Hydrothermal synthesis and luminescence behavior of rare-earth-doped NaLa(WO4)2 powders

Article abstract of DOI:10.1016/j.jssc.2005.01.001

NaLa(WO₄)₂ powders doped with Eu³⁺, Nd³⁺, and Er³⁺ have been synthesized by a mild hydrothermal method and a crystal of exclusive scheelite phase could be obtained at low temperature. From the spectrum of Eu³⁺ it has been concluded that the dopant Eu³⁺ ion occupies a La³⁺ site and mainly takes the site with C2 symmetry. The higher quenching concentration can be observed in the Eu³⁺-doped NaLa(WO₄)₂ powders. The Er³⁺- and Nd³⁺-doped NaLa(WO₄)₂ powders exhibit luminescence in the near infrared (Er³⁺ at 1550 nm, and Nd³⁺ at 1060 nm). The transition mechanism of the up-conversion luminescence of the Er³⁺-doped NaLa(WO₄)₂ powders can be ascribed to two photons absorption process.

Full text of DOI:10.1016/j.jssc.2005.01.001

ARTICLE IN PRESS
Journal of Solid State Chemistry 178 (2005) 825–830
www.elsevier.com/locate/jssc
Hydrothermal synthesis and luminescence behavior of
rare-earth-doped NaLa(WO ) powders
4
2
Ã
Feng Wang, Xianping Fan , Daibo Pi, Zhiyu Wang, Minquan Wang
Institute of Inorganic Materials, Department of Materials Science and Engineering, Zhejiang University, No. 38, Zheda Road, Hangzhou City,
Zhejiang province 310027, PR China
Received 8 November 2004; received in revised form 30 December 2004; accepted 2 January 2005
Abstract
3
+
3+
3+
NaLa(WO
exclusive scheelite phase could be obtained at low temperature. From the spectrum of Eu it has been concluded that the dopant
4 2
) powders doped with Eu , Nd , and Er have been synthesized by a mild hydrothermal method and a crystal of
3
+
3
Eu ion occupies a La site and mainly takes the site with C2 symmetry. The higher quenching concentration can be observed in
+
3+
3
+
3+
3+
4 2 4 2
the Eu -doped NaLa(WO ) powders. The Er - and Nd -doped NaLa(WO ) powders exhibit luminescence in the near
3
+
3+
3+
infrared (Er at 1550 nm, and Nd at 1060 nm). The transition mechanism of the up-conversion luminescence of the Er -doped
NaLa(WO ) powders can be ascribed to two photons absorption process.
r 2005 Elsevier Inc. All rights reserved.
4
2
Keywords: Hydrothermal synthesis; Rare-earth ions; Luminescence; Tungstate
1
. Introduction
It is well known that rare-earth-doped materials can
find a wide variety of applications including phosphors
for lighting, display, and X-ray imaging [14–16],
scintillators [17], laser [3], and amplifiers for fiber-optic
communication [18]. While over a long time the
investigations on the ARe(MO ) family of compounds
Double alkaline rare-earth molybdates and tungstates
(
ARe(MO ) , where A ¼ K, Na, Re ¼ La, Y, and
4
2
M ¼ Mo, W) form a wide variety of inorganic
compounds having tetragonal and monoclinic symme-
tries. The ARe(MO ) doped with trivalent rare earth
4
2
are concentrated on the single crystals to take advantage
of the laser properties and compounds ARe(MO ) with
4
2
(
RE) and transition-metal activators has played an
4
2
important role in quantum electronics for more than
three decades [1]. Their structural diversity provides
these crystals with numerous physical properties. In
addition to the well-known laser potential [2,3], it is also
possible to take benefit of the nonlinearities of the host
as efficient Stokes converters of laser radiation [4–9].
These compounds have been prepared by several
different processes such as the flux growth [10,11],
solid-state reaction [12], and hydrothermal reaction over
an extensive period [13].
small grain size, other uses have received little attention.
Recently, it has been revealed that powder samples of
this family might be promising candidates as phosphors
for visual display [19] and solid-state lighting [20]. But
the traditional synthesis method for prepared powder
samples of ARe(MO ) is usually solid-state reaction
4
2
over 800 1C, involving a complicated procedure and long
reaction time.
Byrappa et al. [13] recently reported preparation of
NaLa(WO ) single crystal via a complicated hydro-
4
2
thermal process that required long reaction time of
several days. In this paper, a simple hydrothermal route
Ã
to synthesis rare-earth-doped luminescent NaLa(WO4)2
Corresponding author. Fax: +86 571 8795 1234.
E-mail address: fanxp@cmsce.zju.edu.cn (X. Fan).
powders was developed and the luminescence behavior
0022-4596/$ - see front matter r 2005 Elsevier Inc. All rights reserved.
doi:10.1016/j.jssc.2005.01.001

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F. Wang et al. / Journal of Solid State Chemistry 178 (2005) 825–830
826
3
of Eu , Nd , and Er ions in NaLa(WO ) powders
was described.
+
3+
3+
HITACH F-4500 fluorescence spectrophotometer with
an external 980 nm LD as the excitation source, instead
of the xenon source in the spectrophotometer. All
measurements were performed at room temperature.
4
2
2
. Experimental
Lanthanum chloride (LaCl ꢀ 4H O) and sodium
3
2
3. Results and discussion
tungstate (Na WO ꢀ 2H O) were used as raw materials.
2
4
2
3
+
Europium chloride (EuCl ꢀ nH O), neodymium chloride
3
2
Fig. 1 shows XRD patterns of the 5 mol% Eu
-
(
NdCl ꢀ nH O), and erbium chloride (ErCl ꢀ nH O)
3
2
3
2
doped samples obtained as-precipitated (A) and after
hydrothermal treatment (B). Only a diffused broad trace
can be found in the Fig. 1A, indicating that only
amorphous NaLa(WO ) particles could be achieved at
room temperature. After the sample was hydrothermal
treated at 180 1C for 2 h in a Teflon-lined autoclave the
XRD pattern shown in Fig. 1B exhibited prominent
peaks well accordant with JCPDS standard card (79-
were prepared by dissolving corresponding rare-earth
oxide (Eu O , Nd O , Er O ) in dilute hydrochloric acid
2
3
2
3
2
3
and then drying in order to remove the unreacted
hydrochloric acid. Solution A was prepared by dissol-
4
2
3
+
ving LaCl ꢀ 4H O and RECl ꢀ nH O (RE ¼ Eu
,
3
2
3+
3
2
3
+
Nd , and Er ) in 90 mL deionized water with the
total metal cations being 3.6 mmol. The solution was
then adjusted to the desired pH 6.0 with NaOH and
stirred for 10 min at room temperature. Another
solution B was prepared by dissolving 2.1375 g of
Na WO ꢀ 2H O (6.48 mmol) in 90 mL deionized water
1118) of tetragonal NaLa(WO ) crystal with no second
4 2
phase. Obviously, the colloidal phase has completely
crystallized during the hydrothermal synthesis. Fig. 2
3
+
2
4
2
shows the SEM photograph of the 5 mol% Eu -doped
NaLa(WO ) powders, which exhibit nearly spherical
morphology. The particle diameter was found to be
with initial pH 10.0 and stirred for 10 min at room
temperature. Solution A was then added to solution B
with vigorous magnetic stirring. The formed suspension
was stirred continuously for 10 min and subsequently
heated at 180 1C for 2 h in a Teflon-lined autoclave of
4
2
about 1–2 mm.
The Eu
3
+
ion is a good probe for the chemical
environment of the rare-earth ion because probability of
5
7
2
50 ml capacity with stirring. The white precipitate was
the D - F transition (allowed by electric dipole) is
0 2
separated by centrifugation and washed with deionized
water and ethanol for several times. And then the
precipitate was dried at 60 1C for 12 h and collected for
characterization. For comparison, a sock of suspension
was stirred under ambient pressure at room temperature
for 2 h. The precipitate was washed and collected with
the same procedure described above, which was
referenced as as-precipitated sample.
The phase and crystallinity were analyzed by X-ray
diffraction (XRD) (Philips XD98) using CuKa radia-
tion. Scanning electronic micrograph (SEM) was taken
on a Philips XL 30. The excitation and emission spectra
in the visible region were recorded on a HITACH F-
very sensitive to relatively small changes in the
5
7
surroundings, but the D - F transition (allowed by
0
1
magnetic dipole) is insensitive to the environment [21].
Thus, the ratio of the two intensities is a good measure
3
+
for the symmetry of the Eu
5
site. In a site with
7
inversion symmetry, the D - F transition is dom-
0
1
inating, while in a site without inversion symmetry, the
5
7
D - F transition is strongest [22]. In addition, the
0
2
5
7
emitting level D0 and the ground state F0 are
nondegenerate, and the D - F emission can give
5
7
0
0
4
500 fluorescence spectrophotometer equipped with a
Xe-arc lamp as excitation source. The luminescence
decay curves were measured with a SP-750 monochro-
mator, a PMT, a BOXCAR and an NCL multi-channel
data collecting analysis system. The sample was excited
by the emission line at 355 nm from the third harmonic
of Xe-lamp pumped Q-switched Nd:YAG laser with
pulse width of 25 ns and beam diameter of 1.5 mm. The
pump repetition rate could be adjusted from single shot
to 20 Hz. The emission spectra in the near-infrared
region were recorded on a TRIAX550 monochromator
(
JOBIN YVON-SPEX) equipped with a PS/TC-1
detector (ELECTRO-OPTICAL SYSTEMS INC.).
Two LDs of 800 and 980 nm were used as excitation
3
+
3+
ions and Er ions, respectively. Up-
3
+
source for Nd
conversion luminescent spectra were taken on the
Fig. 1. XRD patterns of the 5 mol% Eu -doped NaLa(WO
powders (A) as precipitate, (B) after hydrothermal treatment.
4 2
)

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F. Wang et al. / Journal of Solid State Chemistry 178 (2005) 825–830
827
800
600
400
200
0
300
400
500
600
700
Wavelength (nm)
3
+
Fig. 3. Excitation (left, monitored at 614 nm) and emission (right,
Fig. 2. SEM photograph of the 5 mol% Eu -doped NaLa(WO
powders.
4 2
)
3+
excited at 395 nm) spectra of the 5 mol% Eu -doped NaLa(WO
powders.
4 2
)
information about the impurity or if this ion occupies
more than one site symmetry, particularly of the type
C , C , or C [23]. Fig. 3 shows the excitation
1.0
3
3
00 nm excitation
95 nm excitation
nv
n
s
(
3
(
monitored at 614 nm) and emission (excited at
0.8
0.6
0.4
0.2
3
95 nm) spectra of the 5 mol% Eu -doped NaLa
+
WO4)2 powders. The excitation spectrum shows a
broad C–T (O-W) band along with sharp lines of
3
+
Eu
7
ions at ꢁ363 nm, ꢁ395 nm, and ꢁ465 nm
7 5 7 5
5
(
majority of the emission spectrum contains only the
F - D , F - L , and F - D , respectively). The
0 4 0 6 0 2
5
7
D - F transitions, indicating that the higher levels
0
J
were coupled to tungstate stretching vibrations and
5
quenched by cross-relaxation. In addition, the D - F
intensity is about three times more than that of the
5
7
0
2
0
10
20
30
40
50
Mole percent of Eu3+
7
D - F transition, which is allowed even in the free
0
1
5
7
5
D
7
ion case. The dominance of forced electric dipole
transitions implies the loss of inversion symmetry of
Eu
Fig. 4. Dependence of the ratio of
D
0
- F
1
to
0
- F
2
emission
3+
intensity under 300 and 395 nm excitation on mole percent of Eu
ions.
3
+
ions in the NaLa(WO ) powders, which is in
4 2
accordance with the C2 symmetry of the luminescent
rare-earth ions in KRe(WO ) as determined by IR and
4
2
+
Raman spectroscopy [24].
Fig. 4 shows that the ratio of D - F to D - F
2
distribution of Na and RE ions, however, will cause a
reduction of the site symmetry D3d and yield a set of D3d
subgroups (C2; Cs; and C1). Each subgroup of D3d
implies a special set of restrictions on the possible
5
7
5
7
0
1
0
emission intensity is strongly dependent on excitation
3
+
wavelength and doping concentration of Eu
5
ions.
7
+
Similar correlation between the ratio of D - F to
distribution of the Na and RE ions [27]. Thus the
7
variety in the ratio of D - F to D - F emission
0 1 0 2
0
1
5
7
5
7
5
D - F emission intensity and excitation wavelength
0
2
was also observed by Blasse et al. for compounds
intensity under different exciting radiation can be
concluded to be produced by two effects: (1) The
K EuW O ꢀ 18H O [25]. Blasse et al. attributed the
9
10 36
2
3
+
phenomenon to the inhomogeneous composition of the
samples and the diversity of penetration depth for
different exciting radiation. It is well known that the
compound NaLa(WO ) crystallizes in the tetragonal
Eu
including D . (2) Under C–T excitation (O-W,
ions have taken more than one site symmetries
3
d
3
+
300 nm), the Eu
located in the site symmetry D3d
could be excited more effectively. The decrease in the
4
2
5
7
5
7
system of scheelite type with sodium and lanthanum
disordered in the same site [26]. The symmetry of the RE
site is D3d with respect to all ions within the cell opened
by the 12 nearest RE neighbor sites. The statistical
ratio of D - F to D - F emission intensity with
0
1
0
2
3
+
increasing Eu
extent of deviation from D3d symmetry was enhanced at
3+
higher doping concentration of Eu ions.
doping concentration reveals that the

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F. Wang et al. / Journal of Solid State Chemistry 178 (2005) 825–830
828
2000
1500
1000
500
0
10
20
30
40
50
Mole percent of Eu3+
3+
-
Fig. 5. Relationship between the luminescence intensity of the Eu
3+
3+
Fig. 6. Luminescence decay curve of the 5 mol% Eu -doped
NaLa(WO powders.
4 2
doped NaLa(WO ) powders and mole percent of Eu ions.
4 2
)
Fig. 5 shows the relationship between luminescence
intensity of the NaLa1 Eu (WO ) powders by mon-
ꢂx
x
4 2
5
7
itoring the emission of D - F transition at 614 nm
0
2
3
+
and doping concentration of Eu
ions. The higher
concentration quenching can be observed. The concen-
tration quenching can generally be attributed to the
possible nonradiative transfer resulted from resonance
energy transfer between neighboring rare-earth ions [28].
As described above, the NaLa(WO ) crystal adopts a
4
2
sheelite-like tetragonal structure. The structure consists
of WO4 tetrahedron, NaO8 and LaO8 polyhedrons.
LaO polyhedrons are isolated from each other, with
8
+
3
La
spatial arrangement will block resonance energy transfer
ions joined by means of La–O–W–O–La. This
3
+
3+
between Eu
ions, which occupy the La
sites and
result in higher luminescence concentration quenching.
Therefore, the concentration quenching occurs at
5
Fig. 7. Dependence of the D
3+
excited-state lifetime of the Eu in the
0
3
+
3+
NaLa(WO ) powders on mole percent of Eu ions.
higher Eu
powders. Thus, a desirable characteristic of the
concentration in the NaLa1 Eu (WO )
x 4 2
4 2
ꢂx
NaLa1 Eu (WO ) powders could obtained by doping
x
the near infrared. Fig. 8 shows the emission spectra in
3+
ꢂx
more Eu
which was of great benefit to their practical uses.
4 2
3
+
ions into the host NaLa(WO ) powders,
4
the near-infrared of the 5 mol% Er
3
(A, excited at
(B, excited at 800 nm)
2
+
980 nm) and the 5 mol% Nd
doped NaLa(WO ) powders. The emission spectrum of
5
The luminescence decay curves of the D state were
0
4 2
5
7
3+
3+
the 5 mol% Nd -doped NaLa(WO ) powders exhibits
obtained by monitoring the D - F emission of Eu .
Fig. 6 shows the observed luminescence decay curves of
the 5 mol% Eu -doped NaLa(WO4)2 powders. The
luminescence decay curve was approximately fitted to a
0
2
4 2
3
the typical Nd transitions at 880, 1060, and 1330 nm
+
3
+
4
4
( F - I , J ¼9/2, 11/2, and 13/2, respectively). The
3/2
J
3
emission band of the 5 mol% Er -doped NaLa(WO )
+
4
2
4
4
single exponential, yielding the lifetimes of t1
¼
powders at 1550 nm can be attributed to I - I
13/2 15/2
=e
5
3+
3+
1
excited-state lifetime of the Eu
:02 ms: Fig. 7 shows the dependence of the
D
transition of Er . The emission band of the Er has
obviously been split into four peaks (1509, 1540, 1562,
3+
and 1590 nm) in the 5 mol% Er -doped NaLa(WO )
0
3
+
in the NaLa(WO )
4
2
3
+
powders on the doping concentration of Eu ions. The
excited-state lifetime gradually increased with increasing
4
2
3
+
powders in comparison with the other Er -doped
materials [29,30]. It is because tungsten ions have high
charge density due to their small size (0.41 nm [31]) and
high charge (6+), which cause much polarization in the
host material.
3
+
doping concentration of Eu
ions and reached to a
3
maximum at 25 mol% Eu ions.
+
The same procedure was used to synthesize NaLa
WO ) powders doped with rare-earth ions that emit in
4 2
(

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F. Wang et al. / Journal of Solid State Chemistry 178 (2005) 825–830
829
luminescence intensity (525 nm) and P the pumping
power of LD. The slope can be found to be 1.83, which
indicates that the up-conversion luminescence is due to a
two-step excitation process: first excited from the
level by one photon absorp-
1/2
4
ground state to the I
1
4
tion, and then excited to the F7/2 level in large
4
quantities by a second photon. The S
4
levels are populated from F level by multi-phonon
2
and H
11/2
3
/2
7
/2
relaxation, which resulted in luminescence correspond-
2
4
4
4
ing to H - I
and S - I
transitions. This is
15/2
1
1/2
15/2
3/2
3
+
accordance with the up-conversion mechanism of Er
ions which was described by sequential two-photon
absorption or energy transfer up-conversion mechan-
level.
1/2
4
isms [32–34]; both need the population in the I
1
4. Conclusions
Rare-earth-doped NaLa(WO ) powders have been
2
4
synthesized by a simple hydrothermal method. Well-
crystallized scheelite phase could be obtained at low
temperature. The dopant type and concentration can
easily be varied. The higher luminescence quenching
3
+
Fig. 8. Emission spectra in the near infrared of the 5 mol% Er (A,
3+
excited at 980 nm) and the 5 mol% Nd (B, excited at 800 nm) doped
NaLa(WO powders.
3
+
4
)
2
concentration can be observed in the Eu -doped
+
3
3+
NaLa(WO4)2 powders. The Nd - and Er -doped
NaLa(WO4)2 powders exhibit the typical emission
spectra in the near infrared. The up-conversion lumines-
2
4
1
0.00
3+
6
4
2
000
000
000
0
H11/2
I15/2
Slope = 1.83
cence of the Er -doped NaLa(WO ) powders has also
4 2
been observed and the transition mechanism of the up-
conversion luminescence can be ascribed to two photons
absorption process.
9.75
9.50
9.25
6.3
6.4
6.5
6.6
6.7
4
lnP
Acknowledgments
4
S3/2
I15/2
The authors gratefully acknowledge financial support
for this research from the National Nature Science
Foundation of China (No. 50472062, 50272059). The
authors thank Lu Meihua of National University of
Singapore for help with SEM photographs.
5
00
550
600
Wavelength (nm)
650
700
3+
Fig. 9. Up-conversion luminescence spectrum of the 5 mol% Er
excited at 980 nm)-doped NaLa(WO powders. Inset: Relationship
(
4 2
)
between the up-conversion luminescence intensity and the pumping
power of LD.
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[
[
[
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9
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