Crystal growth and characterization of non-linear optical gamma glycine

A. Jayarama, S.M. Dharmaprakash and M. Narayana Bhat

Department of Physics, Mangalore University, Mangalagangothri 574199, India (

Gamma glycine (GG) is one of the polymorphs of amino acid glycine. The other two polymorphs, alpha and beta glycine crystallize in centro symmetric structure, however GG crystallizes in noncentro symmetric space group, making it a candidate for nonlinear optical applications. Glycine is water-soluble and bulk crystals can be grown from aqueous solution. Glycine grows in many forms including the unstable beta form and the growth mechanism of GG is complicated. Single crystals of GG has been grown at ambient temperature by solvent evaporation technique from a mixture of water and sodium/ ammonium salt. Mercury has been used to inhibit the growth of microbes. XRD, DSC, NMR, UV VIS IR laser Raman spectral and indentation techniques were employed to characterize and assess the crystal quality. The Crystal faces were indexed by X-ray measurements. The morphology of the crystals grown from various solvents showed a marked difference. GG changes to alpha glycine at 441K. GG exists as dipolar ions in which carboxyl group is present as carboxylate ion and amino group as ammonium ion. Due to this dipolar nature glycine has a high melting point. The UV cutoff of GG is below 300nm, which is sufficiently law for second harmonic generation (SHG) of laser in blue region, The Vickers hardness number determined was 76kg/sq mm. The functional groups in GG were identified using NMR and laser Raman spectra. The SHG conversion efficiency of GG is two times that of potassium dihydrogen phosphate.