ISSN: Vol. XVII: No.1 & 2 SB Academic Review 2010 : 49-55

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1 INVESTIGATIONS ON THE GROWTH HABITS, THERMAL AND OPTICAL BEHAVIOR OF A PROMISING NLO CRYSTAL- MANGANESE(II) MALONATE DIHYDRATE A. Lincy, V. Mahalakshmi and K.V. Saban * ABSTRACT Well faceted single crystals of manganese malonate dihydrate have been grown by the controlled diffusion of ionic species in hydrosilica gel. Optimum conditions for the growth of defect free crystals were identified. X-ray diffraction studies on single crystals showed that the compound crystallizes in the orthorhombic system with a non centrosymmetric space group Pca2 1. The UV-Vis spectrum indicates good transparency in the visible region. The optical band gap of the material was estimated using diffuse reflectance spectroscopy (DRS). Kurtz and Perry powder test conducted on the material revealed comparable second harmonic generation (SHG) efficiency with that of standard KDP crystals. The optical nonlinearity and SHG efficiency are attributed to the molecular acentricity and intermolecular hydrogen bonding. Thermo gravimetric studies unfolded the decomposition of the material in three stages. INTRODUCTION The coordination of transition metals with various organic counterparts such as dicarboxylic acids would give rise to crystals, which exhibit a variety of interesting properties [1-6]. Being a versatile ligand in crystal engineering and in materials science [7-11], the malonate group can coordinate with various metals leading to the formation of two and three dimensional complexes with an extensive network of hydrogen bonds [12]. These hydrogen bonds between the adjacent units in the structure of such compounds open up a distinctive pathway for magnetic interaction among paramagnetic centers and also enhance optical non-linearity [13-14]. Due to their wide technological importance, recently we have grown a few metal malonate crystals of fairly good size and quality using limited diffusion method [15-18]. This paper reports the diffusion controlled growth of manganese malonate single crystals of appreciable size, along with its linear and nonlinear optical behavior and thermal properties. * Corresponding Author : Smart Materials Analytic Research and Technology (SMART), St. Berchmans College, Changanassery , Kerala, India. Tel , Fax: smartlabindia@gmail.com 49 49

2 MATERIALS AND METHODS Gel matrices were prepared, in straight glass tubes, using sodium meta silicate (SMS) solution acidified with malonic acid [19]. Aqueous manganous chloride was poured over the set gel column and the tubes were hermetically sealed and kept undisturbed at room temperature. Many batches of experiments were carried out corresponding to various ph and concentration of reactants. The specific gravity of the SMS was altered from gm/cc in various trials whereas the ph value of the titrated solution ranged from 2-8. Also, the concentrations of inner and outer reactants were varied from 0.25M - 2M. X-ray diffraction spectrum on powdered samples was recorded on a Bruker AXS D8 Advance X-ray diffractometer with Cu K-á radiation, whereas single crystal diffraction studies were carried out on the Oxford Diffraction Xcalibur-S diffractometer with graphite monochromated Mo-Ká radiation. Optical behavior of the material in the nm range was evaluated using the Cary 5 E UV-VIS- NIR spectrophotometer. The non linear optical response of the crystal was evaluated by Kurtz and Perry powder method [20] using a Q-switched, mode locked Nd:YAG laser beam of wavelength 1064 nm and pulse width 8ns. Thermogram of the sample was traced, in the temperature range 30 C to 1000 C, on a Perkin Elmer made Pyris Diamond TG-DTA analyzer. For this, the sample was heated at a rate of 15 C/ minute in nitrogen atmosphere. Crystal Formation RESULTS AND DISCUSSION Crystal bunches comprising of long crystal spikes were formed at the gelsolution interface and well inside the gel within the first 4 weeks (Fig. 1). The spikes Fig. 1. Manganese malonate crystals grown (a) at the gel- solution interface (b) well inside the gel 50 50

3 grew radially outward into transparent, rectangular rods of increasing cross section. The impact of various parameters on the nucleation is discussed elsewhere [21]. Large transparent crystals of maximum size 12x3x2 mm 3 were resulted in a gel of density 1.05 gm/cc, at ph 6.1. Optimum concentrations of both the inner and outer reactants were 1M. X-ray Diffraction Studies Fig. 2 X-ray powder Diffraction Pattern Powder XRD spectrum of the grown crystals agreed well with the reported data (JCPDS No ) [22]. Single crystal X-ray diffraction analysis revealed that the crystallized compound is manganese (II) malonate dihydrate [MnC 3 H 2 O 4.2H 2 O]. It belongs to the primitive orthorhombic system with a non-centrosymmetric space group Pca2 1. The lattice parameters a = Å, b = Å, c = 7.327Å give shape to a unit cell of volume V = Å 3. Detailed analyses on single crystal data showed exact concurrence with the earlier structure reports [23, 24] on precipitated tiny crystallites. The Mn 2+ ions are coordinated octahedrally with the four carboxylate oxygen atoms from the three ligands in the equatorial plane and two oxygen atoms from two different water molecules in the axial positions. The planar layers made up of Mn 2+ ions are bridged by malonate groups in alternating anti-anti and anti-syn coordination modes. Network of hydrogen bond involving coordinated water molecules and oxygen atom from malonate ligand contribute to the stability of the structure. These hydrogen 51 51

4 bonds can contribute significantly to the magnetic interactions in the compound and to the NLO behavior. Linear and Non Linear Optical studies Fig. 3. Transmittance spectrum of manganese malonate dihydrate The good optical transparency of the crystal in the entire visible region is evident in the UV-visible transmittance spectrum (Fig. 3). The UV cut off wavelength of the crystal is 300 nm. The diffuse reflectance from the sample in the UV-Vis-NIR region is depicted in Fig. 4. From the Kubelka-Munk function, F (R) = (1 R)²/2R =k/s, the optical band gap of the material is determined by plotting ((k/s)hí) 2 versus hí ( inset of Fig. 4) where R, k and s are the reflectance, absorption and scattering coefficients respectively [25-26]. The band gap evaluated from the linear fit of the curve on the energy axis is found to be 1.93 ev. The smaller band gap value is indicative of the relatively dense crystalline nature

5 Fig. 4. DRS spectrum of manganese malonate The NLO behavior of the crystals was evaluated by the Kurtz and Perry method. The emission of green light from the powdered sample, using Nd:YAG Laser beam of 1064 nm, confirmed the second harmonic generation. The powder SHG efficiency is found to be 0.7 times that of KDP. Crystallization in the noncentrosymmetric space group, Pca2 1, and the intermolecular hydrogen bonding are the key factors that contribute to the NLO activity. Thermal Studies Recorded thermograms indicate a decomposition scheme consisting of three steps (Fig. 5). The compound remains thermally stable upto 120 C after which it gets dehydrated. The second stage of the decomposition gets completed at 320 C, yielding manganous carbonate. Ultimately, it gets transformed to stable manganous oxide at 440 C. The mass loss observed in each stage is concurs well with the calculated values (Table 1). Also, the thermal studies stand confirmatory to the proposed crystal structure. Fig. 5. TG-DTG curve of manganese malonate dihydrate. Table 1. The decomposition process of manganese (II) malonate dihydrate. Stage Decomposition Product after Molecules Observed Calculated temperature decomposition evolved mass loss mass loss ( C) (%) (%) I MnC 3 H 2 O 4 2H 2 O II MnCO 3 + C CO+H III MnO+ C CO

6 CONCLUSION Optically transparent single crystals of manganese (II) malonate dihydrate have been grown in inert hydrosilica gel. Better crystals were obtained in a gel of density 1.06 gm/cc at ph 6.1. The compound crystallizes in the orthorhombic system with space group Pca2 1. The material is transparent in the entire visible region and possess second harmonic generation efficiency comparable to that of KDP. Crystallization in acentric space group and the hydrogen bonds present within the structure contribute to the optical non linearity of the material. The dense crystal packing is indicated by the low value of optical band gap energy. The compound is found to be thermally stable up to 120 C. REFERENCES Y. Liu, J.F. Eubank, A.J. Cairns, J. Eckert, V. Kravtsov, R. Luebke, M. Eddaoudi, Angew. Chem. Int. Ed. 46 (2007) B. Moulton, M. Zaworotko, J. Chem. Rev. 101 (2001) A. Erxleben, Coord. Chem. Rev. 246 (2003) 203. G. Amirthaganesan, T. Dhanabal, M. Dhandapani, Indian J. Phys. 84(11) (2010) S.R. Batten, K.S. Murray, Coord. Chem. Rev. 246 (2003) 103. A.C. Sudik, A.P. Cote, A.G. Wong-Foy, M.O. Keeffe, O.M. Yaghi, Angew. Chem. Int. Ed. 45 (2006) Y.R. Martin, M.H. Molina, F.S. Delgado, J. Pasan, C.R. Perez, J. Sanchiz, F. Lloret, M. Julve, Cryst. Eng. Comm. 4(87) (2002) 522. F.S. Delgado, J. Sanchiz, C.R. Perez, F. Lloret, M. Julve, Cryst. Eng. Comm. 5(48) (2003) 280. C.R. Perez, Y.R. Martin, M.H. Molina, F.S. Delgado, J. Pasan, J. Sanchiz, F. Lloret, M. Julve Polyhedron 22 (2003) Y.R. Martin, M.H. Molina, F.S. Delgado, J. Pasan, C.R. Perez, J. Sanchiz, F. Lloret, M. Julve, Cryst. Eng. Comm. 4(73) (2002) 440. F.S. Delgado, M.H. Molina, J. Sanchiz, C.R. Perez, Y.R. Martin, T. Lopez, F. Lloret, M. Julve, Cryst. Eng. Comm. 6(22) (2004) 106. Y.R. Martin, M.H. Molina, J. Sanchiz, C.R. Perez, F. Lloret, M. Julve, Dalton Trans. (2003) C. Desplanches, E. Ruiz, A. Rodriguez-Fortea, S. Alvarez, J. Am. Chem. Soc. 124 (2002) D. Xue, S. Zhang, Chem. Phys. Letters 301 (1995) 449. A. Lincy, V. Mahalakshmi, A.J. Tinto, J. Thomas, K.V. Saban, Physica B 405 (2010) P.A. Varughese, K.V. Saban, J. George, G. Varghese, J. Mater. Sci. 39 (2004)

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