2. growth, structure and chemical...
TRANSCRIPT
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2. GROWTH, STRUCTURE AND CHEMICAL
COMPOSITION
In this Chapter, we deal with the growth of sample crystals and analyzing the
quality along with providing some details of low temperature solution growth
methods.
2.1 Low Temperature Solution Growth
Among the various methods of growing single crystals, solution growth at low
temperature occupies a prominent place owing to its versatility and simplicity.
Materials which decompose on heating or which exhibit any structural transformation
while cooling from the melting point can be grown by solution growth if suitable
solvents are available. This method is more widely used to grow bulk crystals [116].
After undergoing so many modifications and refinements, the process of solution
growth now yields good quality crystals for a variety of applications. Growth of
crystals from solution at room temperature has many advantages over the melt growth
though the rate of crystallization is very low. Since growth is carried out at room
temperature, the concentration of structural imperfections in solution grown crystals is
relatively low [117].
2.1.1 Solution and solubility
Solution is a homogeneous mixture of a solute in a solvent. Solute is the
component which is present in a smaller quantity. For a given solute, there may be
different solvents. The solvent must be chosen taking into account the following
factors to grow crystals from solution. A solvent of choice is the one with:
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1. A good solubility for the given solute
2. A good solubility gradient
3. Less viscosity
4. Less volatility
5. Less corrosion
If the solubility is too high, it is difficult to grow bulk single crystals and too
small a solubility restricts the size and growth rate of the crystals. Solubility gradient
is another parameter which dictates the growth procedure. Neither a flat nor a steep
solubility curve will enable the growth of bulk crystals from solution; while the level
of supersaturation could not be varied by reducing the temperature in the former, even
a small fluctuation in the temperature will affect the supersaturation to a large extent
in the latter disabling the growth of good quality bulk crystals in both cases. If the
solubility gradient is very small, slow evaporation of the solvent is the other option
for crystal growth to maintain the super saturation in the solution.
Growth of crystals from solution is mainly a diffusion controlled process; the
medium must be less viscous to enable faster transference of the growth units from
the bulk solution by diffusion. Hence a solvent with less viscosity is preferable [118].
Supersaturation is an important parameter for the solution growth process. Crystal
grows by the accretion of the solute in the solution as a degree of supersaturation is
maintained. Solubility data at various temperatures are essential to determine the level
of supersaturation. Hence, the solubility of the solute in the chosen solvent must be
determined before starting the growth process.
The solubility of the solute may be determined by dissolving the solute in the
solvent maintained at a constant temperature with continuous stirring. On reaching
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saturation, the equilibrium concentration of the solute may be determined
gravimetrically. A sample of the clear supernatant liquid is withdrawn by means of a
warmed pipette and a weighed quantity of the sample is analysed. The solubility curve
can then be plotted in this way by repeating the above for different temperatures.
2.1.2 Expression of supersaturation
The supersaturation of a system may be expressed in a number of ways. The
basic units of concentration as well as temperature must be specified. The
concentration driving force (∆C) the supersaturation ratio (S) and relative super
saturation (σ) are related to each other as follows:
The concentration driving force ∆C = C-C*
Where C is the actual concentration of the solution and C* is the equilibrium
concentration at a given temperature.
Supersaturation ratio S = C/C*
Relative supersaturation σ = (C-C*)/C*
σ = S-1
If the concentration of a solution can be measured at a given temperature and
the corresponding equilibrium saturation concentration is known, then it is easier to
calculate the supersaturation.
Low temperature solution growth can be subdivided into the following
methods:
1. Slow cooling method
2. Slow evaporation method
3. Temperature gradient method
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2.1.3 Slow cooling method
It is the best way to grow crystals by solution technique. Its main disadvantage
is the need to use a range of temperature. The possible range of temperature is usually
small so that much of the solute remains in the solution at the end of the run. To
compensate this effect, large volumes of solution are required. The use of a range of
temperature may not be desirable because the properties of the grown material may
vary with temperature. Eventhough the method has technical difficulty of requiring a
programmable temperature control, it is widely used with great success. A schematic
diagram of the apparatus is shown in Figure 3.
L - Heater
B - Constant temperature bath
F - Flask
S - Stirrer
T - Thermometer
SG - Stirring gland
Figure 3: Schematic diagram of the apparatus for slow cooling method
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2.1.4 Slow evaporation method
This method is similar to the slow cooling method in view of the apparatus
requirements. The temperature is fixed constant and provision is made for
evaporation. With non-toxic solvents like water, it is permissible to allow evaporation
into the atmosphere. Typical growth conditions involve temperature stabilization to
about ± 0.005 ⁰C and rates of evaporation of a few mm3/hr. The evaporation
techniques of crystal growth have the advantage that the crystals grow at a fixed
temperature. But inadequacies of the temperature control system still have a major
effect on the growth rate. This method is the only one which can be used with
materials which have very small temperature coefficient of solubility. A schematic
diagram of a simple apparatus is shown in Figure 4.
Figure 4: Schematic diagram of a simple apparatus for slow
(free) evaporation method
Lid with holes
Solution
Crystal
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2.1.5 Temperature gradient method
This method involves the transport of the materials from a hot to a cooler
region, where the solution is supersaturated and the crystal grows. The main
advantages of this method are:
1. Crystal grows at fixed temperature
2. This method is insensitive to changes in temperature provided both the source
and the growing crystal undergo the same change
3. Economy of solvent and solute
On the other hand, changes in the small temperature difference between the
source and the crystal zones have large effect on the growth rate. Figure 5 shows a
schematic diagram of the apparatus.
Figure 5: Schematic diagram of the apparatus for the temperature gradient
2.1.6 Crystals grown from solution
Dielectric, nonlinear optical materials attract wide attention due to the
increasing applications in telecommunications, optical information storage and
computing.
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The solution grown crystal was chosen for this piece of work, considering
their potential applications in the fields of nonlinear optics and dielectrics.
2.2 Growth of Sample Crystals
Nonlinear optical (NLO) materials have a significant impact on laser
technology, optical communication and optical storage technology. The search for
new frequency conversion materials over the past decade has led to the discovery of
many organic and semi-organic materials. These materials have large nonlinearity,
high resistance to laser induced damage, low angular sensitivity and good mechanical
hardness.
Recent studies reveal that L-arginine acetate (LAA) possesses excellent,
optical, thermal and mechanical properties, which make it a strong candidate material
for photonic device fabrications.
In order to estimate the perfection of the grown crystals an assessment
technique is required. This will assist us to make rapid progress in the growth process
and also improve the quality of the crystals. Post growth analysis of a crystal provides
information on the process that computer control instruments, the speed, convenience,
accuracy and precision of instrumentation methods have generally improved as well.
According to Elwell and Scheel [119] characterization of crystal essentially consists
of an evaluation of its chemical composition, structure and study of their optical
properties. In the present work, the crystals of pure and doped LAA crystals were
grown and characterized by employing the techniques briefly described.
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2.3 Materials Used
In our present investigation, pure L-arginine acetate (LAA) was synthesized
from AR grade L-arginine and acetic acid in the molecular ratio 1:1. The chemical
reaction is as follows
(NH2)NHCNH(CH2)3CH(NH2)COOH+CH3COOH (NH2)2+CNH(CH2)3
CH(NH3) +COO
-CH3COO
-
To obtain formic acid, hydrochloric acid and oxalic acid doped LAA single
crystals mol% of the respective dopants were added to the parent solution of LAA
separately.
2.4 Growth of LAA Single Crystals
To prepare the supersaturated solutions the amount of solute (m) in grams may
be obtained by using the following formula
m = MXV/1000 (in gram unit),
where
M is the molecular weight of the solute,
X is the concentration in molar unit and
V is the required volume of the solution.
V is taken as 50 cc in the present work. Using the above relation, the required
amounts of L-arginine and acetic acid were dissolved in doubly distilled water and
saturated solution was prepared. The prepared solution was kept over the magnetic
stirrer and stirred for about 45 minutes to attain homogeneity.
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The saturated solutions were then transferred to clean beakers, covered tightly
with polythene covers. The whole setup was kept in a dust free area and monitored.
The period of growth of the crystals ranged from 15-20 days. After the completion of
growth , the crystals were harvested.
2.5 Growth of Doped LAA Crystals
If the solute is an impurity added one, the required amount of the impurity
solute was also added and dissolved along with the pure solute. The amount of
impurity required in grams (m*) was calculated by using the formula:
m* = M* P X V/ 1000 (in gram units).
Where M* is the molecular weight of the impurity considered, P is the molar
concentration (in %) of the dopant.
In the present study, formic acid is maintained at concentrations 0.2, 0.4, 0.6
and 0.8 mol%; hydrochloric acid is maintained at concentrations 0.08, 0.1, 0.3 and 0.4
mol%; and oxalic acid is maintained at concentrations 0.06, 0.08, 0.1 and 0.2 mol%.
In order to maintain the pH between 5 and 6, we have considered different
concentrations for different dopants.
2.6 Characterization Techniques
2.6.1 Single crystal X-ray diffractometer (SXRD)
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 (SXRD) is a
nondestructive tool to analyse crystal structure of compounds, which can be grown as
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single crystals. The molecular structure, atomic coordinates, bond length, bond
angles, molecular orientation and packing of molecules in single crystals can be
determined by X-ray crystallography. Single crystal X-ray diffractometer shown in
Figure 6 collects intensity data required for structure determination.
Accurate measurements of intensities of reflections of all Miller indices within
a specified reciprocal radius (usually 25 ⁰ for MoKα and 68 ⁰ for CuKα) 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 sinθ = nλ where d is the
interplanar spacing of the incident plane, λ 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 group, molecular structure, etc of the crystalline solids and Miller
indexing of the different faces of the crystal are possible. Unit cell parameter is
simply the dimension of the basic molecular brick with which the crystal is built.
Space group tells us the symmetry with which the molecules are arranged within the
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unit cell. All the geometrical features of molecules (bond distances, bond angles,
torsion angles between bonds, dihedral angles between planes, etc.) may be obtained
from coordinates.
X-ray diffraction measurements have greatly contributed to the growth of
successful crystal structure analysis. The statement made by Mahadevan (1986) [120]
on crystal structure determination is:
“Crystal structure determination is as important and necessary in physical,
chemical, material and biosciences as soil test in cultivation”.
In the present study, the single crystal X-ray diffraction analysis was
performed using an Enraf Nonius CAD4 single crystal X-ray diffractomter. ( a
photograph is shown in Figure 6) The shield was equipped with graphite
monchromated MoKα 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 telemicroscope. 25 reflections were
collected from different zones of the reciprocal lattice using random search procedure.
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.
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Figure 6: Photograph of an Enraf Nonius CAD4 Diffractometer
2.6.2 Powder X-ray diffractometer (PXRD)
Powder X-ray diffraction analysis (PXRD) is the primary tool for
investigating the structure of crystalline materials, from atomic arrangement to
crystallite size and inperfections [121]. A schematics of powder method is shown in
Figure 7.
In this technique, a monochromatic X-ray beam is allowed to irradiate a small
specimen of the substance ground to a fine powder and contained in a thin-walled
glass capillary tube. Since the orientation of the minute crystal fragments is
completely random, a certain number of them will lie with any given set of lattice
planes making exactly the correct angle with the incident beam for reflection to occur.
Furthermore, these planes in the different crystallites are randomly distributed about
the axis of the incident beam so that the corresponding reflections from all the
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crystallites in the specimen lie on a cone coaxial with the axis and with a semi-apex
angle of twice the Bragg angle, i.e., with 2θ. The specimen is surrounded by a
cylindrical film and two small portions of each cone are recorded as lines on the film
as shown in Figure 7(c). If the grain size is fairly large (>10-6
m) there is insufficient
room within the irradiated volume for enough crystallites to be in all possible
orientations and the resultant powder lines will be rather ‘spotty’. This spottiness can
be eliminated by rotating the specimen during exposure as this considerably increases
the number of crystallites which can contribute to each powder line [122].
Figure 7: Schematics of powder method
a) Experimental arrangement
b) Diffraction geometry
c) Developed film
The condition for diffraction of a beam of X-rays from a crystal is given by
the Bragg’s equation,
nλ = 2dsinθ
With the help of PXRD we can determine:
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i) the crystalline nature of the materials,
ii) cell parameters,
iii) hkl values, and
iv) The crystal structure.
It gives unambiguous accurate and reliable three dimensional parameters. All
the thirteen samples were characterized by PXRD measurements [123-127].
In the present study, the powder X-ray diffraction analysis was performed
using XPERT PRO powder X-ray diffractometer. Figure 8 shows the photograph of
the diffractometer used.
Figure 8: Photograph of an XPERT pro PXRD diffractometer
2.6.3 FTIR spectrophotometer
Infrared spectroscopy is the study of the interaction of infrared light with
matter. The fundamental measurement obtained in infrared spectroscopy is an infrared
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spectrum, which is a plot of measured infrared intensity versus wavelength (or
frequency) of light.
Fourier transform infrared (FTIR) spectroscopy was first developed by
astronomers in the early 1950s to study the infrared spectra of distant stars has now
been developed into a very powerful technique for the detection of very weak signals
from the environmental noise. It is a simple mathematical technique to resolve
complex wave into its frequency components. The conventional IR spectrometers are
not of much use for the far IR region (20-400 cm-1
) as the sources are weak and
detectors insensitive. It has made the middle infrared (400-4,000 cm-1)
region more
accessible and also more useful [127-130].
The basic components of a Fourier transform spectrometer are given in Figure
9. The source is the usual glower operated at very high temperatures. The Michelson
interferometer consists of a source S, a beam splitter B and two plane mirrors M1 and
M2 as in Figure 10. Mirror M1 is fixed and M2 is capable of to and fro movements.
The beam splitter allows 50% of the radiation of mirror M1 and the other 50% to
mirror M2. The two beams are reflected back to B where they recombine with 50%
going to the source and the other 50% going to the sample. For monochromatic
source, if the path lengths BM1B and BM2B differ by an integral number including
zero of wavelengths, one gets constructive interference of the two beams at B (bright
beam). Destructive interference results when the difference in path lengths is half odd
integral number of wavelengths.
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Figure 9: Block diagram for Fourier transform infrared spectrometer
Thus, if mirror M2 is moved towards or away from B, the sample and detector
will see an alternation in intensity. If two different monochromatic frequencies υ1 and
υ2 are used instead of one, a more complicated interference pattern would follow
while M2 is moved. A Fourier transform of the resultant signal would give the two
originals with the appropriate intensities. Extending this, a white light produces an
extremely complicated interference pattern which can be transformed back to the
original frequency distribution. The recombined beam if directed through a sample,
the sample absorption will show up as gaps in the frequency distribution which on
transformation gives a normal absorption spectrum. In the experiment, the detector
signal is collected into a multichannel computer, while mirror M2 is moved. The
computer then carries out the Fourier transform of the stored data and plots it on a
paper.
Infrared materials are generally single crystals of great technological
importance and have applications in numerous infrared
devices like thermo vision,
Radiation
Source
Interferometer
and sample
Analog to digital
converter
Digitized
interference
pattern converter
Computer of effect
Fourier transform
Digital to analog
converter
Record
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night vision, line scanners and thermometers useful for thermal imaging, target
identification and optical communication [131-136].
Figure 10: Interferometer arrangement of Fourier transform spectrometer
The FTIR spectrum was recorded for all the pure and doped samples (thirteen
samples) prepared in the present study, using a Perkin Elmer RX1-Fourier transform
Infrared spectrometer a photograph of which is shown in Figure: 11.
2.6.4 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 fragment of the substance under
consideration at room temperature. Density can also be calculated by knowing the
mass of the unit cell content and volume of the unit cell. The volume of the unit cell
can be calculated from X-ray diffraction data. The measured density of a substance
Fixed mirror (M1)
Movable Mirror (M2)
B S
-X 0 +X
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Figure 11: Photograph of a Perkin Elmer RX1-FTIR spectrophotometer
may sometimes be different from that calculated from X-ray data. This suggested of
crystal defects, mostly point defects in crystals leading to non-stoichiometry. Density
is also a diagnostic property related to solid solutions, i.e., in a crystal containing two
or more end member compositions within the same crystal structure, e.g. forsterite
(Mg2SiO4) and fayalite (Fe2SiO4) where density increases proportionately with
Fe2SiO4 content. Pressure and temperature affect the density of a substance. For
example, increase in temperature increases the volume without change in mass; hence
a decrease in density or specific gravity. Increase in pressure decreases the volume
without change in mass; hence an increase in density or specific gravity.
Densities of all the crystals grown in the present study were measured by the
floatation method within an accuracy of ±0.008 g/cm3
[137-145]. Carbon tetrachloride
of density 1.592 g/cm3 and hexane of density 0.652 g/cm
3 are respectively the rarer
and denser liquids used.
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2.6.5 CHNS elemental analysis `
CHNS elemental analysis provides a means for the rapid determination of
carbon, hydrogen, nitrogen and sulphur in organic matrices and other types of
materials. They are capable of handling a wide variety of sample types, including
solids, liquids, volatile and viscous samples, in the fields of pharmaceuticals,
polymers, chemicals, environment, food and energy.
The analysers are often constructed in modular form such that they can be set
up in a number of different configurations to determine, for example, CHN, CHNS,
CNS or N depending on the application. This adaptability allows not only flexibility
of operation but also the use of a wide range of sample weights from a fraction of a
milligram to several grams (macro-systems).
Figure 12: Photograph of an Elementar Vario EL III Germany - CHNS analyser
In its simplest form, simultaneous CHNS analysis requires high temperature
combustion in an oxygen-rich environment and is based on the classical Pregl-Dumas
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method. This combustion can be carried out under both static conditions i.e.
introduction of a set volume of oxygen or dynamic conditions, i.e. a constant flow of
oxygen for a set period of time. Often, catalysts are also added to the combustion tube
in order to aid conversion.
Basic principle
In the combustion process (furnace at ca. 1000 oC), carbon is converted to
carbon dioxide; hydrogen to water; nitrogen to nitrogen gas/oxides of nitrogen and
sulphur to sulphur dioxide. If other elements such as chlorine are present, they will
also be converted to combustion products, such as hydrogen chloride. A variety of
absorbents are used to remove these additional combustion products as well as some
of the principal elements, sulphur for example, if no determination of these additional
elements is required.
The combustion products are swept out of the combustion chamber by inert
carrier gas such as helium and passed over heated (about 600 oC) high purity copper.
This copper can be situated at the base of the combustion chamber or in a separate
furnace. The function of this copper is to remove any oxygen not consumed in the
initial combustion and to convert any oxides of nitrogen to nitrogen gas. The gases are
then passed through the absorbent traps in order to leave only carbon dioxide, water,
nitrogen and sulphur dioxide.
Detection of the gases can be carried out in a variety of ways including (i) a
GC separation followed by quantification using thermal conductivity detection (ii) a
partial separation by GC (‘frontal chromatography’) followed by thermal conductivity
detection (CHN but not S) (iii) a series of separate infra-red and thermal conductivity
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cells for detection of individual compounds. Quantification of the elements requires
calibration for each element by using high purity ‘micro-analytical standard’
compounds such as acetanilide and benzoic acid.
Applications of CHNS elemental analysers
CHNS elemental analysers have been used in analytical laboratories for over
thirty years. The method is used extensively across a wide range of applications,
including pharmaceuticals, chemicals, oil-related products, catalysts and food. In the
oil industry, an important application is the regular monitoring of coke build-up on
refinery catalysts to ensure that regeneration procedures (involving controlled burning
of the carbon) are executed at optimal intervals. Since many of these catalyst systems
involve large quantities of noble metals such as platinum, palladium and rhenium,
mismanagement of this testing would entail serious financial losses. In food analysis,
the determination of nitrogen (as a surrogate for protein) is very important for pricing
grain and evaluating meat products, and is increasingly undertaken by combustion
analysis [146-147].
In the present work, Elementar Vario EL III Germany CHNS analyser is used
to analyse the samples and the photograph of the analyser is shown in Figure 12.
2.6.6 EDAX Spectral analysis
Energy Dispersive X-ray Spectroscopy (EDAX) is an analytical technique that
can be of great value in the examination of the chemical composition of the samples.
The technique utilizes a scanning electron microscope, a type of high magnification
microscopy in which the sample is bombarded by electrons. The interaction of an
electron beam with a sample target produces a variety of emissions, including X-rays.
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An energy-dispersive (ED) detector is used to separate the characteristic X-ray of
different elements into an energy spectrum and ED system software is used to analyse
the energy spectrum in order to determine the abundance of specific elements. Energy
dispersive X-ray spectroscopy (EDAX) can be used to find the chemical composition
of materials down to a spot size of a few microns, and to create element composition
maps over a much broader raster area. Together, these capabilities provide
fundamental compositional information for a wide variety of materials [148].
For some samples where only a very few elements are present and their X-ray
lines are widely separated in wavelength, the crystal analyses may be eliminated and
energy dispersive detectors with pulse height discrimination are used in this place. For
concentration greater than 1% and elements separated by a few atomic numbers,
energy dispersion analysis is very useful, because the intensities are increased about
1000-fold. The intensity at the detector is drastically increased because no collimating
slits are required, thus increasing the energy through and the detector can be placed
very close to the sample, again increasing the energy intercepted by the detector. With
such an increase in radiation intensity, weaker primary sources can be used for
example radio isotopes. The resolution of an energy dispersion instrument, however is
as much 50 times less than the wavelength dispersion spectrometer using a crystal;
thus lines from nearby elements may overlap. For precise quantity measurements,
wavelength dispersion followed by energy-dispersive detectors and pulse height
discriminators must be used to get sufficient resolution [149].
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Figure 13: Photograph of an EDS Make-JEOL Model JED-2300 EDS analyser
In the present study, the atom% of Cl2 present in the HCl doped crystals were
analysed using an EDS Make-JEOL Model JED-2300 EDS energy dispersive X-ray
spectrum analyser. A photograph of the instrument is shown in Figure 13.
2.7 Results and Discussion
2.7.1 General features
The pure and doped LAA crystals grown in the present study are found to be
stable, non hygroscopic and transparent. Photographs of the grown pure and doped
LAA crystals are shown in Figures 14-17.
The result experiments indicated that the `acid added LAA single crystals take
short span of time to grow in comparison with that of the pure crystals. Morphologies
of all the impurity added crystals grown are similar to that of the pure LAA. It was
found that as dopant concentration increases the transparency increases.
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Figure 14 : Photograph showing the pure LAA (LAA) crystal
Figure 15: Photograph showing the formic acid doped LAA crystals
(From left: LAAF1, LAAF2, LAAF3, LAAF4)
Figure 16: Photograph showing the hydrochloric acid doped LAA crystals
(From left: LAAH1, LAAH2, LAAH3, LAAH4)
Figure 17: Photograph showing the oxalic acid doped LAA crystals
(From left : LAAO1, LAAO2, LAAO3, LAAO4)
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2.7.2 Lattice parameters
The grown pure and doped crystals were subjected to single crystal X-ray
diffraction measurements to determine the unit cell dimensions and volume, A good
quality crystal was selected for the X-ray diffraction studies. Unit cell parameters and
volumes obtained from SXRD measurements for the grown crystals are entered in
Table 1. The crystals belong to monoclinic crystal system. The observed densities are
also provided in Table 1.
Table 1: Lattice parameters and densities
Crystal a (Ǻ) b (Ǻ) c (Ǻ) β⁰⁰⁰⁰ Volume
(Ǻ3)
Density
(g/cc)
LAA 9.189
(9.174)
5.166
(5.172)
13.050
(13.478)
109.621
(110.8)
584
(600.4) 1.322
LAAF1 9.220 5.178 13.110 109.655 590 1.366
LAAF2 9.226 5.168 13.077 109.55 587 1.367
LAAF3 9.207 5.173 13.080 109.52 587.2 1.449
LAAF4 9.205 5.173 13.079 109.503 587.3 1.469
LAAH1 9.200 5.181 13.080 109.561 587.5 1.339
LAAH2 9.240 5.200 13.130 109.380 595 1.354
LAAH3 9.206 5.178 13.070 109.55 587.3 1.367
LAAH4 9.195 5.155 13.052 109.595 587.8 1.496
LAAO1 9.220 5.175 13.120 109.670 589.00 1.351
LAAO2 9.221 5.183 13.116 109.49 591.00 1.347
LAAO3 9.230 5.180 13.100 109.64 590.00 1.369
LAAO4 9.217 5.177 13.110 109.668 588.9 1.422
Reported values are given in parenthesis [115].
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The lattice parameters and space group obtained for the pure crystal LAA
agree well with the reported values [115]. All the grown crystals belong to the space
group P21 and the monoclinic crystal system. The difference in the lattice volume of
pure and doped LAA crystals observed in the present study are very small to that
there is no lattice distortion. This confirms that the impurity molecules have entered
into the LAA crystal matrix but not distorted the crystal structure.
2.7.3 Powder X-ray diffraction patterns
The powder XRD patterns obtained for the pure doped crystals grown are
shown in Figures 18-20. The peaks were indexed following the procedures of Lipson
and Steeple [150] using the package program UNIT CELL.
The XRD pattern of pure LAA crystal obtained in the present study is
essentially identical with the reported results [115]. This shows that the grown
crystals can be characterized as LAA crystals. The numerous sharp peaks found in the
XRD patterns give a clear cut proof of the crystalline nature of all the grown crystals.
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Figure 18: PXRD patterns of formic acid doped LAA crystals
71
Figure 19: PXRD patterns of hydrochloric acid doped LAA crystals
72
Figure 20: PXRD patterns of oxalic acid doped LAA crystals
73
Comparison of the PXRD patterns shows that most of the reflecting planes are present
in pure and impurity added crystals with slight shift in the Bragg angle due to doping.
2.7.4 FTIR spectra
To analyse the IR spectrum (Figures :21, 22, 23) of LAA, we have to consider
the single crystal structure of LAA, which shows that in crystalline state, the arginine
molecule is deprotonated at the carboxyl group (COO-) and protonated at the guanidyl
(+(H2N)
2CNH and amino (NH
3+) groups. Thus, the structure of the LAA crystal
consists of an L-arginine molecule in the ionized form and an acetate ion. For
simplicity of analysis, we have considered three different regions in the vibrational
spectrum that is obtained for LAA crystals. The wavenumber region (2100-3900 cm-1
)
consists of bands due to NH, NH3+
, NH2, CH, CH2, and CH3 stretching vibrations.
The lower wave number region (below 1000 cm-1
) contains bands due to deformation
vibrations of different groups. The presence of characterization bands at 950, 928,
891, 823, 763, 675, 610, 542, 473 and 463 cm-1
corresponds to COO- in plane
deformation.
COO- wagging mode, and NH3+
torsion mode indicates the protonation of the
NH3+
group and deprotonation of COO-. Besides this presence of absorption band in
the region of 2025 cm-1
is due to combination of asymmetrical –NH3+
bending (1557
cm-1
) vibration and its torsional oscillation (542 cm-1
), and this is an indicator band
for the identification of the charged NH3+
group and this confirms the protonation of
74
Figure 21: FTIR spectra of formic acid doped LAA crystals
75
Table 2: Observed FTIR wave numbers and their vibrational assignments for formic acid doped LAA crystals
LAA
(Reported
[86])
Wave numbers (cm-1
) for
Assignment LAA
(Present
work )
LAAF1 LAAF2 LAAF3 LAAF4
3750-2300 3900-2300 3900-2300 3900-2300 3900-2300 3900-2300 NH and CH stretching vibrations
2027 2025.35 2025.75 2025.74 2025.93 2025.65 Asym.NH3+ bending
1526 1557.39 1560.11 1555.43 1556.04 1557.93 Asymmetric stretching modes of COO-
1395.79 1414.63 1399.87 CH3 symmetric deformation
1322.85 1322.85 1323.04 1356.10 Stretching vibration of CO
1340.38 CH3 wagging
1323.18 CH2 twisting
1228 1229.10 1228.94 1228.82 1229.06 1229.15 C-C-COO vibrations
1196 1197.02 1196.87 1196.22 1197.05 1197.13 -COO vibrations
1089.74 1089.62 1089.56 1089.75 1089.75 C-CN stretching vibrations
928.93 928.94 928.88 928.98 928.89 C-CH bending
914.91 915.09 Overtone of torsional oscillation of
NH3+
675.87 671.32 653.46 671.39 653.16 NH out of plane bending
543 542.88 544.86 543.02 544.58 544.52 Torsional NH oscillation of NH3+
group
76
Figure 22: FTIR spectra of hydrochloric acid doped LAA crystals
77
Table 3 Observed FTIR wave numbers and their vibrational assignments for hydrochloric acid doped LAA crystals
LAA
(Reported
[86])
Wave numbers (cm-1
) for
Assignment LAA1
(Present
work)
LAAH1 LAAH2 LAAH3 LAAH4
3750-2300 3900-2300 3900-2300 3900-2300 3900-2300 3900-2300 NH and CH stretching vibrations
2027 3178.60 3041.05 3051.64 2997.73 2956.28 NH Stretching of NH2 vibration
1526 1557.39 1558.78 1519.99 1542.24 1558.14 Asymmetric stretching modes of
COO-
1.412.44
CH3 symmetric deformation
1322.92 1323.06 1323.17
Stretching vibration of CO
1278.09 1278.13 1278.21 1278.06 1278.07 CH3 wagging
1242.98 1243.02
CH2 twisting
1228 1229.10 1229.04 122912 1229.66 1229.01 C-C-COO vibrations
1196 1197.02 1197.05 1197.17 1196.94 1196.97 -COO vibrations
1089.74 1089.69 1089.79 1089.69 1089.60 C-CN stretching vibrations
928.93 928.94 928.88 928.98 928.89 C-CH bending
915.03 914.82
Overtone of torsional oscillation of
NH3+
675.87 653.15 653.42 653.87 667.85 NH out of plane bending
543 542.88 5444.75 544.22 544.56 543.48 Torsional NH oscillation of NH3
+
group
78
Figure 23: FTIR spectra of Oxalic acid doped LAA crystals
79
Table 4: Observed FTIR wave numbers and their vibrational assignments for oxalic acid doped LAA crystals
LAA
(Reported [86])
Wave numbers (cm-1
) for
Assignment LAA
(present
work )
LAAO1 LAAO2 LAAO3 LAAO4
3750-2300 3900-2300 3900-
2300 3900-2300 3900-2300 3900-2300 NH and CH stretching vibrations
2027 2025.78 2025.78 2025.77 2025.90 2025.79 Asym.NH3+ bending
1526 1557.11 1647.63 1539.56 1580.43 1536.97 Asymmetric stretching modes of
COO-
1400.37 CH3 symmetric deformation
1322.66 1323.00 1322.71 Stretching vibration of CO
1278.09 1277.93 1278.08 1278.01 CH2 twisting
1228 1229.10 1228.82 1228.99 1229.98 1228.94 C-C-COO vibrations
1196 1197.02 1196.87 1196.22 1197.05 1197.13 -COO vibrations
1089.74 1089.51 1089.63 1089.63 1089.57 C-CN stretching vibrations
928.93 928.92 929.00.88 928.94 928.94 C-CH bending
914.83 Overtone of torsional oscillation of
NH3+
675.87 653.24 653.09 6752.19 671.47 NH out of plane bending
543 542 543.48 543.82 543.90 544.46 Torsional NH oscillation of NH3
+
group
80
the amino group. The stretching vibrations of the O-H bonds in the COOH group fall
into the same region as the stretching vibrations of the OH bonds in water molecules
and the deformation vibrations of the water molecules fall into the region of the
deformation vibrations of NH2+
and NH3+
groups, it is difficult to determine with
certainty the presence or absence of water molecules on the basis of IR spectra alone.
Thus with the help of available data on the vibrational frequencies of amino acids all
the molecular groups present in the LAA crystals could be identified [92,98, 105, 108,
114, 151,152].
Table 2 shows the comparison of vibrational bands of formic acid doped LAA
with that of pure LAA and their assignments. Comparing the bands it can be seen that
the FT-IR spectra of doped LAA crystals are similar to that of the pure LAA with
small changes. All the spectra of LAAF1, LAAF2, LAAF3 and LAAF4 show
additional peaks in the frequency band between 1500 cm-1
and 1200 cm-1.
The broad
envelope positioned between 2100 cm-1
to 3500 cm-1
corresponds to the symmetric
and asymmetric stretching modes of NH2. The low frequency absorption bands in the
grown doped crystals were not shifted, but the transmittance percentage has either
increased or have been decreased, and also some additional peaks has been noticed
which is a clear indication of incorporation of more number of formate impurity ions
into the crystal lattice matrix.
Table 3 shows the comparison of vibrational bands of hydrochloric acid doped
LAA with that of pure LAA and their assignments. Comparing the bands it can be
seen that the FT-IR spectra of doped LAA crystals are similar to that of the bands
obtained for the pure LAA with small changes. The frequency of absorption band at
81
1532 cm-1
is changed to 1558, 1519, 1542 and 1558 cm-1
, for LAAH1, LAAH2,
LAAH3, LAAH4 respectively (ie) as the concentration of dopant increases there is
variation in the wave number of the vibrational band assigned as asymmetric
stretching modes of COO-. Also, we can notice the additional peaks between the wave
number ranging between 1500-1200 cm-1
. Thus for LAAH1 absorptions at 1412 cm-1
assigned to be CH3 symmetric deformation, 1322 cm-1
assigned to be stretching
vibration of CO, 1278 cm-1
assigned to be CH3 wagging , 1242 cm-1
assigned to be
CH2 twisting. For LAAH2, the additional peaks are noticed at vibrational bands of
wave numbers 1323, 1278, 1243 and 1243 cm-1
. LAAH3 has the additional peaks in
the vibrational band of wave numbers 1323 and 1278 cm-1
. Finally for LAAH4 the
additional peak can be noticed at the wave number 1278 cm-1
alone. From these
variations, we can expect the chloride ion inclusion into the crystal lattice. The
inclusion of chloride ions into the crystal lattice is further confirmed from the EDAX
data.
The comparison of vibrational bands of oxalic acid doped LAA with that of
pure LAA and their assignments are shown in Table 4. Comparing the bands, it can be
noticed that the spectra of oxalic acid doped LAA crystals are similar to that of the
pure LAA with some changes. The wave number 1557 cm-1
corresponding to
asymmetric stretching modes of COO- has variation and the corresponding band has
been changed to 1647 cm-1
for LAAO1, for LAAO2 the band is at 1539 cm-1
, for
LAAO3 the vibration is at a 1580 cm-1
, 1536 cm-1
is observed for LAAO4. Thus the
addition of oxalic acid at different molar concentrations has disturbed the band at
1557 cm-1
. Additional peaks such as 1322 cm-1
for LAAO1; 1400 cm-1
, 1400 cm-1
and
1323 cm-1
for LAAO2; 1580 cm-1
for LAAO3; 1322 and 1278 cm-1
for LAAO4 have
82
been obtained for oxalic acid doped LAA crystals. From the above variations we can
expect the inclusion of the dopant into the host crystal lattice matrix.
The comparison of vibrational bands of oxalic acid doped LAA with that of
pure LAA and their assignments are shown in Table 4. Comparing the bands, it can be
noticed that the spectra of oxalic acid doped LAA crystals are similar to that of the
pure LAA with some changes. The wave number 1557 cm-1
corresponding to
asymmetric stretching modes of COO- has variation and the corresponding band for
LAAH1 is 1647 cm-1
, for LAAH2 the band is at 1539 cm-1
, for LAAH3 the vibration
is at 1580cm-1
, 1536cm-1
is observed for LAAH4. Additional peaks such as 1322cm-
1, for LAAH1; 1400cm-1
, 1400cm-1
and 1323cm-1
for LAAO2; 1580cm-1
for LAAO3;
1322 cm-1
and 1278cm-1
for LAAO4 have been obtained for oxalic acid doped LAA
crystals. From the above variations we can expect the inclusion of the dopant into the
host crystal lattice.
2.7.5 Densities
It was observed that the difference in densities of crystals grown in the same
container (growth cell) was very small and negligible. The densities observed are
provided in Table 1. The amount of impurity (Cl) content present in the hydrochloric
acid doped crystals have been obtained by performing the EDAX measurements on
those crystals. The density value observed in the present study for LAA is 1.32 g/cm3.
The value reported in the literature is 1.345 g/cm3 [101]. There is good agreement
between the measured and literature values, again confirming the identity of the
substance.
83
The observed variation of density (see Table1) and lattice volume (see Table
1) of LAA crystal caused by the impurity addition indicates that the impurity
molecules have entered into the lattice of LAA crystals. The formate, chloride and
oxalate ions are expected to disturb the acetate ions in LAA which is evidenced by the
FTIR spectral studies (see Section 2.7.4). This is also evidenced by the variation of
the amount of C, H, N of the impurity added crystals from that of the pure crystals,
the results obtained from CHNS analysis (see Section 2.7.6).
The estimated impurity content in the crystal justifies the incorporation of
impurity molecules in the crystal. In the case of HCl added LAA crystals, the EDAX
data obtained confirm the estimated impurity concentrations. It can be noticed that the
chloride content in the crystal is less than that taken in the solution used for the
growth of crystals.
2.7.6 CHNS analysis
CHNS analysis was done using an Elementar Vario EL III Germany to the
powder samples to estimate the carbon, hydrogen and nitrogen contents present in the
grown crystals.
The weight % of carbon, hydrogen and nitrogen present in both pure and
doped LAA are shown in Table 5.
84
Table 5: CHNS analysis data
Crystal Composition (wt. %)
C H N
LAA 40.98 12.82 24.52
LAAF1 40.85 11.89 24.30
LAAF2 40.86 12.52 24.40
LAAF3 40.82 9.317 24.16
LAAF4 40.84 12.48 24.40
LAAH1 40.84 13.16 24.50
LAAH2 40.98 12.59 24.40
LAAH3 40.95 12.08 24.34
LAAH4 40.85 10.53 24.30
LAAO1 40.89 11.40 24.29
LAAO2 40.92 13.16 24.46
LAAO3 40.88 11.66 24.29
LAAO4 40.82 10.03 24.17
Formic acid and oxalic acid also contain carbon, nitrogen and hydrogen atoms.
Variations in C, H, and N concentrations show the incorporation of dopants into the
host crystal matrix.
2.7.7 EDAX spectra
The presence of chloride ions in the crystal lattice of LAA can be verified by
subjecting the samples for energy dispersive X-ray adsorption spectral (EDAX)
studies. The samples are bombarded by electrons. The interaction of an electron beam
with the sample target produes a variety of emission including X-rays. The EDAX
spectra observed are shown in Figure 24. The chloride contents observed are given in
Table 6.
The presence of feeble or trace amount of the element can also be traced out.
The incorporation of the dopant into the host matrix can be noticed from the pattern.
85
EDAX spectrum of LAAH1
2 4 6 8 10 12 14
keV
0
200
400
600
800
1000
x 0.001 cps/eV
C
O N
Cl
Cl
EDAX spectrum of LAAH2
2 4 6 8 10 12 14keV
0.0
0.2
0.4
0.6
0.8
1.0
1.2
cps/eV
C
O N
F Cl
Cl
EDAX spectrum of LAAH3
2 4 6 8 10 12 14
keV
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
cps/eV
O
C
N
Cl
Cl
EDAX spectrum of LAAH4
2 4 6 8 10 12 14keV
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
cps/eV
C O N
F Cl
Cl
Figure 24: EDAX spectra of hydrochloric acid doped LAA crystals
86
Table 6: EDAX analysis
Crystal
Atomic % of Cl present
In the solution In the crystal
LAAH1 0.08 0.03
LAAH2 0.10 0.04
LAAH3 0.30 0.14
LAAH4 0.40 0.16
The Cl peaks observed indicate the presence of the impurity Cl present in the
samples. Higher the peak in the EDAX spectrum, more is the concentration of the element in
the spectrum. An EDAX spectrum plots not only identifies the element corresponding to the
type of X-rays to which it corresponds. A peak corresponding to the amount of energy
possessed by X-rays emitted by an electron in the L-shell going down to the K-shell is
identified as Kα peak.
The chemical composition of the all elements except Chloride ions in HCl
doped LAA crystals were estimated using CHNS analysis. It can be noticed that the
chloride ion content in the doped crystal increases with the increase in doping
concentration used in the solution for the growth of single crystals. Also it is found
that the amount of the chloride ions in the crystals is less than that present in the
solution taken for the growth of the crystals.