2. GROWTH, STRUCTURE AND CHEMICAL COMPOSITIONshodhganga.inflibnet.ac.in/bitstream/10603/73155/8/08_chapte 2.pdf · technique is required. This will assist us to make rapid progress

44

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:

45

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

46

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

47

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

48

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

49

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.

50

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.

51

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.

52

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

53

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

54

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.

55

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

56

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:

57

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

58

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.

59

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

60

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

61

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.

62

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

63

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

64

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.

65

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].

66

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.

67

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)

68

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].

69

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.

70

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.