SYNTHESIS AND CHARACTERISATION OF
PbI2 SINGLE CRYSTALS IN GEL
A Project Report submitted to Mahatma Gandhi University
Kottayam in partial fulfillment for the award of the Degree of
BACHELOR OF SCIENCE IN
PHYSICS
BY
TEENA STEPHAN
REG NO: SBAB10178838
DEPARTMENT OF PHYSICS
ST.ALBERT’S COLLEGE
ERNAKULAM
2012
1
ST.ALBERT’S COLLEGE
ERNAKULAM
(Affiliated to M.G.University, Kottayam)
B.Sc. Physics
Project Report
Name
: Teena Stephan
Reg.No
: SBAB10178838
Certify that this is a bonafide project work entitled “SYNTHESIS AND
CHARACTERISATION OF PbI2 SINGLE CRYSTALS IN GEL” carried
out by Ms Teena Stephan under the guidance of Sri. Justin Paiva,
Associate Professor, St. Albert’s College, Ernakulam during the year
2012 - 2013.
Lawrel Gregory
Head of the department
Ernakulam
2
SYNTHESIS AND CHARACTERISATION OF
PbI2 SINGLE CRYSTALS IN GEL
Certified that this is a bonafide project work carried out
by Ms. TEENA STEPHAN (Reg.no: SBAB10178838) in partial fulfillment
of the requirements for the award of degree of Bachelor of Science in
Physics of St. Albert’s College, Ernakulam,(Affiliated to M.G. University,
Kottayam) during the year 2012 - 2013
Examiners
1.
Date:
2.
3
CERTIFICATE
Certify that this is a bonafide project work entitled
“SYNTHESIS
AND
CHARACTERISATIONOF
PbI2
SINGLE
CRYSTALS IN GEL” carried out by Ms. Teena Stephan under my
guidance in the Department of Physics; St. Albert’s College and has not
been included in any other B.Sc. project report submitted previously for
the award of any degree.
Ernakulam
Sri. Justin Paiva
October 12, 2012
Associate Professor
Ernakulam
4
DECLARATION
I, Teena Stephan hereby declare that the work entitled
“SYNTHESIS
AND
CHARACTERISATION
OF
PbI2SINGLE
CRYSTALS IN GEL” submitted to Mahatma Gandhi University,
Kottayam, in partial fulfillment of the requirements for the award of
Bachelor of Science in Physics is a record of the original work done by
us under the supervision and guidance of Sri. Justin Paiva, project
guide, Associate Professor, St. Albert’s College, Ernakulam.
Place:
Teena Stephan
Date:
5
ACKNOWLEDGEMENT
First and foremost, I thank GOD, for the abundance blessing on me in
the materialization of this project work and I thank Prof. Harry Cleetus,
our principal for providing various facilities for the training.
I am sincere grateful to Sri. Lawrel Gregory, our head of the department
for his continued support extended to me throughout the course of
training.
I express my profound sense of gratitude to Sri. Justin Paiva, our
teacher in charge and my project guide for his valuable guidance,
innovative suggestions and encouragement throughout humble
endeavor. I also thank him for the keen interest he had shown in
training. I would like to remember here, with a sense of gratefulness, the
co-operation forwarded to me by all other respected teachers.
I wish to record my gratefulness to my parents and lab assistants for
their dedicating support in every time.
I also wish to acknowledge to the authors, whose works I have consulted
during the preparation of this project.
Finally, I express my sincere thanks to my dear friends for their support
and cheerful co-operation.
Teena Stephan
6
SYNTHESIS AND
CHARACTERISATION OF PbI2
SINGLE CRYSTALS IN GEL
7
Contents
CHAPTER 1 CRYSTAL GROWTH
9
1.1 Introduction
9
1.2 Methods of Crystal Growth
9
1.2.1 Growth from Solution
10
1.2.2 Growth from Melt
11
1.2.3 Growth from Vapour
12
1.2.4 Gel Growth
12
CHAPTER 2 GEL GROWTH
13
2.1 Introduction
13
2.2 What is Gel?
13
2.3 Preparation of Hydro Silica Gel
14
2.4 Structure of Hydro Silica Gels
15
2.5 Experimental Methods
16
2.6 Growth Mechanism in Gel
19
2.7 Nucleation
19
2.8 Control of Nucleation in Gel Growth
20
2.9 Advantages of Gel Growth
21
2.10 Limitations of Gel Growth
22
CHAPTER 3 LEAD IODIDE CRYSTALS
23
3.1 Introduction
23
3.2 Properties of Lead Iodide
23
3.3 Applications of Lead Iodide
25
CHAPTER 4 SYNTHESIS AND MORPHOLOGY OF PbI2 CRYSTALS IN GEL
26
4.1 Experimental Method
26
4.2 Etching
28
CHAPTER 5 CHARACTERISATION OF PbI2 CRYSTALS
30
5.1 Introduction
30
5.2 X–Ray Powder Diffraction Studies
30
5.3 FTIR
32
5.4 EDX
34
CONCLUSION
36
BIBLIOGRAPHY
37
8
CHAPTER 1
CRYSTAL GROWTH
1.1 Introduction
Crystal growth is a new industry but an old subject. Buckley (1951), Van
Hook (1961), Burke (1966) and Elwell and Scheel (1975) devote sections
to the history of the subject and quote references to the early work in the
field.
Advances in modern solid state technology depend on the availability of
good quality defect free crystalline materials. A good number of crystals
have been grown by different gel techniques. All the methods used to
grow the crystals have their own potentiality and constraints. In spite of
the technological advancement in condensed matter physics, crystal
growing is still an extremely difficult task requiring great expertise and
skill. In this context the gel method has emerged as a convenient growth
technique to grow several crystals having advanced technological
application in the fields of optics, acousto-optics, optoelectronics and
electronics. There are different methods to obtain pure crystals. They
are:
 Solution method
 Vapour growth method
 Melt growth
 Gel growth
All the techniques used for the growth of single crystals from melt;
vapour and solution, those require elevated temperatures have their own
inherent constraints. Defects and lattice strains are frequently
incorporated into the growing matrix. In this context, the gel technique is
found to be promising one, for getting good quality single crystals.
1.2 Methods of Crystal Growth
Growth of crystal ranges from a small inexpensive technique to a
complex sophisticated expensive process and crystallization time ranges
9
from minutes, hours, days and to months. Single crystals may be
produced by the transport of crystal constituents in the solid, liquid or
vapour phase. On the basis of this, crystal growth may be classified into
three categories as follows,
 Solid Growth - Solid-to-Solid phase transformation
 Liquid Growth - Liquid to Solid phase transformation
 Vapour Growth - Vapour to Solid phase transformation
1.2.1 Growth from Solution
Materials, which have high solubility and have variation in solubility with
temperature, can be grown easily by solution method. There are two
methods in solution growth depending on the solvents and the solubility
of the solute. They are
 High temperature solution growth
 Low temperature solution growth
i.
High Temperature Solution Growth
In high-temperature solutions, the constituents of the material to be
crystallized are dissolved in a suitable solvent and crystallization occurs
as the solution becomes critically supersaturated. The supersaturated
may be promoted by evaporation of the solvent, by cooling the solution
or by a transport process in which the solute is made to flow from a
hotter to a cooler region. The high temperature crystal growth can be
divided into two major categories:
 Growth from single component system.
 Growth from multi component system.
This method is widely used for the growth of oxide crystals. The
procedure is to heat the container having flux and the solute to a
temperature so that all the solute materials dissolve. This temperature is
maintained for a ‘soak’ period of several hours and then the temperature
is lowered very slowly.
10
ii.
Low Temperature Solution Growth
Growth of crystals from aqueous solution is one of the ancient methods
of crystal growth. The method of crystal growth from low temperature
aqueous solutions is extremely popular in the production of many
technologically important crystals. It is the most widely used method for
the growth of single crystals, when the starting materials are unstable at
high temperatures and also which undergo phase transformations below
melting point .The main disadvantages of the low temperature solution
growth are the slow growth rate in many cases and the ease of solvent
inclusion into the growing crystal. Low temperature solution growth is a
well-established technique due to its versatility and simplicity. It is
possible to grow large crystals of high perfections as the growth occurs
close to equilibrium conditions. It also permits the preparation of different
morphologies of the same materials by varying the growth conditions.
Even though the method has technical difficulty of requiring a
programmable temperature control, it is widely used with great success.
1.2.2 Growth from Melt
All materials can be grown in single crystal form from the melt provided
they melt congruently without decomposition at the melting point and do
not undergo any phase transformation between the melting point and
room temperature. Depending on the thermal characteristics, the
following techniques are employed.
 Bridgman technique
 Czochralski technique
 Kyropoulos technique
 Zone melting technique
 Verneuil technique
 Electrocrystallisation
11
1.2.3 Growth from Vapour
The growth of single crystal material from the vapour phase is probably
the most versatile of all crystal growth processes. Crystals of high purity
can be grown from vapour phase by sublimation, condensation and
sputtering of elemental materials. To obtain single crystals of high
melting point materials this method is used. Finding a suitable
transporting agent is a formidable, problem in this technique. It is rarely
possible to grow large crystals because of multi-nucleation. The
commercial importance of vapour growth is the production of thin layers
by chemical vapour deposition (CVD), where usually irreversible
reactions e.g. decomposition of silicon halides or of organic compounds
are used to deposit materials epitaxially on a substrate. Doping can be
achieved by introducing volatile compounds of dopant elements into the
reaction region.
1.2.4 Gel Growth
It is an alternative technique to solution growth with controlled diffusion
and the growth process is free from convection. Gel is a two-component
system of a semisolid rich in liquid and inert in nature. The material,
which decomposes before melting, can be grown in this medium by
counter diffusing two suitable reactants. Crystals with dimensions of
several mm can be grown in a period of 3 to 4 weeks. The crystals
grown by this technique have high degree of perfection and fewer
defects since the growth takes place at room temperature.
12
CHAPTER 2
GEL GROWTH
2.1 Introduction
The utilization of gel as a medium of crystal growth was put forward by
Fisher and Siemens in 1926. However it did not evoke much interest of
crystal growers and remained as an unused work till 1962.The fast
developments in the semiconducting materials during the second half of
this century prompted the search for new intelligent in materials. The
increased interest in crystal growth led scientists to turn to the less lot
iced gel technique who realized its capability and advantage in
generating perfect defect free crystals.
2.2 What is Gel?
A gel is a solid, jelly-like material that can have properties ranging
from soft and weak to hard and tough. It is a highly viscous two
component system of semi-solid nature, rich in liquid and having pores in
it. These fine pores may allow the free passage of electrolytes and
sustain nucleation. The gel medium works as a 'Smart' material i.e.,
sensitive to the minutest changes in the ambience.
Gels can also be defined as a substantially dilute cross-linked
system, which exhibits no flow when in the steady-state. The word ‘Gel’
is taken from the Latin ‘gelu’ means freezing, cold, ice or from ‘gelatus’
meaning frozen, immobile. By weight, gels are mostly liquid, yet they
behave like solids due to a three-dimensional cross-linked network within
the liquid. It is the crosslink within the fluid that gives a gel its structure
(hardness) and contributes to stickiness (tack). In this way gels are a
dispersion of molecules of a liquid within a solid in which the solid is the
continuous phase and the liquid is the discontinuous phase.
Gels are prepared from sodium silicate solution, agar, gelatin or
soft soaps. Usually sodium silicate solution is used as silica gel. Gels are
formed from suspensions or solutions by the establishment of a 3dimensional system of cross linkages between the molecules of one
13
component. The second component permeates this system as a
continuous phase. A gel can thus be regarded as a loosely interlinked
polymer.
2.3 Preparation of Hydro Silica Gel
The Sodium Meta Silicate (SMS) powder of AR grade is dissolved
in double distilled water and by changing the hydrogen ion concentration
(pH) of the solution, the desired gel of specific gravity 1.04 can be
prepared. The pH factor is the important parameter, which determines
the rate of polymerization and the speed of gel setting. For maintaining
the hydrogen ion concentration (4-7), SMS is then treated with a suitable
acid in requisite concentration. The pH value is noted using pH paper.
During gelation the pH of the mixture varies and the gelation period
varies from few minutes to hours or days. One can adjust only the initial
pH of the mixture; the subsequent changes are not easily monitored and
controlled. Ageing hardens the gel and decreases the transparency and
easiness of diffusion. The efficiency of the system mainly depends on
the physical quality of the medium. If small bubbles may crippled into the
medium during the gelation it will grow in size and become lenticular in
size. This will diminish the efficiency of the system; therefore great care
has to be taken to prevent the entry of the air bubbles.
One of the most important factors affecting the hardness of the gel
medium is the density of the sodium Meta silicate solution. It is observed
that the range of densities in between 1.03 to 1.06 g/dcc yields better
experimental results in many systems. The optimum density allows the
growth of reasonably bigger crystals by this technique.
14
2.4 Structure of Hydro Silica Gels
It is worth noting that the hydro silica gel is the polymerized form of silicic
acid. When Sodium Meta Silicate is dissolved in water, monosilicic acid
is produced due to the reaction:
Na2SiO3 + 3H2OH4
SiO4 + 2NaOH
This is a reversible process and the by-product, which is the strong alkali
NaOH, remains in the solution. This is the reason for the alkaline habit of
the solution. The monosilicic acid liberates the hydroxyl ions and
polymerizes as shown below:
OH
OH
Si
OH
OH + OH
OH
Si
OH
OH
OH
OH
Si
OH
O
OH
Si
OH+ H2O
OH
This process continues until the entire molecule becomes part of the
three dimensional network. The oxygen silicon linkage is extremely
strong and which is irreversible. A section of the cross-linked polymer is
shown below.
OH
OH
OH
OH
Si
O
Si
O
Si
O
-----
OH
Si
O
Si
O
Si
O
-----
OH
OH
OH
The by-product resulting from the reaction is water and it accumulates
on the top of the gel because it is lighter than the gel. This phenomenon
is called syneresis. In the above structure it can be observed that H3Si04
and H2Si04 are also formed during the process of gelation. The relative
abundance of these products depends on the pH value. When the pH is
high H2Si042ions are abundant and it is more active. The H3Si04 is
favoured by low pH and they are believed to be responsible for triggering
the polymerization. In due course, cross linkages are formed between
15
the chains; and these contribute to the sharp increase of viscosity that is
clearly visible in gelation.
The gel structures were shown up by ultra-microscopy. The estimated
yield effective pore diameter is of the order of 50-160Ao for silica gels.
The gel structure affects the crystal growth characteristics, including
growth rates and ultimate crystal size along with their variables.
2.5 Experimental Methods
The experimental technique to grow crystals by gel diffusion technique is
categorized according to the formation process of crystals,




i.
Growth by chemical reaction
By chemical reduction
Complex dilution method
Solubility reduction method
Chemical Reaction Method
This is one of the widely used methods to grow a large number of
crystals. The basis of the reaction method is the chemical reaction of the
components used for the growth purpose. It is especially suited for
growing crystals which are insoluble or partially soluble and those having
thermal instabilities. There are two types of growth which can take place
in the chemical reaction: one in which the growth takes place by the
reaction of one component with the other and in the other with the
reaction of one component impregnated in the gel medium. In this
method the crystals grow inside the gel. The process is a highly
controlled one because the reactants combine due to the diffusion of
ions through fine pores.
In this procedure, two soluble reactants are allowed to diffuse through
the gel where they react and form an insoluble or sparingly soluble
crystalline product. The basic requirements of this method are: (a) the
gel must remain stable in the presence of reacting solutions and (b) it
must not react with the solutions or the product crystal.
The chemical reaction-taking place can be represented as follows,
16
AX+BY
anions.
AY↓+BX, where A and B are cations, X and Yare
(a) Gel uniformly changed with AX
(b) Gel containing the salt in the solid form
(c) Neutral gel technique
ii.
Crystallisation by gel method employing ‘U’ tube
The Chemical Reduction Method
This is a very good technique exclusively suitable for growing metallic
crystals from gel media. Crystals of copper, nickel, lead selenium, etc.,
have been grown by this method. For growing the copper crystals, a
suitably titrated gel with CuS04impregnated in it is taken in a test-tube.
After the proper setting of the gel a reducing agent such as
hydroxylamine hydrochloride or hypo phosphoric acid is added from the
top as an outer reactant. The chemical reduction of the CuS04 gives the
desired copper crystal within the gel.
iii.
Complex Dilution Method
This method is suitable for a material whose solubility in the presence of
another soluble material increases in a nonlinear way with the
concentration of the soluble material. In this at first a chemical complex
of the material of the crystal is formed with an appropriate substance
(solution) and it is allowed to dissociate to form the required crystal.
Armington and O'Conners have pioneered in developing this technique
for growing cupric halide crystals. They utilized a dumb bell shaped
vessel for this purpose. This method has provided an impetus to grow
the important class of transition metal dichalcogenides by gel, because
17
these materials when crystallized by vapour transport (CVT) methods
show enormous stacking faults.
iv.
Solubility Reduction Method
This method is applicable to water-soluble materials. When the material
of the eventual crystal is dissolved in an acid and resulting solution
allowed to diffuse through gel medium of that pH at which the solubility is
less, then the substance should crystallize by increasing super
saturation. Glocker and Soest were the first to utilize this technique to
grow monobasic ammonium phosphate crystals. They diffused alcohol
into a gel containing the crystal salt solution. The alcohol reduced the
solubility of the compound and thereby created the nucleation leading to
the formation of the crystals. The ferromagnetic crystals such as KDP
and TGS are grown by this method. Single diffusion technique and
double diffusion technique are two basic growth procedures in gel
media.
i.
Single diffusion technique.
In single diffusion technique, one reagent is incorporated in gel mixture
and another is then diffused in gel; leading to high super saturation
nucleation and crystal growth
ii.
Double diffusion technique
In this method, gel is used to separate the solution containing the
reagents by pressing the gel in the bent portion of a U tube and the
reagent in its two arms.
18
Double Diffusion Method
Diffusion Method
2.6 Growth Mechanism in Gel
The gel growth mechanism depends on the environment temperature,
gel pH, gel density, gel aging, gel quality, nature and strength of the
acid, nature, purity and concentration of the reactants, types of
crystallization apparatus etc.
Gel is obviously not impermeable, but the fact that convection currents
are suppressed, above a certain magnitude at any rate, is decided by
Vand, Vedam and Stein with a laser ultra-microscope arrangement.
In the absence of convection, the only mechanism available for the
supply of solute to the growing crystal is diffusion, diffusion of dissolved
matter as a consequence of the casual character of the thermal motion
of molecules. Homogeneous nucleation is favoured by gel in which
super saturation near the growing face of the crystal in gel is usually
high enough for this. It is clear that in the medium the diffusion of the
discharged matter is on the sequence of the chaotic motion of the
molecules.
2.7 Nucleation
Nucleation is an important phenomenon in crystal growth and is the
precursor of the overall crystallization process. Nucleation is the process
of generating within a metastable mother phase, the initial fragments of
a new and more stable phase capable of developing spontaneously into
gross fragments of the stable phase. Nucleation is consequently a study
of the initial stages of the kinetics of such transformations.
19
Nucleation may occur spontaneously or it may be induced artificially.
There are cases are referred to as homogeneous and heterogeneous
nucleations respectively. Both these nucleations are called primary
nucleation and occur in systems that do not contain crystalline matter.
On the other hand, nuclei are often generated in the vicinity of crystals
present in the supersaturated system. This phenomenon is referred to as
secondary nucleation. There are three steps involved in the
crystallization process.
i) Achievement of super saturation or super cooling
ii) Formation of crystal nuclei
iii) Successive growth of crystals to get distinct faces
All the above steps may occur simultaneously at different regions of a
crystallization unit. However an ideal crystallization process consists of a
strictly controlled stepwise program.
2.8 Control of Nucleation in Gel Growth
The facility to control the nucleation is one of the most important features
of the gel growth. At the same time this is a sensitive and crucial aspect
of the gel technique. The diffusion rate can be controlled in this
technique to a great extent, but it is not enough to control the population
of nucleation centers in the gel. The lack of knowledge on actual
structure of the gel prevents one from taking any effective measures for
nucleation control. The commonly used methods to minimize the
spurious nucleation in gels are
1. Optimization of the. Gel density
2. Ageing of the gel
3. Neutral gel technique
4. Concentration of the nutrients
5. Stabilizing the thermal condition
6. Use of additives
7. Field utillsation
.
20
2.9 Advantages of Gel Growth
There are several well-known and well-established methods for crystal
growth, but of all techniques for crystallization at ambient condition, the
gel technique holds the greatest promise. This is due to several
advantageous characteristics of the technique:
 The crystals can be observed practically in all stages of growth
due to the action of gel as a transparent crucible.
 The gel medium prevents the convection currents and turbulence
considerably and thus the crystals formed are defect free or
perfect in nature.
 The gel medium remaining chemically inert and harmless, the gel
framework acts like a three dimensional crucible in which the
crystal nuclei are delicately held in the position of their formation
and growth, thereby preventing damage due to the impact with
either the bottom or the walls of the container.
 The gel being soft and porous yields mechanically to the growing
crystals.
 Since the gel reduces, in effect, the speed of chemical reagents,
crystals could be made to grow too much larger sizes than if, they
were formed by a similar reaction in water or in molten stage by
decomposition process.
 Concentration of the reactants can be easily varied.
 The nuclei are distributed individually in the medium and thereby
the effects of precipitate interaction are drastically diminished.
 The technique is highly economical when compared with other
methods. The grown crystals can be harvested easily without
damaging the crystal faces. It yields good quality crystals with less
expensive equipment.
21
 Crystals of different morphologies and size can be grown by
suitably adjusting growth parameters.
 Nucleation can be controlled.
 Crystal can be observed at all stages of growth.
 The method is very simple and involves low investment.
However the quality of the crystals grown in gel is good but the size is
invariably small compared to other methods
2.10 Limitations of Gel Growth
1. In some crystals the gel trapping during the growth occur, when a
silica gel is used.
2. Growth period is very long.
3. Crystal size is generally small.
22
CHAPTER 3
LEAD IODIDE CRYSTALS
3.1 Introduction
Lead (II) iodide (PbI2) is a bright yellow solid at room temperature that
reversibly becomes brick red by heating. In its crystalline form it is used
as a detector material for high energy photons including x-rays and
gamma rays. Lead iodide is toxic due to its lead content. In the
nineteenth century it was used as an artists' pigment under the name
Iodine Yellow, but it was too unstable to be useful.
3.2 Properties of Lead Iodide
Lead Iodide is a promising material, due to its applicability in various
fields Lead Iodide is a toxic, yellowish solid. It displays a range of colors
with varying temperature from bright yellow at room temperature to brick
red. On cooling, its color returns to yellow. Lead Iodide is direct band
gap layered semiconductor consisting of molecular sheets, each
consisting of a layered cation sandwiched between two layers of
hexagonal closed packed anion. The forces within a sandwiched are
purely ionic in nature, giving a strong binding between an anion and
cation layer, whereas the anion layer in adjacent sandwiches are held
together by weak Vander Waal’s forces of attraction. Lot of work has
been done on different properties of Lead Iodide by different researches.
Lead iodide (PbI2) belongs to one of promising materials for high
efficiency uncool solid state detectors (in the range of 1kev-1MeV)
operating at room temperatures. It can be applied over a wide
temperature range from 200°C up to 130°C in detectors in devices used
within and outside the laboratory, for example, for ecological
measurements (polluted waste water, sewage, etc.), and for improved
diagnostic methods in biology and medicine (radiography and
tomography). Lead iodide is often compared with mercuric iodide.
Especially two important physical properties make PbI2 a more
interesting material for detector applications than HgI2. These are its
lower vapour pressure, and its higher chemical stability. No degradation
was observed in the PbI2 detectors under laboratory ambient in 6 month.
23
The polytypism of PbI2 seems to be also one significant property of this
material. PbI2 has not a structure modification change.
Molecular formula
:
PbI2
Molar mass
:
461.01g/mol
Appearance
:
bright yellow powder
Odour
:
odourless
Density
:
6.16g/cm3
Melting point
:
402oC, 675 K, 756F
Boiling point
:
872oC, 1145K, 1602F
G of PbI2 starts to be negative at 500oC; the chemical reaction is
possible from this temperature
Solubility in water
:
0.044g/100mL
(0oC), 0.063g/100mL (20oC), 0.41g/100mL (100oC)
Solubility product
:
Solubility
:
8.49 x 10-9 Ksp
insoluble
in
ethanol,
cold HCl, insoluble in alkali, KI solution
Crystal structure
:
rhombohedral hP3
Crystallographic system :
Hexagonal type of CdI2
Space group
:
P3ml
Space parameters
:
a=0.4557nm
24
c=0.6979nm
Layered structure of PbI2
PbI 2 Crystal Structure
3.3 Applications of Lead Iodide
Lead Iodide has wide spread applications in the field of electronics
ranging from phosphors to photovoltaic cells due to its photo conducting
nature. Single crystals of photoconductors are preferable because of
relative case of defining the case of pertinent variables
Lead iodide (PbI2) is an excellent and interesting candidate for high
efficiency room temperature detectors working in the medium energy
range of 1 KeV–1 MeV. It is a wide-band- gap semiconductor (Eg> 2eV)
with high environmental stability efficiency It can be widely applied in
medicine, monitoring ecology, nondestructive defectoscopy and X-ray
and gamma spectroscopy. The peculiarities of this material are high
resistivity, ability to work in a wide range of temperatures and high
chemical stability.
25
CHAPTER 4
SYNTHESIS AND MORPHOLOGY OF PbI2
CRYSTALS IN GEL
4.1 Experimental Method
Lead Iodide is an important crystal for nuclear particle detection and is
also used as a photoconductor. The growth of these crystals near
ambient temperature would be advantageous from the view point of the
elimination of structural defects.
A great deal of the work has been done on gel grown PbI2
crystals, partly because of its importance and partly in order to add our
understanding of polytypism
Lead Iodide crystals are grown by the reaction method. The reactants
used are lead acetate and potassium iodide.
The chemical reaction involved in this is as follows.
2 KI + Pb (COOCH3)2
PbI2 + 2 K C2H3O2
For the present project PbI2 crystals are grown in test tubes. Modified
vessels can also be used to control the nucleation and to get larger
crystals.
Cleaning of the tubes are extreme important to control the nucleation. Is
cleaned using chromic acid and then ultrasonically cleaned. Dissolve
97.616gm of Sodium Meta Silicate in 200ml of double distilled water to
obtain the stock solution. 7.5ml of stock solution is diluted with equal
quantity of water. Then 15ml of 2M acetic acid and 6ml of 1M lead
acetate are combined, with continued agitation. The mixture is allowed
to set. It will take around 3 days for gelation depending upon the pH of
the solution. After setting of the gel, potassium iodide solution is allowed
to stand over the set gel. Potassium iodide solution is then diffuse into
the gel. Good hexagonal platelets of lead iodide grow within 3 days with
deep yellowish and luminescent in nature. The crystals will reach a
maximum size within about 6 weeks.
In the test tube growth, the lead acetate is already embedded in the gel
so that when the KI solution is introduced, the rate if reaction in the gel is
fast, resulting in the formation of polycrystalline lead iodide, an yellow
26
salt precipitation near the surface of the gel. As the solution advances
downwards in the test tube, the porosity of the gel decreases due to the
alogging of the pores with reaction products. Therefore the rate of
diffusion of the solution also decreases. The consequent reduced rate of
reaction then enables the PbI2 to crystallize in the gel. Hence in the test
tube growth the crystals are formed only towards the bottom of the tube.
In other test tubes also, the silica gel is prepared in the similar manner
as mentioned above and is allowed to set. After the gel is set, in one of
the test tube 7ml of potassium iodide and 3ml of 1M Zinc Sulphate is
allowed to stand over the set gel. In other test tube, 7ml of KI and 3ml of
0.1M zinc sulphate is allowed to stand over the set gel. Crystals are
formed in both the test tubes. The crystals are allowed to grow to
maximum size.
Golden yellow hexagonal platelet shape crystal and twin shaped crystals
are obtained in this case. Pure PbI2 crystals having width ranging from
1mm to 4mm, Zn doped PbI2 crystals having width ranging from 1mm to
3mm are obtained. Different crystals obtained and the instruments used
are shown below.
Hexagonal shaped crystal
Twin shaped crystal
Ultrasonic Cleaner
Crystals grown in test tubes
Distillation Unit
Magnetic Stirrer
PbI 2 crystals doped
with 1M Zn
Pure PbI2 crystals
PbI 2 crystals doped
with 1M Zn
PbI2 crystals doped
with 0.1M Zn
27
4.2 Etching
When a crystal is in contact with its unsaturated environment or some
corrosive medium it undergoes decrystallization (dissolution) occurs.
This process is called etching. The crystal may show etch figures
(depressions and elevations) and dissolution layers on its surfaces
without disturbing the macroscopic appearances, if the process is carried
out for short durations.
A variety of each figures are observed on etched surfaces of crystal, like
pits, depressions elevations, etch grooves etc. Etch pits are the most
frequently observed figures on etched crystal surfaces. They are formed
at the starting points of dislocations. The symmetry of a crystal face can
be known from the shape of etch pits, the study of the density and
distribution of structural defects like grain boundaries, slip lines,
dislocations, stacking faults etc.
When a crystal is chemically etched, the molecules of the reacting
species must first diffuse through the liquid to the crystal surface. Then
chemical reaction takes place and reaction products are formed. If the
product is not soluble in liquid, further reactions are needed before
soluble material is produced. The soluble products then diffuse away
from the surface.
The etch rate depends mainly on etching time, temperature, nature of
solution, intensity of illumination at the surface (if the etching mechanism
is governed by an oxidation process) etc.
A basic formula for etch rate Vs id described by the Arrhenius equation
Vs = V0e-Ea / KT
Where, Ea is the activation energy,
T – Temperature of the etchant,
K – Boltzmann constant
V0 is the pre-exponential factor including all other parameters affecting
the etch rate.
28
Density of etch pits varies from sample to sample and even from region
to region of the same specimen. This may be due to the fluctuations in
the growth temperatures. The low dislocation density reveals crystalline
perfection and hence the adequacy of the growth technique. Etching can
also be utilized to delinate the nature of the growth.
Result
Etching can also be utilized to delinate the nature of the growth. Etching
studies are carried out on pure PbI2 crystals having width 1190µm and
598.6µm, PbI2 crystals doped with 1M Zn having width 677.77µm and
395.5µm and PbI2 crystals doped with 0.1M Zn having width 839.1µm
and 384.9µm using Conc. HCl and ethyl alcohol. Triangular etch pits
have been observed. From this it is established that the decay of crystals
have been taken place. This may be due to keeping the crystals in the
etching solutions for the long period. The low dislocation density reveals
crystalline perfection and hence the adequacy of the growth technique.
29
CHAPTER 5
CHARACTERISATION OF PbI2 CRYSTALS
5.1 Introduction
In order to confirm and to identify the grown crystal, the characterization
of the crystal is essential. X–ray diffraction is used to characterize the
crystallographic parameters of the crystal.
5.2 X–Ray Powder Diffraction Studies
In powder method, the crystal to be examined is finely powered and
placed in a beam of monochromatic X–rays. The sample consists of
enormous number of tiny crystals, randomnity distributed. Sometimes by
chance, some of the crystals will be aligned in such a way that their
planes fulfill Bragg’s condition for reflection. By using x–rays of known
wavelength λ and measuring, we can determine the spacing ‘d’ of the
various planes in a crystal by using the Bragg equation
nλ= 2d Sinθ
X–ray powder diffraction pattern of PbI2 crystals are recorded with
SHIMADZU XD 610 X–ray diffractometer using CuKα radiation of
wavelength 1.514Ao. The powered samples are scanned over a 2θ
range of 20o to 60o at the rate of 4o per minute.
The d value are calculated using the Bragg equation,
2dSinθ =nλ
X-ray diffractometry is useful in analyzing crystal structure, evaluation of
‘d’ values, cell parameters, system to which the sample under study
belongs, grain size, micro strain, reflecting planes etc. Records of X-ray
powder diffraction patterns of these samples, doped and undoped Lead
Iodide crystals, under identical conditions signifies that the samples
belongs to hexagonal system and are crystalline in nature. Fig. 1, 2, 3,
and 4 represents an X-ray diffractogram of undoped and Zn-doped
Similar X-ray diffractogram were obtained for undoped Lead Iodide
crystals.
30
X-ray diffractogram of undoped PbI 2
X-ray diffractogram of Zn-doped PbI 2 (from bottom 0.1Mand 1M respectively)
Compound
Lattice Parameters
a (A)
c (A)
c/a
V (A)³
Reported
4.575
6.989
1.5337
125.69
Pure PbI2
Crystal
4.575
7.0357
1.5479
128.85
X–ray diffraction provides a convenient and practical means for the
qualitative identification of materials. It finds wide ranging applications in
crystallization phase analysis and in determining unit cell parameters
and space groups.
31
Result
XRDs of Lead Iodide crystals are shown above. The lattice parameters
‘a’ and ‘c’ for Lead Iodide crystals have been computed from the
observed ‘d’ values by successive refinement. Mean values of lattice
parameters are given in the table. XRD confirms the crystallinity and
hexagonal structure of the grown Lead Iodide crystals. Similar results
were obtained for gel grown Zn-doped Lead Iodide crystals.
5.3 FTIR
Fourier Transform Infrared Spectroscopy (FTIR) is a powerful tool for
identifying types of chemical bonds in a molecule by producing an
infrared absorption spectrum. FTIR is most useful for identifying
chemicals that are either organic or inorganic. It can be utilized to
quantitative some components of an unknown mixture. It can be applied
to the analysis of solids, liquids, and gasses.
By interpreting the infrared absorption spectrum, the chemical bonds in a
molecule can be determined. FTIR spectra of pure compounds are
generally so unique that they are like a molecular "fingerprint". Unlike
inorganic compounds, organic compounds have very rich, detailed
spectra. Molecular bonds vibrate at various frequencies depending on
the elements and the type of bonds. Any given bond, vibrate at several
specific frequencies. According to quantum mechanics, these
frequencies correspond to the ground state and several excited states.
One way to cause the frequency of a molecular vibration to increase is to
excite the bond by having it absorb light energy. For any given transition
between two states the light energy must exactly equal the difference in
the energy between the two states [usually ground state (E0) and the first
excited state (E1)].
The energy corresponding to these transitions between molecular
vibration states is generally 1-10 kilocalories/mole which corresponds to
the infrared portion of the electromagnetic spectrum.
Difference in Energy States =
E1 - E0
Energy of Light Absorbed
=
h=Planks constant
32
hc/λ
c=speed of light, and
λ=the wavelength of light.
Fourier Transform Spectrum of Pure PbI 2
Result
In lead Iodide dehydrate sample, the analysis shows that the limited
number of vibrations peaks indicates the absence of second order
harmonic generations while the hydroxyl stretching and bending bands
can be identified by their broadness and strength of the band which
depends on the extended of hydrogen bond. Hydroxyl stretching
vibrations are generally observed in the around 3500 cm-1. Multiple
bands present in the stretching region of water in the spectrum with
weak intensity in the region 2515-2003 cm-1indicates the presence of
hydrogen bonds of various strengths. On deuteration the stretching
bonds of water molecules are shifted towards the low wave number
region.
33
5.4 EDX
Energy-dispersive X-ray spectroscopy (EDS or EDX) is an analytical
technique used for the elemental analysis or chemical characterization
of a sample. It relies on the investigation of an interaction of some
source of X-ray excitation and a sample. Its characterization capabilities
are due in large part to the fundamental principle that each element has
a unique atomic structure allowing unique set of peaks on its X-ray
spectrum. To stimulate the emission of characteristic X-rays from a
specimen, a high-energy beam of charged particles such as electrons or
protons or a beam of X-rays, is focused into the sample being studied.
At rest, an atom within the sample contains ground state electrons in
discrete energy levels or electron shells bound to the nucleus. The
incident beam may excite an electron in an inner shell, ejecting it from
the shell while creating an electron hole where the electron was. An
electron from an outer, higher-energy shell then fills the hole, and the
difference in energy between the higher-energy shell and the lower
energy shell may be released in the form of an X-ray. The number and
energy of the X-rays emitted from a specimen can be measured by an
energy-dispersive spectrometer. As the energy of the X-rays is
characteristic of the difference in energy between the two shells, and of
the atomic structure of the element from which they were emitted, this
allows the elemental composition of the specimen to be measured
EDS makes use of the X-ray spectrum emitted by a solid sample
bombarded with a focused beam of electrons to obtain a localized
chemical analysis. All elements from atomic number 4 (Be) to 92 (U) can
be detected in principle, though not all instruments are equipped for
'light' elements (Z < 10). Qualitative analysis involves the identification of
the lines in the spectrum and is fairly straightforward owing to the
simplicity of X-ray spectra. Quantitative analysis entails measuring line
intensities for each element in the sample and for the same elements in
calibration Standards of known composition.
By scanning the beam in a television-like raster and displaying the
intensity of a selected X-ray line, element distribution images or 'maps'
can be produced. Also, images produced by electrons collected from the
34
sample reveal surface topography or mean atomic number differences
according to the mode selected.
Result
The percentage composition of Lead and Iodine is confirmed by energy
dispersive power by X-rays. The above figure represents the elemental
analysis of Lead Iodide performed by EDX, indicating the proper
proportion of Lead and Iodine.
35
CONCLUSION
Single crystals of pure and Zinc Sulphate doped Lead Iodide have been
grown by gel method. There are hexagonal platelet crystals and twin
shaped crystals. Pure PbI2 crystals having width ranging from 1mm to
4mm, Zn doped PbI2 crystals having width ranging from 1mm to 3mm
are obtained. The size of the crystal obtained is considerably reduced
when it is doped with Zn.
Etching studies are carried out using Conc. HCl and ethyl alcohol.
Triangular etch pits have been observed. From this it is established that
the decay of crystals have been taken place. This may be due to
keeping the crystals in the etching solutions for the long period. No etch
pit rows resembling tilt or twist boundaries were observed. These
observations indicate that the crystals are of high perfection.
Diffractograms of pure and doped PbI2 crystals are taken and it is in well
agreement with the spectrums observed in other studies. The lattice
parameters ‘a’ and ‘c’ for Lead Iodide crystals have been computed from
the observed ‘d’ values by successive refinement. XRD confirms the
crystallinity and hexagonal structure of the grown Lead Iodide crystals.
Similar results were obtained for gel grown Zn-doped Lead Iodide
crystals.
Fourier Transform Infra-Red spectrum of pure and doped PbI2 crystals is
taken and it is in well agreement with the spectrums observed in other
studies.
The percentage composition of Lead and Iodine is confirmed by energy
dispersive power by X-rays. The elemental analysis of Lead Iodide
performed by EDX indicates that there is correct proportion of Lead and
Iodine in the crystals obtained.
36
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