CHAPTER 3 GROWTH AND CHARACTERIZATION OF PURE, UREA AND THIOUREA DOPED HIPPURIC ACID SINGLE CRYSTALS

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1 61 CHAPTER 3 GROWTH AND CHARACTERIZATION OF PURE, UREA AND THIOUREA DOPED HIPPURIC ACID SINGLE CRYSTALS 3.1 INTRODUCTION Nonlinear optical (NLO) applications demand good quality single crystals, which possess large NLO coefficients coupled with improved physical parameters. One attractive system, where there is a potential for realizing very large second order nonlinear coefficient is based on organic crystals. Organic materials have been of particular interest because of the large second order non linear optical coefficients and high laser damage threshold [119]. Generally the organic NLO materials are more favored over the inorganic NLO materials because of the large nonlinear response, as they are often formed by weak Van der Waals and hydrogen bonds and hence possess a high degree of delocalization [120]. The other advantages of organic NLO materials include easiness to synthesise, possibilities for introducing desired characteristics by substitutions, appreciable resistance to optical damage etc. A number of organic materials have been identified and synthesized, showing considerable NLO effects. However, only a few of them could be crystallized and investigated for second order NLO applications. The main reason for enhanced SHG activity in organic materials is the chirality and hydrogen bonds of the material. But, growing organic crystals of large size is still a big challenge. Among organic NLO materials, amino acids display specific features such as molecular chirality, absence of strongly conjugated bonds and zwitter ionic nature of the molecule [121]. Further, amino acids contain a proton-donor carboxyl (-COO) group and the proton-acceptor amino (-NH 2 ) group in them and thereby creating

2 push pull type motif to enhance the NLO response. -glycine is an interesting NLO material [114] and hippuric acid belongs to the glycine family. 62 Hippuric acid is a colourless crystal obtained from the urine of animals and human beings [122]. Hippuric acid (HA) is referred to as benzoyl amino acetic acid and crystallizes in the orthorhombic system with non-centrosymmetric space group P and lattice parameters: a = Å, b = Å and c = Å [123]. Its molecular formula is C 6 H 5.CO.NH.CH 2.COOH. Its structural formula is shown in Figure 3.1 [122]. Figure 3.1 Structural formula of hippuric acid Ringertz et al., [124] have reported the structure of HA crystal. The structure has been determined by direct methods and refined by full matrix least square computations. The final R value is 5.8 %. The benzene ring, the peptide part and the carboxyclic group are planar and twisted with respect to each other. The molecules are held together in three dimensions by one O-H O and one N-H O hydrogen bond to the peptide oxygen atom. The HA molecule is not extended to its maximum length. The carboxyl group has been rotated around the N-C (8) bond forming an approximate right angle with the rest of the molecule. This brings one of the carboxyl oxygen atoms into a hydrogen bonding position with respect to the peptide oxygen atom in the molecule related to the original by twofold screw axis parallel to the a-axis. The molecules are further held together by a N-H O hydrogen bond to the peptide oxygen atom in the c axis screw related molecule. Interestingly, HA is less hygroscopic when compared with KDP and urea and large crystals can be grown with ease. HA crystal possesses high non-linearity, low cut-off wavelength, high conversion efficiency and high laser damage threshold [125].

3 63 Alex et al., [126] reported that the optical transparency of HA crystal is much larger than that of the other organic crystals. Narayan Bhat et al., [123] have observed the growth of transparent and defect free HA crystals of size mm 3 from the HA dissolved in acetone. This crystal was free from macroscopic defects. Nagaraja et al., [125] observed that when HA crystal was irradiated at room temperature with Xe, Pb, Bi and U ions, it changed from colourless to yellow after irradiation. It is also reported that the irradiation with energetic heavy ions caused crater like surface defects as observed by scanning force microscopy. Suresh et al., [127] have studied the variation of dielectric constant and dielectric loss of HA crystals at different temperatures and frequency ranges. It was reported that the dielectric constant decreases with increasing frequency in the lower frequency range. The conductivity was found to increase with temperature. The effect of doping HA crystals with NaCl and KCl was investigated by Suresh Kumar et al., [71] with the vision to improve the physicochemical properties of the crystal. It was found that the doped HA crystals have enhanced mechanical properties than the pure HA crystals. Premanand et al., [128] have studied the influence of benzophenone and iodine in HA crystals. The addition of dopants lead to the reduction of lower cut-off wavelength. Vijayan et al., [129] have grown HA crystals by novel unidirectional solution growth method using dimethyl formamide (DMF) as solvent. The Vickers hardness number (H v ) of HA crystal grown by Vijayan et al., [130], varied between 30 and 50 kg mm -2. Ramachandran et al., [122] have reported the growth of HA crystals in gel by double diffusion method in sodium metasilicate. Rectangular plates of HA of size mm 3 were obtained. Pure HA crystals was synthesized by Jayarama et al., [131] using acetone-water (60:40) as solvent. These HA crystals were found to be stable upto 188 C. Urea and thiourea were reported as excellent nonlinear optical materials. Thiourea and urea crystals finds widespread use as frequency doublers in laser applications [ ]. The addition of urea in KDP crystal was investigated by Pritula et al [26]. They found that nonlinear optical properties and laser damage threshold were increased due to the addition of dopants. Sumithraj et al., [136] have reported the effect of urea and thiourea on the optical and thermal properties of L-arginine phosphate crystals. The nonlinear optical activity of the parent crystal was

4 64 enhanced due to the addition of urea and thiourea. In the present work, urea and thiourea were chosen as dopants expecting the increase in second harmonic generation (SHG) efficiency. The present investigation deals with the growth of pure, urea and thiourea doped HA crystals by slow solvent evaporation technique. The grown crystals have been subjected to XRD, CHNS analysis, FTIR, UV-Vis-NIR, Thermogravimetric analysis, Differential Scanning Calorimetery, Vickers Hardness, Knoop Hardness, Second Harmonic Generation, Dielectric measurements, A.C. Conductivity and etching studies. The results of these studies are discussed in this chapter. 3.2 GROWTH OF PURE, UREA AND THIOUREA DOPED HA CRYSTALS Generally, to grow bulk crystals from solution using isothermal solvent evaporation technique, it is desirable to select a solvent which is moderately soluble. The solubility of HA acid were determined by many researchers [123, 131, 137] in different solvents like water, acetone, dimethyl formamide, acetic acid, methanol, ethanol and ethyl acetate. It was found by Narayan Bhat et al., [123] that the solubility of HA is negligibly small in water and ethyl acetate. Therefore, it is very difficult to grow bulk crystals of HA using these solvents. Solubility of HA was very large in dimethyl formamide. Therefore, dimethyl formamide is not a suitable solvent to grow bulk crystals of HA. The solubility of HA is moderate in methanol, acetic acid and acetone and hence they are potential solvents for growing single crystals of HA. Hippuric acid of analytical grade (AR) was procured from Loba Chemicals and used as such. Slow evaporation method was employed for the growth of pure and doped crystals. In the present work, acetone was employed as the solvent for the growth of HA crystals g of HA was dissolved in 100 ml of acetone and stirred continuously for three hours using a magnetic stirrer to obtain a homogeneous mixture. The solution was filtered twice using ultra micro pore filter paper. The resulting solution was kept in a beaker and covered for controlled evaporation. After a period of 5 days, small crystals were obtained and these crystals were suspended in

5 65 the parent solution to get good quality crystals after a period of 25 days. The percentage of dopants added was 1 mole. In the case of urea doped crystals, 0.6 g of urea was taken and was dissolved in 100 ml of acetone. The resulting solution was stirred continuously for three hours. This solution was mixed with the parent solution and the mixture was stirred well for three hours. The resulting solution was filtered and allowed to evaporate. Seed crystals were obtained over a period of five days and left suspended in the saturated solution. Good quality crystals were obtained over a period of 25 days. The same procedure was followed for thiourea doped HA crystals. The amount of thiourea added as dopant was 0.7 g. The photographs of the as grown crystals of pure, urea doped and thiourea doped HA are shown in Figure 3.2 (a) to 3.2 (c) respectively. (a) (b) (c) Figure 3.2 Photograph of (a) pure HA (b) urea doped HA and (c) thiourea doped HA crystals

6 RESULTS AND DISCUSSIONS Single Crystal X-ray diffraction studies The grown crystals of pure and doped HA were subjected to single crystal XRD studies using ENRAF NONIUS CAD4-F single crystal X-ray diffractometer with MoK ( = Å) radiation. The structure was solved by the direct method and refined by the full matrix least-squares technique using the SHELXL program. The lattice parameters and the cell volume of the pure and doped HA crystals are presented in Table 3.1. It is observed from the X-ray diffraction data that both pure and doped HA crystals belong to orthorhombic system. The lattice parameters of pure HA are in good agreement with the reported values [124, 138]. Premanand et al., [128] observed that there is a slight change in the lattice parameters obtained from single crystal X-ray diffraction in the case of pure, benzophenone and iodine doped HA crystals. A similar effect is observed in the present work. In the case of doped HA crystals, slight variation in the values of lattice parameter and cell volume are observed. The variations in the lattice parameters are due to the incorporation of urea and thiourea molecules in to the crystal lattice. Table 3.1 Single crystal XRD data of pure and doped HA crystals Lattice parameters a (Å) b (Å) c (Å) ( ) ( ) ( ) Pure HA Urea doped HA Thiourea doped HA Crystal System Orthorhombic Orthorhombic Orthorhombic Space Group P P P Volume (Å 3 )

7 Powder X-ray diffraction studies X-ray powder diffraction (XRD) was performed on the grown crystals, to study the effect of doping on HA crystals with urea and thiourea. The powder XRD pattern was recorded using powder SEIFERT X-ray diffractometer with CuK 1 radiation ( = Å). The powdered samples were scanned over the range 10 o - 70 o at a rate of 1 o per minute. The XRD patterns of the pure, urea and thiourea doped HA were indexed with XRDA software. The position of the peaks in the doped crystals were found to be in agreement with that of pure HA crystals. However, there are some peaks in addition to that of the parent crystal, thereby confirming the incorporation of urea and thiourea in the grown crystal. It is found from the powder XRD pattern that the peaks at higher angle of diffraction in pure HA crystal are missing due to the effect of doping. The indexed XRD patterns of the pure and doped crystal are presented in Figure 3.3. Jayarama et al., [131] had performed the powder X-ray diffraction for the HA crystals. The cell parameters obtained by them from powder X-ray diffraction were in good agreement with the single crystal X-ray diffraction data. The powder XRD for pure NaCl and KCl doped HA crystals were analysed by Suresh Kumar et al., [71] and they suggested that the variations in lattice parameters, intensity of peaks and increase in cell volume were attributed to the incorporation of dopants in HA lattice. Similar results were observed in the present work.

8 (0 1 1) (2 0 2) (0 1 1) (1 1 2) (2 0 2) (3 4 0) (2 0 1) (0 2 1) (2 5 0) (0 1 1) (2 0 2) (0 5 1) (c) 100 (1 1 2) (2 0 1) Intensity (cps) (b) 100 (2 1 1) (0 1 2) (a) Two theta (degree) Figure 3.3 Powder XRD for (a) pure HA, (b) urea doped HA and (c) thiourea doped HA crystals CHNS analyses The chemical composition of the grown crystals were determined by using CHNS analysis for the pure and doped hippuric acid crystals. The CHNS analyses were performed by Elementar Model Vario EL III using helium as carrier gas. The percentage of carbon, hydrogen, nitrogen and sulphur is given in Table 3.2. Table 3.2 Percentage of CHNS in pure and doped hippuric acid crystals Crystal C(%) H(%) N(%) S(%) HA (calculated) HA (observed) Urea doped HA Thiourea doped HA

9 69 It is found that the amount of nitrogen and sulphur content varies in the case of urea and thiourea doped HA crystal. This observation clearly indicate the incorporation of the dopants in the lattice FTIR spectral analyses The FTIR spectral analyses were carried out to identify the chemical bonding and molecular structure of the material. The FTIR spectra were recorded by using Bruker IFS-66V spectrophotometer in the region cm -1 using KBr pellet technique. The FTIR spectra for the pure HA, urea doped HA and thiourea doped HA are presented in Figure 3.4 (a) 3.4 (c) respectively. The vibrations appearing in the spectra are in consonance with the literature. Generally, the NH asymmetric stretching vibrations appear at 3344 cm -1 [122, 123, 131] and as expected, this band appears at 3341 cm -1 with strong intensity. The broad and intense peak in the region of cm -1 is usually assigned to O-H stretching modes [131]. The strong absorption at 3073 cm -1 is attributed to OH stretching. The CH stretching of CH 2 is observed at 2938 cm -1. There are less intense resolved bands between cm -1 and these are attributed to hydrogen bonding interaction in the crystal lattice [139]. The peaks at 1491 cm -1 and 430 cm -1 are due to the asymmetric deformation and bending vibrations of the N-H group. The strong band at 1557 cm -1 is due to C-H stretching of aromatic ring. The absorptions at 1181, 1079 and 1029 cm -1 are assigned to in plane deformation of CH and CH 2 groups. The out of plane deformation vibrations of CH and CH 2 groups appear at 850, 723 and 659 cm -1. This confirms the ring structure in mono-substituted benzene. The intense absorption at 1745 cm -1 is assigned to C=O stretching. The C=O vibration of COOH group appears at 1600 cm -1 with strong intensity. The in plane deformation band of C-CO group appears at 547 cm -1. The band at 1416 cm -1 is attributed to CH 2 -CO deformations which support the presence of methyl group.

10 70 (a) (b) (c) Figure 3.4 FTIR spectra of (a) pure HA (b) urea doped HA and (c) thiourea doped HA crystals

11 71 It is found that the characteristic vibrational bands assignments of urea i.e., C=H stretching and NH 2 assymetric stretching coincides with the vibrational bands of HA. Similarly, the C=S, NH 2 and CN vibrations of urea also overlap with the vibrational bands of HA [ ].The detailed band assignments are tabulated in Table Optical studies A good optical transmittance is desirable for an NLO crystal. If there is an absorption at the fundamental or second harmonic of Nd : YAG laser, it leads to a loss of conversion efficiency of SHG. This has been a major problem in organic crystals. Organic crystals with very large NLO coefficients are mostly coloured and they allow considerable absorption in the visible / near UV region. The desired lower cutoff wavelength should be between nm, for generating blue light from diode lasers [16]. The recorded absorption spectra are shown in Figure 3.5. From the spectra, it is found that the cut off wavelength for the pure and doped crystals are around nm. The cutoff wavelength of pure HA is in good agreement with the values reported earlier [131]. Suresh Kumar et al., [71] have determined the cutoff wavelength for NaCl and KCl doped HA crystals in the range of 300 nm. The transmission levels are maximum in the wavelength range nm which are most desirable characteristic of a NLO material for applications. The formation of additional peaks in the spectra are attributed to overtones and combination bands. In the case of thiourea doped HA crystals, these overtones and combination bands give rise to modified absorption in the lower wavelength region which may lead to enhanced SHG efficiency. It is found that the absorption basically increases in the higher wavelength region.

12 72 Table 3.3 FTIR spectral assignments of HA crystal Pure HA ( cm -1 ) Urea doped HA ( cm -1 ) Thiourea doped HA ( cm -1 ) Vibrational Band Assignments NH asymmetric stretching OH stretching of aromatic ring CH stretching of CH NH symmetric stretching C=O stretching C=O stretching CH stretching of aromatic ring Asymmetric deformation / CN stretching CH 2 -CO deformation CH 2 -CO deformation CH 2 -CO deformation CH 2 -CO deformation In plane deformation of CH In plane deformation of CH In plane deformation of CH In plane deformation of CH In plane deformation of CH Out of plane deformation of CH Out of plane deformation of CH Out of plane deformation of CH Out of plane deformation of CH/ C=S vibrations Out of plane deformation of CH NH bending Out of plane deformation of CH/NH bending In plane deformation of C-CO NH bending

13 73 Wavelength (nm) Figure 3.5 UV-VIS-NIR Spectra of (a) pure (b) urea doped and (c) thiourea doped HA crystals

14 Second harmonic generation studies Fine powders of the pure and doped HA crystals were exposed under 1064 nm laser beam from a pulsed Nd:YAG laser having a repetition rate of 10 Hz and pulse width of 8 ns to test the second harmonic generation (SHG) efficiency. An input pulse of 5.8 mj / pulse was supplied. Signal amplitude in millivolts on the oscilloscope indicates the efficiency of the sample. The output for pure HA crystal was 32 mv, whereas the urea and thiourea doped HA crystals yielded an output of 35 mv and 52 mv respectively. The same procedure was adopted for the measurement of SHG for standard KDP crystal. For the same input signal, the output was 25 mv. This shows that pure HA crystals have NLO efficiency nearly 1.3 times greater than that of the standard KDP crystal. This is in consonance with the literature [129]. Premanand et al., [128] have reported that there is a small increase in SHG efficiency when HA was doped with benzophenone and iodine. Similar observations were made by Suresh Kumar et al., [71]. For urea and thiourea doped HA crystals, the SHG efficiencies were found to be 1.4 and 2.08 times that of the standard KDP crystal. The enhanced NLO activity in thiourea doped HA is due to the charge transfer of NH 2 group of thiourea molecules Thermal analyses The thermogravimetric analysis (TGA) deals with the change in the mass of a substance, continuously monitored as a function of temperature when it is heated. The differential scanning calorimetry (DSC) shows the variation of heat flow with temperature. The thermal stability of the grown crystals were studied by using Perkin Elmer Thermal Analysis Instrument and Netzsch Instrument. The TGA was carried out in nitrogen atmosphere at a heating rate of 20 C / minute in the temperature of 50 C to 800 C. The DSC was carried out in the temperature range between 25 C and 220 C. A sample mass of 5 mg was used for the analyses along with alumina as reference material.

15 75 The TGA curves for the pure and doped HA crystals are presented in Figure 3.6. From the thermograms, it is observed that all the three crystals show a single stage decomposition process. For pure HA crystal, the decomposition starts at 225 C and afterwards, a sharp decrease in weight loss is observed upto 290 C. The end residue is only 21 %. There is a slight variation in the decomposition temperature of doped crystals. This is an clear indication that the dopants have altered the thermal stability of HA crystals. The starting decomposition temperature for urea and thiourea doped HA are 240 C and 220 C respectively. The end residues for urea and thiourea doped HA are 11 % and 17 % respectively. The major weight loss is attributed to the expulsion of water molecule and atoms of carbon, hydrogen and oxygen from the chain, leaving behind nitrogen which decomposes at higher temperatures. The DSC curves for the pure and doped HA crystals are presented in Figure 3.7. Weight (%) (a) (c) (b) Temperature ( o C) Figure 3.6 TGA of (a) pure (b) urea doped and (c) thiourea doped HA crystal

16 76 Heat flow endo up (mw/mg) (c) (b) (a) Temperature ( o C) Figure 3.7 DSC of (a) pure (b) urea Doped and (c) thiourea Doped HA crystal It is found that there is a sharp endotherm corresponding at 195 C for pure HA, whereas for urea and thiourea doped HA crystals, the endotherms are shifted to 182 C and 183 C. These values are in good agreement with the melting point values reported by Ramachandran et al [122] Dielectric studies The measurement of dielectric constant as a function of frequency and temperature is of considerable interest. The dielectric measurements were carried out using an LCR meter (Agilant 4284A). The dielectric constants were determined at five different viz., 100 Hz, 1 khz, 10 khz, 100 khz and 1 MHz in the temperature range from 30 o C to 100 o C. Crystals with high transparency and surface defect free were selected and used. The extended portions of the crystals were removed completely and the opposite faces were polished and coated with quality graphite to get a good conductive surface layer. The dimensions of the crystals were measured using a travelling microscope. The readings were taken when the sample was cooled. The air capacitance was also measured.

17 77 The dielectric constant and A.C. Conductivity were calculated using the equations (2.8, 2.9). The values of dielectric constant at different temperatures were calculated for pure and doped crystals and are presented in Figure 3.8 (a) - (c) respectively. From this, it is found that the dielectric constant increases with increase in temperature. From Figure 3.8(a), it is observed that the increase in dielectric constant at lower frequencies is less when compared with that at higher frequencies. For urea and thiourea doped HA crystals, the same trend is observed. Such variations at higher temperature may be attributed to the blocking of charge carriers at the electrodes [140]. The variation of dielectric constant with frequency for pure and doped HA crystals are shown in Figure 3.9 (a) - (c) respectively. The dielectric constant decreases very rapidly at low frequencies and then slowly, as the frequency increases and finally, it becomes almost a constant at higher frequencies. The high value of dielectric constant at low frequencies may be associated with the establishment of polarizations namely; space charge, orientational, electronic and ionic polarization and its low value at higher frequencies are attributed to the loss of significance of these polarizations gradually. At high frequencies, normally orientation and space charge polarization exists. The dielectric loss was also studied as the function of frequency at various temperatures and are presented in Figure 3.10 (a) - (c) for pure HA, urea doped HA and thiourea doped HA respectively. From the Figure 3.10, it is noted that the dielectric loss is strongly dependent on the frequency of the applied field like dielectric constant. In the lower frequency region, dielectric loss is more due to the loss associated with ionic mobility [141]. The variation of A.C. Conductivity ( ac ) with temperature for pure HA, urea doped HA and thiourea doped HA were calculated using equation (2.9) are presented in Figure 3.11 (a) - (c) respectively. At higher temperature, the increased conductivity could be due to the reduction in the space charge polarization. The electrical conduction is mainly a defect controlled process in low temperature region.

18 78 Dielectric Constant Constant ( ( r) r ) 28 BA Hz 1 KHz 10 KHz 100 KHz 1 MHz (a) Temperature ( Oo C) Dielectric Constant ( ( r) r ) BA2 100 Hz 1 KHz 10 KHz 100 KHz 1 MHz (b) Temperature ( Oo C) Dielectric Constant ( r) Dielectric Constant ( r ) BA3 100 Hz 1 KHz 10 KHz 100 KHz 1 MHz (c) Temperature ( O ( C) o C) Figure 3.8 Variation of Dielectric constant with temperature of (a) pure HA (b) urea doped HA and (c) thiourea doped HA

19 79 Dielectric Constant ( r) Dielectric Constant ( r) Dielectric Constant ( r) Dielectric constant ( r ) Dielectric constant ( r ) Dielectric constant ( r ) BA1 368 K 358 K 348 K 338 K 328 K 318 K 308 K log f log f BA2 368 K 358 K 348 K 338 K 328 K 318 K 308 K log f log f (b) (c) BA3 (a) 368 K 358 K 348 K 338 K 328 K 318 K 308 K log f log f Figure 3.9 Variation of Dielectric constant with Frequency for (a) pure HA (b) urea doped HA and (c) thiourea doped HA

20 80 Dielectric loss Dielectric loss BA1 100 Hz 1 KHz 10 KHz 100 KHz 1 MHz (a) Temperature ( O ( o C) 0.6 Dielectric loss Dielectric loss BA2 100 Hz 1 KHz 10 KHz 100 KHz 1 MHz (b) Temperature ( oo C) C) 0.6 Dielectric loss Dielectric loss BA3 100 Hz 1 KHz 10 KHz 100 KHz 1 MHz (c) Temperature ( O ( o C) Figure 3.10 Variation of Dielectric loss with Temperature for (a) pure HA (b) urea doped HA and (c) thiourea doped HA

21 81 ln ac (ohm ln -1 ac cm -1 ) BA1 100 Hz 1 KHz 10 KHz 100 KHz 1 MHz (a) /T ln ac ln (ohm -1 ac cm -1 ) ln ac (ohm ln -1 ac cm -1 ) BA2 100 Hz 1 KHz 10 KHz 100 KHz 1 MHz BA3 100 Hz 1 KHz 10 KHz 100 KHz 1 MHz 1000/T (b) (c) /T Figure 3.11 Variation of A.C. Conductivity with 1000/T for (a) pure HA (b) urea doped HA and (c) thiourea doped HA

22 Etching studies Etching is a technique which is used to reveal the defects in crystals like dislocations, growth bands, twin boundaries, point defects etc. Normally when the crystal is dissolved in the solvent, well defined etch pits are formed. The formation of the etch pits is assumed to be the reverse of growth process. The etching studies were carried out on the grown crystals of pure and doped HA crystals using Carl Zeiss High resolution optical microscope. The surface of the crystals was polished very well before the etching process. Ethanol, methanol and acetone were used as etchants. The photographs were taken with a maximum etching time of 60 seconds, so that the deformation produced by the etchant is maximum. The etch patterns are presented in the Figure respectively. From Figure 3.12(a), it is found that pure HA crystals form no etch pits for t = 60 s, when acetone was used as a etchant. When ethanol was used as the etchant, striations and small etch pits are observed [Figure 3.12 (b)]. In the case of methanol, circular etch pits are observed [Figure 3.12 (c)]. The deformation produced in urea doped HA crystals by acetone as etchant is minimal, though the ends become glassy. When ethanol and methanol were used as etchants (Figure 3.13 and 3.14), the deformations produced are also minimal and only striations are produced. In the case of thiourea doped HA crystals, etch pits are formed to a maximum when ethanol was used as an etchant. These observations from the etching studies clearly indicate that the deformation produced in HA crystals are minimal under the action of different solvents like acetone, ethanol and methanol.

23 83 (a) (b) (c) Figure 3.12 Etch Patterns for pure HA crystals with (a) acetone (b) ethanol and (c) methanol as etchant (60 seconds)

24 84 (a) (b) (c) Figure 3.13 Etch Patterns for urea doped HA crystals with (a) acetone (b) ethanol and (c) methanol as etchant (60 seconds)

25 85 (a) (b) (c) Figure 3.14 Etch Patterns for thiourea doped HA crystals with methanol as etchant (60 seconds)

26 Hardness studies The mechanical characterization of the HA crystals were carried out by Vickers and Knoops hardness test at room temperature. Crystals with flat and smooth faces were chosen for the static indentation tests. The crystals were mounted properly on the base of the microscope. The crystals were indented gently at loads 10 g, 25 g and 50 g for a dwell period of 10 s using both Vickers diamond pyramid indenter and Knoop indenter attached to an incident ray research microscope. The length of the two diagonals was measured by a calibrated micrometer attached to the eyepiece of the microscope after unloading and the average of the diagonals ( d ) was calculated. The variations of Vickers Hardness number and Knoops Hardness number with applied load for pure and doped HA crystals are presented in Figure 3.15 and Figure 3.16 respectively. The Vickers hardness number (H v ) and Knoops hardness number were calculated using the equations (2.2 and 2.3) respectively. The maximum value of the Vickers hardness number is found to be 55 kg/mm 2 and that of Knoops hardness number is, 35 kg/mm 2. The Vickers hardness number increases with increase of applied load. Jayarama et al., [131] have reported the variation of Vickers hardness number with applied load and have compared with glycine. Suresh Kumar et al., [71] observed that the hardness number is enhanced due to the addition of dopants. In the present work, the hardness number decreases slightly due to the addition of urea and thiourea in HA crystals. The minimal decrease in the hardness number in the doped crystals can be attributed to the soft nature of urea and thiourea

27 UREA DOPED HA THIOUREA DOPED HA PURE HA H v (Kg/mm Hv (kg/mm 2 ) 2 ) Load P P (g(g)) Figure 3.15 Variation of Vickers Hardness Number with applied load for Pure and doped Hippuric Acid crystals UREA DOPED HA THIOUREA DOPED HA PURE HA 45 H Hk K ( (kg/mm Kg/mm 2 ) 2 ) Load P (g) Figure 3.16 Variation of Knoops Hardness Number with applied load for Pure and doped Hippuric Acid crystals

28 CONCLUSION Good quality single crystals of pure HA, urea and thiourea doped HA were grown by slow solvent evaporation technique. Its lattice parameters have been found from single crystal XRD analysis. The various planes were indexed using powder XRD. The variations in the composition due to the addition of dopants have been confirmed by CHNS analysis. TGA and DSC studies revealed the dopants changes the thermal stability of the crystals. The melting point of pure and doped HA crystals lies in the range of 180 o C o C. The optical behavior is assessed by UV-Visible studies and it indicates the crystals have transmission in the region nm. The FTIR studies reveals the presence of different functional groups in the crystals. The SHG studies indicate that the thiourea doped HA crystals and urea doped HA crystals have SHG efficiency 2.08 and 1.4 times greater than that of KDP, respectively. The dielectric studies reveal the low dielectric constant and low dielectric loss of the crystals at high frequency range, which is ideal for NLO materials. Vickers and Knoops hardness values were determined inorder to study the mechanical properties of the crystals. The growth pattern was analysed by etching studies.