Synthesis, Growth, and Characterization of L-Arginine Acetate Crystal: A Potential NLO Material Tanusri Pal, † Tanusree Kar,* ,† Gabriele Bocelli, ‡ and Lara Rigi ‡ Department of Materials Science, Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700032, India, and Centro di Studio per la Strutturistica Diffrattometrica del CNR, Parco Area delleScienze 17a, 43100 Parma, Italy Received September 13, 2002; Revised Manuscript ReceivedNovember 5, 2002 ABSTRACT: Single crystals of nonlinear optical material L-arginine acetate (LAA) (C 6 H 14 N 4 O 2 ‚CH 3 COOH), space group P2 1 , were successfully grown for the first time by the temperature-lowering method and also by the slow evaporation method at constant temperature (30 °C) from its aqueous solution with pH at 6 and dimension of 21 × 15 × 3 mm 3 . Initially, solubility tests were carried out for four solvents such as water, water and methanol, water and ethanol, and water and acetone. Among the four solvents, the solubility of LAA was found to be the highest in water, so crystallization of LAA was done from its aqueous solution. Morphological analysis reveals that the crystal is a polyhedron with 16 developed faces with major face forms {100}, {001}, and {102} (pinacoids) parallel to the polar axis. As grown crystals were characterized by chemical analysis, density measurement, and X-ray powder diffraction studies. Infrared spectroscopy, thermogravimetric analysis, and differential thermal analysis measurements were performed to study the molecular vibration and thermal behavior of LAA crystals. Thermal analysis does not show any structural phase transition. Nonlinear optics (NLO) is at the forefront of current research because of its importance in providing the key functions of frequency shifting, optical modulation, optical switching, optical logic, and optical memory for the emerg- ing technologies in areas such as telecommunications, signal processing, and optical interconnections. 1 Organic materials have been of particular interest because the nonlinear optical response in this broad class of materials is microscopic in origin, offering an opportunity to use theoretical modeling coupled with synthetic flexibility to design and produce novel materials. 1,2 Also, organic nonlinear optical materials are attracting a great deal of attention, as they have large optical susceptibilities, inherent ultrafast response times, and high optical thresholds for laser power as compared with inorganic materials. Organic molecules with significant nonlinear optical activity generally consist of a π-electron- conjugated moiety substituted by an electron donor group on one end of the conjugated structure and an electron acceptor group on the other end, forming a “push-pull” conjugated structure. The conjugated π-electron moiety provides a pathway for the entire length of conjugation under the perturbation of an external electric field. The donor and acceptor groups provide the ground state charge asymmetry of the molecule, which is required for second- order nonlinearity. In this context, amino acids are interesting materials for NLO application as they contain a proton donor carboxyl acid (-COO) group and the proton acceptor amino (-NH 2 ) group in them. L-Arginine is an amino acid, which forms a number of complexes on reaction with different acids and has attractive NLO properties as discovered by Monaco et al. 3 L-Arginine acetate (LAA) is one of the most attractive salts that belongs to this arginine complex family. Vijayan et al. 4 solved the single crystal structure of LAA. It crystallized to the monoclinic system with space group P2 1 and Z ) 2. The cell parameters were a ) 9.229(2) Å, b ) 5.178(3) Å, c ) 13.271(4) Å,  ) 114.4(1)°, and d m ) 1.346 gm/cc. Monaco et al. 3 have studied the linear and nonlinear optical properties of this complex using crystals as small as 50-100 µm. Apart from these studies, no other studies on the physical properties of these LAA crystals have been carried out to date and no attempts have been made to grow larger crystals of LAA. So, we have tried to grow large size single crystals of LAA for the first time from the aqueous solution of its salt by solvent evaporation and the slow-cooling method (0.5 °C/day) followed by characteriza- tion by chemical analysis, Fourier transform infrared (FTIR) studies, and X-ray powder diffraction studies. Beside these, we have also studied the morphology and solubility of LAA along with differential thermal analysis (DTA) and thermogravimetric analysis (TGA). For device purposes, it is necessary to assess the quality of the grown crystals by X-ray Lang topography, chemical etching, and other methods. However, work in this direction will be reported in a future paper. LAA was synthesized by dissolving 1 equiv amount of strongly basic amino acid, L-arginine (Lobachemie) (NH 2 )- NHCNH(CH 2 ) 3 CH(NH 2 )COOH, in double-distilled water containing 1 equiv amount of acetic acid. The synthesized salt was then purified by repeated crystallization until optically clear crystals were obtained. The purity of the material was also checked by measuring the melting point (about 221 °C) after every crystallization. The chemical composition of the synthesized salt was then established by CHN analysis. To confirm the identity of the synthesized salt, unit cell parameters were calculated from the X-ray diffraction pattern of the salt and also verified with density measurement by flotation method. The X-ray powder diffraction pattern of LAA (Figure 1) was recorded on a Philips microprocessor-controlled X-ray dif- fractometer (APD1710) using nickel-filtered Cu KR radia- tion (36 KV, 20mA) from a Philips X-ray generator (PW1310). The powdered sample was scanned in steps of 0.02° for a time interval of 2 s over a 2θ range of 10-50°. All of the observed reflections were indexed, and the unit cell parameters were calculated using the computer pro- gram POWD. 5 The density of as grown crystal was mea- sured by the flotation method in a mixture of carbon tetrachloride (CCl 4 ) and chloroform (CHCl 3 ) and found to be 1.345 gm/cc. This agreed well with the theoretical value * To whom correspondence should be addressed. E-mail: mstk@ mahendra.iacs.res.in. † Indian Association for the Cultivation of Science. ‡ Centro di Studio per la Strutturistica Diffrattometrica del CNR. CRYSTAL GROWTH & DESIGN 2003 VOL. 3, NO. 1 13 - 16 10.1021/cg025583y CCC: $25.00 © 2003 American Chemical Society Published on Web 11/19/2002