Growth and characterization of α and γ-glycine single crystals

Growth and characterization of α and γ-glycine single crystals

Journal of Crystal Growth 318 (2011) 762–767 Contents lists available at ScienceDirect Journal of Crystal Growth journal homepage: www.elsevier.com/...

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Journal of Crystal Growth 318 (2011) 762–767

Contents lists available at ScienceDirect

Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro

Growth and characterization of a and g-glycine single crystals T.P. Srinivasan a,b, R. Indirajith a, R. Gopalakrishnan a,n a b

Crystal Research Laboratory, Department of Physics, Anna University, Chennai 600 025, India Department of Physics, K.L.N. College of Engineering, Pottapalayam 630 611, India

a r t i c l e i n f o

a b s t r a c t

Available online 21 December 2010

Single crystals of a- and g-glycine were grown by the slow evaporation solution growth method using deionised water as solvent. The a-glycine was transformed to g-glycine by addition of KNO3 as additive and both the forms of glycine single crystals were grown and the characteristic properties were studied and compared. From the single crystal XRD analysis the grown a- and g-glycine crystals are confirmed. The presence of the functional groups of a- and g-glycine was analyzed from the recorded FT-IR spectrum. The optical transmission was ascertained from UV–vis–NIR spectrum. The lower cut-off wavelengths of a- and g-glycine are 292 and 272 nm, respectively. The second harmonic generation relative efficiency was measured by the Kurtz and Perry powder technique. Group theoretical analysis predicts 120 vibrational optical modes in a-glycine and 90 vibrational optical modes in g-glycine. The TGA, DTA and dielectric studies were carried out to explore information about thermal and dielectric behavior, respectively, for a- and g-glycine. & 2010 Elsevier B.V. All rights reserved.

Keywords: A1. Crystal morphology A2. Growth from solutions B2. Dielectric materials B3. Harmonic generation

1. Introduction Nonlinear optical (NLO) material is an active element for optical communications, optical switching, data storage technology, optical mixing and electro-optic applications [1–4]. Complexes of amino acids with inorganic salts are of interest as materials for second harmonic generation (SHG). All amino acids except glycine contain chiral carbon atoms and perhaps crystallize in the noncentrosymmetric space group [5,6]. Dipolar molecules possess an electron donor group and an electron acceptor group contributes to large second order optical nonlinearity arising from the intramolecular charge transfer between two groups of opposite nature. Although the salts of amino acids like L-arginine [7], L-histidine [8] and L-proline [6,9] are reported to have novel properties, the complexes of glycine with inorganic salts are not explored very much for SHG so far. Glycine is one of the simplest amino acids, which is NLO inactive because it does not possess the asymmetric carbon. Though a number of glycine complexes have already been reported, most of them are not NLO active [10–12]. Glycine has three polymorphic crystalline forms a, b and g [13,14]. Both a and b forms crystallize in centrosymmetric space groups, but g-glycine crystallizes in non-centrosymmetric space group P31 making it a possible candidate for NLO applications and it is found difficult to grow the g-glycine crystals [15,16]. The thermodynamic stabilities

n Corresponding author. Tel.: +91 44 2235 8710/+ 91 44 2235 8707; fax: + 91 44 2235 8700. E-mail addresses: [email protected], [email protected] (R. Gopalakrishnan).

0022-0248/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2010.11.117

of the three polymorphs of glycine at room temperature are in the order g 4 a 4 b [17]. It has been recently reported that complexes of g-glycine can be efficient in optical SHG with inorganic salt sodium nitrate [7]. Ferroelectrocity was discovered in glycine silver nitrate [18]. Glycine combines with CaNO3 [19] and LiNO3 [20] to form single crystals but none of these are reported to have nonlinear optical property. In this article we report the growth of a- and g-glycine single crystals from aqueous solutions in the presence of potassium nitrate as additive in equimolar ratio.

2. Experimental procedure 2.1. Growth of a- and g-glycine single crystals

a- glycine was dissolved in a 100 ml beaker using deionised water as the solvent. The solution was stirred well for about 6 h at room temperature and the saturated solution was filtered with Whatman (grade no. 1) filter paper in clean vessel. The vessels containing the solutions were covered with perforated polythene sheets and housed in the constant temperature bath at 331C. The solution was allowed for slow evaporation. The first speck (nucleation) was observed in 5 days and it was allowed to grow further for 21 days. Crystals of prismatic morphology (20  10  5 mm3) were harvested as shown in Fig. 1(a). The growth solution was prepared using commercially available analytical grade glycine (CH2NH2COOH) (SRL-extra pure), KNO3 (Merck-extra pure) and deionised water. The g-form of glycine single crystals were grown from glycine and KNO3 using deionised

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Table 1 Crystallographic data for a- and g-glycine. Lattice parameters

a-glycine

c-glycine

Present work

Reported value [16]

5.105(3)

5.102

7.033(1)

7.037

11.946(2)

11.970(9)

7.032(2)

7.037

˚ c (A)

5.447(4)

5.457(5)

5.479(1)

5.483

a (deg)

90 111.79 90 308.4

90 111.42 90 309.6(9)

90 90 120 234.8

90 90 120 235.1(4)

˚ a (A) ˚ b (A)

b (deg)

g (deg) V (A3)

Present work

Reported value [16]

Fig. 1. As grown crystals of (a) a-glycine and (b) g-glycine.

water as the solvent. Bulk size crystals were grown by the slow evaporation method. The a-glycine and KNO3 were dissolved in equimolar ratio (1:1) in separate beakers using deionized water and the solutions were continuously stirred well for 6 h. The prepared solutions were mixed together and filtered using Whatman (grade no. 1) filter paper in 100 ml degreased clean beaker. The beaker containing the solution was optimally closed and kept in constant temperature bath at 33 1C. Nucleation was observed in a period of ten days and allowed to grow for four weeks and a harvested crystal of g-glycine (20  10  7 mm3) is shown in Fig. 1(b). Investigation of g-glycine by selective additives has been reported in the literature [21,22]. both in the given Refs. [21,22] and in the present investigation the addition of KNO3 as additive with glycine shows that a critical concentration is required for the transformation from a-glycine to g-glycine form. When the crystallization is carried out in the presence of KNO3 glycine carries reduced hydration. It is due to hydration demanded by additional KNO3. As a result glycine crystallizes rapidly due to high rate of crystallization; hence the tendency to crystallize in centrosymmetric fashion is lost. In contrast the rate of crystallization of glycine without additive KNO3 might be less because of high hydration. As a result they acquire enough time to crystallize in centrosymmetric fashion (a-glycine). Hence reduced solvation of amino glycine and a consequent high rate of crystallization are the reasons for the formation of gamma form of glycine. In addition we like to present that pure glycine requires 21 days, whereas additive KNO3 added glycine needs 7 days for good crystallization. 2.2. Characterization The grown single crystals of a- and g-glycine were confirmed by single crystal X-ray diffraction analysis. The FT-IR spectrum was recorded by JESCO 416 PLUS FT-IR Spectrometer (KBr pellet technique) in the range of 4000–400 cm  1. The UV–visible spectra of a- and g-glycine crystals were recorded between 200 and 800 nm using a (Perkin Elmer Lamda 35) UV–vis–NIR spectrophotometer. The thermogravimetric analysis (TGA) was carried out between 50 and 600 1C in the nitrogen atmosphere at a heating rate of 10 1C/min using a STA 409 C/CD TGA unit. The dielectric behavior was studied with an LCR meter (HIOKI 3635 model) as a function of frequency at different temperatures.

3. Results and discussion 3.1. Single crystal X-ray diffraction The single crystal XRD data of the grown a- and g-glycine crystals were obtained by single crystal X-ray diffractometer

˚ (Model: ENRAF NONIUS CAD4/MACH3) using MoKa (0.71073 A) radiation at room temperature. From the single crystal XRD analysis it is confirmed that the grown a-glycine crystallizes in the monoclinic crystal system with space group P21/n and g-glycine crystallizes in the hexagonal crystal system with space group P31. The determined lattice parameter values of a- and g-glycine crystals are in accordance with the literature [16] and the values are presented in Table 1. 3.2. Fourier transform infrared (FTIR) analysis The functional groups of a- and g-glycine crystals were analyzed by Fourier transform infrared spectroscopy. The recorded FT-IR spectra of a- and g-glycine are shown in Fig. 2(a) and (b), respectively. In the FT-IR spectrum, the peaks observed at 3167 cm  1 of a-glycine and 3165 cm  1 of g-glycine are assigned to NH3 asymmetric stretching mode. This is related to the hydrogen bond strength. The bands observed in a-glycine at 3028 cm  1 were assigned to CH2 stretching mode. The peak in a-glycine at 1109 cm  1 is assignable to rocking vibrational mode of CH2. The peak at 870 cm  1 of a-glycine is due to the C–C stretching mode. The bands observed at 607 and 697 cm  1 of g-glycine are assignable to carboxylate groups, while the absorption peaks at 1111 and 1132 cm  1 are attributed to NH3+ group. Thus carboxyl group is present as carboxylate ion in g-glycine [23]. The symmetric and asymmetric deformation vibrations of the NH3+ group appear in the region between 1680 and 1470 cm  1. The polarized properties of a very strong band at 1518 cm  1 arising from the asymmetric stretching vibration of COO  also appear in this region of the IR spectra of g-glycine. The bands at 1032 cm  1 in the IR spectra are assigned to NO3 ion in g-glycine. Of the remaining peaks, those at 891 and 1132 cm  1 are assigned to CCN group and the band at 910 cm  1 corresponds to CH2 group [5]. 3.3. UV–vis analysis The UV–vis spectrum gives limited information about the structure of the molecule because the absorption of UV and visible light involves promotion of the electrons in the s and p orbitals from the ground state to a higher energy state. A nonlinear optical material can be of practical use only if it has a wide transparency window. It is observed that the lower cut off wavelengths of a- and g-glycine are 292 and 272 nm, respectively. The lower UV cut off wavelength of a compound is decided by chromophores present in it. But the chromophores viz amino and carboxyl groups present in g-glycine are transparent and hardly show any absorption in the visible region. The absorption in g-glycine is in the wavelength region between 330 and 800 nm. It is to be noted that the optical transparency below 330 nm in the UV region is the most desirable

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Fig. 2. FT-IR spectra of (a) a-glycine and (b) g-glycine.

characteristic for any nonlinear optical application such as second harmonic generation. 3.4. Factor group analysis of a-glycine The factor group method provides a basis for the prediction of theoretical IR and Raman spectra of lattice vibrations. The single crystal X-ray diffraction studies on the a-glycine crystal confirm that the crystal belongs to monoclinic system with space group P21/n (C22h). There are totally four atoms per unit cell, which occupy the general sites of C1(4) symmetry. A single molecule of CH2NH2COOH contains 10 atoms in turn give rise to 40 atoms in a unit cell. Group theoretical analysis of the fundamental modes of a-glycine crystal predicts 117 vibrational modes that are seen to decompose into Gtotal ¼30Ag +29Au +30Bg +28Bu apart from three acoustic modes 1Au + 2Bu. The factor group analysis for a-glycine

Table 2 Factor group analysis summary for a-glycine. Factor group species C22h

Site symmetry C1(4)

C

N

H

O

Optical mode

Acoustic mode

Total

0 1 0 2 3

30 29 30 28 117

External Internal Ag Au Bg Bu

3T 3R 2T 3R 3T 3R T 3R 9T 12R

24 24 24 24 96

6 3 6 3 6 3 6 3 24 12

15 6 30 15 6 30 15 6 30 15 6 30 60 24 120

crystal was performed by following the procedures outlined by Rousseau et al. [24]. The summary of the factor group analysis of a-glycine is given in Table 2.

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3.4.1. Vibrational analysis of a-glycine The vibrational analysis of a-glycine reveals information on the nature of bonding. The vibrations of a-glycine could be due to lattice vibrations and internal vibrations of the coordinated compounds. The formal calculations of fundamental modes of a-glycine reveal 96 internal vibrations and 21 external modes contributed by 9 translational and 12 rotational modes. The bands observed between Table 3 Correlation scheme for a-glycine. Factor group symmetry

Activity

C22h

IR

Ag Au Bg Bu

– Z – X,Y

Raman

axx ayy, azz, axy

Site symmetry C1(3)

C

N H

4R 3T 5R 3T 9R 6T

36 36 72

3.4.2. Internal vibrations The internal vibrations of a-glycine are those arising from the NH3 asymmetric stretching and CH2 symmetric modes of vibrations. The internal modes of a-glycine ions split into four components of Au(Z) and Bu(X,Y), which are IR active and Ag (XX, YY, ZZ, XY) and Bg (XZ, YZ) are Raman active. In the title compound a-glycine, the NH3 vibrations of absorption bands at 3167 cm  1 arise due to asymmetric stretching. The CH2 vibrations of a-glycine crystal have their absorption bands at 3028 cm  1 due to symmetric stretching.

axz ayz –

O

Optical mode

Acoustic mode

Total

1 2 3

43 44 87

External Internal A E

4000 and 400 cm  1 (Fig. 2a) are due to internal vibrations of the co-ordinated compounds and the peaks below 500 cm  1 arise from the deformational vibrations and the translational and rotational modes of the compounds.



Table 4 Factor group analysis summary for g-glycine. Factor group species C3

765

6 3 15 6 45 6 3 15 6 45 12 6 30 12 90

Table 5 Correlation scheme for g-glycine. Factor group symmetry

Activity

C3

IR

Raman

A E

Z X,Y

axx + ayy, azz (axx–ayy  axy), (axz, ayz)

3.4.3. External vibrations The external vibrations are mainly due to the bands observed below 500 cm  1, which are due to the rotational and translational modes of vibrations of a-glycine. The rotational modes are expected to have higher frequency and intensity than those of translational modes in the Raman spectra. However, the translational modes are more intense in IR spectra [25,26]. a-glycine is found to have 21 external modes and those vibrations can be achieved experimentally by polarized Raman measurements. The correlation scheme for a-glycine is given in Table 3.

3.5. Factor group analysis of g-glycine The single crystal X-ray diffraction analysis of g-glycine (NH2CH2COOH) confirms that the crystal belongs to the hexagonal crystal system with the space group P31(C23). There are totally three atoms per unit cell, which occupy the general sites of C1(3) symmetry. A single molecule CH2NH2COOH contains 10 atoms, which in turn give rise to 30 atoms in a unit cell. Group theoretical analysis of the fundamental modes of g-glycine predicts that there are 90 vibrational optical modes. They are seen to decompose into G87 ¼43A+44E optical modes apart from three acoustic modes 1A+2E. The summary of the factor group analysis of g-glycine is given in Table 4.

Fig. 3. Variations of dielectric constant with frequency of (a) a-glycine and (b) g-glycine.

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3.5.1. Vibrational analysis of g-glycine The formal calculation of fundamental modes of g-glycine predicts 90 optical modes. They are seen to decompose into G87 ¼44A+ 43E and 15 external modes contributed by 6 translational and 9 rotational modes apart from three acoustic modes. The bands between 4000 and 400 cm  1 (Fig. 2b) are due to the internal vibrations of the co-ordinated compounds and the peaks below 500 cm  1 arise from the deformational vibrations and the translational and rotational modes of the compounds. 3.5.2. Internal and external vibrations As the g-glycine molecule does not have any symmetry, the internal vibrations exhibited are both IR and Raman active exclusive of acoustic mode. The internal vibrations of g-glycine can be classified as those arising from the NH3, CH2 and NO3 functional groups. These vibrations are strongly coupled between themselves. The g-glycine is found to have 15 external modes that can be achieved experimentally by polarized Raman measurements, which are below 500 cm  1. The correlation scheme for g-glycine

obtained by following the procedures of Fateley et al. [27] is given in Table 5. 3.6. Dielectric studies The dielectric studies were carried out using HIOKI 3532-50 LCR HITESTER. Fig. 3(a) and (b) shows the variation of dielectric constant as a function of frequency for a- and g-glycine. It is found that the dielectric constant of a- and g-glycine is high at low frequencies and decreases with increase in frequency. This may be attributed to space charge polarization owing to charged lattice defects. But at a fixed frequency, the dielectric constant of g-glycine is more than that of a-glycine. The g-glycine is more polarized and hence has high dielectric constant. For a-glycine the dielectric constant has a high value of 705 at 100 Hz and decreases to 189 at 5 MHz. Similarly for g-glycine the dielectric constant has a high value of 965 at 100 Hz and decreases to 228 at 5 MHz. The characteristics of low dielectric loss at high frequencies for g-glycine suggest that the crystal possesses enhanced optical quality with lesser defects. For a

Fig. 4. TGA and DTA curves of (a) a-glycine and (b) g-glycine.

T.P. Srinivasan et al. / Journal of Crystal Growth 318 (2011) 762–767

particular frequency, the dielectric loss of g-glycine is lesser than that of a-glycine, which indicates that the g-glycine possesses enhanced optical quality and less defects. 3.7. Second harmonic generation studies The SHG conversion efficiency of g-glycine was determined by the Kurtz and Perry technique [28]. A Q-switched Nd:YAG laser of 1064 nm was used as a source for illuminating the powder sample g-glycine. Intense green light was observed. KDP sample was used as the reference material and the output power intensity of g-glycine was comparable with the output power of KDP. The SHG conversion efficiency of g-glycine is 1.43 times greater than that of KDP.

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The theoretical factor group analysis of a-glycine predicts 120 optical modes that decompose into Gtotal ¼30Ag + 29Au + 30Bg + 28Bu modes apart from three acoustic modes Gacoustic ¼1Au + 2Bu. Similarly g-glycine predicts 90 optical modes that decompose into Gtotal ¼43A+ 44E along with three acoustic modes Gacoustic ¼ 1A+2E.

Acknowledgement The authors thank Prof. P.K. Dass, Indian Institute of Science, Bangalore for providing SHG test facility.

3.8. Thermal analysis

References

The recorded thermograms of TG and DTA for a- and g-glycine are shown in Fig. 4(a) and (b). The TG and DTA curves of the grown a-glycine did not indicate any change in the heat flow until the melting transition that occurs at 257 1C. The melting transition starts at 230 1C and ends at 265 1C with a sharp melting band of 35 1C. Also no weight loss was observed until the melting transition for a-glycine whereas TG and DTA curves of g-glycine show an endothermic peak at 134.13 1C before its melting. This peak represents certainly a phase transformation of this crystal from g to possibly the a-form [29]. Actually, the melting starts at 315.33 1C and ends at 331.39 1C with a sharp melting band of about 24.06 1C. No appreciable weight loss was observed before this melting transition.

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4. Conclusions The single crystals of a- and g-glycine were grown by slow evaporation solution growth method. The XRD data for the grown a- and g-glycine confirmed the crystal structure. The transparency of g-glycine is observed in the wavelength region between 200 and 800 nm. It should be noted that the optical transparency below 272 nm (g-glycine) in the UV region is the most desirable characteristic for any nonlinear optical application such as second harmonic generation. TG and DTA studies made on the grown a- and g-glycine crystals show that the g-glycine changes its form to a at around 134.13 1C. The dielectric studies indicate that the g-glycine possesses good optical quality with lesser defects compared to a-glycine. The dielectric loss of g-glycine was found to be less than that of a-glycine. The presence of intermolecular hydrogen bonding was confirmed from the FT-IR spectrum that could enhance the nonlinear property of the g-glycine. The relative SHG efficiency of the g-glycine is 1.42 times greater than that of KDP.