35

CHAPTER 2

CHARACTERIZATION TECHNIQUES

2.1 INTRODUCTION

A complete description of the physical and chemical properties of a

material of interest is termed as characterization of that material. Characterization of a

crystal essentially consists of determination of chemical composition, structure,

defects, study of their optical properties etc [99].

The study of growth defects includes the defects such as inclusions,

mechanical stress etc., that result due to poor control of crystal growth parameters.

Standard techniques such as etching, chemical analysis, X-ray topography, scanning

electron microscopy etc., can be used for defect characterization. The measurement of

optical properties includes the study of optical transmission and absorption of the

crystal, SHG conversion efficiency, NLO coefficients, electro-optical coefficients and

structural dependence of these properties. Characterization of NLO crystals can

generally be divided into three types:

i) Crystal structural analysis

ii) Investigation of growth defects and

iii) Measurement of optical properties

The single crystals chosen for the present investigation were subjected to

the following studies:

Collection of XRD data by single crystal X-ray diffraction analysis

and Powder X-ray diffraction.

36

FTIR spectral analysis to confirm the various functional groups

present in the compound.

CHNS analyses to identify the percentage of carbon, hydrogen,

nitrogen and sulphur in the sample.

UV-Vis-NIR analysis for the detection of the transparency region

and cutoff wavelength.

Thermal analysis to study the thermal stability.

Hardness studies to estimate the hardness.

Dielectric measurements and AC conductivity studies to investigate

the dielectric response of the crystal with varying frequency.

Etching studies to study the behavior of the crystals under different

solvents.

SHG test to check the nonlinear response of the crystal to the

incident coherent light.

Laser damage threshold studies to determine the resistance provided

by the crystal to the laser beam.

2.2 SINGLE CRYSTAL X-RAY DIFFRACTION ANALYSIS

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

technique in which X-rays are employed to determine the actual arrangement of atoms

within a crystalline specimen. Single crystal X-ray diffraction 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 angles, molecular

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

crystallography. Single crystal X-ray diffractometer collects intensity data required

for structure determination.

37

Accurate measurements of intensities of reflections of all Miller indices

within a specified reciprocal radius (usually 25 for MoK and 68 for CuK ) is

needed to find the structure, while unit cell parameters depend only on direction of

reflections. As the name implies, a crystalline sample is required. For single-crystal

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 sin = n where ‘d’ is the

interplanar spacing of the incident plane, ‘ ’ is the wavelength of X-rays and ‘n’ 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.

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

groups, molecular structure of the crystalline solids and Miller indexing 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

performed using an ENRAF NONIUS CAD4-F single crystal X-ray diffractometer.

The shield was equipped with graphite monochromated MoK radiation. Since the

crystal was transparent, the single crystallinity was studied with Leica polarizing

microscope. Single crystal of suitable size was cut and mounted on the X-ray

goniometer. The sample is mounted on a thin glass fiber that is attached to a brass pin

and mounted on a goniometer head. Adjustment of the goniometer head in the X, Y

and Z orthogonal directions allows centering of the crystal in the X-ray beam. The

38

unit cell dimensions (a, b, c, , , ) can be determined from the accurately measured

values (the angular deviation from the direct undeviated beam of about 25

reflections. The reflections were indexed using method of short vectors followed by

least square refinements. The unit cell parameters thus obtained were transformed to

correct Bravais cell.

2.3 POWDER X-RAY DIFFRACTION ANALYSIS

The powder method of diffraction 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 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 K excitation potential of the target material.

Selection of K renders the incident beam to be a highly monochromatised

one. The focusing monochromatic geometry results in narrower diffracted peaks and

low background at low angles. The sample is mounted vertically to the

Seemann-Bohlin focusing circle with the scintillation counter tube moving along the

circumference of it. It is possible to record the diffracted beam from 2 to 160 degrees.

The diffractometer is connected to a computer for data collection and analysis. The

scintillation counter tube can be moved in step of 0.01 degree by means of a stepper

motor and any diffracted beam can be closely scanned to study the peak profile.

High Resolution GUINIER Powder X-ray diffractometer (SEIFERT,

Germany) has been used for phase analysis as well as for calculation of the lattice

parameters. This diffractometer can record diffraction pattern at lower angles (starting

from 1°) with very little background and good resolution. The fine line focused beam

(0.4 mm wide and 0.8 mm in height) generated from the Cu-target, is used at the focal

plane in the present study. A bent primary quartz monochromatic situated close to the

target is used to select the CuK radiation.

39

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. For the purpose of comparison, many standards are

available, some of which are, Willars Hand book, Joint Committee on Powder

Diffraction Standards (JCPDS) Pcpdfwin and National Bureau of Standards.

2.4 FOURIER TRANSFORM INFRARED (FTIR) SPECTRAL

ANALYSIS

Fourier Transform Spectroscopy is a simple mathematical technique to

resolve a complex wave into its frequency components. The conventional IR

spectrometers are not of much use for the far IR region, as the sources are weak and

the detectors are insensitive. FTIR has made this energy-limited region more

accessible. It has also made the mid infrared more useful. Conventional spectroscopy,

called the frequency domain spectroscopy, records the radiant power as a function of

frequency. In the time domain spectroscopy, the change in radiant power is recorded

as a function of time. In the Fourier Transform Spectrometer, a time domain plot is

converted into a frequency domain spectrum. The actual calculation of the Fourier

transform of such systems is done by means of high – speed computers.

The FTIR spectrometer consists of an infrared source, a sample chamber

with a provision for holding solids, liquids and gases, monochromator, a detector and

a recorder, which are integrated with a computer. At present, all commercially

available infrared spectrophotometers employ reflection gratings rather than prisms as

dispersing elements. Interferometric multiplex instruments employing the Fourier

transform are now finding more general applications in both qualitative and

quantitative infrared measurements. The interference pattern is obtained from a two

beam interferometer, as the path difference between the two beams is altered, then

Fourier transformed output gives rise to the spectrum. The transformation of the

interferogram into spectrum is carried out mathematically with a dedicated online

computer. The spectrometer consists of globar and mercury vapour lamp as sources.

An interferometer chamber comprising of KBr and mylar beam splitters is followed

40

by a sample chamber and detector. The schematic diagram of a FTIR spectrometer is

shown in Figure 2.1. The spectrometer works under vacuum condition. Solid samples

are dispersed in KBr or polyethylene pellets depending on the region of interest. This

instrument has resolution of 0.1 cm–1. Signal averaging, signal enhancement, baseline

correction and other spectra manipulations are possible with multitasking OPUS

software on the dedicated PC/AT 486. Spectra are plotted on a HP plotter and data

can be printed.

Figure 2.1 Schematic diagram of a FTIR spectrometer

Recording of IR spectra of solid sample is more difficult because the

particles reflect and scatter the incident radiation and therefore transmittance is

always low. Three different techniques are employed commonly in recording such

spectra. For solid compounds, that are insoluble in the usual solvents, a convenient

sampling method is the Pressed Pellet Technique. A few milligrams of the sample are

ground together in an agate or mullite mortar with about 100 times the quantity of a

material (the matrix) transparent to the infrared. The usual material is KBr, although

Collimated lamp for visual alignment

Oscillating mirror ‘C’

Oscillating mirror ‘A’

Oscillating mirror ‘B’

Interferometer

IR Source

Sample component

Reference holder

Sample holder

TGS Detector

Microsampling position

41

other compounds such as CsI, TlBr and Polyethylene are used in special

circumstances. The ground powder is finally introduced into a mini pressing

arrangement made from two half-inch diameter stainless steel bolts and a stainless

steel nut. The ends of the bolts must be polished, flat and parallel. One bolt is

inserted about half way into the nut and the KBr plus sample mixture. The second bolt

is then screwed into the nut and pressure is applied by tightening the bolts

together. When the bolts are carefully withdrawn, a pellet suitable for infrared

transmission work, remains. The pellet is not removed from the nut, which acts as a

holder in the spectrometer [100].

Molecular vibrations that oscillate with the same frequency absorb IR

light. The frequency of the vibration and the probability of absorption are influenced

by intra and intermolecular effects. Thus, information about structure and

environment can be deduced from the spectral parameters, bandwidth and absorption

coefficient. There are no rigid rules for interpreting the vibration spectrum. Certain

requirements, however, must be met before an attempt is made to interpret a

spectrum :

i) The spectrum must be adequately resolved and of adequate

intensity.

ii) The spectrum should be that of a reasonably pure compound.

iii) The spectrometer should be calibrated.

iv) The method of sample handling must be specified. If a solvent is

employed, the solvent, concentration and the cell thickness should be

indicated.

A precise treatment of the vibrations of a complex molecule is not feasible;

thus, the spectrum must be interpreted from empirical comparison of spectra and

extrapolation of studies of similar molecules. Many of the group frequencies of

organic compounds vary over a wide range. Because the bands arise from complex

interacting vibrations within the molecule, absorption bands may, however represent

predominantly a single vibration mode. Important details of structure may be revealed

42

by the exact position of an absorption band within a narrow region. Shifts in

absorption position and changes in band contours, accompanying changes in

molecular environment, may also suggest important structural details

Some of the general uses of FTIR spectra are :

i) Identification of all types of organic and many types of

inorganic compounds

ii) Determination of functional groups in organic materials

iii) Determination of the molecular composition of surfaces

iv) Identification of chromatographic effluents

v) Quantitative determination of compounds in mixtures

vi) Determination of molecular conformation (structural isomers) and

stereochemistry (geometrical isomers)

vii) Determination of molecular orientation (polymers and solutions)

2.5 CHNS ANALYZER

Elemental analyzer technique determines the presence of the elements like

carbon, nitrogen, hydrogen and sulphur in a given sample and gives the result as

percentage amount of these against the total weight. This technique specially

determines these three elements and is also called “CHNS analyzer”. Most of the

organic compounds are made up of these three elements and oxygen; hence after

determining these three elements the percentage weight of oxygen can be easily

calculated [101].

This is an important study when it comes to a newly synthesized

compound to prove its exact composition. Solid samples weighing from less than

1 mg up to 200 mg are packed in tin boats. Samples are dropped into the combustion

tube automatically at user selected temperature i.e. 1000 ºC by combustion to tin

boats. The principle of the instrumentation is that the substance under study is

combusted under oxygen stream in a furnace at high temperatures (1000 °C). In the

43

combustion process, carbon is converted to carbon dioxide; hydrogen to water;

nitrogen to nitrogen gas/oxides of nitrogen and sulphur to sulphur dioxide. That is, the

end product of the combustion would be mostly the oxides of the concerned elements

in the form of gases. These are then separated and carried to the detector using inert

gases helium or argon. The Vario EL III-Germany CHNS analyser was used in the

present study.

It gives a clear quantitative measurement of the carbon, hydrogen and

nitrogen. It finds applications in almost every field of chemistry like the analysis of

organics, polymers, pharmaceuticals, energy fuels, environmental studies etc.

2.6 UV - VISIBLE SPECTROSCOPY

Ultraviolet-visible spectroscopy (UV/ VIS) is also known as electronic

spectroscopy. Ultraviolet (200 - 400 nm) and visible (400 - 800 nm) absorption

spectroscopy is the measurement of the attenuation of a beam of light after it passes

through a sample or after reflection from a sample surface. The schematic

representation of a UV-Vis spectrophotometer is shown in Figure 2.2. It uses light in

the visible and adjacent near ultraviolet (UV) and near infrared (NIR) ranges. In this

region of energy space molecules undergo electronic transitions.

Ultraviolet and visible light are energetic enough to promote outer

(valence) electrons to higher energy levels. Valence electrons are found in three types

of electron orbitals namely bonding orbitals, bonding orbitals and non-bonding

orbitals (n-lone pair electrons). Sigma ( ) bonding orbitals tend to be lower in energy

than bonding orbitals, which in turn are lower in energy than non-bonding orbitals.

The unoccupied or anti bonding orbitals ( * and *) are the orbitals of highest energy.

An energy level diagram showing electronic transitions is depicted in Figure 2.3. Of

the six transitions outlined, only the two lowest energy ones (left-most, coloured blue)

are achieved by the energies available in the 200 to 800 nm spectrum. As a rule,

energetically favored electron promotion will be from the highest occupied molecular

orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO) and the

resulting species is called an excited state.

44

Figure 2.2 Schematic representation of a UV-Vis-NIR spectrophotometer

Figure 2.3 Energy level diagram with electronic transitions

EN

ER

GY

n * *

n *

*

* (anti – bonding)

* (anti – bonding)

n (non – bonding)

( bonding)

(bonding)

45

In particular, absorption bands results from transitions ( *) and n *

involving -orbitals and lone pairs (n = non-bonding) are important and so

UV-Vis spectroscopy is of most use for identifying conjugated systems which tend to

have stronger absorptions. Absorption bands can also arise from certain molecules

where the energy required for removing an electron from one atom and placing it on

another falls within the UV/Vis region. This process is known as a charge transfer

excitation. Molecules with the ability to exhibit the above types of electronic

transitions are said to possess chromophores. An isolated functional group not in

conjugation with any other group is said to be a chromophore if it exhibits absorption

of a characteristic nature in the ultraviolet or visible region. The most common are

(C=C) and (C=O) which exhibits * and n * transitions respectively.

If a series of compounds have the same functional group and no

complicating factors are present, all of them will generally absorb at very nearly the

same wavelength. Thus, it is readily seen that the spectrum of a compound, when

correlated with data from the literature for known compounds, can be a very valuable

aid in determining the functional groups present in the molecule.

Samples are typically placed in a transparent cell, known as a cuvette. The

sample holders (cuvettes) are the rectangular shaped quartz or glass cells of about

10 mm path length. The transmitted light radiation is received at the photomultiplier

tube alternately from the reference and the sample beams. A photoelectric signal

timing system is synchronized with the alternate pulses which permits the comparison

of signals from the two beams. The difference between the two signals is recorded

with the help of a motor driven pen or is interfaced with a PC-XT and stored there for

easy reference. Samples in solid form, powder, pellets are dissolved in suitable

solvents to form the contents of the sample cell and the solvents are taken in the

reference cell. In the present work, the UV-Visible spectra was recorded using Perkin

Elmer Lamda Instrument.

46

2.7 THERMAL STUDIES

Thermal analysis is a very essential method to study the thermal behaviour of materials and finds widespread applications in diverse industrial and research fields. It is a general term, which covers a group of related techniques in which the temperature dependence of the parameters of any physical property of a substance is measured. In addition to providing valuable information on the thermal stability of the compounds and the decomposition products, these studies often provide an insight into their mode of decomposition.

Thermoanalytical methods involve the measurement of various properties of materials subjected to dynamically changing environments under predetermined conditions of heating rate, temperature range and gaseous atmosphere or vacuum. In many cases, the use of a single thermoanalytical technique may not provide sufficient information to solve the problem on hand and hence, the use of other thermal techniques, either independently or simultaneously for complementary information becomes necessary.

Thermoanalytical analysis incorporates the following three closely related techniques:

i) Thermogravimetric analysis (TGA), which involves monitoring weight while varying temperature.

ii) Differential thermal analysis (DTA), which involves comparing the precise temperature difference between a sample and an inert reference material, while heating both.

iii) Differential scanning calorimetry (DSC), similar to DTA except that electrical energy is used to restore the cooler of the two materials to the same temperature as the other. This allows direct measurement of energy changes.

These techniques are useful for determining glass points, phase changes, water of crystallization and mixtures where the components have different melting or decomposition points. Among the thermal methods, the most widely used techniques

47

are TG, DTA and DSC, which find extensive use in all fields of inorganic and organic chemistry, metallurgy, mineralogy and many other areas. In the present work, thermal behaviour of the grown crystals has been investigated using DTA and TG-DTG techniques.

Thermogravimetry is a technique in which the mass of a substance is

measured as a function of temperature or time, while the substance is subjected to a

controlled temperature program. The curve obtained in a thermogravimetric analysis

is called a thermogram (TG) and its first derivative is called derivative thermogram

(DTG). The inflexion point in the program corresponds to the peak point in the

derivative thermogram. Modern commercial TG instruments consist of the following:

i) A sensitive analytical balance

ii) A temperature programmable furnace

iii) A pure gas system for providing suitable gas atmosphere and

iv) A microprocessor for instrument control, data acquisition and display

Even though different types of balance mechanism are available today,

those employing null-point-weighing mechanism are favoured as the sample remains

in the same zone of furnace irrespective of changes in mass. The furnace is normally

an electrical resistive heater and the temperature range for most of the furnace is from

ambient to 1000 - 2000 ºC. Thermogravimetry is widely used to determine the

thermal stability, decomposition temperature, temperature of desorption and drying,

oxidative stability, etc.

Differential scanning calorimetry (DSC) is similar to DTA except that

electrical energy is used to restore the cooler of the two materials to the same

temperature as the other. This allows direct measurement of energy changes. DSC

deals with recording of the energy necessary to establish the zero temperature

difference between a sample and a reference material against time or temperature

when both are heated or cooled at a predetermined rate. DSC monitors the heat effects

associated with phase transitions and chemical reactions as a function of temperature.

48

It is developed from DTA. In a power compensated DSC, the heat to be measured is

compensated with electrical energy, by increasing or decreasing adjustable heat

energy. The measuring system consists of two micro furnaces of the same type made

of a Platinum-Iridium alloy, each of which contains a platinum resistance

thermometer as a temperature sensor and a heating resistor made of platinum wire.

Both micro furnaces-separated from each other, are positioned in an aluminium block

of constant temperature. The maximum heating power of a micro furnace is about

14 W; the maximum heating rate is 500 K/min. The measuring range extends from

-175 ºC to 725 ºC.

During the programmed heating, some heating powder is supplied to both

micro furnaces through a control circuit in order to change with their mean

temperature in accordance with the preset heating rate. If there is ideal thermal

symmetry, the temperature of both the furnaces is always the same. Whenever

asymmetry occurs, for example as a result of a reaction in the sample, a temperature

difference results between the furnaces and the system adjusts the power supplied to

both the furnaces in such a way that the temperature difference is maintained at zero.

Two control circuits employed to obtain differential thermogram are:

i) Differential temperature control and

ii) Average temperature control

In the average temperature control circuit, the programmer generates an

electrical signal proportionate to the desired temperature of the signal and reference

holders, which is recorded on a chart. The programmed signal is then compared with

the average signal from the temperature sensors in two holders and the resultant signal

difference is amplified by the average temperature amplifier.

If the programmed temperature (Tp) is greater than the average temperature

(Tp + Tr) / 2, more power will be fed to the two furnaces. If it is lower, the power to

the furnaces will be reduced. In this manner, the average temperature of the furnaces

is made to follow closely the programmer command and the same power is fed to the

two micro furnaces.

49

In the differential temperature control circuit, signals representing the

temperatures of the sample and reference holders are compared and are fed to the

amplifier. The amplifier output adjusts the power input to the two furnaces in such a

way that their temperatures are kept identical. In the absence of any reaction taking

place in the sample, the differential power input to the sample and reference furnaces

is almost zero. When a reaction takes place in the sample, a differential power input is

fed to the furnaces. Depending upon whether the reaction is exothermic or

endothermic, the differential power input increases the power to one furnace while

decreasing it to other furnaces. This differential power usually in mW is plotted as a

function of temperature. In the present work, Netzch Thermal TGA/DSC equipment

was used with alumina as reference material.

2.8 MICROHARDNESS STUDY

Hardness is a physicochemical property that characterizes the state of the

material under test and gives information on some specific features of the material

such as the character of the chemical bonding. It is the resistance which the material

offers to indentation by a much harder body and may be termed as a measure of the

resistance against lattice destruction or permanent deformation or damage. As the

hardness properties are basically related to the crystal structure of the material and the

bond strength, microhardness studies have been applied to understand the plasticity of

the crystals. Hardness tests are commonly carried out to determine the mechanical

strength of materials and it correlates with other mechanical properties like elastic

constants and yield stress [102]. Hardness measurements can be defined as macro,

micro and nano according to the forces applied and displacement obtained [103].

The various methods using which hardness measurement can be carried

out are classified as follows:

i) Static indentation test

ii) Dynamic indentation test

iii) Scratch test

50

iv) Rebound test and

v) Abrasion test

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

known 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

variables are 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 a hardness steel sphere or a diamond

pyramid or cone is employed. A pyramid indenter is preferred as geometrically

similar impressions are 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 indentation and

knowing the applied load, empirical hardness number may be calculated. This method

is followed by Brinell, Meyer, Vickers, Knoop and Rockwell tests [104].

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:

i) Comparison test, in which one material is said to be harder than

another, if the second material is scratched by the first

ii) 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.

51

In the abrasion test, a specimen is loaded against a rotating disk and the

rate of wear is taken as a measure of hardness. Generally good sized which are

polished are used for the hardness studies.

2.8.1 Vickers Hardness and Anisotropy Studies

Vickers hardness method is the reliable and most common among the various methods of hardness measurement discussed above. In this method, microindentation is made on the surface of a specimen with the help of diamond pyramidal indenter. Smith et al., [105] have proposed that a pyramid be substituted for a ball in order to provide geometrical similitude under different values of load.

The Vickers pyramid indenter where opposite faces contain an angle ( = 136 ) is the

most widely accepted pyramid indenter. A pyramid indenter is suited for hardness tests due to the following reasons [106].

i) The contact pressure for a pyramid indenter is independent of indent size and

ii) 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)th 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

22 2

vp sin ( / )H

d (2.1)

where ‘ ’ is the apex angle of the indenter ( = 136 ). The Vickers hardness number

is thus calculated using the relation,

22

1.8544v

pH kg/mm d

(2.2)

52

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 behaviour is the summation of a number of

effects involved in the materials response to the indentation pressure during loading,

in the final measurement of the residual impression.

The importance of microhardness study lies in the possibilities of making

an indirect estimate of mechanical characteristics of materials such as yield strength

and toughness having a specific correlation with the hardness.

The hardness measurements may depend upon the orientation of the

indented crystals. To study the hardness anisotropy present in crystals, the crystals are

initially mounted on the stage of microscope properly and indented. The initial

position is 0 degree. The stage of the microscope was then rotated keeping the

indenter fixed and ‘Hv’ was measured at every 30 degrees interval till the original

position. No distortion in shape of indenter will be observed with crystal orientation.

When the variation of ‘Hv’ with angular displacement is periodic, then it brings the

anisotropic nature of crystals.

2.8.2 Knoops Hardness Studies

Knoop hardness can be treated as an alternative to the Vickers test,

particularly for very thin layers, Fredrick Knoop developed a low-load test with a

rhombohedral-shaped diamond indenter. The long diagonal is seven times

(7.114 actually) as long as the short diagonal. With this indenter shape, elastic

recovery can be held to a minimum. Knoop tests are mainly done at test forces from

10 g to 1000 g, so a high powered microscope is necessary to measure the indent size.

Because of this, Knoop tests have mainly been known as microhardness tests. The

magnifications required to measure Knoop indents dictate a highly polished test

surface. To achieve this surface, the samples are normally mounted and

metallurgically polished, therefore Knoop is almost always a destructive test.

53

The Knoop indented impressions are approximately rhombohedral in

shape. The average diagonal length (d) was considered for the calculation of the

Knoop hardness number (Hk) using the relation,

214.229

k PH

d (2.3)

where P is the applied load in kg, d in mm and Hk is in kg/mm2. In the present work,

MUTUTOYO M112 Japan, Instrument was used.

2.9 DIELECTRIC STUDIES

Dielectric measurement is one of the useful characterizations of electrical

response of solids. A study of the dielectric properties of solids gives information

about the electric field distribution within the solid. The frequency dependence of

these properties gives a great insight into the materials applications. The different

polarization mechanisms in solids can be understood from the study of dielectric

constant as a function of frequency and temperature.

Polarization ‘P’ of a dielectric is the numerical magnitude, which

describes the phenomenon of polarization of a dielectric in an external electric field.

In the absence of an external electric field, each element in the volume of a dielectric

has no electric moment. The action of an electric field brings the charges of the

molecules of the dielectric into a certain ordered arrangement in space. The study of

dielectric constant of a material gives an insight into the nature of bonding in the

material. The study of the electrical and other properties of dielectrics in relation to

their chemical composition and structure will lay the basis for obtaining new materials

with new properties. A lot of work had been carried out on dielectric measurement for

a variety of materials including ceramics and single crystals by many authors yielding

valuable information [107 - 109].

54

The relative dielectric constant ( r) is defined as,

0

r (2.4)

It is known that, CdA

(2.5)

Hence, 0r ACd

(2.6)

where, ‘A’ is the area of the sample and ‘d’ is the thickness of the sample.

The relative permittivity ( r) is usually known as permittivity. It is always greater

than unity. The capacitance ‘C0’ of a parallel plate condenser having a capacitance ‘C’

in air can be given by,

C = r C0 (2.7)

Thus, 0

rC

C (2.8)

‘ r’ can be found by the measurement of capacitance. The dielectric

constant of a substance is a property of the constituent ions. Major contributions to the

dielectric constant are from,

i) The extrinsic nature of the material,

ii) The electronic polarizability,

iii) The ionic polarizability and

iv) The deformation of the ions

55

In the present work, HIOKI 3532-50 LCR HITESTER meter was used for

the dielectric study which is shown in Figure 2.4.

The A.C. Conductivity is given by:

ac = 2 f tan 0 r (2.9)

where f is the frequency.

Using the above equation (2.9), the A.C. Conductivity at different

temperatures were calculated and the graph between ac and 1/T are plotted.

Figure 2.4 Photograph of HIOKI 3532-50 LCR HITESTER

2.10 ETCHING STUDIES

Etching studies have been carried out to understand the growth mechanism

and to assess the perfection of the grown crystals. Chemical etching is the oldest,

easiest and most widely used method to evaluate the quality of the crystal. Etching is

selective dissolution of the crystal which is used to reveal the crystal symmetry and

lattice defects.

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The establishment of the relation between etch pits and the dislocation

array present in imperfect crystals by employing the etching technique has turned out

to be a powerful tool for the preparation and characterization of single crystals for

electronic and optical devices [110, 111].

The rate of reaction of a solution with a solid surface can depend distinctly

on the crystallographic orientation on the reacting surfaces. This rate dependence is

the basis of etching and brought about intentionally by specific chemical attack on the

crystal surface. When a surface is etched, well defined etch patterns of negative

volume are produced at the dislocation sites according to Thomas and Renshaw [112].

Etching may be considered as the reversal of crystal growth; therefore, a surface step,

for example, a screw dislocation, will dissolve more easily than a flat surface as

reported by Burton et al., [113]. The relative rates of removal of atoms along different

directions determine the geometry of etch pits. The shape of the etch pits may be

changed by varying the solvent. Moreover, factors such as temperature of etching,

stirring and adsorption of impurities or reaction products, which alter the absolute

value of these rates, also lead to the change in the geometry of etch pits [114]. Since

the geometrical variation of etch pits strongly depends on the arrangement of ions or

atoms which comprises the crystal faces, the shapes of the etch pits also differ for

various crystal faces. The shape of the etch pits may be changed by varying the

amount of the solvent. The etch patterns obtained also depend upon the nature of the

etchant. In the present work, the etching studies were carried out using Carl Zeiss

optical microscope (Model Imager AIM).

2.11 REFRACTIVE INDEX MEASUREMENTS

The refractive index of the crystals can be determined by Brewster’s angle

method using He-Ne laser of wavelength 632.8 nm. A polished flattened single crystal

of is mounted on a rotating mount at an angle varying from 0 to 90 degrees. The

angular readings on the rotary stage was observed, when the crystal is perfectly

perpendicular to the intracavity beam. The crystal is rotated until the laser oscillates

and the angle has been set for maximum power output. Brewster’s angle ( p) for the

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crystal is measured. The refractive index is calculated using the equation, n = tan p,

where p is the polarizing angle.

2.12 NLO TEST – 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 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 oriented samples. Kurtz and Perry

proposed a powder SHG method for comprehensive analysis of the second order

nonlinearity. Employing this technique, Kurtz surveyed a very large number of

compounds.

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 2.5. A Q-switched, mode locked Nd:YAG laser was used to generate about

6 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 multishot

mode, the 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 mirror and a 50/50 beam splitter (BS) to

generate a beam with pulse energies about 6 mJ. The input laser beam was passed

through an IR reflector and then directed on the microcrystalline powdered sample

packed in a capillary tube of diameter 0.154 mm.

The photodiode detector and oscilloscope assembly measure 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 harmonic

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.

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Figure 2.5 Schematic experimental setup for SHG efficiency measurement

2.13 LASER DAMAGE THRESHOLD STUDY

An important related property of NLO crystals is the threshold for catastrophic laser induced damage. Laser induced damage in optical materials is a phenomenon involving interaction of high power laser radiation with matter and various physical, chemical, mechanical, optical and other aspects of materials that come into play. It is evident that the harmonic conversion efficiency is proportional to the power density of the fundamental beam. Hence, a convenient way to increase the efficiency is to focus the beam into the crystal. But, this often leads to breakdown of the materials, catastrophically damaging the crystal. It is then useful to prescribe the maximum permissible power for a particular crystal, defined as damage threshold.

The minimum power level which causes damage to at least 50% of sites irradiated is defined as the single shot (1-on-1) damage threshold (P1). However, power levels much lower than this can cause damage to the material under continuous exposure. Hence, at n pulses, damage may occur to the material at power levels Pn < P1. Following the definition of Nakatani et al., [115], the multiple shot (n-on-1) damage threshold ( ,) is the maximum power level below which the crystal does not suffer damage even after 1800 pulses. The origin of laser induced damage is highly complex. However, there are some processes which are basic to the phenomenon of laser damage.

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Laser damage threshold is an important material parameter, the knowledge of which is essential for using the crystal as an NLO element in various applications involving large laser input power like frequency doubling, optical parametric processes, etc. In fact, laser induced damage in optical materials remains the limiting factor in the development of high power laser systems and optoelectronic devices. Although the optical damage has been studied in materials ever since the advent of high power lasers, much still remains to be learnt about the interaction of a high intensity laser beam with materials [116]. It is believed that the formation of an ionized region of dense plasma is the first and the most important step in a damage event [117]. In recent times, there have been several effects to model the laser damage processes by taking the pulse shape and duration in to account [118]. Laser damage in general manifests itself as a localized microscopic entity. Microscopic examination of the damage sites created in different materials often suggests the mechanism responsible for such a process. The damage in NLO materials like KDP and Deuterated Potassium Phosphate (DKDP), manifests itself as micro-cavity whose sizes are insensitive to the wavelength of the laser beam, suggesting a wavelength independent absorption mechanism. However, the damage mechanisms vary from material to material and hence damage studies become important for newly discovered materials. Laser damage measurement was carried out on the crystal at 1064 nm. The single shot surface damage thresholds have been determined. The laser damage threshold depends on pulse duration, focal spot geometry, sample quality, previous history of the sample, experimental technique employed etc. It may be noted that in the present case 13 ns pulse was used for the laser damage threshold measurement. Recent investigations into laser damage in various optical materials by nanosecond pulses have shown that the temperature reached at the damage site could be as high as 12000 K. One can expect the damage to be of thermal origin. However, one cannot rule out other mechanisms being operative simultaneously, as the damage mechanism is quite complex and depends on the nature of the material and various experimental parameters.

The experimental set-up used for the measurement of laser damage of the samples is shown in Figure 2.6. A Q-switched Nd:YAG laser (Continuum USA, Model: Surelite-III) of wavelength 1064 nm and pulse width 13 ns was used. The energy of the laser pulses was controlled by an attenuator (combination of / 2 plate

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and glan polarizer) and delivered to the test sample located near the focus of a plano-convex lens of focal length 30 cm. The occurrence of single pulse damage was observed by monitoring the fall of transmitted intensity as detected by a fast PIN type Si photodiode and traced in a digital storage oscilloscope (Tektronix:TDS 3054B). A reference pyro-electric energy meter records the energy of the input laser pulse for which the crystal gets damaged. The laser damage threshold was calculated using the relation,

P = (E / A) (2.10)

where P is the Laser damage threshold in GW/cm2, E is the energy required to break the crystal, is the response time in ns and A represents the beam area.

Figure 2.6 Functional Block Diagram of the Setup used for damage threshold measurement

2.14 CONCLUSION

Single crystals grown by the various methods need to be characterized to that assess the suitability of the crystal for various applications including NLO device application. In order to understand the behaviour of any solid material, firstly, the structural characterization was carried out. Secondly, the optical characterization to check the transparency window and cut-off frequency of the crystal was performed. Since the entire thrust is with respect to NLO application, the SHG property was estimated. In addition, the mechanical, thermal, dielectric, refractive index and laser damage threshold of the grown crystal were also investigated.