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Structural, optical and dielectric studies of glycinium trifluoroacetate single crystal
Physica B 406 (2011) 2152–2157
Contents lists available at ScienceDirect
Physica B journal homepage: www.elsevier.com/locate/physb
Structural, optical and dielectric studies of glycinium trifluoroacetate single crystal Neelam Singh, Binay Kumar n Crystal Lab, Department of Physics and Astrophysics, University of Delhi, Delhi-7, India
a r t i c l e i n f o
a b s t r a c t
Article history: Received 30 December 2010 Received in revised form 4 March 2011 Accepted 9 March 2011 Available online 15 March 2011
An organic material glycinium trifluoroacetate (GTFA) has been re-synthesized and large single crystals have been grown by solution technique. Complete structure of GTFA has been redetermined from single crystal XRD data. FTIR confirmed the presence of various functional groups. Melting point (152.44 1C), thermal stability and specific heat were studied from TG/ DTA and DSC. In UV absorption spectra, a lower cutoff value as 220 nm and a wide band gap as 4.86 eV for GTFA were observed. The dielectric studies, dielectric constant and loss were measured at different temperatures (30–90 1C) in the frequency range 100 Hz–2 MHz. & 2011 Elsevier B.V. All rights reserved.
Keywords: Solution growth Single crystal X-ray diffraction Crystal structure Dielectric properties Optical properties Infrared spectroscopy
1. Introduction Organic single crystals constructed by p-conjugated molecules have attracted great attention in the field of organic optoelectronics [1]. Materials of this type generally have low dielectric constants which increase their efficiency as high frequency modulators. Amino acids are vital components of a variety of biological, industrial and environmental samples. They exist as zwitterions in aqueous solution within a wide range of pH-value. Ohno et al. [2] designed zwitterionic-type ionic liquids to prevent migration of the cation of ionic liquids under the influence of an electric field by tethering anion and cation. Amino acids are used to synthesize room temperature ionic liquids (RTILS), which are under investigation as solvents for technological applications such as metal surface finishing, batteries, capacitors, fuel cells, electro-synthesis and nuclear waste treatment [3]. Glycine (NH2CH2COOH) is a particularly interesting case in all amino acids, as it can exist in three distinct crystalline phases (polymorphs) known as a, b and g glycine. The three polymorphs of glycine can be formed under different solution conditions. a-glycine is formed by spontaneous nucleation of pure aqueous glycine, while g glycine can be formed from acidic or basic aqueous solution, because at high or low pH, the zwitterions become protonated or de-protonated, respectively, making cyclic dimer formation unfavorable [4]. b-glycine, the least stable forms, can be
formed from mixed solvents such as methanol or ethanol and water. The a- and b-forms of glycine crystallizes in the monoclinic system with centro-symmetric space groups P21/m and P21, respectively, and the g-form crystallizes in the trigonal–hexagonal system with non centrosymmetric space group P31 [5]. This makes g glycine a suitable candidate for non-linear optical applications [6]. Very recently two additional polymorphs, d- and e-glycine, have been discovered under high pressure conditions [7]. Glycine has many commercial uses, such as flavor masking and enhancement (i.e., carbonated soft drinks based on saccharin), pH buffering and stabilization applications (in antiperspirants, cosmetics and toiletries), pharmaceutical applications (as an excipient) [8] and as a chemical intermediate and in atmospheric aerosol droplets, representative for remote marine areas, consisting of either pure water or glycine/water mixtures [9]. Glycine is reported to form various complex; some of them show non linear optical properties like glycine sodium nitrate [10], glycine lithium sulfate [11] and benzoyl glycine [12] while those formed of glycine with CaCl2 [13], BaCl2 [14], H2SO4 [15] and CoBr2 [16] did not show non linearity. In the present work, the title compound has been re-synthesized from glycine with trifluoroacetatic acid as additives and the grown crystal were characterized by various techniques. 2. Experimental procedure 2.1. Synthesis of GTFA and crystal growth
n
Corresponding author. Mobile: þ91 9818168001; fax: þ 91 011 27667061. E-mail addresses: [email protected], [email protected] (B. Kumar). 0921-4526/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2011.03.020
Glycine from Sigma–Aldrich ( 499%) and trifluoroacetatic acid from Sigma–Aldrich ( 499%) were dissolved in distilled water in a
N. Singh, B. Kumar / Physica B 406 (2011) 2152–2157
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molar ratio of 1:1. The calculated amounts of the reactants were stirred well for about 5 h using a magnetic stirrer to yield a homogeneous mixture. The solution was kept in constant temperature bath at 50 oC (accuracy71 1C). After several days, the synthesized salt glycinium trifluoroacetate (GTFA) was obtained and then purified by successive re-crystallization. To avoid decomposition, enough care is taken while heating the solution and maintained below a temperature of 75 1C. This compound has been used to grow bulk crystals. The purified compound stirred continuously resulted in a homogenous solution, which was then kept undisturbed at room temperature to obtain seed crystals by spontaneous nucleation. Seed perfection is very important for growing single crystals with high purity because defects in the seed crystal could cause spurious crystallization and flaws. The solution was allowed to evaporate at room temperature, which yielded colorless crystal after several weeks.
2.2. Characterization The chemical composition of the re-synthesized salt GTFA was established by Vario EL III Elemental CHNS analyzer. X-ray diffraction (XRD) allows for unambiguous determination of the chemical species and arrangement (and spacing) of atoms in a crystalline material. Diffraction data were taken using an OxfordDiffraction XCallibur with sapphire CCD detector and Enhance diffractometer (Mo Ka radiation, graphite monochromator, ˚ to determine the crystal structure of the rel ¼0.71073 A) synthesized GTFA single crystals. XPS measurements were carried out using a Perkin Elmer (Model no. 1257). X-ray photoelectron spectrometer operated under a base pressure of 5 10–10 Torr. The X-ray source, of Al Ka (1486.6 eV) was used for the present analysis. The FTIR spectra of the samples were recorded in KBr pallet in the frequency region of 400–4000 cm 1, using a Perkin-Elmer Spectrum BX under a resolution of 4 cm 1 and with a scanning speed of 2 mm/s. The transmission spectra for the specimens were recorded using a Perkin Elmer UV spectrophotometer in the region 190–1100 nm. Thermal analysis was performed using a Diamond Perkin Elmer simultaneous TG/DTA analyzer in the temperature range 30–500 1C in a nitrogen atmosphere with a heating rate of 10 1C/min. The variation of dielectric constant at different frequency (100 Hz–2 MHz) of as grown samples was studied from room temperature to 90 1C using Agilent E 4980 A impedance analyzer. High grade silver paste was coated on both sides of GTFA crystal and then placed between the electrodes of Agilent 16084 A sample holder to form the parallel plate capacitor.
3. Result and discussion 3.1. Crystal growth of GTFA GTFA single crystals were grown after purifying the materials by repeated recrystallization process. In the present case both glycine and trifluoroacetic acid are used as solute in water. Optically good quality single crystals (30 4 5 mm3) were obtained in a period of over one month by slow evaporation technique. Earlier, Rodrigues et al. [17] have used glycine as a solute and trifluoroacetic acid as a solvent because of which the solution is more acidic resulting in faster growth rate (growth period few hours). Slower rate of crystal growth in the present case has resulted in large sized crystals. The photograph of asgrown single crystals is shown in Fig. 1(a; inset).
Fig. 1. (a) Structure of GTFA showing 50% probability ellipsoids and grown crystal of GTFA (inset). (b) Projection along the a-axis of GTFA crystal.
3.2. CHNS analysis The percentage of carbon, hydrogen and nitrogen contents in the single crystal of glycinium trifluoroacetate was analyzed using Vario EL III Elemental CHNS analyzer. The formation of glycinium trifluoroacetate was confirmed by CHN analysis. The sample of weight 6.7360 mg is taken for CHN analysis. The calculated theoretical values are C¼25.408%, H ¼3.198%, N¼7.408%; observed experimental values are C¼25.400%, H ¼3.514%, N¼7.512%. Both are in good agreement with each other. 3.3. Single crystal X-ray diffraction Single-crystal X-ray diffraction analysis for the grown GTFA crystal has been carried out to confirm the crystallinity and also to identify the unit cell parameters. It exhibits monoclinic crystal system with the space group of P121/n1. The crystal structure was solved and refined using the program SIR-92(WINGX) [18] and SHELXL-97 (WINGX) program [19]. The results are in agreement with the earlier reported values [17]. The crystallographic data and structure refinement of GTFA are given in Table 1. The ORTEP drawing (Fig. 1a) was performed with ORTEPIII program [20]. The projection along a-axis of GTFA is shown in Fig. 1b. 3.4. XPS analysis In order to see the composition on the surface, XPS measurements have been carried out using Al Ka (1486.6 eV) as an X-ray source. Fig. 2 shows the XPS survey scan of the GTFA, which manifest the presence of the fluorine (F) in the grown crystal as well as the presence of oxygen, nitrogen and carbon. The core
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Table 1 Crystal data and structure refinement for GTFA. Crystal Data Identification code Empirical formula Formula weight Temperature Wavelength
GTFA C4H6F3NO4 189.1 293(2) K 0.71073 A˚
Crystal system, space group Unit cell
Monoclinic, P121/n1 ˚ b¼ 12.262 (15) A, ˚ a¼ 4.9506 (5) A, ˚ a ¼ g ¼ 901, b ¼ 97.5471 c ¼11.5664 (13) A,
Volume Z, Calculated density Absorption coefficient, m Radiation type F(0 0 0) Theta range for data collection Crystal form, color Crystal size
696.05(14) A˚ 3 4, 1.805 Mg/m3 0.153 mm 1 Mo Ka 288 3.55–32.651 Rod, color less 0.31 0.30 0.28
Data Collection Diffractometer Data collection method Absorption correction Tmin Tmax Criterion for observed reflections Rint ymax (1) Limiting indices
Enhance o scans Multi scan 0.56914 1.00000 I 42 s(I) 0.0412 32.65 7r h r 7, 17r kr 17, 17 r lr 16
Refinement Refinement method Final R indices [I4 2 Sigma (I)] Goodness-of-fit on F2 No. of parameters H-atom treatment Extinction method Extinction coefficient
Full-matrix least-squares on F2 R1¼ 0.0533, wR2¼0.1354 1.019 131 Mixed Shelxl 1.31(5)
the UV–vis–NIR transmittance spectrum were recorded. The UV cut-off wavelength for the lysinium trifluoroacetate, argininium trifluoroacetate, histidinium trifluoroacetate crystals was found to be at 234, 232 and 242 nm, respectively, [21–23]. From the spectrum (Fig. 3a) of GTFA crystal, it is noted that the UV transparency cut off occurs at 220 nm and this is much lower than other materials with trifluoroacetic acid. GTFA crystal show low absorption throughout the entire visible region is one of the most desired properties for the fabrication of optoelectronic devices. 3.5.1. Determination of optical constants The extinction coefficient (K) can be obtained from the following relation: K¼
al 4p
The reflectance in terms of the absorption coefficient can be derived from the equation pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 1 7 ð1expÞðatÞ þ expðatÞ R¼ ð1expÞðatÞ From the above data the refractive index n can be derived as pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ðRþ 1Þ 7 3R2 þ10R3 n¼ 2ðR1Þ Fig. 3b represents the dependence of reflectance and the extinction coefficient on absorption coefficient. The internal efficiency of the device also depends upon the absorption coefficient. Hence by tailoring the absorption coefficient and tuning the band gap of the material, we can achieve the desired material that is suitable for fabricating various layers of the optoelectronic devices as per our requirements [24]. 3.6. FTIR analysis
Fig. 2. XPS spectra of GTFA crystals.
level spectra for this sample have been shown as an inset of the Fig. 2.
To analyze the presence of functional groups in GTFA qualitatively, FTIR spectra were recorded in the range 400–4000 cm 1 using Perkin-Elmer grating infrared spectrometer and the recorded spectrum is shown in Fig. 3c. In the high energy region, there is a broad band between 2100 and 3500 cm 1. For GTFA crystal, the broad band at 3081 cm 1 in the IR spectrum is assigned to asymmetric stretching mode of NH3þ . The symmetric and asymmetric deformation vibrations of the NH3þ group appear in the region between 1680 and 1470 cm 1. The strong absorption band due to NH3þ asymmetric deformation occurs at 1527 cm 1 [25–27]. The band caused by this type deformation of CH2 group is observed at 1440 cm 1 as a strong line in the IR spectra [25,27]. For the complex, the infrared bands of COO symmetric str. are observed at 1377 cm 1 as strong lines. For free trifluoroacetic acid, there are two characteristic stretching vibrations to the bands at 1255 and 1181 cm 1 in the infrared spectrum [28]. In the present infrared spectrum the bands at 1255 and 1178 cm 1 as strong lines are assigned to C–F stretching. The strong lines of C–F stretching mode indicate the existence of trifluoroacetate anion. We assign the strong band at 1037 cm 1 to asymmetric stretching CCN. The COO scissoring (652 cm 1), wagging (603 cm 1) and NH3þ torision (520 cm 1) modes are also observed and assigned [25,29]. 3.7. Thermal analysis
3.5. Optical properties To determine the optical transmittance range and hence to know the suitability of GTFA single crystal for optical applications,
TG and DTA analyses are of immense importance as they provide information about thermal stability and melting point of the material, which is significant for device fabrication and
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Fig. 3. (a) Optical transmission spectrum & Optical band gap determination using. UV–vis absorption data (inset). (b) Absorption coefficient versus reflectance, refractive index and extinction coefficient (inset). (c) FTIR spectra of GTFA Crystal.
application. The powdered GTFA with sample weight 7.985 mg was taken in an alumina crucible for the measurement. The TG/DTA spectra are shown in the Fig. 4a. A gradual loss of weight in the region between 101 and 449 1C in TGA was observed. The DTA trace shows an endothermic peak at 152.44 1C corresponding to the melting of the sample. This is immediately followed by sharp endothermic peak at 215.53 1C coinciding with the major weight loss due to decomposition which/and can be assigned to the absorption of energy for the breaking of bands resulting in complex formation of the residues. The sharpness of these endothermic peaks confirms a high degree of crystallinity of the sample. It is observed that there is no weight loss in the sample upto 101 1C implying thermal stability of the sample upto this temperature. Variation of specific heat with temperature in the range RT-450 1C is shown in Fig. 4b. In this curve also the same peaks are observed as in the DTA curve. The specific heat at room temperature is found to be 0.0791 Jg 1 1C 1. The specific heat corresponding to melting and complex formation peaks are calculated as 21.29 and 14.84 Jg 1 1C 1, respectively.
3.8. Dielectric studies The dielectric constant of the sample was measured in the applied frequency range 100 Hz–2 MHz at different temperatures (30–90 1C). Fig. 5a shows the variation of dielectric constant with frequency and temperatures. It is also observed that dielectric constant increases with increase in temperature which is attributed to the presence of space charge polarization near the grain boundary interfaces, which depends on the purity and perfection of the crystal [30]. In general, the dielectric constant decreases with increasing frequency and attains a constant value, depending on the fact that beyond a certain frequency of the electric field, the dipole does not follow the alternating field [31,32]. The dielectric constant has a higher value (9.08) in the lower frequency region and it then decreases (1.46) with high frequency at 90 1C. Fig. 5b suggests that the dielectric loss strongly depends on the frequency of the applied field, similar to what commonly happens with the dielectric constant. The dielectric constant of materials is due to the contribution of electronic, ionic, dipolar and space charge polarizations, which
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Fig. 4. (a) TG-DTA spectra of GTFA crystal. (b) Specific heat curve for GTFA.
depend on frequencies [33]. At lower frequencies, all these polarizations are active. The space charge polarization is generally active at lower frequencies and at high temperatures [34]. In the case of GTFA crystal, the dielectric loss is as low as 0.012 at 2 MHz. The behavior of dielectric losses in the low frequency range depends on many factors which can be controlled (growth rate, defects, size of crystal, etc.) [35]. Low dielectric loss shows that the crystals are of enhanced optical quality.
4. Conclusion Single crystals of glycinium trifluoroacetate, an organic material of the amino-carboxyl acid family, were successfully re-synthesized and grown by aqueous solution slow evaporation technique. The crystal is transparent and colorless. Single crystal XRD shows that the obtained GTFA crystals have the same crystal structure as previously reported. FTIR spectral analysis confirms the presence of functional groups constituting GTFA such as NH2þ , CH2, C–N and COO . An optical study confirms that glycinium trifluoroacetate crystal is transparent in the wavelength region 190–1100 nm with the UV transparency cutoff at 220 nm, which is much lower than other materials with trifluoroacetic acid. The optical investigations such as extinction
Fig. 5. (a) Variation of dielectric constant with frequency at different temperature of GTFA crystal. (b) Variation of dielectric loss with frequency at different temperature of GTFA crystal.
coefficient (k) ,refractive index (n) and reflectance (R) indicate the high transparency of the crystal and confirm its suitability for optical device fabrication. TG/DTA reveals that this compound is stable up to 152.44 1C. The specific heat at room temperature is found to be 0.0791 Jg 1 1C 1. The specific heat corresponding to melting and complex formation peaks are found to be 21.29 and 14.84 Jg 1 1C 1, respectively. The dielectric constant has a higher value at high temperature in the lower-frequency region and it then decreases at room temperature with the applied high frequency.
Acknowledgment Financial support received through DU DST PURSE Grant for ‘‘Growth of single crystalsy. various methods’’ is thankfully acknowledged. Neelam Singh is thankful to Council of Scientific and Industrial Research (CSIR), India for providing the Senior Research Fellowship (SRF).
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References [1] A. Saeki, S. Seki, T. Takenobu, Y. Iwasa, S. Tagawa, Adv. Mater. 20 (2008) 920. [2] H. Ohno, M. Yoshizawa, W. Ogihara, Electrochim. Acta 48 (2003) 2079. [3] Tetsuya Tsuda, Charles L. Hussey, The Electrochemical Society Interface, Springer, 2007. [4] C.S. Towler, R. Davey, R.W. Lancaster, C.J. Price, J. Am. Chem. Soc. 126 (2004) 13347. [5] K. Srinivasan, J. Cryst. Growth 311 (2008) 156. [6] M.N. Bhat, S.M. Dharmaprakash, J. Cryst. Growth 242 (2002) 245. [7] A. Dawson, D.R. Allan, S.A. Belmonte, S.J. Clark, W.I.F. David, Cryst. Growth Des. 5 (2005) 1415. [8] Roberte Taylor, Concepts Magn. Reson. A 22A (2) (2004) 79. ˚ [9] X. Li, T. Hede, Y. Tu, C. Leck, H. Agren, Atmos. Chem. Phys. Discuss. 10 (2010) 23169. [10] M. Narayan Bhat, S.M. Dharmaprakash, J. Cryst. Growth 235 (2002) 511. [11] T. Balakrishnan, K. Ramamurthi, Cryst. Res. Technol. 41 (2006) 1184. [12] H.S. Nagaraja, V. Upadhyaya, P. Mohan Rao, S. Aithal, A.P. Bhat, J. Cryst. Growth 193 (1998) 674. [13] S. Natarajan, J.K.Mohan Rao, Z. Kristallogr. 152 (1984) 179. [14] P. Narayanan, S. Venkataraman, Z. Kristallogr. 142 (1975) 52. [15] S. Hoshino, T. Mitsui, F. Jona, R. Pepinsky, Phys. Rev. 107 (1957) 125. [16] K. Ravikumar, S.S. Rajan, Z. Kristallogr. 171 (1985) 201. [17] V.H. Rodrigues, J.A. Paixao, M.M.R.R. Costa, A.Matos Beja, Acta Crystallogr. C 58 (2002) 658. [18] A. Altomare, G. Gascarano, C. Giacovazzo, A. Guagliardi, J. Appl. Crystallogr. 26 (1993) 343.
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[19] G.M. Sheldrick, Acta Crystallogr. A: Found. Crystallogr. 64 (2008) 112. [20] M.N. Burnett, C.K. Johnson, ORTEPIII, Report ORNL-6895, Oak Ridge National Laboratory, Tennessee, USA, 1996. [21] Z.H. Sun, D. Xu, X.Q. Wang, G.H. Zhang, G. Yu, L.Y. Zhu, H.L. Fan, Mater. Res. Bull. 44 (2009) 925. [22] D. Xu, X.Q. Wang, W.T. Yu, S.X. Xu, G.H. Zhang, J. Cryst. Growth 253 (2003) 481. [23] G.H. Sun, X.T. Sun, Z.H. Sun, X.Q. Wang, X.J. Liu, G.H. Zhang, D. Xu, J. Cryst. Growth 311 (2009) 3904. [24] G. Shankar, P.S. Joseph, M. YosuvaSuvakin, A. Sebastiyan, Physica B 405 (2010) 4231. [25] Henryk Jan Baran, Ratajczak, Spectrochim. Acta A 61 (2005) 1611. [26] Shigeko Suzuki, Takehiko Shimanouchi, Spectrochim. Acta 19 (1963) 1195. [27] Joseph S. Alper, Hoang Dothe, Marian A. Lowe, Chem. Phys. 161 (1992) 199. [28] N. Fuson, M.L. Josien, E.A. Jones, J. Chem. Phys. 20 (1952) 1627. [29] K. Nakamoto, Infrared and Raman Spectra of Inorganic and Coordinate Compounds, 3rd ed., John Wiley & Son, New York, 1978. [30] C.P. Smyth, Dielectric Behaviour and Structure, McGraw-Hill, New York, 1965, p. 132. [31] A.K. Batra, C.R. Carmichale-Owens, M. Simmons, M.D. Aggarwal, R.B. Lal, Cryst. Res. Technol. 40 (2005) 757. [32] N.E. Hill, W.E. Vaughan, A.H. Price, M. Davies, Dielectric Properties and Molecular Behavior, Van Nostrand Reinhold, London, 1969. [33] D. Xue, K. Kitamura, Solid State Commun. 122 (2002) 537. [34] B.W. Betterman, H. Cole, Rev. Mod. Phys. 36 (1964) 681. [35] J. Przesiawski, T. Lglesias, J.A. Gonzalo, Solid State Commun. 96 (1995) 195.
Contents lists available at ScienceDirect
Physica B journal homepage: www.elsevier.com/locate/physb
Structural, optical and dielectric studies of glycinium trifluoroacetate single crystal Neelam Singh, Binay Kumar n Crystal Lab, Department of Physics and Astrophysics, University of Delhi, Delhi-7, India
a r t i c l e i n f o
a b s t r a c t
Article history: Received 30 December 2010 Received in revised form 4 March 2011 Accepted 9 March 2011 Available online 15 March 2011
An organic material glycinium trifluoroacetate (GTFA) has been re-synthesized and large single crystals have been grown by solution technique. Complete structure of GTFA has been redetermined from single crystal XRD data. FTIR confirmed the presence of various functional groups. Melting point (152.44 1C), thermal stability and specific heat were studied from TG/ DTA and DSC. In UV absorption spectra, a lower cutoff value as 220 nm and a wide band gap as 4.86 eV for GTFA were observed. The dielectric studies, dielectric constant and loss were measured at different temperatures (30–90 1C) in the frequency range 100 Hz–2 MHz. & 2011 Elsevier B.V. All rights reserved.
Keywords: Solution growth Single crystal X-ray diffraction Crystal structure Dielectric properties Optical properties Infrared spectroscopy
1. Introduction Organic single crystals constructed by p-conjugated molecules have attracted great attention in the field of organic optoelectronics [1]. Materials of this type generally have low dielectric constants which increase their efficiency as high frequency modulators. Amino acids are vital components of a variety of biological, industrial and environmental samples. They exist as zwitterions in aqueous solution within a wide range of pH-value. Ohno et al. [2] designed zwitterionic-type ionic liquids to prevent migration of the cation of ionic liquids under the influence of an electric field by tethering anion and cation. Amino acids are used to synthesize room temperature ionic liquids (RTILS), which are under investigation as solvents for technological applications such as metal surface finishing, batteries, capacitors, fuel cells, electro-synthesis and nuclear waste treatment [3]. Glycine (NH2CH2COOH) is a particularly interesting case in all amino acids, as it can exist in three distinct crystalline phases (polymorphs) known as a, b and g glycine. The three polymorphs of glycine can be formed under different solution conditions. a-glycine is formed by spontaneous nucleation of pure aqueous glycine, while g glycine can be formed from acidic or basic aqueous solution, because at high or low pH, the zwitterions become protonated or de-protonated, respectively, making cyclic dimer formation unfavorable [4]. b-glycine, the least stable forms, can be
formed from mixed solvents such as methanol or ethanol and water. The a- and b-forms of glycine crystallizes in the monoclinic system with centro-symmetric space groups P21/m and P21, respectively, and the g-form crystallizes in the trigonal–hexagonal system with non centrosymmetric space group P31 [5]. This makes g glycine a suitable candidate for non-linear optical applications [6]. Very recently two additional polymorphs, d- and e-glycine, have been discovered under high pressure conditions [7]. Glycine has many commercial uses, such as flavor masking and enhancement (i.e., carbonated soft drinks based on saccharin), pH buffering and stabilization applications (in antiperspirants, cosmetics and toiletries), pharmaceutical applications (as an excipient) [8] and as a chemical intermediate and in atmospheric aerosol droplets, representative for remote marine areas, consisting of either pure water or glycine/water mixtures [9]. Glycine is reported to form various complex; some of them show non linear optical properties like glycine sodium nitrate [10], glycine lithium sulfate [11] and benzoyl glycine [12] while those formed of glycine with CaCl2 [13], BaCl2 [14], H2SO4 [15] and CoBr2 [16] did not show non linearity. In the present work, the title compound has been re-synthesized from glycine with trifluoroacetatic acid as additives and the grown crystal were characterized by various techniques. 2. Experimental procedure 2.1. Synthesis of GTFA and crystal growth
n
Corresponding author. Mobile: þ91 9818168001; fax: þ 91 011 27667061. E-mail addresses: [email protected], [email protected] (B. Kumar). 0921-4526/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2011.03.020
Glycine from Sigma–Aldrich ( 499%) and trifluoroacetatic acid from Sigma–Aldrich ( 499%) were dissolved in distilled water in a
N. Singh, B. Kumar / Physica B 406 (2011) 2152–2157
2153
molar ratio of 1:1. The calculated amounts of the reactants were stirred well for about 5 h using a magnetic stirrer to yield a homogeneous mixture. The solution was kept in constant temperature bath at 50 oC (accuracy71 1C). After several days, the synthesized salt glycinium trifluoroacetate (GTFA) was obtained and then purified by successive re-crystallization. To avoid decomposition, enough care is taken while heating the solution and maintained below a temperature of 75 1C. This compound has been used to grow bulk crystals. The purified compound stirred continuously resulted in a homogenous solution, which was then kept undisturbed at room temperature to obtain seed crystals by spontaneous nucleation. Seed perfection is very important for growing single crystals with high purity because defects in the seed crystal could cause spurious crystallization and flaws. The solution was allowed to evaporate at room temperature, which yielded colorless crystal after several weeks.
2.2. Characterization The chemical composition of the re-synthesized salt GTFA was established by Vario EL III Elemental CHNS analyzer. X-ray diffraction (XRD) allows for unambiguous determination of the chemical species and arrangement (and spacing) of atoms in a crystalline material. Diffraction data were taken using an OxfordDiffraction XCallibur with sapphire CCD detector and Enhance diffractometer (Mo Ka radiation, graphite monochromator, ˚ to determine the crystal structure of the rel ¼0.71073 A) synthesized GTFA single crystals. XPS measurements were carried out using a Perkin Elmer (Model no. 1257). X-ray photoelectron spectrometer operated under a base pressure of 5 10–10 Torr. The X-ray source, of Al Ka (1486.6 eV) was used for the present analysis. The FTIR spectra of the samples were recorded in KBr pallet in the frequency region of 400–4000 cm 1, using a Perkin-Elmer Spectrum BX under a resolution of 4 cm 1 and with a scanning speed of 2 mm/s. The transmission spectra for the specimens were recorded using a Perkin Elmer UV spectrophotometer in the region 190–1100 nm. Thermal analysis was performed using a Diamond Perkin Elmer simultaneous TG/DTA analyzer in the temperature range 30–500 1C in a nitrogen atmosphere with a heating rate of 10 1C/min. The variation of dielectric constant at different frequency (100 Hz–2 MHz) of as grown samples was studied from room temperature to 90 1C using Agilent E 4980 A impedance analyzer. High grade silver paste was coated on both sides of GTFA crystal and then placed between the electrodes of Agilent 16084 A sample holder to form the parallel plate capacitor.
3. Result and discussion 3.1. Crystal growth of GTFA GTFA single crystals were grown after purifying the materials by repeated recrystallization process. In the present case both glycine and trifluoroacetic acid are used as solute in water. Optically good quality single crystals (30 4 5 mm3) were obtained in a period of over one month by slow evaporation technique. Earlier, Rodrigues et al. [17] have used glycine as a solute and trifluoroacetic acid as a solvent because of which the solution is more acidic resulting in faster growth rate (growth period few hours). Slower rate of crystal growth in the present case has resulted in large sized crystals. The photograph of asgrown single crystals is shown in Fig. 1(a; inset).
Fig. 1. (a) Structure of GTFA showing 50% probability ellipsoids and grown crystal of GTFA (inset). (b) Projection along the a-axis of GTFA crystal.
3.2. CHNS analysis The percentage of carbon, hydrogen and nitrogen contents in the single crystal of glycinium trifluoroacetate was analyzed using Vario EL III Elemental CHNS analyzer. The formation of glycinium trifluoroacetate was confirmed by CHN analysis. The sample of weight 6.7360 mg is taken for CHN analysis. The calculated theoretical values are C¼25.408%, H ¼3.198%, N¼7.408%; observed experimental values are C¼25.400%, H ¼3.514%, N¼7.512%. Both are in good agreement with each other. 3.3. Single crystal X-ray diffraction Single-crystal X-ray diffraction analysis for the grown GTFA crystal has been carried out to confirm the crystallinity and also to identify the unit cell parameters. It exhibits monoclinic crystal system with the space group of P121/n1. The crystal structure was solved and refined using the program SIR-92(WINGX) [18] and SHELXL-97 (WINGX) program [19]. The results are in agreement with the earlier reported values [17]. The crystallographic data and structure refinement of GTFA are given in Table 1. The ORTEP drawing (Fig. 1a) was performed with ORTEPIII program [20]. The projection along a-axis of GTFA is shown in Fig. 1b. 3.4. XPS analysis In order to see the composition on the surface, XPS measurements have been carried out using Al Ka (1486.6 eV) as an X-ray source. Fig. 2 shows the XPS survey scan of the GTFA, which manifest the presence of the fluorine (F) in the grown crystal as well as the presence of oxygen, nitrogen and carbon. The core
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Table 1 Crystal data and structure refinement for GTFA. Crystal Data Identification code Empirical formula Formula weight Temperature Wavelength
GTFA C4H6F3NO4 189.1 293(2) K 0.71073 A˚
Crystal system, space group Unit cell
Monoclinic, P121/n1 ˚ b¼ 12.262 (15) A, ˚ a¼ 4.9506 (5) A, ˚ a ¼ g ¼ 901, b ¼ 97.5471 c ¼11.5664 (13) A,
Volume Z, Calculated density Absorption coefficient, m Radiation type F(0 0 0) Theta range for data collection Crystal form, color Crystal size
696.05(14) A˚ 3 4, 1.805 Mg/m3 0.153 mm 1 Mo Ka 288 3.55–32.651 Rod, color less 0.31 0.30 0.28
Data Collection Diffractometer Data collection method Absorption correction Tmin Tmax Criterion for observed reflections Rint ymax (1) Limiting indices
Enhance o scans Multi scan 0.56914 1.00000 I 42 s(I) 0.0412 32.65 7r h r 7, 17r kr 17, 17 r lr 16
Refinement Refinement method Final R indices [I4 2 Sigma (I)] Goodness-of-fit on F2 No. of parameters H-atom treatment Extinction method Extinction coefficient
Full-matrix least-squares on F2 R1¼ 0.0533, wR2¼0.1354 1.019 131 Mixed Shelxl 1.31(5)
the UV–vis–NIR transmittance spectrum were recorded. The UV cut-off wavelength for the lysinium trifluoroacetate, argininium trifluoroacetate, histidinium trifluoroacetate crystals was found to be at 234, 232 and 242 nm, respectively, [21–23]. From the spectrum (Fig. 3a) of GTFA crystal, it is noted that the UV transparency cut off occurs at 220 nm and this is much lower than other materials with trifluoroacetic acid. GTFA crystal show low absorption throughout the entire visible region is one of the most desired properties for the fabrication of optoelectronic devices. 3.5.1. Determination of optical constants The extinction coefficient (K) can be obtained from the following relation: K¼
al 4p
The reflectance in terms of the absorption coefficient can be derived from the equation pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 1 7 ð1expÞðatÞ þ expðatÞ R¼ ð1expÞðatÞ From the above data the refractive index n can be derived as pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ðRþ 1Þ 7 3R2 þ10R3 n¼ 2ðR1Þ Fig. 3b represents the dependence of reflectance and the extinction coefficient on absorption coefficient. The internal efficiency of the device also depends upon the absorption coefficient. Hence by tailoring the absorption coefficient and tuning the band gap of the material, we can achieve the desired material that is suitable for fabricating various layers of the optoelectronic devices as per our requirements [24]. 3.6. FTIR analysis
Fig. 2. XPS spectra of GTFA crystals.
level spectra for this sample have been shown as an inset of the Fig. 2.
To analyze the presence of functional groups in GTFA qualitatively, FTIR spectra were recorded in the range 400–4000 cm 1 using Perkin-Elmer grating infrared spectrometer and the recorded spectrum is shown in Fig. 3c. In the high energy region, there is a broad band between 2100 and 3500 cm 1. For GTFA crystal, the broad band at 3081 cm 1 in the IR spectrum is assigned to asymmetric stretching mode of NH3þ . The symmetric and asymmetric deformation vibrations of the NH3þ group appear in the region between 1680 and 1470 cm 1. The strong absorption band due to NH3þ asymmetric deformation occurs at 1527 cm 1 [25–27]. The band caused by this type deformation of CH2 group is observed at 1440 cm 1 as a strong line in the IR spectra [25,27]. For the complex, the infrared bands of COO symmetric str. are observed at 1377 cm 1 as strong lines. For free trifluoroacetic acid, there are two characteristic stretching vibrations to the bands at 1255 and 1181 cm 1 in the infrared spectrum [28]. In the present infrared spectrum the bands at 1255 and 1178 cm 1 as strong lines are assigned to C–F stretching. The strong lines of C–F stretching mode indicate the existence of trifluoroacetate anion. We assign the strong band at 1037 cm 1 to asymmetric stretching CCN. The COO scissoring (652 cm 1), wagging (603 cm 1) and NH3þ torision (520 cm 1) modes are also observed and assigned [25,29]. 3.7. Thermal analysis
3.5. Optical properties To determine the optical transmittance range and hence to know the suitability of GTFA single crystal for optical applications,
TG and DTA analyses are of immense importance as they provide information about thermal stability and melting point of the material, which is significant for device fabrication and
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Fig. 3. (a) Optical transmission spectrum & Optical band gap determination using. UV–vis absorption data (inset). (b) Absorption coefficient versus reflectance, refractive index and extinction coefficient (inset). (c) FTIR spectra of GTFA Crystal.
application. The powdered GTFA with sample weight 7.985 mg was taken in an alumina crucible for the measurement. The TG/DTA spectra are shown in the Fig. 4a. A gradual loss of weight in the region between 101 and 449 1C in TGA was observed. The DTA trace shows an endothermic peak at 152.44 1C corresponding to the melting of the sample. This is immediately followed by sharp endothermic peak at 215.53 1C coinciding with the major weight loss due to decomposition which/and can be assigned to the absorption of energy for the breaking of bands resulting in complex formation of the residues. The sharpness of these endothermic peaks confirms a high degree of crystallinity of the sample. It is observed that there is no weight loss in the sample upto 101 1C implying thermal stability of the sample upto this temperature. Variation of specific heat with temperature in the range RT-450 1C is shown in Fig. 4b. In this curve also the same peaks are observed as in the DTA curve. The specific heat at room temperature is found to be 0.0791 Jg 1 1C 1. The specific heat corresponding to melting and complex formation peaks are calculated as 21.29 and 14.84 Jg 1 1C 1, respectively.
3.8. Dielectric studies The dielectric constant of the sample was measured in the applied frequency range 100 Hz–2 MHz at different temperatures (30–90 1C). Fig. 5a shows the variation of dielectric constant with frequency and temperatures. It is also observed that dielectric constant increases with increase in temperature which is attributed to the presence of space charge polarization near the grain boundary interfaces, which depends on the purity and perfection of the crystal [30]. In general, the dielectric constant decreases with increasing frequency and attains a constant value, depending on the fact that beyond a certain frequency of the electric field, the dipole does not follow the alternating field [31,32]. The dielectric constant has a higher value (9.08) in the lower frequency region and it then decreases (1.46) with high frequency at 90 1C. Fig. 5b suggests that the dielectric loss strongly depends on the frequency of the applied field, similar to what commonly happens with the dielectric constant. The dielectric constant of materials is due to the contribution of electronic, ionic, dipolar and space charge polarizations, which
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Fig. 4. (a) TG-DTA spectra of GTFA crystal. (b) Specific heat curve for GTFA.
depend on frequencies [33]. At lower frequencies, all these polarizations are active. The space charge polarization is generally active at lower frequencies and at high temperatures [34]. In the case of GTFA crystal, the dielectric loss is as low as 0.012 at 2 MHz. The behavior of dielectric losses in the low frequency range depends on many factors which can be controlled (growth rate, defects, size of crystal, etc.) [35]. Low dielectric loss shows that the crystals are of enhanced optical quality.
4. Conclusion Single crystals of glycinium trifluoroacetate, an organic material of the amino-carboxyl acid family, were successfully re-synthesized and grown by aqueous solution slow evaporation technique. The crystal is transparent and colorless. Single crystal XRD shows that the obtained GTFA crystals have the same crystal structure as previously reported. FTIR spectral analysis confirms the presence of functional groups constituting GTFA such as NH2þ , CH2, C–N and COO . An optical study confirms that glycinium trifluoroacetate crystal is transparent in the wavelength region 190–1100 nm with the UV transparency cutoff at 220 nm, which is much lower than other materials with trifluoroacetic acid. The optical investigations such as extinction
Fig. 5. (a) Variation of dielectric constant with frequency at different temperature of GTFA crystal. (b) Variation of dielectric loss with frequency at different temperature of GTFA crystal.
coefficient (k) ,refractive index (n) and reflectance (R) indicate the high transparency of the crystal and confirm its suitability for optical device fabrication. TG/DTA reveals that this compound is stable up to 152.44 1C. The specific heat at room temperature is found to be 0.0791 Jg 1 1C 1. The specific heat corresponding to melting and complex formation peaks are found to be 21.29 and 14.84 Jg 1 1C 1, respectively. The dielectric constant has a higher value at high temperature in the lower-frequency region and it then decreases at room temperature with the applied high frequency.
Acknowledgment Financial support received through DU DST PURSE Grant for ‘‘Growth of single crystalsy. various methods’’ is thankfully acknowledged. Neelam Singh is thankful to Council of Scientific and Industrial Research (CSIR), India for providing the Senior Research Fellowship (SRF).
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