Hydrothermal synthesis of BaYF5:Yb3+/Er3+ upconversion luminescence submicrospheres by a surfactant-free aqueous solution route

Article abstract of DOI:10.1016/j.jpcs.2010.12.013

BaYF₅:Yb³⁺/Er³⁺ upconversion (UC) luminescence submicrospheres have been synthesized by the hydrothermal synthesis method. The samples were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), scanning probe microscope (SPM), transmission electron microscope (TEM), laser diffraction particle analyzer (LDPA) and UC emission spectra. The as-prepared highly crystalline BaYF₅:Yb ³/Er³ submicrospheres are of uniform size depending on different reaction temperatures and reaction times. It is found that the usage of fluoride source NaBF₄ plays the crucial key in the formation of submicrosphere. Under the 980 nm excitation, the UC emission transitions for ⁴F_9/2→⁴I_15/2 (red), ²H_11/2, ⁴S_3/2→ ⁴I_15/2 (green) in the BaYF₅:Yb ³/Er³ submicrospheres came from two-, two-, and two-photon UC processes, respectively. Further, the effects of Yb³ ion concentration, size and surface of as-prepared submicrospheres, and pumping power on the UC luminescence properties of BaYF₅:Yb ³/Er³ have also been discussed.

Full text of DOI:10.1016/j.jpcs.2010.12.013

Journal of Physics and Chemistry of Solids 72 (2011) 207–213
Contents lists available at ScienceDirect
Journal of Physics and Chemistry of Solids
journal homepage: www.elsevier.com/locate/jpcs
3
+
3+
Hydrothermal synthesis of BaYF :Yb /Er upconversion luminescence
5
submicrospheres by a surfactant-free aqueous solution route
n
Jiayue Sun, Jianbo Xian, Haiyan Du
College of Chemistry and Environmental Engineering, Beijing Technology and Business University, Beijing 100048, China
a r t i c l e i n f o
a b s t r a c t
3
+
3+
Article history:
BaYF :Yb /Er upconversion (UC) luminescence submicrospheres have been synthesized by the hydro-
5
Received 3 May 2010
Received in revised form
thermal synthesis method. The samples were characterized by X-ray diffraction (XRD), scanning electron
microscope (SEM), scanning probe microscope (SPM), transmission electron microscope (TEM), laser
diffraction particle analyzer (LDPA) and UC emission spectra. The as-prepared highly crystalline
1
7 June 2010
Accepted 22 December 2010
Available online 29 December 2010
3+
3+
BaYF
reaction times. It is found that the usage of fluoride source NaBF
of submicrosphere. Under the 980 nm excitation, the UC emission transitions for
5
:Yb /Er submicrospheres are of uniform size depending on different reaction temperatures and
4
plays the crucial key in the formation
Keywords:
4
4
2
F9/2- I15/2 (red),
11/2
H ,
A. Optical materials
B. Chemical synthesis
D. Luminescence
4
4
3+
3+
S
3/2- I15/2 (green) in the BaYF
5
:Yb /Er submicrospheres came from two-, two-, and two-photon UC
3+
processes, respectively. Further, the effects of Yb ion concentration, size and surface of as-prepared
3
+
3+
submicrospheres, and pumping power on the UC luminescence properties of BaYF
5
:Yb /Er have also
been discussed.
&
2010 Elsevier Ltd. All rights reserved.
3
+
3+
1
. Introduction
LaF
3
:Yb /Er . Furthermore, BaYF
5
is also an ideal host for the
3
+
3+
Yb /Tm codopants in nanoscale [10]. Based on this, BaYF is an
ideal host for UC materials by Yb /Er codopants and need to
study the shape- and size-dependent UC luminescent behavior
5
3
+
3+
In the past years, there has been a surge of publications on
upconversion nano- and submicron scale materials owing to their
potential applications, such as lasers, three-dimensional displays,
light emitting devices, biological detection and many others
of BaYF
5
.
3
+
3+
Herein, in this paper, BaYF
5
:Yb /Er submicrospheres have
[
1–7]. Such applications are highly dependent on size, morphology,
been synthesized via a solution-based method in a hydrothermal
environment without any surfactant addition, among which
structure and other fundamental factors that govern the lumines-
cence behavior. Phosphors with spherical morphology of a fine size
are advantageous in achieving high packing densities and low light
scattering, which can significantly improve screen brightness. There-
fore, synthesis of spherical phosphors via a simple and facile
approach may promise a future large-scale synthesis of this nano-
size material for some important applications in nanotechnology, or
may help to unlock a realm of new possibilities in practical
applications through a simple one-pot technique [8]. Typically in
upconversion materials, fluorides are efficient hosts due to their
appropriate energy phonons to produce strong UC fluorescence. As
an important category of fluorides, barium yttrium fluoride (BaYF
is a particularly promising host material for doping with Ln ions for
UC fluorescence and has rarely been studied in detail at submicron
scale [9]. In the bulk state, it is reported that BaYF
excellent upconverting material. In addition, the UC efficiency of
4
NaBF fluorine source plays an important role in the formation
of submicrospheres. To our knowledge, there are rarely reports on
3
+
3+
5
the controlled synthesis of BaYF :Yb /Er submicrospheres, the
major reason might be that the rapid precipitation reaction
between soluble barium salts and rare-earth salts (yttrium,
4 2
ytterbium and erbium) and NaF/NH H F in aqueous solutions
makes it difficult to achieve a controlled nucleation and growth
process, which is prerequisite to obtain uniform and well-defined
3
+
submicron scale. Further, the effects of Yb ion concentration,
size and surface of as-prepared submicrospheres, and pumping
3
+
3+
5
)
power on the UC luminescence properties of BaYF
5
:Yb /Er
3
+
were investigated in detail.
3
+
3+
5
:Yb /Er is an
2. Experimental
3
+
3+
the green emission in BaYF
5
:Yb /Er
is stronger than that in
3
+
3+
2
.1. Synthesis of BaYF : Yb / Er submicrospheres
5
n
2 3 2 3 2 3 4 3 2
Chemicals: Analytical grade Er O , Yb O , Y O , NaBF , Ba(NO )
Corresponding author. Tel./fax: +86 10 6898 5467.
E-mail address: jiayue_sun@126.com (H. Du).
and HNO from Beijing Chemical Reagents, China. Deionized water
3
0022-3697/$ - see front matter & 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.jpcs.2010.12.013

2
08
J. Sun et al. / Journal of Physics and Chemistry of Solids 72 (2011) 207–213
was used throughout. Er
dilute HNO by heating to prepare the stock solutions of Er(NO
Yb(NO and Y(NO . In a typical procedure for the preparation
of BaYF
added to 10 mL of aqueous solution containing 0.02 mmol Er (NO
.2 mmol Yb (NO and 0.78 mmol Y (NO
for 30 min, 20 mL of aqueous solution containing 5 mmol of NaBF
2
O
3
and Yb
2
O
3
were separately dissolved in
recorded on a Hitachi F-4500 spectrophotometer, and an external
980 nm semiconductor laser was used as the excitation source.
3
3 3
) ,
3
)
3
3 3
)
3
+
3+
5
3 2
:20%Yb /2%Er , 10 mL containing 1 mmol Ba(NO ) was
)
3
,
3. Results and discussion
3
0
3
)
3
3 3
) . After vigorous stirring
3.1. Morphologies and structure
4
was introduced into the solution. The pH of the mixture was
adjusted to 9 with diluted NH .H O. After additional agitation for
5 min, the as-obtained mixing solution was transferred into a
0 ml Teflon bottle held in a stainless steel autoclave, sealed and
maintained at the selected temperature of 220 1C for 16 h. As the
autoclave cooled to room temperature naturally, the precipitates
were separated by centrifugation, washed with ethanol and deio-
nized water in sequence, and then dried in vacuum at 80 1C for 12 h.
Other samples were prepared by a similar procedure except that
3
+
3+
Fig. 1(a–e) shows SEM images of the BaYF
5
:Yb /Er
sub-
3
2
1
5
microspheres under different experimental conditions of reaction
temperature and reaction time, the size distribution images of
the submicrospheres corresponding to Fig. 1(a–e) are shown in
3
+
Fig. 2(a–e). First, reaction temperature dependent BaYF
5
:Yb
/
3
+
Er morphologies have been studied. As-obtained products are
spheres in shape (Fig. 1(a)) with a diameter of 160760 nm
(Fig. 2(a)), when the reaction condition was controlled at 160 1C
for 16 h. When the reaction temperature was elevated to above
180 1C, as illustrated in Fig. 1(b), well-crystallized and relative
uniform submicrospheres with a narrow size distribution were
obtained. From the size distribution diagram (Fig. 2(b)), it can be
observed that the as-obtained submicrospheres are uniform in
shape with a diameter of 180740 nm, and the size is increased
with increase in temperature. As can be seen in Fig. 1(c), when the
4 4 4 4
NaBF was replaced by NaF, NH BF and KBF . In addition, different
hydrothermal temperatures and treatment times were selected
to investigate the size and morphological evolution process of
3
+
3+
5
BaYF :Yb /Er products.
3
+
3+
2.2. Characterization
temperature was elevated to 200 1C, BaYF :Yb /Er submicro-
5
spheres with an average size of about 240 nm were synthesized. It
The morphologies of the samples were recorded using a
can also be observed from the size distribution image (Fig. 2(c))
that the average diameter is 240720 nm. To gain more insight
into the formation of the submicrospheres, time-dependent
experiments were carried out at 200 1C with different reaction
scanning electron microscope (SEM, JEOL and JSM-6490). The
size distributions for the samples were measured by a Beckman
Coulter LS13-320 laser diffraction particle analyzer. Transmission
electron microscopy (TEM) and high-resolution transmission
electron microscopy (HRTEM) images were measured with a
HITACHI-H-7605 operated at 80 kV and a JEOL 2010 operated at
3
+
times. As the reaction time was only 4 h, the size of BaYF
5
:Yb
/
3
+
Er submicrospheres was in a large size discrepancy, as shown in
Figs. 1(d) and 2(d). When the time was prolonged to 8 h, it
seemed that the small submicrospheres grew faster and thus the
size distribution was reduced (Figs. 1(e) and 2(e)). When the
reaction time was prolonged to 16 h, the obtained submicro-
spheres became uniform in size, as illustrated in Fig. 2(c). As could
be seen from Figs. 1(c–e) and 2(c–e), it can be accepted that the
2
00 kV, respectively. Scanning Probe Microscope (SPM) images
were measured with a SHIMADZU model SPM-9600 Scanning
Probe Microscope. X-ray diffraction (XRD) analyses were per-
formed on a SHIMADZU model XRD-6000 X-ray powder diffract-
ometer with Cu K
a radiation. UC luminescence spectra were
3
+
3+
3+
3+
Fig. 1. SEM images of BaYF
submicrospheres treated for 4 h at temperatures of 200 1C (d) and 8 h (e), and (f) gives the X-ray diffraction patterns of BaYF
different conditions: (i) 160 1C, 16 h; (ii) 180 1C, 16 h; (iii) 200 1C, 16 h; (iv) 200 1C, 4 h and (iv) 200 1C, 8 h and the JCPDS no. 46-0039 is also given for comparison.
5
:Yb /Er submicrospheres treated for 16 h at different temperatures: (a) 160 1C, (b) 180 1C and (c) 200 1C, SEM images of BaYF
5
:Yb /Er
3
+
3+
5
:Yb /Er submicrospheres obtained at

J. Sun et al. / Journal of Physics and Chemistry of Solids 72 (2011) 207–213
209
3
+
3+
Fig. 2. Size distribution images of BaYF
iv) 200 1C, 8 h.
5
:Yb /Er submicrospheres obtained at different conditions: (i) 160 1C, 16 h; (ii) 180 1C, 16 h; (iii) 200 1C, 16 h; (iv) 200 1C, 4 h and
(
3
+
3+
3+
3+
formation of the uniform BaYF
5
:Yb /Er submicrospheres may
crystalline structure of the BaYF
5
:Yb /Er
submicrosphere.
undergo an Ostwald-ripening process in which small and irregu-
lar submicrospheres dissolved and big ones grew simultaneously
Fig. 3(f) shows a high-resolution TEM image of a single submicro-
sphere where the lattice fringes are clearly observed alluding to the
highly crystalline nature of the material. The distance between the
[
11]. This is understandable, with prolonged ripening time, the
3
+
3+
˚
numerous BaYF
5
:Yb /Er nuclei are formed and quickly aggre-
lattice fringes (d-spacing) is measured to be 3.2 A.
4
Other fluoroborate complexes (NH BF and KBF ) and the
gated into submicrospheres to minimize their surface energy. As a
result, the small submicrospheres possessing higher surface
energy had a faster growth rate than the large ones, which make
the size uniformity possible after a long ripening process [12,13].
4
4
binary fluoride NaF have also been examined as fluoride sources
with the aim to investigate their effects on the morphology of
3
+
3+
BaYF
fluoride sources were indexed to the tetragonal-phase BaYF
(JCPDS no. 46-0039) according to their XRD patterns in Fig. 4(a).
5
:Yb /Er . All the products obtained from these different
3
+
3+
Fig. 1(f) shows XRD patterns of the BaYF
5
:Yb /Er submicro-
5
spheres under different experimental conditions. All the diffrac-
tion peaks are very coincident with the standard card tetragonal-
3
+
However, one can see from the SEM image that the BaYF
5
:Yb
/
3
+
phase BaYF
the longer 2
5
(JCPDS no. 46-0039) except for a small shift toward
Er sample synthesized by NaF is composed on a large scale of
microcrystals with broad size distribution (Fig. 4(b)), which
3
+
3+
y
side, because the radii of Er
and Yb ions are
3
+
smaller than the radius of Y ions. As a result of the similarity of
lanthanide elements (Er, Yb and Y), it is thought that different
lanthanide ions can occupy the same sites in crystal without
changing the crystal structure [10]. So it is feasible to consider
confirms that the NaBF
microspheres. This phenomenon should be ascribed to the rapid
precipitation reaction between soluble barium salt (Ba(NO
and rare-earth nitrate salts (Er(NO , Yb(NO and Y(NO ) and
4
has a key role in the formation of the
3 2
) )
3
)
3
3
)
3
3 3
)
3
+
3+
3+
that the Er , Yb and Y ions are accommodated in the same
crystal sites. On the basis of the above results, we conclude that
varying temperature and reaction time cannot change the crystal
structure of the products, but they can tune the size distribution
of the products, as also given in the above SEM images.
NaF in aqueous solutions makes it difficult to achieve a controlled
ꢀ
nucleation and growth process. In contrast, F ions could be
released slowly from NaBF
barium salt (Ba(NO ) and rare-earth nitrate salts (Er(NO
Yb(NO , Y(NO ) to form the BaYF
a prerequisite to obtain uniform and well-defined microspheres.
4
in aqueous solution and reacted with
3
)
2
3 3
) ,
3
+
3+
3
)
3
3
)
3
5
:Yb /Er nuclei, which is
Further, the size and morphology of the as-prepared product
synthesized at 200 1C for 16 h were examined by scanning probe
microscopy (SPM) and transmission electron microscopy (TEM), as
given in Fig. 3. The SPM image displayed in Fig. 3(a) indicates that
the submicrospheres have rough surfaces and could be aggregated
together easily. The magnified SEM image of the products shown in
Fig. 3(b) also clearly displays uniform submicrospheres with rough
surfaces, and the average size is approximately 240 nm. In Fig. 3(c),
the TEM image demonstrates that the submicrospheres are discre-
tely freestanding with a uniform size distribution. A careful observa-
tion of a single submicrosphere can also find the uneven fringes of
the submicrosphere, as given in Fig. 3(d). In Fig. 3(e), the selected
area electron diffraction (SAED) pattern demonstrates the high
crystalline nature of the material and confirms the tetragonal
3
+
3+
Fig. 4(c) and (d) shows the SEM images of BaYF
pared using NH BF and KBF , respectively. Similar to that
prepared from NaBF , the product from NH BF is sphere in shape
with an average size of 500 nm. However, the product prepared
from KBF is different from those obtained using NaBF and
NH BF , which is octahedron in shape with an average size of
5
:Yb /Er pre-
4
4
4
4
4
4
4
4
4
4
300 nm. Indeed, it has been recognized that not only the fluoride
sources can regulate the crystal growth but also cations can tune
the crystal growth by adsorbing on certain surfaces. For example,
it is reported that various morphologies of microstructured NaYF
produced using NH F and NaF as fluoride sources are different, as
4
4
+
4
+
the cations, NH
and Na , in the reaction system have a different
crystals [14]. It has
guiding effect on the formation of the NaYF
4

2
10
J. Sun et al. / Journal of Physics and Chemistry of Solids 72 (2011) 207–213
Fig. 3. SPM image (a), SEM image (b) and TEM images (c and d) of the products obtained at 200 1C for 16 h, (e) SAED pattern and (f) HRTEM image of the single
submicrosphere corresponding to (d).
+
+
+
+
also been revealed that the cations of XF (X¼K , H , NH
4
, Na ,
under 980 nm semiconductor laser excitation are presented in Fig. 5.
In the UC spectra, strong green upconversion emissions are observed
to be centered at around 520 and 540 nm, which are attributed
+
+
Rb or Cs ) play important roles in the formation of EuF
nanoparticles with different morphologies [15].
3
2
4
4
4
to the
11/2
H -
I15/2 and
S3/2- I15/2 transitions, respectively.
Simultaneously, relatively weak red upconversion emission at
approximately 654 nm is also observed, which originates from the
3
+
3+
3
.2. Upconversion spectra of the BaYF
5
:Yb /Er submicrospheres
4
4
F
9/2- I15/2 transition. It is found in the upconversion emission
3
+
3+
4
4
It is widely accepted that Yb -sensitized Er ion is an efficient
system for near-infrared to visible frequency upconversion. The
upconversion emission spectra of the selected samples doped with
spectra that the green emission intensity ( S3/2- I15/2 transition)
3
+
3+
changes with the dopant Yb ion concentration. As the Yb ions
concentration increases from 10 to 20 mol%, the green emission
intensity drastically increases and arrives at the maximum when the
3
+
3+
different Yb ion and constant Er ion concentrations (2 mol%)

J. Sun et al. / Journal of Physics and Chemistry of Solids 72 (2011) 207–213
211
3
+
3+
3+
3+
Fig. 4. (a) X-ray diffraction patterns of BaYF
5
:Yb /Er samples obtained by different fluoride sources: (i) NaF, (ii) NH
4
BF
4
, (iii) KBF
4
; SEM images of BaYF
5
:Yb /Er
samples obtained by different fluoride sources: (b) NaF, (c) NH
4
BF
4
and (d) KBF
4
.
2H +4S - I
4
4S3/2-4I15/2
4S3/2-4I15/2
4S -4I
11/2
3/2 15/2
4
4
3
/2 15/2
F9/2- I
15/2
0 % Yb3+
(c)
2
(
e)
2
5 % Yb3+
5 % Yb3+
(d)
1
(
b)
1
0% 15% 20% 25% 30%
Yb3+ content
(a)
(b)
(c)
(d)
(e)
3
0 % Yb3+
2H11/2-4I15/2
2
4
H11/2- I15/2
4F9/2-4I
4F9/2-4I15/2
(a)
15/2
1
0 % Yb3+
5
00
550
600
Wavelength(nm)
650
700
500
550
600
650
700
Wavelength(nm)
3
+
3+
3+
3+
3+
Fig. 5. Yb concentration dependent upconversion spectrum of BaYF
submicrospheres synthesized at 200 1C for 16 h.
5
:Yb /Er
Fig. 6. Upconversion spectrum of the BaYF :Yb /Er submicrospheres synthe-
5
sized at different experimental conditions: (a) 160 1C, 16 h; (b) 180 1C, 16 h;
(
c) 200 1C, 16 h; (d) 200 1C, 4 h and (e) 200 1C, 8 h, and the inset shows integrated
2
4
4
4
4
green ( H11/2+ S3/2- I15/2) to red ( F9/2- I15/2) upconversion emission intensity
for corresponding samples. (For interpretation of the references to color in this
figure legend, the reader is referred to the web version of this article.)
3
+
Yb ion concentration is 20 mol%. However the green emission
3
+
almost drastically decreases when the Yb
ion concentration
further increases and the emission appears yellow to the naked
3
+
eye, as also given in the inset of Fig. 5. At higher Yb concentrations,
times than that of the products synthesized at 160 1C for 16 h. The
3
+
4
4
it is possible that the pairing or aggregation of donor Yb ions may
turn some of these ions into quenchers and thus induce a quenching
effect following Er -Yb back transfer [16,17]. Namely, at higher
Yb concentrations the Yb ions may act as trapping centers and
dissipate energy nonradiatively [16].
green upconversion emission ( S3/2- I15/2 transition) intensity
increases greatly with increase in size. Interestingly, as also given
in the inset of Fig. 6, a decrease in the ratio between the
3
+
3+
3
+
3+
2
4
4
4
4
intensities of green ( H11/2+ S3/2- I15/2) and red ( F9/2- I15/2)
is observed with a decrease in their size. Many factors might be
responsible for different UC luminescence properties and the
Fig. 6 shows the UC luminescence spectrum of the as-prepared
3
+
3+
2
4
4
BaYF
5
:20%Yb /2%Er submicrospheres under different experi-
ratios of the intensities of green ( H11/2+ S3/2- I15/2) and red
4
4
mental conditions. It is clear from the upconversion emission
spectra that the luminescence intensity changes with the size.
The submicrospheres obtained at 200 1C for16 h has the greatest
green emission intensity than other samples, which is about three
( F9/2- I15/2), such as crystal structure, size, shape, excitation
power, preparation temperature, phonon confinement, on which
no final conclusion has yet been reached [18]. On one hand, in this
work, this behavior can be attributed to high energetic vibrations

2
12
J. Sun et al. / Journal of Physics and Chemistry of Solids 72 (2011) 207–213
ꢀ
of bound OH groups that can effectively bridge the energetic
gaps between different levels, which results in the enhanced
population of the red emitting level [17–20]. On the other hand,
as the particles become smaller, there will be more number of
as the UC mechanism are presented in Fig. 8, the first photon of
2
infrared irradiation elevates an electron to
F5/2 and transfer the
3
+
4
4
energy to Er , then it can promote an electron from I15/2 to I11/2
4
4
3+
2
and from I11/2 to F7/2 by injecting another Yb ( F5/2) excitation.
3
+
3+
4
Er ions located at the submicrospheres surface, and therefore
there are more surface vibrational sites to support the red
emission than the green emission [18,21–22]. In addition, the
The electron of Er in the F7/2 state may decay nonradiatively to
slightly lower energy states and the transmissions of the electron
from these lower energy states to the ground state may result in
the UC fluorescence emissions. The emission bands at 520, 540
and 654 nm could be assigned to the transitions from the excited
3
+
3+
Er ꢀEr distance becomes small and the cross-relaxation (CR)
2
4
4
4
process ( H11/2+ I15/2- I9/2+ I13/2) occurs. As a result, the green
2
4
4
2
4
4
4
3+
band ( H11/2
,
S
3/2- I15/2) will start to be quenched easily with
states
H
11/2
,
S3/2 and
F9/2 to the ground state
I15/2 of Er
,
4
decreasing size, but the red band ( F9/2-4I15/2) will be quenched
harder [16].
respectively [24].
Furthermore Fig. 7 gives the pumping power dependent upcon-
3
+
3+
version spectra of BaYF
5
:Yb /Er
submicrospheres treated at
4. Conclusion
2
00 1C for 16 h, the UC emission intensities increased gradually
3
+
3+
with increase in pumping power. The relationship between pump
power and emission intensity can be approximately expressed as
follows: Ioutp(Iexc) [23], where n is the number of infrared photons
In conclusion, BaYF :Yb /Er submicrospheres have been
5
synthesized following a simple and facile approach. The temperature
and reaction time in these actions play an important role in
controlling the size distribution. Furthermore, the usage of fluoride
source NaBF is the crucial key in the formation of submicrosphere
morphology. Under the 980 nm excitation, the UC emission
n
absorbed for emitting a visible photon; n¼1.40, 1.71 and 1.38
2
4
4
4
4
for the H11/2, S3/2- I15/2 and F9/2- I15/2 emissions, respectively
4
2
(
inset of Fig. 7). This means that the population of the states H11/2
S
,
4
4
2
4
4
4
4
3/2 and F9/2 came from two-, two- and two-photon UC processes,
transitions for H1 ( S )– I
(green), and F9/2– I
15/2
(red) in
1/2
3+
3/2
15/2
3
+
3+
3+
respectively. The energy level diagrams of Er and Yb ions as well
the BaYF :Yb /Er submicrospheres came from two-, two- and
5
two-photon UC processes, respectively. The UC intensity influenced
3
+
by the size and surface, Yb concentration, pumping power has
been discussed.
4S3 - I
4
/2
15/2
n = 1.71
Acknowledgments
4
4
Power
F9/2- I
15/2
4
2
4
n = 1.38
n = 1.40
S
H
- I
3
/2 15/2
This work was supported by the National Natural Science
Foundation of China (nos. 20976002 and 20876002), the Beijing
Natural Science Foundation (nos. 2091002 and 2082009) and
Funding Project for Academic Human Resources Development in
the Institution of Higher Learning Under the Jurisdiction of Beijing
Municipality.
4
- I
1
1/2 15/2
0
.2
0.3
0.4 0.5
2H11/2-4I15/2
2
Log(Powder Density(W/cm ))
4F9 - I
4
/2
15/2
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