International Journal of Current Research and Review (IJCRR)

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IJCRR - Vol 04 Issue 17, September

Pages: 122-132

Date of Publication: 14-Sep-2012

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Author: N.Balasundari , P.Selvarajan, S.Lincy Mary Ponmani D.Jencylin

Category: Technology

Abstract:The growth of picric acid-doped Zinc tris (thiourea) sulphate ( ZTS) single crystals were grown from aqueous solution by slow evaporation technique. 1.5 mol% of picric acid was added in saturated solution of ZTS. When picric acid was added as dopant, morphological alterations were noticed in the grown crystals. Cell parameters of the grown crystals were obtained from the XRD analysis and the presence of functional groups was identified by FTIR study. Its optical properties were examined by UV-vis analysis. The microhardness values were measured for the grown crystals and discussed.

Keywords: ZTS; doped ; Solubility; crystal growth; FTIR; microhardness;SHG.

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Zinc (tris) thiourea sulphate (ZTS) is a semiorganic NLO material for second harmonic generation from metal complexes of thiourea. High-damage threshold and wide transparency make it a better alternative for KDP crystals in frequency-doubling and laser fusion experiments [1,2]. SHG efficiency of ZTS crystal is 1.2 times more than that of KDP [3,4]. ZTS crystal belongs to the orthorhombic system with the space group Pca21. The growth of ZTS crystals from aqueous solution and its characterization have been reported in a number of recent publications [5-9]. Picric acid is one of the organic compounds having tendency to form the stable picrate compounds with various organic molecules. It has been reported that doping NLO crystals with organic and inorganic additives can alter various physical properties and doped–NLO crystals may find wide applications in optoelectronic devices compared to pure NLO crystals [10,11]. Keeping this in mind, pure and picric aciddoped Zinc (Tris) Thiourea sulphate (ZTS) crystals have been crystallized and studied in the present work. The results of the growth, solubility, XRD studies, FTIR studies, UV-Vis analysis, and microhardness studies of the grown crystals are reported in this paper. 2.Experimental methods and instrumentation 2.1.Synthesis and Growth Pure crystal of Zinc (tris) Thiourea sulphate (ZTS) was synthesized by dissolving high purity AR grade Zinc Sulphate heptahydrate and thiourea in molar ratio 1:3 in double distilled water. The solution was stirred by a magnetic stirrer and white crystalline ZTS salt was obtained on heating at 45 oC for a long time. The salt was purified by repeated recrystallization. To obtain picric acid-doped ZTS salt, 1.5 mol% of picric acid was added to solution of ZTS and the solution was heated at 60 oC . Single crystals of ZTS and picric aciddoped ZTS were grown from saturated solutions of synthesized salts by the solution growth employing slow evaporation technique at room temperature (31 oC). The good quality transparent and colorless pure ZTS crystal was harvested in 20 -25 days. Transparent and pale yellow colored picric acid-doped ZTS crystals were harvested within 20 days. The photograph of grown crystals is shown fig. 1.

2 Instrumentation Single crystal X-ray diffraction studies were carried out for the grown crystals of this work using ENRAF NONIUS CAD-4 X-ray diffractometer with MoKα (λ=0.71069 Å) radiation to evaluate the structural properties. The transmission properties of the crystals were examined using Lambda 35 model Perkin Elmer double beam UV-vis-NIR spectrometer in the range from 190 nm to 1100 nm. Optically polished single crystal of thickness 2 mm was used for this study. The Fourier transform infrared spectrum (FTIR) of the sample was recorded in the region 400-4000 cm-1 with Perkin Elmer Fourier transform infrared spectrometer (Model : Spectrum RXI) using KBr pellet. Microhardness measurement was carried out using Vickers microhardness tester fitted with a diamond indentor. The Second Harmonic Generation (SHG) conversion efficiency was tested using a set-up of Kurtz and Perry [12] and it was carried out using Qswitched mode locked Nd:YAG laser with first harmonic output at 1064 nm. The grown sample was powdered with uniform particle size of about 600 ?m using a ball mill and the powdered sample was packed densely between two transparent glass slides. The fundamental laser beam of 1064 nm wavelength, 8 ns pulse with 10 Hz pulse rate was made to fall normally on the sample cell. The emitted green light from the sample was detected by a photomultiplier tube (PMT) and displayed on a Cathode Ray Oscilloscope (CRO). KDP crystal was powdered into identical size as that of the sample of this work and it was used as reference material in the SHG measurement. 3.Results and discussion 3.1.Solubility studies Solubility study for the synthesized salts was carried out using a hot-plate magnetic stirrer and a digital thermometer. The procedure for finding solubility is reported elsewhere [13]. The variation of solubility with temperature for the samples is shown in figure 2. It is observed from the results that the solubility increases with temperature for both the samples and it is found to be more for picric acid doped ZTS sample. It is clear that for the picric acid -doped sample, the solvent is able to accommodate a marginally increased amount of solute for the saturation at the same temperature. Since solubility increases with temperature, the samples of this work have positive temperature coefficient of solubility. The increase in solubility for the picric acid-doped sample may be responsible for the change in the growth rate and morphological change observed in the present work.

3.2. Single crystal XRD studies
The grown crystals were subjected to single crystal XRD analysis and single crystalline data were obtained. From the data, it is observed that pure and picric acid-doped ZTS crystals crystallize in orthorhombic system and the unit cell parameters are listed in table 1. The obtained values of lattice parameters for the pure ZTS crystal are found to be in good agreement with the reported literature [5]. The changes in the lattice parameters are due to incorporation of picric acid in the lattice of ZTS crystal.

3.2 Powder XRD Analysis
The powder X-ray diffraction (XRD) pattern picric acid-doped ZTS crystal are shown in the figure 3. The well-defined peaks at specific 2θ values show high crystallinity of the grown crystals. All the reflections of powder XRD patterns of this work were indexed using the TREOR software package following the procedure of Lipson and Steeple [14]. The values of hkl, relative intensity and 2 theta values for the reflection peaks of the powder XRD pattern are given table 2. Using the values in the table 2 and the UNITCELL software package, the cell parameters were found and it is observed that the values of unit cell parameters obtained from powder XRD method are in comparable with those obtained from single crystal XRD method.

3.3 FTIR Spectral studies The FTIR analysis was carried out by Perkin Elmer FTIR spectrometer by KBr pellet technique in the range 500-4000 cm-1 . The FTIR spectra of pure and picric acid mixed ZTS samples are shown in figure 4. In ZTS complex, there are two possibilities by which the coordination with metal can occur. It may be either through nitrogen or through sulfur. From spectra, the N-H, absorption bands in the high frequency region in thiourea were not shifted to lower frequencies on formation of metal thiourea complex, thus coordination of thiourea occurs through sulfur in ZTS [11]. The NH, C=S and N-C-N stretching vibrations were also seen. The comparison of the samples shows slight shift in characteristic vibrational frequencies of 1.5 mol% picric acid doped ZTS with respect to pure ZTS. This confirms the addition of picric acid in grown crystal. The spectral assignments for the picric acid mixed ZTS sample are provided in the table 3.

3.4.UV-Visible spectral study The study of optical transmission range of grown crystals is important because the crystals are mainly used for optical applications. The UV–visible study of grown crystal was carried out by Varian-Cary 5E UV-vis Spectrometer in a range 200 nm to 1100 nm. The absorption spectrum of 1.5 mol% picric acid–doped ZTS is shown in fig.6 and the inset figure shows UV spectrum of pure ZTS for comparison. The absorption spectrum reveals that the crystal has lower cutoff wavelength at around 290 nm. The absorption near UV region is associated electron with transition within thiourea units of ZTS. Doping of 1.5 mol % of picric acid in ZTS does not destroy the transparency of the crystal. From spectra it is observed that the lower cutoff wavelength is almost the same for picric acid doped ZTS and pure ZTS crystals. The wide range of transparency in UV-visible and IR region enables good transmission of the second harmonic frequencies of Nd:YAG laser. This is an added advantage of the grown crystals of this work in the field of optoelectronic applications.

3.5.Measurment of microhardness
Microhardness of a crystal is its capacity to resist indentation. Physically hardness is the resistance offered by a material to the localized deformations caused by scratching or by indentations[15].The selected smooth surfaces of the grown crystals were used for microhardness measurement at room temperature using a Vickers microhardness tester fitted with a diamond indenter. The hardness of the crystal is calculated using the relation The hardness of the crystal is calculated using the relation Hv = 1.8544 P / d2 kg / mm 2 where P is the applied load in kg and d is the length of indentation impression in millimeter and 1.8544 is a constant of a geometrical factor for the diamond pyramid [16]. A plot drawn between the hardness value and corresponding loads is shown in figure 6. It is noticed that Vickers hardness number (Hv) increases with the applied load satisfying the indentation size effect. When a ZTS crystal is doped with picric acid, the hardness decreases and this decrease in the hardness value of doped sample can be attributed to the incorporation of picric acid in the lattice.


4. Conclusion
Single crystals of pure and picric aciddoped Zinc (Tris) Thiourea sulphate(ZTS) were synthesized and solubility studies were carried out for the prepared samples in de-ionized water in the temperature ranging from 30 to 50 oC. In accordance with the solubility data, saturated solutions were prepared for growing pure and picric acid doped ZTS crystals by slow evaporationtechnique. Morphological alterations have been observed when ZTS crystals are doped with picric acid. The powder and single crystal XRD studies confirms the orthorhombic structure of the grown crystal..The unit cell parameters of pure ZTS crystals are in agreement with the literature values. The FTIR spectrum confirms the presence of all the functional groups in the grown crystals. UV-visible study reveals the picric acid doped ZTS crystal has lower cutoff wavelength at around 290 nm. Mechanical property of the grown crystals has been studied by microhardness test and noticed that there is a decrease of microhardness number for 1.5 mol% of picric acid-mixed ZTS crystals. Acknowledgement The supports extended in the research by Sophisticated Analytical Instrumentation Facility (SAIF), IIT, Chennai, RRL, Trivandrum and M.K.University, Madurai are gratefully acknowledged. We thank authorities of Aditanar College of Arts and Science, Tiruchendur and Infant Jesus College of Engineering, Keela Vallanadu, Tuticorin for the encouragement given to us to carry out the research work.


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