Abstract Pure and rare-earth (Nd 3+ , Tm 3+ and Er 3+ )-doped strontium tungstates of good optical quality with sizes of about f20 mm Â 50 mm were grown successfully by the Czochralski technique. The (0 0 1) orientation was found to be the favorable direction for crystal growth. X-ray powder diffraction (XRD), differential thermal analysis (DTA) and differential scanning calorimeter (DSC) of pure strontium tungstate were measured. The concentrations of Nd 3+ , Tm 3+  and Er 3+ were measured and their segregation coefficients were also calculated. The absorption and emission spectra of rare-earth-doped crystals as a function of the s and p polarizations were presented and discussed. Favorable values of the absorption cross section centered at about 800 nm suggest that Nd 3+ -and Tm 3+ -doped strontium tungstates are promising candidates for laser diode (LD) pumping. 

Baichang Wu

Chaoyang Tu

Guohua Jia

Jianfu Li

Yang Wang

Zhaojie Zhu

Zhenyu You

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ARTICLE IN PRESS0022-0248/$ - se
doi:10.1016/j.jcr
Correspondi
+86-591-371-49
E-mail addreJournal of Crystal Growth 273 (2004) 220–225
www.elsevier.com/locate/jcrysgroCzochralski technique growth of pure and rare-earth-doped
SrWO4 crystals
Guohua Jiaa,b, Chaoyang Tua,b,, Zhenyu Youa, Jianfu Lia, Zhaojie Zhua,
Yang Wanga, Baichang Wua
aFujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, China
bGraduated School of Chinese Academy of Sciences, 100039, Beijing
Received 26 June 2004; accepted 30 July 2004
Communicated by M. Schieber
Available online 2 October 2004Abstract
Pure and rare-earth (Nd3+, Tm3+ and Er3+)-doped strontium tungstates of good optical quality with sizes of about
f20mm 50mm were grown successfully by the Czochralski technique. The (0 0 1) orientation was found to be the
favorable direction for crystal growth. X-ray powder diffraction (XRD), differential thermal analysis (DTA) and
differential scanning calorimeter (DSC) of pure strontium tungstate were measured. The concentrations of Nd3+,
Tm3+ and Er3+ were measured and their segregation coefficients were also calculated. The absorption and emission
spectra of rare-earth-doped crystals as a function of the s and p polarizations were presented and discussed. Favorable
values of the absorption cross section centered at about 800 nm suggest that Nd3+- and Tm3+-doped strontium
tungstates are promising candidates for laser diode (LD) pumping.
r 2004 Elsevier B.V. All rights reserved.
PACS: 42.70.Hj; 78.20.e; 81.10.Fq; 65.40.De
Keywords: A2. Czochralski method; A2. Growth from melt; B1. Strontium compounds1. Introduction
Pure and rare-earth-doped strontium tungstates
have received a great deal of interest in the past
decades in the fields such as electron paramagnetice front matter r 2004 Elsevier B.V. All rights reserve
ysgro.2004.07.095
ng author. Tel.: +86-591-371-1368/2122; fax:
46.
ss: tcy@ms.fjirsm.ac.cn (C. Tu).resonance [1,2], dielectric properties [3], micro-
structure [4,5], Raman spectra characteristics [6]
and Raman self-conversion media [7]. SrWO4
belongs to the scheelite family; the unit cell
parameters are: a ¼ 5:4168 (A; c ¼ 11:951 (A; V ¼
350:66 (A3; z ¼ 4; Dc ¼ 6:35 g=cm
3 and the space
group is I41/a [8].
A. Peckter et al. [9], had described the crystal-
lization of SrWO4 crystal from solution in lithiumd.
ARTICLE IN PRESS
Table 1
Growth parameters of SrWO4 crystals
Parameters Property Parameters Property
Seeding
temp. (1C)
1540 Atmosphere N2
Soaping
temp. (1C)
1590 Pulling rate
(mm/h)
1–1.2
Soaping time (h) 2 Rotate rate
(rpm)
12–15
Crucible size
(mm)
+=50 50 Annealing rate
(1C/h)
12–30
Crucible Iridium
G. Jia et al. / Journal of Crystal Growth 273 (2004) 220–225 221chloride melts. However, a detailed growth meth-
od of pure and rare-earth-doped strontium tung-
states crystals, to our knowledge, has not yet been
reported. In this paper, we present a series of rare-
earth-doped strontium tungstate crystals growth
from melt by the Czochralski method. Growth
methods were presented; measurements of X-ray
powder diffraction (XRD) and differential thermal
analysis (DTA) were recorded. In addition,
polarized absorption and emission spectra of these
rare-earth-doped crystals were investigated and
assigned to the corresponding transitions.Table 2
Properties of pure and rare-earth-doped SrWO4 crystals
Crystals Size Color Ions
concentration
Segregation
(1019/cm3) coefficient
(%)
Pure SrWO4 40 12 Colorless — —
Nd3+:SrWO4 60 20 Sky-blue 8.5 20.09
Er3+:SrWO4 50 20 Red 8.0 20.28
Tm3+:SrWO4 50 15 Colorless 6.2 15.782. Experimental procedure
The initial chemicals of analytical grade SrCO3,
WO3 and spectral grade Ln2O3 (here Ln=Tm, Nd
and Er) were mixed separately in the molar ratio in
an agate motor and then charged into a platinum
crucible. They were heated to 1100 1C and main-
tained at this temperature for 7 days. After that,
the chemicals were deposited in an iridium crucible
and were placed in the DJL400 furnace under
nitrogen atmosphere. The compounds were heatedFig. 1. As-grown pure and rare-earth-doped SrWO4 crystals.to a temperature of 50 1C higher than the crystal-
lization temperature for about 2 h to allow the
melt to mix completely and homogeneously. Pure
SrWO4 seed was selected from small crystals
obtained by spontaneous crystallization and was
used to grow bulky SrWO4 crystal. The seeds used
in the subsequent experiments were oriented
parallel to the c-axis. The rotate and pulling rates
were 12–15 rpm and 1–1.2mm/h, respectively.
When these procedures were over, the crystals
were drawn out of the melt and cooled down to
room temperature at a rate of 12–30 1C/h. Grown
crystals are shown in Fig. 1 and the growth
parameters are displayed in Table 1.
The rare-earth ions concentrations were measured
by the ICP–AES method. Nd3+, Er3+ and Tm3+
concentrations of these crystals were 0.33, 0.31 and
0.24 at%, respectively. The effective coefficients
were determined by the following formula:
KLn ¼
ðmolesLn=ðmolesLnþmolesSrWO4ÞÞcrystal
ðmolesLn=ðmolesLnþmolesSrWO4ÞÞmelt
:
ARTICLE IN PRESS
10 20 30 40 50 60 70 80
0
1×104
2×104
3×104
4×104
In
te
ns
ity
 (
C
ou
nt
s)
2theta (deg.)
4.
94
50
 (
10
1)
3.
22
77
 (
11
2)
2.
98
96
 (
00
4)
2.
71
09
 (
20
0)
2.
37
52
 (
21
1)
2.
00
65
 (
20
4)
1.
91
60
 (
22
0)
1.
76
76
 (
11
6)
1.
64
72
 (
31
2)
1.
61
28
 (
22
4)
1.
49
32
 (
00
8)
1.
30
79
 (
20
0)
1.
29
87
 (
31
6)
1.
24
88
 (
33
2)
1.
21
15
 (
42
0)
1.
17
77
 (
22
8)
Fig. 2. XRD of a pure SrWO4 crystal.
0 200 400 600 800 1000 1200 1400
-1.5
-1.0
-0.5
0.0
0.5
1.0
1.5
2.0
2.5
DSC
DTA
A
rb
. u
ni
t
Temperature (°C)
-2.5
-2.0
-1.5
-1.0
-0.5
0.0
0.5
1.0
1.5
W
ei
gh
t L
os
s 
(1
0-
4 g
)
Fig. 3. DTA and DSC graphs of a pure SrWO4 crystal.
400 800 1200 1600 2000 2400
20
40
60
80
100
T
ra
ns
iti
on
 /%
Wavelength (nm)
Fig. 4. Transmission spectrum of a pure SrWO4 crystal
recorded at room temperature.
Fig. 5. Absorption spectra recorded at room temperature (a)
Tm3+:SrWO4, (b) Nd
3+:SrWO4, (c) Er
3+:SrWO4.
G. Jia et al. / Journal of Crystal Growth 273 (2004) 220–225222
ARTICLE IN PRESS
G. Jia et al. / Journal of Crystal Growth 273 (2004) 220–225 223Here Ln is the lanthanide such as Nd3+, Tm3+
and Er3+. Table 2 presents the properties of these
pure and rare-earth-doped SrWO4 crystals.
XRD investigations were carried out with a
CAD4 diffractometer equipped with CuK a
radiation (l ¼ 1:054056 (A). The data were col-
lected using a Ni-filtered Cu-target tube at room
temperature in the 2y range of 51–851. Fig. 2 shows
the XRD patterns of a pure SrWO4 single crystal.
The XRD pattern was in good accordance with the
standard XRD powder card of SrWO4.
DTA and differential scanning calorimeter
(DSC) graphs were measured by Netzsch
STA449C in the temperature range of
30–1300 1C in a N2 atmosphere at heating rates
of 10 1C/min (shown in Fig. 3). On the heating
curve, there was only one endothermic peak
(1300 1C) of the DTA curve corresponding to the
weight loss of the DSC curve. This may be a result
of the sublimation of the extra WO3 in the sample;
a similar phenomenon was also found for
La2(WO4)3 and Sm2(WO4)3 crystals [10,11]. Thus,
there is no high-temperature phase transition in
the range of 30–1320 1C for SrWO4 crystal.3. Spectrum characteristics
Room temperature transmission and absorption
spectra of these crystals were recorded by a
Perkin–Elmer UV–VIS–NIR Spectrometer
(Lambda-35). The transmission spectrum of aTable 3
Comparison between the FWHM and absorption cross section in Tm
Crystals Polarization Wavele
spectra (nm)
5 at% Tm:YVO4 p 797.5
0.5 at% Tm:LiTaO3 s 795
0.7 at% Tm:KYb(WO4)2 p 793.6
s 801.8
0.24 at% Tm:SrWO4 p 795
s 795
1.5 at% Nd3+:Gd0.8La0.2VO4 p 808.5
3.81 wt% Nd3+:LaB3O6 Unpolarization 799
0.33 at% Nd:SrWO4 p 805
s 801pure SrWO4 crystal is shown in Fig. 4. It is
transparent up to 2700 nm and its ultraviolet
absorption edge is at 300 nm. This broad transmis-
sion range enables the study of rare-earth ion
transitions in the visible and infrared region.
Fig. 5 (a), (b) and (c) presents Tm3+-, Nd3+-
and Er3+-doped SrWO4 crystals polarized absorp-
tion spectra recorded at room temperature. Each
absorption band was assigned to the correspond-
ing transitions from the ground state of Tm3+,
Nd3+ and Er3+, respectively. The most interesting
aspects of the absorption spectra in Fig. 5 (a) and
(b) were the intensive absorption bands at about
800 nm, which were suitable for pumping of
commercial GaAlAs laser diode. The FWHM of
the s and p spectra absorption peak of
Tm3+:SrWO4 crystal at about 795 nm is about
10 and 7 nm, respectively. As for Nd3+: SrWO4
crystal, the s spectra absorption peak is at 801 nm
and the FWHM is 17 nm while the the p spectra
absorption peak is at 805 nm and the FWHM is
14 nm.The absorption cross section sa can be
determined by the following equation:
sa ¼ a=Nc;
where a is the absorption coefficient, a=A/L log e,
A is the absorbance, L is the thickness of the
polished crystal, and Nc is the rare-earth ions
concentration in atoms. The absorption cross
section of Tm3+:SrWO4, Nd
3+:SrWO4 and other
crystals are compared in Table 3. The moderate
absorption cross sections and the larger FWHM
indicate that Tm3+:SrWO4 and Nd
3+:SrWO43+- and Nd3+-doped crystals
ngth FWHM Cross section Ref.
(nm) (1020 cm2)
5 2.5 [12]
9.5 6 [13]
3.5 8.8 [14]
1.4 4.8
7 7.01 This work
10 5.83
6.2 14.74 [15]
16 3.37 [16]
14 4.46 This work
17 4.48
ARTICLE IN PRESS
Fig. 6. Emission spectra recorded at room temperature (a)
Tm3+:SrWO4 (805 nm pumping), (b) Nd
3+:SrWO4 (805 nm
pumping) and (c) Er3+:SrWO4 (521 nm pumping).
G. Jia et al. / Journal of Crystal Growth 273 (2004) 220–225224crystals are preferable to be pumped by the
GaAlAs laser diode.
Polarized emission spectra of these crystals were
recorded at room temperature by using an
Edinburgh Instruments FLS920 spectrophot-
ometer (shown in Fig. 6). Each emission band
was also assigned to the corresponding transitions
between the inter-levels of these rare earth ions. In
Fig. 6 (a), (b) and (c) the intense emission peaks
are centered at 1457 nm, 1061 nm and 1536 nm
corresponding to the transition 3H4–
3F4 of Tm
3+
ions, the transition 3F3/2–
4I11/2 of Nd
3+ ions and
the transition 4I13/2–
4I15/2 of Er
3+ ions, respec-
tively. In addition, the intensity of s emission
spectra was much stronger than that of p emission
spectra, indicating larger dependence of the emis-
sion spectra on polarization.4. Results and Discussions
Transparent and crack free of pure and rare-
earth (Tm3+, Nd3+ and Er3+)-doped SrWO4
crystals were grown successfully by the Czochrals-
ki technique. A pure SrWO4 crystal was grown by
spontaneous crystallization along the b-axis direc-
tion, while Tm3+-, Nd3+- and Er3+-doped SrWO4
crystals were grown along the c-axis orientation.
Crystals grown along the b-axis were more easily
cracked than those grown along c-axis directions;
thus the (0 0 1) orientation should be the favorable
direction. This result was in good accordance with
the growth properties of BaWO4 crystals reported
in Ref. [17]. In addition, both comparative lower
pulling rates and suitable annealing program also
contributed to the cracking free of these crystals.
We presented polarized absorption and emission
spectra of rare-earth (Tm3+, Nd3+ and Er3+)-
doped SrWO4 crystals. Results showed that there
were hardly any differences between the p polar-
ized and the s polarized absorption spectra, which
implied that the absorption spectra showed little
depended on the polarization. On the contrary, the
intensity of s polarized emission spectra was much
stronger than that of the p polarized emission
spectra, indicating high dependence of the emis-
sion spectra on the polarization.
ARTICLE IN PRESS
G. Jia et al. / Journal of Crystal Growth 273 (2004) 220–225 225We measured the FWHM and calculated the
absorption cross section centered at about 800 nm
of Tm3+- and Nd3+-doped SrWO4 crystals.
Favorable values of FWHM and absorption
cross-section suggest that Tm3+:SrWO4 and
Nd3+:SrWO4 crystals are promising candidates
for LD pumping.Acknowledgement
This project was supported by the Natural
Science Foundation of China and the Natural
Science Foundation of the Fujian Province of
China (2002I016) and (2001H107).References
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Czochralski technique growth of pure and rare-earth-doped SrWO 4 crystals

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Czochralski technique growth of pure and rare-earth-doped SrWO 4 crystals

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