CHAPTER 3 EFFECT OF L -LYSINE MONOHYDROCHLORIDE DIHYDRATE ON THE GROWTH AND PROPERTIES OF ADP SINGLE CRYSTALS

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1 55 CHAPTER 3 EFFECT OF L -LYSINE MONOHYDROCHLORIDE DIHYDRATE ON THE GROWTH AND PROPERTIES OF ADP SINGLE CRYSTALS 3.1 INTRODUCTION As mentioned in the previous chapter, ammonium dihydrogen orthophosphate having superior properties has been exploited for variety of applications. It is a representative of hydrogen-bonded materials, which possess important piezoelectric, ferroelectric, dielectric, electro-optic and nonlinear optical properties (Xue and Siyuan 1996, Xue and Zhang 1999). Applications of integrated electro-optics include high-speed modulation and switching of optical signals for telecommunications and signal processing (Xu and Xue 2006). The process of crystallization represents an important separation and purification technique for the chemical industry, so its control in the manufacture has been an intriguing field for a long time and has recently received considerable attention from both theoretical and experimental standpoints (Xu and Xue 2006). Several researchers have carried out a lot of studies on pure and doped ADP crystals (Yang et al 1999, Asakuma et al 2008). Also, some of the amino acids are used as dopants and they enhanced the material properties like nonlinear optical and ferroelectric properties, for example, enhancement in SHG efficiency has been reported in L- arginine doped KDP crystals (Parikh et al 2007). The effects on various properties of L-threonine,

2 56 dl-threonine and L-methionine admixtured TGS crystals were studied and the authors reported that, the admixtured TGS crystal has different properties compared to pure TGS crystal (Meera et al 2004). Semiorganic crystals are playing essential roles in NLO applications. Several semiorganic crystals like L-histidine tetrafluoroborate, L-arginine phosphate (Dhanuskodi et al 2004), L-arginine tetrafluoroborate (Aggarwal et al 2003) and L-histidine nitrate (Zhang et al 2008) were grown and reported for their enhanced NLO efficiency. L-arginine monohydrochloride added KDP crystals were grown, in which, increased growth rate, enhancement in metastable zone width and induction period and higher hardness have been achieved (Dhanaraj et al 2008a). L-Lysine monohydrochloride dihydrate is a potential material to produce semi-organic crystals for nonlinear optical applications (Ramesh Babu et al 2006). However, there is no report available in literature on the effect of L-Lysine monohydrochloride dihydrate doping on the crystal growth and various properties of ADP. The L-lysine monohydrochloride dihydrate crystallizes in monoclinic crystal system with P21 space group (Ramesh Babu et al 2006). ADP is tetragonal with the space group I 42m. It is interesting and important to study the semi organic doping in inorganic material. Hence, in our laboratory efforts were made to grow pure and L-Lysine monohydrochloride (L-LMHCl) dihydrate added ADP single crystals from its aqueous solutions. In order to explain the effect of the addition of L-LMHCl dihydrate, it is required to have data for various properties. This chapter presents the growth, structural, optical, thermal and dielectric properties of L-Lysine monohydrochloride dihydrate added ADP single crystals.

3 CRYSTAL GROWTH The commercially available ADP and L-LMHCl dihydrate were used for growth after 2 times of recrystallization process. Single crystals were grown from aqueous ADP solution containing 1, 2 and 3 mol% of L-LMHCl dihydrate using deionized water as a solvent by the slow evaporation technique. All the crystals were harvested after 10 days and there was not significant variation seen in the growth rate of the crystals. But physical observation of the grown crystals showed that the 2 mol% of L-LMHCl dihydrate added crystal has good transparency and morphology. Keeping this in our mind the saturated solution of 2 mol% L-LMHCl dihydrate added ADP was prepared at 42 C and the selected seed crystal of size 5 x 4 x 3 mm 3 grown by the slow evaporation technique was placed inside the solution and kept in the constant temperature bath and the growth run was carried out between the temperatures 42 and 35 o C with the cooling rate 0.5 o C per day. A colourless bulk crystal of size 35 x 12 x 15 mm 3 was harvested after 14 days. The growth rate was 1 mm along the (0 0 1) direction and 0.5 mm per day along the (1 0 0) direction. It is observed that the growth rate along the (0 0 1) direction is normally higher than the (1 0 0) direction. The grown crystal of L-LMHCl dihydrate added ADP is shown in Figure 3.1. Using the same material ingredients pure ADP was grown and it is shown in the Figure 3.2 (a). 2-mol% L-LMHCl dihydrate added ADP grown by the slow evaporation method is shown in the Figure 3.2 (b).

4 58 Figure 3.1 L-LMHCl added ADP single crystal by slow cooling method (a) (b) Figure 3.2 Crystals of (a) pure ADP (b) L-LMHCl added ADP 3. 3 CHARACTERIZATION X-Ray Diffraction Analysis Lattice parameters were calculated using the single crystal XRD. The unit cell parameters obtained from the single crystal XRD for pure ADP

5 59 are a = b = Å, c = Å, = = = 90 and it belongs to tetragonal system, and for doped ADP the unit cell parameters are a = b =7.504 (2) Å, c = 7.558(2) Å = = = 90 V = 425.1(2) Å 3. The powder X-ray diffraction studies were done. The spectrum is shown in the figure 3.3. The prominent peaks of pure ADP are (1 0 1), (2 0 0), (1 1 2), (2 0 2), (3 0 1) and (3 1 2). The obtained peaks for the doped crystals are similar with the pure ADP crystal. Comparing the XRD spectrum of the pure and L-LMHCl added ADP crystal, slight variation in the intensity is observed. But no shift in the angle (2 ) is observed. It reveals that the structure of ADP is not distorted when 2 mol% L-LMHCl dihydrate is added with ADP solution. Both single and powder crystal XRD studies show that the dopant has not entered into the lattice sites of ADP. The observed values are in good agreement with the reported values (Xu et al 2005, Xu and Xue 2008a) 2000 L-LMHCl dihydrate added ADP [1 1 2] Intensity (CPS) L-LMHCl dihydrate added ADP [1 0 1] [2 0 0] [2 0 2] [3 0 1] [3 1 2] [2 0 0] Pure ADP [1 0 1] [1 1 2] [2 0 2] [3 0 1] [3 1 2] (Degree) Figure 3.3 XRD patterns of pure and L-LMHCl added ADP single crystal

6 FTIR Analysis The FT-IR spectrum was recorded for pure and doped crystals using JASCO FT-IR 410 spectrometer by the KBr pellet technique in the range cm -1 and it is shown in figure 3.4. The FTIR spectrum of L-LMHCl dihydrate doped ADP was identical to that of pure ADP and hence no change in the functional groups could be detected. This again confirms that L-LMHCl has not entered into the crystal lattice. Similar results were reported in the L-arginine monohydrochloride added KDP crystals (Dhanaraj et al 2008a). L-LMHCl added ADP Transmittance (%) Pure ADP Wavenumber (cm -1 ) Figure 3.4 FTIR Spectra of pure and L-LMHCl added ADP single crystal

7 UV-Vis-NIR Spectral Analysis The UV-Vis-NIR spectral studies were carried out using Perkin- Elmer Lambda UV-Vis-NIR spectrometer in the range nm. L-LMHCl added ADP crystal plates were cut from the various parts of the crystal with a thickness of about 3 mm, polished at <1 0 0> direction without any antireflection coating for optical measurements. The higher transmittance has been observed in the entire visible region for all the plates and confirms that the quality of the crystal is good. The large transmission in the entire visible region enables it to be a good candidate for electro-optic applications. The spectrum is shown in the Figure Transmittance (%) Pure ADP L-LMHCl added ADP Wavelength(nm) Figure 3.5 UV-Vis-NIR spectra of pure and L-LMHCl added ADP single crystal

8 Thermal Analysis Figure 3.6 shows the TG/DTA spectra for pure and L-LMHCl added ADP crystals. The DTA curve shows an endothermic peak at 215 C for the pure ADP and at 204 C for the L-LMHCl dihydrate added ADP. At the same time weight loss started for the pure ADP at 197 C and for the L-LMHCl dihydrate added ADP at 194 C. It is observed from the figure that the decomposition temperature of the ADP is decreased by 11 C. The measurement was repeated several times and same results were observed Weight % Pure ADP L-LMHCl added ADP Microvolt endodown ( V) o C 215 o C Temperature ( o C) Figure 3.6 TG-DTA curves of pure and L-LMHCl added ADP single crystal

9 Dielectric Measurements To study the effects of the addition of L-LMHCl dihydrate on the antiferroelectric properties of ADP, dielectric measurements were performed. The cut and polished single crystal (8x8x3mm 3 ) of L-LMHCl dihydrate added ADP was used for dielectric studies. Two opposite surfaces across the breadth of the sample were treated with good quality silver paste in order to obtain good Ohmic contact. Using the LCR meter, the capacitances of these crystals were measured for frequencies 100, 1k, 100k and 1 MHz at various temperatures. The dielectric constants of the crystal were calculated using the formula given in the equation (1.3) Hz 1 KHz 100 KHz 1 MHz Dielectric Constant Temperature ( 0 C) Figure 3.7 Dielectric constant of the L-LMHCl added ADP single crystal

10 Hz 1 KHz 100 KHz 1 MHz Dielectric Loss Temperature ( o C) Figure 3.8 Dielectric loss of the L-LMHCl added ADP single crystal Figures 3.7 and 3.8 show respectively the dielectric constant ( r ) and the dielectric loss of the doped crystal for different frequencies with various temperatures. From the figure, it is found that the values of dielectric constant and dielectric loss increase with the increase in temperature and decrease with the increase of frequency. This may be due to the contributions of all the four polarizations such as electronic, ionic, dipolar and space charge, which are predominant in the lower frequency region (Meena and Mahadevan 2008). Also, in the lower frequency at higher temperature the space charge polarization is active. At 120 o C, the dielectric constant is 21 and at 40 o C it is only 12 for 100 Hz. This may be due to the term contributing to dielectric constant from ion-dipole interactions being compensated by the thermal energy leading to the relaxation polarization. It is also observed that the dielectric constant of the L-LMHCl added ADP is increased compared to the

11 65 pure ADP. The low values of dielectric loss indicate that the grown crystal contains minimum defects. The obtained values are in agreement with the reported values (Anandha babu et al 2008) Microhardness Studies The good quality crystals are needed not only with good optical performance but also with good mechanical behaviour (Marchewka et al 2003). The indentation hardness was measured as the ratio of applied load to the surface area of the indentation. The grown pure and L-LMHCl dihydrate added crystal of size 10 x 10 x 4 mm 3 with smooth and dominant face (1 0 0) was selected for microhardness studies. Indentations were carried out using Vickers indenter for varying loads Pure ADP L-LMHCl added ADP 62 Hardness Kg/mm Load (g) Figure 3.9 Vickers hardness for pure and L-LMHCl added ADP single crystal

12 66 For each load (p), several indentations were made and the average value of the diagonal length (d) was used to calculate the microhardness. Vickers microhardness number was determined using H v = p/d 2. The hardness number was found to increase with the load. A plot drawn between the hardness value and corresponding loads is shown in Figure 3.9. It is observed from the figure that hardness increases with increase in load and the cracks have been observed at 100 g for both of the crystals. It is noted that, L-LMHCl dihydrate has not entered into the lattice of pure ADP and the hardness value is increased. Similar behaviour was already reported in the L-arginine monohydrochloride added KDP crystals (Dhanaraj et al 2008a). 3.4 CONCLUSIONS Good quality ADP crystals were grown from 2 mol% of L-Lysine monohydrochloride dihydrate added ADP solution. The cell parameters estimated in this work agree well with the reported values. From the XRD study it is found that the addition of L-LMHCl dihydrate does not change the structure of ADP and the crystallinity is good. The optical studies show that the crystal is transparent in the region of nm and the decomposition temperature is decreased. Dielectric studies reveal that the grown crystal has dielectric constant slightly higher than the pure ADP. Higher hardness value is obtained for the L-Lysine monohydrochloride dihydrate added ADP than the pure ADP crystal.