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CHAPTER 6

GROWTH AND CHARACTERIZATION OF

g AND γ- GLYCINE SINGLE CRYSTALS

6.1 INTRODUCTION

In the field of nonlinear optical crystal growth, amino acids are

playing a vital role. Many numbers of natural amino acids are individually

exhibiting the nonlinear optical properties because they have a donor NH2 and

acceptor COOH and also intermolecular charge transfer is possible

(Davydov et al 1970). Especially natural amino acids of aspartic, glutamic,

arginine, lysine, L-alanine (Razzetti et al 2002) and γ-glycine are evidently

showing NLO activity because of an additional COOH group in the first and

NH2 group in the second. Also, some of the amino acids are used as dopants

and they enhance the material properties like ferroelectric properties (Meera

et al 2004, Mohan Kumar et al 2001). A series of semi-organic compounds

such as L-histidine tetrafluroborate, L-arginine diphosphate crystals have

been reported (Aggarwal et al 1999, Reena Ittyachan and Sagayaraj 2002)

with moderately high mechanical and chemical stability.

Complexes of amino acids with inorganic salts have been of

interest as materials for second harmonic generation (SHG). All amino acids

except glycine contain chiral carbon atoms and perhaps crystallize in the

non-centrosymmetric space group (Narayan Bhat and Dharmaprakash 2002).

Dipolar molecules possessing an electron donor group and an electron

acceptor group contribute to large second order optical nonlinearity arising

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from the intramolecular charge transfer between two groups of opposite

nature. Although the salts of amino acids like L-Arginine (Eimerl et al 1989),

L-Histidine (Aggarwal et al 1999) and L-Proline (Hudson et al 2002) are

reported to have novel properties, the complexes of glycine with inorganic

salts are not explored very much for optical SHG so far, since glycine, the

simplest amino acid, does not possess the asymmetric carbon, it is NLO

inactive. Glycine has three polymorphic crystalline forms α, く and γ

(Albrecht and Corey 1939). Both α and β forms crystallize in

centrosymmetric space groups ruling out the possibility of optical second

harmonic generation. But γ-glycine crystallizes in non-centrosymmetric space

group P31 making it a possible candidate for NLO applications and it is found

difficult to grow the γ-glycine crystals (Iitaka 1961).

The thermodynamic stabilities of the three polymorphs of glycine

at room temperature are in the order γ > α > く (Isabelle et al 2005). It has

been recently reported that complexes of the γ- glycine can be efficient in

optical SHG with inorganic salt sodium nitrate (Narayan Bhat and

Dharmaprakash 2002). Ferroelectricity was discovered in glycine silver

nitrate (Pepinsky et al 1957). It was also reported that glycine combines with

LiNO3 (Baran et al 2003) to form single crystals but none of these are

reported to have nonlinear optical property. In this chapter the growth aspects

of g- and け-glycine single crystals from aqueous solutions in the presence of

potassium nitrate as additive in equimolar ratio are presented.

6.2 GROWTH OF g- AND γ-GLYCINE SINGLE CRYSTALS

6.2.1 Growth of g-glycine

In the g-polymorph zwitter-ions are linked via hydrogen bonds

NH…O in double antiparallel layers, the interactions between these double

119

layers being purely van der Waals. In the け-polymorph zwitter-ions form

polar helixes linked with each other via extra NH…O hydrogen bonds to give

a three-dimensional polar network.

In solution growth technique, selection of a solvent (water, ethanol,

methanol, mixed solvents, etc.,), in which solute is moderately soluble, plays

a major role. The solubility test was carried out using deionised water. The

commercially available Glycine (GLY) was used for solubility study, after

repeated recrystallization process. The solubility experiment was carried out

in Constant temperature bath (CTB) (accuracy ±0.01°C) for the temperatures

of 30°C, 35°C, 40°C, 45°C, 50°C and 55°C. The temperature dependence of

solubility of glycine is shown in Figure 6.1. The growth experiments were

performed using deionised water and mixed solvents with the help of

solubility curve, by slow evaporation technique.

The saturated solution was prepared using commercially available

glycine (CH2 NH2 COOH). g- glycine was dissolved in a 100 ml beaker using

deionised water as the solvent. The solution was stirred well for about six

hours at room temperature and the saturated solution was filtered with

Whatman (Grade No. 1) filter paper in clean vessel. The solubility of

g- glycine was estimated at different temperatures as shown in Figure 6.1.

The vessels containing the solutions were covered with perforated polythene

sheets and housed in the constant temperature bath at 33°C. The solution was

allowed for slow evaporation and nucleation was observed in 5 days and it

was allowed to grow further for 21 days. Crystals with prismatic morphology

(20 mm × 10 mm × 5 mm) were harvested as shown in Figure 6.2. The

molecular structure of g-glycine is shown in Figure 6.3.

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Figure 6.1 Solubility of g-glycine in deionised water

Figure 6.2 As grown crystal of α- glycine

121

Figure 6.3 Molecular Structure of g-glycine

6.2.2 Synthesis and Growth of け-glycine

The saturated solution was prepared using commercially available

analytical grade glycine (CH2NH2COOH) (SRL-extra pure), KNO3

(Merck-extra pure) and deionised water. Prior to solution preparation for

crystal growth the solubility of γ-glycine in deionised water at different

temperature was estimated by the gravimetric method (Figure 6.4). It is

observed that the title compound exhibits good solubility and a positive

solubility temperature gradient in deionised water as solvent.

The γ-form of glycine single crystals were grown from glycine and

KNO3 as additive using deionised water as the solvent. Bulk size crystals

were grown by the slow evaporation solution growth method. The equimolar

ratio (1:1) of g-glycine and KNO3 were dissolved in separate beakers using

deionized water and the solutions were continuously stirred well for six hours.

The prepared solutions were mixed together and filtered using Whatman

(grade No. 1) filter paper in 100 ml degreased clean beaker. The beaker

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containing the solution was optimally closed and kept in constant

temperature bath at 33oC. Nucleation was observed in a period of ten days and

allowed to grow for four weeks and a harvested crystal of け-glycine

(20 mm × 10 mm × 7 mm) is shown in Figure 6.5. Investigation of γ-glycine

by selective additives has been reported in the literature (Srinivasan 2008,

Srinivasan and Arumugam 2007). Both in the given references (Srinivasan

2008, Srinivasan and Arumugam 2007) and present investigation the addition

of KNO3 as additive with glycine show that a critical concentration for the

transformation from g-glycine to γ-glycine form. It appears that the pH value

(in our case 6) is required for the transformation. The powder X-ray

diffraction studies show that there is no inclusion of additives in the host

lattice. Figures 6.6 and 6.7 show the powder X-ray diffraction pattern of

g-glycine and γ-glycines respectively.

When the crystallization is carried out in the presence of KNO3

glycine carries reduced hydration. It is due to hydration demanded by

additional KNO3. As a result glycine crystallizes rapidly due to high rate of

crystallization, hence the tendency to crystallize in centrosymmetric fashion is

lost. In contrast the crystallization of glycine without additive KNO3, the rate

of crystallization might be less, because of high hydration. As a result they

acquire enough time to crystallize in centrosymmetric fashion

(α-glycine). Hence reduced solvation of amino glycine and a consequent high

rate of crystallization are the reasons for the formation of gamma form of

glycine. In addition it is inferred that pure glycine requires 21 days, whereas

additive KNO3 added glycine needs 7 days for good crystallization.

123

Figure 6.4 Solubility of γ-glycine in deionised water

Figure 6.5 As grown crystal of γ-glycine

124

6.3 CHARACTERIZATION OF g-AND け-GLYCINE CRYSTALS

The grown single crystals of g- and け-glycine were confirmed by

single crystal and powder X-ray diffraction analyses. The FT-IR spectrum

was recorded by JESCO 416 PLUS FT-IR Spectrometer (KBr pellet

technique) in the range of 4000 cm-1 to 400 cm-1. The UV-Visible spectra of

g- and け-glycine crystals were recorded between 200 nm and 800 nm using a

(PERKIN ELMER LAMDA 35) UV-Vis- NIR spectrophotometer. The

thermogravimetric analysis (TGA) was carried out between 50°C and 600°C

in the nitrogen atmosphere at a heating rate of 100C/min using a STA

409 C/CD TGA unit. The dielectric behavior was studied with an LCR meter

(HIOKI 3635 model) as a function of frequency at different temperatures. The

characterization results of g- and け-glycine are presented in the following

sections.

6.3.1 X-ray Diffraction Studies

The grown crystals of g- and け-glycine were confirmed by both

single crystal and powder X-ray diffraction analyses.

6.3.1.1 Single crystal X-ray Diffraction

The single crystal XRD data of the grown α- and γ-glycine were

obtained by single crystal X-ray diffractometer (Model: ENRAF NONIUS

CAD4/MACH3) using MoKα (0.71073Å) radiation at room temperature by

the least square refinement of the setting angles of 25 reflections. From the

single crystal XRD analysis it is confirmed that the grown α-glycine

crystallizes in the monoclinic crystal system with space group P21 /n and

γ-glycine crystallizes in hexagonal crystal system with space group P31. The

determined lattice parameter values of α- and γ-glycine crystals are in

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accordance with the literature (Iitaka, 1961) and the values are presented in

Tables 6.1 and 6.2.

Table 6.1 Crystallographic data for α-glycine

Lattice parameters Present workReported value

(Iitaka 1961)

A 5.105(3)Å 5.102(0)Å

B 11.946(2)Å 11.970(9)Å

C 5.447(4)Å 5.457(5)Å

α 90° 90°

β 111.79° 111.42°

γ 90° 90°

V 308.4 Å3 309.6(9) Å3

Table 6.2 Crystallographic data for γ-glycine

Lattice

parametersPresent work

Reported value

(Iitaka 1961)

a 7.033(1)Å 7.037 Å

b 7.032(2)Å 7.037 Å

c 5.479(1)Å 5.483 Å

α 90° 90°

β 90° 90°

γ 120° 120°

V 234.8(1) Å 3 235.1(4) Å 3

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6.3.1.2 Powder X-ray Diffraction analysis

The grown g- and γ-glycine crystals were subjected to powder

X-ray diffraction studies. The powder X-ray diffraction pattern of g- and

γ-glycine were recorded from 10-60° using XPERT-PRO diffractometer

which are shown in Figures 6.6 and 6.7. The maximum intensity peaks of

(021), (040) of g-glycine and (110) for γ-glycine are observed in the patterns.

Figure 6.6 Powder X-ray diffraction pattern of α-glycine

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Figure 6.7 Powder X-ray diffraction pattern of γ-glycine

6.3.2 Spectral Analysis

6.3.2.1 Factor group analysis of g-glycine

The factor group method provides a basis for the prediction of

theoretical IR and Raman spectra of lattice vibrations. The single crystal

X-ray diffraction studies performed for the g-glycine (NH2CH2COOH) crystal

confirms that the crystal belongs to monoclinic crystal system with the space

group of P21/n ( 22h

C ). There are totally four atoms per unit cell, which

occupies the general sites of C1 (4) symmetry. There are 10 atoms in a single

molecule CH2NH2COOH which in turn gives rise to (10 × 4 × 3) 120 modes.

Group theoretical analysis of the fundamental modes of g-glycine crystal

predicts that there are 120 vibrational optical modes which are seen to

decompose into d117 = 30Ag+29Au+30Bg+28Bu apart from three acoustic

modes 1Au+2Bu. The factor group analysis for α-glycine crystal was

128

performed by following the procedures outlined by Rousseau et al (1981).

The summary of the factor group analysis of g-glycine is given in Table 6.3.

Table 6.3 Factor group analysis of g-glycine - summary

Factor

group

species

22h

C

Site

symmetry

C1(4)

Ext Int

C N H O

Optical

mode

Acoustic

mode Total

Ag 3T 3R 24 6 3 15 6 30 0 30

Au 3T 3R 24 6 3 15 6 30 1 29

Bg 2T 3R 24 6 3 15 6 30 0 30

Bu T 3R 24 6 3 15 6 30 2 29

9T 12R 96 24 12 60 36 120 3 117

6.3.2.1.1 Vibrational analysis of g-glycine

The vibrational analysis reveals the structures of the co-ordination

compounds, information on the nature of bonding and confirmation of the

material. The calculated vibrations of g-glycine could be due to lattice

vibrations and internal vibrations of the co-ordinated compounds mostly the

g-glycine groups. The formal calculations of fundamental modes of g-glycine

reveals 96 internal vibrations which can be attributed as

30Ag+29Au+30Bg+28Bu and 21 external modes contributed by 9 translational

and 12 rotational modes. The bands observed between 4000 cm-1and 400 cm-1

is due to the internal vibrations of co-ordinated compounds and the peaks

below 500 cm-1 arise from the deformational vibrations and the translational

and rotational modes of the compounds.

129

6.3.2.1.2 Internal vibrations

The internal vibrations of g-glycine are those arising from the NH3

asymmetric stretching and CH2 symmetric modes of vibrations. The internal

modes of g-glycine ions split into four components of Au (Z) and Bu (X,Y)

are IR active and Ag (XX,YY,ZZ,XY) and Bg (XZ,YZ) are Raman active. In

the title compound g-glycine, the NH3 vibrations of absorption at 3161 cm-1 is

due to asymmetric stretching. CH2 vibrations of the g-glycine have their

absorption at 3028 cm-1 is due to symmetric stretching.

6.3.2.1.3 External vibrations

The external vibrations are mainly due to the bands observed below

500 cm-1 which are due to the rotational and translational modes of vibrations

of g-glycine. The rotational modes are expected to have higher frequency and

intensity than translational modes in the Raman spectra. However the

translational modes are more intense in IR spectra (Bhattacharjee 1990,

Hanuja and Fomitsev 1980). g-glycine is found to have 21 external modes and

those vibrations can be achieved experimentally by polarized Raman

measurements. The correlation scheme for g-glycine is given in Table 6.4.

Table 6.4 Correlation scheme for g-glycine

ActivityFactor group

symmetry

22h

CIR Raman

Ag -- g xx gyy , gzz, gxy

Au Z --

Bg -- g xz gyz

Bu X,Y --

130

6.3.2.2 Factor group analysis of け-glycine

The single crystal X-ray diffraction studies performed for the

け-glycine (NH2CH2COOH) crystal confirms that the crystal belongs to

monoclinic crystal system with the space group P31 ( 23C ). There are totally

three atoms in the unit cell, which occupies the general sites of C1(3)

symmetry. There are 10 atoms in a single molecule CH2NH2COOH which in

turn gives rise to (10 × 3 × 3) 90 modes. Group theoretical analysis of the

fundamental modes of け-glycine crystal predicts that there are 90 vibrational

optical modes and are seen to decompose into d90 = 44A + 43E apart from

three acoustic modes 1A+2E. The factor group analysis for

け-glycine crystal was performed by following the procedures outlined by

Rousseau et al (1981). The summary of the factor group analysis of け-glycine

is given in Table 6.5.

Table 6.5 Factor group analysis of け-glycine - Summary

Factor

group

species

23C

Site

symmetry

C1(3)

Ext Int

C N H O

Optical

mode

Acoustic

mode Total

A 3T 3R 36 6 3 15 6 45 1 44

E 3T 6R 36

6T 9R 72

6

12

3

6

15

30

6

12

45

90

2

3

43

87

6.3.2.2.1 Vibrational analysis of け-glycine

The calculated vibrations of け-glycine could be due to lattice

vibrations and internal vibrations of the coordinated compounds mostly the

け-glycine groups. The formal calculations of fundamental modes of け-glycine

131

reveals 76 internal vibrations which can be attributed as 44A + 43E and 15

external modes contributed by 6 translational and 9 rotational modes. The

bands observed between 4000 cm-1 and 400 cm-1 is due to the internal

vibrations of co-ordinated compounds and the peaks below 500 cm-1 arise

from the deformational vibrations and the translational and rotational modes

of the compounds.

6.3.2.2.2 Internal vibrations

As the け-glycine molecule does not have any symmetry the internal

vibrations exhibited are of both IR and Raman active exclusive of acoustic

mode. The internal vibrations of け-glycine may be classified as those arising

from the NH3, CH2 and NO3- functional groups. These vibrations are strongly

coupled between themselves.

6.3.2.2.3 External vibrations

The external vibrations are mainly due to the bands observed below

500 cm-1 which are due to the rotational and translational modes of vibrations

of け-glycine in the present case. The rotational modes are expected to have

higher frequency and intensity than translational modes in the Raman spectra.

However the translational modes are more intense in IR spectra

(Bhattacharjee 1990, Hanuja and Fomitsev 1980). け-glycine is found to have

15 external modes and those vibrations can be achieved experimentally by

polarized Raman measurements. The correlation scheme obtained by

following the procedures of Fateley et al (2001) is given in Table 6.6.

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Table 6.6 Correlation scheme for け-glycine

ActivityFactor group

symmetry

23C

IR Raman

A Z gxx + gyy, gzz

E X,Y (gxx – gyy, gxy) , (gxz, gyz)

6.3.3 Infrared Spectral Analysis

Infrared spectroscopy finds application in determining the site of

co-oridnation, the nature of metal-ligand bonding as well as for the

elucidation of structures of co-ordination compounds. The FT-IR spectrum

was recorded on Bruker IFS – 66 V spectrophotometer in the regions

4000 cm-1 – 400 cm-1 by KBr pellet technique.

The recorded FTIR spectra were compared with the standard

spectral data of the functional groups. The hydrogen bond bridges two atoms

that have higher electro negativity (such as O, N) than hydrogen. Although

more symmetrical interaction occurs in organic molecular optical crystals,

there is interaction between a hydrogen bond to a sp3 nitrogen or oxygen and

an oxygen atom with more s-bond character. The profound influences of

hydrogen bonding will be different depending on whether the hydrogen bond

is intramolecular or intermolecular. The functional groups of g- and け-glycine

crystals were analyzed by Fourier Transform Infrared Spectroscopy. The

recorded FT-IR spectra of α and γ-glycine are shown in Figures 6.8 and 6.9.

In the FT-IR spectrum, the peaks observed at 3167 cm-1 of g-glycine and

3165 cm-1 of け-glycine are assigned to NH3 asymmetric stretching mode. This

133

is related to the hydrogen bond strength. The bands observed in g-glycine at

3028 cm-1 are assigned to CH2 stretching mode. The peak observed in

g-glycine at 1109 cm-1 is assignable to rocking vibrational mode of CH2. The

peaks observed at 870 cm-1 of g-glycine are due to the C-C stretching mode.

The bands observed at 607 cm-1 and 697 cm-1 of け-glycine are assignable to

carboxylate groups, while the absorption peaks at 1111 cm-1 and 1132 cm-1

are attributed to NH3+ group. Thus carboxyl group is present as carboxylate

ion in け-glycine (Baran Jan and Ratajczak Henryk 2005). The symmetric and

asymmetric deformation vibrations of the NH3+ group appear in the region

between 1680 and 1470 cm-1. The polarized properties of a very strong band

at 1518 cm-1 arising from the asymmetric stretching vibration of

COO- appears also in this region of the IR spectra of け-glycine. The bands at

1032 cm-1 in the IR spectra are assigned to NO3- ion in け-glycine. Of the

remaining peaks, those at 891 cm-1 and 1132 cm-1 are assigned to CCN group

and band at 910 cm-1 corresponds to CH2 group (Narayanan Bhat and

Dharmaprakash 2002a).

Figure 6.8 FT-IR spectrum of g-glycine

134

Figure 6.9 FT-IR spectrum of γ-glycine

6.4 OPTICAL STUDIES OF g- AND け-GLYCINE CRYSTALS

The UV-Vis spectrum gives limited information about the structure

of the molecule because the absorption of UV and visible light involves

promotion of the electrons in the j and ヾ orbital from the ground state to a

higher energy state. It is important for any NLO material because a nonlinear

optical material can be of practical use only if it has a wide transparency

window. Figure 6.10 shows the UV-Vis spectrum recorded with transparent

single crystal of g and け-glycine. It is observed that the lower cutoff

wavelengths of g- and け-glycine are 292 nm and 272 nm, respectively. The

lower UV cut off wavelength of a compound is decided by chromophores

present in it. But the chromophores viz. amino, carboxyl groups present in

け-glycine are transparent and show almost no absorption in the visible region.

γ-glycine absorption observed in the wavelength region between 330 nm and

800 nm. It is to be noted that the optical transparency below 330 nm in the

135

UV region is the most desirable characteristics for any nonlinear optical

application such as second harmonic generation.

Figure 6.10 UV-Vis Spectrum of g and γ - glycine

Figure 6.11 Plot between photon energy and (ghち)1/2

of け-glycine

136

The experimentally determined absorbance value permits one to

determine the band gap of the material. From the wavelength corresponding

to the band gap of material in the absorbance curve, a sudden rise in the

absorbance is expected. The threshold at which the absorption data showing

abrupt rise determined graphically, can be an indicative of the band gap of

material. Working on this hypothesis, and Equation (6.1) and utilizing the

absorbance data, the wavelength threshold was determined by plotting tangent

to the absorption threshold at which the absorption rises abruptly the optical

band gap of the crystal was determined (Chaudhary et al 2004, Joshi

et al 2001).

Eg = eV (6.1)

The absorption coefficient (g) of the crystals was calculated by

Mclean’s formula given in Equation (6.2).

Tr = (6.2)

where g is the absorption coefficient, d is the crystal thickness in mm and R is

the reflectance which is calculated from the transmittance and absorption

data. Since the product g is large, the second term in the denominator is

neglected and Equation (6.2) becomes

Tr = (1-R2) e-gd (6.3)

g = cm-1 (6.4)

137

The value of optical band gap (Figure 6.11) of け-glycine is 4.2 eV.

The internal efficiency of the device also depends upon the absorption

coefficient. Hence by tailoring the absorption coefficient and tuning the band

gap of the material, one can achieve the desired material which is suitable for

fabricating various layers of the optoelectronic devices as our requirements.

6.5 DIELECTRIC STUDIES

The dielectric studies were carried out using HIOKI 3532-50 LCR

HITESTER. The samples were prepared and mounted between the electrodes.

In order to ensure good electrical contact between the crystal and the

electrodes, the crystal faces were coated with silver paint. Extreme care was

taken that the silver paint does not spread to the sides of the crystal. The

capacitance and dissipation factor of the parallel plate capacitor formed by the

copper plate and electrode having the sample as a dielectric medium have

been measured. Figures 6.12 and 6.13 show the variation of dielectric

constant as a function of frequency for g- and け-glycine. It is found that the

dielectric constant of g- and け-glycine is higher at low frequencies and

decrease with increase in frequency. This may be attributed to space charge

polarization due to charged lattice defects. But at a fixed frequency, the

dielectric constant of γ-glycine is more than that of α-glycine. The γ-glycine

is more polarized and hence has high dielectric constant. For g-glycine

dielectric constant has a high value of 705 at 100 Hz and decreases to

189 at 5 MHz. Similarly for γ-glycine the dielectric constant has a high value

of 965 at 100 Hz and decreases to 228 at 5 MHz.

138

Figure 6.12 Variation of dielectric constant of g-glycine with frequency

Figure 6.13 Variation of dielectric constant of γ-glycine with frequency

139

Figures 6.14 and 6.15 show the variation of dielectric loss as a

function of frequency of α- and γ-glycine. The value of dielectric loss

indicates that the grown crystals have lesser defects. In accordance with

Miller rule, the lower value of dielectric constant at higher frequencies is a

suitable parameter for the enhancement of SHG coefficient

(Von Hundelshausen 1971). The characteristics of low dielectric loss at high

frequencies for γ-glycine suggest that it possesses enhanced optical quality

with lesser defects. For a particular frequency, the dielectric loss of γ-glycine

is lesser than α-glycine, which indicates that the γ-glycine possesses enhanced

optical quality and less defects.

Figure 6.14 Variation of dielectric loss of g- glycine with frequency

140

Figure 6.15 Variation of dielectric loss of γ-glycine with frequency

Figures 6.16 and 6.17 show the variations in conductivity of g- and

け-glycine for different frequencies and different temperatures. At higher

frequencies, the increased conductivity could be due to the reduction in the

space charge polarization. The increase in conductivity at higher frequencies

for a given temperature confirms small polaron hopping in the title crystal.

The conductivity of g- and け- glycine increases upon increasing temperature.

It is ascertained from Figures 6.16 and 6.17 that the electrical conductivity of

g and け- glycine crystals is low at low temperature, owing to the trapping of

some carriers at defect sites (Srinivasan et al 2006). The electrical conduction

in dielectrics is mainly a defect controlled process in the low temperature

region. The presence of impurities and vacancies mainly determine this

region. The energy needed to form the defect is much larger than the energy

needed for its drift. The conductivity of the crystalline material in the higher

temperature region is determined by the intrinsic defects caused by the

thermal fluctuations in the crystal (Gowri and Sahaya Shajan 2006).

141

Figure 6.16 Variation of conductivity of g- glycine with log f

Figure 6.17 Variation of conductivity of け-glycine with log f

142

6.6 TGA–DTA ANALYSIS OF g- AND け- GLYCINE CRYSTALS

The recorded TG and DTA thermograms for α- and γ-glycine are

shown in Figures 6.18 and 6.19. The TG and DTA curves of the grown

α-glycine did not indicate any change in the heat flow until the melting

transition that occurs at 257°C. The melting transition starts at 230°C and

ends at 265°C with a sharp melting band of 35°C. Also no weight loss was

observed until the melting transition for g-glycine whereas TG and DTA

curves of γ-glycine show an endothermic peak at 134.13°C, before its melting

transition. This peak represents certainly a phase transformation of this crystal

from γ to possibly the α-form (Perlovich et al 2001). Actually, the melting

starts around 315.33°C and ends at 331.39°C with a sharp melting band of

about 24.06°C. No appreciable weight loss was observed before this melting

transition.

Figure 6.18 TGA and DTA curves of g- glycine

143

Figure 6.19 TGA and DTA curves of γ - glycine

6.7 SECOND HARMONIC GENERATION OF γ-GLYCINE

CRYSTAL

The SHG conversion efficiency of γ-glycine was determined by the

modified version of the powder technique developed by Kurtz and Perry

(1968). A Q-switched Nd: YAG laser of 1064 nm was used as a source for

illuminating the powder sample of γ-glycine. Intense green light was

observed. KDP sample was used as the reference material and the output

power intensity of γ-glycine was comparable with the output power of KDP.

The SHG conversion efficiency of γ-glycine is given in Table 6.7.

144

Table 6.7 Comparison of SHG Signal energy Output

Input power (mJ / pulse) 1.45

KDP (mV) 95

γ-glycine (mV) 135

6.8 CONCLUSION

The single crystals of α- and γ-glycine were grown by slow

evaporation solution growth method. The XRD data for the grown α- and

γ-glycine confirmed the crystal structure. In け-glycine, transparency is

observed in the wavelength region between 200 nm and 800 nm. It is to be

noted that the optical transparency below 272 nm (け-glycine) in the UV region

is the most desirable characteristics for any nonlinear optical application such

as second harmonic generation. TG and DTA studies made on the grown

α- and γ-glycine crystals showed a tendency of the γ-glycine crystal changes

its form to α- at around 134.13°C. The dielectric studies indicate that the

γ-glycine possesses good optical quality with lesser defects compared to

α-glycine. The dielectric loss of γ-glycine was found to be less than that of

g-glycine. From the FTIR spectrum the presence of intermolecular hydrogen

bonding, which could enhance the nonlinear property of the γ-glycine material

was confirmed. The relative SHG efficiency of the け- glycine is 1.42 times

greater than that of KDP. The theoretical factor group analysis of α-glycine

predicts 120 optical modes that decompose into

Γtotal = 30Ag+29Au+30Bg+28Bu modes apart from three acoustic modes

Γacoustic = 1Au+2Bu. Similarly 90 optical modes were predicted for γ-glycine

that decompose into Γtotal = 43A + 44E along with the three acoustic modes

Γacoustic = 1A+2E.