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Effect of KCl addition on crystal growth and spectral properties of glycine single crystals
Spectrochimica Acta Part A 74 (2009) 1160–1164
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
Effect of KCl addition on crystal growth and spectral properties of glycine single crystals C. Sekar ∗ , R. Parimaladevi Department of Physics, Periyar University, Salem 636011, Tamilnadu, India
a r t i c l e
i n f o
Article history: Received 10 February 2009 Received in revised form 9 August 2009 Accepted 12 September 2009 Keywords: Non-linear optical materials Growth from solution FTIR spectrum Thermal properties of crystal
a b s t r a c t Glycine usually crystallizes as the metastable ␣-polymorph from pure aqueous solution. The polymorph, ␥-form of glycine can be crystallized only in presence of additive. In the present work, ␥-glycine has been crystallized by using potassium chloride (KCl) as additive at ambient temperature by solvent evaporation method. The form of crystallization is confirmed by X-ray powder diffraction method. Spectroscopic and thermal studies have been carried out for analyzing the presence of functional groups, thermal stability and decomposition of the sample. The results indicate that the KCl is doped into the ␥-glycine. The optical transparency of the ␥-glycine in the ultraviolet–visible region has been studied by recording the optical transmission spectrum. Second harmonic generation (SHG) conversion efficiency has been estimated as 120 mV and the output power by the crystal is less than that of potassium dihydrogen phosphate crystal. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Great efforts have been made to the research and design of highly efficient non-linear optical (NLO) materials due to widespread applications such as high speed information processing, optical communication and optical data storage [1]. Optical non-linearity is highly dependent on the geometrical arrangement of the molecules and it favors organic crystals more to exhibit large non-resonant optical linearities and it also become more attractive because of its large electro-optic co-efficient with low frequency dispersion and low dielectric constant. Materials based on amino acids are widely utilized because they not only contain chiral carbon atoms directing the crystallization in non-centrosymmetric space group, but also possess zwitterions nature favoring crystal hardness [2]. Glycine, NH2 CH2 COOH, is the simplest of the 20 protein aminoacids which function as a neurotransmitter and one of the principle components of structural proteins, enzymes and hormones. It is the only protein forming aminoacid without a center of chirality. Glycine crystallizes in three polymorphic forms, viz., ␣,  and ␥. The polymorphs have similar structure but differ in the way molecules are packed (the space group P21 /n, P21 and P31 for ␣-, - and ␥-glycine, respectively) [3]. The carboxylic acid group present in the ␥-glycine donates its proton to the amino group to form the structure (NH3 + CH2 COO− ). Thus in the solid state ␥-glycine exist as a dipolar ion in which carboxyl group is
∗ Corresponding author. E-mail address: [email protected] (C. Sekar). 1386-1425/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.saa.2009.09.026
present as carboxylate ion and amino group present as ammonium group. The dipolar nature exhibits peculiar physical and chemical properties in glycine, thus making it as ideal candidate for NLO, piezoelectric and pyroelectric applications. Another advantage of glycine is the presence of chromopores namely amino group and carboxyl group which makes it as transparent in the ultraviolet–visible region [4]. The metastable ␣-glycine grown from aqueous solution transforms into ␥-form spontaneously. The least stable -form is always obtained from water–alcohol mixed solvent. The -form transforms rapidly to ␣- and ␥-forms in presence of moisture at room temperature. The more stable ␥-glycine crystals are grown from aqueous solution or gel in the presence of additives. The ␥-form transforms to ␣-form on heating around 170 ◦ C [5]. Srinivasan and Arumugam [6] reported that the incorporation of NaCl in glycine solution have changed the morphology of the ␣-glycine to ␥-glycine. Narayan Bhat and Dharmaprakash [7] reported that the morphology of the glycine crystals grown from various solvents such as sodium hydroxide, sodium fluoride, sodium nitrate and sodium acetate showed a marked difference in transition temperature and second harmonic conversion (SHG) efficiency. The SHG efficiency of ␥-glycine grown in presence of NaF, NaOH, NaNO3 and NaCH2 COOH were reported to be 1.3, 1.4, 1.6 and 1.2 times higher than that of potassium dihydrogen phosphate (KDP) crystal. The same authors have reported that the ␥-crystal grown from a mixture of glycine and NaOH undergoes a transition from ␥- to ␣-form at 172 ◦ C. In the present work, we have grown ␥-glycine single crystals by incorporating various amounts of potassium chloride in the solution and studied the changes in the growth, spectral, thermal and non-linear properties of glycine crystal.
C. Sekar, R. Parimaladevi / Spectrochimica Acta Part A 74 (2009) 1160–1164
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were recorded on a PerkinElmer (Spectrum RX1) spectrometer and PerkinElmer Lambda 25 spectrometer in transmission mode. Powder X-ray diffraction studies were performed to confirm phase purity and to estimate the cell parameters. The XRD patterns were recorded on Bruker diffractometer within the 2 range of 10–80◦ using Cu K␣ as X-ray source ( = 1.5406 Å). Thermogravimetry of samples was performed using TA instruments NETZSCHSTA409. Second harmonic generation efficiency of the samples was determined using Kurtz powder method. A Q switched Nd:YAG laser beam of wavelength 1064 nm was used with an input energy of 6.2 MJ/pulse and pulse width of 8 ns, the repetition rate being 10 Hz. The SHG radiations of 532 nm (green light) emitted were collected by a photomultiplier tube (PMT – Philips photonics model 8563) and the optical signal incident on the PMT was converted into voltage output at the CRO (Tektronix – TDS 3052B). 3. Results and discussion Fig. 1. Solubility curves of glycine (a) without and with incorporation of KCl, (b) 4 g, (c) 12 g and (d) 18 g.
2. Experimental 2.1. Solubility studies The growth rate of a crystal depends on its solubility and growth temperature. Solubility of a material governs the amount of material, which is available for the growth and hence defines the total size limit. ␣-Glycine was crystallized by taking the analar grade 1 M glycine (99%). In another series ␥-glycine was synthesized by taking 1 M glycine with various concentration of KCl (1–18 g in 100 ml of water). The solubility of glycine has been determined at various temperatures in double distilled water. For this, a saturated solution was prepared in a well-controlled thermal environment with excess of solute. The solubility was then measured gravimetrically (Fig. 1). There is significant increase in the solubility of glycine due to the KCl addition. In aqueous solution, glycine molecules exist in the form of zwitterions with double charges. When an electrolyte is present, complexes could be formed through specific interactions among the zwitterions of glycine, the electrolytes and the water molecules. The formation of these complex interactions would contribute to the concentration change of glycine [8]. Thus, the addition of KCl leads to the increase of glycine solubility in double distilled water. However, the increase in KCl concentration does not affect the solubility much. The positive slope of the solubility curve enables growth by solvent evaporation method.
3.1. Crystal growth Fig. 2 shows the two polymorphs of glycine crystal which differ in crystal habits. As seen in the picture, ␣-form shows prismatic shape while the ␥-form crystal exhibit as bipyramids. ␣-Form of crystal has been grown with dimension of 3 mm × 10 mm × 2 mm, while ␥-form of crystal have approximate dimension of 10 mm × 10 mm × 3 mm. ␣-Glycine have long growth along the ‘c’ direction than ‘a’. The additive, KCl selectively inhibits growth of crystal faces with favorable adsorption site and effectively changes the morphology. Further the additive is prefentially adsorbed on (0 1 1) faces of ␣-glycine crystal and subsequently
2.2. Crystal growth Initially, ␣-glycine solution was prepared by mixing synthesized glycine salt in double distilled water. This solution was stirred using a magnetic stirrer until the salt dissolves completely. The impurity content in the glycine was minimized by successive recrystallization process. The solution was then filtered using Whatmann filter paper of 100 mm porosity. The pH value of the final solution was 4. The final solution was taken in the beaker and it is covered with perforated sheets and kept in dust free environment. After 2–3 days of solvent evaporation, the solution becomes supersaturated and tiny crystal was found. It was allowed to grow for maximum possible dimension and then harvested after 20 days. In another series, ␥glycine was prepared by mixing it in double distilled water. The pH value increases with increase in KCl concentration, i.e., 5.86, 6.15 and 6.50 for 4, 12 and 18 g, respectively. ␥-Glycine crystals were grown by slow evaporation method as described above and the crystals were harvested after about 15 days. The as-grown crystals were subjected to the following characterization studies. The FTIR spectrum and UV–visible spectrum
Fig. 2. As-grown (a) ␣-glycine crystal and (b) ␥-glycine crystals grown from aqueous solution of glycine containing KCl (12 g).
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Fig. 3. Simulated powder XRD pattern of ␥-glycine.
inhibits its growth along c-axis. It changes the actual morphology to the extent that it enhances the growth along the ‘a’ direction and reduces the growth along ‘c’. The nucleation of ␥-glycine in presence of KCl is probably due to a modification of molecular arrangements resulting from the columbic interactions between KCl and glycine molecules. The crystallization of charged glycine molecules enhanced by KCl can result in direct nucleation of ␥form [8]. The crystals grown in presence of lower concentration of KCl (4 g) was ␣-glycine. But it tends to change their transparency after few weeks of time even at ambient conditions and become opaque and appear white in color. This change indicates the possibility of phase transformation from ␣-form to the ␥-form and was confirmed by XRD (Fig. 3). Srinivasan [9] reported that when the NaCl concentration in the solution exceeds certain level (>12 g) the solution yields only crystals with much destructed morphologies with both ␣- and ␥-glycine. Contrary to this, KCl addition of upto 18 g in the solution yielded good quality ␥-glycine crystals with defined morphology. 3.2. X-ray diffraction analysis The crystal structure of glycine was reported by Dawson et al. [10]. The ␣- and ␥-glycine crystallizes in monoclinic and hexagonal structure with space groups of P21 /n and P31 , respectively. The lattice parameters of ␣-glycine are reported as a = 5.11 Å, b = 11.98 Å, c = 5.47 Å and ˇ = 111.78◦ . XRD pattern of ␥-glycine crystals grown with various concentration of KCl are shown in Fig. 3. The results agree well with simulated XRD pattern (Fig. 4) of ␥-glycine based on the data of Dawson et al. [10]. Appearance of sharp and strong peaks confirms the good crystallinity of the grown samples. The characteristic peak at 25.3◦ (2) corresponds to ␥-glycine. The prominent faces of ␥-glycine crystals are (1 1 0), (1 0 1), (1 1 1), and (2 0 1). It is clear that the powder XRD pattern of all the crystals grown from different concentration remained the same except for a small change in the intensity level of the peaks and peak position at the higher angle side. The calculated lattice parameters of ␥-glycine grown with incorporation of various concentration of KCl are shown in Table 1. There is slight increase in lattice constants with increasing KCl concentration which may be due to the doping of KCl into ␥-glycine crystals.
Fig. 4. Powder XRD patterns of ␥-glycine crystals grown from aqueous solution of glycine with (a) 4g, (b) 12g and (c) 18g of KCl.
Table 1 Lattice parameters of ␥-glycine crystals grown with various concentration of KCl. KCl concentration used for growing ␥-glycine
a = b (Å)
c (Å)
4 g in 100 ml water 12 g in 100 ml water 18 g in 100 ml water
6.955 6.998 7.019
5.427 5.475 6.399
[11]. The observed frequencies and their assignment of the glycine crystals are given in Table 2. The peak around 2899 and 2612 cm−1 corresponds to CH2 stretching. The broad band in the higher energy region around 2500–2200 cm−1 is due to NH3 + stretching vibration. Two overlapped bands at around 1612 and 1404 cm−1 could be attributed to the asymmetric and symmetric stretch modes at the COO− group. The peaks around 1328, 1033, 690 and 502 cm−1 have been attributed to CH2 wagging, CN+ stretching, COO− bending and COO− rocking, respectively. The presence of carboxylate and ammonium ion clearly indicates that the glycine molecule exists in zwitter ionic form in ␥-glycine crystal. XRD results suggest that the KCl acts not only as additive, which transforms ␣- to ␥-form of glycine but also gets doped at higher concentration. The KCl concentration was not quantitatively measured. However, it was
3.3. FTIR analysis The recorded FTIR spectra of powdered glycine crystals grown with and without KCl are depicted in Fig. 5. FTIR spectrum in the mid-region for the glycine samples agree well with reported result
Fig. 5. FTIR spectra of (a) ␣-glycine and ␥-glycine crystals grown from aqueous solution of glycine with (b) 4 g, (c) 12 g and (d) 18 g of KCl.
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Table 2 FTIR analyses of ␥-glycine crystals grown in presence of various concentration of KCl. ␣-Glycine (cm−1 )
KCl concentration used for growing ␥-glycine −1
4 g in 100 ml water (cm 2899, 2612 2270, 2135 1502 1612, 1404 1328 1119 1033 892 690 607 502
)
2922, 2617 2268, 2121 1511 1622, 1406 1329 1113 1030 894 695 608 503
12 g in 100 ml water (cm 2930, 2620 2210, 2119 1508 1630, 1405 1330 1114 1030 894 696 608 506
indirectly established by FTIR spectra. The singlet peak at 890 cm−1 was changed into doublet at 910 cm−1 in the crystals grown with higher concentration (18 g) of KCl. The peak at 607 cm−1 which corresponds to COO− wagging got suppressed with increase in the concentration of KCl. In addition, the broad nature of spectra in the spectral region between 2000 and 3500 cm−1 got widened with increase in KCl concentration which may be due to the co-existence of KCl in the crystal lattice. 3.4. Thermal analysis Figs. 6 and 7 illustrate the TG–DTA curve of ␣- and ␥-glycine crystals recorded in the temperature range of 30–1100 ◦ C at the rate of 20 ◦ C/min in nitrogen atmosphere. The TGA curves show no change in weight before 230 ◦ C for both forms, which eliminate the possibility of hydrate or solvate formation of crystals. The major weight loss around 230–300 ◦ C could be due to the sublimation and decomposition of samples which can be confirmed by the sharp endothermic peak in DTA. The weight loss at first stage may due to the release of NH3 and CO molecules. Appearance of a peak at temperature 170 ◦ C in DTA curve of ␥-glycine crystal corresponds to the phase transformation of ␥- to ␣-glycine [12]. This phase transition is not seen in the DTA curve of glycine sample grown in absence of additive which reveals it is ␣-glycine. Close observation of TG results show that there is a significant difference with weight loss (24 wt%) of ␥-glycine when compared to that of ␣glycine (21 wt%) in the higher temperature range (550–750 ◦ C). The excess weight loss may be due to the release of KCl from ␥-glycine crystal. The DTA curve of ␥-glycine shows two additional exothermic peaks at 609 and 732 ◦ C which could be due to the presence of KCl which in ␥-glycine samples. These peaks were absent in the
Fig. 6. TG–DTA curve of ␣-glycine crystal.
Tentative assignment −1
)
−1
18 g in 100 ml water (cm
)
2892, 2605 2228, 2173 1498 1635, 1394 1325 1125 1042 910 682 608 502
CH2 stretching NH3 + stretching COO− stretching CH2 wagging NH2 rocking C–N stretching C–C stretching COO− bending COO− wagging COO− rocking
DTA curve of ␣-glycine. The DTA curve reveals that no endothermic/exothermic reaction occurs below 170 ◦ C in glycine crystals. This ensures the suitability of the material for possible application in laser, where the crystal is required to withstand high temperature. 3.5. Optical transmission spectral analysis The UV–visible spectrum gives information about the structure of the molecule that the absorption of UV and visible light involves in the promotion of electrons in and orbital from the ground state to higher energy state. The UV transmission spectrum of ␥-glycine is shown in Fig. 8. Since these crystals are mainly used in optical application, the determination of UV transmission range and the transparency cutoff are important. It can be seen from the transmission curve that the lower cutoff wavelength lays nearly 325 nm for ␥-glycine crystals. The absence of absorption in the visible region clearly indicates that the glycine crystals can be used as window material in optical instruments. The peak at 327 nm is due to the n–* transition [13]. High transmittance % is observed from 400 nm which clearly indicates that the crystal possess good optical transparency for SHG of Nd:YAG laser. 3.6. Powder SHG measurement In order to confirm the NLO property, the grown crystals were powdered and subjected to Kurtz and Perry powder technique, which is a powerful tool for initial screening of materials for SHG [14]. The second harmonic signal generated in the crys-
Fig. 7. TG–DTA curve of ␥-glycine crystal grown from aqueous solution of glycine containing KCl (12 g).
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for application in laser. In FTIR spectra the presence of carboxylate and ammonium ion clearly indicates that the glycine molecule exists in zwitter ionic form in ␥-glycine crystal. The grown glycine crystals have higher percentage of transmission in the UV region. The measured amplitude of second harmonic green light for glycine crystal grown with KCl is 120 mV against 360 mV for KDP crystal. The decrease in SHG value has been attributed to the possible KCl doping in the glycine crystals. The co-existence of KCl in the crystal lattice of ␥-glycine was also confirmed by FTIR, TGA and XRD analyses. Acknowledgements
Fig. 8. UV spectra of ␥-glycine crystal grown from aqueous solution of glycine containing KCl (12 g).
One of the authors (R.P.) thank Periyar University for providing University Research Fellowship (URF) and the authors thank Prof. V. Krishnakumar of Periyar University for useful discussions. The authors thank E. Ramya of Sacred Heart’s College, Tirupattur for her immense help in experimental works. References
talline sample was confirmed from the emission of green radiation ( = 534 nm) from the crystal. The measured amplitude of second harmonic green light for ␥-glycine is 120 mV against 360 mV for KDP crystal. This value is relatively low when compared to the SHG values reported for ␥-glycine crystals grown with other additives. This poor lasing performance may due to the possible KCl doping into ␥-glycine as suggested by XRD and TGA results. 4. Conclusion We have grown ␥-glycine crystal in the presence of KCl at various concentrations by slow evaporation method. The XRD studies confirm the good crystalline nature and the phase formation of ␥glycine crystals. TG–DTA studies reveal that the ␥-glycine crystals are stable upto 170 ◦ C which suggests the suitability of this material
[1] J. Hernandez-Paredes, D. Glossman-Mitnik, H.E. Esparza-Ponce, M.E. AlvarezRamos, A. Duarte-Moller, J. Mol. Struct. 875 (2007) 295. [2] R. Ramesh Babu, N. Vijayan, R. Gopalakrishnan, P. Ramasamy, Cryst. Res. Technol. 41 (2006) 405. [3] P. Langen, S.A. Mason, D. Myles, B.P. Schoenborn, Acta Cryst. B 58 (2002) 728. [4] K. Ambujam, S. Selvakumar, D. Prem Anand, G. Mohamed, P. Sagayaraj, Cryst. Res. Technol. 41 (2006) 671. [5] X. Sun, B.A. Garetz, A.S. Myerson, Cryst. Growth Des. 6 (2006) 684. [6] K. Srinivasan, J. Arumugam, Opt. Mater. 30 (2007) 40. [7] M. Narayan Bhat, S.M. Dharmaprakash, J. Cryst. Growth 242 (2002) 245. [8] X. Yang, J. Lu, X.-J. Wang, C.-B. Ching, J. Cryst. Growth 310 (2008) 604. [9] K. Srinivasan, J. Cryst. Growth 311 (2008) 156. [10] A. Dawson, D.R. Allan, S.A. Belmonte, S.J. Clark, W.I.F. David, P.A. McGregor, S. Parsons, C.R. Pulham, L. Sawyer, Cryst. Growth Des. 5 (2005) 1415. [11] R.K. Khanna, M. Horak, E.R. Lippincott, Spectrochim. Acta 22 (1966) 1759. [12] E. Ramachandran, K. Baskaran, S. Natarajan, Cryst. Res. Technol. 42 (2007) 73. [13] W. Kemp, Organic Spectroscopy, Palgrave Macmillan, 1991. [14] S.K. Kurtz, T.T. Perry, J. Appl. Phys. 39 (1968) 3798.
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
Effect of KCl addition on crystal growth and spectral properties of glycine single crystals C. Sekar ∗ , R. Parimaladevi Department of Physics, Periyar University, Salem 636011, Tamilnadu, India
a r t i c l e
i n f o
Article history: Received 10 February 2009 Received in revised form 9 August 2009 Accepted 12 September 2009 Keywords: Non-linear optical materials Growth from solution FTIR spectrum Thermal properties of crystal
a b s t r a c t Glycine usually crystallizes as the metastable ␣-polymorph from pure aqueous solution. The polymorph, ␥-form of glycine can be crystallized only in presence of additive. In the present work, ␥-glycine has been crystallized by using potassium chloride (KCl) as additive at ambient temperature by solvent evaporation method. The form of crystallization is confirmed by X-ray powder diffraction method. Spectroscopic and thermal studies have been carried out for analyzing the presence of functional groups, thermal stability and decomposition of the sample. The results indicate that the KCl is doped into the ␥-glycine. The optical transparency of the ␥-glycine in the ultraviolet–visible region has been studied by recording the optical transmission spectrum. Second harmonic generation (SHG) conversion efficiency has been estimated as 120 mV and the output power by the crystal is less than that of potassium dihydrogen phosphate crystal. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Great efforts have been made to the research and design of highly efficient non-linear optical (NLO) materials due to widespread applications such as high speed information processing, optical communication and optical data storage [1]. Optical non-linearity is highly dependent on the geometrical arrangement of the molecules and it favors organic crystals more to exhibit large non-resonant optical linearities and it also become more attractive because of its large electro-optic co-efficient with low frequency dispersion and low dielectric constant. Materials based on amino acids are widely utilized because they not only contain chiral carbon atoms directing the crystallization in non-centrosymmetric space group, but also possess zwitterions nature favoring crystal hardness [2]. Glycine, NH2 CH2 COOH, is the simplest of the 20 protein aminoacids which function as a neurotransmitter and one of the principle components of structural proteins, enzymes and hormones. It is the only protein forming aminoacid without a center of chirality. Glycine crystallizes in three polymorphic forms, viz., ␣,  and ␥. The polymorphs have similar structure but differ in the way molecules are packed (the space group P21 /n, P21 and P31 for ␣-, - and ␥-glycine, respectively) [3]. The carboxylic acid group present in the ␥-glycine donates its proton to the amino group to form the structure (NH3 + CH2 COO− ). Thus in the solid state ␥-glycine exist as a dipolar ion in which carboxyl group is
∗ Corresponding author. E-mail address: [email protected] (C. Sekar). 1386-1425/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.saa.2009.09.026
present as carboxylate ion and amino group present as ammonium group. The dipolar nature exhibits peculiar physical and chemical properties in glycine, thus making it as ideal candidate for NLO, piezoelectric and pyroelectric applications. Another advantage of glycine is the presence of chromopores namely amino group and carboxyl group which makes it as transparent in the ultraviolet–visible region [4]. The metastable ␣-glycine grown from aqueous solution transforms into ␥-form spontaneously. The least stable -form is always obtained from water–alcohol mixed solvent. The -form transforms rapidly to ␣- and ␥-forms in presence of moisture at room temperature. The more stable ␥-glycine crystals are grown from aqueous solution or gel in the presence of additives. The ␥-form transforms to ␣-form on heating around 170 ◦ C [5]. Srinivasan and Arumugam [6] reported that the incorporation of NaCl in glycine solution have changed the morphology of the ␣-glycine to ␥-glycine. Narayan Bhat and Dharmaprakash [7] reported that the morphology of the glycine crystals grown from various solvents such as sodium hydroxide, sodium fluoride, sodium nitrate and sodium acetate showed a marked difference in transition temperature and second harmonic conversion (SHG) efficiency. The SHG efficiency of ␥-glycine grown in presence of NaF, NaOH, NaNO3 and NaCH2 COOH were reported to be 1.3, 1.4, 1.6 and 1.2 times higher than that of potassium dihydrogen phosphate (KDP) crystal. The same authors have reported that the ␥-crystal grown from a mixture of glycine and NaOH undergoes a transition from ␥- to ␣-form at 172 ◦ C. In the present work, we have grown ␥-glycine single crystals by incorporating various amounts of potassium chloride in the solution and studied the changes in the growth, spectral, thermal and non-linear properties of glycine crystal.
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were recorded on a PerkinElmer (Spectrum RX1) spectrometer and PerkinElmer Lambda 25 spectrometer in transmission mode. Powder X-ray diffraction studies were performed to confirm phase purity and to estimate the cell parameters. The XRD patterns were recorded on Bruker diffractometer within the 2 range of 10–80◦ using Cu K␣ as X-ray source ( = 1.5406 Å). Thermogravimetry of samples was performed using TA instruments NETZSCHSTA409. Second harmonic generation efficiency of the samples was determined using Kurtz powder method. A Q switched Nd:YAG laser beam of wavelength 1064 nm was used with an input energy of 6.2 MJ/pulse and pulse width of 8 ns, the repetition rate being 10 Hz. The SHG radiations of 532 nm (green light) emitted were collected by a photomultiplier tube (PMT – Philips photonics model 8563) and the optical signal incident on the PMT was converted into voltage output at the CRO (Tektronix – TDS 3052B). 3. Results and discussion Fig. 1. Solubility curves of glycine (a) without and with incorporation of KCl, (b) 4 g, (c) 12 g and (d) 18 g.
2. Experimental 2.1. Solubility studies The growth rate of a crystal depends on its solubility and growth temperature. Solubility of a material governs the amount of material, which is available for the growth and hence defines the total size limit. ␣-Glycine was crystallized by taking the analar grade 1 M glycine (99%). In another series ␥-glycine was synthesized by taking 1 M glycine with various concentration of KCl (1–18 g in 100 ml of water). The solubility of glycine has been determined at various temperatures in double distilled water. For this, a saturated solution was prepared in a well-controlled thermal environment with excess of solute. The solubility was then measured gravimetrically (Fig. 1). There is significant increase in the solubility of glycine due to the KCl addition. In aqueous solution, glycine molecules exist in the form of zwitterions with double charges. When an electrolyte is present, complexes could be formed through specific interactions among the zwitterions of glycine, the electrolytes and the water molecules. The formation of these complex interactions would contribute to the concentration change of glycine [8]. Thus, the addition of KCl leads to the increase of glycine solubility in double distilled water. However, the increase in KCl concentration does not affect the solubility much. The positive slope of the solubility curve enables growth by solvent evaporation method.
3.1. Crystal growth Fig. 2 shows the two polymorphs of glycine crystal which differ in crystal habits. As seen in the picture, ␣-form shows prismatic shape while the ␥-form crystal exhibit as bipyramids. ␣-Form of crystal has been grown with dimension of 3 mm × 10 mm × 2 mm, while ␥-form of crystal have approximate dimension of 10 mm × 10 mm × 3 mm. ␣-Glycine have long growth along the ‘c’ direction than ‘a’. The additive, KCl selectively inhibits growth of crystal faces with favorable adsorption site and effectively changes the morphology. Further the additive is prefentially adsorbed on (0 1 1) faces of ␣-glycine crystal and subsequently
2.2. Crystal growth Initially, ␣-glycine solution was prepared by mixing synthesized glycine salt in double distilled water. This solution was stirred using a magnetic stirrer until the salt dissolves completely. The impurity content in the glycine was minimized by successive recrystallization process. The solution was then filtered using Whatmann filter paper of 100 mm porosity. The pH value of the final solution was 4. The final solution was taken in the beaker and it is covered with perforated sheets and kept in dust free environment. After 2–3 days of solvent evaporation, the solution becomes supersaturated and tiny crystal was found. It was allowed to grow for maximum possible dimension and then harvested after 20 days. In another series, ␥glycine was prepared by mixing it in double distilled water. The pH value increases with increase in KCl concentration, i.e., 5.86, 6.15 and 6.50 for 4, 12 and 18 g, respectively. ␥-Glycine crystals were grown by slow evaporation method as described above and the crystals were harvested after about 15 days. The as-grown crystals were subjected to the following characterization studies. The FTIR spectrum and UV–visible spectrum
Fig. 2. As-grown (a) ␣-glycine crystal and (b) ␥-glycine crystals grown from aqueous solution of glycine containing KCl (12 g).
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Fig. 3. Simulated powder XRD pattern of ␥-glycine.
inhibits its growth along c-axis. It changes the actual morphology to the extent that it enhances the growth along the ‘a’ direction and reduces the growth along ‘c’. The nucleation of ␥-glycine in presence of KCl is probably due to a modification of molecular arrangements resulting from the columbic interactions between KCl and glycine molecules. The crystallization of charged glycine molecules enhanced by KCl can result in direct nucleation of ␥form [8]. The crystals grown in presence of lower concentration of KCl (4 g) was ␣-glycine. But it tends to change their transparency after few weeks of time even at ambient conditions and become opaque and appear white in color. This change indicates the possibility of phase transformation from ␣-form to the ␥-form and was confirmed by XRD (Fig. 3). Srinivasan [9] reported that when the NaCl concentration in the solution exceeds certain level (>12 g) the solution yields only crystals with much destructed morphologies with both ␣- and ␥-glycine. Contrary to this, KCl addition of upto 18 g in the solution yielded good quality ␥-glycine crystals with defined morphology. 3.2. X-ray diffraction analysis The crystal structure of glycine was reported by Dawson et al. [10]. The ␣- and ␥-glycine crystallizes in monoclinic and hexagonal structure with space groups of P21 /n and P31 , respectively. The lattice parameters of ␣-glycine are reported as a = 5.11 Å, b = 11.98 Å, c = 5.47 Å and ˇ = 111.78◦ . XRD pattern of ␥-glycine crystals grown with various concentration of KCl are shown in Fig. 3. The results agree well with simulated XRD pattern (Fig. 4) of ␥-glycine based on the data of Dawson et al. [10]. Appearance of sharp and strong peaks confirms the good crystallinity of the grown samples. The characteristic peak at 25.3◦ (2) corresponds to ␥-glycine. The prominent faces of ␥-glycine crystals are (1 1 0), (1 0 1), (1 1 1), and (2 0 1). It is clear that the powder XRD pattern of all the crystals grown from different concentration remained the same except for a small change in the intensity level of the peaks and peak position at the higher angle side. The calculated lattice parameters of ␥-glycine grown with incorporation of various concentration of KCl are shown in Table 1. There is slight increase in lattice constants with increasing KCl concentration which may be due to the doping of KCl into ␥-glycine crystals.
Fig. 4. Powder XRD patterns of ␥-glycine crystals grown from aqueous solution of glycine with (a) 4g, (b) 12g and (c) 18g of KCl.
Table 1 Lattice parameters of ␥-glycine crystals grown with various concentration of KCl. KCl concentration used for growing ␥-glycine
a = b (Å)
c (Å)
4 g in 100 ml water 12 g in 100 ml water 18 g in 100 ml water
6.955 6.998 7.019
5.427 5.475 6.399
[11]. The observed frequencies and their assignment of the glycine crystals are given in Table 2. The peak around 2899 and 2612 cm−1 corresponds to CH2 stretching. The broad band in the higher energy region around 2500–2200 cm−1 is due to NH3 + stretching vibration. Two overlapped bands at around 1612 and 1404 cm−1 could be attributed to the asymmetric and symmetric stretch modes at the COO− group. The peaks around 1328, 1033, 690 and 502 cm−1 have been attributed to CH2 wagging, CN+ stretching, COO− bending and COO− rocking, respectively. The presence of carboxylate and ammonium ion clearly indicates that the glycine molecule exists in zwitter ionic form in ␥-glycine crystal. XRD results suggest that the KCl acts not only as additive, which transforms ␣- to ␥-form of glycine but also gets doped at higher concentration. The KCl concentration was not quantitatively measured. However, it was
3.3. FTIR analysis The recorded FTIR spectra of powdered glycine crystals grown with and without KCl are depicted in Fig. 5. FTIR spectrum in the mid-region for the glycine samples agree well with reported result
Fig. 5. FTIR spectra of (a) ␣-glycine and ␥-glycine crystals grown from aqueous solution of glycine with (b) 4 g, (c) 12 g and (d) 18 g of KCl.
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Table 2 FTIR analyses of ␥-glycine crystals grown in presence of various concentration of KCl. ␣-Glycine (cm−1 )
KCl concentration used for growing ␥-glycine −1
4 g in 100 ml water (cm 2899, 2612 2270, 2135 1502 1612, 1404 1328 1119 1033 892 690 607 502
)
2922, 2617 2268, 2121 1511 1622, 1406 1329 1113 1030 894 695 608 503
12 g in 100 ml water (cm 2930, 2620 2210, 2119 1508 1630, 1405 1330 1114 1030 894 696 608 506
indirectly established by FTIR spectra. The singlet peak at 890 cm−1 was changed into doublet at 910 cm−1 in the crystals grown with higher concentration (18 g) of KCl. The peak at 607 cm−1 which corresponds to COO− wagging got suppressed with increase in the concentration of KCl. In addition, the broad nature of spectra in the spectral region between 2000 and 3500 cm−1 got widened with increase in KCl concentration which may be due to the co-existence of KCl in the crystal lattice. 3.4. Thermal analysis Figs. 6 and 7 illustrate the TG–DTA curve of ␣- and ␥-glycine crystals recorded in the temperature range of 30–1100 ◦ C at the rate of 20 ◦ C/min in nitrogen atmosphere. The TGA curves show no change in weight before 230 ◦ C for both forms, which eliminate the possibility of hydrate or solvate formation of crystals. The major weight loss around 230–300 ◦ C could be due to the sublimation and decomposition of samples which can be confirmed by the sharp endothermic peak in DTA. The weight loss at first stage may due to the release of NH3 and CO molecules. Appearance of a peak at temperature 170 ◦ C in DTA curve of ␥-glycine crystal corresponds to the phase transformation of ␥- to ␣-glycine [12]. This phase transition is not seen in the DTA curve of glycine sample grown in absence of additive which reveals it is ␣-glycine. Close observation of TG results show that there is a significant difference with weight loss (24 wt%) of ␥-glycine when compared to that of ␣glycine (21 wt%) in the higher temperature range (550–750 ◦ C). The excess weight loss may be due to the release of KCl from ␥-glycine crystal. The DTA curve of ␥-glycine shows two additional exothermic peaks at 609 and 732 ◦ C which could be due to the presence of KCl which in ␥-glycine samples. These peaks were absent in the
Fig. 6. TG–DTA curve of ␣-glycine crystal.
Tentative assignment −1
)
−1
18 g in 100 ml water (cm
)
2892, 2605 2228, 2173 1498 1635, 1394 1325 1125 1042 910 682 608 502
CH2 stretching NH3 + stretching COO− stretching CH2 wagging NH2 rocking C–N stretching C–C stretching COO− bending COO− wagging COO− rocking
DTA curve of ␣-glycine. The DTA curve reveals that no endothermic/exothermic reaction occurs below 170 ◦ C in glycine crystals. This ensures the suitability of the material for possible application in laser, where the crystal is required to withstand high temperature. 3.5. Optical transmission spectral analysis The UV–visible spectrum gives information about the structure of the molecule that the absorption of UV and visible light involves in the promotion of electrons in and orbital from the ground state to higher energy state. The UV transmission spectrum of ␥-glycine is shown in Fig. 8. Since these crystals are mainly used in optical application, the determination of UV transmission range and the transparency cutoff are important. It can be seen from the transmission curve that the lower cutoff wavelength lays nearly 325 nm for ␥-glycine crystals. The absence of absorption in the visible region clearly indicates that the glycine crystals can be used as window material in optical instruments. The peak at 327 nm is due to the n–* transition [13]. High transmittance % is observed from 400 nm which clearly indicates that the crystal possess good optical transparency for SHG of Nd:YAG laser. 3.6. Powder SHG measurement In order to confirm the NLO property, the grown crystals were powdered and subjected to Kurtz and Perry powder technique, which is a powerful tool for initial screening of materials for SHG [14]. The second harmonic signal generated in the crys-
Fig. 7. TG–DTA curve of ␥-glycine crystal grown from aqueous solution of glycine containing KCl (12 g).
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for application in laser. In FTIR spectra the presence of carboxylate and ammonium ion clearly indicates that the glycine molecule exists in zwitter ionic form in ␥-glycine crystal. The grown glycine crystals have higher percentage of transmission in the UV region. The measured amplitude of second harmonic green light for glycine crystal grown with KCl is 120 mV against 360 mV for KDP crystal. The decrease in SHG value has been attributed to the possible KCl doping in the glycine crystals. The co-existence of KCl in the crystal lattice of ␥-glycine was also confirmed by FTIR, TGA and XRD analyses. Acknowledgements
Fig. 8. UV spectra of ␥-glycine crystal grown from aqueous solution of glycine containing KCl (12 g).
One of the authors (R.P.) thank Periyar University for providing University Research Fellowship (URF) and the authors thank Prof. V. Krishnakumar of Periyar University for useful discussions. The authors thank E. Ramya of Sacred Heart’s College, Tirupattur for her immense help in experimental works. References
talline sample was confirmed from the emission of green radiation ( = 534 nm) from the crystal. The measured amplitude of second harmonic green light for ␥-glycine is 120 mV against 360 mV for KDP crystal. This value is relatively low when compared to the SHG values reported for ␥-glycine crystals grown with other additives. This poor lasing performance may due to the possible KCl doping into ␥-glycine as suggested by XRD and TGA results. 4. Conclusion We have grown ␥-glycine crystal in the presence of KCl at various concentrations by slow evaporation method. The XRD studies confirm the good crystalline nature and the phase formation of ␥glycine crystals. TG–DTA studies reveal that the ␥-glycine crystals are stable upto 170 ◦ C which suggests the suitability of this material
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