STUDIES ON THE GROWTH ASPECTS OF SEMI-ORGANIC AMMONIUM BORODILACTATE: A PROMISING NEW NLO CRYSTAL

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1 STUDIES ON THE GROWTH ASPECTS OF SEMI-ORGANIC AMMONIUM BORODILACTATE: A PROMISING NEW NLO CRYSTAL T. Panchanathan 1, S. Nalini Jayanthi 2, P.Sagayaraj 3, K. Thamizharasan 4 1 Department of Physics, Vellayan Chettiyar Hr. Sec. School, Chennai, India 2 Department of Physics, AIHT, Chennai, India 3 Department of Physics, Loyola College, Chennai, India 4 Department of Physics, Sir Theagaraya College, Chennai, India ABSTRACT A new second order Non-linear optical semi organic single crystals of pure and Neodymium(Nd 3+ ) doped Ammonium borodilactate (ABL) has been grown by aqueous solution slow evaporation technique (SET). In the present study, to improve the device characteristics of ABL crystals, metal dopant was incorporated into the pure crystals. The grown crystals are Non- Hygroscopic and good transparent in the visible region, the solubility of the grown crystals was found. The cell parameters were estimated by single crystal X-ray diffraction pattern. UV- Vis-NIR spectrum was recorded to study the optical transparency of the grown crystal. The pure and doped crystals were characterized by thermal studies. The mechanical behavior was studied by Vickers micro hardness test, dielectric and photoconductivity studies were also carried out for the pure and doped ABL crystals. KEYWORDS: Solution growth, ABL, Micro Hardness, Dielectric, Photoconductivity. I. INTRODUCTION Nonlinear optical (NLO) materials which can generate highly efficient second harmonic blue-violet light are great interest for various applications including optical, optical computing, optical information processing, optical disk data storage, laser remote sensing, colour display, etc.[1,2]. In the recent past, there have been extensive efforts to develop new inorganic, organic and semi organic nonlinear optical (NLO) materials that posses several attractive properties such as high threshold, wide transparency range and high nonlinear coefficient which make them suitable for frequency doubling [3, 4]. In view of this, there has been considerable interest in the synthesis of semi-organic materials having high mechanical and thermal stability. Semi-organic materials gain importance over inorganic materials because of their polarizability, wide transmission window and high damage threshold [5]. The low temperature solution growth is an important technique because most of the semi-organic non linear- optical crystals are being grown by this technique. Due to the inherent limitation of these techniques, the size of the crystals grown by these methods is small. In the present investigation, we report the growth of Neodymium (Nd 3+ ) doped ammonium bordilactate (ABL) along with pure ABL crystals by slow evaporation technique. The grown crystals are subjected to single crystal X-ray studies to estimate the crystal structure and space group. The content of the dopant was determined by ICP analysis. UV- Vis-NIR, thermal, micro hardness, dielectric and photoconductivity studies were carried out from the grown pure and doped crystals. 298 Vol. 6, Issue 1, pp

2 II. RELATED WORKS The effect of sodium chloride, borax and boric acid of different concentrations on the growth rate of ammonium pentaborate octahydrate crystals (APBO) was measured by O. Sahin et.al. [6]. The effect of ammonium malate on the growth rate, structural, optical, thermal, mechanical, dielectric properties, crystalline perfection and second harmonic generation (SHG) efficiency of ammonium dihydrogen phosphate single crystals grown by the slow cooling method has been investigated by P. Rajesh et. al [7]. They found SHG efficiency was enhanced by the dopant. Ammonium acetate doped ammonium dihydrogen phosphate single crystals have been grown by slow cooling along with bidirectional seed rotation method by P.Rajesh et.al.[8] III. EXPERIMENTAL PROCEDURE 3.1 Crystal growth The synthesis of ammonium borodilactate with chemical formula (NH 4) + (C 6H 8BO 6) - was done by stoichiometric incorporation of ammonium carbonate, boric acid and lactic acid. Taken in the ratio 1:2:4. ABL salt was synthesized according to their relation. (NH4) 2 CO 3 + H 3BO 3 + C 6H 6O 3 (NH 4) + (C 6H 8 BO 6) - Nd 3+ doped ABL salt was also synthesized by adding 3 mole % of the dopant. To increase the purity of the crystal, recrystalization was carried out using doubled distilled water more than three times. 3.2 Solubility Studies The solubility of pure and Nd 3+ added ABL in double distilled water was measured at different temperature (30, 35, 40 and 45 º C) using a constant temperature bath of accuracy ± 0.01ºC. The solvent and an excess amount of ABL were added to a 250ml glass crystallizer. Experiments were repeated for several times at each temperature. Similar experimental procedure was followed for Nd 3+ at each temperature added ABL material. The solubility of pure and Nd 3+ added ABL in double distilled water was plotted as a function of temperature (Fig.1). Fig.2 shows the photograph of as grown pure and doped crystals in a period of 45 days. Figure 1. Solubility curves of pure and Nd 3+ doped ABL crystal Figure 2. Photograph of as grown (a) pure and (b) Nd 3+ doped ABL crystal 3.3 Characterization analysis The grown crystals of pure and doped ABL have been subjected to single crystal X-ray diffraction studies using ENRAF NONIUS CAD-4 single crystal X-ray diffractometer with Cu K(λ=1.541Ǻ) radiation. The structure of the grown ABL crystal was solved by direct method and retired by the full matrix-least squares technique using SHELXL program. The optical absorption spectrum was recorded for samples of about 4-6 mm thickness using a Varian carry 5E model dual beam spectrometer in the wave length range from 200 to 2000 nm. Single crystals of pure and doped ABL crystals were subjected to thermo gravimetric analysis (TGA) and differential thermal analysis (DTA) simultaneously between 20 C and 1400 C in the nitrogen atmosphere at the heating rate of 10K/min using STA 409 C instrument. The electric constant was measured along the direction perpendicular to 299 Vol. 6, Issue 1, pp

3 the (010) face of low frequency at room temperature (28 C). The single crystal (dimensions: thickness 1.04mm and area 8.5 mm 2 ) using a LCR meter (HIOKI LCR HITESTER). In the frequency range 100Hz 6MHz. IV. RESULT AND DISCUSSION 4.1 Single crystal XRD analysis It is observed that both pure and doped ABL single crystals belongs to orthorhombic system with a non-centrosymmetric space group C222 1 with four molecules per unit cell (Z = 4). The lattice parameters values of ABL are measured as a= Ǻ, b= ǻ, c= Ǻ and cell volume V= Ǻ 3 and agree well with the reported values [9]. There slight variations in the lattice parameters and cell volume of the doped crystals. These variations may be due to the incorporation of the dopant in the ABL crystal lattice. 4.2 Inductively Coupled Plasma Analysis In order to determine the weight percentage of dopant in doped ABL crystal, 10mg of fine powder of the doped crystal was dissolved in 100ml of triple distilled water. This prepared solution was taken for the ICP analysis. The results obtained from ICP show that 2.16% of Nd 3+ (216 µg/100ml) was present in the solution. It is observed that the amount of dopant incorporated into the crystal lattice is below its original concentration (3%) in the solution. 4.3 UV-Vis-NIR spectral analysis UV-Vis-NIR spectrum was as shown in Fig.3. For optical application, especially for SHG, the crystal must be transparent in the wavelength region of interest. The grown pure and Nd 3+ doped ABL sample shows high transparency (85%) in the range from 230 to 1300 nm and a sharp UV cut off wave length observed at 230nm and 240nm for pure and doped ABL is due to - * transition in this material.from the spectra, it is seen that doped ABL crystals have better lower cut-off wavelenghths than the pure crystals. The high transmission in the entire visible region on short cutoff wave length facilities it to be a potential NLO material for second and third harmonic of Nd: YAG laser. Figure 3. Absorption spectrum of pure and Nd 3+ doped ABL crystal 4.4. TGA and DTA studies Fig.4 shows the resulting TGA and DTA traces of the pure and doped crystals. ABL was thermally stable around C and C respectively. The sharp weight loss of the material starts around C. The DTA trace of ABL shows that a sharp endothermic matching with the decomposition of ABL. The Nd 3+ doped ABL crystal shows the same features as that of pure ABL. 300 Vol. 6, Issue 1, pp

4 Figure 4. TGA and DTA curves of pure ABL crystal 4.5 Micro hardness studies Micro hardness behaviour of the pure and doped crystals were tested by employing Vicker s micro hardness test on the (010) plane. Measurements were taken by varying the applied loads from 5 to 50 g. Micro cracks were developed at higher loads, therefore the maximum applied load was restricted to 50g only. The plot of variation of Vicker s hardness number (H V) with applied load for (010) plane of pure and doped ABL is shown in (Fig.5). From the plot, it is noted that the hardness number (H V) of the crystal decreases with increasing load. This type of behaviour wherein the hardness number decreases with increasing applied load is called normal indentation size effect (ISE). The workhardening coefficient n is calculated using log P versus log d graph. The value of work hardening coefficient, n is found to be less than 2 for both pure and doped crystals. This further confirms the normal ISE behaviour [10]. The hardness number H V has improved in the case of doped crystal. Figure 5. Variation of H V with load for pure and Nd 3+ doped ABL 4.6 Dielectric studies The opposite parallel faces of the crystals were coated with high-grade silver paste placed between the two copper electrodes and thus a parallel plate capacitor was formed. The capacitance of the sample was measured by varying the frequency from 100 to 6MHz. The dielectric constant (ε r) was calculated on capacitance, electrode area, and sample thickness. Fig.6 shows the plot of dielectric constant (ε r) verses applied frequency. The dielectric constant has high values in the lower frequency region and then it decrease with the applied frequency. The dielectric constant has a high value of 6.4 at 100 Hz and decreases to 2.7 at 6 MHz. The dielectric constant of materials may be due to the contribution of all the four polarizations, namely, space charge, dipolar, electronic and ionic polarization, which depend on the frequencies. The variation of dielectric loss with frequency is shown in Fig.7. The dielectric loss has low value of at high frequency (6MHz). The effect of inclusion of dopant is found to decrease the dielectric constant. The behavior of doped crystal is very similar to that of undoped crystal except having lower values of dielectric constant. The low value of dielectric loss at high frequency for these samples suggest that samples possesses enhanced optical quality with lesser defects and this parameter is of vital importance for NLO materials in their application [11]. 301 Vol. 6, Issue 1, pp

5 Figure 7. Variation of dielectric loss with frequency for pure and doped ABL single crystals Figure 6. Variation of dielectric constant with frequency for pure and doped ABL single 4.7 Photoconductivity Studies Fig.8 and 9 shows the field dependence of dark and photo currents in doped and pure ABL crystals. It is observed that both dark and photo currents of crystals increase linearly with the applied electric field but the photo current of both pure and doped crystals is less than the dark current which is termed as negative photoconductivity. The loss of water molecules can also lead to decrease in conductivity. However, in the present case the contribution of water molecules to negative photoconductivity is ruled out as the loss of water molecules for pure and doped ABL crystals begins at C and C respectively. Hence, the negative photoconductivity in the present case is attributed to the reduction in the number of charge carriers or their life time, in the presence of radiation [12]. Figure 8. Field dependent photoconductivity of doped ABL single crystals Figure 9. Field dependent photoconductivity of pure ABL single crystals V. CONCLUSIONS AND FUTURE WORK Pure and Nd 3+ doped ABL crystals have been grown for pure and Neodymium (Nd 3+ ) - added growth solution by the slow evaporation method. The structure of the grown crystals confirmed with single crystal X- ray analysis, it is obvious that the pure and Nd 3+ doped ABL crystals retain the orthorhombic structure and the calculated lattice parameter values are comparable with the reported values of pure ABL. The presence of Nd 3+ in ABL crystal was confirmed by inductively coupled plasma analysis. The transparency nature of the crystal in the visible and infrared region that form the absorption spectrum confirms the NLO property of the crystal. From Vickers microhardness studies, the VHN value of this pure ABL crystal is less than that of the doped crystal, and revealed that the micro hardness number decreases linearly with increasing load for both pure and doped. ABL crystals were calculated and found to be less than two for both pure and doped crystals of ABL were measured. At low frequency range both dielectric constant and dielectric loss are found to decrease 302 Vol. 6, Issue 1, pp

6 with the increase of frequency. In general, ABL shows higher dielectric constant and dielectric loss than its doped system. The crystals with low dielectric constant lead to minimum losses as they have less number of dipoles per unit volume and hence doped crystals will be more useful for high speed electro optic modulations as compared to pure crystals. Photoconductivity studies of both pure and doped ABL crystals. It is clearly observed that both pure and doped crystals exhibit negative photoconductivity. Basic studies shows Pure and Neodymium (Nd 3+ ) added crystals are suitable for high speed electro optic modulation techniques. Further studies on Second Harmonic Generation, Photo Luminescence analysis, DC conductivity measurements can lead to some concrete conclusion regarding the use of this crystal in NLO devices. REFERENCES [1]. H.S. Nalwa, Seizo Miyata, Nonlinear Optics Molecules and Polymers,CRC Press NewYork, [2]. P.N. Prasad, D.J. Williams, Introduction to Nonlinear Optical Effects In organic Moleculeas and Polymers, John Wiley and Sons Inc. New York, USA,1991. [3]. N. Vijayan, R. Ramesh Babu, R. Gopalakrishnan, P. Ramasamy,J.Cryst.Growth 267 (2004). [4]. R. Mohan Kumar, D. Rajan Babu, D. Jayaraman, R. Jayavel, K. Kitamura, J.Cryst. Growth, 275 (2005) [5]. R. Bairava Ganesh, V. Kannan, K. Meera, N.P. Rajesh,P.Ramasamy, J.Cryst.Growth, 282 (2005) 429. [6]. Ö. Şahin, M. Özdemir, N. Genli, Journal of Crystal Growth, 270 (2004) [7]. P. Rajesh,P. Ramasamy, G. Bhagavannarayana, Journal of Crystal Growth, 311 (2009) [8]. P. Rajesh, K. Boopathi, P. Ramasamy, Journal of Crystal Growth, 311 (2011) [9]. K.Thamizharasan, S.Xavier Jesuraja, Francis P.Xavier, P. Sagayaraj, J. Cryst. Growth, 218 (2000) 323. [10]. S. Dhanuskodi, P.A. Angeli Mary, J. Cryst. Growth, 253 (2003) 424. [11]. M.D. Shahabuddin Khan, G. Prasad. G.S. Kumar, Cryst. Res. Tech, 27 (1992) K28. [12]. V.N. Joshi, Photoconductivity, Marcel Dekker, New York, AUTHORS T. Panchanathan received his B.Sc., M.Sc., and M.Phil. in Physic from Madras University. He is currently doing his Ph.D., in Bharathiyar University, Coimbatore. He has 2 years of teaching experience in college and 20 years of teaching experience in school. He published more than 4 research papers in various National and International conferences. S. Nalini Jayanthi received her B.Sc., Degree in Physics (2000), M.Sc., Degree in Physics (2002), M.Phil., Degree in Physics (2003) from Bharathidasan University. She is currently working as Assistant Professor in Anand Institute of Higher Technology, Chennai She has published more than 6 research papers in various National and International Conferences. Her research interest includes Ultrasonics and Spectroscopy. P. Sagayaraj received his Ph.D degree from Madras University (1996). He have 30 years of teaching experience. Now he is working as Dean of Reasearch in Loyola College, Chennai. He published more than 94 international research papers and 25 national research papers. He published more than 180 national and international conference papers. He completed two research projects and two other projects are undergoing. He guided 50 M.Phil., students and 21 Ph.D., research scholars/ Currently he is guiding 2 M.Phil., students and 8 Ph.D., research scholars. K. Thamizharasan received his Ph.D., degree from Madras University (2000). He has 30 years of teaching experience. Now, he is working as Associate professor and Head of Physics department in Sir Theagaraya College, Chennai-21. He published more than 20 papers in Nation/International conferences. He published nearly 25 papers in international Journals. He guided nearly 15 M. Phil., students and guiding 5 Ph.D., scholars. His area of Interest is Crystal growth. 303 Vol. 6, Issue 1, pp