class="t m0 x15 he y7e ff3 fs8 fc0 sc0 ls4 ws25">023764 0.323 - 10 20H 0.4557 0.6979
042510 0.316 2.40 12 24R 0.4557 0.6979
060328 0.316 2.40 15 30H 0.4557 0.6979
042511 0.316 2.50 18 36R 0.4557 0.6979
90˚θ
90˚θ
90˚+θ90˚+θ
Figure 1. The near octahedral [PbI6]4. It has six equal Pb, I
distances, six I-Pb-I angles that are smaller than 90˚ by
,
and six 1-Pb-1 angles that are larger than 90˚ by the same
deviation. The angle
is a measure of deviation from octa-
hedral symmetry.
The actual mechanism of polytype formation in PbI2 is
complex. There is still a great amount of dispute about
the phase transformation even amongst the simple poly-
types 2H, 12R and 4H which can co-exist in the same
crystal in different proportions. Even higher order PbI2
polytypes could also exist in hexagonal or rhombo-
hedral structure (Table 1). According to existing litera-
ture the structural defects, imperfections native/extrinsic
impurities in PbI2 may provoke the co-existence of va-
rious polytypes. It is very difficult to detect any differ-
rence between various polytypes by measuring physical
properties which always reflects the average property of
the material.
Earlier Chaudhary & Kaur [9] have given a mecha-
nism of phase transition between polytypes due to impu-
rities. It is to be noted that the mechanism is only valid at
a local scale in the vicinity of the impurity and the atoms
rearrange within single layers. With this, it is difficult to
explain the transformation of the whole crystal from one
form to the other. The possible mechanism could be that
transformation may spread over the whole crystal from a
single nucleation centre or the whole structure may be-
come unstable and atoms may become mobile enough to
create a new stacking which is the most stable one.
3. Studies on Storage of Lead Iodide Crystals
PbI2 structure consists of various stackings of PbI2 sheets
in each of which a layer of Pb ions is sandwiched be-
tween two close packed layers of iodine ions and I-Pb-I
sandwich being the repeat unit. The binding within a
sandwich believed to be largely ionic, is quite strong but
two adjacent sandwiches are bound together with weak
van der Walls forces.
As our interest goes for the device fabrication that may
give reliable results for a long time, it becomes necessary
to examine carefully the results of storage of PbI2 crys-
tals (Table 2).
Out of seven categories there are four categories where
the crystals do not transform to another structure after
storage.
Copyright © 2012 SciRes. CSTA
S. K. CHAUDHARY 23
Table 2. Results of X-ray characterization of PbI2 crystals
and storage.
No. of crystals Structure of as grown
crystals Storage*
1) Crystals grown from gel (Soudmand & Trigunayat (1989) [14])
Undoped crystals (20)
40 X-ray photographs All 2H (40)
After heating converted
to 12R but restored to
2H after storage
AgI doped crystals (15)
30 X-ray photographs
2H (21), 3H (3), 12H
(2), 16H (1), 12R (3)
Do not change to 2H
after heating and storing
at room temperature
2) Crystals grown from vapour (Jain & Trigunayat (1996) [15])
Undoped crystals (20)
40 X-ray photographs All 12R (40) No change after storage
AgI doped crystals (20)
40 X-ray photographs
12R (13)
(4H + 12R) (16)
(2H + 12R) (11)
All crystals transform
to 2H
3) Crystals grown from melt (Chaudhary & Trigunayat (1987) [16])
Very pure crystals
39 X-ray photographs
(20 zone passes)
All 12R (39) No change after storage
More pure crystals 46
X-ray photographs
(12-14 zone passes)
12R (36)
4H + 12R (8)
2H + 12R (2)
All crystals transform
to 2H
Less pure crystals
75 X-ray photographs
(6 - 8 zone passes)
12R (51)
12R + 4H (24) No change after storage
*Storage time in all the cases is few months.
1) Undoped crystals grown from gel (all 2H)
2) Undoped crystals grown from vapour (all 12R)
3) Very pure crystals grown from melt (all 12R)
4) Less pure crystals grown from melt (12R), (12R +
4H)
These observations clearly establish a link between
purity of material and phase transitions between poly-
types. The explanation for the choice of best crystals for
detector is as follows.
In gel growth, the impurities can enter the crystals
from both reactants and the gel medium whereas in va-
pour growth, comparatively pure crystals can be grown
as only volatile impurities can enter into the crystal. Fur-
ther streaking and arcing has frequently been reported in
the gel grown crystals [7]. However in both the cases,
size of the grown crystal is small. However, vapour
grown crystals could be a better option for the devices.
When we examine crystals grown from melt, it is ob-
served that structure of very pure and less pure crystals
do not change after storage. If the degree of impurities is
high, (less pure crystals) that may cause excessive distor-
tion of octahedron, that makes the iodine planes uneven
which leads to interlocking and blockage of iodine planes
which in turn do not allow the various structures to
transform to 2H. In highly pure crystals that are deprived
of impurities, there is no distortion of octahedron, thus no
room temperature transformation may be expected. For
the crystals containing intermediate amount of impurities
(“more pure crystals” in Table 2) the iodine layers are
not as close packed as should have been in absence of
impurities. These layers are prone to translation/rotation
due to weak binding and are responsible for the forma-
tion of polytypes. Wahab and Trigunayat have reported
that such transformations can take place by layer dis-
placements during crystal growth and high temperature
treatment of polytypes [10,11]. The above observation
indicates that phase transformation can also take place by
suitable impurity and/or time effects. Though “less pure”
crystals have not transformed after storage but it is ex-
pected that they may get transformed after a long period
of storage. Thus less pure crystals should also be ignored
for device fabrication.
Now we are left with two options, very pure crystals
grown from melt and the crystals grown from vapour.
Out of the two choices, it is very clear that material of
melt grown crystals has been purified by zone refining
technique and impurities present in the starting material
like Ag and Mn have been reduced to <<1 ppm and
crystal has grown as a perfect one. An X-ray diffraction
showing the reflections of photographs 12R structure is
reproduced in Figure 2. The photograph is free of
streaking and arcing (that measure the degree of disorder
and tilt boundaries in the crystal). Further in melt growth
(using zone refining system) one can better manipulate
the impurities if required, and have a control on the size
of the crystal. Thus it is suggested that pure melt grown
crystals are the best choice for the fabrication of the de-
tectors as there is no change in structure during long
storage and of course vapour grown crystals can be the
next choice.
Further, the crystal growth improvements are required
in order to reduce the structural defects and to increase
electron/hole mobilities needed to improve the perfor-
mance of lead iodide radiation detectors.
4. Future Studies
In most of the earlier studies made on PbI2 crystals, the
role played by impurities and the vacancies was being
ignored. The degree and the distribution of vacancies are
also important parameters to be looked for the fabrication
of devices from PbI2 crystals. It is expected that atoms
Figure 2. An a-axis 15˚-oscillation photograph of PbI2 crys-
tal showing the reflections of polytype 12R. CuKα radia-
tions; 3 cm camera.
Copyright © 2012 SciRes. CSTA
S. K. CHAUDHARY
Copyright © 2012 SciRes. CSTA
24
surrounded by a large number of vacancies should be
considered to be more mobile than others along the lay-
ers. Further, the density of vacancies and their distribu-
tion play an important role in semi conducting properties
of the materials. In a recent theoretical investigation Ito
et al. [12] have calculated that for SiC polytype with Si
vacancy, 6H structure is formed and 4H structure is fa-
vored in C vacancy condition. Thus showing that vacan-
cies in SiC play an important role in stabilizing a par-
ticular structure. Similar calculations have been made on
ZnS polytypic crystals [13].
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