CHAPTER 2 SYNTHESIS, GROWTH AND CHARACTERIZATION OF AMINO ACIDS ADMIXTURED L-ARGININE PHOSPHATE MONOHYDRATE SINGLE CRYSTALS

28 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 130 o C with the free energy of 250 J/g. Direct observation of heated LAP shows that the crystal begins to soften above 100 o C and then liquefies at 120 140 o C. Above 15 o C, 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 mm 3 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 P2 1 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: (NH 2 ) NHCNH (CH 2 ) 3 CH (NH 2 ) COOH + H 3 PO 4 + H 2 O (H 2 N) 2 + CNH(CH 2 ) 3 CH (NH 3 ) + COO -. H 2 PO 4. H 2 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. [(NH 2 ) NHCNH (CH 2 ) 3 CH (NH 2 ) COOH] (1-x) + [CH 3 CH(NH)COOH] x + H 3 PO 4 + H 2 O (H 2 N) 2 + CNH(CH 2 ) 3 CH (NH 3 ) + COO -. H 2 PO 4. H 2 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 [(NH 2 ) NHCNH (CH 2 ) 3 CH (NH 2 ) COOH] (1-x) + [CH 2 (NH 2 )COOH] x + H 3 PO 4 + H 2 O (H 2 N) 2 + CNH(CH 2 ) 3 CH (NH 3 ) + COO -. H 2 PO 4. H 2 O

33 The same procedure was adopted for synthesis of valine mixed LAP (VLAP) as per the reaction: [(NH 2 ) NHCNH (CH 2 ) 3 CH (NH 2 ) COOH] (1-x) + [(CH 3 ) 2 CH(NH)COOH] x + H 3 PO 4 + H 2 O (H 2 N) 2 + CNH(CH 2 ) 3 CH (NH 3 ) + COO -. H 2 PO 4. H 2 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)

38 Table 2.1 Statistics of growth rates along the crystallographic axes of pure and amino acid mixed LAP crystals Crystals Growth rates (mm/day) 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 H 2 O 2 and CHCl 3 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, H 2 O 2 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 Crystals Lattice Parameters Å 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

41 4000 3500 (21-1) (a) LAP Intensity (arb.units) 3000 2500 2000 1500 1000 (101) (111) (300) (012) (320) 500 (200) (11-1) (120) (220) (221) (311) (313) 0 10 20 30 40 50 60 70 80 Diffraction angle, 2 deg) 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 NH 2, NH + 2, NH + 3, COO - and HPO 4 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 NH 2 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 H 2 O 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.

44 100 (a) LAP Transmittance ( % ) 80 60 40 20 0 4000 3500 3000 2500 2000 1500 1000 500 Wavenumber (cm -1 ) 100 (b) ALAP Transmittance (%) 80 60 40 20 0 4000 3500 3000 2500 2000 1500 1000 500 Wavenumber (cm -1 ) Figure 2.8 (Continued)

45 100 (c) GLAP Transmittance (%) 80 60 40 20 0 4000 3500 3000 2500 2000 1500 1000 500 Wavenumber (cm -1 ) 100 (d) VLAP Transmittance (%) 80 60 40 20 0 4000 3500 3000 2500 2000 1500 1000 500 Wavenumber(cm -1 ) 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 (H 2 O) 3331 1 (H 2 O) 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 (H 2 O) 1409 COO - symmetric stretching 1372 C C H in plane deformation 1334 CH 2 wagging 1320 CH 2 wagging 1286 P OH angular 1245 P = O stretching 1176 NH + 3 rocking 1160 NH + 2 wagging 1130 NH 2 wagging 1088 3 (PO 4 ) 1045 1 (PO 4 ) 1035 P OH deformation 950 P OH stretching 901 CH 2 rocking 872 C C stretching, P OH stretching 782 NH 2 rocking 760 COO - scissoring 699 NH 2 out of plane bending

47 2.5.3 HRXRD Analysis of LAP Single Crystal Before recording the diffraction curve, to remove the noncrystallized 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. Diffracted X-ray intensity [c/s] 1200 LAP(Pure) (300) Planes MoK 800 400 6" 0-100 -50 0 50 100 Glancing angle [arc s] 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. Diffracted X-ray intensity [c/s] 600 400 200 52" 92" 84" 130" 88" GLAP (300) Planes MoK 70" 112" 0-400 -200 0 200 400 Glancing angle [arc s] Figure 2.10 High-resolution X-ray diffraction curve recorded for a 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. Diffracted X-ray intensity [c/s] 1200 800 400 19" 300" 157 10" 162" ALAP (300) Planes MoK 0-200 0 200 400 Glancing angle [arc s] Figure 2.11 High-resolution X-ray diffraction curve recorded for a 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. Diffracted X-ray intensity [c/s] 600 400 200 VLAP (300)Planes MoK 202" 142" 166" 60" 54" 40" 68" 0-500 -250 0 250 500 Glancing angle [arc s] Figure 2.12 High-resolution X-ray diffraction curve recorded for a 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

Figure 2.14 (Continued) 54

Figure 2.14 (Continued) 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.