31

CHAPTER - 2

CRYSTAL GROWTH AND CHARCTERIZATION

TECHNIQUES

2.1 Introduction

In this chapter, a brief description of solution growth method of synthesizing the

semi metallo organic pure L-alanine cadmium chloride, K" , Na* and Zn^^ ions doped

LACC crystals and the various experimental methods used for the characterization of them

are discussed here.

2.2 Low temperature solution growth

The method of growing crystals from solutions may be used for substances fairly

soluble in a solvent and not react with it. Moreover, growth of crystals from solutions is the

only method for the crystallization of substances which undergo decomposition before

melting. Growth of crystals from aqueous solution is one of the ancient methods of crystal

growth. The method of crystal growth from low temperature aqueous solutions is extremely

popular in the production of many technologically important crystals. Much attention has

been paid to understand the growth mechanism of the process.

Materials having moderate to high solubility in temperature range, ambient to

100 °C at atmospheric pressure can be grown by low temperature solution growth method.

This method is the most widely used method for the growth of single crystals, when the

starting materials are unstable at high temperature [9]. This method is widely used to grow

32

bulk crystals, which have high solubility and have variation in solubility with temperature

[170]. Growth of crystals from solution at room temperature has many advantages over

other growth methods though the rate of crystallization is slow. Since growth is cai-ried out

at room temperature, the structural imperfections in solution grown crystals are relatively

low [12].

The low temperature solution growth technique also allows variety of different

morphologies and polymorphic forms of the same substance, which can be grown by

variations of growth conditions or of solvent. The proximity to ambient temperature

reduces the possibility of major thermal shock to the crystal both during growth and on

removal from the apparatus. »

The low temperature solution growth technique has the merits such as

(a) Sunple growth apparatus

(b) Growth of strain and dislocation free cr>'stals

(c) Permits the growth of prismatic crystals by varying the growth conditions

(d) This is the only method which can be used for substances that undergo

decomposition before melting

The main disadvantages of the low temperature solution gi'owth are the slow growth

rate in many cases and the ease of solvent inclusion into the growing ciystal. Under the

controlled conditions of growth, the solvent inclusion can be minimized and the high

quality of the grown crystal can compensate the disadvantage of much longer growtli

periods. After undergoing so many modification and refinements, the process of solution

growth now yields good quality crystals for a variety of applications.

33

Low temperature solution growth can be subdivided into the following methods

(i) Slow cooling method

(ii) Slow evaporation method

(iii) Temperature gradient method

2.2.1 Slow Cooling Method

This is the most suitable method among various methods of solution growth.

However, the main disadvantage of slow cooling method is the need to use a range of

temperature. The possible range of temperature is usually naiTow and hence much of the

solute remains in the solution at the end of the growth run. To compensate this effect, large

volume of solution is required. The use of wide range of tempei-atures may not be desirable

because the properties of the grown crystals may vary with temperature. Temperature

stability may be increased by keeping the solution in large water bath or by using a vacuum

jacket. Achieving the desired rate of cooling is a major technological hurdle. This

technique needs only a vessel for the solution in which the crystals grow. The height, radius

and volume of the vessel are so chosen as to achieve the required thermal stability. Even

though this method has technical difficulty of requiring a programmable temperature

controller, it is widely used with great success. In general, the crystals produced are small

and the shapes of the crystals are unpredictable.

34

2.2.2 Slow Evaporation Method

In slow evaporation and slow cooling methods the apparatus are similar to each

other. In this method, the saturated solution is kept at a particular temperature and

provision is made for evaporation. If the solvent is non-toxic like water, it is permissible to

allow evaporation into the atmosphere. Typical growth conditions involve a temperature

stabilization of about 0.05 °C and rate of evaporation of a few mm^/h. The evaporation

technique has an advantage, viz. the crj'stals grow at a fixed temperature. But inadequacies

of the temperature control system still have a major effect on the growth rate. This method

can effectively be used for materials having ver>' low temperature coefficient of solubility.

But the crystals tend to be less pure than the crystals produced by slow cooling technique

because, as the size of the crystal increases, more impurities find place in the crystal faces.

Evaporation of solvent from the surface of the solution produces high local super saturation

and formation of imwanted nuclei. Small crystals are also formed on the walls of the vessel

near the surface of the liquid from the material left after evaporation. These tiny crystals

fall into the solution and hinder the growth of the crystal. Another disadvantage lies in

controlling the rate of evaporation. A variable rate of evaporation may affect the quality of

the crystal. In spite of all these disadvantages, this is a simple and convenient method for

growing single crystals of large size.

2.2.3 Temperature Gradient Method

Temperature gradient method involves the transport of the materials from hot

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

supersaturated and the crystal grows.

35

The main advantages of the method are

(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.

2.3 Optimizing solution growtli

The growth of good quality single ciystals by slow evaporation and slow cooling

techniques require the optimized conditions and the same may be achieved with the help of

the following norms: (i) Material purification, (ii) Solvent selection, (iii) Solubility, (iv)

Solution preparation, (v) Seed preparation, (vi) Agitation, (vii) Crystal habit, and (viii)

Cooling rate.

2.3.1 Material Puriflcation

An important prerequisite for growing good quality crystals is the availability of the

highest purity material. Solute and solvents of liigh purity are required, since impurity may

be incorporated into the crystal lattice resulting in the formation of flaws and defects.

Sometimes impurities may slow down the crystallization process by being absorbed on the

growing face of the crystal which changes the crystal habit. A careful repetitive use of

36

standard purification methods of recrystallization followed by filtration of the solution

would increase the level of purity.

2.3. 2 Solution and Solubility

A solution is homogeneous mixture of a solute in a solvent. Solute is the

component, which is present in a smaller quantity and the one which gets dissolved in the

solvent. For a given solute, there may be different solvents. The solvent must be chosen

taking into account the following factors to grow crystals from solution. A solvent of

choice is the one with

(i) a good solubility for the given solute,

(ii) a good temperature coefficient of solute solubility,

(iii) less viscosity,

(iv) less volatility,

(v) less corrosion and non-toxicity, and

(vi) cost effective.

It is known that the choice of solvent provides some control over crystal habit and

this effect depends on the interaction of the surface of the crystal as it grows and the

solvent molecules. Sometimes this is sufficient to result in the precipitation of a new

crystalline phase. Also this effect is related to the influence of impurities or additives upon

habit. Solvent commonly used include water, both light (H2O) and heavy (D2O), ethanol,

methanol, acetone, carbon tetrachloride, hexane, xylene and many others. Solvents having

all the above characteristics together, however, do not exist. Almost 90% of the ciystals

produced from low temperature solutions are grown by using v/ater as solvent. Probably no

other solvent is as generally useful for growing crystals as water. Some properties that

account for this are its high solvent action, which is related to its high dielectric constant,

its stability, its low viscosity, its low toxicity and its availability. For crystal growth, high

purity water is needed.

Solubility of the material in a solvent decides the amount of the material, which is

available for the growth and hence defines the total size limit. If the solubility is too high, it

is difficult to grow bulk single crystals and lower solubility restricts the size and grow'lh

rate of the crystals. Solubility gradient is another important parameter, which dictates the

growth procedure. Neither a flat nor a steep solubility curve will enable the growth of bulk

crystals from solution. If the solubility gradient is very small, slow evaporation of the

solvent is the other option for crystal growth to maintain the supersaturation in the solution.

Low temperature solution growth is mainly a diffiision-controlled process; the

mediimi must be less viscous to enable faster transfer of the growth units from the bulk

solution by diffusion. Hence a solvent with less viscosity is preferable. Super saturation is

an important parameter for the solution growth process. The solubility data at various

temperatures are essential to determine the level of super saturation. Hence, the solubility

of the solute in the chosen solvent must be determined before starting the growth process

[171].

The solubility of the solute can be determined by dissolving the solute in the solvent

maintained at a constant temperature with continuous stirring. On reaching saturation,

equilibrium concentration of the solute can be determined gravimetrically. A sample of the

38

clear supernatant liquid is withdrawn by means of a warmed pipette and a weighed quantity

of the sample is analyzed. By repeating the above procedure for different temperatures, the

solubility curve can be plotted. Solubility of most substances increases with temperature

(the temperature coefficient of solubility is positive).

In the present study the solubility of pure LACC in double distilled water was

determined at 30 °C. It was found to be 12.25 gram/10ml. This was in good agi-eement with

the reported value [154].

2.3.3 Growth of pure and K , Na"*'and Zn " ions doped LACC crystals

Growth of pure and K"*, Na" and Zn " ions doped LACC crystals were

carried out by slow evaporation technique. The analytical reagents (AR) grade L-alanine,

Cadmixmi chloride; Potassium chloride, Sodium chloride and Zinc chloride along with

doubly distilled water are used for the gro^vth.

In the present work the amount of solute (m) required to prepare the solution

for the growth of pure LACC is find out using the following relation

m = (M X X X V)/ 1000 (in gram units)

Where, M - is the molecular weight of the particular salt (L-alanine or Cadmium

chloride)

X - is the concentration in molar units

V - is the required volume of the solution.

Pure solution based on the above calculation was added with potassium chloride;

sodium chloride in two different concentrations (2 and 4 mole percentage) and zinc

chloride in two different concentrations (1 and 3 mole percentage) were added separately to

CTRt t h e dnnRd p.rvstals

39

In these cases the amount of dopant solute (mO to be added is calculated using the relation

mi = (Ml X P X V)/ 1000 (in gram units)

Where,

Ml = molecular weight of the dopant(Potassium chloride, Sodium chloride and

Zmc chloride)

P = 0.01 for 1 mole percentage

P = 0.02 for 2 mole percentage

P = 0.03 for 3 mole percentage

P = 0.04 for 4 mole percentage

The amount of reactants (solute) are calculated using the above relations and are taken

for the preparation of the required solution.

Pure LACC was synthesized from L-alanine and cadmium chloride

monohydrate taken in the equimolar ratio. An adduct was formed according to the

following reaction.

CH3CHNH2COOH + CdCl2 ^ CHjCHNHzCOOH.CdCh

The calculated amounts of the reactants were thoroughly dissolved in double distilled water

and stirred well for about 2 hours using a magnetic stirrer to obtain a homogeneous

mixture. Then the mixture was evaporated to dryness, by heating below an optimum

temperature of 333 K, to prevent possible decomposition. It was then dissolved thoroughly

in double distilled water to form a saturated solution. The solution was filtered well to

remove the suspended impurities and allowed to crystallize by slow evaporation of solvent

at room temperature of about 303 K for about four weeks at a pH of 5.0. Well defined

40

single crystals (size up to 1 2 x 7 x 2 mm ) of good transpaiency were collected in four

weeks [172].

For the potassium and sodium doped LACC crystals, two and four mole

percentage of KCl and NaCl were added to the initial solution as the dopant. Again to

obtain the zinc doped LACC crystals one and three mole percentage of ZnCl2 were added

to the initial solution as the dopant (the experimental procedures are similar to the pure

material). The single crystals of pure K , Na^ and Zxi* ions doped LACC crystals were

harvested in proper time. The size and duration of growth of pure and doped LACC

crystals are tabulated in Table 2.1

41

Table 2.1: Size and duration of growth of pure and doped LACC crystals

Crystal type

Pure LACC crystal

KCl doped LACC (2 mole %)

KCl doped LACC (4 mole %)

NaCl doped LACC (2 mole %)

NaCl doped LACC (4 mole %)

ZnCl2doped LACC (Imole %)

ZnCl2 doped LACC (3mole %)

Size (dimension) in mm^

1 2 x 7 x 2

1 3 x 9 x 3

1 3 x 9 x 4

12x11x3

15x14x4

3 5 x 3 2 x 4

12x10x4

Duration in days

25 - 30

18-21

15-20

25-30

25-30

26-20

16-20

In all the pure and doped LACC crystals, as a result of nucleation srtiall crystals

appeared first then they grow bigger on slow evaporation. After the completion of growth

good quality crystals, which are free from imperfections are selected for various

characterizations. The grown crystals are shown in figure 2.1.

Figure 2.1 Photograph displaying the grown pure and potassium chloride, zinc chloride and sodium chloride doped crystals.

2.4 Characterization Techniques

In the present study the pure and K ", Na" and Zn'" ions doped LACC

crystals were subjected to various characterizations such as X - Ray diffraction

(XRD) analysis, Fourier Transform Infra Red (FTIR) spectroscopy, Atomic

Absorption Studies (AAS), Scanning Electron Microscopy (SEM), Energy Dispersive

Analysis by X-ray (EDAX), floatation method, Thermo Gravimetric/Differential

Thermal (TG/DT) analyses. Vicker's microhardness test, UV- Visible optical

analysis, Second Harmonic Generation (SHG) measurements and the electrical

studies. In the remaining of this chapter a brief description of the instnamentation

adopted is presented.

2.4.1. X-Ray Diffraction (XRD) Analyses

X- ray diffraction is a tool for the investigation of the fine structure of

matter. This technique had its beginnings in von Laue's discovery in A.D.1912 that

crystals diffract X - rays, the manner of the diffraction revealing the structure of the

crystal. At first X- ray diffraction was used only for the determination of crystal

structure. Later on, however the other uses were developed, and today the method is

applied not only to structure determination, but to such diverse problems as chemical

analysis and stress measurement, to the study of phase equilibria and the

measurement of particle size, to the deteimination of the orientation of one crystal or

the ensemble of orientations in a polycrystalline aggregate. When we are focused of

structure determination the single crystal and powder X- ray diffraction techniques

are used.

44

2.4.1. (a) Single crystal x-ray diffraction analysis

Single crystal X-ray diffraction (X-ray crystallography) is an analytical technique in

which X-rays (wavelength range from 0.1 to 10 A) are employed to determine the actual

arrangement of atoms within a crystalline specimen. Single crystal X-ray diffraction (XRD)

is a non-destructive tool to analyze crystal structure of compounds, which can be grown as

single crystals. The molecular structure, atomic coordinates, bond lengths, bond cmgles,

molecular orientation and packing of molecules in single crystals can be determined by X-

ray crystallography. Single crystal X-ray diffractometer (Figure 2.2) collects intensity data

required for structure determination.

Accurate measurements of intensities of reflections of all Miller indices within a

specified reciprocal radius (usually 25° for MoIQi and 68° for CuK^) is needed to find the

structure, while unit cell parameters depend only on direction of reflections. As die name

implies, a crystalline sample is required. For single-ciystal work, the specimen should be

smaller than cross section diameter of the beam. Larger crystals can be cut down to proper

size and smaller crystals may be suitable if they contain strongly diffracting elements.

The monochromatic X-rays incident on a plane of single crystal at an angle theta

are diffracted according to Bragg's relation 2d sind = nX where d is the interplanar spacing

of the incident plane, X is the wavelength of X-rays and « is a positive integer. The intensity

of the diffracted rays depends on the arrangement and nature of atoms in the crystal.

Collection of intensities of a full set of planes in the crystal contains the complete structural

information about the molecule. Fourier transformation techniques are used to determine

the exact coordinates of atoms in the unit cell from this data.

45

Figure2.2.Instrumental setup of Enraf Nonius CAD4-F Diffractometer

^ 46

With the set of X-ray diffraction data collected, unit cell parameters, space group,

molecular structure, etc of the crystalline solids and Miller indexing of the different faces

of the crystal are possible. Unit cell parameter is simply the dimension of the basic

molecular brick with which the crystal is built. Space group tells us the symmetry with

which the molecules are arranged within the unit cell. All the geometrical features of

molecules (bond distance, bond angles, torsion angles between bonds, dihedral angles

between planes, etc.) may be obtained from coordinates.

In the present study, the single crystal X-ray diffraction analysis was perfoiTned

using an Enraf Nonius CAD4 single crystal X-ray diffractometer.

2.4.1. (b) Powder X-ray diffraction analysis

The powder diffraction of a substance is characteristic of the substance and

forms a sort of fingerprint of the substance to be identified. The peaks of the X-ray

diffraction pattern can be compared with the standard available data for the confirmation of

the structure. The powder X- ray diffraction method was devised independently by Debye

and Scherrer. It is the most useful of all diffraction methods and when properly employed,

can yield a great deal of structural information about the material under investigation.

Powder X-ray diffraction method involves the diffraction of monochromatic X-rays by a

powder specimen. Monochromatic usually means a strong K^ characteristic component of

the filtered radiation from an X- ray tube operated above the Ka excitation potential of the

target material.

Selection of Ka renders the incident beam to be a highly monochromatised one. The

focusing monochromatic geometry results in narrower diffracted peaks and lovv'

47

background at low angles. In the present study the powder fi '•. ' ' imple holder was kept on the

goniometer cell of XRD system (Philips, model: X'Pert p'-o) employing X'celerator and a

monochromator on diffracted beam side. Scans were made in 29" usually at 0.01 7step. Most of the

scans were done by applying 40 kV tube potential differences and 30 mA current. CuKal radiation

(A, = 1.540566 A) was used for all mea j'l • nents.

Powder X-ray diffraction is useful for confirming the identity of a solid material and

determining crystallinity and phase purity. Many books give details of principles and

methods involved in the determination of crystal and molecular structures of inorganic and

organic substances [173-177] ' -

2.4.2 Fourier Transform Infrared (FTIR) spectroscopic technique

Infrared spectroscopy is the study of the interaction of infrared light with matter. The

fundamental measurement obtained in infrared spectroscopy' is an infrared spectrum, which

is a plot of measured intensity versus wavelength (or wave number) of light. An instmment

used to obtain an infrared spectrum is called an infrared spectrometer. There are several

kinds of spectrometers in the world used to obtain infrared spectra. The most prevalent type

of spectrometer is called a Fourier Transform Infrared Spectrometer (FTIR). FTIR

technique is most usefiil for identifying functional gi-oiips organic and inorganic

compounds. It can be applied to the analysis of solids, liquids and gases. The term Fourier

Transform Infrared spectroscopy refers to fairly recent development in the manner in

which the data is collected and converted from an interference pattern to a spectmm.

Today's FTIR instruments ai'e computerized vi'hich makes them faster and more sensitive

than the older dispersive instruments.

48

Infrared spectroscopy deals with the study of vibrational spectra of molecules.

An infrared absorption spectrum originates from molecular vibrations (vibrations of bonds)

which cause a change in the dipole moment of the molecule. The vibrational frequencies,

their relative intensities and shapes of the infrared bands recorded in a double beam

specfrometer are used for the qualitative chai'acterization of a sample.

FTIR spectroscopic technique is based on the principle of a Michelson

Interferometer with a sensitive infrared detector and a digital minicomputer, FTIR

spectrometers provide higher resolution, total wavelength coverage, higher accuracy in

frequency and intensity measurement^. The instruments also possess greater ease and speed

of operation. By interpreting the infrared absorption spectrum, the fianctional groups of a

compound and chemical bonds in a molecule can be detennined. FTIR spectra of pure

compounds are generally so unique that they are like a fingerprint. While organic

compoimds have very detailed specfra, inorganic compounds are usually much simpler.

Samples of FTIR can be prepared in a number of ways. For liquids samples, the easiest is

to place one drop of sample between two plates of sodium chloride (salt), which is

transparent to infrared light. The drop forms a thin film between the plates. Solid samples

can be milled with potassium bromide (KBr) to form a very fine powder. This powder is

then compressed into a thin pellet which can be analyzed. KBr is also transparent in the IR.

Alternatively solid samples can be dissolved in a solvent such as methylene chloride and

the solution placed into a single plate. The solvent is then evaporated off leaving a thin

film of the original material on the plate. This is called a cast film and is frequently used for

polymer identification [178-182].

49

In the present work The FTIR spectrum of the sample was recorded with a Fourier

transform infrared spectrometer in the range of 400 - 4000 cm"' following KBr pellet

technique.

2.4.3 Scanning Electron Microscopy (SEM)

The Scanning Electron Microscopy (SEM) teclmique is widely used for the

observation of surface morphology of different kinds of materials with high magnification

and resolution, hi this technique an image of very small area of the specimen under

investigation is formed by the secoij^ary electrons (SE) ejected from the surface of the

specimen by bombarding the specimen with primary electron (PE) beam. The SEs are

collected by a scintillator, which transmit the signal to a photomultiplier and finally to a

view cathode ray tube which scans simultaneously with the PE beam. Apart from the SE,

the PE beam results in the emission of back scatted (or reflected) electrons from the

specimen. Back scattered electrons possess more energy than SE and have a definite

direction. Back scattered electron imaging is more usefUl in distinguishing one material

from another, since the yield of the collected backscattered electrons increases

monotonically with the specimen's atomic number.

In the present study, the surface morphological features of all the samples have

been observed using a SEM {JEOL-JSM5600 LV), in back scattered mode. It is operated at

20 kV and the magnification range lOOOX, 5000X and lOOOOX.

50

2.4.4 Energy dispersive analysis by X-ray (EDAX)

X-rays are produced when materials are irradiated with a high energy electron

beam. The wavelengths and hence the energies of the x-rays are characteristic of the

electron shell energies of the respective atoms and hence the spectrum of X-rays can be

used to identify different elements and also possible to measure the amount of different

elements present in the material.

In energy dispersive spectrometer, the diffraction is not involved, the various

wavelengths in which the radiation is emitted by the sample aie separated by means of a

silicon-lithium (Si-Li) detector and each signal is collected, amplified and corrected for

absorption and other effects, to give both qualitative and quantitative analysis of the

composition of the specimen as a whole or an area of interest (for elemevits of atomic

number greater than 11). This technique is known as Energy Dispersive Analysis by X-ray

(EDAX or EDX). The EDX {Phoenix) analysis system works as an integrated feature of a

SEM and can be operated on its own.

2.4.5 Floatation method

The floatation method was employed for the precise determination of

density and this method is sensitive to point defects and insensitive to dislocation of

crystals. Bromoform (density: 2.89 g/cc) and carbon tetrachloride (density: 1.59 g/cc) were

used for the experiment. After mixing bromoform and carbon tetrachloride in a specific

gravity bottle in suitable proportion, a small piece of a crj'Stal was immersed in the mixture

of the liquids. Change the quantities of bromoform and carbon tetrachloride so that the

sample was attained a mechanical equilibrium, at tliis stage the density of the crystal would

51

be equal to the density of the mixture of liquids. Then the density v/as calculated using the

relation

p = (W3-Wi)/(W2-Wi) g/CC

Where Wi is the weight of the empty specific gravity bottle, W2 is the weight of the

specific gravity bottle with full of water and W3 is the weight of the specific gravity bottle

with full of the mixture of bromoform and carbon tetrachloride.

The density of the pure LACC crystal can also be calculated from the

crystallographic data using the relation p - (M.Z)/(N .V)

where M is the molecular weight of the material used, Z is the number of molecules per

unit cell. N is the Avogadro's number and V is the volume of the unit cell [183].

2.4.6 Thermal Analyses

Thermal analysis is usefiil in both quantitative and qualitative analyses. Samples

may be identified and characterized by qualitative investigations of their thermal

behaviour, hiformation concerning the detailed structure and composition of different

phases of a given sample is obtained fi-om the analysis of thermal data. Thermal methods

are based upon the measurement of the dynamic relationship between temperature and

some property of a system such as mass, heat of reaction of volume. Of the various thermal

methods, Thermo Gravimetric Analysis (TGA) and Differential Thermal Analysis (DTA)

are most important [184, 185].

The TGA provides a quantitative measurement of any weight changes associated with

thermally induced transitions. For example, TGA can record directly the loss in weight as a

52

function of temperature or time (when operating under isothermal conditions) for

transitions that involve dehydration or decomposition [186, 187]. Thermo gravimetric

curves are characteristic of a given compound or material due to the unique sequence of

physical transitions and chemical reactions that occur over definite temperature ranges.

TGA data are usefiil in characterizing materials as well as in investigating the

thermodynamics and kinetics of the reaction and transitions that result from the application

of heat to these materials.

In TG analysis, the mass of the sample is recorded continuously as a function of

temperature as it is heated or cooled at a controlled rate. A plot of mass as a function of

temperature provides both quantitative and qualitative information. The apparatus required

for TG analysis includes a sensitive recording analytical balance, a furnace, a temperature

controller and a recorder that provides a plot of sample mass as a function of temperature.

Often an auxiliary equipment to provide an inert atmosphere for sample is also needed.

Change in the mass of the sample occurs as a result of the nipture formation of various

physical and chemical bonds at elevated temperature that lead to the evolution of volatile

products or formation of reaction products. Thus the TGA cui-ve gives infonnation

regarding the thermodynamics and kinetics of various chemical reactions, reaction

mechanisms, intermediate and final products.

hi DTA, the difference in temperature between the sample and a thennally inert

reference material is measured as a function of temperature (usually the sample .•V

temperature). Any transition that the sample undergoes results in liberation or absorption of

energy by the sample with a corresponding deviation of its temperature from that of the

53

reference. As the temperature of both is increased at a constant rate [188], the

corresponding deviation of the sample temperature from that of reference (AT) versus the

programmed temperature (T) is recorded and it explains whether the transition is

endothermic or exothermic. The DTA studies along with TGA provide detailed information

regarding the dehydration, decomposition and the phase transitions of a material during

heating.

In the present study the thermal characteristics of the grown crystals were

studied using Perkin TG/DTA analyzer. The temperature range selected for the present

study was from ambient temperature to 1073 K.

2.4. 7 Micro hardness Measurements

Hardness is one of thp important properties which crystals should possess

for device applications. Hardness is a measure of resistance against lattice destruction or

the resistance offered to permanent deformation or damage. Measurement of hardness is a

non-destructive testing method to determine the mechanical behavior of the materials

[189]. The term hardness is having different meanings to different people depending upon

their area of interest. For example, it is the resistance to wear for a lubrication engineer,

the resistance to cutting for a machinist, the resistance to wear for a penetration for a

metallurgist and a measure of flow stress for a design engineer. All these actions are

related to the plastic stress of the material. For hard and brittle materials, the liardness test

has proved to be a valuable technique in the general study of plastic deformation. Meyer

established a relationship between indentation hardness and work hardening capacity of a

material [190]. Again hardness test provides useful information on the strength and

deformation characteristics of material [191].

54

The hardness depends not only on the properties of the materials under test but also

largely on the conditions of measurement. Micro hardness tests have been applied to fine

components of clock and instrument mechanisms, metallic fibers, thin galvanic coatings,

artificial oxide films, thin metal strip, foils, wires, etc. The micro hardness method is

widely used for studying the individual structural constituent elements of metallic alloys,

minerals, glasses and enamels.

2.4.7.1 Methods of Hardness Testing

There are various methods using which hardness of the material can be measured, they

are named as follows:

(i) Static indentation test

(ii) Dynamic indentation test

(iii) Scratch test

(iv) Rebound test

(v) Abrasion test

In the above tests the most popular and simplest test is the static indentation test,

wherein an indenter of specific geometry is pressed into the surface of a test specimen

under a knovm load. The indenter may be a ball or diamond cone or diamond pyramid. A

permanent impression is retained in the specimen after removal of the indenter. The

hardness is calculated from the area or the depth of indentation produced. The vaiiable is

the type of indenter or load. The indenter is made up of a very hard material to prevent its

deformation by the test piece, so that it can cover materials over a wide range of hardness.

For this reason, either an indenter .pf steel sphere or a diamond pyramid or cone is

employed. A pyramid indenter is preferred as geometrically similar impressions are

55

obtained at different loads. In this static indentation test, the indenter is pressed

perpendicularly to the surface of the sample by means of an applied load. By measuring the

cross sectional area or depth of the faidentation and knowing the applied load, empirical

hardness number may be calculated. This method is followed in Brinell, Meyer, Vickers,

Knoop and Rokwell tests [192-194].

In the dynamic indentation test, a ball or a cone (or a number of small spheres) is

allowed to fall from a definite height and the hardness number is obtained from the

dimensions of the indentation and the energy of impact.

The scratch test can be classified into two types:

(a) Comparison test, in which one material is said to be harder than another, if the

second material is scratched by the first; and

(b) A scratch test is made with a diamond or steel indenter on the surface at a steady rate

and under a definite load. The hardness number is expressed in terms of the width or

depth of the groove formed.

In the rebound test, an object of standard mass and dimension is bounced from the test

surface and the height of rebound is taken as the measure of hardness. In the abrasion

test, a specimen is loaded against a rotating disk and the rate of wear is taken as a

measure of hardness.

2.4.7.2 Vickers hardness Test

Vickers hardness test belong to static indentation test. This method is the reliable

and most common among the various methods of hardness measurement discussed above.

In this method, micro-indentation is made on the surface of a specimen with the help of

56

diamond pyramidal indenter. In Vickers hardness test a pyramid is substituted for a ball in

order to provide geometrical similitude under different values of load [195].

The Vickers pyramid indenter whose opposite faces contained an angle

(a = 136°) is the most widely accepted p3Tamid indenter. A pyramid indenter is suited for

hardness tests due to the following two reasons [196].

• The contact pressure for a pyramid indenter is independent of indent size

• Pyramid indenters are less affected by elastic release than other indenters.

The base of the Vickers pyramid is a square and the depth of indentation

corresponds to 1/7"" of the indentation diagonal. Hardness is generally defined as the ratio

of the load applied to the surface area of the indentation. The Vickers hardness number Hv

of Diamond Pyramid Number (DPN) is defined as

Hv = 2Psin(a/2)/d^ kg-mm-^

where, a is the apex angle of the indMiter (a = 136°). The Vickers hardness number is thus

calculated using the relation

Hv =1.8544P/d^ kg-mm"'

where, P is the applied load in kg and d is the diagonal length of the indentation mark in

mm. Hardness values are always measured from the observed size of the impression

remaining after a loaded indenter has penetrated and has been removed from the surface.

Thus, the observed hardness behavior is the summation of a number ol" elTects

involved in the material's response to the indentation pressure during loading, in the final

measurement of the residual impression.

57

In the present study the microhai-dness measurements were

made using a Leitz microhardness tester fitted with a diamond pyramidal indentor.

2.4.8. UV-Visible Spectrophotometery

UV-Vis- NIR spectral measurements are done in the case of grown crystals, so as

to find how far the crystals can be used in these spectral regions. If the sample transmits the

incident light in the entire UV-Vis-NIR regions with out any significant absoiption then

that sample would be a good candidate for optical applications in those regions.

UV-Vis-NIR spectroscopy might be defined as the measurement of the absoiption

or emission of radiation associated with changes in the spatial distribution of electrons in

atoms and molecules. In practice, the electrons involved are usually the valence or bonding

electrons, which can be excited by absorption of UV or Visible or near IR radiation.

Excitation of a bound electron from the Highest Occupied Molecular Orbital increases the

spatial extent of the electron distribution, making the total electron density larger and more

diffuse, and often more polarizable. A vibrationally excited state of the molecule contains

rotational excitation and electronically excited state of a molecule also contains vibrational

excitation.

The probability for electronic transitions determines the intensity of spectral lines.

There must be large overlap between the vibrational states in the initial and the iinal

electronic states to have a large absorption cross-section, or high probability that the

molecule will absorb/emit radiation. Electronic transitions are possible for a wide range of

vibrational levels within the initial and final electronic states.

58

An UV-Visible spectrophotometer allows light of a given frequency to pass through

a sample and detects the amount of transmitted light. The instrument compaies the intensity

of the transmitted light with that of the incident light. The source of radiation in the UV-

Visible spectrophotometer is tungsten, hydrogen or deuterium lamp. A source of radiation

must be provided with each spectral region having its own requirements. All

spectrophotometers include some way to discriminable between different radiation

frequencies either they use as filters, prisms or gratings. The polychromatic radiation is

separated into its component wavelength using monochromators which consist of a prism

and a plane transmission grating. The sample absorbs a portion of the incident radiation and

the reminder is transmitted on to a detector where it is changed into an electrical signal and

displayed, usually after amplification, on a meter, chart recorder or some type of readout

device.

Automatic instruments gradually and continuously change the frequency or

wavelength. The spectrum of a compound represents a group of either wavelength or

frequency, contmuously changing over a portion of the electromagnetic spectrum versus

either percent fransmission (%T) or absorbance (A) [197 - 199].

In the present study the UV-Visible transmittance/absorbance spectra of the grown

pure and doped LACC crystals were recorded using a Varian Caiy 5E UV-Vis- NIR

specfrophotometer in the range 200 - 1200 nm to find the transmission range and hence

their suitability for optical applications.

59

2.4.9 Nonlinear Optical Phenomenon - Kurtz Powder SHG Method

Nonlinear Optics (NLO) is the study of the interaction of intense

electromagnetic field with materials to produce modified fields that are different from the

input field in phase, firequency or amplitude [200]. An optical process in which an input

optical wave is converted into an output wave of twice the input frequency is called Second

harmonic generation (SHG) [201]. This process occurs within a nonlinear medium, usually

a crystal. Such fi-equency doubling process is commonly used to produce green light (532

nm) fi-om, for example, a Nd:YAG (yttrium-aluminium-gamet) laser operating at 1064 nm.

The light propagated through a crystalline solid, which lacks a center of symmetry

(noncentrosymmetric), generates light at second and higher harmonics of the applied

fi"equency.

Nonlinear optics is completely, a new effect in which light of one wavelength is

transformed to light of another wavelength. The creation of light of new wavelength can

be best imderstood, as we think about the electrons in nonlinear crystal. Electrons in a

nonlinear crystal are bound in potential well, which acts like a spring, holding the electrons

to lattice point in the crystal .If an external force pulls an electron away IVom its

equilibrium position the spring pulls it back v/ith a force proportional to the displacement.

The spring's restoring force increases linearly with the electron displacement from its

equilibrium position. The electric field in a light wave passmg thiough the crystal exerts a

force on the electrons and pulls them away fi-om their equilibrium position. In an ordinary

optical material, the electrons oscillate about their equilibrium position at the frequency of

this electronic field. According to the fimdamental law of physics, an oscillating charge

60

will radiate at its frequency of oscillation, hence these electrons in the ciystal "generate"

light at the frequency of the original light wave.

The nonlinear material is different from the linear material in several aspects. We

can think of a nonlinear material as the one whose electrons are bound by veiy short

springs. If the light passing through the material is intense enough, its electric field can

pull the electrons so far that they reaCh the end of their springs. The restoring force is no

longer proportional to the displacement and then it becomes non-linear. The electrons are

jerked back roughly rather than pulled back smoothly and they oscillate at frequencies

other than the driving frequency of the light wave. These electrons radiate at the new

frequencies, generating the new wavelength of light. The exact values of the new

wavelengths are determined by conservation of energy. The energy of the new photons

generated by the nonlinear interaction must be equal to the energy of the photon used.

Materials in crystalline form have special optical and electrical properties, in many

cases improved properties over randomly arranged materials. Many organic and inorganic

materials are highly polarizable and are good candidates for NLO study. However, the net

polarization of a material depends on its symmetry properties, with respect to the

orientation of the impinging fields. Thus materials for second order NLO application must

be orientationally non-centrosymmetric to be functional. No such restriction applies to third

order materials.

Second order NLO materials are used in optical switching (modulation), frecfuency

conversion (SHG, wave mixing), and electro-optic applications, especially in EO

modulators. All of these applications rely on the manifestation of the moleculai- hyper

61

polarizability of the materials. NLO materials will be the key elements for future photonic

technologies based on the fact that photons are capable of processing information with the

speed of light. In the beginning, studies were concentrated on inorganic materials such as

quartz, potassium dihydrogen phosphate (KDP), lithium niobate (LiNb03), and

semiconductors such as cadmium sulfide, selenium, and tellurium.

Inorganic materials are much more matured in their application to second order

NLO materials than organics. Most commercial materials are inorganics especially, for

high power use. However, organic materials are perceived as being structurally more

diverse and therefore are believed to have more long term promise than inorganics. Recent

studies indicate that semiorganic/metal-organic crystals possess superior NLO properties

and thus they have opened a new era in this area of research.

Kurtz Powder SHG Method

Growth of large single crystal is a slow and difficult process. Hence, it is highly

desirable to have some technique of screening crystal structures to determine whether they

are non-centrosymmetric 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 oriented samples. Kurtz and Perry [202] proposed a powder SHG

method for comprehensive analysis of the second order nonlinearity. Employing this

technique, Kurtz [203] surveyed a very large number of compounds.

In this method the nonlinear optical property of the grown single crystal is tested by

passing the output of Nd:YAG Quanta ray laser through the crystalline powder sample. The

schematic of the experimental setup used for SHG studies is shown in figure 2.3. A Q-

62

switched, mode locked Nd:YAG laser was used to generate about X mJ/pulse at the 1064

nm fundamental radiation. This laser can be operated in two modes. In the single shot mode

the laser emits a single 8 ns pulse. In the multi shot mode tlie laser produces a 'continuous

train of 8 ns pulses at a repetition rate of 10 Hz. In the present study, a single shot mode of

8 ns laser pulse with a spot radius of 1mm was used. This experimental setup used a miiTor

and a 50/50 beam splitter (BS) to generate a beam with pulse energies about 5.3 mJ.

IR reflector

Glass plates

Beam splitter 50/50

AG •

CuSo sol"

Samples holder

Interference 4 filter

I CI >— j \ Oscilloscope

I \ BG-38 \

Photodiode

Figure 2.3 Schematic experimental setup for SHG efficiency measureinertt

The input laser beam was passed through an IR reflector and then

directed on the micro crystalline powdered sample packed in a capillary tube of a diameter

0.154 mm. The photodiode detector and oscilloscope assembly measured the light

emitted by the sample. Microcrystalline powder of urea or KDP is taken in a similar

capillary tube sealed at one end for comparison. The intensity of the second haimonic

output from the sample is compared with that of either KDP or urea. Thus, the figure of

merit of SHG of the sample is estimated.

63

2.10 Electrical conductivity and dielectric measurements

Electrical conductivity of solid substance is a characteristic property. It is due to the

mobility of electrons or ions and imperfections which are charged. Since the ionic solids

are insulators, the electrons are so tightly bound to the atoms that at ordinary temperatures,

they cannot be dislodged either by thermal vibrations or with ordinar>' fields. The positive

and negative charges in each part of the cr\'stal can be considered to be centered at the

same point, and, since no conductivity is possible, the localized charges remain that way

essentially forever. When an electric field is applied to the crystal, the centers of positive

charges are slightly displaced in the direction of the applied field and the centers of

negative charges are slightly displaced in the opposite direction. This produces local

dipoles throughout the crystal, and process of inducing such dipoles in the c/yi'tal is called

polarization [1].

Ionic conduction occurs either through the migration of positive and negative ions

in an extemal electric field (the ions originate either in the material in question or in

interstice impurities) or through the motion of ions in vacancies which reflects the

migration of vacancies. If the sample is placed in a stationary electric field, the carriers may

be considered to be contained in an enclosure bounded by the capacitor plates. As the

carriers may not leave the enclosure" they accumulate in the regions close to the plates

which cause a concentration gradient to be formed and this gradient feeds diffusion cunent.

At equilibrium the diffiision current density equals that of the drift cuirent. Charge

accumulation is related to inhomogeneities of the material, the agglomeration of impurity

ions by diffusion in the vicinity of electrodes or chemical changes in layers close to

electrodes.

64

2.10.1 DC Electrical conductivity measurements using parallel plate capacitor

method

For the DC electrical conductivity measurements crystals with high

transparency and large surface defect-free (i.e. without any pit or crack or scratch on the

surface tested with a traveling microscope) were selected and used. The extended portions

of the crystals were removed completely and the opposite faces were polished and coated

with good quality graphite to obtain a good conductive surface layer. The dimensions of

the crystals were measured using a traveling microscope (L.C. = 0.001 cm).

The DC conductivity measurements were carried out along the b-axis of the pure

and K , Na^ and Zn^* ions doped LACC crystals using the conventional two-probe

technique at various temperatures ranging fi'om 40 - 110°C. The DC resistances of the

crystals were measured using a million - meg ohmmeter. The observations were made both

for heating and cooling of the samples. The DC conductivity (ojc) of the crystal was

calculated using the relation

Ode = d/(RA) mho.

where R is the measured resistance, d is the thickness of the sample and A is the face area

of the crystal in contact with the electrode.

2.10.1a DC Activation energy

DC activation energy of a substance is the minimum energy required for the atoms

or molecules in the compound to activate wliile d.c voltage is applied. The dependence of

DC conductivity on temperature is usually obeying the well known relation [204].

Ode = o exp (- Edc/KT)

65

where K is the Boltzmann's constant, T is the absolute temperature , a is the constant

depending on the material and Edc is the DC activation energy. The above equation can be

written as

In Ode = IncT - Edc/KT

This is equation for a straight Ime. A plot of hi Odc versus (1/T) gives Edc/KT as the slope

and Ina as the y- intercept. It is customary to plot In Odc versus (1000/T), from the slope

of which the activitation energy (Edc) can be calculated.

2.10.2 Dielectric and AC conductivity measurements using LCR meter

Dielectrics are non-metallic materials having high resistance and negative

temperature coefficient of resistance. The dielectric constant of a material is related to

electronic, ionic, dipolar and space charge polarizations. All these are active at low

frequencies, hi fact, the nature of variation of dielectric constant with frequency indicates

the type of polarization present. The space charge polarization will depend on the purity

and perfection of the crystals. Its influence is negligible at low temperatures and is

noticeable in the low frequency region. The dipole orientational effect can sometimes be

seen in some materials even up to lO' Hz. The ionic and electronic polarizations always

exist below lO' Hz. Dielectric constant is a number relating the ability of a material to »

carry alternating current to the ability of vacuum to carry alternating current. The

capacitance created by the presence of the material is directly related to the dielectric

constant of the material. Knowuig the dielectric constant of a material is needed to properly

design and apply instruments such as level controls using radar, RF admittance, or

capacitance technologies.

66

Various polarization mechanisms in solids such as atomic polarization

of the lattice, orientational polarization of dipoles and space charge polarization can be

understood very easily by studying the dielectric properties as a function of frequency and

temperature for crystalline solids. The frequency dependence of these properties gives a

great insight into the material's applications. The dielectric constant determines the shaie of

the electric stress which is absorbed by the material without any dielectric breakdown. The

dielectric loss is a measure of the energy absorbed by a dielectiic. There are also analytical

reasons to know the dielectric constant of a material.Operation of electro-optic devices is

based on the Pockel's effect, in which the change in the dielectric constant, Asr, is a linear m

function of the applied field. So the dielectric characterization may yield some useful initial

information. Microelectronics industry needs replacement of dielectric materials in

multilevel interconnect structures with new low dielectric constant (s ) materials, as an

interlayer dielectric (ILD) which surrounds and insulates intercoimect wiring. Lowering the

8r values of the ILD decreases the RC delay, lowers power consumptions and reduces

'cross-taUc' between nearby interconnects

In order to carry out dielectric measurements, selected samples of the pure and K ,

Na"* and Zn " ions doped LACC ciystals were polished in proper size and for good

electrical contact opposite faces of the sample cr,'stals were coated with good quality

grq)hite [205, 206]. The samples were annealed in the holder assembly at 383 K before

making observation. The dimensions of the crystals were measured using a traveling

microscope and the area of contact (1 x b) of the sample with the electrodes were

calculated. The capacitance (Ccrys) and dielectric loss factor (tan 5) are measured to an

67

accuracy of+2% using Agilent 4284A Precision LCR meter with five different frequencies,

viz. 100 Hz, 1 kHz, 10 kHz, 100 kHz and 1 MHz from temperature 303 to 383 K along the

b-axis of the crystal in a way similar to that followed by Goma et al [207] and Manonmani

et al [208]. Temperature was controlled to an accuracy of+l°C.The samples were prepared

and annealed in a way similar to that followed for the resistance measurement. Air

c^acitance (Cair) was also measured.

The dielectric constant of the crystal (Sr) was calculated by using the relation (as

the crystal area was smaller than the plate area of the cell)

£. = (AA(C^,.-C,A^-A,^JAJ

c„

where Acrys is the area of the crystal touching the electrode and Aair is the area of the

electrode.

The AC conductivity (Oac) was calculated using the relation

where EQ is the permittivity of free space (8.85 x 10''^ C N'' m' ) and (o is the angular

frequency.

2.10.2 AC Activation energy

AC activation energy of a substance is the minimum energy required for the atoms

or molecules in the compound to activate while an AC voltage is applied. The dependence

of AC conductivity on temperature is usually obey the well known relation [204]

a exp (- Eac/KT)

68

where K is the Boltzmann's constai^t, T is the absolute temperature , a is the constant

depending on the material and Eac is the AC activation energy.

The above equation can be written as

hi Gac = Ina - Eac/KT

This is equation for a straight line. A plot of In Oac versus (1/T) gives Eac/KT as the slope

and Ina as the y- intercept. It is customary to plot In Odc versus (1000/T), from the slope

of which the activation energy (Eac) can be calculated.

Actually this is the energy required for the process such as direct

interchange between the two atoms, migration of an interstitial atom subsequent to

formation of a Frenkel defect, atomic displacement into a neighboring vacancy subsequent

to formation of a Schottky defect, diffusion of pairs of vacancies by means of atomic

movements in to either of vacancies, that is the sum of energies of defect formation and

subsequent migration is called the activation energy [1]