CHAPTER - 1

INTRODUCTION

Crystals have been attracting mankind in the past due to their aesthetic beauty.

Examples for them are the most valuable diamond to artificial stones like American

diamond. Though the things are like above, their scientific applications were understood

only in the last century, i.e., 20th century. Recently, single crystals have been used

extensively in solid state devices.

The search for new materials and their single crystal growth have always been

given top priority by the scientists throughout the world. Since 1970, one could see that

there are lot of developments in science and technology, especially in the fields of

electronics, fiber-optic communications and lasers. These things became possible due to

the availability of single crystals. Nowadays, it is possible to synthesis artificially in the

laboratory almost all naturally occurring crystals, and new crystals from the elements in

the periodic table and organic crystals.

Their smooth surfaces with scintillating reflections of light, their enchanting

colours, and their definite and varied shapes with sharp edges, their deep transparency

altogether aroused the aesthetic since of early men who used these crystals as ornaments.

As science and technology grows, the curiosity of mankind to understand quantitatively

about crystals also grows. Thus their utility range from ornaments to several useful

applications in fiber-optics, etc.

The fantasy of their external beauty was understood by the natural laws of

mathematics, physics and chemistry. Their contents and [inside] were probed, analyzed

and understood by the modern methods of diffraction and spectroscopic techniques.

Their external plane, shape and colours are correlated with the internal atomic content

and their arrangement. Thus grew a separate field of science, the study of crystals -

crystal growth of characterization. The word crystal originates from the Greek word

CRYSTTALOS, which means clear transparent ice. In the middle ages, this word was

extended to include quarts are rock crystals which were believed to have been formed by

the intense freezing of water on the Alps mountains into permanent form of ice. To a

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present day of student of crystal growth, the crystal is a chemical compound in the shape

of a solid polyhedron bounded by definite planes. Their shape and symmetry is a

manifestation of the internal atomic arrangements extending in three dimensions, in an

orderly way.

Nowadays crystal growth technology is largely on crystals such as NLO crystals,

piezo-electric crystals, Ferro-electric crystals, sensitive crystals and crystalline films.

Preparation of single crystals of such materials has resulted in growing realization for the

importance of crystal growth, in the theoretical and experimental aspects. The

preparation of single crystal is mainly based on the availability and nature of the starting

materials and their physico-chemical properties. Fields as diverse as physics, chemistry,

chemical engineering, mineralogy and biology have contributed much to the field of

crystal growth, also these fields benefited from crystal growth. Crystal growth concept

has been fundamental to many areas of science and technology.

We can grow crystals in any one of the following four transformations. [Pamplin,

B.R., 1979].

1. Solid-state reaction involving solid-solid phase transition.

2. Solution growth process involving Liquid-solid phase transition.

3. Vapour growth process involving vapour-solid phase transition.

4. Melt growth process involving liquid- solid phase transition.

The general condition for all the above mentioned process is that the growing

crystals must have lower free energy than the initial state of the system.

1.1 SOLID GROWTH TECHNIQUES

Solid state growth requires atomic diffusion except in case of martenstic

transformations. At normal temperatures such diffusion is usually very slow except in the

case of superionic materials where the small cation is quite mobile. Thus solid state

growth techniques are seldom employed when other methods can be used. Fig. 1.1 shows

that most of these techniques were known and used before 1900 and compared with the

other growth methods, have not been changed very substantially this century. Solid state

growth techniques for the production of single crystal are of very small significance;

however, annealing, heat treatment, sintering, and quenching are, of course,

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metallurgical processes of great importance in tailoring the properties of materials.

[Brice J.C., 1986].

Fig. 1.1. Evolution of solid growth technique

1.2 ANNEALING TECHNIQUES

If a polycrystalline metal rod is held at an elevated temperature below the melting

point for many hours some grains may grow at the expense of their neighbors. Since

grain boundaries contain more free energy than bulk crystal. This process can be seen to

lower the free energy of the rod. However, such growth is unreliable and incomplete.

Two main techniques were introduced early this century to improve it.

1.3 STRAIN ANNEALING

It was found that cold worked metals showed more grain growth than unstrained

samples. Some favoured grains grow at the expense of others occasionally the whole

sample may become a single crystal. Thirteen “laws of grains growth” were enunciated

in a book published in 1924. It became clear that a critical strain can induce nucleation

and growth of new grains. Secondary recrystallization: Below the critical strain only

normal coarsening or primary crystallization occurs. Above it, many new grains

nucleate and grow. At the critical strain ideally one grain nucleates and grows to

encompass the whole specimen before there is time for any substantial statistical chance

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of a second nucleation. By this method single crystal aluminum bolts and other shaped

crystals have been produced. Temperature gradient strain annealing use of a two-zone

furnace was successful in causing just one grain to grow down specimens of aluminum,

lead, copper and tin. This has been called the grain boundary migration technique.

Rutter and Aust welded a single crystal of lead to a polycrystalline specimen and

used a strain anneals technique to cause this seed crystal to grow downs the bar. This

neatly avoids the nucleation problem but cannot easily be used for high melting point

metals.

1.4 ZONE HEATING

In the early days of tungsten filament lamps it was found that wires which were

single crystals or of a large grain size held up better and suffered less from “filament

sag”. Bottger describes, how finely divided tungsten plus a binder was extruded into a

wire and passed through a temperature in excess of 2000oC to produce the commercial

wire sold by the Pintsch Company in Germany in early days of this century.

Fig. 1.2. Zone heating method for the production of single crystal wires

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In 1937 Andrade modified this pintsch process to produce single crystal wire of

molybdenum and tungsten. This is illustrated in Fig. 1.2. The wire is electrically heated

in Cacuo and a small traveling heater passes a hot zone up and down the wire. Provided

the wire is fine enough to have a single grain across its cross-section, the whole length

can be grown into a single crystal. The zone-heating method is a forerunner of the

techniques of zone melting, zone refining, zone levelling and floats zone refining.

1.5 SINTERING AND HOT PRESSING

The annealing of a pre-compressed powder called sintering and the annealing of a

powder under pressure is hot pressing and can lead to the homogenization of alloys and

to grain growth. Both have been used for many years in metallurgy as tools for studying

phase diagrams and as production techniques for producing ingots or shaped components

of different alloys. Neither offers much scope for the growth of good crystals.

They are complicated process often encompassing a variety of interrelated

phenomena, such as recrystallization, solid state diffusion and crystal growth from solid,

liquid and gas phases as well as changes in porosity, density and mechanical strength. In

the case of mixtures, chemical reactions may also occur. By sintering, intermediate

phases – ordered compositions existing some 500oC below the solids – were found in

copper-gold alloys like Cu3Au, CuAu3 and CuAu II (which is super-super lattice). These

phases are formed by carefully annealing which permits atomic diffusion to the ordered

structure. Such ordering is generally achieved quicker in small particles and grains than

in a single crystal.

Sintering was used intensively in the field of pnictide and chalcogenide alloy

semiconductors, as a method of obtaining homogenous equilibrium samples of difficult

solid solutions of mixed III-V and II-VI compounds and III-V – III2 – VI3 and II-VI – III2

- VI3 alloys in the initial pioneering studies of the 1950’s by the groups led by

Goryunova and Woolley. Pampkin has worked out the compositions at which ordering

is most likely in these adamantine alloys. No ordering in the Ge-Si III V – III1V1 or IIVI

– II1VI systems has yet been observed, but it is common when vacancies are plentiful on

the cationic sub lattice. This is the case in IIVI – III2 –VI3 alloys illustrated in Fig. 1.3

[HgTe – In2-Te3 is an example].

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Fig. 1.3. Phase diagram of an adamantine pseudo binary system

on Hg3Te3 In2Te3

In this hypothetical diagram the phases, and melt concurrently and so

crystals of these may be melt grown. The phase is a peritectic, which may perhaps be

grown by a suitable vapour phase or molten metal solution method. The third

intermediate phase can only really be prepared by a solid state annealing or sintering

method. Since it only exists below 600o, the ordered end component phase is also

solid state grown, and it can be grown by a polymorphic phase transition from the zinc

blende structure and phase. Notice that initial freezing of melts of composition X and Y

results in solid material of compositions a & b respectively.

Crystals YIG, BeO, Al2O3 and ZnO grown from sintered powders are mentioned

by Aust and Laudise and recently crystals of mm dimensions of a new superionic spine

AgInSnS4 were grown on top of a sintered powder of compositions Ag3In3SnS8 cycling

of the temperature during sintering sometimes called mineralization has been used. This

is an example of pendelofen technique. This is a German word, meaning oscillating

oven.

1.6 MELT GROWTH TECHNIQUES

1.6.1 INTRODUCTION

Melt growth is undoubtedly the best method for growing large single crystals of

high perfection relatively rapidly. It has been used for a great many metals, semi-

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conductors, ionic crystals and a few organic compounds. Often, especially with semi

conductors and laser host crystals, impurities (dopants) can be deliberately added and

homogenously dispersed in a large percentage of the grown crystals. So these techniques

have been developed largely in the electronics, optics and synthetic gemstone industries

and a vast body of detailed expertise has been built up [Brice J.C., 1972].

However, melt growth normally requires that the material melt congruently (that

is it does not decompose below or near its melting point) and has a manageable vapour

pressure at its melting point. Thus a great many materials cannot be grown from the

melt. These include many hydrated and anhydrous salts, most organic crystals and

virtually all biological materials. Because, good ideas and techniques developed in one

technique are often carried over and applied to other methods, it is not possible to

classify melt growth techniques in an unambiguous way. However, it is convenient here

to divide melt growth into four main groups of techniques.

Normal Freezing : Ingot gradually frozen from one end.

Crystal pulling: Crystal grows on a seed withdrawn from the melt.

Zone melting : A molten zone is passed through an ingot.

Flame fusion (or) : Crystal grows below a melt which is fed from above.

Pedestal growth

The super cooling and nucleation problem is eliminated where possible by using

a seed crystal. This is usually possible in the latter three groups but is often difficult to

achieve in the normal freezing of a boule of melt.

1.6.2 Normal Freezing, Directional Freezing or Bridgman – Stock Barger Method

The most straight forward and inexpensive melt growth technique is normal

freezing – molten ingot is gradually frozen from one end to the other (Fig 1.4). When

this is achieved by the use of a two-zone furnace, it is called Bridgman – Stock Barger

method. The usual configuration is vertical with the melt in an ampoule being lowered

slowly from the hot zone to the cooler zone which is below the melting point. This was

the method used in the United States by Bridgman and others, for the growth of large

metal single crystal and later by Stock Barger for the growth of optical quality alkali

halide crystals for prisms and lenses. It was also developed independently in Europe by

Tammann & Obreimov and Schubnikov.

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The nucleation problem can be overcome by a variety of devices Fig. 1.5.

1. Use of a conical bottom to the crucible so that the coolest region is essentially a

point. A cooled gas stream can be directed at this point or a conducting rod [of

quartz or copper] attached thereto.

2. use of a capillary, the initial rapid growth after nucleation before near equilibrium

conditions are reached takes place in the capillary and hopefully a single seed

grows out into the bulk of the melt.

3. Necking, one or more bulbous extensions are provided below the main ampoule

to contain the initial rapid growth and to allow only one seed of fast growing

orientation to pass into the melt.

4. Melt back, sometimes the damaging effects of rapid initial growth can be

alleviated by raising the temperature [or reversing the pulling] to melt some of

the nucleated crystallites.

Fig 1.4. Normal freezing also called directional freezing

Fig. 1.5. Techniques for nucleating single crystals from the melt (a) Conical bottom (b) Capillary (c) Necking

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If the material expands on freezing, would crack the vertical ampoule or if it is

desirable to use a graphite or vitreous carbon boat, horizontal normal freezing is used.

This is also the more convenient configuration when the vapour pressure of a component

is to be controlled by either liquid encapsulation or having a connection to a source at a

fixed temperature.

There are three ways of moving the freezing interface:

1. Moving ampoule : The melt is drawn through the temperature gradient.

2. Moving furnace: The melt is stationary and the furnace moves.

3. Static freeze: When the temperature gradient is a approximately linear as Fig 1.4.

the temperature of a furnace may be gradually reduced causing the freezing

isotherm to run down the ingot. The term directional freezing is sometime

restricted to this technique, which was oriented by Stober.

If in a normal freeze situation with constant cross- section as in Fig 1.4. there is

present an impurity with segregation constant k and initial concentration co, then the

composition c of the material freezing when a fraction g of the melt has solidified is

given by,

c = kco (1 –g) k-1

This is called the normal freeze equation and applies if there is complete mixing

in the liquid phase but no diffusion in the solid phase and K is a constant. Since for most

impurities K is a appreciably different from unity, the middle region of the crystal is

purified impurities, like P in Ga, which have a preference for the solid phase, freeze out

early while those, including most dopants in Si, which prefer the liquid phase, are sweft

to the end. This is a basic principle of zone refining (see below), but it also means that

crystals have graded impurity content especially in the last part to freeze.

Sometimes directional freezing is used to obtain graded solid solution material in

system where complete solid solution occurs. This is illustrated in Fig 1.6, which applies

to a great many pairs of adamantine compounds like InAs – GaAs, to Ge – Si, and many

metal systems like Cu- Au. When a boule of composition C is frozen, the first to freeze

has composition D, richer in component B than the liquid. Thus the liquid become richer

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in component A and the composition of the liquid at the freezing interface moves the

direction shown by the arrow on the liquid at C, the composition of freezing solid given

by the arrow on the solidus at D, provided growth is slow enough to avoid constitutional

super cooling. The resulting ingot will be graded in composition.

Fig. 1.6. Production of a graded composition ingot. The diagram illustrates

complete solid solution phase diagram of A & B.

1.6.3 COOLED SEED METHOD

Nacken used a seed crystal attached to copper rod which was cooled by a stream

of air in a jacket at the other end to initiate growth in a melt. Kyropoulous some 10 years

later used a similar cooled seed method for the growth of alkali halide. He dipped an air

cooled platinum tube into the melt and then withdrew it so that just the bottom most seed

crystal remained in contact with the melt to nucleate growth.

Adams and Lewis used the cooled seed method to grow large ice crystal and the

method has been used commercially for alkali halides. It can be used to grow much fatter

crystals than the pulling method and used simpler apparatus however this latter method

has achieved far greater popularity and success.

1.6.4 CRYSTAL PULLING

Crystal pulling dates from Czochralski's work on the speed of crystallization of metals

published in 1918. Its importance however starts in the early 1950s when Teal and Little

at Bell Labs. Developed for the production of pure and doped crystals of germanium and

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silicon and even grew in junctions. It has subsequently been used for some III-V semi-

conductors using. When there is a high vapour pressure of arsenic or phosphorus, the

liquid encapsulation technique (LEC pulling). In the early 1960s pulling in air was

pioneered for many laser host materials like CaWO4.

Fig. 1.7 Necking

Dislocation free crystal has been grown by necking (Fig 1.7) a technique

originally introduced to produce a single crystal from a polycrystalline seed. Dash grew

dislocation free Si in the late fifties and since then dislocation free pulled crystal of Ge,

GaAs, InP, GaP, AI, Cu, Ag and Ni have been reported. Because of the crystal growth is

strain free, it is difficult to control its shape. Automated automatic diameter control

methods have been introduced. Recently the use of die to aid shape controlled crystal

pulling has proved promising. The method has been given in the clumsy named “Edge

defined film-fed crystal growth” or “EFG pulling” for short.

1.6.5 ZONE MELTING

Zone heating was a forerunner of zone melting or zone refining. It was

developed by plan in 1952 for Germanium. It may be classified as an S-L-S process.

Fig. 1.8. shows a zone melting situation where there is a volatile component such as As

in InAs or GaAs. The tube containing the boat of material for zone refining is kept at a

suitable ambient temperature. Fig. 1.8. shows zone refining (also called zone melting).

The molten zone, heated here by r.f. induction is moved along the ingot. The separate

boat at a lower temperature contains a volatile compound of the temperatures are

adjusted to prevent any net transport to or from the molten zone. And a movable heater

melts a molten zone which is passed down the ingot. A seed crystal can be introduced at

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the starting end if single crystal growth is desired. A second boat containing the volatile

components is kept at just the right temperature to ensure no net loss of that component

from the molten zone.

Fig. 1.8. Zone refining (zone melting)

Float zone refining is used with silicon rod vertical of the zone free from contact

with any crucible materials to produce the purest single crystal known to man. It is a

vital stage in the production of substrate for integrated circuits. Sometimes the growing

crystal above the molten zone and the feed ingot below it are rotated and pushed or

pulled independently. If a fat source ingot is pushed slowly into the molten zone of a

slimmer crystal is pulled out, the process is sometime called pedestal pulling or crystal

pushing or differential pulling. Float zone refining can be thought of as a crucible less

crystal pulling method.

1.6.6 FLAME FUSION TECHNIQUES

Like float zone refining just mentioned, the Verneuil method has the greater

advantage of being crucible less. It was originated before the turn of the century. The

essential features are a seed crystal, the top of which is molten which is fed with molten

drops of source material coming usually as a powder through a flame or plasma. The

original interest was centered on the synthesis of sapphires and rubies gem stones based

on alumina, corundum, Al2O3 and often the work was clandestine because jewelers much

preferred natural stones through these were less perfect. Michel’s book give details of

improvement to Vermeil’s technique of the dopants needed. For example, chromic oxide

must be added to the alumina powder for rubies plus a little iron to imitate the rubies

from Thailand. Iron and titanium oxides are both required for sapphires. Many other

natural gemstones can be imitated. Verneuil and his successors used a flame of

hydrogen burning in oxygen to melt the falling powder the technique is still often called

“Flame Fusion”. Burner design was improved by Merker who introduced the tricone in

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which hydrogen passes down the inner tube and two outer tubes carry oxygen.

Essentially the same technique is still used to produce rubies and sapphires for watch

bearing and similar applications, although crystal pulling is now the preferred method for

oxide crystals of high quality, other methods of heating include are imaging, plasma

heating and electron beam heating.

Drabble and Palmer, have described an arc method which can be classified as a

variant of flame fusion technique. The arc is struck between a nickel cathode and a NiO

anode. The top of which is kept molten by the heat of the arc. Nickel is transported

from cathode to anode oxidizing as it comes. The method is restricted to a few oxides

(CO, Fe, Ti, and U) and ferrites. High melting point oxides such as MgO, CaO, SrO,

ZrO2 and BaO may be grown as single crystal inside a charge of their own powders. The

electrodes are buried in the charge, which melts near their tips. When this melt is

allowed to freeze slowly, large single crystal volumes are frequently found. This is a

crucible less normal freezing method.

1.7 SOLUTION GROWTH TECHNIQUES

Melt growth often merges in practice almost imperceptibly into solution growth.

If one considers the segregation (rejection of impurity atoms), which commonly occurs

in melt growth, it is evident that the growing crystal is in equilibrium not with pure melt

but with a solution or liquid alloy. However the distinction between the two groups of

techniques is usually clear enough in melt growth, the solvent (major component) freezes

whereas in solution growth it is the solute, which crystallizes usually, well below its

melting point (assuming it even has one). It was divided basically by the nature of the

solvent. This is justified by the fact that generally speaking crystals are only grown from

one type of solvent-aqueous, organic, flux (usually an ionic compound), or metal. Then

electro crystallization and gel growth seem to need separate classification, but it is all

rather arbitrary [Bardui, P., 1987].

This is yet another useful classification. Apart from the two last-named

categories solution growth can be classified by the method of obtaining super saturation

into three groups of methods.

a) Temperature change: Cooling (or in rare cases heating) the solution.

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b) Solvent extraction: Usually by evaporation (in rare cases solvent addition is

required.

c) Circulation: A two temperature system in which the solvent passes

nutrient from source to seed. Unlike the other two this

method may often be made a more or less continuous

process as opposed to a batch process, and is thus favoured

in industry.

Solution growth methods are at least an order of magnitude slower than melt

growth generally speaking and, of course, generally less pure. However, for many

commercial materials like salt, sugar, some fertilizers, hydrated materials and some

compounds that decompose before melting, it is the only viable process. Also much high

purity, high perfection materials for the electronics and optical industries are produced

from solution. KDP and ADP are grown from aqueous solution, quartz hydro-thermally,

magnetic bubble domain materials from fluxes and IIIV alloys from molten metals.

Aqueous solution growth has produced the largest crystals known to man. Buckley

mentions that Bounds has produced the world’s biggest artificial crystals (e.g.) an

Octahedron of alum weighing 240 Ib. This crystal is said to have been taken over three

years to grow and was turned over in its bath daily to ensure reasonable symmetry of its

growth habit.

With other solvents, fluxes and metals particularly, liquid phase epitaxy (LPE) is

a technique of major importance. Here, by contrast a layer only a few microns thick, is

grown on to the parent crystals slice. Epitaxial growth of IIIV compounds and alloys is

an important production tool for devices like light-emitting diodes (LEDs).

1.8 VAPOUR PHASE GROWTH

Sublimation (e.g.) of sulphur, was practiced by the archemists and subsequent

chemists more as a purification process than a crystal growth method. This is the

simplest and the only pure vapour growth method. It has been used in recent decades for

the production of high quality bulk crystals of materials like CdS and HgI 2 in which all

elements present are volatile at readily attainable temperatures. I has also led to vacuum

evaporation and the exciting a new technique of MBE (Molecular Beam Epitaxy).

Which is because of its growing importance for integrated optics and special semi-

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conductor devices? Piper and Polich and subsequently others have used elaborate

sublimation methods for Cds. Fig. 1.9. illustrates the principles. ZnS, CdI2 and other

volatile materials may be grown as large single crystals by careful use in this method of

which there are several variants, but indicated above, it is not of wide application. It also

suffers from the disadvantage of being a closed tube method. Also developed during

sublimating was the temperature oscillation method (TOM) pioneered by Schultz in

Germany, this Pendelofen techniques has been applied particularly to HgI2.

Fig .1.9. Growth by sublimation method

It has two variants either Periodic Oscillation of the Source Temperature (POST),

Periodic Oscillation of the Crystal Temperature (POCT). The periodic temperature

oscillation is designed ideally to cause and keep a single nucleation. Minor secondary

nucleations evaporate during the heating cycle. Schultz has tried linear and radial

temperature gradients. There is currently considerable discussion of the merits of

temperature oscillation and "melt back" in crystal growth generally, although current

practice in most growth methods favours as steady temperature as possible. Contrasting

with these physical vapour deposition methods are various chemical methods. This,

which has been called "impure vapour growth”, is to sublimation as solution growth is to

melt growth. Other chemical elements are present besides those in the wanted crystal.

These are divided into halogen transport methods and gas phase reactions. Another

useful division is into open tube and closed tube methods. Usually sublimation and

halogen transport are done in closed tubes and CVD in open tubes, which have an

obvious commercial advantage.

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Chemical transport methods are widely used in the study of new semi-

conducting, insulating and magnetic crystals. They were pioneered by Schafer and

Nitsche and are widely used in closed tube arrangements, and are discussed by Kaldis.

For e.g. small crystals of most adamantine compounds may be grown in sealed quartz

ampoules when about 5mg cc-1 of I2 are added to a powdered charge of the compound in

vacuum, crystals grow at the cold end of the tube. Crystals of CuGaSnSe4 and Cu2GeSe3

have recently been grown in this apparatus with pendelofen of the crystals showing that

it is possible to have simultaneous transport of three non-volatile metals.

The functions of the iodine is to transport the non-volatile metal atoms by

formation of the volatile iodide at the low temperature end and its decomposition in the

higher temperature growth zone of a directional of a two-zone furnace. The tube should

be tilted by some 10o to enhance convection, which is the principal feed mechanism.

Most III-V, II-VI, IIII-VI2 and III-VV2 compounds can be grown this way, among many

other examples. Solid solution material may also be grown as in our work and that of

Yamamoto. Iodine is not the only transporting agent used for non-volatile metals.

Fig 1.10 Iodine vapours transport of components containing non-volatile metals

These elements usually have volatile chlorides, fluorides and bromides too. HCl

and Cl2 were used recently to prepare single crystal sample of U1-x ThxO2 solid solution.

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These closed tube methods are very useful in research but too costly for production,

where gas phase reactions, open tube methods are preferred. Several, like the Silane

process for epitaxial silicon or the gas phase reaction between GaCl3 and As4 to give

epitaxial GaAs are of considerable commercial importance.

1.9 CHOOSING A CRYSTAL GROWTH METHOD

For bulk growth of high quality single crystal material seeded melt growth (eg.

crystal pulling (or) float zone melting) is undoubtedly the best method available today

for congruently melting materials. As in the cases of Si, GaAs and GGG it is fast,

efficient and can be automated, and it produces the most perfect crystals possible. The

use of micro-gravity conditions of space processing, are exciting for the future. Since

the elimination of gravity induced problems could compensate for the increased expense

especially if the products are to be used in space. When a material is wanted in thin film

form with accurately controlled doping, composition and quality on available substrate

material as in the semiconductor and photonics industries (eg. III-V alloys), vapour

deposition, especially CVD or MBE is an appropriate method, although sometimes

solution growth is preferred.

In other cases solution growth is the next best choice. Using a seed crystal if

possible. Failing this chemical transport is often a good method, particularly if

Pendelofen is used to limit nucleations. This is particularly a useful method of obtaining

small crystals of new materials for scientific research. Sometimes, as with biological

crystal growth, and gel growth, the conditions cannot readily be chosen, they are fixed by

the system [Hopper R.M., et al., 1980].

1.10 THREE-DIMENSIONAL LATTICE TYPES

In two dimensions the point groups are associated with five different types of

lattices. In three dimensions the point symmetry groups require the 14 different (One

general and 13 special) lattice types. The general lattice type is the triclinic lattice.

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Fig.1.11.Crystal Lattices in three dimensions

The 14 lattice types are of conveniently grouped into seven systems according to

the seven types of conventional unit cells: triclinic, monocline, orthorhombic, tetragonal,

cubic, trigonal, and hexagonal. The division into systems is summarized in terms of the

special axial relations for the conventional unit cells. The axes a, b, c and called the

lattice parameters. The cells shown in Fig.1.11 are the conventional cells, and they are

not always primitive cells. Sometimes a non-primitive cell has a more obvious

connection with the point symmetry elements than has a primitive cell.

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In the cubic system there are three lattices: the simple cubic (sc) lattice, the

body-centered cubic (bcc) lattice, and the face-centered cubic (fcc) lattice. A primitive

cell of the body-centered cubic lattice is shown in Fig.1.11. The primitive translation

vectors of the face-centered cubic lattice are shown in Fig.1.11. The primitive cells

contain only one lattice point, but the conventional cubic cells contain two lattice points

(bcc) or four lattice points (fcc). The positions of a point in a cell is specified by (3) in

items of atomic coordinates x, y, z in which each coordinate is a fraction of the axial

length a, b, or c in the direction of the coordinate, with the origin taken at a corner of a

cell. Thus the coordinates of the body center of a cell are ½½½. The face-centers include

½½0; 0½½; ½0½. The coordinates of atoms in fcc and bcc lattices are usually given in

items of the conventional cubic cell [Charles Kittel, 1987].

1.11 BRAGG LAW

Fig.1.12 Schematic diagram of Bragg’s law

W.L. Bragg presented a simple explanation of the different bear from a crystal.

Suppose that the incident waves are reflected from specular parallel planes of atoms in

the crystal, with each plane reflector only a very small fraction of the radian, like a

lightly silvered mired. The diffracted beams are found when the reflections from parallel

plan of atoms interfere constructively, as in Fig.1.12. We treat elastic scattering, which

the energy of the x-ray is not changed on reflection. Inelastic scattering, with the

excitation of elastic waves, is discussed at the end of the chapter.

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Consider parallel lattice planes spaced apart. The radiation was incident in the

plane of the paper. The path difference for rays reflective from adjacent interference of

the radiation from successive planes occurs when the path difference is an integral

number n of wavelengths , i.e. 2d sin = n.

This is the Bragg law. Although the reflection from each plane is specular for

only certain values of will the reflections from all parallel planes as up in phase to give

a strong reflected beam. In each perfectly reflecting plane, only the first plane of a

parallel set would see the radiation and wavelength would be reflected. But each plane

reflected 10-3 to 10-5 of the incident radiation.

The Bragg law is a consequence of the periodicity of the lattice. The law does not

refer to the arrangement of atoms in the basis associated with each lattice point. The

composition of the basis determines the relative intensity of the various orders n of

diffraction from a given set of parallel planes. Bragg reflection can occur only

wavelength 2d. This is why we cannot use visible light.

1.12. EXPERIMENTAL DIFFRACTION METHODS

The Bragg law requires that and be matched; monochromatic x-rays of

wavelength striking a three-dimensional crystal at an arbitrary angle of incidence will

not in general be reflected. To satisfy the Bragg law requires an accident, and to create

the accident it is necessary to scan in either wavelength or angle. The standard methods

of diffraction used in crystal structure analysis are designed expressly to accomplish this.

We describe three simple, order methods, still used by physicist; but for professional

crystallography these techniques have been replaced by complicated precession camera

methods.

1.13. LAUE METHOD

In the Laue method (Fig.1.13), a single crystal is stationary in a beam of x-ray or

neutron radiation of continuous wavelength. The crystal selects and diffracts the discrete

values of for which planes exist of spacing d and incidence angle satisfying the

Bragg law. A source is used that produces a beam of x-rays over a wide range of

20

wavelengths, perhaps from 0.2 Ǻ to 2Ǻ. A pinhole arrangement produces a well-

collimated beam. The dimensions of the single-crystal specimen need not be greater than

1 mm. Flat film receives the diffracted beams. The diffraction pattern consists of a series

of sports. The pattern will show the symmetry of the crystal; if a crystal has a fourfold

axis of symmetry parallel to the beam, the Laue pattern will show fourfold symmetry.

The Laue method is widely used to orient crystals for solid state experiments [Banwel,

C.N., 1996].

Fig.1.13.Crystal structures are determined experimentally by X-Ray diffraction

1.14. ROTATING-CRYSTAL METHOD

In the rotating-crystal method, a single crystal is rotated about a fixed axis in a

beam of monoenergetic x-rays or neutrons. The variation in the angle brings different

atomic planes into position for reflection. The film is mounted in a cylindrical holder

concentric with a rotating spindle crystal mount. The incident x-ray beam is

monochromatized by a filter or by reflection from an earlier crystal. The beam is

diffracted from a given crystal plane when in the course of rotation the value of

satisfies the Bragg equation. Beams from all planes parallel to the vertical rotation axis

will lie in the horizontal plane. Planes with other orientations will reflect in layers above

and below the horizontal plane. The intensity distribution of the radiation from a 30 keV

x-ray tube with a molybdenum target and the distribution of neutrons are emerging from

a nuclear reactor. If we reflect the beam from a monochromatic crystal, we get the

crosshatched distribution.

21

Several variations are common use. In oscillating-crystal photographs the crystal

is oscillated through a limited angular range, instead of being rotated through 3600. The

limited range reduces the possibility of overlapping reflections. The precession camera

developed by M. J. Buerger gives a picture of the various levels of the reciprocal lattice.

Modern diffractometers use scintillation counters or proportional counter tubes to data,

needed because complex structures may exhibit 10,000 diffracted rays.

Nearly all crystals with simple structures were solved by x-ray analysis long ago.

One present center of interest in x-ray structure analysis is in the determination of the

configuration of enzymes with a molecular weight between 10,000 and 100,000. The

crystallization of an enzyme and the subsequent x-ray analysis of the structure of the

crystal is the most effective method for the determination of the shape of the molecule.

The coordinate 500 to 5000 atoms in a cell are wanted, so at least this number of x-ray

reflection lines is required. Computer programs have enormously simplified the problem

of structure determination.

1.15. POWDER METHOD

In the Powder method the incident monochromatic radiation strikes a finely-

powdered specimen or a fine-grained polycrystalline specimen contained in a thin-walled

capillary tube (Fig. 1.14). The distribution of crystallite orientations will be nearly

Fig. 1.14. The Debye-Scherrer Camera

continuous. The powder method is convenient precisely because single crystals are not

required. Diffracted rays go out from individual crystallites that happen to be oriented

with planes making an incident angle with the beam satisfying the Bragg equation.

22

Diffracted rays leave the specimen along the generators of cones concentric with the

original beam. The generators make an angle of 2 with the direction of the original

beam, where is the Bragg angle. The cones intercept the film in a series of concentric

rings. (Fig. 1.15).

Fig. 1.15. Photographic film after developing

1.16. FOURIER TRANSFORM INFRARED SPECTROSCOPY

Fig. 1.16. Schematic diagram of a Fourier transform infra-red spectrometer

Infra-red spectroscopy extends outside the limits we have discussed so far in this

chapter, and in particular a good deal of useful molecular information is contained in

spectra below 400cm-1, i.e. the far infra-red region, from about 400cm-1 to 20cm-1 or

10cm-1. Because sources are weak and detectors insensitive, this region is known as

23

‘energy-limited’ and difficulty is experienced in obtaining good signal-to-noise ratios by

conventional means. The advent of Fourier transform spectroscopy has made the far

infra-red much more accessible, and has considerably speeded and improved

spectroscopy in the infra-red region in general.

In this region Fourier transform (FT) methods are used in absorption. The

apparatus derives from the classical attempt by Michelson to measure the ‘ether wind’ by

determining the velocity of light in two perpendicular directions. A parallel beam of

radiation is directed from the source to the interferometer, consisting of a beam splitter B

and two mirrors M1 and M2 (Fig. 1.16). The beam splitter is a plate of suitable

transparent material (e.g. potassium bromide) coated so as to reflect just 50 per cent of

the radiation falling on it. Thus half the radiation goes to M1 and half to M2, returns from

both these mirrors along the same path, and is then recombined to a single beam at the

beam splitter (clearly half the total radiation is sent back to the source, but this is

immaterial).

It is well known (and the essence of the Michelson experiment) that if

monochromatic radiation is emitted by the source, the recombined beam leaving B shows

constructive or destructive interference, depending on the relative path lengths B to M1

and B to M2. Thus if the path lengths are identical or differ by an integral number of

wavelengths, constructive interference gives a bright beam leaving B, whereas if the

difference is a half-integral number of wavelengths, the beams cancel at B. As the mirror

M2 is moved smoothly towards or away from B, therefore, a detector sees radiation

alternating in intensity. It is fairly easy to imagine that if the source emits two separate

monochromatic frequencies, v1 and v2, then the interference pattern (beat pattern) of v1

and v2 would overly the interface caused by M1 and M2; the detector would see a more

complicated intensity fluctuation as M2 is moved, but computing the Fourier Transform

of the resultant signal is a very rapid way of obtaining the original frequencies and

intensities emitted by the source. Taking the process further, even ‘white’ radiation

emitted by the source produces an interference pattern, which can be transformed back to

the original frequency distribution [Nakamoto, K., 1995].

A typical interference pattern or interferogram for a ‘white’ source, where the

wide range of frequencies causes a rapid diminishing of signal away from the position at

24

which both mirrors are at an equal distance from the beam splitter (the so-called zero

retardation peak). No real source is truly white. The variation in total intensity caused by

varying source output and beam splitter efficiency across the IR range for a typical FT

spectrometer. Since FT infra-red spectroscopy is carried out as a single-beam technique,

this background variation must be taken into account for each spectrum. If the beam

from such a source is directed through a sample before reaching the detector, sample

absorptions cause gaps in the frequency distribution which, after transformation, will

appear as down-going peaks.

The production of a spectrum, then, is a two-stage process, which may be thought

of as follows. Firstly, without a sample in the beam, mirror M2 is moved smoothly over a

period of time (e.g. one second) through a distance of about 1cm, while the detector

signal-the interferogram is collected into a multi-channel computer (it may be, for

instance, that the detector signal is monitored every thousandth of a second during the

mirror traverse, and each piece of information put serially into one of a thousand

different storage locations in the computer); the computer carries out the Fourier

Transformation of the stored data to produce the background spectrum. Secondly, a

sample interferogram is recorded in exactly the same way, Fourier transformed, and then

ratioed against the background spectrum for plotting as a transmittance spectrum.

Alternatively, the sample and background spectra may each be calculated in absorbance

forms and the latter simply subtracted from the former to give an absorbance spectrum of

the sample alone.

25

CHAPTER – 2

NONLINEAR OPTICS

2.1 INTRODUCTION

Optics is already preferred for many applications owing to its wide bandwidth

and autonomy from electromagnetic interference. Non-linear optical process are useful

in optical communications and signal processing, laser surgery, parallel image

processing and integrated optics.

Non-linear optics (NLO) gained importance with the advent of lasers followed by

a demonstration of harmonic generation in quarts by Franken. It deals with interaction of

intense electromagnetic fields in a suitable medium, producing magnified fields that are

different from the input field in frequency, phase or amplitude. NLO is essentially a

material phenomenon and the usual non-linear medium is a crystal. Hence search for

new non linear materials with superior properties acquired importance in recent years.

Fig. 2.1. Schematic diagram showing the arrangement forSHG in quartz crystal

2.2 HARMONIC GENERATION

Franken and coworkers observed ultraviolet light at twice the frequency of a ruby

laser light, when the light was made to traverse a quartz crystal. This experiment

attracted widespread attention and marked the beginning of the experimental and

theoretical investigation of nonlinear optical properties. A simplest scheme for this

experiment is shown in Fig.2.1. A ruby laser beam, with average power of the order of

10kW is focused on a quartz slab. The transmitted light then was passed through a filter,

26

Quartz Crystal

( = 694.3 nm)

2 ( = 347.15 nm)

Incident light Ruby laser

= 694.3 nm

which cuts off the red light and allows uv light to pass through. The emerging light was

incident on a photocell. Radiation with wavelength =3471Å and the power of the order

of 1mW was observed in the transmitted light.

In a electric material the influence of all electric field causes distortion in the

spatial distribution between the electrons and nucleus. These distortions cause electric

dipole which in turn, manifest as polarization. At very low fields induced polarization is

directly proportional to the electric field [Laud, B.B., 2001] .

P = 0 (1)E ………. (2.1)

Where,

(1) is the linear susceptibility of the material.

E is the electric field vector

0 is the permittivity of the free space.

However at intense electric fields, polarization becomes independent of the field

but the susceptibility becomes field dependent. The induced polarization is capable of

multiplying the fundamentals frequency to second, third order and even higher

harmonics. The reradiation from the oscillating dipoles differs in amplitude with respect

to the incident sinusoidal electric field. As a consequence, the distorted reradiated waves

contain different frequencies from that of the incident wave.

The induced polarization is now described by the relation

P = 0 ( (1)E + (2)E + (3)E + ….) ……. (2.2)

where (1), (2), (3), . . .are nonlinear susceptibilities of the medium. If the field is low,

as it is the case of ordinary light sources, only the first term of the above equation can be

retained. It is for this reason that the prelaser optics is known as linear optics. The

medium of which the polarization is described by a nonlinear relation of the type (13.3)

is called a “nonlinear medium”.

27

To realize the non linear effect a suitable medium is required. A crystal which

exhibits the following properties is required for non linear device fabrication,

High effective non linear optical coefficient

Good optical quality

Wide transparency region

Good mechanical and chemical stability

Low absorption

Easy device fabrication

2.3 SECOND HARMONIC GENERATION

A polarization oscillating at frequency 2 radiates an electromagnetic wave of

the same frequency, which propagates with the same velocity as that of the incident

wave. The wave, thus, produced, has the same characteristics of directionality and

monochromacity as the incident wave and is emitted in the same direction. This

phenomenon is known as the second Harmonic Generation (SHG). In most crystalline

materials, the nonlinear polarizability (2) depends on the direction of propagation,

polarization of the electric field and the orientation of the optic axis of the crystal. Since

in such crystalline materials the vectors P and E are not necessarily parallel the

coefficients must be treated as tensors. The second order polarization, therefore, may

be represented by the relation of the type

Pi(2) = 0 (2)

ijk Ej Ek ………. (2.3)

where i,j,k represent the coordinated x,y,z,. Most of the coefficients ijk however, are

usually zero and we have to deal only with one or two components. The second

harmonic generation represented by (Eq. 2.3) occurs only in certain type of crystals.

Consider, for example, a crystal that is isotropic. In this case ijk is independent of

direction and, hence, is a constant. If we now reverse, the direction of the axis (x-x,

y-y, z-z) leaving electric field and dipole moment unchanged in direction, the sign of

these two must change. Second harmonic generation, therefore, cannot occur in an

isotropic medium such as liquids or gases nor in centro-symmetric crystals. In the case

of non-centro-symmetric materials (e.g. anisotropic crystals, such as uniaxial crystals)

28

both the quadratic and cubic terms are present. However, generally, the cubic term is

substantially smaller than the second order term and may be ignored [Laud, B.B., 2001].

For such materials, we can write

P = 0 ((1) E + (2) E.E) ………. (2.4)

2.4 PHASE MATCHING

It was observed that the efficiency of the generation of harmonics depends not

only on the intensity of the exciting radiation, but also on its direction of propagating in

crystals. Suppose a plane wave at frequency and the second harmonic wave at

frequency 2 driven by it are propagating along the z-direction through a material of

length L. The amount of second harmonic radiation produced within a slab of width dz

located at z will be proportional to the width and to the second harmonic dipole moment

per unit volume induced at frequency 2, i.e., P2(z) which, in turn, is proportional to the

square of the electric field E.

The field of the second harmonic generation will be maximum when,

L = / (2k1 - k2) = / 4 ( - 2) ………. (2.5)

The magnitude of L given by above equation is called the coherence length for

the second harmonic radiation. The expression for the intensity of SHG at the exit

surface of the material of length L is given as follows.

Sin2 ((2k1 - k2)/2) LI ………. (2.6)

((2k1 - k2)/2)2

is maximum when ((2k1 - k2)/2) L = 0 ………. (2.7)

i.e. k2 = 2k1 ………. (2.8)

For efficient frequency doubling, this relation must be satisfied. This

requirement is known as phase-matching criterion.

29

Since k2 = 22 / c and k1 = / c the above equation reduces to

2 =

Fig 2.2 Indicatrix for a Negative uniaxial crystal

Now, the phase matching criterion becomes a refractive-index criterion. In a

negative uniaxial crystal, i.e., a crystal for which the refractive index for the ordinary ray

is greater than that for the extraordinary ray. Fig.2.2 shows a section through the

refractive index surfaces (indicatrix) for one such crystal. The dotted curve represents

the surface corresponding to the frequency 2 and the solid curve for frequency . OX is

the optic axis of the crystal. The refractive index surface of the ordinary wave and that

for the extraordinary wave intersect at A. This means, that for the waves propagating in

the direction OA

0 () = e (2) ………. (2.9)

That is, the incident and the second harmonic waves propagating in this direction are

phase matched. The importance of second harmonic generation lies in the fact that

30

it is one of the principal methods of effective conversion of infrared radiation into visible

and visible into ultraviolet.

2.5 THIRD HARMONIC GENERATION

The induced polarization in the material, in the case of centrosymmetric

materials, will lack terms in even powers of E and it will have the form in vector

notation.

P = 0 ((1) E + (3) E.E.E + …) ………. (2.10)

Third harmonic generation (THG) is, therefore, possible in crystals that exhibit

inversion symmetry. The development of Q-switched lasers had made it possible to

generate third harmonic in crystals. However, the energy conversion efficiency in such

cases is very low. For example, in calcite the maximum energy conversion efficiency in

the third harmonic was 0.01%.

Experiments for observation of the third harmonic were also performed by Maker

and Terhune using giant pulse lasers. Zwernemann and Beeker have observed

experimentally the enhancement of third harmonic generation (THG) at 9.33m in CO

by having the interaction take place in a waveguide. They have presented a theoretical

determination of the most suitable waveguide in which the interaction can take place.

2.6 OPTICAL MIXING

It is proved that a no polarization term may result from the interaction of two

fields with different frequencies and we can express

20 (2) E1 E2 cos1 t cos2 t = 0 (2) E1 E2 (cos (1 + 2 )t + cos (1 - 2 )t)

………. (2.11)

This shows that the nonlinear polarization and, therefore, emitted radiation

contains frequencies 1+2 and 1-2. The energy conversion between the beams can

take place over significant distances only if the beams travel in the same direction and at

the same velocity. The sum and difference frequencies can be observed experimentally.

Generation of difference optical frequencies was first observed by mixing a beam from a

31

ruby laser with an incoherent beam of a mercury lamp. The efficiency with which the

difference frequency was generated was negligible. Here 1 - 2 may fall in to the range

of acoustic frequencies. Therefore, frequency mixing is an optical method of generating

ultrasonic waves.

As in the case of second harmonic generation, phase-matching condition is also

important in frequency mixing. In fact, it is more stringent in the latter case, because of

the number of frequencies involved. In the case of second harmonic generation, it is

necessary to find a direction in crystals such that k1=k2. In the case of sum or difference

frequencies, three waves must be matched. If

3 = 1 2 ………. (2.12)

the condition to be satisfied is k3 = k1 k2 ……… (2.13)

2.7. Parametric Generation of Light

In electronics, parametric phenomena occur in circuits involving nonlinear

capacitors. Similar processes occur in optics when nonlinear crystals are used as

parametric media. The process is known as parametric generation of light, and is based

on “optical mixing” discussed in the preceding section. If a powerful signal at frequency

p(pump frequency) is applied to a parametric medium and a small signal at frequency

s (signal frequency) is introduced at one end. The fields at the original frequencies are

regarded as fixed parameters. “Mixing” of the signal and the pump frequency may result

into a secondary wave at frequency i given by

i = P - S ………. (2.14)

which is known as “idler” frequency. The corresponding field strength being

proportional to EPES = Ei

In view of the nonlinear properties of the medium, further mixing may occur. In

particular, the field generated by the polarization component oscillating at the idler

frequency and the original pump field, when mixed, would make a contribution to the

signal field. Thus

P - i = P - (P - S ) = S ………. (2.15)

32

The strength of the contribution is proportional to

EP Ei = EP2 ES ………. (2.16)

The above equation shows that the strength is proportional to E s in accordance with

the usual requirement for parametric amplification. Thus, the secondary light waves at

frequencies s and i can be excited parametrically at the expense of the part of energy

of the pumping wave. Initial signals required for triggering the process of parametric

generation are always available in any crystal in the form of spontaneous photons.

2.8. Self-Focusing of Light

The refractive index of a nonlinear medium is proportional to the square of the

amplitude of the field, that is, to the intensity. Now the intensity of a laser beam is not

constant over its cross-section. It peaks at the axis of the beam and falls off gradually

away from the axis. The velocity of the light wave is given by =c/. Since decreases

owing to the falling of the intensity of the light beam, the velocity increases with the

distance away from the axis. Consequently, a plane wave-front incident on material

becomes concave as it propagates through the medium and contracts towards the axis

(Fig.2.3). In other words, it self-focusses, after which it propagates as a narrow light

fibre [Laud, B.B., 2001].

Fig.2.3 Self focusing

The distance L0 over which the beam self-focuses can be approximately estimated

using the formula,

Lo = D / (nl Eo2) ………. (2.17)

33

where D is the diameter of the beam. Self-focussing occurs when intensity reaches a

certain limiting value. This threshold value is estimated from the formula

Ithresh = 2 / (l2 nl D2) ………. (2.18)

The formula shows that for higher frequencies and for materials with greater

nonlinear susceptibilities, the threshold intensity is lower.

Experimental investigations in self-focussing have been carried out in liquids:

Carbon disulphide, benzene, acetone, etc. For a beam diameter of 0.5m, the self-

focussing distance, is about 10cm and the observed light fibres were 30 to 50 m, in

diameter. It has been further established that the observed light fibre has still a finer

structure; it consists of a number of still thinner filaments with diameters of about 5m.

Self-focussing, on the whole, is a complicated phenomenon.

2.9. APPLICATION OF NON-LINEAR OPTICAL MATERIALS

During the last few years, nonlinear optics technology has developed at a very

fast pace, in terms of both new materials and new applications. Simultaneously, there is

a new and growing need in the industry and medicine for new kinds of tools with high

precision and capabilities that are being found through the use of nonlinear optics.

Telecommunications are feeling the impact of the global information revolution, and are

in need of the high speed provided by photonics and electro-optic systems just to catch

up with the demand. The applications of non-linear optical materials in the areas are

given below :

Industry

Medicine

Entertainment

Remote sensing and analysis

Basic research

Others, including instrumentation, image recording, inspection, military optical

data storage and Laser and Radar (LIDAR)

Telecommunications

34

Nonlinear optical materials utilize the non-linear dependence of the refractive

index on the applied electric field to produce other frequencies. This results in either

harmonic generation or a frequency shifting. The development of the field was enhanced

in parallel with the introduction of lasers, because the laser beams posses the energy

density necessary to produce nonlinear effects. Today there is a large number of

nonlinear optical materials, for specific wavelengths, with various damage thresholds,

and with various optical characteristics [Nalwa, H et al., 1997].

35

CHAPTER – 3

SOLUTION GROWTH

3.1 INTRODUCTION

In this chapter, we are going to discuss the apparatus and conditions for the

growth of materials of moderate to high solubility in the temperature range ambient to

353K at atmospheric pressure. The main advantage of crystallization from solution at

low temperature range in the proximity to ambient temperature and the degree of control,

which can be exercised over the growth conditions. i.e., supersaturation can be precisely

controlled by stabilizing the temperature at 0.01K to 0.001K. These factors and the ease

of agitation of the growing crystal and solution reduce fluctuations of all kinds to a

minimum. Also the proximity to ambient temperature reduces the possibility of major

thermal shock to the crystal both during growth and on removal from the apparatus. This

low temperature solution growth technique is particularly suited to those materials,

which suffer from decomposition in the melt or in the solid at high temperatures and

which undergo phase transformation above the present working range. There are number

of organic and inorganic materials which fall within the categories. This method also

permits the preparation of a variety of different morphologies and polymorphic forms of

the same substance by variation of growth conditions or variation of solvent.

3.2 BASIC REQUIREMENTS

The fundamental requirement for the crystal growth is the availability of pure

materials. In our case, i.e. solution growth, it is more easy to succeed when attempting to

grow crystals from relatively impure materials than in the case of other growth methods,

with reasonable, careful, pre-purification, such as crystallization, sublimation, and zone

refining of the solute and similar treatment, coupled with fractional distillation and

spinning band distillation of the solvent, materials of high and well-defined materials, the

prime essential is the choice of a suitable solvent.

3.2.1 Choice of suitable solvent

The ideal solvent should

1) yield a prismatic habit in the crystal,

2) high solute solubility,

36

3) high positive temperature co-efficient of solute solubility,

4) Low volatility

5) Density less than that of the bulk solute,

6) Low viscosity.

The last two factors simplify apparatus design, since it is desirable that the

growing crystal should not float and that it should be well agitated. The use of low

volatile solvents reduce the possibility of uncontrolled loss of solvent during lengthy

growth periods. The first three properties are the most important characteristics [Bordui,

P., 1987].

3.2.2 Crystal habit

The most useful crystals are those, which grow at approximately equivalent rates

in all dimensions. Growth of this type results in a large bulk of material from which can

be cut into samples of any desired orientation. If dislocations and other defects

propagate, they do so from the nucleus or seed along specific directions in the bulk of the

crystal. If the crystal grows with a bulky habit, these imperfections usually become

isolated into defective regions surrounded by large volumes of high perfection. In needle

like or plate-like crystals the growth dislocations follow the principal growth directions

and the crystal remains dominantly imperfect. The situation is obviously worse for the

needle-like specimens. Where the defects continuously propagate into the whole of the

crystal bulk.

3.2.3 Solubility

Having selected solvents which produce a suitable habit, the next problem is to

determine whether they will yield a satisfactory rate of growth. The principal

experimental factors involved in this decision are the solubility of the material in the

solvent and its temperature dependence. The former governs the amount of material

which is available for growth and hence defines the total size limit. Both factors define

the supersaturation which is the driving force which governs the rate of crystal growth.

37

3.2.4 Supersaturation

The super saturation = C / Co ………. (3.1)

where,

Co - the equilibrium concentration of solute at the temperature of growth

and C - the increment by which the true concentration exceeds this.

The saturation can be supersaturated in a number of ways. The most commonly

used method is to lower the temperature of the solution below the equilibrium saturation

temperature. If this is done continuously at a controlled rate (temperature lowering

method), C is determined by the rate of lowering of the temperature. Linear variations

will yield constant values of C over small temperature intervals, provided that the

solubility - temperature curve is not changing slope too rapidly. Even if this is not the

case, it is relatively easy task to match the temperature lowering rate to the shape of the

solubility curve and hence to achieve a constant supersaturation over a wider temperature

range. This is probably the most versatile and easy method to operate.

If the temperature is suddenly lowered, C is the difference between the two

equilibrium values. C and consequently the rate of growth will decrease as growth

progresses. This is not desirable and can be overcome by constantly replenishing the

solution saturated at the upper temperature as growth proceeds at constant temperature

differential method. Though this method is more complicated than the previous method

it can be more appropriate under some circumstances and it does provide for growth

under rigidity constant temperature at supersaturation conditions.

When neither temperature lowering method nor constant temperature differential

method is appropriate an alternate method is to allow the slow evaporation of the solvent

at constant temperature (Solvent evaporation technique]. In practice, this is a difficult

method. The inevitable fluctuations will lead to poor quality growth.

The choice of method of supersaturation is, to a large extent, dependent on the

shape of the solubility curve and the magnitude of the solubility. So we need accurate

solubility - temperature data. The solubility curve in the Fig. 3.1. represents the rapid

change in solubility with temperature. Each region (A, B, C, D and E) may, however, be

38

typical of the behaviour of any particular solvent in the temperature range, which we are

considering. Let us assume that, the region B, which we would define as being of

moderate to high solubility and moderate solubility - temperature gradient is a

satisfactory region in which, we can work with the temperature lowering method.

Fig.3.1 Generalized solubility curve

In the solubility range 200-1000gm solute per 1000gm solvent and the ratio of

solubility temperature gradient to solubility (solubility ratio) lies in the range 0.03 - 0.01,

then excellent crystals can be grown at temperature lowering rates of 0.5o to 1oK per day,

i.e. at supersaturation 2%. This implies a certain temperature precision (0.005K) to

prevent bursts of rapid and uncontrolled growth, which leads to imperfection in the

resulting crystals.

When the solubility gradient and solubility exceeds this range, e.g. at A, the

demands on the precision of the system become too great, (i.e. at 0.001K). Minor

fluctuations in temperature yield large fluctuations in solubility, supersaturation and

growth rate. This precision can be achieved by resorting to the constant temperature

differential method where the temperatures of both source and growing crystal are

maintained at precisely controlled constant values.

It is not desirable to operate in a region of rapidly changing solubility gradient

such as at C in Fig. 3.1. Constant growth conditions are different to define for any but

short periods of time. In region D, the gradient is shallow and C attainable either by the

temperature lowering technique or by constant temperature differential technique is

significantly reduced. Consequently, growth rates will be low and minor fluctuations

39

would again interfere considerably with growth parameters. Of these two methods, the

latter could be more suitable where solubility remains high. It is necessary to resort to

solvent evaporation to yield reasonable supersaturations. The solvent evaporation

method is at its best where the overall solubility is low. At low solubilities the inevitable

fluctuations yield only small fluctuation in supersaturation and better results are ensured.

3.2.5 EXPRESSION OF SUPERSATURATION

In order to grow crystals, the solution must be saturated; the concentration of the

solution is more than the equilibrium concentration. Supersaturation is the driving force,

which governs the rate of crystal growth. The supersaturation of a system may be

expressed in a number of ways. The degree of supersaturation of a solution is defined

using the concept of absolute supersaturation.

= C - Co ………. (3.2)

where C is the concentration of the dissolved substance at a given moment and Co is its

solubility limit. The degree of supersaturation can also be defined as the relative

supersaturation.

C - Co = = ………. (3.3)

Co Co

or as the coefficient of supersaturation

C = = + 1 = + 1 ………. (3.4)

Co Co

The quantities , and are obviously inter related (Khamshii, 1969).

A typical solubility diagram is shown in Fig. 3.2. The whole concentration-

temperature field is separated by the saturated-solution line (solubility curve) into two

regions; unsaturated and supersaturated solutions. Saturated solutions are those

mixtures, which can retain their equilibrium indefinitely in contact with the solid phase

with respect to which they are saturated. The solubility of most substances increases

with temperature (the temperature coefficient of the solubility is positive). Crystals can

40

be grown only from supersaturated solutions, which contain an excess of the solute

above the equilibrium value. The region of supersaturated solutions can be divided into

two sub-regions, metastable (stable) and labile (unstable) zones.

Figure 3.2 Solubility diagram showing different levels of saturation

Nucleation will occur spontaneously in the labile zone. Metastable zone refers to

the level of supersaturation where spontaneous nucleation cannot occur and a seed

crystal is essential to facilitate growth.

3.3 Methods of crystallization

Low temperature solution growth can be subdivided into the following methods.

i) Slow cooling method

ii) Slow evaporation method

iii) Temperature gradient method

3.3.1 Crystallization by slow cooling of solutions

This is the best method among others to grow bulk single crystals from solution.

In this method, supersaturation is produced by a change in temperature usually

throughout the whole crystallizer. The crystallization process is carried out in such a

41

Co

n c

e n

t r

at

io

n

Temperature

Labile C"

C'

Metastable

B" B'

ABCStable

BB' - Solubility curveAB"C" -

Evaporation andcooling

CC' - Super solubility curve

way that the point on the temperature dependence of the concentration moves into the

metastable region along the saturation curve in the direction of lower solubility. Since

the volume of the crystallizer is finite and the amount of substance placed in it is limited,

the supersaturation requires systematic cooling. It is achieved by using a thermostated

crystallizer and volume of the crystallizer is selected based on the desired size of the

crystals and the temperature dependence of the solubility of the substance. The

temperature at which such crystallization can begin is usually within the range 45-75oC

and the lower limit of cooling is the room temperature [Meirs, H.A. et al., 1987].

3.3.2 Crystallization by solvent evaporation

In this method, an excess of a given solute is established by utilizing the

difference between the rates of evaporation of the solvent and the solute. In contrast to

the cooling method, in which the total mass of the system remains constant, in the

solvent evaporation method, the solution loses particles, which are weakly bound to

other components, and, therefore, the volume of the solution decreases. In almost all

cases, the vapour pressure of the solvent evaporates more rapidly and the solution

becomes supersaturated. Usually, it is sufficient to allow the vapour formed above the

solution to escape freely into the atmosphere. This is the oldest method of crystal growth

and technically, it is very simple. Typical growth conditions involve temperature

stabilization to about 0.005oC and rates of evaporation of a few mm3/hr [Pamplin,

B.R., 1979].

3.3.3 Temperature gradient method

This method involves the transport of the materials from a hot region containing

the source material to be grown to a cooler region where the solution is suspended and

the crystal grows. The main advantages of the method are that

a. crystal grows at fixed temperature

b. this method is insensitive to changes in temperature, provided both the source

and the growing crystal undergo the same change

c. economy of solvent and solute

On the other hand, changes in the small temperature difference between the

source and the crystal zones have a large effect on the growth rate.

42

In production systems, the cool growth zone is separated from the hot saturator

and the solution is pumped from one vessel to the other. Supersaturated solutions tend to

nucleate when pumped, and if the solution saturated at T + T is pumped directly to

growth vessel, undissolved particles are transferred to the growth region. To overcome

such problems crystallizers having three-vessel growth system is normally used.

The temperature in the saturator vessel will be 10oC above the crystallizer and the

solution temperature in the superheater vessel will be much higher than the saturator.

During the growth run the solution flows from superheater vessel to the crystallizer and

then to the saturator and returns to the superheater vessel. The solution pumps fitted in

the saturator and superheater vessels are fitted with filters of size 100m and 0.6m

respectively.

3.3.4 Crystal growth system

The equipment required for the growth of crystals can be broadly classified as the

following :

a. Thermostated water bath with provision for ramping the temperature.

b. Crystallizer with seed holder and stirrer

c. Motor drives for

i. ramping the temperature in the case of thermostatic baths with contact

thermometer.

ii. rotating the seed holder / stirrer.

d. Filtration assembly

e. Crystal cutting, lapping and polishing facilities to fabricate seeds.

3.3.5 Constant temperature bath

Since the temperature affects the driving force of crystallization very much,

highly stable temperature maintenance is essential through out the growth process. To

achieve this, an active thermostating system with fine controlling accuracy is necessary.

A general design of a thermostat with a control system, also known as Constant

Temperature Bath (CTB) includes a thick walled glass chamber filled with water, heating

element, temperature sensor, control relay, temperature indicator, stirrer and illuminating

43

lamp. The long periods necessary to grow crystals and the need to avoid any interruption

of the temperature control process mean that special measures must be taken to ensure

that the control system is reliable.

The details of Constant Temperature Baths normally designed and fabricated in

some laboratories is as follows. Instead of using general purpose water heaters, which

always give temperature fluctuations due to thermal inertia, an optical heating system has

been employed. The system has three 250W Philips infrafil lamps energised through a

relay circuit and the lamps are situated at the bottom of the chamber. The power of these

lamps is controlled by a triac based electronic circuit comprising active and passive

components. The circuit is shown in the Fig. 3.3.

Fig. 3.3 Circuit used in constant temperature bath

Fig.3.4. Basic apparatus for solution growth

44

For initial heating an immersion heater of 500W is fixed in the bottom of the

chamber. The system uses a contact thermometer for temperature control. The desired

temperature is set in the Jumo contact thermometer. The sensor converts the variable

into a suitable signal that can be accepted by the controller / indicator. The controller is

a contactor with an on-off switch. The CTB has provision to set and read the

temperature with an accuracy of 0.01oC by means of three and half segment digital

display. In addition, it has the capacity of controlling the temperature with an accuracy

of 0.01oC in the temperature range from ambient to 100oC. The constant temperature

bath is shown in the Fig. 3.4.

3.3.6 Crystallizer

Criteria in the selection of Crystallizer includes the following :

i. Range of operating temperature

ii. Selection and control of supersaturation levels.

iii. Establishment of defined solution hydrodynamic conditions

iv. Ability to measure linear growth rates

v. Facility to remove crystal after completion of growth cycle and

vi. Long term reliability of mechanical and electronic equipment.

The constant temperature water bath mentioned above is of 20 litre capacity and

easily accommodates a crystallizer of 10 litres of maximum capacity. Hermetic sealing

of crystallizer is an important factor, since secondary nucleation is more probable in low

temperature solution growth methods, which leads to the formation of stray crystals and

they often reduce the growth of the main crystal and create many defects in it. One of

the main reasons for this cause is the contamination of mother solution by foreign

impurities. In addition to that, the exposure of the mother liquor to the atmosphere will

allow the solvent for evaporation, which leads to the formation of stray crystals due to

uncontrolled changes in the concentration of the solution. In this way, the stability of

mother liquor is reduced very much. To avoid this crystallizing vessel should be

completely sealed throughout the growth process.

45

3.3.7 Filtration assembly

The filtration of solutions is necessary to free them from suspended particles.

The presence of impurities may scratch and damage mechanically a rapidly revolving

crystal and they may split off small fragments from a growing crystal, which can give

rise to stray crystals. Moreover, foreign particles may themselves act as growth nuclei.

At the same time, filtration of supersaturated solutions is very difficult because

crystallization starts in the filter and the liquid ceases to pass through it. Thus, a filtered

solution must be poured into another vessel and heated again to dissolve the precipitate.

Therefore, only unsaturated solutions, heated to a temperature 10-15oC above the

saturation temperature, can be filtered. All these process results in excessive loss of the

solute and solvent. It results in considerable change in the concentration of the solution.

A filtration assembly consisting of a peristaltic pump with silicon tubes and a

filter is used to overcome the above difficulties. Peristaltic pump enables the filtration in

a faster rate by pressurizing the solution into the filter. Whatmann filter paper No. 1 and

41 are used for pre-filtration and a nuclear track membrane with a pore size of 0.05m is

used for final filtration.

The nuclear track filter has distinguishing features when compared to the

“classical” type filters produced by chemical methods. They are having the uniformity

of pore size, high selectivity with respect to the component to be extracted, a very low

absorption of components by the membrane surface and biological passivity (non-

pyrogenity). The filter assembly with nuclear track membrane of pore size 0.05 m is

used.

3.3.8 Seed, seed mount platform and crystal revolution unit

Seed crystals are best prepared by slow cooling or slow evaporation of a saturated

solution in a clean, controlled temperature enclosure kept specifically for this purpose.

Alternatively, an excellent source is the base of the flask in the main crystallizer. Small

crystals inevitably form here during the growth of the larger specimens. They are often

of excellent perfection.

46

The seed crystals prepared should be suspended on a smooth support. The use of

a rough support provides many centres on which further nucleation can occur. Growth

of these additional nuclei can often interfere with the growth of the original crystals. In

some cases, this effect can be used to advantage, and the suspension of a smoother thread

(e.g. nylon thread) in the solution can yield a few seed crystals of excellent quality firmly

attached on the line. The crystals, still attached to the thread, can be separated and used

as seed for growth. Alternative methods of suspension are to tie the crystal with wire, to

attach it to the stirring rod.

The forced motion of a solution around a crystal makes it possible to increase

considerably the rate of its growth without deterioration of its optical homogeneity.

Among various types of crystal revolving methods, the one in which the crystal holder

can be rotated clockwise and anti-clockwise about its axis is proved as effective and

giving best results. A seed mount platform can be easily designed and fabricated using

acrylic materials. At the centre point of the platform, a depression of one cubic

centimeter is made to fix the seed of the same dimensions. Fine graduation is made on

the outer bottom surface to observe the expansion of the crystal dimensions during the

growth. Adequate care has to be taken to avoid sharp edges of the acrylic surfaces of the

platform and the axle rod and the components should be well polished without any kinks,

pits essentially to avoid secondary nucleation on them.

Any AC/DC motor can be used to impart rotation to the seed holder. A tape

recorder motor, wiper motor, is some readily available examples. A stepper motor

driven by a microprocessor based programmable controller is preferred. The preference

owes due to the easy maneuverability of the rotation rates; clockwise and anticlockwise

with an interval just by finger touch controls. In addition, microprocessors –

programming controls permits alternation of rotation rates as the crystal grows.

3.4 Scope of the thesis

ZTC is a promising semiorganic NLO material for efficient SHG of Nd: YAG

laser. Some special features of ZTC are:

(i) moderate second harmonic generation conversion efficiency

47

(ii) low UV cut–off

(iii) crystals can be grown in bulk form at optimized growth conditions

and

(iv)good mechanical properties

ZTC crystals have been successfully grown in bulk and different

morphological forms at optimized growth conditions.

(i) growing bulk single crystals of ZTC and ZTCP by slow cooling

technique.

(ii) studying the effect of solution pH on the growth rate and morphology

of ZTC crystals

(iii) studying the effect of phosphate substitution on the growth rate and

morphology of ZTC crystals.

(iv) Powder XRD was taken to study the crystalline quality of grown

crystals ZTC and ZTCP.

(v) FTIR was taken to reveal the functional groups present in the grown

crystals ZTC and ZTCP.

(vi) Vickers micro hardness was taken to study the mechanical strength of

ZTC and ZTCP.

(vii) Kurtz nonlinear test was taken to study the nonlinearity of the grown

crystals ZTC and ZTCP.

48

CHAPTER – 4

GROWTH AND CHARACTERIZATION OF ZTC, ZTCP SINGLE CRYSTALS

4.1. INTRODUCTION

A number of studies on metal coordination compounds of thiourea have been

reported previously. Recently the engineering of a new class of organo–mineral salts

paid much attention to crystals of the M [tu]2 X2 type, where M= Cd, Co, Hg, Pb, Ti and

Zn, tu is thiourea and X is a halogen. These materials have been found to exhibit good

nonlinear optical properties. The idea of combining the inorganic distorted polyhedron

with asymmetric conjugate organic molecules results in new nonlinear optical materials.

Motivated by these considerations, crystals of ZTC were synthesized.

ZTC is an efficient semiorganic nonlinear material for SHG applications.

Vibrational spectroscopic studies of ZTC crystalline powder have been reported. They

have grown the ZTC crystals by slow evaporation technique, and the size of the crystals

were very small (mm). In the present investigation, large size crystals have been

successfully grown by slow cooling method. The influence of pH on the growth

morphology and substitution of phosphate on ZTC has been studied and bulk crystals

have been grown with optimized growth parameters.

4.2. GROWTH OF ZTC CRYSTALS

The impurity content of the synthesized salt was minimized by purifying the

solution by repeated recrystallization processes. Most of the growth runs were carried out

from solution saturated at 45C. Saturated solution at different pH values were prepared

from the solubility data. The saturated solution was further purified by filtering through

glass filter provided with fine pores of size ranging from 4 to 7 μm. Before starting growth,

to ensure homogenization of the solution, it was heated to a few degrees above the

saturation temperature. The temperature was reduced at a rate of 0.02 C to 0.2 C per day

as the growth progressed. The seeds obtained from slow evaporation were employed for the

growth. The seeds were seasoned at the growth temperature before initiating the growth.

For the static growth experiments, the seeds were hung using a nylon thread and allowed to

grow to a considerable size. The periods of growth ranged from 40 to 50 days. After

completion of growth, crystals were harvested. Effect of solution pH on the growth and

49

morphology of ZTC crystal has been studied. Good quality bulk crystals of size 4.5 x 5.5 x

2.0 cm3 were grown with an optimized pH condition (Rajasekaran et al 2003).

4.3. EFFECT OF pH ON THE GROWTH AND MORPHOLOGY OF

ZTC CRYSTALS

In the present study, all the parameters, T, S, and C were kept constant during the

growth and the effect of pH on the growth rate has been studied. To investigate the

influence of solution pH on the morphology and the growth rate of ZTC crystals, growth

experiments were carried out with pH values ranging from 5.3 to 3.0. The crystals grown at

pH values 5.3, 4.0 and 3.0 are shown in Fig. 4.1. Crystal grown at pH 5.3 had inclusions.

The morphology of ZTC crystals grown at different pH values (5.3, 4.0 and 3.0)

are shown in Fig. 4.2. Crystal grown with pH 5.3 possesses 12 faces with elongated a-

axis as indexed in Fig. 4.1a. When the pH is reduced to 4.0, the {010} face disappeared

and {021} and {0 1} became narrow (Fig. 4.1b). When the pH is reduced to 3.0, the

{010} face disappeared completely with a further reduction in growth rate along a-axis

leading to a bulk crystal as shown in Fig. 4.1c. The increase in growth rate along b-axis

is attributed to the formation of chloro-zinc complexes due to the addition of HCl. The

formation of chloro-zinc complexes would decrease the concentration of zinc thiourea

chloride (ZTC) complex.

Fig. 4.1. Grown crystals of zinc thiourea chloride at different pH values:(a) 5.3 (b) 4.0 and (c) 3.0.

50

Fig. 4.2 Morphology of ZTC crystals grown at different pH values

(a) 5.3 (b) 4.0 and (c) 3.0

4.4. GROWTH OF ZTCP CRYSTALS

Since the ionic radii of chlorine and phosphorus are close to each other, it is of

our interest to investigate the effect of partial substitution of chlorine with phosphate in

ZTC (Ushasree et al 1999a). Phosphate substituted ZTC (ZTCP) single crystals have been

grown for different mol % of phosphate (1.0, 2.0 and 3.0). Morphological changes were

observed with increase in phosphate concentration. The crystals grown at different mol %

of phosphate are shown in Fig. 4.3.

(a)

51

(a) pH – 5.3 (b) pH – 4.0 (c) pH – 3.0

(b)

(c)

Figure 4.3 ZTCP crystals grown at different mol % of phosphate (a) 1.0 mol % (b) 2.0 mol % and (c) 3.0 mol %

4.5. CHARACTERIZATION OF ZTC AND ZTCP CRYSTALS

4.5.1 INTRODUCTION

Researchers in material science and device engineers want to know the degree of

purity and perfection of crystals to interpret structure dependent properties in order to

determine whether the material can be successfully employed in the experiments or

device fabrication process. It is also important to know the nature and distribution of the

52

imperfections present in the crystals. Detailed studies of the crystals can provide

information to deduce how the growth techniques should be modified so that the

perfection of the crystal may be increased.

Characterization of the crystal consists of its chemical compositions, structure,

defects and the study; of their electrical, mechanical and optical properties. The

measurement of optical properties includes the study of optical transmission and

absorption of the crystal and SHG conversion efficiency.

Characterization of NLO crystals can be divided into following topics.

1. Structural analysis of a crystal.

2. Measurement of optical properties.

3. Measurement of thermal and electrical properties.

In the present work, the grown crystals are ZTC and ZTCP were subjected to the

following studies.

1. Crystal structure analysis by powder X-ray diffraction techniques.

2. FTIR analysis to reveal the metal complex co-ordination and to conform the

chloride and phosphate in ZTC and ZTCP crystals.

3. Powder SHG studies by Kurtz method to find the SHG conversion efficiency.

4. Micro hardness studies to evaluate the mechanical hardness of the crystals.

4.5.2. Powder X-Ray diffraction analysis

Finely crushed powders of ZTC and ZTCP were subjected to powder X-ray

diffraction analysis using a rich Seifert diffractometer with Cu-K ( = 1.5418 Ao

radiation).

The sample was scanned over the range 10o to 70o at a scan ratio of 1o/minute.

The recorded x-ray spectra of ZTC and ZTCP are shown in Fig. 4.3-4.9. Some

additional peaks, small peak shift and variation of intensity in the peaks were observed

for ZTCP crystals. This is due to the incorporation of phosphate ion in the crystal lattice

of ZTC. The d values and intensity ratio (I/Io) are calculated and is shown in the Tables

4.1 – 4.7.

53

Fig. 4.4. Powder XRD pattern of ZTC (pH 5.3)

54

Table 4.1. Powder XRD of ZTC (pH 5.3)

Sl. No.

2 d = /2Sin Å

I / Io x 100 %

1. 12.6 6.30 7.02515 19.02. 13.0 6.50 6.80988 19.53. 13.7 6.85 6.46346 32.04. 13.9 6.95 6.37091 37.55. 14.6 7.30 6.06699 19.06. 15.4 7.70 5.75358 91.07. 16.5 8.25 5.37240 55.58. 17.6 8.80 5.03903 23.09. 18.3 9.15 4.84783 24.510. 19.6 9.80 4.52912 34.511. 20.3 10.15 4.37450 42.012. 21.5 10.75 4.13298 24.013. 22.4 11.20 3.96892 19.014. 23.0 11.50 3.86672 20.015. 24.1 12.05 3.69267 21.016. 24.6 12.30 3.61873 58.017. 25.5 12.75 3.49302 32.518. 26.1 13.05 3.41406 22.019. 26.8 13.40 3.32646 29.020. 27.5 13.75 3.24336 100.021. 28.0 14.00 3.18657 28.022. 29.4 14.70 3.03793 29.423. 30.3 15.15 2.94972 34.024. 21.4 10.70 4.15207 30.025. 32.2 16.10 2.77987 21.026. 32.8 16.40 2.73038 23.027. 33.1 16.55 2.70632 23.028. 33.6 16.80 2.66718 24.029. 34.3 17.15 2.61433 23.030. 34.8 17.40 2.57791 31.031. 36.0 18.00 2.49468 18.032. 36.5 18.25 2.46165 22.033. 36.9 18.45 2.43588 20.034. 37.9 18.95 2.37388 27.035. 38.3 19.15 2.35000 27.536. 39.1 19.55 2.30374 20.037. 39.7 19.85 2.27030 23.538. 40.7 20.35 2.21680 17.539. 41.5 20.75 2.17589 18.540. 44.3 22.15 2.04465 25.041. 47.7 23.85 1.90655 20.042. 49.6 24.80 1.83787 21.543. 50.9 25.45 1.79395 25.0

55

Fig. 4.5. Powder XRD pattern of ZTC (pH 4.0)

56

Table 4.2. Powder XRD of ZTC (pH 4.0)

Sl. No.

2 d = /2Sin Å

I / Io x 100 %

1. 12.50 6.25 7.08113 39.00

2. 12.80 6.40 6.91583 75.50

3. 15.60 7.80 5.68026 100.00

4. 16.80 8.40 5.27714 51.00

5. 18.60 9.30 4.77031 36.00

6. 19.70 9.85 4.50636 42.00

7. 20.60 10.30 4.31147 50.00

8. 25.00 12.50 3.56173 100.00

9. 27.70 13.85 3.22039 100.00

10. 28.20 14.10 3.16442 42.00

11. 30.50 15.25 2.93083 40.00

12. 31.40 15.70 2.84885 39.00

13. 34.90 17.45 2.57075 36.00

14. 36.60 18.30 2.45515 34.50

15. 41.70 20.85 2.16592 36.50

16. 44.20 22.10 2.04904 46.00

17. 49.70 24.85 1.83441 33.50

18. 51.00 25.50 1.79066 38.00

19. 53.30 26.65 1.71869 33.50

20. 57.50 28.75 1.60274 30.00

21. 60.40 30.20 1.53254 30.00

57

Fig. 4.6. Powder XRD pattern of ZTC (pH 3.0)

58

Table 4.3. Powder XRD of ZTC (pH 3.0)

Sl. No.

2 d = /2Sin Å

I / Io x 100 %

1. 10.70 5.35 8.26796 33.00

2. 11.10 5.55 7.97089 33.00

3. 12.00 6.00 7.37502 32.00

4. 12.20 6.10 7.25457 32.00

5. 12.80 6.40 6.91583 32.50

6. 13.30 6.65 6.65694 32.00

7. 14.00 7.00 6.32563 35.00

8. 15.40 7.70 5.75358 44.00

9. 19.50 9.75 4.55212 41.00

10. 20.50 10.25 4.33227 34.50

11. 21.60 10.80 4.11407 31.50

12. 24.80 12.40 3.59 41.00

13. 25.60 12.80 3.4796 32.00

14. 27.40 13.70 3.25497 84.00

15. 28.20 14.10 3.16442 33.50

16. 28.40 14.20 3.14259 33.00

17. 31.30 15.65 2.85772 35.00

18. 32.30 16.15 2.7715 31.00

19. 33.70 16.85 2.65949 33.00

20. 38.00 19.00 2.36786 32.50

21. 41.40 20.70 2.18092 34.00

22. 42.80 21.40 2.11277 35.00

23. 44.20 22.10 2.04904 37.50

24. 46.60 23.30 1.94895 33.00

25. 49.70 24.85 1.83441 35.00

26. 51.00 25.50 1.79066 41.00

27. 53.10 26.55 1.72469 36.50

28. 54.70 27.35 1.67797 36.50

29. 57.50 28.75 1.60274 31.00

30. 59.40 29.70 1.55593 35.50

31. 60.80 30.40 1.52342 34.00

59

Fig. 4.7. Powder XRD pattern of ZTC (pH 1.5)

60

Table 4.4. Powder XRD of ZTC (pH 1.5)

Sl. No.

2 d = /2Sin Å

I / Io x 100 %

1. 12.50 6.25 7.08113 54.00

2. 13.80 6.90 6.41685 100.00

3. 15.50 7.75 5.71668 100.00

4. 16.70 8.35 5.30851 55.00

5. 19.60 9.80 4.52912 41.50

6. 20.50 10.25 4.33227 62.00

7. 21.70 10.85 4.09534 35.00

8. 24.90 12.45 3.57581 100.00

9. 25.70 12.85 3.46629 42.00

10. 26.40 13.20 3.37595 37.50

11. 26.90 13.45 3.31432 37.00

12. 27.60 13.80 3.23183 100.00

13. 28.40 14.20 3.14259 38.00

14. 29.40 14.70 3.03793 31.00

15. 29.60 14.80 3.01786 31.50

16. 30.50 15.25 2.93083 35.00

17. 31.00 15.50 2.88469 30.00

18. 32.80 16.40 2.73038 33.00

19. 33.70 16.85 2.65949 33.00

20. 37.90 18.95 2.37388 33.00

21. 39.80 19.90 2.26482 46.00

22. 41.80 20.90 2.16097 40.00

23. 44.20 22.10 2.04904 57.50

24. 51.00 25.50 1.79066 41.00

25. 53.30 26.65 1.71869 33.00

26. 54.50 27.25 1.68365 32.00

61

Fig. 4.8. Powder XRD pattern of ZTC P2

62

Table 4.5. Powder XRD of ZTC P2

Sl. No.

2 d = /2Sin Å

I / Io x 100 %

1. 12.40 6.20 7.13801 71.002. 15.00 7.50 5.90609 100.003. 15.60 7.80 5.68026 41.504. 16.40 8.20 5.40493 66.505. 17.40 8.70 5.09649 41.006. 19.30 9.65 4.59884 58.007. 19.90 9.95 4.46152 59.008. 21.20 10.60 4.19078 40.009. 22.60 11.30 3.93424 40.0010. 23.00 11.50 3.86672 40.0011. 23.90 11.95 3.72311 41.0012. 24.40 12.20 3.64794 65.0013. 25.30 12.65 3.52017 46.0014. 26.50 13.25 3.36343 44.0015. 27.10 13.55 3.29031 100.0016. 27.80 13.90 3.20903 44.5017. 29.10 14.55 3.06857 39.5018. 29.60 14.80 3.01786 37.0019. 30.80 15.40 2.90296 36.0020. 31.40 15.70 2.84885 35.5021. 32.20 16.10 2.77987 47.0022. 33.20 16.60 2.69839 46.0023. 34.00 17.00 2.63671 39.0024. 34.70 17.35 2.58511 39.0025. 35.50 17.75 2.52867 42.0026. 36.60 18.30 2.45515 45.0027. 37.40 18.70 2.40446 36.5028. 37.80 18.90 2.37993 37.0029. 38.90 19.45 2.31513 37.5030. 39.80 19.90 2.26482 41.5031. 40.10 20.05 2.24857 45.0032. 41.50 20.75 2.17589 39.5033. 43.40 21.70 2.08494 41.0034. 44.50 22.25 2.03592 42.5035. 45.90 22.95 1.97703 45.5036. 46.50 23.25 1.95291 38.0037. 47.00 23.50 1.9333 37.5038. 47.70 23.85 1.90655 38.0039. 48.70 24.35 1.86971 38.0040. 49.20 24.60 1.85188 37.5041. 49.60 24.80 1.83787 36.5042. 50.60 25.30 1.80387 35.5043. 51.50 25.75 1.77445 41.0044. 52.70 26.35 1.73683 43.00

63

Fig. 4.9. Powder XRD pattern of ZTC P4

64

Table 4.6. Powder XRD of ZTC P4

Sl. No.

2 d = /2Sin Å

I / Io x 100 %

1. 13.50 6.75 6.55876 25.002. 13.70 6.85 6.46346 26.503. 14.70 7.35 6.02594 15.504. 14.90 7.45 5.94551 22.005. 15.20 7.60 5.82883 83.006. 16.50 8.25 5.3724 51.007. 17.50 8.75 5.0676 16.008. 17.90 8.95 4.95525 16.009. 18.30 9.15 4.84783 16.0010. 18.80 9.40 4.72001 13.0011. 19.40 9.70 4.57536 36.0012. 20.30 10.15 4.3745 40.0013. 21.40 10.70 4.15207 19.0014. 22.20 11.10 4.00422 15.5015. 22.90 11.45 3.88338 13.0016. 23.20 11.60 3.83384 12.0017. 24.60 12.30 3.61873 56.5018. 25.40 12.70 3.50654 23.0019. 26.30 13.15 3.38855 12.0020. 26.80 13.40 3.32646 17.5021. 27.40 13.70 3.25497 100.0022. 28.00 14.00 3.18657 19.5023. 29.30 14.65 3.04807 16.5024. 30.30 15.15 2.94972 25.0025. 31.20 15.60 2.86665 21.0026. 32.10 16.05 2.78831 11.0027. 32.70 16.35 2.7385 15.0028. 33.40 16.70 2.68269 21.0029. 34.20 17.10 2.62175 15.0030. 34.60 17.30 2.59235 14.5031. 37.80 18.90 2.37993 14.5032. 38.20 19.10 2.35592 14.0033. 39.60 19.80 2.2758 11.5034. 41.40 20.70 2.18092 9.5035. 42.60 21.30 2.12222 12.0036. 44.10 22.05 2.05346 16.5037. 49.70 24.85 1.83441 12.0038. 50.90 25.45 1.79395 14.0039. 51.90 25.95 1.76171 10.00

65

Fig. 4.10. Powder XRD pattern of ZTC P6

66

Table 4.7. Powder XRD of ZTC P6

Sl. No.

2 d = /2Sin Å

I / Io x 100 %

1 11.10 5.55 7.97089 20.00

2 11.50 5.75 7.69453 16.00

3 12.50 6.25 7.08113 16.00

4 13.30 6.65 6.65694 17.50

5 13.70 6.85 6.46346 31.00

6 15.10 7.55 5.86721 73.50

7 15.60 7.80 5.68026 18.00

8 16.50 8.25 5.3724 58.00

9 18.30 9.15 4.84783 24.50

10 19.30 9.65 4.59884 37.00

11 20.20 10.10 4.39593 40.50

12 20.70 10.35 4.29086 14.50

13 21.40 10.70 4.15207 21.00

14 22.80 11.40 3.90018 18.00

15 24.50 12.25 3.63328 50.00

16 25.40 12.70 3.50654 26.00

17 26.30 13.15 3.38855 17.00

18 26.60 13.30 3.35101 21.00

19 27.30 13.65 3.26666 100.00

20 28.00 14.00 3.18657 23.00

21 28.80 14.40 3.09984 14.50

22 29.30 14.65 3.04807 21.50

23 30.30 15.15 2.94972 30.50

24 30.80 15.40 2.90296 19.00

25 31.20 15.60 2.86665 27.50

26 32.10 16.05 2.78831 17.00

27 32.60 16.30 2.74667 20.50

28 33.60 16.80 2.66718 20.00

29 34.10 17.05 2.62921 19.00

67

30 34.70 17.35 2.58511 24.00

31 36.20 18.10 2.48136 14.50

32 36.80 18.40 2.44227 13.00

33 37.70 18.85 2.38601 22.00

34 38.20 19.10 2.35592 20.00

35 39.60 19.80 2.2758 17.50

36 40.90 20.45 2.20642 14.50

37 41.30 20.65 2.18597 14.00

38 42.00 21.00 2.15114 12.00

39 42.50 21.25 2.12698 23.00

40 43.30 21.65 2.08952 12.00

41 44.00 22.00 2.05789 23.00

42 45.00 22.50 2.01446 15.50

43 47.00 23.50 1.9333 13.00

44 48.70 24.35 1.86971 14.00

45 49.50 24.75 1.84135 14.00

46 50.90 25.45 1.79395 17.00

47 51.80 25.90 1.76487 15.00

48 52.10 26.05 1.75542 13.00

49 52.50 26.25 1.74298 13.00

50 53.20 26.60 1.72168 13.50

68

4.5.3. FTIR STUDIES

The ZTC single crystals are grown and the results are obtained from FTIR

spectra for solution of different pH values .The spectrum of all the crystals are

similar except a small shift in the peak positions and hence the crystals are

expected to preserve nearly the same interactions among the group and ions. The

NH group frequency in thiourea is seen to form a broad envelope between 3500

and 1600 cm-1 due to hydrogen bonding interaction with chloride site of the

neighbouring complex. The C=S stretch of thiourea is slightly shifted to lower

value than pure thiourea due to its coordinate interaction Zn2+. C=S coordination

to metal ion enhances the resonance delocalization of NH2 lone pair electrons.

Thus providing more double bond character for N-C-N and hence both symmetric

and asymmetric N-C-N stretching is lifted to higher values.

For pure thiourea C=S is bonded to NH2 and therefore C-S bonding made

is also shifted to lower values from 730 to 713 cm-1. The NH2 bonding forms

intense and broadband at 500 cm-1. From FTIR analyses, the N-H stretching

region is sharpened with decreasing pH values, illustrating well packing of the

crystal in the more acidic pH range.

In the frequency region 2700 to 3500 cm -1 the NH2 group stretching

vibrations of thiourea and its metal complexes are assigned to the absorption

band. The NH2 symmetric stretching vibrations and asymmetric stretching

vibrations are appear between 2700 to 3100 cm-1 in FTIR.

For NH2 asymmetric bending vibrations have the peaks at 1611 per cm and

1631 per cm with very strong intensity in FTIR spectra. The strong absorption

bands at 514 cm-1 (pH 5.3), 515 cm-1 (pH 4.0) and 518 cm-1 (pH 3.0) in the FTIR

are due to symmetric NH2 bending vibrations. The peaks with strong intensity at

554 cm-1 (pH 3.0) in the FTIR spectra are attributed symmetric NH2 lagging

vibrations.

69

For NH2 rocking vibrations the strong bend centered at 1105 cm-1 in FTIR

spectra for all pH values. For ZTC the frequency is slightly increased comparing

to the N-C-N stretching vibrations of thiourea occur at 1470 cm-1 because of

greater double bond character of C-N bond. A strong peak appears at 1495

cm-1 for all pH values due to N-C-N asymmetric stretching vibrations. In the

ZTCP spectrum there is a shift in the absorption band, due to the incorporation of

phosphate ions in the crystal lattice of pure ZTC crystals.

70

Fig. 4.11. FTIR spctra of ZTC crystals grown at different pH values. (a) 5.3 (b) 4.0 and (c) 3.0

4.5.4. KURTZ POWDER TEST FOR NON LINEARITY

Growth of large single crystals is a slow process. Hence, it is highly desirable to

have some technique of screening crystal structures to determine whether they are

noncentrosymmetric and it is also equally important to know whether they are better than

those currently known. Such a preliminary test should enable us to carry out the activity

without requiring for oriented samples. Kurtz and Perry (1968) proposed a powder SHG

method for comprehensive analysis of the second order nonlinearity. Employing this

technique Kurtz surveyed a very large number of compounds .

To confirm the nonlinear property, Kurtz powder SHG test was carried out for

ZTC and ZTCP crystals. In this technique, the sample was packed as polycrystalline

powder into a cell sandwiched between two glass slides (Fig. 4.11). The crystal powder

must be so finely ground that the average size of the particle is much less than the

coherence length for the second harmonic generation, otherwise there would be no

signal. Typical value of coherence length is 2m. The sample is then subjected to a Q-

switched Nd; YAG emitting 1.064 m, 10 ns laser pulses with spot radius of 1mm to

assess the SHG intensity. The result shows the crystals are having the nonlinear effect

comparable to the well known Zinc Thiourea sulphate (ZTS) crystals.

71

Fig. 4.12 Apparatus used for study of second harmonic generation in powders

4.5. MICRO HARDNESS TESTING

Hardness is an important solid-state property, which determines the mechanical

strength of materials and the resistance to locate deformation. The hardness index can be

controlled also with other mechanical properties such as elastic constants and yield

strength. Like etching, hardness is also a tool to indicate the dislocation content and

plasticity of the crystals.

Fig. 4.13. Vickers Indenter

72

Hardness testing techniques involve pressing a hard indenter into a sample

surface and measuring the area of the resulting indentation. This technique has been

employed industrially using different shapes of indenters such as spherical, conical and

pyramidal. In the case of crystals, the practice employed for hardness measurement is

called micro indentation hardness. This involves creating small indentations at low,

enabling many measurements to be made on one sample. During the micro hardness

test, a diamond indenter is lowered at a controlled rate on to the specimen for a specific

interval of time, during which plastic flow occurs, resulting in the formation of

permanent indentation of the same shape as that of the indenter as shown in Fig. 4.12.

The indentation pressure is directly proportional to the uniaxial flow stress of the

material. As the indenter is removed from the surface, there is some elastic recovery, the

extent of which depends on the elasticity of the material and the shape of the indenter.

The magnitude of the diagonal length of the indentation gives indication of the hardness

number, which has units of stress.

Hardness is an important solid state property, which determines the mechanical

strength of materials and the resistance to local deformation. Mechanical properties such

as elastic constant and yield strength can be determined from the hardness index. Fairly

good values are required to achieve a good polish and for robustness of the device made

out of the grown crystals [Venkataraman, V. et al., 1997].

The polished face of ZTC and ZTCP are subjected to static indentation tests at

room temperature using Leitz Wetzlar hardness tester fitted with Vicker's diamond

pyramidal indenter. Vicker's microhardness number was then evaluated from the

relation;

Hv = 1.8544 P/d2 kg/mm2 ……(4.1)

Where Hv is the Vicker's microhardness number, several indentations were made on the

face of the crystal (Fig. 4.13 & 4.14). The distance between any two indentations was

maintained to be greater than five times the diagonal length in order to avoid any mutual

influence of the indentations. The diagonal length of the indentations was measured

using a micrometer eyepiece.

73

0 10 20 30 40 500

50

100

150

200

250

300

350

400

(100) plane

pH 5.3 pH 4.0 pH 3.0

Hv (

kg m

m-2)

Load (g)

Fig. 4.14. Vicker’s hardness number (VHN) of ZTC single crystals of(100) plane grown at different pH values

Fig. 4.15. Vicker’s hardness number (VHN) of ZTC P4 single crystals of (100) plane

74

CHAPTER - 5

SUMMARY & CONCLUSION

ZTC single crystals had been successfully grown in bulk and different

morphological forms at optimized growth conditions.

The effect of pH on the growth rate and morphology of ZTC crystals were

studied.

Powder X-ray Diffractions were taken to study the crystalline quality of grown

crystals ZTC and ZTCP, the shifting of peaks indicated the presence of phosphate ions,

and the lattice distances (d values) were calculated. The FTIR spectrum obtained for the

grown crystals confirmed the crystals to be ZTC.

The mechanical strength of ZTC and ZTCP were studied by micro hardness test

by using Vickers indenter.

Kurtz's non-linearity test reveals that the ZTC and ZTCP have the non-linear

nature. Thus we can use the ZTC and ZTCP crystals for the following non-linear

applications : Industry, Medicine, Entertainment, remote sensing and analysis, LIDAR

and Telecommunications.

75

CHAPTER - 6

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