1

CHAPTER - I

INTRODUCTION

1.1. Crystalline Materials

Matter exist in three different states normally solids, liquids and gases. The

fourth state of matter is plasma is not included here. Matter in the solid state can be

classified into three types such as crystalline, amorphous and quasi crystalline.

The scientific meaning of the word ‘Crystal‘ is limited to the description of

any solid with an ordered atomic arrangement with a particular structure .Thus

crystals are solids which have a regular periodic arrangement of their constituent

atoms extending over a large volume of them. Real crystals often exhibit a variety of

imperfections in the regularity arrays like dislocations, twinning and other kind of

defects. If a crystal has inner boundaries along with external boundary it is called

polycrystalline irregular packing of atoms.

Amorphous crystals does not have crystalline structure in the condensed state

i.e. the atoms of the material are not arranged in a regular periodic pattern. Under

definite conditions amorphous substances change the state. Ordinary glass, sulphur,

selenium and most of the high polymers can exist in the amorphous state.

Quasi crystals are solids which contain atoms in order of arrays but the

patterns they assume are stable and do not occur at regular intervals.

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1.1.1. Importance of Crystalline Materials

The beauty of the crystalline materials are symmetry, the smoothness of

surface, the colour and the brightness of naturally occurring crystals found all over the

earth’s crust have been appreciated by man from time immemorial. It is admired

because of their ornamental value. From the Blue John Mine in Derbyshire, fluorite

crystals were valued by Romans as decorative objects. From early time’s diamonds,

emeralds, rubies and sapphire have been very precious. The size of the crystals that

Berly discovered in America were 1.2 meter long and 0.6 meters thick and in many

parts of the world diamond weighing about 5 tonnes to almost tiny microscopic one

were found. Now-a-days artificial crystals are being made to replace natural one,

since natural materials are sometimes scarce and highly valuable.

Man made crystals are not less perfect than natural ones. Quartz crystals were

synthesized by man more than hundred years ago. Crystalline quartz insoluble in

water, but it is slightly soluble in hot water when grind finely. When a seed crystal is

introduced in this solution, growth of a perfect quartz crystal is promoted. These can

be used as time keepers.

Crystals find an important place in the present technology. In the field of

electro-optics, the crystals work as frequency controlled oscillators, transistors,

transducers, polarizer, radiation detectors, masers and lasers. They are used in

computers, watches and many other machines. New materials like titanium doped

sapphire which guarantee longer life and give more stable output are used as tuneable

lasers.

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In the field of electronics, silicon continues to dominate. The phenomenon of

photo-refractive effect is being exploited for recording hologram and development of

phase conjugated optics. The increasing demand for storage and processing of data

has encouraged interest with crystals such as bismuth silicate and barium titrate, it

produces a small change in the refractive index is being exploited for recording

holograms and the development of phase-conjugate optics.

The most exciting contribution to molecular biology made by crystallographer

(by growing single crystal of biological molecules and analyzing their structure by

X-ray diffraction methods) has been the direct visualization at atomic resolution of

nucleic acid and variety of proteins with which it interacts has been made possible by

crystallographers. This helps the understanding of molecular biology and various

chemical terms of the processes by which genetic events are initiated, mediated and

regulated. Also cellular processing can be interpreted in a structurally consistent and

rational manner.

Now-a-days industries, science and technology handy make any progress

without the development of new materials of enhanced performance. Artificial

crystals are grown every day. Semiconductors and dielectrics are manufactured today

with the help of versatile methods of chemical synthesis, various treatments such as

artificial growth of monocrystals, formation of their films on electromagnetic field

and ionizing radiations, etc.,

We are now concerned in the discovery of materials for the purpose of

industrial and academic uses.

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1.2. Alkali halide mixed crystals

Alkali halide compounds are formed by the combination of alkali atoms Li,

Na, K, Rb and Cs of the first group (IA) and halogens (F, Cl, Br and I) of the seventh

(VII A) group. The alkali halides crystallize either in the NaCl (Sodium Chloride) or

in the CsCl (Caesium Chloride) structure. CsBr and CsI crystallize in the CsCl

structure and the other crystallizes in NaCl structure.

The simple structures and knowledge of the chemical bonding in the alkali

halide have made them the favourite system for verifying theories. Thus the alkali

halides were the first system (or one of the first system) on which the Debye theory of

specific heat, Gruneisen’s theory of thermal expansion, Born’s theory of cohesion,

Kellermann’s lattice dynamics and Cowdin’s very first application of quantum

mechanics in crystal elasticity were tested. The general properties of NaCl, NaBr

[1-3] are listed in Table .1.

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Table.1. General properties of Alkali Halides

Property /Parameters Alkali Halides

NaCl NaBr

Molecular Weight (M) 58.45 102.91

Colour Colourless Colourless

Density (d) (g/cc) 2.1617 3.1997

Refractive index(n measured at λ =5893Å) 1.5443 1.6412

Melting point (T) (°C) 801 747

Boiling point (T) (°C) 1465 1447

Crystal System Cubic Cubic

Lattice System f.c.c f.c.c

Space group Fm3m Fm3m

Point group m3m m3m

Coordination numbers 6 6

Number of molecules per unit 4 4

Lattice Constant (a) (Å) 5.6402 5.9772

Structure Type NaCl NaBr

Interionic distance (r) (Å) 2.82 2.9865

Molar Volume (Vm) (cm3) 27.012 32.083

Molecular Volume (Vm) (Å3) 44.854 53.274

Compressibility ( ) (10-12 cm2/dyne) 4.17 5.02

Mean Debye Waller Factor (B) (Å2) 1.53(2) 1.25(5)

Debye Temperature θD ( °K)

From X-ray/Neutron Diffraction 278(2) 202(6)

From Elastic Constant 322 224

From Compressibility 292 241

From Micro Hardness 0.216 0.129

Static dielectric Constant (ε0) for 1 KHz at 300° K

5.8949 6.3957

Electronic dielectric constant (εα) for 1 KHz at 290 °K

2.33 2.6

Activation energy of Ionic Conduction Eav (eV) 0.83 0.8

Solubility in Water (S) (g per 100gm solvent)

At 30 °C 36.1

98.4

At 40 °C 36.4

107

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Seitz made a statement a statement in 1946 on alkali halides which remain

relevant even now “In the field of solids, the properties of alkali halides have an

enduring interest, since these crystals have continuously yielded to persistent

investigation and have gradually provide with a better and better understanding of

the most interesting properties of solids”.

In the last few decades the alkali halides emerged as crystals with useful

application ranging from X-ray monochoromotors to tuneable lasers. Several alkali

halides either pure or doped are employed in energy detection in the X-ray, γ-ray and

Cerenkov region. Alkali halides like LiF and NaCl have been employed as

monochoromotors for X-rays, KCl-KBr mixed crystals as neutron monochoromotors.

Harmonic generation and super conductivity have also observed in the alkali halides.

Several reports are also available on the simple and doped alkali halide

crystals [4-70]. Recently Boudino and his co-workers [71-73] have found that the

alkali halide crystals can act as a good medium for the preparation of II-VI compound

nano composites.

All the alkali halides (except LiF and NaF) are soluble in water and can (in

principle) be crystallized in from aqueous solution. All the alkali halides have

congruent melting point and therefore their crystal can be formed from their melts.

Growth of single crystals has been reported by using a variety of melt growth

technique. But only few reports are available for the growth of alkali halides from

aqueous solution.

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1.2.1. Doped and Mixed crystals

The physical property that limits the utility of alkali halides as device

materials is their low hardness. Armington et al (1973) [74] discussed two methods of

improving the hardness of alkali halides

(i) Solid solution hardening

(ii) Impurity hardening.

When two substances A and B have closely similar structures (isomorphic

structures) with not much different dimensions, it is found that the atoms of one can

replace those of the other discriminately in the lattice resulting in a mixed crystal AB

or solid solution [75].

There are several ways in which solid solution can form. Accordingly they are

classified in to the following three categories [76].

i) Substitutional solid solutions in which replacement of one atom for

another takes place.

Azaroff [77] observed that substitution solid solution can occur only if the

radius of the larger atom does not excess that of the smaller atom by more than 15

percent. Here, some of the normal lattice sites in the solvent crystal are occupied by

solute atoms and structure of the solvent remains unchanged. KCl and KBr give solid

solutions of any compositions between the two extremes.

ii) Interstitial solution in which limited number of solute atoms occupy

interstitial position in the solvent crystal. Solid solutions CxF2-YF3

provide examples of crystals containing interstitial ions.

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iii) Defect solid solutions to which some sites in the lattice of one of the

components remain vacant. Defect solid solution are formed typically

in chemical compounds of transition elements as well as sulphides,

selenides and some oxides

The above 3 kinds of mixed crystals (solid solution) are described

schematically in Fig.1.

Fig.1.1 Types of solid solutions

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Two compounds or elements are said to form a continuous solid solution if a

single lattice parameter as measured by X-ray powdered photographs can be assigned

to the solid solution at all compositions.

The conditions for the formation of mixed crystals,

i) The structure of the two crystals should be similar type.

ii) The bonds in the two crystals should be similar type.

iii) The radii of the substituent atoms should not differ by more than 15%

from that of the smaller one and the difference between their lattice

parameter should be less than 6%.

The formation of mixed crystals, obviously not limited to a pair of substances

that belongs to a chemically coherent group. Whatever the case may be similarity in

size and shape is prerequisite [78].

A mixed crystal has physical properties analogous to those of the pure crystal.

The composition dependence varies from system to system and property to property.

In many cases, the property changed monotonically with composition in a linear or

nearly linear manner. Once the trend in composition dependence established, we have

a means to have tailor-made crystals with a desired value for a physical property. In a

few properties, the composition dependent is highly non-linear and in some cases, the

magnitude of the physical property for the mixed crystal even exceeds the values of

the end members. In such a case, it is as if we have new crystal in the family. Such

behaviour is shown for instances in the micro hardness of alkali halide mixed crystal.

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In some instances mixed crystals show exciting behaviour. One such example

is the appearance of first order Raman spectrum in mixed crystals of alkali halides

which is absent in the pure crystals [79].

1.2.2. Recent interest in alkali halides

Alkali halides are widely used as laser window materials, neutron

monochoromotors, infra-red prisms, infra-red transmitters etc. But the uses are limited

by their mechanical properties and hence there exist the need to strengthen them. The

mixed and impurity added (doped) crystals of alkali halides are found to be harder

than the end members and so they are more useful in their applications. In addition

mixed alkali halides find their application in optical, opto-electronic devices. In view

of this it becomes necessary and useful to prepare binary and ternary mixed crystals

regardless of miscibility problem and characterize them by measuring their physical

properties.

Several reports are available on binary mixed crystals but only a few reports

are available for ternary and for quaternary mixed crystals (including multiphase

ones) of alkali halides.

1.2.2.1. Growth of Alkali halide Crystals

Sodium and Potassium are soluble in water. It is possible to grow, in certain

cases of mixed crystals by evaporation of aqueous solution. However, the melt

technique is commonly employed and single crystals with linear dimension of several

centimetres have been obtained.

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Veeresham et al [80] have grown mixed crystals of KCl-KBr, KCl-KI, KBr-KI

and KCl-NaCl and found that the dislocation density increases with the degree of

mixing and is maximum at equimolar composition. Freund et al [81] grew KCl-KBr

single crystals with a continuous various composition from one end to the other.

Padiyan and Mohanlal [82] have grown quaternary mixed crystals of

K0.5Rb0.5Cl0.5Br0.5 from alkali halides. Tobolsky [83] showed that for ionic crystals

like alkali halides, complete miscibility is possible only above a particular

temperature T is given by T=4.5δ2, where δ being the percentage deviation in the

lattice parameter. As per this, alkali halide solutions have got only limited miscibility

at room temperature.

Mahadevan and his co-workers [84] obtained larger and more stable crystals

from NaClxKCl0.9xKBr0.1 solutions than from NaxK1-xCl. They grew the crystals from

aqueous solutions only. Though the miscibility problem was there, their studies have

made one to understand that a KBr addition to NaCl-KCl system may yield a new

class of stable material.

If the mixed crystals are grown from solution, there can be considerable

difference between the composition of the starting mixture and that of the resulting

crystal. The different is much less when the melt method is employed for the growth

of single crystals. However significant difference in composition does exist from

region to region of a crystal. Lattice variation in composition up to 20% was observed

in KCl-KBr crystals [83].

Composition dependence of properties of mixed crystals find an important

place, while carrying out the growth and characterization studies on mixed crystals.

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So accurate determination of the composition is as important as the determination of

the property itself.

For alkali halide mixed crystals with anionic substitution, the potentiometric

titration method [85] can be used for composition determination. The techniques of

atomic absorption spectroscopy [86] and X-ray fluorescence [87] are useful in the

case of cationic substitution. Since the lattice constant can be determined accurately

and the law of composition dependence of lattice constant is fairly well established

for highly miscible systems. It affords a simple but reliable method for composition

estimation which can be used for mixed crystals of highly miscible system with

anionic as well as cationic substitution [88-89]. Measured macroscopic densities,

assuming an additive rule can also be used for the composition determination [83].

Rao et al [90] proposed a method of composition estimation from the Crompton

scattering of gamma rays. This method is non destructive but time consuming (seven

days for a sample).Nowadays Energy dispersive analysis x-ray spectroscopy (EDAX)

is used for the determination of compositions accurately.

1.2.2.2. Lattice parameter

The composition dependence of lattice constant in a mixed crystals series can

be expressed by a general relation of the type.

an = !"#$ + (1 − !)"'

$ ……… .. – (1)

This equation which predicts a linear composition dependence was suggested

empirically by Vegard’s [91] and is known as Vegards law.

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If the volumes are assumed to be additive we get

a3 = !"#( + (1 − !)"'

( ………………….. (')

This equation is known as Retgers rule [92] and represents an ideal mixed

crystal. Theoretical investigation of Durham and Hawkins [93] also predicted that

n=3, Grimm and Hertzfeld [94] on the basis of theoretical argument predicted n=8,

Zen [95] pointed that if the difference between a1 and a2 is very small, equation (2) is

indistinguishable from equation (1).

NaCl- NaBr System

Nickels et al [96] have found a deviation of about 8.4x10-3 Å from Vegards’s

law at equimolar composition of NaCl- NaBr system. The difference in the lattice

constant was being 0.3319 Å. The system was completely miscible at room

temperature. Avericheva et al [97] determined the lattice parameters of different

composition of NaCl-NaBr system referenced in Table.2.

Table.2. Lattice Constant (Å) of NaClxBr(1-x) Crystals [97]

X Lattice Constant ( A˚)

0.000 5.956

0.215 5.884

0.370 5.840

0.495 5.710

0.740 5.658

1.000 5.638

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Bhima Shankaram [98] also determined the same by using Debye-Schemer

powder method. From these values it appeared that there are slight deviations from

Vegards’s law, the deviation being more in crystal having higher NaCl content and

less in crystals have higher NaBr content.

NaCl-KCl system

Barrot and Wallace [99] determined the lattice parameters of NaxK(1-x)Cl

crystals , refer Table.3. In this system the deviation from Vegard’s law has been

found to be about 0.4%.

Table.3. Lattice Constant (Å) of NaxK(1-x)Cl Crystals [99]

X Lattice Constant (A˚)

0.000 6.2996

0.100 6.2354

0.300 6.1155

0.383 6.0654

0.500 5.9913

0.504 5.9883

0.598 5.9256

0.699 5.8571

0.824 5.7705

0.900 5.750

1.000 5.6400

The system does not form a continuous series. Later Yulvesnin and

S.P.Zakaryashin [100] measured the lattice parameters of NaCl-KCl and 10 solution

of these salts in the temperature of 20 to 780°C.

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The whole equilibrium decay curve of NaCl-KI has been determined, it was

shown that it agree the empherical rule of constancy of molar volumes sum at

conjugate points on the decay curve.

KCl-KBr System

The lattice parameter of the solid solution KClxBr(1-x) have been measured by

Havisburst et al [101], Oberlies [102] Slagle and Mckinstry [103] studied the lattice

parameter to define the dependency of the KCl-KBr mixed crystal series with

composition.

The variation of the lattice parameters with composition expressed by them as

equation.

"))$ = "#

$ *# + "'+ *'

where ass , a1and a2 are the lattice parameters of the solid solutions KCl and

KBr respectively. C1 and C2 are the respective concentration (molar fractions) and n is

an arbitrary power describe the variation.

They plot the difference between the observed lattice constant a and the value

of lattice constant ac calculated from the laws of Vegards and Retger’s rule. These

plots are in shown in Fig. 2.

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Fig.2. Plots of Lattice Constants (a0) as function of composition x; [ inset (a0-ac) vs x]

It is seen that their values shows positive deviation from Vegard’s law and

negative deviation from Retger’s rule. But the deviation from Vegard’s law is much

less than those from Retger’s rule.

The best fit was found to be for n=3.26. Subbarao and Haribabu [104],

Nair [85] determined the lattice parameter of various compositions of KClxKBr(1-x)

mixed crystals using Debye Scherer powder method. Cohen’s method [105] was

employed to get the best value of the lattice parameters see Table.4 for the values.

Also, they have used these lattice parameters for micro hardness calculations.

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Table.4. Lattice constant (Å) of KClxBr(1-x) crystals [105]

X Lattice constants (Å)

0.000 6.6008

0.150 6.5623

0.286 6.5064

0.460 6.4594

0.615 6.4096

0.864 6.3360

1.000 6.2741

KBr-KI Systems

Nair and Walker [85] determined the lattice parameters of KBr(1-x)Ix mixed

crystals using the conventional scherer method. The lattice parameter variation of

KBr-KI with composition is shown in Fig.3. The average composition indicated was

determined by chemical methods.

Fig.3. Lattice parameter variation for KBr-I with composition[85]

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The Straight line shows Vegards’s law joining the lattice parameters of KI and

KBr for which the value obtained are 7.005Å and 6.575Å respectively. It was

observed that for the extreme concentration range x<0.3 and x>0.7, the system was

characterized by single f.c.c lattice parameter while in inter medial region three f.c.c

phases characterized by lattice parameters. Thus the KBr(1-x)Ix results clearly indicated

the Vegard’s law in the single phase region and the existence of three phase in the

samples of intermediate composition.

KBr-NaBr Systems

Mixed crystals of KBr-NaBr of different compositions were grown using slow

evaporation technique by Anandakumari and Chandramani [106]. The lattice

parameters of the crystals were determined for various compositions by powder

diffraction method experimentally [106]. The values of lattice constants of the mixed

crystals of KBrxNaBr(1-x) have also been estimated using Vegard’s [81] law. There is a

good agreement between the experimental and theoretical value of lattice parameters.

The lattice constant values obtained from experiment and Vegard’s law is shown in

Table.5.

NaCl- KCl- KBr

Jeyakumari and Mahadevan [107] has grown single crystals of

NaClxKCl(y-x)KBr(y-x) for various values of x and y by the Czochralski method.

Refractive indices and densities of the all the crystals were determined and also used

for the estimation of bulk composition in the crystal. The lattice parameters almost

obey the Retger’s rule extended to ternary mixed crystals. But for higher

concentration of NaCl, the system exhibits two separate f.c.c. phases aggregated to

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Table.5. Lattice Constant (Å) of KBrxNaBr(1-x) Crystals[ 106]

Mole % of

KBr

Lattice Constants determinations

From Experiment From Vegards Law

0 5.980 5.980

20 6.120 6.101

40 6.261 6.225

50 6.296 6.287

70 6.420 6.411

80 6.4790 6.472

90 6.537 6.534

100 6.590 6.590

form the crystal of which one is nearly corresponds to the pure NaCl and the other

corresponds to KCl-KBr mixed system.

KCl- KBr- KI systems

Perumal and Mahadeven [108] have grown single crystals of KClxBr(y-x)I(1-y)

systems for various values of x and y by the Czochralski method and estimated the

lattice parameters by the X-ray diffraction method. Their study reveals that the ternary

mixed crystals exhibits two separate f.c.c phases aggregated to form the crystal of

which one nearly corresponds to the pure KI phase and the other nearly correspond to

the KCl- KBr mixed system.

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NaCl- KBr- KI Systems

Poly crystals of NaClxKBr(y-x)KI(1-y), for various values of x and y were

prepared by the melt method and their lattice parameters determined by making X-ray

diffraction measurements by Selvarajan and Mahadevan [109]. Their study shows that

the ternary solid solution exhibits three f.c.c phases instead of single f.c.c phase each

nearly and corresponds to NaCl KBr and KI mixed systems.

NaCl- NaBr- NaI Systems

N.Neelakandapillai and Mahadeven [110] have grown crystals of pure and

CdS added mixed crystals of NaClxBr(y-x)I(1-y) for various values of x and y, grown

crystals were characterized by carrying out density, refractive index, A.A.S, X-ray

diffraction and A.C and D.C electrical measurements. X-ray diffracts analysis

indicates that the mixed crystals have single f.c.c. phases and lattice parameters obeys

Vegard’s law. The Thermal and electrical parameters are found to have non linear

influence with the bulk compositions.

The deviation of lattice parameter determined from X-ray diffraction data

from those calculated from Vegard’s law is less for CdS added system than the pure

mixed system. Lattice parameter and Debye temperature together with initial (taken

for crystallization) and final composition are provided in Table.6.

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Table.6. Lattice Constant (Å) and Debye temperature (°K) of NaClx(Br(y-x)I(1-y)

crystals[ 110]

System Lattice Parameter

Debye Temperature (K)

NaCl 5.596 253

NaBr 5.937 203.4

NaI 6.493 162.3

NaCl0.1Br0.7I0.2 5.844 197.5

NaCl0.3Br0.5I0.2 5.802 198.9

NaCl0.5Br0.3I0.2 5.707 170.3

NaCl0.7Br0.1I0.2 5.650 219.1

NaCl0.1Br0.5I0.4 5.863 169.9

NaCl0.3Br03I0.4 5.799 145.8

NaCl0.5Br0.5I0.4 5.651 196

NaCl- KCl- KBr Systems

Multi phased mixed binary and ternary NaClxKCl(y-x)KBr(1-y) were grown by

melt method for various value of x and y by C.M.Padma and C.K. Mahadevan [111].

The crystals obtained were characterized by X-ray diffraction and electrical

measurements. The thermal parameters viz Debye-waller factor, mean square

amplitude of vibration, Debye frequency and Debye temperature were obtained by

them. The thermal and electrical parameters were found to have non linear influence

with the bulk compositions. X-ray diffraction analysis shows that the

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NaClxKCl(y-x) KBr(1-y) crystals have lattice parameters none linear with the

composition.

NaCl- KCl- KI Systems

The poly crystalline ternary mixed crystals of NaClxKCl(y-x)KI(1-y) were grown

for various values of x and y by M. Priya and C.K.Mahadevan [112]. The crystals

obtained were characterized by x-ray powder diffraction. X-ray diffraction indicates

that the ternary crystals exhibit three f.c.c phases and nearly correspond to NaCl, KCl

and KI. They also reported lattice parameter, Debye Waller factor, Debye frequency,

Debye temperature of NaClxKCl(y-x)KI(1-y) crystals were non linear with composition.

1.2.3.3 Thermal Parameters

1.2.3.3.1. Thermal expansion

Although thermal expansion is an important physical property, considerable

work has not been reported on the thermal expansion of alkali halide mixed crystals.

Kantola [113] ad Salimuki [114] independently made measurement on three

compositions in the KCl-KBr system. Positive deviations from linearity with

composition have been found.

1.2.3.3.2. Debye-waller Factor

It has been shown theoretically, the Waller factor (B) is related to the mean

square amplitude of vibration <µ2> and also to the Debye temperature θD [115].

The B value of KCl0.5Br0.5 were obtained by Wasestjerna[116] and Ahtee et al [117]

from X-ray diffraction. Mohan Lal et al [118] determined the B values for two

compositions in the KCl-KBr system from neutron diffraction intensities.

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Geetakrihsna et al [119] determined the B value of NaClxBr(1-x) crystals from x-ray

diffraction measurements see Table.7.

Table.7. Debye-waller factor (B) and Debye temperatures θD of NaClxBr(1-x) [119]

X B ˚A θD ˚k

0.00 1.67(10) 202(6)

0.10 1.7(9) 204(5)

0.17 1.73(9) 206(5)

0.31 1.70(10) 215(6)

0.37 1.72(10) 217(6)

0.46 1.74(10) 221(6)

0.60 1.73(11) 231(7)

0.63 1.70(10) 235(6)

0.82 1.65(11) 253(8)

0.86 1.61(12) 260(9)

1.00 1.56(11) 278(8)

All these studies indicate that the Debye-waller factor of mixed crystals is

larger than those expected form additivity. In fact the B values in the equimolar

region are considerably larger than those for end members. That is, the B value is

found to vary nonlinearly with the composition with positive deviation from linearity.

In a disordered mixed crystals, in which two kinds of atoms or ions are arranged on a

set of atomic sites, small local distortion in the lattice area because of the atoms of

different sizes. The enhanced Debye-waller factor is a consequence of size effect.

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1.2.3.3.3. Debye temperature

Debye temperature is derivable from experimental data like specific heats,

elastic constants, microhardness, compressibility, X-ray and neutron diffraction

intensities etc. Various method of determination of Debye temperature have been

discussed in review by Blackmann [120], Herbstain [121], Mitra [122] and

Alers [123].

Several relation have been proposed either semi theoretically or empirically to

describe the composition dependence of Debye temperature of mixed crystals [124].

By assuming the additivity of specific heats and assuming the Debye theory

expression for specific heat at low temperatures (The Debye T3 expansion) the

following relation was obtained.

θ-3 = x,#

-(+ (1-x) ,'-( - (1)

where θ1 and θ2 are the Debye temperature of the end members and θ is the

Debye temperature of the crystal. This relation is known in literature as Kopp-

Neumann relation [125]. Following the same relation but employing the high tangent

expression for specific heat, Nagaiah and Sirdeshmukh [126] obtained the relation

θ2 = x,#

'+ (1-x) ,'' - (2)

Karlsson [127], Nagaiah and Sirdeshmukh [128] respectively , proposed the

following relation ,from above equations and considering

θ-2 = x,#

-'+ (1-x) ,'-' - (3)

θ-1 = x,#

-#+ (1-x) ,'-# - (4)

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Recently Geeta Krishna et al [119] have found that seven alkali halide mixed

systems they satisfy the Kopp- Neumann relation for the Debye temperature values of

the Debye temperatures determined from X-ray diffraction data for the NaClxBr(1-x)

system are presented in the Table.7.

Recently Mahadeven and his co-workers [107-111], has reported Debye-

waller factor, Debye frequency for their works in alkali halide mixed crystals.

A summary of reports available in the Debye temperature of Sodium and

Potassium halide mixed crystal is given in Table8.

Table.8. Summary of reports on Debye Temperature

System Methods Conclusion regarding composition

dependence

KCl- KBr Specific heat Equation (3) found suitable [127]

KCl- KBr Elastic

Constant Negative non additivity observed [128]

KCl- KBr Elastic

Constant Equations 1, 2 and 4 tested and equation (4) found most suitable. [126]

KCl- NaCl Elastic

constant

Deviation from linearity , largest amount alkali halides mixed crystal systems and attributed to low stability [129]

KBr- KI Elastic

Constant Negative non additivity [130]

KBr- KI Specific heat Single composition KBr0.53 I0.47 studied equation (3) found suitable [131]

NaCl-NaBr Elastic

Constant Equation (1) found suitable [132]

NaCl-NaBr X-ray

diffraction Negative non additivity [119]

26

1.2.3.4. Microhardness

It is a known fact that simple crystals of alkali halide are of considerable

interest of use as infrared window materials [134], one of the main drawback of these

halides is their low mechanical strength. Attempts have been made to improve the

strength by precipitation hardening and solid solution hardening in different alkali

halide systems [134-137]. Results of a detailed study of hardness and defects such as

dislocations, vacancies, impurity vacancy dipoles in KCl-KBr mixed crystal over the

entire composition range made by Subba Rao and Haribabu [138] have been reported.

Microhardness measurements have also been carried out in KCl-KBr, KCl-KI and

NaCl mixed systems to investigate the effects of the ionic size on micro hardness in

crystals. It was found that the formation of a mixed crystal was accompanied by an

increase in hardness and the microhardness attained a maximum at an intermediate

composition. Also, the change in microhardness was found to be decreasing order

from KCl-NaCl, KCl-KI and KCl-KBr system respectively.

The non-linear variation of microhardness with composition in KCl-KBr

system was thought to be due to the presence of imperfections. The imperfections

may be vacancies, impurity-vacancy dipoles, dislocations, low angle grain boundaries

and etc. The results on discussion morphology studies [139] showed that the density

of the dislocations and grain boundaries appeared to be the dominant imperfections.

The decrease in hardness observed in aged samples [138] may be due to annealing of

vacancies. It was also suggested that microhardness in mixed crystals depends upon

the difference in the size of the ions in the lattice of the mixed crystals and not on the

nature of the ions substituted. Similar results have been obtained for KBr-KI mixed

crystals also [140], that are provided in Table.9.

27

Table.9. Microhardness values obtained for the NaCl-NaBr [98] , KCl-KBr[138]

and KBr-KI [140] systems.

NaClxBr(1-x) KClxBr(1-x) KBrxI(1-x)

Composition x

Hv Composition

x Hv

Composition x

Hv

0.17 29.81 0.15 16.3 0.10 15.5

0.30 36.51 0.29 20.9 0.20 20.9

0.45 40.45 0.39 23.3 0.40 27.6

0.64 42.66 0.62 24.7 0.60 29.6

0.74 38.40 0.87 19.4 .071 26.3

0.83 35.75 0.94 14.6 0.78 24.9

0.90 32.5 - - 0.85 23.7

- - - - 0.90 17.8

Studies on hardening by radiation produced defects [141-144] have shown that

severe hardening was observed, after irradiation that produces low concentration

(x10-5) of point defects. In this the hardening has been attributed to cluster of defects,

rather than to individual point defects.

Veeresham et al [80] have made radiation hardening studies on KCl-KBr

mixed crystals. Plots on increase in hardness drawn against time of irradiation for

KCl, KBr, 38.5 and 71.4 mole% KBr and KCl crystals shows that

28

(i) Microhardness increase due to X-irradiation both in end member and

mixed crystals.

(ii) In the case of KCl-KBr the increase in hardness is rapid in the beginning

and attains saturation after nearly 8 hours of X-irradiation.

In mixed crystals, the increase is gradual and no saturation could be seen even

after irradiation for 14 hours [145]. For all doses of X-irradiation, the increase in

hardness in mixed crystals was found to be less when compared to that in end member

crystals. Also, it was found that the increase in hardness due to X-irradiation varied

non-linearly with composition, attaining a minimum value at an intermediate

composition.

Results of various studies on KCl-KBr mixed crystal have shown that

dislocation have an important role on the irradiation hardening of alkali halides mixed

crystals [145]. The hardening studies on KBr-KI mixed crystals [146] showed similar

results except one difference. The rate of increase in hardness due to irradiation was

found to be more in KCl-KBr system when compared to that found in KBr-KI system.

Recently, G.Maruthi and R.Chandhramani [146] have grown ternary mixed

crystals of KClxKBr(0.9-x)NaI0.1 for various values of x from aqueous solution and

characterized by EDAX , XRD , microhardness and dielectric measurements. They

found the hardness values vary non-linearly with compositions. Also, they found the

dielectric constant increases with decrease in frequency.

C.V.Somasundari et at [147] have grown undoped and doped mixed crystals

of alkali halides. They grown NaxKl(1-x)Cl for various value of x by slow evaporation

method. They found that the hardness number of mixed crystals is found to be greater

29

than those for the end member crystals. For their system, the hardness number varies

non-linearly with compositions.

A.Lesly Fathima and N.Neelakandapillai [148] have grown pure and ZnO

added binary crystals of KClxBr(1-x) from the aqueous solution for various values of x

and characterized them by taking EDAX , XRD and microhardness measurements.

They also reported that the microhardness value varies non-linearly with composition.

1.2.3.5. Electrical Conductivity

Ionic conductivity studies provide valuable information on the state of point

imperfection [150]. The ionic conductivity at temperatures not very close to the

melting point. The electrical conduction in ionic crystal is a defect controlled

property. Wallace and Flinn [149] and Wollam and Wallace [150] have shown

through density measurements on KCl-KBr and NaCl- NaBr mixed crystals, that these

mixed crystals should contain as much as one percentage of vacancies more than in

pure crystals. Since conduction in alkali halide crystals occurs by motion of

vacancies, the alkali halide mixed crystals then exhibit good electrical conductivity

when compared with the pure end member crystals. However, results of the electrical

conductivity studies of Ambrose and Wallace [151] on KCl-KBr mixed crystals in the

temperature range of 400 to 500°C, did not indicate abnormal population of

vacancies. The conductivity of the mixed crystals was found to be never far outside

the range of conductivity fixed by the pure components.

Measurements of electrical conductivity of samples KCl-KBr and their solid

solution made by Annenkov et al [152] showed that the value of conductivity

exponentially increase with increase in temperature . The value of the activation

30

energy for migration of current carriers obtained from the slope of the conductivity-

temperature plot was correlated to the melting point of the solid solutions. The

activation energy was found to be less in sodium solution having a smaller melting

point.

Results of the above study also indicated that the conductivity of the mixed

crystals of KCl and KBr does not much exceed that of the end members. This

indicates that the vacancy concentration in mixed crystals was not so high as the

density measurement indicate. This discrepancy has been explained by saying that the

vacancies in mixed crystals may exist probably as aggregates which do not contribute

to the value of the electrical conductivity. As the conductivity in the temperature

region studied was mainly controlled by divalent metal impurities, the researchers

have pointed that the conductivity of mixed crystals may also be controlled by capture

coefficient of uncontrolled impurity, the value of which is different from those of end

member crystals.

Ionic conductivity measurement done by Schultze [153] on KCl-KBr mixed

crystals indicated that the concentration of vacancies in mixed crystals slightly

exceeds the pure components. Results of the investigation of Smakula et al [154] on

KCl-KBr mixed crystals indicated that those crystals may contain either vacancies or

intertestitials.

In view of the uncertainty about the nature of the defects responsible for the

ionic conductivity in alkali halide mixed crystals, Haribabu and Subbarao also made

electrical conductivity measurements on KCl-KBr and KBr-KI mixed crystals [155]

[139]. They found that the variation of conductivity with composition was found to be

non-linear, attaining a maximum value at an intermediate composition and also the

31

activation energy calculated in both the regions showed a non-linear variation with

composition.

Bhima Sankaram and Bansigir [156] have observed, in NaCl-NaBr mixed

system, slightly higher conductivity than the pure end member crystals in the

temperature range of 42 to 450°C.

The activation energy associated with the migration of cation vacancy has

been found to vary non-linearly with composition. These results are in agreement with

the results of KCl-KBr systems [139].

Recently Mahadevan and Co-workers [157-159,107,109] have made

conductivity measurement for their ternary mixed system. They found that it varies

nonlinearly with composition and it is maximum in the intermediate composition.

1.2.3.5.1. Dielectric constants

Fertal and Perry [160] were the first to determine the static dielectric constants

of KCl-KBr systems, from Kramers-Kronig analysis of infra-red reflectivity data.

They reported that the dielectric constant variation with composition was found to be

haphazard. Kamiyohsi and Nigara [161] measured dielectric constant using

immersion method , the dielectric constant of five mixed systems viz KCl-KBr,NaCl-

NaBr, RbCl-RbBr, KCl-RbCl, and KI-RbI were determined. They observed a non-

linear variation of dielectric constant with composition in all the cases. Large

difference in the value of dielectric constant for the KCl-KBr system was observed

when compared to the value obtained by Fertal and Perry [160]. As a cross check

Prameela Devi [162] determined the dielectric constant of KCl-KBr mixed crystals

for various compositions at room temperatures. Her results favours the values

32

obtained by Kamiyoshi and Nigara [161] and differ considerably from those of Fertal

and Perry[160]. Later Sathiah [163] determined dielectric constant and loss at

elevated temperature up to about 673°C as a function of frequency and also a function

of composition of KCl-KBr and RbCL-RbBr mixed crystals. Also, he has analysed

the results semi theoretically [163]. However there is no report of dielectric

measurements available on ternary mixed crystals of alkali halides.

1.2.3.6. Spectroscopy Studies

K.Jeyakumari et al [164], S.Perumal [165], R.Selvarajan [166] have studied

UV visible characterization of their systems namely NaClxKCl(1-x),

NaClxKBr(1-x), KClxKBr(1-x) and NaClxKCl(y-x)KBr(1-y) (for various values of x & y) ,

KClxKBr(y-x)KI(1-y) single crystals for various values of x and y , NaClxKBr(y-x)KI(1-y)

for various values of x and y respectively. Their result indicates a non-linear variation

with the bulk composition of wavelength corresponding to maximum transmittance.

Kruger et al [167] were the first to record the IR spectra of alkali halide mixed

crystals. They studied NaCl-KCl systems. Mitsuishi [168] recorded the IR spectra of

mixed KCl-KBr and KCl-RbBr systems. The KCl-KBr system was also studied by

Ferraro et al [169]. Angress et al [170] have recorded IR spectra for the KCl-RbCl and

KBr-RbBr systems. In all these studies it has been observed that the frequency of

transverse optical mode, vary linearly with composition. Fertal and Perry [160] in IR

frequency, from the reflectivity data of the KCl-KBr system and reported a slightly

non dependence in composition.

An interesting observation was made by Fertal and Perry [160] in their study

of the KI-RbI system. For this system they observed not a single frequency as in the

33

case of pure crystals, but two frequencies close to those of the pure crystals. Angress

et al [170] observed two frequencies in the KCl-RbBr system. This is a new

phenomenon and is referred to as “two mode behaviour”. Chang and Mitra [171]

obtained a criteria whether the given mixed crystal of type ABxC(1-x) will exhibit a one

or two mode behaviour .

The criteria

mB>µac – one mode behaviour

mB<µac – two mode behaviour

where mB is the mass of the atom B and µac is the reduced mass of Ac.

Recently Somasundari and Neelakandapillai [172] recorded FTIR spectra for

their NaCl-KCl system and calculated force constant. They observed two-mode

behaviour for their mixed system. Also, Lesly Fathima and N.Neelakandapillai [173]

have recorded FTIR spectra for their KCl-KBr system, they also observed two mode

behaviour as observed by Fertal and Perry[160].

Pure alkali halide show only second order Raman spectra [85, 88,174], but in

the mixed crystals , the addition of one of the alkali halides to another alkali halides

disturbs the symmetry of the pure crystals and a first order Raman spectra is observed

in the mixed crystals. Thus, the appearance of a first order Raman spectra are a new

phenomenon displayed by mixed crystals but not displayed by the pure numbers.

Nair and Walker [85] first studied the Raman spectrum of mixed crystals of

the KCl-KBr system. Subsequently, they studied the KBr-KI, KCl-KI and KCl-RbCl

systems [161]. The KCl-KBr, KBr-KI and KCl-KI systems involve negative -ion

34

substitution. For these systems Nair and Walker found that the T2g phonon did not

show much variation but A1g phonon was found to be vary linearly with composition.

But the KCl-RbCl system involves positive -ion. Here, the Raman spectra contain Eg

and T2g phonons.

In this systems Eg phonons was found to vary linearly with composition but

not the T2g. The features in the observed 1st order Raman spectra of alkali halide

mixed crystals have been satisfactorily explained on the basis of a lattice dynamical

model by Massa et al [175-176]. Koichiro Tanaka et al [177] studied the luminescent

properties of excitons relaxed on Br- ions in KCl crystals containing a small amount

of Bromine impurity (KCl:Br) in KCl(1-x)Brx mixed crystals. It is confirmed that both

of 3.6ev and 4.88 ev emission bonds in KCl:Br originate from relaxed excitons

localised in Br- ion pairs. No monomer band appears.

Experimental results of KCl(1-x)Brx mixed crystals suggest that the initial state

of π , emission band in KBr should be accompanied with another type of lattice

distortion in addition to the Vk type distortion. A candidate for their type of distortion

will be off-centre displacement of Br-2 core for the self trapped excitons.

P.Eswaran et al [178] studies the photoluminescence phenomenon on

RbBr(1-x)Ix TII mixed crystals. They found that the optical absorption spectra

RbBr(1-x)Ix : TII(0.1 mol%) mixed crystals exhibited an inhomogeneous broadening of

the A-band towards low energy side with increase of I-ion composition. Recently

Eswaran et al [179] studied the optical absorption spectra of KBr: Ti+ (0.0125 mol%)

single crystals, that A, B and C bands around 258, 220,and 210 nm respectively. In

KCl0.1Br0.9 Ti+ (0.0125 mol %) mixed crystals exhibit slightly broadening of the A-

band towards lower wavelength side. The broadening of the absorption spectra are

35

suggested to be due to some complex Ti+ centres involving Br- and Cl- ions formed in

the mixed crystals. When excited at A, B and C bands of Ti+ ions, PL of KBr:Ti+

showed emission band around 320 nm with a prominent shoulder around 365 nm. In

KCl0.1Br0.9 mixed crystals, the shoulder around 365 nm is not prominent due to the

perturbing influence of Cl- ions.

1.3 Methods of Crystal Growth

A wide range of techniques have been developed for the growth of single

crystals from the melt, vapour phase and solution. The method chosen for the growth

principally depends on the characteristics of the material like melting point, nature of

melting and other physico-chemical properties.

Various methods of Crystals Growth

Generally the growth methods are classified according to their phase

transformation as:

1. Solid growth – Solid to Solid phase transition.

2. Solution growth – Liquid to Solid phase transition.

3. Vapour growth – Vapour to solid phase transition.

4. Melt growth – Liquid to solid phase transition.

Low and high temperature solution growth and melt growth fall in the

category of growth of single crystals from liquids. In vapour-solid growth, the growth

is by sublimation process. There are a number of growth methods in each category.

The principal requirements of the crystals are that they should be of large size free

36

from strain and imperfections and should be grown in a short time scale. These limit

the methods of production by low temperature solution and melt growth. Vapour

growth is the slow process and the method which involves the growth on a substrate,

inevitable causes a gradient of strain in the crystal.

Melt growth by the Bindgeman technique involves preparation of the raw

material in closed vessels under extreme temperature gradients around the melting

temperature.

Low temperature solution growth near ambient temperature offers the best

prospect of producing crystals under near equilibrium conditions and free from strain

and dislocations. It also permits the preparation of a variety of different morphologies

of the same material by varying the growth conditions. The present thesis deals with

the growth of crystals from aqueous solution. Among the different methods employed

for crystal growth, solution growth at low temperature occupies a prominent place

owing to its versatility and simplicity. The principal advantage of this technique is

that the growth occurs close to equilibrium conditions and hence crystals with high

perfection can be grown.

1.3.1. Low temperature solution Growth

Solution growth can be adopted that are highly unstable in the melt and

undergo decomposition upon melting. The major advantage of these techniques is

convenience, simplicity and avoidance of complex growth apparatus. This method

envisages that the material must crystalline from solution with prismatic morphology.

The basic growth procedure involves seemed growth from a saturated solution

and then regulating its growth by control of temperature, concentration and

37

temperature gradients. Crystal growth requires the production of a correct

supersaturated solution at the growth face. This super saturation is achieved by

lowering the temperature of the solvent.

1.3.2. Purification of Materials

An essential prerequisite for success in crystal growth is the availability of

material of the highest purity attainable solute and solvents. If high purity is required

impurities may be incorporated into the crystal lattice resulting in the formation of

flaws and defects. Sometimes, impurities may slow down the crystallization process

by having absorbed on the crystal habit which changes the crystal. A careful repetitive

use of standard purification methods of recrystallization followed by filtration of the

solution would increase the level of purity

1.3.3. Solvent Solution

Once the method has been defined and high purity starting material has been

obtained, the next requirement is that a solvent should be chosen which allows

prismatic growth in which the solute has high solubility. An ideal solvent should

possess the following characteristics.

i) High and positive temperature co-efficient of solute solubility.

ii) Low Volatility

iii) Density less than that of the bulk solute

iv) Low viscosity

v) Low toxicity

38

1.3.3.1. Crystal habit

The growth of the crystal at approximately equivalent rates along all the

directions is a prerequisite for its accurate characterization. This will result in a large

built crystal from which samples of any desired orientation can be cut. Further such

large crystal should also be devoid of dislocations and other defects. These

imperfections become isolated defective regions surrounded by large volumes of high

perfection, when the crystal grows with a bulk habit. In the crystal which grow as

needles or plates, the growth dislocations propagate along the principal growth

directions and the crystal remain imperfect. Needle like crystals have very limited

applications and plate like crystals need to be favourably oriented.

Changes of habit in such crystals which naturally grow as needles or plates

can be achieved by any one of the following ways.

i) Changing the temperature of growth.

ii) Changing the PH of the solution

iii) Adding a habit modifying agent , and

iv) Changing the solvent.

1.3.3.2. Preparation of the solution

The most important part of solution growth is preparation of the solution,

before the seed crystal is inserted. It is desired to prepare a saturated solution at the

initial growth temperature to prevent the dissolution of the seed or rapid initial growth

to prepare a saturated solution. It is necessary to have an accurate solubility

temperature data of the material. This solution is saturated at the desired temperature.

39

This solution is filtered and the filtered solution is taken in a growth vessel,

the growth vessel is sealed to prevent solvent evaporation, the solution is tested for

saturation by suspending a small crystal in the solution. If the system is not in

equilibrium the crystal will either dissolve or the solute will crystallize on the seed.

By varying the temperature the equilibrium is acquired. The test seed is then

withdrawn from the solution and a good quality seed is inserted. The temperature of

the solution is slightly raised above the saturated temperature to dissolve any

unwanted nuclei or any surface damage of the seed. The temperature is then lowered

to the equilibrium temperature and the growth commences.

1.3.3.3. Seed Preparation

The defects present in an imperfect seed propagate into the bulk composition

of the crystal, hence seed crystals are prepared with care .The quality of the crystal is

usually slightly better than that of the seed. The seed crystals are prepared by slow

evaporation of a saturated solution. During this process the surface of the seed

inevitably gets damaged before commencing the growth cooling rate.

Supersaturating is achieved by lowering the temperature of the solution.

Temperature and supersaturation have to be preciously controlled for desirable

results. The growth rate is maintained linear in order to grow large crystals. This

requires an increase in the supersaturation level and linear cooling not provide this.

Hence after the initial growth the rate of temperature lowering is increased, a large

cooling rate changes the solubility beyond the Meta stable limit rate. Further

fluctuations in super saturation may encourage solution inclusions flow into growing

crystals. Hence a balance between a temperature lowering rate and the growth rate has

to be maintained.

40

1.3.3.4. Crystal Perfection

The perfection of the final crystal is based on

i) The purity of the starting material

ii) The Quality of the seed crystal

iii) Cooling rate employed

iv) The efficiency of agitation.

1.4. Present work

Alkali halides are formed by the combination of alkali atoms of the first group

and halogens of the seventh group. They crystallize in either of the two structures viz

NaCl and CsCl structure see Fig.4. In ambient conditions CsCl, CsBr and CsI assume

the CsCl structure and the rest crystallize with the NaCl structure.

Alkali halide crystals are widely used as Laser window materials, neutron

monochoromotors, infrared prisms, infrared transmittance etc., But their uses are

limited by their mechanical properties and hence there exists need to strengthen them.

Armington et al [74] discussed two methods of improving the hardness of alkali

halides i. solid solution hardening ii. Impurity hardening. Mixed and the impurity

added crystals are found to be harder than the end members and so they are more

useful in these applications. In addition, mixed alkali halides find their applications in

optical, opto-electronic and electronic devices. In view of these, it becomes necessary

and useful to prepare mixed and doped mixed crystals regardless miscibility problem

and characterize them by measuring their physical properties.

41

Fig.4. Structure of NaCL

Fig.5. Structure of CsCL

42

Mahadevan and his coworkers [84] have found that the transparency of the

crystals is reduced, when the crystals are cooled from high temperature to room

temperatures due to the introduction of thermal defects. Mahadevan and his

coworkers [84] obtained larger and more stable crystals from NaClxKCl(0.9-x)KBr0.1

from solution growth. They grew the crystals from slow evaporation technique.

A research program on the preparation and properties of Pure and ZnS doped

binary mixed crystals of sodium chloride and Sodium bromide from the aqueous

solutions was planned in this laboratory and investigations were undertaken.

In the present study, single crystals of (NaCl)x(Br)(1-x) with out and with

adding ZnS have been grown from aqueous solution by slow evaporation technique

and characterized. Single crystals of NaCl and NaBr have been grown for the

comparison purposes. A total of 16 crystals (12 mixed and 4 end members) have been

grown under identical conditions.

The grown crystals have been chemically characterized by determining the

bulk composition and ZnS contents using Edax spectrum taken.

X-ray diffraction data for the powdered sample were collected using

automated X-ray diffractometer. The reflection data were indexed and the lattice

parameters determined by using the standard methods. Thermal parameters like

Debye-waller factor, mean square amplitude of vibration, Debye temperature and

Debye frequency were determined from the X-ray diffraction (integrated) intensity

data.

The hardness and melting point of all the grown crystals were determined by

microhardness measurement and TG/DTA thermal analysis respectively. Debye

43

temperatures have also been determined from the hardness values and melting point

temperatures.

The dielectric parameters like dielectric constant, dielectric loss (tangent) and

A.C. conductivity were measured by parallel plate capacitor method at different

frequencies viz 20Hz, 100Hz, 1kHz, 10kHz, 100kHz and 1MHz at various

temperature ranging from 40 to 150 °C. Activation energy, relaxation time and Debye

temperature are also determined from the conductivity data. Cole- Cole plot also been

drawn from the complex dielectric constant values.

Optical measurements like UV-vis, FTIR and Photoluminescence studies have

also been carried out for all the grown crystals.

The results obtained in the present study indicate that the crystals grown are

two phased as expected to be highly useful. The hardness and stability have increased

with the ZnS addition. Dielectric constant increased not only with the mixed crystals

but also with dopant addition.

A report of the present research work is provided in this thesis. The thesis is

divided into nine chapters including ‘Reference ‘section. The present work is

introduced in the first chapter. A brief review of various studies made on alkali halide

mixed crystals in the near past is also provided in Chapter I. Chapter II contains the

details of growth of crystals along with SEM, EDAX and the estimation of bulk

composition.

X-ray diffraction measurement along with the calculation of thermal

parameters and results obtained are discussed in Chapter III. Chapter IV deals with

44

microhardness measurement along with Debye temperature calculation. Thermal

analysis is made and the result obtained is given in Chapter V.

Chapter VI contains A.C electrical measurement along with results obtained.

The optical studies including UV-Vis, FTIR and Photoluminescence are provided

along the results obtained in Chapter VII. Chapter VIII contains Summary and

Conclusion along with Future scope. We have provided the Reference section in

Chapter IX.