of MoO4 involving the charge
transition will dominate; on the contrary, when a Eu3+ ion site is non-
transfer state (CTS) from O2− to Mo6+. This is also confirmed by the
distribution of PDOS(Section 3.2). In addition, there are some strong
sharp lines in the longer wavelength range of 350-500 nm and the
sharp lines are associated with the typical intra-4f transitions of Eu3+
ion (Figure 4b).28
5
centrosymmetric, the D0–7F2 electric dipole transitions will be the
strongest in the emission. In this case, NaY(MoO4)2 has a tetragonal
scheelite structure with space group I41/a, in which Y3+ is coordi-
nated with eight oxygen atoms and has a S4 point symmetry with
no inversion center. Upon 466 nm excitation, characteristic emission
peaks of Eu3+ within the wavelength range from 580 nm to 655 nm
Generally, the emission spectra of Eu3+ are sensitive to the sur-
rounding environment. A variation in the environment of the optically

Journal of The Electrochemical Society, 158 (12) J387-J393 (2011)
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(a)
λex=454nm
λem=575nm
8
6
4
7
2
3
5
1
200 250 300 350 400 450
500
550
600
650
700
Figure 5. (a) the excitation(left) and emission(right)
spectra of Dy3+-doped NaY(MoO4)2; (b) the transitions
Wavelength/nm
depicted from 1 to 8 of Dy3+
.
(b)
40
35
30
25
20
15
10
5
CTS
4L17/2
4I15/2
6P7/2
4I11/2
6P5/2
4F7/2,4I13/2
4G11/2
4I15/2
4F9/2
6F1/2
6F3/2
6F5/2
6F7/2
6H5/2
6H7/2,6F9/2
6H9/2,6F11/2
6H11/2
6H13/2
6H15/2
0
1
7
3
6
2
8
4 5
are assigned to transitions from the excited 5D0 state to 7FJ (J = 0–3)
luminescence is close to white. Figure 5a shows the excitation and
emission spectra of NaY(MoO4)2:Dy3+. A similar excitation spec-
trum in the wavelength range from 200–500 nm as Eu3+ is obtained,
in which the principal features are a strong excitation band extend-
ing from 200 to 320 nm with broad band maximum at 283 nm and
levels (see Figure 4b). The emission spectrum is dominated by the
5
red D0–7F2 transition of Eu3+ at 617 nm, which corresponds to the
electric dipole transition and indicates that the Eu3+ ions occupy a site
without inversion symmetry in the crystal lattice. The emission color
was also analyzed and confirmed with the help of CIE chromatic-
ity coordinates. The chromaticity then corresponds to a red region
in the CIE diagram (Figure 7a). In summary, the NaY(MoO4)2:Eu3+
phosphor in our experiment showed an intensive red emission under
466 nm excitation, indicating that it may be applied as an excellent
red component for blue chip white LEDs.
some sharp peaks induced by the f–f transitions of Dy3+ 29
.
Among
all the excitation bands, the broad band (283 nm) corresponds to
host absorption, while the peak located at 454 nm (6H15/2–4I15/2
)
has the maximum intensity. The 454 nm radiation excites the Dy3+
ions to the 4I11/2 level and then quickly relaxes nonradiatively to pop-
4
ulate the F9/2 level.30,31 The yellowish white light is composed of
blue (480 nm) and yellow (575 nm) regions (Figure 5b). They corre-
4
6
Photoluminescent properties of NaY(MoO4)2:Dy3+.— It is well
spond to the emission from the F9/2 excited state to the H15/2 and
6H13/2 ground states, respectively. The 4F9/2–6H13/2 is a hypersensitive
known that the color of the trivalent dysprosium (4f9 configuration)

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Journal of The Electrochemical Society, 158 (12) J387-J393 (2011)
5
(a)
λem=565nm
λex=406nm
12
13
14
1
6
3
2
9
8
10
4
7
11
200
300
400
600
700
800
Wavelength/nm
Figure 6. (a) the excitation(left) and emission(right)
spectra of Sm3+-doped NaY(MoO4)2; (b) the transitions
depicted from 1 to 14 of Sm3+
.
40
(b)
CTS
4I9/2
35
30
25
20
15
10
5
4M21/2,4L15/2
4D3/2
6P7/2
4K11/2
4L13/2,4F7/2
4M19/2,6P5/2
4G9/2,4I15/2
4M17/2
4I13/2,4F5/2
4I11/2
4M15/2
44G
3/2
4G5/2
7/2
F
6F11/2
6F9/2
6F7/2
6F5/2
6F1/2,6H15/2,6F3/2
6H13/2
6H11/2
66H9/2
H
7/2
6H5/2
0
4
12
11
9
14
13
3
5 6
7 8
1 2
10
transition (ꢀJ = 2), which is strongly influenced by the surrounding
377, 406, 421, 441, 465, 482 and 490 nm, which are attributed to the
f–f transition of Sm3+ ions (Figure 6b). In addition, there is a wide
excited band at the region of 200–350 nm with broad band maximum
at 283 nm. Similarly, it is well known that this band is attributed to
the host absorption. The emission spectrum of NaY(MoO4)2:Sm3+
consists of three groups of narrow band emissions including 550–
575, 580–625 and 630–670 nm. All these emissions belong to the
4
environment around the Dy3+ ion. The F9/2–6H13/2 forced electric
dipole transition is predominant only when Dy3+ ions are located at
low-symmetry sites with no inversion centers in NaY(MoO4)2, which
is in accordance with the S4 point symmetry. The CIE chromaticity
coordinates for Dy3+-doped NaY(MoO4)2 phosphors is located at (x
= 0.336, y = 0.328), as indicated in the CIE diagram in Figure 7b.
It is obvious that the coordinate is very close to the standard white
illuminate.
characteristic emission of Sm3+ 22
. The reason of generating the three
emissions is the radiation transition from excited state of Sm3+ ion to
three lower energy states, that is to say, from 4G5/2 to 6H5/2, 6H7/2 and
6H9/2 (Figure 6b). The luminous colors of the phosphors are reddish
orange as result of complex spectra. In general, luminous color is rep-
resented by color coordinates and color ratios. At the present work, the
values of chromaticity coordinate of the phosphor NaY(MoO4)2:Sm3+
has been calculated using the CIE system. It has been found that the
sample has chromaticity coordinates of x = 0.518 and y = 0.314,
which shows reddish orange light emission(Figure 7c).
Photoluminescent properties of NaY(MoO4)2:Sm3+.— Trivalent
samarium with 4f 5 configuration has complicated energy levels and
various possible transitions between 4f levels. The transitions between
these 4f levels are highly selective and of sharp line spectra. The
excitation and emission spectra of NaY(MoO4)2:Sm3+ are given in
Figure 6a. We can see from Figure 6a that the excitation spectrum
of sample is composed of a series of narrow peaks including 365,

Journal of The Electrochemical Society, 158 (12) J387-J393 (2011)
J393
for the Fundamental Research Funds for the Central Universities
(no. 450060326085) and by the Science and Technology Innova-
tion Projects of Jilin Province for overseas students and by the
Korea Research Foundation Grant funded by the Korean Govern-
ment (2010-0022540) and by the National Core Research Center
Program from MEST and NRF(2010-0001-226) and by the Open
Project of State Key Laboratory of Rare Earth Resources Utilization,
Changchun Institute of Applied Chemistry, Chinese Academy of Sci-
ences (RERU2011005).
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In summary, novel phosphors, Eu3+-, Dy3+-, Sm3+-doped
NaY(MoO4)2 were synthesized by hydrothermal reaction with further
calcinations. The crystal structure, morphology, and luminescence
properties were characterized by XRD, FE-SEM, PL and diffused
reflectance spectrum, respectively. It is conceived that their optical
absorption of NaY(MoO4)2 at low energy region can be mainly as-
cribed to the charge transitions from O-2p to Mo-4d states. Our cal-
culated value of the direct band gap for NaY(MoO4)2 is 3.46 eV by
using DFT calculations, which is higher than the experimental value
of 3.28 eV from the diffused reflectance spectra. Under blue excita-
tion, the Eu3+-, Dy3+-, Sm3+-doped NaY(MoO4)2 show strong red,
white light and reddish orange, respectively. These phosphors can be
used as a potential candidate for white-light-emitting diode and other
display devices.
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