3. growth and characterization...
TRANSCRIPT
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3. GROWTH AND CHARACTERIZATION
TECHNIQUES
This chapter briefly presents the low-temperature solution methods used for
the growth of single crystals. It enumerates the significance of the semiorganic
materials used as dopants in KDP, their synthesis, along with growth of pure and
semiorganic doped KDP single crystals. The experimental methods and mechanism
adopted for the characterization of the grown crystals are also described.
3.1 LOW TEMPERATURE SOLUTION GROWTH Growth of single crystals from solutions at low temperatures is the only
method for the crystallization of substances which undergo decomposition before
melting. This method may be used for substances fairly soluble in a solvent and non
reactive with it. Single crystals intended for device applications are expected to have a
well developed morphology and to contain a low density of defects. For this, it is
necessary to consider the thermodynamic and kinetic parameters which characterize
the overall growth conditions. The thermodynamic parameters determine the growth
mechanism and the kinetic parameters determine the growth kinetics space and the
generation of defects.
Nucleation is one of the most important phenomenon in crystal growth. In a
supersaturated (or super cooled) system when few atoms or molecules join together in
the form of a cluster, a change in energy takes place. This cluster which consists of
such atoms or molecules is normally termed as an embryo. An embryo may grow or
disintegrate and disappear completely. If the embryo grows to a particular critical
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size, then it is known as “critical nucleus”, and there is a greater probability for the
nucleus to grow.
Growth of single crystals ranges from a small inexpensive technique to a
complex sophisticated expensive process and crystallization time changes from
minutes to several months. It is a non-equilibrium process, at the same time the crystal
growth process must be as near equilibrium and as near to a steady state process as
possible. So, control of the crystal growth environment and consideration of growth
kinetics both at the macroscopic and atomic levels are of vital importance to the
success of a crystal growth experiment.
Crystal growth from solution is an ancient crystal growth method permitting
crystal growth at a temperature well below the melting point. Materials which
decompose on heating and/or which exhibit any structural transformation while
cooling from the melting point can be grown by low temperature solution growth
technique, when suitable solvents are available. The supersaturation may be attained
by evaporation of the solvent, by cooling the solution or by a transport process in
which the solute is made to flow from a hotter to a cooler region.
If the crystal is in dynamic equilibrium with its parent phase, the free energy is
at a minimum and no growth will occur. For growth to occur this equilibrium must be
disturbed by a change in temperature, pressure, chemical potential, electrochemical
potential or strain. The system may then release energy to its surrounding to
compensate for the decrease in entropy accompanied by the ordering of atoms in the
crystal and the evolution of heat of crystallization. Normally, just one of these
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parameters is held minimally away from its equilibrium value to provide a driving
force for the growth of crystal.
Selection of a solvent is a critical step in crystal growth from low temperature
solutions. Proper selection of a solvent requires a chemical similarity between the
solvent and the material to be grown. For example, crystals of nonpolar organic
compounds can be easily grown from nonpolar organic solvents. The chemical
similarity also determines the crystal solubility in the solvent. Consequently, because
there is the contact between the surface of a growing crystal and the solvent
molecules, the solvent also provides a control over the crystal habit.
Organic solvents are in general, volatile, toxic and inflammable. For growing
crystals of many materials, water is a good solvent because of its low viscosity, low
toxicity, easy availability in the pure state and cheapness. It is also inert to a variety of
glasses, plastics and metals and provides a wide range for the selection of growth
temperature in comparison with other solvents. A solvent in which the solute has
solubility between 10 to 60 % may be considered suitable for crystal growth. In the
case of very high solubility (i.e. solutions containing a large number of solute) growth
rate may be very low due to the increased solution viscosity which renders the system
diffusion controlled or the resulting crystals are of unwanted morphology. Similarly,
solvents in which a solute is less soluble also provide low growth rates due to the low
solubility. In both these cases, it is desirable to use solution modifiers to change
solubility or viscosity.
We have, in the present study, used one of the low-temperature solution
methods (the free-evaporation method) for the growth of sample crystals. So, we
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briefly provide here some information regarding the three major methods used at low
temperatures for the growth of single crystals.
3.2 METHODS OF CRYSTALLIZATION FROM SOLUTION GROWTH
Low temperature solution growth can be sub-divided into the following
methods such as slow cooling method, slow evaporation method and temperature
gradient method.
In slow cooling method, supersaturation is produced by a change in
temperature usually throughout the whole crystallization. Since the volume of the
crystallization is finite and the amount of substance placed in it is limited, the
supersaturation requires systematic cooling. It is achieved by using a thermo stated
crystallizer and the volume of the crystallizer is selected based on the desired size of
the crystals and the temperature dependence of the solubility of the substance. The
lower limit of the temperature is the room temperature.
In slow evaporation method, an excess of a given solute is established by
utilizing the difference between the rates of evaporation of the solvent and the solute.
In this method the solution loses particles, which are weakly bound to other
component and therefore the volume of the solution decreases. The vapour pressure of
the solvent above the solution is higher than the vapour pressure of the solute and
therefore the solvent evaporates more rapidly and the solution becomes supersatured.
It is sufficient to allow the vapour formed above the solution to escape freely into the
atmosphere. This method of crystal growth is the oldest and technically very simple.
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Temperature gradient method involves the transport of the materials from a hot
region containing the source materials to be grown to a cooler region where the
solution is supersaturated and the crystal grows. The main advantages of this method
are that, this method is insensitive to changes in temperature provided, both the source
and growing crystal undergo the same change with the crystal growing at a fixed
temperature and there is economy of the 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.
3.3 SIGNIFICANCE OF THE SEMIORGANIC DOPANTS USED
The quest for new frequency conversion materials is presently concentrated
on semiorganic crystals due to their large nonlinearity, high resistance to laser
induced damage, low angular sensitivity and good mechanical hardness [209]. Amino
acid exists as dipolar ion in which the carboxyl group is present as carboxylate ion
and this dipolar nature of amino acids make them ideal candidates for NLO
applications. Complexes of amino acids with inorganic salts are promising materials
for optical second harmonic generation as they tend to combine the advantage of the
organic amino acid with that of the inorganic salt [210]. A large number of
semiorganic materials have been formed using amino acids such as glycine,
L-arginine, L-alanine, L-proline, L-lycine with a wide combination of inorganic salts
and they have been explored for a variety of applications.
Transition metal thiourea (TU) complexes are also potentially useful
candidates for such organometallic systems. As ligands with potential S and N donors,
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the TU molecule is interesting not only due to the structural chemistry of their
multifunctional coordination modes, but also because of the possibility of formation
of organometallic coordination complexes with NLO activities. In the case of metal–
organic coordination complexes, the organic ligand is usually more dominant in the
NLO effect. As for the metallic part, focus is on the group - IIB metals, (Zn, Cd and
Hg) as these compounds usually have a high transparency in the UV region, because
of their closed d10 shell configuration. Regarding the organic ligands, small � electron
systems such as Urea [OC(NH2)2], and thiourea [SC(NH2)2] have been used with
remarkable success. These ligands and their metal (group IIB) complexes are always
colourless. Potential NLO materials like zinc tris(thiourea) sulphate (ZTS), thiourea
zinc chloride (ZTC), bis thiourea zinc chloride (BTZC), triallyl-thiourea cadmium
chloride (ATCC), bis thiourea zinc acetate (BTZA), are examples of this approach.
Zinc tris(thiourea) sulphate (ZTS) is a metal-organic crystal which plays an
important role in the emerging photonic and optoelectronic technologies. It is a
promising semiorganic NLO material which has a high laser damage threshold, a low
cut off and is 1.2 times more nonlinear than KDP [209]. It is a noncentrosymmetric
orthorhombic crystal with space group Pca21 [210].
Bis thiourea zinc chloride (BTZC) is a potential nonlinear optical material
which crystallizes in the noncentrosymmetric orthorhombic space group Pn21a with
SHG efficiency 0.66 times that of KDP. It has a large transmittance window in the
visible region which enables very good optical transmission of the second harmonic
frequencies of Nd:YAG lasers [211].
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Urea thiourea magnesium sulphate (UTMS) is a relatively new semiorganic
material with good thermal stability.and transmitting ability in the entire visible
spectrum. It belongs to the orthorhombic system and has an SHG efficiency which is
1/4th of KDP [212].
Glycine sodium nitrate (GSN) is one of the complexes of glycine and it
possesses both non-linear optical (NLO) and ferroelectric properties with very high
degree of transparency. It belongs to the monoclinic system with space group Cc and
its SHG efficiency is twice that of KDP [213].
L-alanine sodium nitrate (LASN) is an outstanding NLO material of potential
applications with good lower cut-off wavelength and optical transmission window� It
belongs to the orthorhombic system and has space group P212121, and has an SHG
efficiency which is two times that of KDP [214].
3.4 SYNTHESIS OF SEMIORGANIC DOPANTS
In the present study Zinc tris(thiourea) sulphate (ZTS), Bis thiourea zinc
chloride(BTZC), Urea thiourea magnesium sulphate (UTMS) which are potential
thiourea complexes, Glycine sodium nitrate (GSN) and L-alanine sodium nitrate
(LASN) - the complexes of amino acids - have been synthesized and crystallized. The
identity of these grown semiorganic crystals was confirmed by FTIR analysis, and
their powdered crystalline samples were used as dopants.
3.4.1 Synthesis of Zinc tris(thiourea) sulphate (ZTS) Pure ZTS salt was synthesized by stoichiometric incorporation of AR grade
thiourea and zinc sulphate taken in the molar ratio 3:1. The component salts were very
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well dissolved in deionized water and the solution was stirred to get a saturated
solution using a magnetic stirrer and the mixture was heated at 50° C till the white
crystalline salt of ZTS was obtained. Temperature was maintained at 50° C to avoid
decomposition. The resultant precipitate of ZTS was dried and purified by
recrystallization. The mechanism of the reaction is as follows.
3CS[NH2]2 + ZnSO4 → Zn[CS(NH2)2] 3SO4
From the saturated aqueous solution prepared using the synthesized salt of
ZTS and deionised water, small transparent colourless single crystals of ZTS were
grown within 20-25 days employing slow evaporation technique at constant room
temperature [210].
3.4.2 Synthesis of Bisthiourea zinc chloride (BTZC) Bisthiourea zinc chloride was synthesized using AR grade thiourea and zinc
chloride in the molar ratio 2:1. The component salts were very well dissolved in
deionised water and thoroughly mixed using a magnetic stirrer and the mixture was
heated at 50° C till the white crystalline salt of BTZC was obtained. The mechanism
of the reaction is as follows
2CS[NH2]2 + ZnCl2 → Zn[CS(NH2)2]2 Cl2
From the saturated aqueous solution prepared using the synthesized salt of
BTZC and deionised water, small transparent colourless single crystals of BTZC were
grown within 15-20 days employing slow evaporation technique at constant room
temperature [211].
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3.4.3 Synthesis of Urea thiourea magnesium Sulphate (UTMS) The required quantities of the component salts i.e. equimolar amounts of AR
grade reagents urea, thiourea and magnesium sulphate were very well dissolved in
double distilled water and thoroughly mixed for about 4 hours using a magnetic stirrer
to ensure homogeneous concentration throughout the volume of the solution. The
saturated solution taken in a beaker was covered with a perforated cover and left
undisturbed for slow evaporation. Good quality single crystals were grown within
three weeks [212].
3.4.4 Synthesis of Glycine sodium nitrate (GSN) Glycine sodium nitrate crystals were grown from aqueous solution by slow
evaporation technique. The starting materials were analytical grade reagents glycine
and sodium nitrate. The solution was prepared by dissolving equimolar amounts of
glycine and sodium nitrate in deionized water. The solution was stirred continuously
for an hour to get a saturated solution. Then it was filtered and transferred to a beaker
covered with a perforated cover. Small transparent single crystals with perfect
external form were grown in a few weeks by the slow evaporation technique at
constant room temperature from a saturated solution obtained through spontaneous
nucleation. A possible reaction mechanism of the chemical synthesis is as follows:
C2H5NO2 + Na(NO)3 → Na(NO3) . C2H5NO2
Selecting macro defect free crystals as seeds, crystals with larger dimensions
were obtained by slow evaporation at ambient temperature after a period of 40- 45
days by slow evaporation technique [213].
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3.4.5 Synthesis of L-alanine sodium nitrate (LASN) The semiorganic dopant L-alanine sodium nitrate was synthesized using AR
grade L-alanine and sodium nitrate taken in the molar ratio 1:1. The calculated
amount of L-alanine and sodium nitrate salts were very well dissolved in deionised
water and thoroughly mixed using a magnetic stirrer to get a saturated solution. Then
it was filtered and transferred to a beaker covered with a perforated cover. Small
transparent single crystals of LASN were grown within 40-45 days with perfect external
form by the slow evaporation technique at a room temperature [214].
3.5 GROWTH OF PURE AND SEMIORGANIC DOPED KDP SAMPLE
CRYSTALS
Single crystals of pure and semiorganic-doped KDP single crystals were
grown by solution growth at room temperature by the slow evaporation of aqueous
solutions. To grow pure KDP single crystals, re-crystallized salt of KDP (Merk-
Germany) was used to prepare the supersaturated aqueous solution. The amount of
solute (m) in grams is given by the following relation
m = (M × X × V ) /1000 (in gram units)
where M is the molecular weight of the solute, X is the supersaturated concentration
in molar units (1M in the present work) and V is the required volume of the solution.
A volume of 200 ml of pure KDP solution was prepared and the pH was noted as 3.8.
The solution was constantly stirred for about 3 hours using a magnetic stirrer and then
filtered using Whatmann filter paper. The filtered solution was transferred to a borosil
glass beaker which was porously sealed and placed in a dust free atmosphere for slow
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evaporation. The grown crystals were harvested after a growth period of about 15 to
20 days depending on the temperature of the surroundings.
Potassium dihydrogen phosphate was doped with semiorganic material Zinc
tris(thiourea) sulphate (ZTS), Bis(thiourea)zinc chloride (BTZC), Urea thiourea
magnesium sulphate (UTMS), Glycine sodium nitrate (GSN), and L-alanine sodium
nitrate (LASN) in five different molar ratios. For the formation of the doped crystals,
the supersaturated solution was prepared by dissolving the dopant solute along with
the pure KDP solute. If the molecular ratio of the pure substance and dopant is 1: P,
then the amount of dopant solute (m1) to be added is calculated using the formula
m1 = (M1 × X × V × P ) / 1000 (in gram units).
where M1 is the molecular weight of the dopant. For the dopants ZTS, BTZC, UTMS,
GSN and LASN, the doping ratios used in the present study are
Pure KDP : Dopant (ZTS, BTZC, UTMS, GSN and LASN)
1 : 0.00 (for pure KDP)
1 : P (for P = 0.002, 0.004,0.006, 0.008, 0.01)
where the dopants are added to KDP in five different molar ratios. Figure 3.1(a) & (b)
shows the photographs of the grown pure and doped crystals.
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Pure KDP crystal
0.2 mol% 0.4 mol% 0.6 mol% 0.8 mol% 1.0 mol%
KDP crystals doped with ZTS
0.2 mol% 0.4 mol% 0.6 mol% 0.8 mol% 1.0 mol%
KDP crystals doped with BTZC
Figure 3.1(a): Photographs of the grown pure and semiorganic doped
KDP crystals
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0.2 mol% 0.4 mol% 0.6 mol% 0.8 mol% 1.0 mol%
KDP crystals doped with UTMS
0.2 mol% 0.4 mol% 0.6 mol% 0.8 mol% 1.0 mol%
KDP crystals doped with GSN
0.2 mol% 0.4 mol% 0.6 mol% 0.8 mol% 1.0 mol%
KDP crystals doped with LASN
Figure 3.1(b): Photographs of the grown semiorganic doped KDP crystals
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3.6 EXTERNAL FEATURES OF THE GROWN CRYSTALS
All the grown crystals of pure and semiorganic doped KDP were found to be
colourless and transparent with good quality.
The external morphology of ZTS doped KDP crystals shows a tapered habit.
This may be due to the adsorption mechanism of the impurities. A significant
extension takes place in the prism sector for concentrations of 0.2, 0.4 and 0.6 mol%
of ZTS in the doped KDP crystals. When the concentration of impurity is increased
the propagation rate of the extension of the habit face is slowed down and tapering in
the prism sector appears. The thickness and cross section of the prism sector
decreases. The growth along (001) direction is several times greater than the growth
rate along (100) direction and observations show that the tapering effect is more in the
case of 0.8 and 1.0 mol% ZTS doped KDP crystals. Similar behavior has been
reported when impurities with large molecular dimensions as in the case of
metaphosphate, boric acid and quaternary ammonium cations have been added in
KDP which has resulted in tapering [125].
Earlier studies have also reported that selective adsorptions of metallic cation
suppress the growth of surfaces like the prismatic section (100) or pyramidal section
(101) of KDP crystals [47, 147, 215-216]. Also it was found during the crystal growth
of the N, N’ dimethyl urea doped KDP crystals that growth rate along c-axis is found
to be decreased and tapering of faces occurred along c-axis in the doped crystals. It
gives evidence that the metallic cations and dyes influence the growth of prismatic
(100) section and pyramidal (101) section of KDP crystals and changes the KDP
crystal habit. The growth rate decreases with increasing doping level. This decrease in
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the growth rate is attributed to the attachment of the larger size cations on prismatic
face (100) [31].
In the case of UTMS doped crystals for lower concentrations of the dopant up
to 0.6 mol% the crystals appear transparent without defects. At higher concentrations
i.e with 0.8 and 1.0 mol% dopant concentrations of UTMS the doped KDP crystals
exhibit tapering in the prismatic faces. The presence of mother liquor inclusions is
also observed in the UTMS doped KDP crystals for these higher dopant
concentrations.
BTZC, GSN and LASN doped KDP crystals do not show much variation in
external morphology compared to the pure KDP crystal. Growth rate is found to be
enhanced with these dopants and bigger crystals with better crystalline perfection
have been obtained.
3.7 STUDIES ON PURE AND SEMIORGANIC DOPED KDP CRYSTALS 3.7.1 Measurement of Solubility Solubility corresponds to saturation i.e. the equilibrium between a solid and its
solution at a given temperature and pressure. The solubility data of a material governs
the amount of material, which is available for growth and hence, defines the total size
limit. Solvent and solubility factor define supersaturation, which is the driving force
or the rate of crystal growth.
Re-crystallized salt was used to measure the solubility of KDP in deionised
water. Solubility study was carried out using a hot plate magnetic stirrer, oven and
digital thermometer. The temperature was controlled to an accuracy of within ± 0.1°C
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using a voltage regulator attached to the magnetic stirrer. KDP salt was added in small
amounts to 50 ml of solution taken in a 100 ml beaker, Initially, the solution was kept
at 30oC and stirred continuously using the magnetic stirrer till a white precipitate was
formed, which confirms the supersaturation condition. After attaining this condition,
the solubility was determined gravimetrically. In the same manner the solubility of the
semiorganic impurity (ZTS, BTZC, UTMS, GSN and LASN) doped KDP salts
dissolved in 100 ml at 35, 40, 45 and 50 °C were determined for different impurity
concentrations (0.2, 0.4, 0.6, 0.8 and 1 mol%). The solubility curves for pure and
semiorganic doped KDP crystals are shown in figures 3.2 to 3.6.
25 30 35 40 45 50 55
24
26
28
30
32
34
36
38
40
42
44
So
lub
ilit
y (
g/1
00 m
l w
ate
r )
Temperature (oC)
PURE KDP
KDP+0.2mol% ZTS
KDP+0.4mol% ZTS
KDP+0.6mol% ZTS
KDP+0.8mol% ZTS
KDP+1.0mol% ZTS
Figure 3.2: Solubility of pure and ZTS doped KDP in water
In the case of doped KDP crystals it was found that when semiorganic impurity
was added in the KDP solution in 0.2, 0.4, 0.6, 0.8 and 1.0 mol% and dissolved, the
temperature of KDP crystal saturation was raised because the presence of small amount
of the semiorganic dopants in water changed the dissolution equilibrium of KDP.
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25 30 35 40 45 50 55
24
26
28
30
32
34
36
38
40
42
44
So
lub
ilit
y (
g/1
00
ml w
ate
r)
Temperature (oC)
PURE KDP
KDP+0.2mol% BTZC
KDP+0.4mol% BTZC
KDP+0.6mol% BTZC
KDP+0.8mol% BTZC
KDP+1.0mol% BTZC
Figure 3.3: Solubility of pure and BTZC doped KDP in water
25 30 35 40 45 50 55
24
26
28
30
32
34
36
38
40
42
44
So
lub
ilit
y (
g/1
00
ml
wa
ter)
Temperature (oC)
PURE KDP
KDP+0.2mol% UTMS
KDP+0.4mol% UTMS
KDP+0.6mol% UTMS
KDP+0.8mol% UTMS
KDP+1.0mol% UTMS
Figure 3.4: Solubility of pure and UTMS doped KDP in water
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25 30 35 40 45 50 55
24
26
28
30
32
34
36
38
40
42
44
So
lub
ilit
y (
g/1
00 m
l w
ate
r)
Temperature (oC)
PURE KDP
KDP+0.2mol% GSN
KDP+0.4mol% GSN
KDP+0.6mol% GSN
KDP+0.8mol% GSN
KDP+1.0mol% GSN
Figure 3.5: Solubility of pure and GSN doped KDP in water
25 30 35 40 45 50 55
24
26
28
30
32
34
36
38
40
42
44
So
lub
ilit
y (
g/1
00 m
l w
ate
r)
Temperature (oC)
PURE KDP
KDP+0.2mol% LASN
KDP+0.4mol% LASN
KDP+0.6mol% LASN
KDP+0.8mol% LASN
KDP+1.0mol% LASN
Figure 3.6: Solubility of pure and LASN doped KDP in water
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The observed results in this work are found to be similar as reported in the
literature earlier [132]. It is also observed that solubility increases with increase in
temperature and decreases with increase in concentration. The solvent was able to
accommodate fairly more solute between the temperature ranges of 30 to 50o. In all
cases the cases positive slope of solubility curve enables growth by slow evaporation.
The results confirm that KDP grown in the presence of the semiorganic dopants was
stable and less water soluble than KDP at room temperature.
3.7.2 Density measurement The density of a substance is defined as mass per unit volume, i.e. ρ = M/V
where M is the mass and V is the volume of the substance under consideration at
room temperature. Density can be calculated by knowing the mass of the unit cell
content and the volume of the unit cell. Volume of the unit cell can be estimated from
X-ray diffraction data. The measured density of a substance may sometimes be
different from that estimated from the X-ray diffraction data. This is suggestive of
crystal defects, mostly point defects in crystals leading to non-stoichiometry.
Densities of all the crystals grown in the present study were measured by the
floatation method within an accuracy of ± 0.008 g/cm3. Carbon tetrachloride of
density 1.594 g/cm3 and bromoform of density 2.890 g/cm3 are respectively the rarer
and denser liquids used. About 20 ml of carbon tetrachloride was taken in a test tube
and the crystal for which the density has to be determined was dropped into it. The
crystal was found to be sinking. Bromoform was then gradually added until the crystal
was in a suspended state. Now the density of the mixed solution was equal to that of
the crystal. Density of the solution was determined by finding the mass of 20 ml of
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the solution and using the relation ρ = M/V where the symbols bear the same meaning
mentioned earlier.
3.8 INSTRUMENTATION
Characterization of a crystal essentially consists of an evaluation of its
chemical composition, structure, defects and study of their optical properties. This
will assist, to make rapid progress in the growth process and also improve the quality
of the crystal. In order to estimate the perfection of the grown crystals an assessment
technique is required, and post growth analysis provides information on the process
that occurred during crystal growth. As the technology of instrumentation analysis has
improved with the advent of analytical balances, automated titrators and computer
controlled instruments, the speed, accuracy and precision of the characterization
techniques have improved. In the present work, the grown crystals of pure and
semiorganic impurity doped KDP crystals have been analyzed employing the
following characterization techniques
3.8.1 X-ray Diffraction analysis X-ray Diffraction analysis is a versatile, non-destructive analytical technique
for identification and quantitative determination of various crystalline phases of
powder or solid samples of any compound. 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.
3.8.1.1 Single crystal X-ray diffraction analysis
Single crystal X-ray diffraction is an analytical technique in which X-rays are
employed to determine the actual arrangement of atoms within a crystalline specimen.
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Single crystal X-ray diffraction (XRD) is used to analyze crystal structure of single
crystals.
The monochromatic X-rays incident on a plane of single crystal at an angle �
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.
In the present study, the single crystal X-ray diffraction analysis was
performed using Four-circle Enraf Nonius CAD4 single crystal X-ray diffractometer.
The shield was equipped with graphite monochromated MoK� (� = 0.71073 Å)
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 crystal was optically centered at the sphere of confusion
using the built in tele-microscope. 25 reflections were collected from different zones
of the reciprocal lattice using random search procedure. The reflections were indexed
using the method of short vectors followed by least square refinements. The unit cell
parameters thus obtained were transformed to correct Bravais cell.
3.8.1.2 Powder X-ray diffraction analysis
X-ray powder diffraction (PXRD) is an instrumental technique used to study
crystalline materials. When an X-ray beam hits the three-dimensional structure of
non-amorphous material which is defined by regular, repeating planes of atoms, part
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of the beam is transmitted, part is absorbed by the sample, part is refracted and
scattered and part is diffracted. From the diffracted beam, we can measure the
distance between planes of the atoms that constitute the sample by applying Bragg’s
law 2d sin � = n�. Since we know �, we can measure angle � and calculate d spacing.
The characteristic set of d spacing and their intensity generated in a typical X-ray scan
provide ample information. When interpreted by comparison with standard reference
patterns and measurements, this “finger print” allows for identification of the
material. From X-ray powder diffraction data we can get the angle of scattering and
the corresponding intensities of diffracted beams for each reflection. The ease and
precision with which X-ray intensity measurements are made by means of fully
automated diffractometer has greatly contributed to the growth of successful structure
analysis.
Powder XRD patterns of the grown crystals were recorded using an automated
X-ray powder diffactometer (PANalytical XPERT-PRO MPD).This is a fully
computerized X-ray diffactometer which is employed for X-ray diffraction studies.
This is a versatile, sensitive and high resolution X-ray diffractometer. The intensity of
the diffracted beam against 2�o is recorded in the range 10-70o with CuK� radiation
(� = 1.54056 �). Using the observed 2� (Bragg angle) and d (interplanar spacing), all
the reflections of the powder XRD pattern for pure and impurity added KDP have
been indexed using the ‘TREOR’ software package following the procedure of Lipson
and Steeple [217].
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3.8.2 Atomic absorption studies
Atomic absorption analysis (AAS) is the most powerful analytical technique
for the quantitative determination of a particular metal element within a sample. Most
solid samples are first dissolved and converted to solutions to facilitate analysis.
Atomic absorption technique is specific because the atoms of a particular element can
only absorb radiation of their own characteristic wavelength.
The technique makes use of a flame to atomize the sample. The electrons of
the atoms in the flame can be promoted to higher orbitals for an instant by absorbing a
set of quantity of energy. This amount of energy is specific to a particular electron
transition in a particular element. As the quantity of energy put into the flame is
known and the quantity remaining at the other side can be measured, it is possible to
calculate how many of these transitions take place, and thus get a signal that is
proportional to the concentration of the element being measured. Direct sampling of
solids may be accomplished using an electro-thermal furnace. Atomic absorption
studies of semiorganic material doped KDP crystals were carried out using an atomic
absorption spectrometer (Model: AA-6300) to confirm the presence of specific metals
in the doped KDP crystals.
3.8.3 Energy dispersive spectrum analysis Energy Dispersive X-Ray Spectroscopy (EDS or EDAX) is a chemical
microanalysis technique used in conjunction with scanning electron microscopy
(SEM). This technique detects X-rays emitted from the sample during bombardment
by an electron beam to characterize the elemental composition of the analyzed
volume. The data generated by EDAX analysis consist of spectra showing peaks
78
corresponding to the elements making up the true composition of the sample being
analyzed.
When the sample is bombarded by the SEM’s electron beam, electrons are
ejected from the atoms comprising the sample’s surface. The resulting electron
vacancies are filled by electrons from a higher state, and an X-ray is emitted to
balance the energy difference between the two electrons’ states. The X-ray energy is
characteristic of the element from which it was emitted. The EDAX X-ray detector
measures the relative abundance of emitted X-rays versus their energy. The spectrum
of X-ray energy versus counts is evaluated to determine the elemental composition of
the sampled volume. Features or phases as small as 1 �m or less can be analyzed. The
sample X-ray energy values from the EDAX spectrum are compared with known
characteristic X-ray energy values to determine the presence of an element in the
sample. Elements with atomic numbers ranging from that of beryllium to uranium can
be detected. The minimum detection limits vary from approximately 0.1 to a few
atom percent, depending on the element and the sample matrix.
In the present study EDAX studies on the grown pure and semiorganic
impurity (1 mol%) doped KDP crystal samples were performed using the EDAX
detector (model-Thermoelectron Corporation with superdry/II) equipped in Hitachi
model S-3000H scanning electron microscope.
3.8.4 Microhardness measurement
Microhardness studies find wide applications in the study of material properties
of solids. Hardness testing has been widely used to study the strength and deformation
characteristics of materials. It measures the mean contact pressure when an indenter
79
is pressed on to the surface of a flat specimen, thus providing a simple and non-
destructive means of assessing the resistance of the material to plastic deformation.
Hardness is defined as the ratio of the load applied to the surface area of the
indentation.
Vickers microhardness test is found to be the most suitable (among various
types of hardness measurements available) for the measurement of microhardness of
crystals. Hardness measurement is to be performed on a limited area with small
damage to the area being measured and must yield extremely reliable results. A
hardness tester fitted with a diamond pyramidal indenter attached to an incident light
microscope is used for this study. The diamond indenter is in the form of a square
pyramid, whose opposite faces make an angle 136° with one another. The indenter
can be pressed on the sample under a load (P) of 25, 50, 100,150, 200 g etc. The
duration of the indentation time was kept constant (10 seconds). For each load
several indentations were made and the average value of the diagonal length of the
indentation mark was considered to calculate the microhardness. The impression of a
square pyramid has a superficial area of d2 / 2 sin (�/2) where d is the diagonal
length of the indentation and � is the apex angle of the indenter (� = 136°). The area
of impression is related to hardness as Vickers Microhardness number Hv,
Hv = load / area of impression = 2P sin (�/2) /d2 = 1.8544 P /d2 kg/mm2
where P is the load in kilograms and d is diagonal length of indentation in mm.
80
In the present study, microhardness measurements were done on a plane of the
grown pure and impurity doped KDP crystal surfaces using a Vickers microhardness
indenter.
3.8.5 UV-Vis -NIR spectral measurements
UV-Vis spectroscopy is one of the most important analytical and
characterization techniques which is useful in characterizing the absorption,
transmission, and reflectivity of a variety of technologically important materials. The
Ultraviolet-Visible-Near infrared (UV-Vis-NIR) spectroscopy measures the
absorption or emission of radiation associated with the changes in the spatial
distribution of electrons in atoms and molecules. In practice, the electrons involved
are usually the valence or the bonding electrons, which can be excited by absorption
of UV or visible or near IR radiation. The quantity of absorption depends on the
wavelength of the radiation and the structure of the compound. The radiation
absorption is due to the subtraction of energy from the radiation beam when electrons
in orbitals of lower energy are excited into orbitals of higher energy. Since this is an
electron excitation phenomenon, it is also called as electronic spectroscopy. After the
sample absorbs a portion of the incident radiation, the remainder is transmitted on to a
detector where it is changed into an electrical signal and displayed after amplification.
The transmission spectrum shows what percentage of the incoming light that actually
makes it through the sample.
The spectrum of a compound represents a group of either wavelength or
frequency, continuously changing over a small portion of the electromagnetic
spectrum versus either percent transmission (%T) or absorbance (%A).
81
Transmission spectra are important for any NLO material because, the material can be
of use only if it has a wide transparency range to know the suitability for optical
applications. In the present study, the UV-Vis-NIR transmission spectra were
recorded for the pure and impurity doped KDP single crystals of 2 mm thickness
using a Perkin-Elmer Lambda 35 UV-Visible spectrometer in the range 190-1100 nm.
3.8.6 Fourier Transform Infrared (FTIR) analysis This technique is one of the most important and widely used spectroscopic
techniques for analyzing quantitatively the structural units of the unknown
compounds. It helps to identify the functional units, internal structure of the molecules
and nature of the chemical bonds of a compound. Fourier transform infrared (FTIR)
spectroscopy is an ideal analytical technique for the study of condensed phase
materials, if chemical specificity and selectivity are sought. A complementary
technique to Raman spectroscopy, FTIR has gained widespread acceptance as a
characterization tool primarily due to its universality and versatility. The vibrational
spectrum of a molecule is considered to be a unique physical property and is
characteristic of the molecule. As such, the IR spectrum can be used as a fingerprint
for identification by the comparison of the spectrum from an "unknown" with
previously recorded reference spectra.
Absorption of infrared radiations is confined largely to molecular species for
which small energy differences exist between various vibrational and rotational states.
When the frequency of the incident radiation coincides with the vibrational frequency
of the molecules, absorption of energy takes place. When the molecules return from
the excited state to the ground state the absorbed energy is released resulting in
82
distinct peaks in the IR spectrum. This IR absorption bands reveal the state of the
molecules present in the sample.
In this study, the KBr pellet method was used to record the IR spectra of the
grown crystals, where crushed powder of the grown crystals was mixed with KBr and
pelletised using a hydraulic press. In Fourier Transform IR spectroscopy, the infrared
radiation is analyzed by means of a scanning interferometer. The interferogram
containing all the information is constructed into the spectrum with the help of the
mathematical programming called Fourier Transformation.
The FTIR spectra of pure and semiorganic impurity added (0.2, 0.4, 0.6, 0.8
and 1 mole% ) KDP crystals grown in the present study have been recorded in the
range of 400-4000 cm-1 using Perkin Elmer Fourier transform infrared spectrometer
(Model : Spectrum RXI) using KBr pellet method. The spectra were used to analyze
the presence of different constituents and their bonding properties qualitatively.
3.8.7 Second harmonic generation measurements The study of second harmonic generation in crystals yields useful information
on the nonlinear property of material. 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 in exhibiting NLO property
than those currently known. Such a preliminary test should enable us to carry out the
activity without requiring oriented samples. Kurtz and Perry powder technique [218]
is extremely useful for testing of materials for second harmonic generation. Here the
difficulty in requirement of large single crystal of optical quality is removed.
83
The fundamental beam of wavelength 1064 nm, from a Q-switched Nd:YAG
laser is used to test the second harmonic generation (SHG) property. The pure and
semiorganic impurity added KDP crystals were ground into fine powder and packed
in micro capillary tubes mounted in the path of laser pulses with pulse width 6 ns and
repetition rate 10 Hz, having an input energy of 0.68 mJ/pulse. The second harmonic
generation was confirmed by the green emission of wavelength 532 nm from the
samples. The output energy for pure and semiorganic doped KDP samples was
measured. Microcrystalline powder of 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 pure KDP. Thus, the figure of merit of SHG of the
sample is estimated.
3.8.8 Thermal analysis - TGA/DTA techniques Thermal analysis is defined as a group of techniques in which the physical
property of a substance is measured as a function of temperature, while the substance
is subjected to a controlled temperature program. The importance of thermal analysis
in quality control, failure analysis and material research and development is well
established.
Thermal analysis is useful in both quantitative and qualitative analysis.
Samples may be identified and characterized by qualitative investigations of their
thermal behaviour. Information concerning the detailed structure and composition of
different phases of a given sample is obtained from the analysis of thermal data.
Thermogravimetry (TG) or Thermogravimetric Analysis (TGA) provides a
quantitative measurement of any weight changes associated with thermally induced
84
transitions. For example, TG can record directly the loss in weight as a function of
temperature or time (when operating under isothermal conditions) for transitions that
involve dehydration or decomposition. Thermogravimetric 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. TG data are useful 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. The usual temperature range for TG study is from ambient to 1200 °C in
either inert or reactive atmospheres. In TG, the weight of the sample is continuously
recorded as the temperature is increased. Samples are placed in a crucible or shallow
dish that is positioned in a furnace on a quartz beam attached to an automatic
recording balance. Linear heating rates from 5 to 10 °C/min are typical. The amount
of sample required is from 10 to 300 mg. Computer software allows the computation
of weight change which is important in kinetic interpretations of reactions and
processes.
In Differential Thermal Analysis (DTA), the difference in temperature
between the sample and a thermally inert reference material is measured as a function
of temperature (usually the sample 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 reference. A plot of the
differential temperature, �T versus the programmed temperature, T, indicates the
transition temperature and whether the transition is exothermic or endothermic. DTA
and TG analyses are often run simultaneously on a single sample. The thermal effects
are observed as peaks whose sequence (on the temperature scale), sign (endothermic
85
or exothermic), magnitude and shape reflect the physical or chemical changes taking
place. Since any change in the chemical or physical state of a substance is
accompanied by changes in energy that are manifested as heat changes, the DTA
method is applicable to all studies listed for TG and also to phase transformations
including polymerization, phase equilibrium and chemical reactions.
In the present work DTA and TG studies have been carried out on the grown
crystals in the temperature range 10-1000 °C using SDTQ 600 V 8.2 thermal
analyzer.
3.8.9 Dielectric measurements 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 material’s applications. Various
polarization mechanisms in solids such as atomic polarization of the lattice,
orientational polarization of dipoles and space charge polarizations can be understood
by studying the dielectric properties as a function of frequency and temperature for
crystalline solids. The frequency dependence of these properties gives insight into the
material applications. The dielectric constant is one of the basic electrical properties
of solids. The dielectric constant determines the share 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 dielectric. The
capacitance (Ccrys) and dielectric loss factor (tan �) measurements were carried out to
an accuracy of ± 2% using an LCR meter (Agilent 4284 A) for a fixed frequency of
86
1 kHz at various temperatures ranging from 35-150 °C in a way similar to that
followed by Mahadevan and his co-workers [15, 108, 219]. Temperature was
controlled to an accuracy of ± 1 C. The samples were prepared and annealed in a
way similar to that followed for the resistance measurement. Air capacitances (Cair)
were also measured for the dimensions equal to that of the crystals. Since the
variation of air capacitance with temperature was found to be negligible, air
capacitance was measured only at room temperature.
The dielectric constant of the crystal was calculated using the relation
εr = ��
���
�
air
crys
C
C. As the crystal area was smaller than the plate area of the cell, the above
relation was modified to account for the air capacitance around the crystal within
plate area as
εr =
�����
�
�
�����
�
����
�
�−−
���
�
���
�
air
air
crysaircrys
crys
air
C
A
A1CC
A
A
where Ccrys is the capacitance with crystal, Cair the capacitance of air, Acrys is the area
of the crystal touching the electrode and Aair is the area of the electrode.
3.8.9.1 AC Electrical Conductivity
The AC conductivity (ac) was calculated using the relation ac = �o�r � tan � where ε0 is the permittivity of free space and is equal to 8.854×10−12 C2 N-1m-2, �r is
the dielectric constant, tan � is the dielectric loss and � is the angular frequency.
87
3.8.9.2 Activation energy
The general relation proposed by Arrhenius for the temperature variation of
conductivity is given by
σ = σ0 exp ��
���
�−
(kT)E
.
where o is a constant depending on the material, E is the activation energy, T is the
absolute temperature and k is the Boltzmann’s constant. The above equation may be
rewritten as
ln σ = ln σ0 (kT)
E−
A plot of ln versus T1
gives ���
�
�−
(k)E
as the slope and ln 0 as the intercept.
It is customary to plot ln versus 1000/T, from the slope of which the activation
energy (E) can be calculated.
Values of ln were plotted against T
1000 for all the grown samples and the
AC activation energies were calculated from the slope of the straight line best fitted
by least square analysis.