Dehydration and oxidation in the preparation of Ce-doped LaCl3 scintillation crystals

Article abstract of DOI:10.1016/j.jallcom.2007.11.139

Ce³⁺ activated LaCl₃ is a promising scintillation crystal for radiation detection. The starting raw materials used to grow LaCl₃ crystals are rather hygroscopic and XRD proves that LaCl₃·7H₂O and LaCl

Full text of DOI:10.1016/j.jallcom.2007.11.139

Journal of Alloys and Compounds 467 (2009) 120–123
Dehydration and oxidation in the preparation of Ce-doped
LaCl scintillation crystals
3
∗
Guohao Ren , Xiaofeng Chen, Yu Pei, Huanying Li, Hongxiang Xu
Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 201800, China
Received 23 October 2007; received in revised form 27 November 2007; accepted 28 November 2007
Available online 5 December 2007
Abstract
Ce3+ activated LaCl
3
is a promising scintillation crystal for radiation detection. The starting raw materials used to grow LaCl
hygroscopic and XRD proves that LaCl O and LaCl ·3H
·7H
3
crystals are rather
◦
3
2
3
2
O are two main compounds in the raw materials. Five endothermal peaks, 100.4 C,
◦
◦
◦
◦
1
56.0 C, 181.0 C, 210.6 C and 840 C, are observed in the DTA curves, they are corresponding to the thermal dehydration effect of crystallized
water and melting. The process of dehydration is described to be 7 → 6 → 3 → 1 → 0. In the dehydration process, little amount of LaOCl phase
can be identified, its formation is related to the oxidation of LaCl at high temperature: LaCl
3
3
·nH O → LaOCl + HCl↑. In addition, LaOCl phases
2
also appear in the grown crystals, they are suggested to be responsible for the transmission loss of the crystals. The oxidation temperature of LaCl
with oxygen is much lower than that with water.
3
©
2007 Elsevier B.V. All rights reserved.
Keywords: X-ray diffraction; Growth from melt; Halides; Rare earth compounds; Scintillator materials
1
. Introduction
Up till now, most of the researchers devote their effort on the
crystalgrowthandtheevaluationofthescintillationpropertiesof
LaCl3 crystal. There is no report on the treating of the raw mate-
rials. In fact, the raw material processing plays more important
role than crystal growth. In this paper, we will present the dehy-
dration and oxidization phenomena appeared in the preparation
process of LaCl3 raw materials.
LaCl3 doped with Ce3+ was discovered to be a new kind of
scintillation crystal in 1999 [1]. It has a very high light output
(
(
∼50,500p/MeV), good energy resolution and fast decay time
3
20 ns) [2]. Even though its density is only 3.86 g/cm , it is very
attractive in the application fields where demands of energy res-
olution and time resolution are over density and radiation length.
These properties make it a very promising material for detection
of low energy ␥-ray spectroscopy of energetic photons as well
as neutrons.
2
. Experimental
LaCl3 polycrystalline powder usually contains some crystallized water or
However, this compound is very hygroscopic when it is
exposed to an atmosphere, so that the compound always con-
tains more than 30 (wt.%) crystallized water or moisture. Since
the crystallized water will vaporize at high temperature, it is
unacceptable for these aqueous materials to be used as an initial
charge to grow crystals. However, the anhydrous LaCl3 com-
pounds in the market are too expensive to meet the needs of
mass production. Therefore, it is necessary to search an effec-
tive way to remove crystallized water from the raw materials
before growing crystals.
absorbed water. Heating the raw materials within an ampoule for several hours
is a common way to remove the moisture and crystallized water before they are
charged into a crucible. Since the LaCl3 crystal melts congruently at 860 C
◦
[
3], Bridgman method can be used to grow LaCl3 crystals. Initially, crucibles
with taped tips were used to ensure that only single crystal propagates. LaCl3
powders, as well as a small amount of oxygen scavenger, a chemical to combine
with oxygen, were filled into the crucible made of platinum and then were heated
to the temperature higher than the melting point of LaCl3 crystal. During the
entire growth process, the temperature gradient at the interface between the solid
◦
and melt was kept in the range of 25–30 C/cm. The lowering rate of crucible is
2–3 mm/h.
The DTA and TG curves of the samples were measured with STA 409C
thermal analyzer of NETZSCH. The structure of the crystal boule and sam-
ples obtained at various stages were examined by D/max 2550 V powder
X-ray diffractometer. The transmission was measured by Shimadzu UV-2501PC
Spectrophotometer. X-rayexcitedluminescence(XEL)measurementswerecon-
∗
Corresponding author. Tel.: +86 21 69987740; fax: +86 21 59927184.
E-mail address: rgh@mail.sic.ac.cn (G. Ren).
0
925-8388/$ – see front matter © 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2007.11.139

G. Ren et al. / Journal of Alloys and Compounds 467 (2009) 120–123
121
Fig. 1. XRD pattern of LaCl3·7H2O raw materials.
Fig. 2. DTA/TG curves of LaCl3 raw materials measured in N2 atmosphere.
ductedon asetup, X-rayExcited Luminescence Spectrometer, FluorMain, where
F-30 movable X-ray tube (W anticathode target) was used as X-ray source.
Luminescence spectra were obtained by a 44 W plate grating monochromator
Hamamatsu R928-28 photomultiplier and data auto-acquisition system.
al. proposed that the anhydrous LaCl3 can be obtained by three
steps of dehydration of LaCl3·7H2O, that is 7 → 5 → 1 → 0 [5].
Hong and Sundstrom once proved that the dehydration scheme
of LaCl3·7H2O is 7 → 3 → 1 → 0 [6]. And in our measure-
ment, the four endothermal peaks existing in Fig. 2 demonstrate
that the dehydration is not finished by three steps, but by
four steps. As the calculated mass loss ratio of LaCl3·7H2O
removed water molecules from 7 to 6 to 3 to 1 to zero is 4.85%,
14.55%, 9.7% and 4.85%, respectively (Table 1), they are quite
fit for the ratio measured in Fig. 2. Therefore, it is suggested
that the dehydration process of LaCl3·7H2O can be expressed
as LaCl3·7H2O → LaCl3·6H2O → LaCl3·3H2O → LaCl3·H2O
→ LaCl3. The total weight loss ratio is 34.2%, nearly reaches
the ratio of 33.95%, calculated by the dehydration reaction
LaCl3·7H2O → LaCl3 + 7H2O. This small difference may result
from the little amount of absorbed water in the starting materials.
3
. Result and discussion
3
.1. Dehydration process of LaCl3·7H2O
The crystal structure of the raw materials used to grow
crystals was measured with D/max 2550 V powder X-ray
diffractometer. XRD pattern reveals that the raw materials
are composed of hydrated LaCl3, which contains seven and
three crystallized water molecules, respectively, in the formula
LaCl3·7H2O and LaCl3·3H2O. As it is too complicated to ana-
lyze the dehydration process of a mixture containing different
number of crystallized water, the raw materials were dissolved
into distilled water, so that LaCl3·3H2O can be transferred into
LaCl3·7H2O (Fig. 1).
One hundred and twenty-eight milligrams of aqueous
LaCl3·7H2O powders were charged into an alumina crucible and
3.2. Oxidization of LaCl3 during the crystal growth
◦
heated in the atmosphere of nitrogen at the rate of 10 C/min.
The dehydrated LaCl3 processed in Section 3.1 and anhy-
drousCeCl3 fromSigma–Aldrichwereusedasstartingmaterials
for growing LaCl3:Ce (5 at.%) crystal. They were mixed uni-
formly and then charged into a platinum crucible for crystal
growth. Small amount of scavenger was put into the crucible
to exhaust the oxygen or oxide impurity in the raw materi-
als. Usually, the grown crystals are colorless and transparent,
but sometimes, the grown crystals will loss transparency and
become marble-like solid (Fig. 3a). This kind of solid was
◦
◦
◦
◦
Five endothermal peaks, 105 C, 165 C, 191 C, 222 C and
50.7 C, can be observed in the DTA curve. The endothermal
◦
8
◦
peak at 850.7 C is corresponding to the melting point of LaCl3
crystal, eventhoughitislowerthanthatreportedbyHigginsetal.
◦ ◦
[
4]. The other four endothermal peaks from 100 C to 300 C are
corresponding to the mass loss effects, 4.65%, 13.17%, 10.4%
and 5.8% in the TG curve. This means that the dehydration
process of aqueous LaCl3 is finished by several steps. Chen et
Table 1
Mass loss of LaCl3·7H2O at different temperatures
◦
◦
Temperature range ( C)
Temperature peak ( C)
Measured mass loss (%)
Dehydration
Calculated mass loss (%)
9
0–135
105
165
186
223
–
4.65
13.17
10.4
5.8
LaCl3·7H2O → LaCl3·6H2O
LaCl3·6H2O → LaCl3·3H2O
LaCl3·3H2O → LaCl3·H2O
LaCl3·H2O → LaCl3
Total
4.85
14.55
9.7
4.85
33.95
1
1
2
35–175
75–210
10–270
–
34.2

1
22
G. Ren et al. / Journal of Alloys and Compounds 467 (2009) 120–123
Fig. 5. X-ray excited luminescence spectra of the deposited substance.
growth. It is conjectured that the oxidization can be caused by
the remained moisture or crystallized water in the starting mate-
rials, e.g. LaCl3 + H2O → LaOCl + HCl. In addition, the escape
of scavenger from the crucible can also lead to the damage of
no-oxygen atmosphere and the oxidization of LaCl3.
The formation of LaOCl multiple crystalline not only causes
the transmission loss but also deteriorates the luminescence
properties of the crystals. Fig. 5 presents the luminescence
spectrum of LaOCl multiple crystalline powder excited under
X-ray. It shows that the spectrum can be fitted into three compo-
nents with peaking at 338 nm, 366 nm and 405 nm, respectively.
Compared with the emission spectrum of transparent LaCl3:Ce
crystals [7], both components at 338 nm and 366 nm in Fig. 5 are
Fig. 3. As-grown LaCl3 crystal (a) and the deposited substance (b) filtered from
its aqueous solution.
2
2
3+
attributed to the emission 5d → F and 5d → F7/2 of Ce
5/2
dissolved into the distilled water and then the water became
milk-like solution. After being filtered from the solution, some
white powder can be obtained (Fig. 3b). Its XRD pattern shows
that the deposited substance is composed of not only LaCl3, but
also LaOCl multiple crystalline (Fig. 4). This reveals that LaCl3
compounds were partly oxidized into LaOCl during the crystal
ions, and the 405 nm component to the self trapped exciton
2
(
STE). However, the emission wavelengths from 5d → F
5
/2
2 3+
and 5d → F7/2 of Ce ions were shifted 4 nm and 10 nm
toward red, respectively. Besides, the luminescence intensity
2
2
from 5d → F is much weak than 5d → F7/2 (Fig. 5), it means
5/2
2
that most of photons from the transition 5d → F were reab-
5/2
sorbed by the crystal itself. This absorption will result in the
2
decrease of the light output, because 5d → F emission is the
5/2
main contributor to the total light yield of LaCl3 crystals. The
variation of the spectra shape, relative luminescence intensity
and the wavelength result from the existence of LaOCl or partly
from substitution of Cl by oxygen in LaCl3 crystals.
3.3. Oxidation of LaCl3 during heating process
Since the existence of LaOCl oxide can eliminate the scintil-
lation efficiency of LaCl3 single crystal, it necessary to research
its forming condition and controlling procedure. Fig. 6 presents
the XRD pattern of LaCl3 measured at different temperatures.
The LaCl3 crystal powder was put in a small chamber and
◦
◦
◦
◦
heated in air to 293 C, 381 C, 476 C and 656 C and kept
at above temperature for 2 h, respectively. It was identified that
◦
LaOCl phase appears at temperature of 293 C and its content
will increase until temperature reaches 565 C where all LaCl3
◦
Fig. 4. XRD pattern of the deposited substance.

G. Ren et al. / Journal of Alloys and Compounds 467 (2009) 120–123
123
powder dried from LaCl3 aqueous solution at different tempera-
◦
ture and found that the phase at 300 C is simple LaCl3, and from
5
◦
◦
00 C to 800 C the progressive oxidation of LaCl3 to LaOCl
is going on [8]. Here the oxidation temperature of LaCl3 crystal
in air is much lower than that in H2O. This may be caused by
the more active chemical properties of oxygen than water.
In addition, based on the characteristic that LaOCl is not
dissolved in water, six parts of LaCl3 powder were heated to
◦
◦
◦
◦
◦
temperature of 90 C, 135 C, 165 C, 200 C and 230 C in
an atmosphere and then kept at the above temperature for 2 h.
The powders were then dissolved in water, some substance
will deposit from the solution. After being filtered, dried and
weighed, the deposit was proved to be LaOCl phase by XRD
(Fig. 7a) and its mass is proportional to the temperature (Fig. 7b).
It can be found that the lowest temperature at which LaOCl
appears is 135 C, much lower than that we got from Fig. 6.
Fig. 6. XRD patterns of LaCl3 crystals measured at different temperatures.
◦
This means that the oxidation of LaCl3 in air is really much
easier than that in moist atmosphere.
4. Conclusions
Lanthanum chloride is a kind of hygroscopic compound.
XRD pattern proves that there are three or seven crystallized
water molecules in every LaCl3 formula, and LaCl3·3H2O
can transform into LaCl3·7H2O in aqueous solution. With the
increase of temperature, the scheme of dehydration can be
described in four stages: 7 → 6 → 3 → 1 → 0. During crys-
tal growth, transmission loss phenomenon can appear in some
crystals, its cause is related to the oxidation of LaCl3 at high tem-
perature, and the temperature at which the oxidation takes place
in atmosphere is much lower than that at nitrogen atmosphere.
Therefore, the favorite condition for dehydration reaction should
be limited in an atmosphere without oxygen.
Acknowledgement
This work was supported by National Scientific Foundation
of China (NSFC) with number of 50672109.
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Fig. 7. XRD pattern of LaOCl deposit (a) and its dependence on the temperature
b).
(
[
[
[
[
transform into LaOCl. Chen et al. proposed that this reaction,
LaCl3 + H2O → LaOCl + HCl, will take place at temperature of
◦
3
65 C [5]. Marsal et al. once measured the XRD of the calcined