Structural , mechanical , optical and second harmonic generation ( SHG ) studies of gamma glycine single crystal

Single crystals of γ-glycine were grown by submerged seed solution method, hanging seed solution growth method and solvent evaporation method. Powder X-ray diffraction (XRD) studies were carried out to confirm the crystalline gamma phase and to find out the lattice parameters of the grown crystals. The presence of various functional groups was determined by Fourier transform infrared (FTIR) spectroscopic analysis. Thermal analysis was carried out to study the thermal stability and phase transition of the grown crystals. The Vickers microhardness test was performed. UV-visible transmittance spectra showed wide transparency window in visible and near infrared (NIR) region. The second harmonic generation (SHG) relative efficiency of the grown crystals was measured by Kurtz and Perry powder technique using Nd: YAG laser.


INTRODUCTION
Organic nonlinear materials will be the key elements for future photonic technologies.It is known that glycine, the simplest non-essential amino acid, exhibits in three different polymorphic forms namely α-glycine, β-glycine, and γ-glycine.Among the three forms, γ-glycine exhibits strong piezoelectric and nonlinear optics (NLO) effect (Prasad and Williams, 1991).Recently, growth and characterization studies of γ-glycine crystals have been reported by many authors and it is observed that γglycine single crystals have been grown using several additives (Narayana et al., 2005;Balakrishnan et al., 2008;Dhanraj and Rajesh, 2009;Narayan and Dharmaprakash, 2002;Litaka, 1958;Khanna and Miller, 1970;Perlovich et al., 2001;Litaka, 1961;Sakai et al., 1992;Narayan and Dharmaprakash, 2002;Litaka, 1954;Narayan and Dharmaprakash, 2002;Meera et al., 2004;Esthaku and Ramasamy, 2010;Xia et al., 2008;Ashok et al., 2012;Anbuchezhiyan et al., 2010;Dillip et al., 2011;Sekar and Parimaladevi, 2009;Parimaladevi and Sekar, 2010;Anbuchudar and Ganesan, 2012).Polymorphism is important in the development of pharmaceutical ingredients.Many drugs receive regulatory approval for only a single crystal form or polymorph.Paracetamol powder has poor compression properties: this poses difficulty in making tablets, so a new polymorph of paracetamol is discovered which is more compressible.Due to differences in solubility of polymorphs, one polymorph may be more active therapeutically than another polymorph of same drug.Ostwald suggested that the first solid formed on crystallization of a solution or a melt would be the least stable polymorph.This can be explained on the basis of irreversible thermodynamics, structural relationships, or a combined consideration of statistical thermodynamics and structural variation with temperature.Ostwald's rule is not a universal law but is only a possible tendency in nature.Polymorphs have different stabilities and may spontaneously convert from a metastable form to the stable form at a particular temperature.They also exhibit different melting points, solubilities, X-ray crystal and diffraction patterns.Polymorphism in materials science is the ability of a solid material to exist in more than one form or crystal structure.Polymorphism can potentially be found in any crystalline material including polymers, minerals, and metals, and is related to allotropy, which refers to chemical elements.The complete morphology of a material is described by polymorphism and other variables such as crystal habit, amorphous fraction or crystallographic defects.Polymorphism is relevant to the fields of pharmaceuticals, agrochemicals, pigments, dyestuffs, foods, and explosives.When polymorphism exists as a result of difference in crystal packing, it is called packing polymorphism.Polymorphism can also result from the existence of different conformers of the same molecule in conformational polymorphism.
In pseudopolymorphism, the different crystal types are the result of hydration or solvation.An example of an organic polymorph is glycine which is able to form monoclinic and hexagonal crystals.Chemical compounds regularly display the ability to crystallize in to more than one structural form and these different forms are known as polymorphs.Polymorphs can exhibit different mechanical, thermal and physical properties such as compressibility, melting point, solubility and crystal habit, which can have a great influence on the bioavailability, filtration and tableting processes of pharmaceutical food and specialty materials.The control of crystal habit and growth rate has great importance in the field of chemical and pharmaceutical industries (Balakrishnan et al., 2008;Dhanraj and Rajesh, 2009;Narayan and Dharmaprakash, 2002;Litaka, 1958).At ambient conditions, neutral glycine exists in three polymorphic forms, with the order of their thermodynamic stability being γ > α > β.While α-and γ-glycine are enantiotropically related, βglycine is monotropically related to α-and γ-glycine.Although γ-glycine is the stable form at room temperature, α-glycine readily crystallizes from solutions, while γ-glycine can be crystallized acidic or basic solutions or in the presence of small amount of additives or gel method.The metastable β-glycine is known to crystallize from water-alcohol mixtures or from highly supersaturated solutions.The crystal structures of the α-and γ-forms of glycine are very different: the head to tail chains of glycine zwitterions + NH 3 -CH 2 -COO -in the αpolymorph form double centrosymmetric layers, whereas they are linked in the triple helices additionally connected in a three dimensional network in the polar chiral structure of the γ-form (Narayan and Dharmaprakash, 2002;Meera et al., 2004;Esthaku and Ramasamy, 2010;Xia et al., 2008;Ashok et al., 2012;Anbuchezhiyan et al., 2010;Dillip et al., 2011;Sekar and Parimaladevi, 2009;Parimaladevi and Sekar, 2010;Anbuchudar and Ganesan, 2012).In the present work, we report the crystallization of γ-glycine from aqueous solutions of glycine and ammonium sulfate as an additive by three methods for the first time.

Azhagan and Ganesan 7
EXPERIMENTAL Single crystals of γ-glycine were grown from glycine and ammonium sulfate taken in the equimolar ratio in aqueous solution by solution growth method, hanging seed solution growth and submerged seed solution growth method.Optically clear and well-shaped crystals were obtained and were used as seed crystals .In submerged seed solution method, the seed crystal was kept at the bottom of the vessel containing the saturated solution of glycine and ammonium sulfate.Bulk crystals were grown from the seeds by using a saturated solution of glycine and ammonium sulfate in a modified crystal growth apparatus, using submerged seed solution growth method.A crystal size of 15 mm × 12 mm × 4 mm 3 was obtained in a period of about 2 weeks as shown in Figure 1a.A crystal grown from solvent evaporation method is shown for comparison.It is evident that, γ-glycine single crystal grown by solvent evaporation method is very large compared to submerged seed solution method as shown in Figure 1b.Submerged seed solution method produced transparent γ-glycine crystal.In contrast, solution growth method also produced bulk transparent crystal but soon became opaque after a period of 1 week.In hanging seed solution growth method, a saturated solution of glycine and ammonium sulfate was prepared and the seed crystal was hung inside the solution that was optimally closed for controlled evaporation.A good quality bulk transparent crystal was obtained in a period of 4 weeks.From Figure 1c, it is clear that the end portions of the crystal are of good quality than the middle portion which is not transparent and has lot of multiple faces of defects.It is due to the internal stress applied by the thread which was used to hang the seed crystal.To avoid it, submerged solution method is to be adopted.

X-ray powder diffraction study
Powder X-ray diffraction (XRD) study was carried out by employing a Bruker D8 advance powder X-ray diffractometer using CuKα radiation of wavelength 1.5406 Ǻ.The powder sample was scanned over the range of 10 to 70° at the rate of 1° min -1 .The indexed powder XRD pattern of the grown crystal is given in Figure 2. The reflection peaks corresponding to different crystal planes in the recorded XRD spectrum were indexed and the digital data obtained from the spectrum such as angle 2θ, d-value, hkl, and relative intensity of every prominent peak in the spectrum are given in Table 1.From the XRD data, the lattice parameters of the grown γ-glycine crystal were determined and are given in Table 2 along with literature values.Appearance of sharp and strong peaks confirmed the good crystallinity of the grown crystals.The lattice parameters have been calculated using UNIT CELL software package.The observed lattice parameter values are in good agreement with the data available in JCPDS Card N0: 06-0230.

Fourier transform infrared (FTIR) spectral analysis
The FTIR spectrum was recorded using a Perkin Elmer grating Infrared spectrometer by KBr pellet technique.
The recorded spectrum of Pure (Raw material: Glycine) and γ-glycine single crystal are shown in Figures 3 and 4.
The absorption due to carboxylate group of pure glycine (Raw material) was observed at 501, 888 and 1591 cm -1 respectively.In γ-glycine single crystal, these peaks are shifted to 506, 890 and 1632 cm -1 , respectively.Similarly, the absorption peaks due to NH 2 + group of glycine (Raw material) are observed at 1041.2, 1124.8 and 1487.5 cm - 1 , respectively.These are shifted to 1037, 1121 and 1497 cm -1 , respectively.The observed vibrational frequencies and their tentative assignments are listed in Table 3, and are in close agreement with those in literature (Khanna and Miller, 1970;Silverstein et al., 1981).

Differential scanning calorimetry (DSC) analysis
The DSC thermogram of powdered sample of γ-glycine crystals were carried out by employing NETZSCH DSC 200F3 instrument between 30 and 600°C.The recorded DSC thermogram is shown in Figure 5.An alumina crucible was used for heating the sample and analyses were carried out in the atmosphere of nitrogen at a heating rate of 10°C min -1 in the temperature range of 30 to 600°C.The first endothermic peak in DSC curve at 178°C corresponds to the phase transition from γ-phase of glycine to α-phase of glycine.Perlovich et al. (2001) reported that the transition temperature between γ to α can range between 165 and 201°C.The melting point of the grown crystal is 246°C.Thus, in the present work, γglycine crystal grown from a mixture of glycine and ammonium sulfate is structurally stable up to 178°C.

Microhardness study
Vickers microhardness test were carried out on γ-glycine crystal using Ultra Microhardness Tester fitted with a diamond indentor.The indentations were made using a Vicker pyramidal indentor for various loads from 20 to 80 g.Vickers microhardness number (H v ) was calculated using the relation, Hv = 1.8544 (P/d 2 ) kg/mm 2 , where P is the indentor load (kg) and d is the diagonal length of the impression (mm).Figure 6 shows the variation of Vicker hardness values with load.The hardness value increases up to a load of 80 g cracks develop around the indentation mark above the load of 80 g.This may be due to internal stress generated locally by indentation.

UV-Visible-NIR spectrum
The UV-Vis-NIR transmission spectrum is shown in Figure 7.The lower cut-off wavelength of γ-glycine is well below 300 nm which is an intrinsic property of all amino acids.The absence of absorption in the region between and 1100 nm shows that these crystals are useful for the second harmonic generation (SHG) of Nd: YAG laser of wavelength λ = 1064 nm.The lower cut-off wavelength is found to be 191 nm.The band gap is estimated using the formula E g = 1240/λ (nm).It is found to be 6.49eV.The observed UV spectra and band gap value closely agree well with literature values (Dhanraj and Rajesh, 2009;Narayan and Dharmaprakash, 2002;Esthaku and Ramasamy, 2010;Parimaladevi and Sekar, 2010).

Powder SHG measurements
The SHG intensity of the grown crystal was tested using the Kurtz and Perry powder technique (Kurtz and Perry, 1968).The second harmonic signal generated in the crystalline sample was confirmed from the emission of green radiation (λ = 532 nm) from the crystal.A second harmonic signal of 0.650 mV/pulse was obtained, while the standard potassium dihydrogen phosphate (KDP) crystal gave a SHG signal of 0.452 mV/pulse for the same input energy.The SHG efficiency of the γ-glycine of this work is 1.43 times that of KDP.

Conclusion
Transparent crystals of γ-glycine was successfully grown using submerged seed solution method, solvent evaporation method and hanging seed solution growth method at room temperature and characterized by powder XRD.FTIR spectrum confirms the functional groups of γ-glycine.Thermal studies showed that the crystals are thermally stable up to 178°C.Microhardness study shows the mechanical behaviour of the grown crystal.Optical studies showed that the crystal is transparent to the fundamental and second harmonic of Nd-YAG laser.The SHG efficiency of crystal is about 1.43 times that of KDP which is suitable for photonic device applications.

Table 1 .
X-ray powder diffraction data of the γ-glycine single crystal.