CHAPTER 2 SYNTHESIS, GROWTH AND CHARACTERIZATION OF...

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CHAPTER 2

SYNTHESIS, GROWTH AND CHARACTERIZATION OF

AMINO ACIDS ADMIXTURED L-ARGININE PHOSPHATE

MONOHYDRATE SINGLE CRYSTALS

2.1 INTRODUCTION

The development of semiorganic nonlinear optical (SONLO)

materials leads to novel applications, such as frequency conversion by NLO

crystals, light amplitude and phase modulation by Electro optic (EO) crystals

and phase conjugation by photorefractive crystals. Most of the organic NLO

crystals are constituted by weak van der Waals and hydrogen bonds with

conjugated electrons. Hence, they are soft in nature and hence difficult to

cut and polish the crystals and also they have intense absorption in UV region.

In view of these difficulties, a new type of NLO materials have been built

from organic-inorganic complexes, which form stronger ionic bond than the

van der Waals and hydrogen bonds, thus increasing the mechanical strength

of these semiorganic crystals. The invention of L-arginine phosphate

monohydrate enhanced the search for new materials in the single crystalline

form to fulfill the needs in the field of nonlinear optics and comparable with

potassium dihydrogen phosphate (Xu et al 1983). It is very important for an

NLO material to have excellent phase matching to get more conversion

efficiency. LAP crystals posses phase matching angles varying from 29 to

35 , since the phase matching angles are depending on the wavelength used

and the temperature. Attempts have been made on the synthesis of other salts

29

of L-arginine because of their excellent nonlinear optical properties. Monaco

et al (1987) identified several salts of L-arginine derivatives with better NLO

properties than KDP and verified the linear and nonlinear optical properties.

Fuchs et al (1989) reported the mechanical and thermal properties of LAP

crystals. It has been reported that the surface compliances of LAP are 2 to 3

times larger than those of KDP and the volume compressibility of LAP is

about twice as large as that of KDP. These mechanical differences are

probably attributable to the open, hydrogen bonded network structure of LAP.

LAP is strongly plastic material and it is 2.5 times softer than KDP. Even a

small stress applied during handling of the crystals will make deformation at

the surface. LAP undergoes an irreversible endothermic transition at around

130oC with the free energy of 250 J/g. Direct observation of heated LAP

shows that the crystal begins to soften above 100oC and then liquefies at 120

– 140oC. Above 15oC, other volatile substances are produced due to ammonia

molecule. Colored solution is formed by thermal as well as photo-induced

disintegration of L-arginine molecule.

The quest for various frequency conversion materials is

concentrated on semiorganic crystals due to their large nonlinearity, high

resistance to laser induced damage, low angular sensitivity and good

mechanical hardness (Haja Hameed et al 1999, 2003 and Packiam Julius

2004). Effect of amino acid mixing on the properties of various NLO crystals

have been investigated (Arunmozhi et al 1997, Aravazhi et al 1997 and Haja

Hameed et al 2005a). Growth and characterization of some amino acid doped

L-arginine phosphate crystals have been investigated by Joseph Arulpragasam

et al (2006). Mohan kumar et al (2001) investigated the effect of L-lysine in

TGS and TGSP Single crystals. The aim of the present work was to study the

influence of amino acids on the characteristics of LAP crystals. The neutral

amino acids - glycine, L-alanine and L-valine have been separately mixed

with L-arginine phosphate and the crystals of dimensions around 20x17x7

30

mm3 have been grown. The doping effect on growth and characterization

studies such as X- ray powder diffraction (XRD), Fourier transform infrared

(FTIR) and thermo gravimetric analyses and differential thermo gravimetric

analyses (TG & DTG) has been studied.

2.2 STRUCTURE OF L-ARGININE PHOSPHATE

MONOHYDRATE (LAP)

The structure of L-arginine phosphate monohydrate crystal reported

by Aoki et al (1971) is shown in Figure 2.1. It can be regarded as layers of

phosphate ions, arginine ions and water molecule stacked along ‘a’ axis.

While the arginine-phosphate layers are held together both by ionic forces and

a large number of hydrogen bonds, the arginine-water-arginine layers are

linked only by the bridging of water molecules and van der Waals forces

between the aliphatic segments of adjacent arginines. It is likely that these

weakly bonded layers are the cause of the easy cleavage parallel to (100)

observed in LAP crystals (Fuchs et al 1989). In LAP crystal, the guanadyl and

amino groups are protonated and hence have positive charges, which balance

the negative charges of the carboxylate and dihydrogen phosphate ions. It is

likely that the birefringence and the nonlinear coefficients of LAP crystals are

largely determined by the geometric arrangement of these ionic units (Eimerl

et al 1989). It follows that the primary role of the phosphate ion is to control

the structure. Its influence on the macroscopic NLO coefficient lies in its

control on the orientation of the protonated groups and not in intrinsic

nonlinearity. This crystal belongs to the monoclinic system with the space

group P21 with two formula units per unit cell. The cell dimensions are a =

10.85 Å, b = 7.91 Å, c = 7.32 Å, the angle between ‘a’ and ‘c’ axes is 98°

(Aoki et al 1971). Figure 2.2 shows the molecular structure of L-arginine

phosphate monohydrate.

31

Figure 2.1 Crystal structure of L-arginine phosphate monohydrate

(Aoki et al 1971)

Figure 2.2 Molecular structure of L-arginine phosphate monohydrate

32

2.3 EXPERIMENTAL PROCEDURE

2.3.1 Material Synthesis

L-arginine phosphate was synthesized from the strongly base amino

acid, L-arginine and orthophosphoric acid as per the following reaction:

(NH2) NHCNH (CH2)3 CH (NH2) COOH + H3 PO4 + H2 O

(H2N)2+ CNH(CH2)3 CH (NH3)

+ COO-. H2 PO4 . H2 O

L-arginine (98%) and orthophosphoric acid (87%) were taken in

equimolar ratio and dissolved in millipore water (resistivity of 18.3 M cm)

as per the above reaction. The starting material was prepared by evaporating

excess water to almost dry at room temperature.

For synthesis of alanine mixed LAP, 0.8, 0.2, 1 molar concentration

of L-arginine (98%), L-alanine (99%) and orthophosphoric acid (87%) were

taken and dissolved in millipore water. The mixed solution was kept in hot

plate below 60 C and allowed to evaporate the excess water. As per the

following reaction, dried salt of ALAP was obtained after evaporating the

water.

[(NH2) NHCNH (CH2)3 CH (NH2) COOH](1-x) + [CH3CH(NH)COOH]x +

H3 PO4 + H2 O (H2N)2+ CNH(CH2)3 CH (NH3)

+ COO-. H2 PO4 . H2 O

For the synthesis of glycine mixed LAP, L-arginine, glycine and

ortho phosphoric acid with molar ratio 0.8:0.2:1 respectively were taken in

addition to the excess of millipore water. The reaction is as follows

[(NH2) NHCNH (CH2)3 CH (NH2) COOH](1-x) + [CH2(NH2)COOH]x +


+ COO-. H2 PO4 . H2 O

33

The same procedure was adopted for synthesis of valine mixed

LAP (VLAP) as per the reaction:

[(NH2) NHCNH (CH2)3 CH (NH2) COOH](1-x) + [(CH3)2CH(NH)COOH]x +


+ COO-. H2 PO4 . H2 O

Synthesized salts of LAP, ALAP, GLAP and VLAP were used for

the growth of bulk crystals. The prepared salts were further purified by

repeated recrystallization process.

2.3.2 Multicrystal X-ray Diffractometry

The crystalline perfection of the grown single crystals was

characterized by HRXRD by employing a multicrystal X-ray diffractometer

developed at National Physical Laboratory (NPL) (Lal and Bhagavannarayana

1989). Figure 2.3 shows the schematic diagram of the multicrystal X-ray

diffractometer. The divergence of the X-ray beam emerging from a fine focus

X-ray tube (Philips X-ray Generator; 0.4 mm x 8 mm; 2kWMo) is first

reduced by a long collimator fitted with a pair of fine slit assemblies. This

collimated beam is diffracted twice by two Bonse-Hart type of

monochromator crystals and the thus diffracted beam contains well resolved

MoK 1 and MoK 2 components (Bonse and Hart 1965). The MoK 1 beam is

isolated with the help of fine slit arrangement and allowed to further diffract

from a third (111) Si monochromator crystal set in dispersive geometry (+, -, -).

Due to dispersive configuration, though the lattice constant of the

monochromator crystal and the specimen are different, the dispersion

broadening in the diffraction curve of the specimen does not arise. Such an

arrangement disperses the divergent part of the MoK 1 beam away from the

Bragg diffraction peak and thereby gives a good collimated and

monochromatic MoK 1 beam at the Bragg diffraction angle, which is used as

34

incident or exploring beam for the specimen crystal. The dispersion

phenomenon is well described by comparing the diffraction curves recorded

in dispersive (+,-,-) and non-dispersive (+,-,+) configurations

(Bhagavannarayana 1994). This arrangement improves the spectral purity

( / 10-5) of the MoK 1 beam. The divergence of the exploring beam in

the horizontal plane (plane of diffraction) was estimated to be 3 arc sec.

The specimen occupies the fourth crystal stage in symmetrical Bragg

geometry for diffraction in (+, -, -, +) configuration. The specimen can be

rotated about a vertical axis, which is perpendicular to the plane of diffraction,

with minimum angular interval of 0.4 arc sec. The diffracted intensity is

measured by using an in-house (NPL) developed scintillation counter. To

provide two-theta (2 B) angular rotation to the detector (scintillation counter)

corresponding to the Bragg diffraction angle ( B), it is coupled to the radial

arm of the goniometer of the specimen stage.

The rocking or diffraction curves were recorded by changing the

glancing angle (angle between the incident X-ray beam and the surface of the

specimen) around the Bragg diffraction peak position B (taken as zero for the

sake of convenience) starting from a suitable arbitrary glancing angle. The

detector was kept at the same angular position 2 B with wide opening for its

slit, the so-called scan. This arrangement is very appropriate to record the

short range order scattering caused by the defects or by the scattering from

local Bragg diffractions from agglomerated point defects or due to low angle

and very low angle structural grain boundaries (Bhagavannarayana and

Kushwaha 2010).

35

Figure 2.3 Schematic line diagram of Multicrystal X-ray diffractometer

designed, developed and fabricated at NPL, New Delhi

2.4 GROWTH OF LAP CRYSTALS

2.4.1 Crystal Growth

Optical quality seed crystals grown by slow evaporation technique

were used to grow bulk crystals by slow cooling method. After saturating the

mother solution at 40 C, the growth has been initiated at 39.95 C with a

cooling rate of 0.2 C/day by using a constant temperature bath with a control

accuracy of 0.01 C. The constant temperature bath used in this study is

shown in the Figure 2.4. Millipore water was used for the preparation of the

growth solutions of all the four salts. Optical quality bulk crystals of various

dimensions were harvested after a typical growth period of about 20 days.

Growth rates were determined along the crystallographic axes (a, b, c) by

using digital vernier caliper.

36

Figure 2.4 Photograph of constant temperature bath used to grow the

LAP crystals

2.4.2 Growth Aspects

Sangwal et al (1995) reported various planes present in the LAP

crystals. In the ALAP crystal (100), (110), (-110), (010), (101), (011), (2-1-1),

(011) are the prominent planes, which have been identified. (100), (101),

(110), (-110), (011), (0-11), (2-1-1) (201) are the planes which have been

identified in GLAP. (100), (110), (-110), (-2-11), (-2-1-1), (1-11), (-1-11) are

the main planes observed in the VLAP. The as-grown crystals of LAP,

GLAP, ALAP and VLAP are shown in Figure 2.5. The morphology of the

LAP is shown in the Figure 2.6. Growth rates along the three crystallographic

axes (a, b, c) have been studied for pure and amino acids mixed LAP crystals.

For the pure LAP crystal, the growth rate along the ‘b’ axis has been observed

to be more than that of the other two axes. Among the axes ‘a’ and ‘c’ the

growth rate along the ‘c’ axis has been observed to be more than that of the

‘a’ axis. When compared to pure LAP crystal, the growth rate of ALAP and

GLAP along ‘a’ and ‘b’ axes has not been affected notably, but the c-axis

growth rate is significantly increased. The growth rates of ALAP along all the

37

three axes have been observed to be slightly more compared to GLAP. On the

other hand, the growth rates of VLAP along ‘a’ and ‘b’ axes have been found

to be changed considerably. The increased growth rates have been observed in

‘a’ and ‘b’ axes, whereas the growth rate in ‘c’ axis remain almost same as pure

LAP. Tapering effect has also been observed along the three crystallographic

axes and hence, the shape of the crystal resembles triangle. Due to the

tapering effect the quality of the crystal has been found to be affected. GLAP

is observed to have more transparency than that of other LAP crystals. The

growth rates along the crystallographic axes are tabulated in Table 2.1.

Figure 2.5 As-grown crystals of LAP, GLAP, ALAP and VLAP

Figure 2.6 Morphology of the LAP crystal ( Sangwal et al 1995)

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Table 2.1 Statistics of growth rates along the crystallographic axes of

pure and amino acid mixed LAP crystals

Growth rates(mm/day)Crystals

a b c

LAP 0.35 0.64 0.49

GLAP 0.33 0.66 0.72

ALAP 0.37 0.7 0.78

VLAP 0.56 0.84 0.46

Standard Deviation 0.11 0.09 0.16

Standard Error 0.05 0.05 0.08

2.4.3 Microbial Contamination

The microbial contamination of the solution is one of the major

problems in the growth of L-arginine phosphate family crystals due to the

incorporation of endotoxin during the long growth period. This problem

affects the quality of the crystals limiting the growth of large size crystals by

the conventional slow cooling method. Moreover the incorporation of

microbes into the crystals reduces the laser damage threshold of the crystals

when subjected to laser radiation. Since, L-arginine is an amino acid, which is

rich in nutrient; microbes grow on the surface of the solution when it is

exposed to atmosphere. After a few days, the microbes that grew on the

surface fall into the solution and hence the solution is contaminated. A

number of additives have been tried to inhibit the microbial growth. Yokotani

et al (1990) reported that the addition of H2O2 and CHCl3 in the DLAP

solution inhibited the microbial growth. Dhanaraj et al (1991) employed a

thick layer of n- hexane of about 2-4 cm height over the solution surface to

overcome this problem. In the present study, H2O2 additive has been added to

prevent the microbial formation in solutions. For the entire growth period, the

39

growth of microbes has been monitored. Till the crystals have been harvested

(after twenty five days) no microbes have been observed in all the four

solutions. But after thirty days, a layer of thin white colored microbes were

observed in LAP; In the case of ALAP a thinner layer of light white colored

microbes was observed. A thick layer of dark brown colored microbes was

observed in GLAP. However, in the VLAP solution, the microbes were

observed to be very less and also it is not in the form of layer. This

observation is the evidence of presence of neutral amino acids in the

respective solutions.

2.4.4 Coloration

The coloration of the solution with time is another major problem

as reported by Sasaki et al (1989). The coloration of the solution is due to the

fact that the chemical decomposition takes place in L-arginine molecule when

exposed to light. This coloration is known as photodecomposition. By using

different illumination conditions Dhanaraj et al (1991) have prevented the

coloration of the solution during the growth period. When the crystals were

grown under light proof containers at very low temperatures, no coloration

was observed. It was concluded that the decomposition of L-arginine is the

combined effect of thermal and photo induced decomposition. In the present

study thermal decomposition was avoided by using the low growth

temperature range between 40 C and 30 C for the growth of crystals.

At the time of growth run pure, alanine, glycine and valine, mixed

LAP solutions were colorless. When harvesting the crystals after considerable

size i.e. after 20 days, the solutions of LAP and ALAP become pale yellow.

The solution of GLAP was light brown and the VLAP solution remains

colorless. Hence, the photo-induced decomposition did not take place in

VLAP solution.

40

2.5 CHARACTERIZATION STUDIES

2.5.1 Powder X-ray Diffraction Studies

Powder X-ray diffraction patterns were recorded at room

temperature by using XPERT-PRO diffractometer system for LAP, ALAP,

GLAP and VLAP crystals as shown in Figures 2.7a, 2.7b, 2.7c and 2.7d

respectively. The interplanar distance (d) was calculated for all the prominent

peaks using Bragg’s equation. Using the values of ‘d’, and 2 the hkl values

for all the reflections were assigned. The lattice parameter values were

calculated using the monoclinic crystallographic equation for = 98 . It is

observed that the addition of amino acids alters the lattice parameter values.

Among the four diffraction patterns, the number of peaks observed for VLAP

is less compared to LAP, GLAP and ALAP. The calculated lattice parameters

are shown in Table 2.2.

Table 2.2 Lattice parameter values of pure and amino acids mixed LAP

crystals

Lattice ParametersÅCrystals

A b c

Pure LAP 10.8841 7.9424 7.3408

GLAP 10.9192 8.0866 7.3065

ALAP 10.9191 8.0866 7.2746

VLAP 10.8328 8.0613 7.2477

Standard Deviation 0.0408 0.0689 0.0402

Standard Error 0.0204 0.0345 0.0201

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10 20 30 40 50 60 70 80

0

500

1000

1500

2000

2500

3000

3500

4000

(311)

(221

)

(120

)

(11-1

)(2

00)

(21-1

)(1

11)

(101

)

(012

)(2

20

)

(313

)

(32

0)

Inte

ns

ity

(a

rb.u

nit

s)

Diffraction angle, 2 deg)

(300)

(a) LAP

Figure 2.7 (Continued)

42

Figure 2.7 X-ray diffraction patterns of (a) LAP (b) ALAP (c) GLAP

and (d) VLAP

43

2.5.2 FTIR Spectral Analysis

In order to analyze the presence of various functional groups in the

grown crystals FTIR spectra for LAP, ALAP, GLAP and VLAP were

recorded at room temperature using Thermo Nicolet 380 FTIR spectrometer

as shown in the Figures 2.8a, 2.8b, 2.8c and 2.8d respectively. LAP, ALAP,

GLAP and VLAP were crushed separately and a few milligram of each salt

was used for making pellets with KBr. The spectra were found to be complex,

because of various functional groups present in the crystals. The bands due to

stretching vibrations of NH2, NH2+, NH3

+, COO- and HPO4 have been

observed in the high frequency range. In the range of lower frequency, the

bands due to the deformation vibrations of various functional groups have

been observed. In FTIR spectra, the bands have been formed due to the

internal vibration of the arginine molecule, phosphate group, and hydrated

group in the mixed crystals. Moreover, rocking mode, symmetric deformation

and asymmetric bending have been found from all the three spectra. The

rocking mode of NH2 group is at 782 cm-1 in GLAP, 780 cm-1 in ALAP and at

781 cm-1 in VLAP. In the formation of phosphate ion, P-OH stretching has

been observed at 950 cm-1 in all the crystals. P-OH deformation bands are due

to the internal vibration of the phosphate group, which has been identified at

408 cm-1 in all the crystals. The normal mode frequencies have been identified

at 3637 cm-1 and 3891 cm-1 due to the presence of H2O molecules in the

grown crystals. Overlaping of bands due to the hydrogen bonding in the high

frequency range has been observed in the all the spectra. Variation of

intensity has been observed for the pure and doped crystals and the wave

numbers and its assignments are given in Table 2.3.

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4000 3500 3000 2500 2000 1500 1000 500

0

20

40

60

80

100

Tra

nsm

itta

nce (

% )

Wavenumber (cm-1

)

(a) LAP

4000 3500 3000 2500 2000 1500 1000 5000

20

40

60

80

100

Tra

nsm

itta

nc

e (

%)

Wavenumber (cm-1

)

(b) ALAP


45

4000 3500 3000 2500 2000 1500 1000 5000

20

40

60

80

100

Tra

ns

mit

tan

ce

(%

)

Wavenumber (cm-1

)

(c) GLAP

4000 3500 3000 2500 2000 1500 1000 5000

20

40

60

80

100

Tra

nsm

itta

nce (

%)

Wavenumber(cm-1)

(d) VLAP

Figure 2.8 FTIR Spectra of (a) LAP (b) ALAP (c) GLAP and (d) VLAP

crystals

46

Table 2.3 Wave numbers and assignments for LAP crystal

Wave number (cm-1) Assignments

3449 3 (H2O)

3331 1 (H2O)

3166 NH+3 asymmetric stretching

2411 NH+3 symmetric stretching

1690 C = N stretching

1652 NH+2 deformation

1615 C = O stretching

1568 COO - asymmetric stretching

1524 NH+3 asymmetric deformation

1454 2 (H2O)

1409 COO - symmetric stretching

1372 C – C – H in plane deformation

1334 CH2 wagging

1320 CH2 wagging

1286 P – OH angular

1245 P = O stretching

1176 NH+3 rocking

1160 NH+2 wagging

1130 NH2 wagging

1088 3 (PO4)

1045 1(PO4)

1035 P – OH deformation

950 P – OH stretching

901 CH2 rocking

872 C – C stretching, P – OH stretching

782 NH2 rocking

760 COO - scissoring

699 NH2 out of plane bending

47

2.5.3 HRXRD Analysis of LAP Single Crystal

Before recording the diffraction curve, to remove the non-

crystallized solute atoms remained on the surface of the crystal and also to

ensure the surface planarity, the specimen was first lapped and chemically

etched in a non-preferential etchant. This process also ensures to remove

surface layers, which may sometimes form on the surface of the crystal due to

organic additives (Bhagavannarayana et al 2006).

High-resolution rocking/diffraction curve (DC) was recorded for a

typical undoped LAP crystal grown by SEST (slow evaporation of solvent

technique) method using (300) diffracting planes in symmetrical Bragg

geometry by employing the multicrystal X-ray diffractometer with MoK 1

radiation and is shown in Figure 2.9.

-100 -50 0 50 1000

400

800

1200 LAP(Pure)

(300) Planes

MoK

Diffr

acte

d X

-ra

y in

ten

sity [

c/s

]

Glancing angle [arc s]

6"

Figure 2.9 High-resolution X-ray diffraction curve recorded for a

typical undoped LAP single crystal specimen using (300)

diffracting planes

As seen in the figure, the DC contains a single sharp peak, which

indicates that the specimen is free from structural grain boundaries. The

48

FWHM (full width at half maximum) of the curve is 6 arc s, which is very

close to that expected from the plane wave theory of dynamical X-ray

diffraction (Batterman and Cole 1964) for an ideally perfect crystal.

2.5.4 HRXRD Analysis of GLAP Crystal

The high resolution X-ray diffraction curve was recorded for (300)

diffraction planes of GLAP crystal using MoK 1 radiation as shown in

Figure 2.10.

-400 -200 0 200 4000

200

400

600

Diffr

acte

d X

-ra

y inte

nsity [

c/s

]


84" 130" 88"

52" 92"70" 112"

GLAP

(300) Planes

MoK


typical GLAP single crystal specimen using (300) diffracting

planes

The solid line (convoluted curve) is well fitted with the

experimental points represented by the filled circles. On deconvolution of the

diffraction curve, it is clear that the curve contains three additional peaks,

which are 88, 130 and 214 arc s away from the main peak (at zero glancing

angle). These three additional peaks correspond to three internal structural

low angle boundaries (tilt angle > 1 arc min but less than a degree)

49

(Bhagavannarayana et al 2005), whose tilt angles (tilt angle may be defined as

the disorientation angle between the two crystalline regions on both sides of

the structural grain boundary) are 88, 130 and 84 arc s from their adjoining

regions. The FWHM of the main peak and the very low angle boundaries are

respectively 70, 112, 92 and 52 arc s. The broader low intensity peaks with

112 and 92 arc s FWHM show that the grain corresponding to these peaks

contains mosaic blocks disoriented to each other by at least few tens of arc

seconds.

2.5.5 HRXRD Analysis of ALAP Crystal

Figure 2.11 shows the high resolution X-ray diffraction curve

recorded for (300) diffraction planes of ALAP crystal using MoK 1 radiation.

The solid line (convoluted curve) is well fitted with the experimental points

represented by the filled circles.

-200 0 200 4000

400

800

1200

Diffr

acte

d X

-ra

y in

ten

sity [c/s

]


ALAP

(300) Planes

MoK

300"19"

157 10" 162"


typical ALAP single crystal specimen using (300) diffracting

planes

50

On deconvolution of the diffraction curve, it is clear that the curve

contains two additional peaks, which are 19 and 300 arc s away from the main

peak (at zero glancing angle). Out of these two peaks, the peak at 19 arc s

away from the main peak shows a very low angle boundary, whose tilt angle

is 18 arc s. The other additional peak at 300 arc sec corresponds to a low

angle boundary whose tilt angle is 300 arc s. The FWHM of the main peak

and the very low angle boundaries are respectively 10, 157 and 162 arc s. The

broadness of the additional peaks show that the grains corresponding to these

peaks contain mosaic blocks disoriented to each other by at least few tens of

arc seconds similar to GLAP crystal.

2.5.6 HRXRD Analysis of VLAP Crystal

High resolution X-ray diffraction curve recorded for (300)

diffraction planes of VLAP crystal is shown in Figure 2.12.

-500 -250 0 250 5000

200

400

600

Diffr

acte

d X

-ra

y in

ten

sity [c/s

]


60"

142" 166"

68"40"54"202"

VLAP

(300)Planes

MoK


typical VLAP single crystal specimen using (300) diffracting

planes

51

It is observed that the diffraction curve contains three additional

peaks, which are 166, 226 and 368 arc s away from the main peak (at 225 arc

s glancing angle). These three additional peaks correspond to three internal

structural low angle boundaries whose tilt angles are 166, 60 and 142 arc s

from their adjoining regions. The FWHM of the prominent peak and the very

low angle boundaries are 68, 40, 54 and 202 arc s respectively. The broader

low intensity peak with 202 arc s FWHM shows that the grain corresponding

to this peak contains mosaic blocks disoriented to each other by at least few

tens of arc seconds.

The multi peeks observed in all the doped crystals ensure the

presence of amino acids in the LAP lattice. Since the tilt angles are in the

order of a few arc sec, one cannot presume these grain boundaries as twins. A

complete description of such boundaries with X-ray topographs has been

described by Bhagavannarayana and Kushwaha (2010). However, the

background scattering indicate the presence of point defects like interstitials

and vacancies.

2.5.7 UV-VIS-NIR Spectral Analysis

Since the absorption, if any in the NLO material near the

fundamental or second harmonic will lead to loss of conversion efficiency, it

is necessary to have good optical transparency in the NLO crystal in the UV

region. Optical absorption spectra were recorded in the range between 200 to

1200 nm using 2 mm thick c-cut crystal plates of GLAP, ALAP and VLAP

crystals. The recorded absorption spectra are shown in Figures 2.13a, 2.13b

and 2.13c. From the spectra, it is evident that GLAP and ALAP crystals are

having strong absorption only below 240 nm. However, VLAP has at 230 nm.

Hence, these crystals are useful for optoelectronic applications and the second

harmonic generation from the Nd:YAG Laser which is sufficient for SHG

laser radiation of 1064 nm or other applications in the blue region.

52

Figure 2.13 UV-VIS-NIR absorption spectra of (a) GLAP(b) ALAP and

(c) VLAP

53

2.5.8 Thermal Analysis

Thermal analyses are of immense importance as far as fabrication

technology is concerned, as they provide thermal stability of the material for

fabrication, where a considerable amount of heat is generated during the

cutting process. Thermal analysis has been performed on the grown crystals to

study the thermal stability and melting point. The powdered samples of doped

crystals were subjected to thermo gravimetric analysis (TGA). Differential

thermo gravimetric analysis (DTG) curves were also obtained for the doped

samples. TGA and DTG curves for GLAP, ALAP and VLAP are shown in

Figure 2.14 (a, b and c). The curves were obtained for the temperature range

of 35 C to 800 C. The percentage of weight loss due to temperature for each

sample were observed to be increased for these crystals compared to pure

LAP. The first decomposition due to dehydration of water takes place at

157.3 C in GLAP and ALAP, whereas, for VLAP the first decomposition

temperature is 161 C. This decomposition temperature corresponds to the

melting points of the crystals. However, in the case of LAP, it is nearly 137 C

(Haja Hameed et al 1999). Thermal stability of the doped crystals are found to

be increased. Melting points of the grown crystals are presented in Table 2.4.

Table 2.4 Melting points of pure and amino acids mixed LAP crystals

Crystals Melting Point

LAP 137.0

ALAP 157.3

GLAP 157.3

VLAP 161.0

Standard Deviation 10.91

Standard Error 5.45

54


55


56

Figure 2.14 TG/DTG curves of (a) glycine mixed L-arginine phosphate

monohydrate crystal (b) alanine mixed L-arginine

phosphate monohydrate crystal (c) valine mixed L-arginine

phosphate monohydrate crystal

57

2.6 CONCLUSION

Single crystals of pure and amino acids mixed L-arginine

phosphate crystals such as GLAP, ALAP and VLAP have been grown from

aqueous solution by slow cooling method. Growth rates of these crystals

along the three crystallographic axes of pure and amino acids mixed LAP

crystals have been found to be varied. This growth rate variation is due to the

partial substitution of glycine, alanine and valine in the respective LAP

crystals. The variation of lattice parameters has also been observed from

powder X-ray diffraction studies, which confirmed the incorporation of amino

acids in the LAP crystal lattice. Presence of amino acids in the lattice is also

confirmed by the observation of multi-peaks in the high resolution X-ray

diffraction curves. Various functional groups of LAP and amino acids mixed

LAP crystals have been identified by FTIR studies. Suitability of the grown

crystals for the applications in the blue region is confirmed by optical

absorption study. The incorporation of amino acids in pure LAP significantly

enhances the thermal stability. It has been observed that L-valine has more

influence on the properties of the grown crystals. The presence of L-valine in

the solution completely prevents the formation of microbes and coloration.

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