CHAPTER 3 GROWTH AND CHARACTERIZATION OF NONLINEAR OPTICAL -GLYCINE SINGLE CRYSTAL FROM LITHIUM ACETATE AS SOLVENT

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1 56 CHAPTER 3 GROWTH AND CHARACTERIZATION OF NONLINEAR OPTICAL -GLYCINE SINGLE CRYSTAL FROM LITHIUM ACETATE AS SOLVENT 3.1 INTRODUCTION Extensive studies have been made on the synthesis and crystal growth of nonlinear optical materials over the past decade because of their potential applications in the field of telecommunications, optical signal processing and optical switching. Optical nonlinearity of the crystals with O H bond has been extensively studied (Xue et al 1996, Xu et al 2008). Crystallization of a molecule into a noncentrosymmetric structure can be achieved either by attaching to it suitable substituents which cause a steric hindrance, or by combining it with another molecule that has a favourable crystal structure (Jain et al 1981). Among the organic crystals for NLO applications, amino acids display special features of interest (Chemla et al 1987), such as molecular chirality which secures acentric crystallographic structure, wide transparency in the visible and UV range, zwitterionic nature of the molecule which favours the hardness of the crystal etc. In solid state, amino acid contains the donor and acceptor groups, which provide the ground state charge asymmetry of the molecule, required for second order nonlinearity (Rodrigues et al 2003, Pal et al 2005, Natarajan et al 2006). These enhanced properties of amino acids are due to the presence of a proton donor carboxylic acid (COOH)

2 57 group and the proton acceptor amino (NH 2 ) group. Due to this dipolar nature, amino acids have physical properties which make them ideal candidates for applications. Hydrogen bonds have also been used in the possible generation of noncentrosymmetric structures, which is a prerequisite for an effective NLO crystal (Frankenbach et al 1992). The amino acid family crystals have been subjected to extensive investigation by several researchers because of the excellent characteristics of these materials. Glycine (NH 2 CH 2 COOH: amino acetic acid) is the simplest amino acid and it can exist in zwitterionic form. Unlike other amino acids, it has no centre of chirality and is optically inactive. While glycine can exist as a neutral molecule in the gas phase, it exists as a zwitterion in solution and in the solid state. Certain chemical compounds show the ability to crystallize into more than one structural form with different physical properties, known as polymorphs. A series of works are available in the literature on the polymorphs of different compounds and their studies (Ivanova et al 2004, Kolev et al 2006). Ivanova et al (2006) reported solid-state linear-dichroic infrared spectroscopic and theoretical analysis of glycine-containing peptides and their hydrochlorides. Under different conditions, glycine crystallizes in three kinds of polymorphs with different relative stabilities,, and which exhibits different characteristics. The form consists of hydrogen bonded double layers of molecules, which are packed by van der Waals forces. The least stable form is monotropically related to the other two polymorphs. It is always obtained from a water alcohol mixed solvent and can transform rapidly to the form in the presence of water or upon heating (Ferrari et al 2003). Both and forms crystallize in centrosymmetric space group P2 1 /c ruling out the possibility of optical second harmonic generation (Albrecht et al 1939). But -glycine crystallizes in noncentrosymmetric space

3 58 group P3 1 making it a potential candidate for piezoelectric and NLO applications (Iitaka 1958). In the -glycine, the presence of carboxylic acid group donates its proton to the amino group to form the structure NH + 3 CH 2 COO. Thus in solid state, -glycine exists as a dipolar ion in which carboxyl group is present as a carboxylate ion and amino group is present as ammonium ion. Due to this dipolar nature, glycine has high melting point. In addition to this, the presence of chromophores namely amino group and carboxyl group which makes the -glycine crystal transparent in the UV vis region. Narayan Bhat et al (2002, 2002 a) carried out a detailed study on the growth of -glycine crystals from aqueous solutions of glycine with (i) sodium chloride, (ii) sodium hydroxide, (iii) sodium fluoride, (iv) sodium nitrate and (v) sodium acetate and analyzed the effect of these solvents on the various physical characteristics. The present chapter reports the growth and characterization studies of -glycine crystals grown from aqueous solution of glycine in the presence of lithium acetate for the first time, since some properties of the reported -glycine crystals are enhanced by lithium acetate in solvent. 3.2 SOLUBILITY, METASTABLE ZONE WIDTH AND CRYSTAL GROWTH OF -GLYCINE The starting materials glycine (Merck) and lithium acetate (Alfa Aesar) were taken in the equimolar ratio. The calculated amount of salts was dissolved in Millipore water of resistivity 18.2 M cm at room temperature. The solution stirred well for about 6 h using a magnetic stirrer to obtain a homogeneous mixture. Then the mixture was evaporated to dryness, by heating below an optimum temperature of 50 o C to prevent decomposition. The material was repurified from the aqueous solution by recrystallization processes.

4 59 The nucleation studies were carried out in a constant temperature bath with cooling facility of an accuracy of 0.01 o C. The solubility at 30 C was determined gravimetrically. The metastable zone width was measured by adopting the conventional polythermal method (Nyvlt et al 1970). The saturated solution (100 ml) at 30 C was prepared according to the presently determined solubility data. The nucleation temperature was measured with cooling rate of 4 C/h. Then experiments were repeated for different saturation temperatures (35, 40, 45 and 50 C) and the corresponding solubility and metastable zone widths were measured. The variation in solubility along with the metastable zone width for different temperatures is shown in Figure 3.1. Good quality single crystals of considerable size are essential for the realization of practical applications. To choose a suitable solvent for single crystal growth from a solution, the solubility provides valuable information. Figure 3.1 illustrates that title material exhibits good solubility and a positive solubility temperature gradient in water and solution growth technique can be followed for crystal growth. The metastable zone width of -glycine in aqueous solution as a function of temperature indicates that the zone width decreases with temperature. Concentration (g/100 CC) Saturation curve Nucleation curve Temperature ( o C) Figure 3.1 Solubility and metastability limit curves

5 60 Slow evaporation method was employed for growing the single crystals of -glycine. The recrystallized salt was used for the preparation of saturated solution at room temperature. The solution was filtered by filtration pump and Whatman filter paper of pore size 11 µm and kept undisturbed in a dust free environment. The nonhygroscopic single crystal of -glycine grown by slow evaporation method is shown in the Figure 3.2. Figure 3.2 Photograph of as grown crystal 3.3 X-RAY DIFFRACTION ANALYSIS The grown crystal was subjected to single crystal X-ray diffraction study using NONIUS CAD-4/MACH 3 diffractometer with MoK radiation in the wavelength Å to identify the system and to estimate the lattice parameter values. At room temperature, the cell parameters are a = b = (2) Å (7.037 Å), c = (2) Å (5.483 Å) and the volume of the unit cell is (1) Å 3 (235.1 Å 3 ). The determined lattice parameter values are in-line with the literature values (Iitaka 1958) given in the parentheses which confirms that the grown crystal is in the -phase and belongs to the hexagonal system. Further it

6 61 is evident that lithium acetate is not incorporated into the lattice of grown crystal, but its presence in the aqueous solution yielded -glycine. Powder X-ray diffraction study was carried out by employing SEIFERT, 2002 (DLX model) diffractometer with CuK = Å) radiation using a tube voltage and current of 40 kv and 30 ma respectively. The grown crystals were finely powdered and have been subjected to powder XRD analysis. The sample was scanned over the range o at the rate of 1 o /min. The indexed powder X-ray diffraction pattern of the grown crystal is given in Figure (110) 250 Intensity (cps) (100) (101) (200) (111) (201) (210) (211) (202) (301) (400) Diffraction angle 2 Figure 3.3 Powder X-ray diffraction spectrum of -glycine 3.4 HRXRD ANALYSIS Crystalline perfection has a strong influence on the efficiency of the physical properties of crystals (Bhagavannarayana et al 2005 a). A multicrystal X-ray diffractometer has been used to study the crystalline perfection of the single crystal. Before recording the diffraction curve, to

7 62 remove the non-crystallized solute atoms which 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 of water and acetone mixture in 1:2 ratio. Figure 3.4 shows the high-resolution diffraction curve (DC) recorded for a typical -glycine single crystal specimen using (110) diffracting planes in symmetrical Bragg geometry by employing the multicrystal X-ray diffractometer with MoK 1 radiation. The curve is reasonably sharp having Full Width at Half Maximum (FWHM) is 15 arc s as expected for a nearly perfect crystal from the plane wave dynamical theory of X-ray diffraction (Batterman et al 1964). Absence of additional peaks shows that the specimen crystal does not contain any internal structural grain boundaries and indicates that the crystalline perfection is very good. Diffracted X-ray intensity (c/s) glycine 15" Glancing angle (arc s) Figure 3.4 Diffraction curve recorded for -glycine single crystal for (110) diffracting planes with MoK 1 radiation

8 FTIR SPECTRAL ANALYSIS The FTIR spectrum of -glycine was recorded using a Perkin-Elmer FTIR spectrum RXI spectrometer by KBr pellet technique. The Figure 3.5 shows the FTIR spectrum recorded in the range cm -1 at room temperature. In the spectrum, the broad band centered around 3300 cm -1 is due to C H stretching. It overlaps with the characteristic broad band due to N H asymmetrical stretching observed at 3427 cm -1. The prominent band near 2172 cm -1 may be assigned to the combination of the asymmetrical NH 3 + bending vibration and the torsional oscillations of the NH 3 + group (Silverstein et al 1981). The NH + 3 stretching region shows broad band characteristics of hydrogen bonding (3917) (3771) Transmittance (%) (3427) (2603) (2172) (1629) (1489) (1395) (1326) (1124) (1040) (927) (887) (683) (607) (501) Wavenumber (cm -1 ) Figure 3.5 FTIR spectrum of -glycine The peaks observed at 683, 607 and 501 cm -1 indicates the presence of carboxylate group. The NH 3 + group is revealed by the peaks observed at 2603, 1489 and 1124 cm -1 (Nakamoto 1970). The single crystal structure of

9 64 -glycine (Iitaka 1961) shows that glycine molecule in crystalline state exists as a dipolar ion in which the carboxyl group is present as a carboxylate ion and the amino group is present as an ammonium ion. Thus the presence of above stated peaks confirms the crystal structure of -glycine. The observed vibrational frequencies and their tentative assignments are listed in Table 3.1. The absorption peaks characterizing various functional groups are in good agreement with those in literature (Khanna et al 1970). Table 3.1 FTIR spectral data of -glycine Wave number (cm -1 ) Tentative assignments 3427 N H asymmetric stretch NH COO asymmetric stretch NH COO symmetric stretch 1326 CH 2 twist 1124 NH + 3 rock 1040 CCN asymmetric stretch 927 CH 2 rock 887 CCN symmetric stretch 683 COO bend 607 COO wag 501 COO rock 3.6 THERMAL STUDIES Differential thermal analysis (DTA) and thermogravimetric analysis (TGA) of -glycine crystal were carried out simultaneously employing Perkin-Elmer Diamond TG/DTA. A platinum crucible was used for heating the sample and analyses were carried out in an atmosphere of

10 65 nitrogen at a heating rate of 10 o C/min in the temperature range o C. The TGA-DTA curves of -glycine are illustrated in Figure 3.6. The first endothermic peak in DTA at 182 o C corresponds to the phase transition from -phase to -phase. Iitaka (1961) reported that the transition point between and -glycine exists at 165 ± 5 o C. Narayan Bhat et al (2002 a) observed that the -glycine crystal grown from a mixture of glycine and sodium hydroxide changes to -phase at 172 o C. But Perlovich et al (2001) concluded from their studies that the transition temperature between these two polymorphs of glycine can range between 165 and 201 o C. Thus in the present work, -glycine crystal grown from a mixture of glycine and lithium acetate is structurally stable up to 182 o C. The weight loss occurring between 260 and 280 o C suggests that the sample melts and then decomposes rapidly. This is also indicated by the broad endothermic peak in the DTA curve of the sample at 268 o C TGA Weight (%) DTA Heat Flow (mw) Temperature ( o C) Figure 3.6 TGA-DTA curves of -glycine

11 DIELECTRIC STUDIES For dielectric measurements, a good quality -glycine crystal of 2 mm thickness was electroded on either side with graphite coating to make it behave like a parallel plate capacitor. The capacitance (C crys ) and dielectric loss (tan ) were measured using the conventional parallel plate capacitor method with the frequency range 20 Hz to 1 MHz using the Agilent 4284A LCR meter at various temperatures ranging from 313 to 373 K. The observations were made while cooling the sample. Figures 3.7 and 3.8 show the variation of dielectric constant and dielectric loss respectively with frequency at different temperatures. It shows that the dielectric constant and dielectric loss exhibits similar variation with frequency (Ponpandian et al 2002). It is observed that dielectric constant and dielectric loss decreases very rapidly at low frequencies, and then slowly as the frequency increases, and finally it becomes almost a constant at higher frequencies. It is also indicates that the value of dielectric constant and dielectric loss increases with increase in temperature. The high value of dielectric constant at low frequencies may be associated with the establishment of polarizations namely; space charge, dipolar, electronic and ionic polarization and its low value at higher frequencies may be due to the loss of significance of these polarizations gradually. Space charge polarization is generally active at lower frequencies and higher temperatures and indicates the perfection of crystals. The low value of dielectric loss at high frequency implies that the crystal possesses good optical quality with lesser defects and this parameter is of vital importance for NLO materials in their applications (Rao et al 1965).

12 K 333 K 353 k 373 K Dielectric constant log frequency Figure 3.7 Variation of dielectric constant with frequency of -glycine Dielectric loss K 333 K 353 K 373 K log frequency Figure 3.8 Variation of dielectric loss with frequency of -glycine

13 UV vis NIR SPECTRAL STUDIES In a crystalline material, the region of transparency to electromagnetic radiation defines the intrinsic loss mechanism and also theoretical transmittance achievable within the region (Kanagadurai et al 2008). The optical transmission spectrum of -glycine crystal was recorded in the range 200 to 1100 nm using Lambda 35 spectrophotometer. The UV vis NIR spectrum recorded with the optically transparent single crystal of -glycine of thickness 2 mm is shown in the Figure 3.9. As the UV cutoff wavelength of -glycine is at 240 nm, makes it suitable for SHG laser radiation of 1064 nm or other applications. It is seen from the spectrum that the crystal is transparent in the entire range without any absorption peak, which is an essential parameter for NLO crystals. The absence of absorption of light in the visible range of the electromagnetic spectrum is an intrinsic property of all the amino acids. 50 Transmission (%) Wavelength (nm) Figure 3.9 UV vis NIR spectrum of -glycine

14 NLO PROPERTY Kurtz and Perry SHG test was performed to estimate the NLO property of -glycine crystal. A Q-switched Nd: YAG laser (DCR11) was used as light source. A laser beam of fundamental wavelength 1064 nm, 8 ns pulse width, with 10 Hz pulse rate was made to fall normally on the sample cell. The powdered KDP crystal was used as reference material in the SHG measurement. The input laser energy incident on the powdered sample was 1.35 mj/pulse. The -glycine second harmonic signal of 186 mv was obtained, while the KDP gave an SHG signal of 55 mv for the same input beam energy. Thus SHG relative efficiency of -glycine crystal was found to be 3.4 times higher than that of KDP. The SHG efficiency of -glycine grown in this work is compared in Table 3.2 which shows that it is higher than the reported values. Table 3.2 Comparison of SHG efficiencies of -glycine crystals grown from different solvents Solvent selected for growing -glycine SHG efficiency with respect to KDP Reference Water and NaCl 2 Narayan Bhat et al 2002 Water and NaF 1.3 Narayan Bhat et al 2002 a Water and NaOH 1.4 Narayan Bhat et al 2002 a Water and NaNO Narayan Bhat et al 2002 a Water and NaCH 2 COOH 1.2 Narayan Bhat et al 2002 a Water and NH 4 NO Narayana Moolya et al 2005 Water and NaCl/KCl 1.5 Ambujam et al 2006 Water and LiBr 3 Balakrishnan et al 2008 Water and LiCH 2 COOH 3.4 Present work The main contributions to the higher nonlinear optical properties of -glycine are the presence of the hydrogen bond and the vibrational part due

15 70 to very intense infrared bands of the hydrogen bond vibrations (Narayana Moolya et al 2005). The high polarizing power of lithium ion plays an important role in the enhancement of SHG efficiency of the -glycine crystal grown in the present study. This is confirmed by the nearer SHG efficiency of the -glycine crystal grown with LiBr as solvent (Balakrishnan et al 2008) and comparatively smaller SHG efficiencies of -glycine crystals grown from the poor polarized sodium salts (Narayan Bhat et al 2002, Narayan Bhat et al 2002 a, Ambujam et al 2006) MECHANICAL HARDNESS STUDIES Mechanical strength of the materials plays a key role in the device fabrication. The microhardness measurements were done by using Shimadzu HMV-2 Vickers indentation tester at room temperature and the indentation time was kept at 10 s. The microhardness value was taken as the average of the several impressions made. The variations of Vickers microhardness number (H V ) with load on the -glycine crystal are shown in Figure Hardness Number H V (kg/mm 2 ) Load (g) Figure 3.10 Microhardness number versus load for -glycine

16 71 The microhardness measurements at different loads show that as grown crystal exhibits the reverse indentation size effect in which the hardness value increases with the increasing load. At low loads, the indenter pierces only top surface layers generating dislocations results in the increase of hardness (Pandya et al 1983, Arora et al 2007). When load increases, the indenter penetrates through surface and inner layer. So mutual interaction between dislocations is created and they restrict the motion of dislocations and high resistance is offered by crystal lattice to the motion of dislocations. Samples exhibit the formation of cracks above 100 g due to the release of internal stress generated locally by indentation CONCLUSIONS Single crystals of -glycine were grown from aqueous solution of glycine and lithium acetate by slow evaporation method. The solubility and metastable zone width were measured from aqueous solutions. The crystal lattice parameters were calculated by single crystal X-ray diffraction studies and they agree well with the reported values. HRXRD analysis substantiates that crystalline perfection of the grown -glycine crystal is very good. The various functional groups were confirmed through FTIR spectral analysis. Microhardness and thermal analysis were performed to study the mechanical and thermal behavior of the as grown crystals. The dielectric study revealed the low dielectric constant and low dielectric loss of the crystal in the high frequency region. The grown crystal has good transmission window in the UV vis NIR region between 240 and 1100 nm, suitable for NLO applications. The SHG test confirms the second harmonic conversion efficiency of the crystal and it is found to be 3.4 times that of KDP.