Growth and characterization of urea-thiourea non-linear optical organic mixed crystal

Transcription

1 Indian Journal of Pure & Applied Physics Vol. 48, April 2010, pp Growth and characterization of urea-thiourea non-linear optical organic mixed crystal G Madhurambal 1 & M Mariappan 2 * 1 Dean of Science, A D M College for Women, Nagapattinam , Tamil Nadu 2 Assistant Professor in Chemistry, EGS Pillay Engineering College, Nagapattinam * mmari_101@yahoo.com, mmari101@gmail.com Received 15 July 2009; revised 7 December 2009; accepted 6 January 2010 Mixed crystals of urea-thiourea in various proportions have been grown in solution by slow evaporation technique at room temperature (30 C). The solubility studies have been carried out at room temperature. In the presence of high concentrations of the thiourea in the medium, the solubility decreases appreciably. X-ray diffraction study has been carried out to find the crystal system and unit cell parameters. The diffraction patterns reveal that there is change in basic structure of urea and thiourea. The presence of title compound in the crystal lattice has been qualitatively determined by FTIR analysis. Slight broadening is observed in FTIR of 0.66 and 0.9 urea-thiourea mixed crystal in the range cm 1. It is found that weak hydrogen bond between urea and thiourea is present in higher proportions of urea-thiourea mixed crystal. Keywords: Urea-thiourea mixed crystals, Solubility, FTIR, Non-linear optical material 1 Introduction Non-linear optic (NLO) materials showing second harmonic generation have been in demand over the last few decades due to technological importance in the fields of optical communication, signal processing and instrumentation 1-3. Among them, the nonlinearity 4 of urea is comparable with that of another important commercial NLO material potassium dihydrogen phosphate. Until the last decade the materials explored for NLO application were mostly inorganic. However, it was realized that inorganic materials have lower probability for a centric structure and consequently scientists focused their attention on organic materials. Urea crystals attract the attention of both theoreticians and experimentalist due to the nonlinear optical piezoelectric properties. Urea is representative of one class of materials, which are applicable to photonics and reference material in the DMOS (diffusive mixing of organic solution) experiment in microgravity carried out by NASA. They are potentially useful materials for frequency doubling of near IR laser radiation single crystals of the materials which have very high laser damage threshold. Non-linear optics is playing a major role in emerging photonic and optoelectronic technologies. New non-linear optical frequency conversion materials have a significant impact on laser technology and optical data storage. Thiourea is an interesting inorganic matrix modifier due to its large dipole moment 5 and its ability to form an extensive network of hydrogen bonds. It belongs to the orthorhombic crystal system. However, most of the thiourea complexes crystallize in centro symmetric form at room temperature and do not show second harmonic generation (SHG). Only a few of thiourea complexes viz, zinc thiourea sulphate 5-8, ATCC, cadmium thiourea acetate 9, bis thiourea cadmium chloride 10-13, allyl thiourea mercury bromide 14, thio semicarbazide cadmium bromide 15,16 and bismuth thiourea bromide 17 crystallize in non centro symmetric structure and show second harmonic generation. The search for new and efficient NLO materials has resulted in the development of a new class of materials called organic mixed crystals. In this paper, urea-thiourea mixed crystal is synthesized and grown by slow evaporation technique at room temperature. 2 Experimental Details The 0.1, 0.66 and 0.9 urea mixed thiourea crystals were grown by slow evaporation technique at room temperature. Urea (1 g) and thiourea (9 g), urea (3.4 g) and thiourea (6.6 g), urea (1 g) and thiourea (9 g) (AR Merck grade) were dissolved in triple distilled water. The proportion of the mixed crystal is expressed interms of less soluble salt. Mixed solutions were properly stirred for two hours and filtered to remove the suspended particles. The filtered solutions were poured into Petri dishes and kept for evaporation at ambient temperature. In solution growth technique, the size of a crystal depends on the amount of the material available in the solution, which in turn is decided by the solubility of the material in the

2 MADHURAMBAL & MARIAPPAN: GROWTH OF NON-LINEAR OPTICAL ORGANIC MIXED CRYSTAL 265 solvent. The solubility of synthesized 0.1, 0.66 and 0.9 urea-thiourea mixed crystal has been determined in water (Fig. 1). This was performed by adding water maintained at constant temperature to a known quantity of the material until the material was dissolved. Using this technique, the magnitude of solubility of various proportions of urea-thiourea mixed crystal has been evaluated for various temperatures between C. Morphology of urea, thiourea and urea-thiourea mixed crystals is shown in Fig. 2. Infrared spectroscopic studies were carried out on the grown crystal to understand the structure and bonding in them. The FTIR spectra of various proportions of urea-thiourea mixed crystal (Figs 3-5) was recorded on a Shimadzu spectrometer using KBr pellet technique in the wave number range cm 1. The powder XRD pattern of the grown crystal was recorded using Cu Kα radiation (λ = Å) (Figs 6-10). Fig. 1 Solubility of urea-thiourea mixed crystals Fig. 2 Morphology of (a) urea, (b) thiourea, (c) 0.1 urea-thiourea mixed crystal, (d) 0.66 urea-thiourea mixed crystal and (e) 0.9 urea-thiourea mixed crystal

3 266 INDIAN J PURE & APPL PHYS, VOL 48, APRIL 2010 Fig. 3 FTIR spectrum for 0.1 urea-thiourea mixed crystal Fig. 4 FTIR spectrum for 0.66 urea-thiourea mixed crystal

4 MADHURAMBAL & MARIAPPAN: GROWTH OF NON-LINEAR OPTICAL ORGANIC MIXED CRYSTAL 267 Fig. 5 FTIR spectrum for 0.9 urea-thiourea mixed crystal Fig. 6 XRD pattern for urea 2.1 Solubility The organic urea-thiourea salts are non-linear optic materials. The salts used for the experiments are Analar Merck grade. It is essential to increase purity to a respectable level before proceeding further. Considerably recrystallization will produce material which is pure for crystal growth. Ureathiourea salts are recrystallized with distilled water. Saturated solutions of urea, thiourea was prepared at 40 C. The solutions were filtered to avoid any insoluble impurities by using heated apparatus to Fig. 7 XRD pattern for thiourea prevent nucleation. The solutions were cooled down to room temperature to obtain maximum yield. The resulted crystals were recrystallized further. The fine powdered crystals were filtered off by suction and were air dried at room temperature. The purity of the dried crystals was then checked by standard method analysis like melting point. Triple distilled water was used in all experiments. First of all the solubility of the salts were determined. The experimental procedure adopted was as follows:

5 268 INDIAN J PURE & APPL PHYS, VOL 48, APRIL 2010 Fig. 8 XRD pattern for 0.1 urea-thiourea mixed crystal Fig. 10 XRD pattern for 0.9 urea-thiourea mixed crystal Table 1 Comparison of IR bands of urea-thiourea mixed crystals Proportions Urea-thiourea mixed crystal Wavenumber cm 1 Thiourea Urea Assignments ν as NH ν as NH δ s NH ν as CN δ s NH ν as NH δ s NH ν as C=S ν as CN ν as NH δ s NH ν as C=S ν as CN as-asymmetric stretching s-scissoring of the crystals were expressed in terms of less soluble salt. Fig. 9 XRD pattern for 0.66 urea-thiourea mixed crystal The saturated solutions of the salts are prepared individually and the empty silica crucible was weighed. Saturated solution (5 ml) was taken in a crucible and it was also weighed. From this the solubility of urea in 100 ml of water was found as g and that of thiourea was g. Similarly the solubility of 0.1, 0.66 and 0.9 urea-thiourea mixed crystals was 91, and g. It was observed that the salt thiourea is less soluble so the proportions 3 Results and Discussion The FTIR spectrum of mixed crystal was carried using a Bruker IFS 66 V FTIR spectrometer by KBr pellet technique in the range cm 1. The observed bands along with their vibrational assignments have been tabulated in Table 1. Here the characteristic vibrational absorption of urea-thiourea mixed crystals has been compared with those of thiourea and urea 9. The high wavenumber γ as NH 2 absorption bands in urea, 0.1 urea-thiourea mixed crystal , cm 1 was shifted to lower

6 MADHURAMBAL & MARIAPPAN: GROWTH OF NON-LINEAR OPTICAL ORGANIC MIXED CRYSTAL 269 Table 2 XRD data for 0.1 urea-thiourea mixed crystal Table 4 XRD data for 0.9 urea-thiourea mixed crystal 2θ d (Aº) h k l Thiourea Urea 2θ d (Aº) h k l Thiourea Urea Table 3 XRD data for 0.66 urea-thiourea mixed crystal 2θ d (Aº) h k l Thiourea Urea , wavenumbers 3368, 3364 cm 1 in 0.66, 0.9 ureathiourea mixed crystals. In this spectrum the strong hydrogen bonding between urea, thiourea in 0.1 thiourea mixed crystal causes the bands to be sharper and its position gets shifted to higher wavenumbers ( cm 1 ). The γ as NH 2 stretching, is generally, broader in 0.66, 0.9 thiourea mixed crystals because of much weaker tendency to form hydrogen bonding compared to 0.1 thiourea mixed crystals. The bending vibration (δ s NH 2, γ C=S ) of thiourea 1627, 1417 cm 1 shifted to lower frequency in 0.66, 0.9 indicate the formation of urea-thiourea mixed crystals. 3.1 X-ray diffraction The X-ray diffraction pattern of urea-thiourea mixed crystals 0.1, 0.66, 0.9 (Thiourea proportion) are given below. The American Chemical Society for Testing and Materials (ASTM) data of urea and thiourea are also represented. From the ASTM values, the different planes absorbed are identified for all the mixed crystals that are produced. Urea belongs to tetragonal and D 2 2D-P4 2 1 M and thiourea belongs to D 2H 16 -P 8 MM 1 orthorhombic. From the Tables 2-4, it is understood that as proportion of thiourea decreased, the number of planes and d-values also decreased. This may be due to the overlapping of planes of urea crystals and therefore there is reorientation in the structure and this brings at different morphology of the crystals itself. This is very much striking phenomena observed and this mixed combinations of mixed crystals will produce a wide range of laser active crystals. 4 Conclusions Urea-thiourea mixed crystals were synthesized and its solubility analysed at room temperature. The solubility curve indicates decrease in solubility with increase in proportions of thiourea in urea-thiourea mixed crystals. Mixed crystals of urea-thiourea have been grown by slow evaporation technique at room temperature. X-ray diffraction of urea-thiourea mixed crystals has been used to index the planes. The results clearly show the overlapping of planes of urea crystals with the thiourea crystals and this brings the different morphology of the crystals itself. It is also evident from FTIR studies that there is binding of urea with thiourea from the frequency assignments. References 1 Qui J & Lan L, Mater Sci B, 133 (2006) Shah C, J Phys Condens Matter, 15 (2005) L Oliver S A, App Phys Lett, 76 (2000) Chen Y & Tomokazu, J Appl Phys, 100 (2006) Ram S, J Magn Magn Matter, 80 (1989) 241.

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