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62 CHAPTER 4 GROWTH OF SEMIORGANIC TRIGLYCINE CALCIUM DIBROMIDE CRYSTAL AND ITS CHARACTERIZATION 4.1 INTRODUCTION Amino acids with inorganic compounds are promising materials for nonlinear optical applications, as the high optical nonlinearity of the purely organic amino acid tends to combine with the favorable mechanical and thermal properties of the inorganic salt. Nonlinear optical (NLO) materials are used in optical computing, optical communication, harmonic generators, medical diagnostics, frequency mixing and optical switching (Prasad and Williams 1991, Iwai et al 1997, Lin et al 1999). The potential development of optoelectronic devices based on the nonlinear polarization of molecular materials has aroused much recent interest (Eaton 1991, Dorn et al 1992). The search for large second-order electric susceptibilities (that is, proportional to the square of an applied electric field) has concentrated on acentric organic or organometallic chromophores with an organic -electron system coupling electron donor and acceptor groups (Laidlaw et al 1993). Theoretical studies also show that hydrogen bonds can strongly enhance the nonlinear optical responses of bulk materials (Dongfeng Xue and Siyuan Zhang 1998, Dongfeng Xue and Siyuan Zhang 1999, Dongfeng Xue and Henryk Ratajczak 2003, Dongfeng Xue and Henryk Ratajczak 2005), which widely exist in semiorganic crystals. Many semiorganic nonlinear optical materials have been grown by slow solvent evaporation technique, which are attracting a great 63 deal of attention in the nonlinear optical field from application point of view. L-Hystidine Tetra-Fluoro-Borate (L-HFB) is a semiorganic nonlinear optical material whose single crystal exhibits more NLO properties than that of inorganic crystals like KDP, BBO, LBO and many other semiorganic materials (Sinha et al 2009). 4.2 PRESENT WORK The crystal structures of many addition compounds of glycine with inorganic acids and metallic salts and also many metal complexes of glycine are known. Because of the biological and chemical importance of both glycine and calcium, a series of glycine complexes with calcium halides, viz CaC12, CaBr2 and CaI2, were crystallized and their structures were studied (Mohana Rao and Natarajan 1980). In this chapter we are presenting growth and characterization of triglycine calcium dibromide such as XRD, optical, thermal, dielectric and mechanical studies. 4.3 GROWTH OF TRIGLYCINE CALCIUM DIBROMIDE Single crystals of triglycine calcium dibromide were grown from a saturated aqueous solution containing glycine (Merck) and calcium dibromide (Lobo) in the molar ratio 1:1. A necessary quantity of glycine is taken in a beaker and dissolved in Millipore water of resistivity 18.2M cm at room temperature until it attains saturated condition. After preparing saturated solution of glycine, the proportionate amount of calcium dibromide was added little by little with continuous stirring of the solution for bringing a homogeneous mixture. Due to exothermic reaction, the temperature of the solution increases to 50 oC. The prepared solution of pH value 5.4 was filtered and allowed to evaporate slowly at ambient temperature. Colorless good optical single crystals of triglycine calcium dibromide of size 20 × 12 × 4 64 mm3 were harvested in forty days. The photograph of the as grown single crystals of the same is shown in the Figure 4.1. The title compound triglycine calcium dibromide is formed from the starting 1:1 molar ratio solution of glycine and calcium dibromide due to the more lattice energy benefit for 3:1 than 1:1 coordination of glycine and calcium dibromide. The expected chemical reaction for triglycine calcium dibromide is as follows: 3NH2CH2COOH + 3CaBr2 [Ca(C2H5NO2)3]2+.2Br¯ + 2CaBr2 (in solution) (4.1) Figure 4.1 Photograph of as grown triglycine calcium dibromide crystal 4.4 CHARACTERIZATION STUDIES 4.4.1 Single Crystal X-ray Diffraction X-ray diffraction studies were carried out to reveal the crystal structure of the grown crystal. The unit cell parameters obtained are a = 9.095(6)Å, b = 14.752(3)Å, c = 20.217(20)Å, = 90.34(6)°, 65 = 90.46(7)°, = 90.07(4)° and volume = 2713(3) Å3. Triglycine calcium dibromide belongs to orthorhombic crystal structure with the space group Pbc21. When the obtained cell parameters are compared, the result is in good agreement with the reported values (Mohana Rao and Natarajan 1980). 4.4.2 Fourier Transform Infrared Analysis Theoretical studies carried out by both Phillips-Van Vechten- Levine-Xue bond theory and chemical bonding theory of single crystal growth indicate that some important functional groups such as hydrogen bonds and ammonia group play a very important role in crystallization and nonlinear optical performances (Dongli Xu and Dongfeng Xue 2008, Xiue Ren et al 2008, Zdzislaw Latajka et al 2009). The Fourier transform infrared analysis of triglycine calcium dibromide single crystal was carried out between 4000 and 450 cm-1 using Perkin Elmer spectrum one FT-IR spectrometer. Figure 4.2 shows the resulting spectrum of that analysis, in which the functional groups present in the molecules, can be identified by stretching vibration. In this spectrum the broad and strong band obtaining between 3049 and 2598 cm-1 is due to the NH3+ stretching vibration (Robert Silverstein and Francis Webstar 1998). A weak asymmetrical NH3+ bending band occurs at 1627 cm-1 and a fairly strong symmetrical bending band occurs at 1522 cm-1. The carboxylate ion group COO- absorbs strongly at 1591 cm-1 and more weakly at 1426 cm-1 (John Coats 2000, Sharma 2007). These bands result from asymmetrical and symmetrical C( O)2 stretching respectively. This observation confirms that glycine molecules exist in zwitterionic form. The absorption at 515 cm-1 is due to the presence of oxygen-calcium bond (Bharat Parekh and Mihir Joshi 2007). Absorption peaks characterizing different functional groups are shown in the Table 4.1. 66 515 504 670 528 897 40 1125 2689 2598 60 3431 Transmittance % 1040 595 1830 80 2017 2386 2266 100 3049 1653 1627 15911522 142614801415 1331 20 0 4000 3500 3000 2500 2000 1500 1000 500 -1 Wavenumber (cm ) Figure 4.2 FTIR spectrum of triglycine calcium dibromide crystal Table 4.1 Frequencies of the fundamental vibrations of TGCB crystal Frequency in wavenumber (cm-1) 3049 Assignment of vibrations NH3+ stretching 1653 N-H bending 1627 NH3+ weak asymmetrical bending 1591 COO- asymmetric stretch 1522 NH3+ strong symmetrical bending 1426 COO- symmetric stretch 1331 C-H bending 1040 C-N stretching 897 O-H out of plane bending 670 C-Br stretching 515 Ca-O bond 504 torsional oscillations 67 4.4.3 UV-Vis-NIR Studies In order to reveal optical properties of the triglycine calcium dibromide single crystal, UV-Vis-NIR transmission spectrum was recorded in the range of 200 to 1100 nm using Perkin Elmer Lamda 35 UV/VIS spectrometer. The as grown crystal of 2mm thickness was used for recording the spectrum. Figure 4.3 shows the transmittance curve, in which the lower cut off region is obtained at 240 nm. Further it is found that the maximum transmittance of the grown triglycine calcium dibromide single crystal is 52% and it has almost more than 45% transmittance from 400 to 1100 nm. 60 50 Transmittance (%) 40 30 20 10 0 200 400 600 800 1000 1200 Wavelength (nm) Figure 4.3 UV-Vis-NIR spectrum of triglycine calcium dibromide crystal 4.4.4 Laser Damage Threshold Studies Laser damage threshold studies were made on the as grown triglycine calcium dibromide crystal of 2 mm thickness using Nd:YAG laser of 532 nm wavelength and spot size about 140 m in the method of multiple 68 shots mode. A lens of focal length 8 cm was used to focus the light spot on the crystal. The pulse width and repetition rate of the laser pulse was adjusted to 7 ns and 10 pulse/sec respectively. When the laser beam of energies 10 mJ and 20 mJ were made to be incident on the crystal for 30 seconds respectively, there were no remarkable changes. But, when the beam energy was adjusted to 30 mJ for 32 seconds, the crystal got damaged. 4.4.5 NLO Studies Since the process of second-harmonic generation (SHG) is relevant to new laser technology, and the electro-optic effect, SHG efficiency of the grown crystal was determined by Kurtz powder technique. The crystalline sample was ground into very fine powder and tightly packed in a microcapillary tube. Then the tube was placed in the path of Nd:YAG laser beam of energy 1.95 mJ/pulse. When potassium dihydrogem phosphate crystal in the form of powder was used as a reference material, the transmitted laser beam voltage was 62 mV. But it was observed that the output voltage was 32.8 mV for the triglycine calcium dibromide crystal. Hence the SHG efficiency of triglycine calcium dibromide crystal is half of that of KDP crystal. 4.4.6 Thermal Studies Simultaneous thermogravimetric analysis (TGA) and differential thermal analysis (DTA) were carried out between 30 °C and 1100 °C in nitrogen atmosphere at a heating rate of 10 °C/min using NETZSCH STA 409 C/CD TG/DTA instrument for the as grown crystals to determine the melting point and the thermal stability of the crystal. Figure 4.4 shows the resulting traces of TG/DTA for the triglycine calcium dibromide crystal. In the spectrum of thermogravimetric analysis, there is no weight loss upto 69 305 oC. Hence the triglycine calcium dibromide crystal is devoid of any physically adsorbed water on it and also it is observed that there is a sudden weight loss occurring at the temperature 305 °C. The DTA response curve too shows a sharp endothermic peak at the same temperature. Further a small quantity of the fine powdered sample was taken in a microcapillary tube and heated up using Monatch melting point apparatus for confirming the thermal stability. 2 305.128 80 Weight % 0 60 -2 40 Microvolt Exo Down (µv) TGA DTA 100 20 0 200 400 600 800 1000 -4 1200 o Temperature ( C) Figure 4.4 TG/DTA curves of triglycine calcium dibromide crystal From the thermal analysis, it is observed that triglycine calcium dibromide decomposes with melting at 305 °C temperature. Hence the grown triglycine calcium dibromide crystal is stable upto the temperature 305 °C and can be designed for device application upto the limitation of that temperature. 70 4.4.7 Dielectric Studies Dielectric studies of triglycine calcium dibromide single crystal were carried out as a function of temperature for various frequencies using Precision LCR meter AGILENT 4284A model. The as grown transparent crystal of 2 mm thickness was selected as a sample for studying dielectric constant. Typical area of the sample for the experiment was 63 mm2. In order to make a contact with the electrodes, both the crystal surfaces were coated with fine graphite powder. The prepared sample was placed between the two electrodes and heated from room temperature to 160 °C using thermostat. Then the crystal was annealed at thermostat itself. Capacitance and dielectric loss measurements were carried out with the sample as a function of temperature for the range of frequencies from 100 Hz to 1 MHz. Dielectric constant ‘ r’ was calculated directly using the formula r = Ccrystal / Cair as the area of the crystal is equal to area of the electrode. Figures 4.5 and 4.6 show the plots of dielectric constant and dielectric loss factor respectively. The current investigations showed that dielectric constant is maximum at 100 Hz since all types of polarization such as electronic, ionic, orientation and space charge polarizations occur at lower frequency. Dielectric loss factor too corresponds to the dielectric constant plots at the given experimental frequencies ranging from 100 Hz to 1 MHz. Further it is found that dielectric constant is 6.6 at 160 °C temperature. The orientation and ionic contributions are small at high frequencies due to the inertia of the molecules and ions (Charles Kittel 1993). So, the resultant amount of polarization increases with the decrease of frequencies. 71 7.0 6.5 100 Hz 1 KHz 10 KHz 100 KHz 1 MHz Dielectric Constant ( r) 6.0 5.5 5.0 4.5 4.0 3.5 3.0 2.5 40 60 80 100 120 140 160 o Temperature ( C) Figure 4.5 Dielectric constant of triglycine calcium dibromide crystal 0.7 100 Hz 1 KHz 10 KHz 100 KHz 1 MHz 0.6 Dielectric Loss 0.5 0.4 0.3 0.2 0.1 0.0 40 60 80 100 120 140 160 o Temperature ( C) Figure 4.6 Dielectric loss of triglycine calcium dibromide crystal 72 4.4.8 Mechanical Studies Mechanical properties of the grown triglycine calcium dibromide crystals were studied using HMV2T Microhardness tester. The Vickers hardness number of the crystal was calculated using the relation Hv = 1.8544 P/d2 kg/mm2, where Hv is the Vickers microhardness number, P is the applied load in kg and d is the average diagonal length of the impression in mm. The corresponding trace is shown in the Figure 4.7, from which it is observed that the hardness increases with the increase of load up to 100 g and crack occurs at that load. The maximum hardness obtained in this material is 90 kg/mm2. In order to find work hardening coefficient (n) of the grown crystal, another graph (Figure 4.8) was drawn between logarithmic values of load and diagonal length of indentation. Work hardening coefficient or Meyer index was calculated from the relation P = adn, where ‘a’ is the constant for the given material. The work hardening coefficient “n” was calculated as 3.3. According to Onitsch (Onitsch 1947), if n lies between 1 and 1.6, then the grown crystal will be a harder material and it is more than 1.6 for soft materials (Ramesh Babu et al 2003, Rajesh et al 2004). Since the calculated work hardening coefficient ‘n’ is more than 1.6, the grown crystal is suggested that it comes under the category of soft material. Yield strength of the grown triglycine calcium dibromide single crystalline material was also calculated using the formula (Ramesh Babu et al 2003) where y y = ( Hv /3)(0.1)n-2, is the yield strength, Hv is the Vicker’s microhardness and n is the logarithmic exponent. It was found to be 1.5 MPa from the relation and hence the grown triglycine calcium dibromide single crystal has low mechanical strength. 73 2 Vicker's Hardness (kg/mm ) 90 80 70 60 50 40 20 30 40 50 60 70 80 90 100 110 Load (gram) Figure 4.7 Hardness Vs load graph of triglycine calcium dibromide crystal 1.66 1.64 Log d (mm) 1.62 1.60 1.58 1.56 1.54 1.52 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2.0 Log P (gram) Figure 4.8 Plot of log P and log d of triglycine calcium dibromide crystal 74 4.5 CONCLUSION Semiorganic nonlinear optical crystal triglycine calcium dibromide has been grown by slow solvent evaporation technique from aqueous solution of glycine and calcium dibromide at room temperature. The unit cell parameters estimated by single crystal X-ray diffraction analysis agree with the reported values. Various functional groups present in the crystal were identified using FTIR spectrum. Optical studies reveal that the maximum transmittance under UV-Vis-NIR radiation is 52% and the material has good transparency in the entire visible region. The crystal gets damaged at 30 mJ laser beam energy when it was subjected to Laser damage threshold testing using multiple shots mode method. The grown crystal has NLO efficiency half of that of KDP crystal and has good thermal and mechanical stabilities. Its decomposition and melting temperature is at 305 °C suggesting that it has higher thermal stability. From the microhardness investigations made on the grown crystalline material, the crystal has maximum surface hardness about 90 kg/mm2 at 100 g load and is soft material and having low yield strength.