Research on phase transition and crystal growth of lead iodide

Article abstract of DOI:10.1016/j.jallcom.2010.05.106

Based on Pb-I phase diagram, phase transition of PbI₂ immiscible melt (L₂ + L₃) was analyzed. It indicates that PbI ₂ crystal growth is accompanied with precipitation of excessive Pb. Hereby, with an improved gr

Full text of DOI:10.1016/j.jallcom.2010.05.106

Journal of Alloys and Compounds 505 (2010) 34–36
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Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jallcom
Research on phase transition and crystal growth of lead iodide
∗
Xing-hua Zhu , Hai-bo Zhang, Ding-yu Yang, Zhao-rong Wei, Xiu-ying Gao, Jun Yang
College of Optics & Electronics, Chengdu University of Information Technology, Chengdu 610225, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Based on Pb-I phase diagram, phase transition of PbI2 immiscible melt (L + L ) was analyzed. It indicates
2
3
Received 8 February 2010
Received in revised form 16 May 2010
Accepted 22 May 2010
that PbI2 crystal growth is accompanied with precipitation of excessive Pb. Hereby, with an improved
growth ampoule, an intact and translucent PbI2 crystal was growth by vertical Bridgman technique. X-
ray diffraction (XRD) analysis shows that the structure of the grown crystal is 2H with hexagonal space
group (P3m1). Energy dispersive X-ray (EDX) microanalysis shows the precipitated material is 100% Pb.
The experimental results accord with the phase transition analysis and indicate a convenient way to grow
single crystals from immiscible melt.
Available online 4 June 2010
PACS:
8
8
7
2
1.10.−h
1.30.−t
4.62.Bf
9.40.−n
©
2010 Elsevier B.V. All rights reserved.
Keywords:
PbI2 crystal growth
Phase transition of L2 + L3
Precipitation of excessive Pb
1
. Introduction
the precipitated material were respectively investigated by XRD
and EDX.
PbI2 single crystal is one of the important room temperature
radiation detector materials [1]. The high atomic number (ZPb = 82,
ZI = 53) promises its ability to stop X-ray. The large band gap
(
2
. Phase transition
2.30 eV) and theoretic resistivity at room temperature (1012 ꢀ cm)
In Pb-I phase diagram (Fig. 1), one temperature falling path was
lead to higher energy resolution and detection efficiency [2–4].
chosen to be 0 → 1 → 2 → 3, which covers L + L . The four numbers
2
3
Nevertheless, PbI detector has not been applied widely in the fields
2
denote temperature of the melt at T , T , T , and T respectively.
0
1
2
3
of nuclear physics, radiology, non-destructive testing and so on,
which is resulted from the growth difficulty of this crystal. The
propertiesof crystalsgrown byphysical vapor depositiontechnique
or sol–gel method cannot meet for the fabrication of detectors [5,6].
Usually, PbI2 crystal can be grown from melt for it does not
undergo phase transition below its melting point (679 K). But, there
are still many unsolved problems in crystal growth by vertical
Bridgman technique and zone melting technique [7,8], which is
probably due to absence of investigation on the phase transition
of Pb-I system. In this paper, we expounded phase transition of
PbI2 immiscible melt (L + L ) during crystallization based on Pb-
The PbI2 immiscible melt rich in Pb crystallizes along with this
path.
As T > T , the melt does not show immiscible behavior. When
0
T = T , the melt presents to be immiscible, depicted as L → L + L .
0
2
3
From T to T , the composition of L and L change along with liquid
0
1
2
3
lines. L2 sinks under L3 because of L2 is rich lead due to a larger
density.
As T = T1 (melting point of PbI , 679 K), PbI2 crystal is separated
2
from L3 till exhaustion, denoted as L → L + PbI . From T to T2,
3
2
2
1
PbI2 crystal is separated from L2 melt, and the composition of L2
changes along with ‘eb’ line.
2
3
I phase diagram proposed previously by us [9]. And an improved
growth ampoule was employed to grown PbI2 single crystal using
vertical Bridgman technique. To verify the phase transition theo-
retic analysis results of Pb-I system, the grown crystal ingot and
As T = T2 (eutectic point of Pb-I system, 641 K), the composition
of L2 melt moves from point ‘b’ to ‘a’, with three phase coexistence
of PbI , L and L . It can be expressed as L → L + PbI . Iodine ingre-
2
2
1
2
1
2
dient reduces gradually in PbI grains and lead ingredient increases
2
in L . From T2 to T , PbI2 separation from L1 proceeds and the
2
3
composition of L1 goes along with ‘ad’ line.
∗ Corresponding author. Tel.: +86 2885966382; fax: +86 2885966382.
E-mail address: cuitzxh@163.com (X.-h. Zhu).
As T = T3 (melting point of Pb, 600 K), L1 is liquid lead namely Pb
(L). From T to room temperature, Pb (S) separates out in Pb (L) until
3
0
925-8388/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2010.05.106

X.-h. Zhu et al. / Journal of Alloys and Compounds 505 (2010) 34–36
35
Fig. 3. The XRD spectrum of the PbI2 crystal, ꢁ(Cu K␣) = 0.154178 nm.
3. Experiment and results
Fig. 1. The chosen path in Pb-I phase diagram.
3.1. Growth process
Starting material (99.999%) of lead and iodine was weighed with
excessive Pb, and then loaded into the improved quartz ampoule,
evacuated for 3 h and sealed under 1.5 × 10 Pa. After this, the
ampoule was put into a horizontal tubular furnace. PbI2 polycrys-
talline material was synthesized by vapor transporting method
all excessive Pb curdles and precipitates under PbI , this process is
2
−
3
expressed as Pb(L) → Pb(S).
So, when the immiscible PbI melt with excessive Pb is cooled to
2
room temperature, it can crystallize to be single-phase PbI , and the
2
(VTM) to avoid explosion of ampoule. Using the same quartz
excessive Pb can precipitate under the crystal. For crystal growth,
we can improve the seeding pocket of growth ampoule to ensure
crystal grains competition and growth. The traditional ampoule as
shown in Fig. 2(a) cannot guarantee development of single crystal.
The improved ampoule, as shown in Fig. 2(b), was fabricated to
attach a hollow bulb under the seeding pocket. The bulb’s size was
suitable for holding the precipitated excessive Pb. And the conical
part between the seeding pocket and the bulb can ensure crystal
grains compete and grow to be single crystal.
ampoule, PbI2 single crystal was grown by vertical Bridgman tech-
nique in a two zone tubular furnace. Temperatures of the upper
◦
◦
and the lower were controlled at 450 C and 200 C separately by
the FP93. Temperature gradient of the solid liquid interface is about
◦
2
5 C/cm. Descending velocity of the ampoule is 10 mm/day. After
growth, the ampoule was cooled to room temperature naturally.
The grown crystal ingot is nacarat, translucent and intact. Some of
crystal sample and the precipitated material in the bulb were taken
out for further investigation.
3.2. Characterization results
The grown crystal was cut along its natural cleavage face, which
is smooth and shining. XRD spectrum of the cleavage face was
obtained by DX-2000, as shown in Fig. 3. Compared with PDF cards,
four diffraction peaks of high intensity were founded to be {0 0 1}
faces of PbI . Furthermore, rocking curve of the (0 0 1) cleaving face
2
of the crystal was obtained also, as shown in Fig. 4. The rocking
◦
peak is situated at ꢂ = 6.231 , and its full width at half maximum
Fig. 4. The XRD rocking curve of (0 0 1) cleaving face of the PbI2 crystal.
Fig. 2. Sketch diagram of two different growth ampoule and crystallization progress.

3
6
X.-h. Zhu et al. / Journal of Alloys and Compounds 505 (2010) 34–36
Fig. 5. The SEM topography of the incision section plane of the deposit lead.
4. Conclusions
We analyzed phase transition of the L + L immiscible melt dur-
2
3
ing crystallization according to Pb-I phase diagram. It indicates PbI2
crystal growth is accompanied with precipitation of excessive Pb,
and the excessive Pb can precipitate under single-phase PbI . With
2
an improved growth ampoule, PbI2 single crystal was grown by
vertical Bridgman technique. The XRD results indicate the grown
crystal is 2H structure, and The EDX result indicates the precipitated
material in the bulb is lead. The growth experiment and the inves-
tigation on the samples are consistent with the phase transition
analysis, which indicate a convenient way to grow single crystals
from immiscible melt.
Fig. 6. The EDX spectrum of the deposited lead.
References
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(
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