Influence of Isoelectric pH on the Growth Linear and Nonlinear Optical and Dielectric Properties of L-Threonine Single Crystals G. Ramesh Kumar, †,‡ S. Gokul Raj, † R. Mohan,* ,† and R. Jayavel § Department of Physics, Presidency College, Chennai-600 005, India, and Crystal Growth Centre, Anna UniVersity, Chennai-600 025, India ReceiVed August 24, 2005; ReVised Manuscript ReceiVed March 17, 2006 ABSTRACT: Single crystals of L-threonine have been grown from aqueous solutions of different pH values. The crystal grown from the solution of isoelectric pH (PI) was found to have the best linear and nonlinear optical properties. The optical quality of L-threonine was improved greatly in the case of the crystal grown at PI. The morphology of the title crystal has been discussed in detail. The crystalline powder SHG efficiency of L-threonine crystals grown at PI was found to be 1.2 times that of KDP. Capacitance and dielectric loss measurements were also carried out, and the dielectric constant was calculated at room temperature in the frequency range 100 Hz-5 MHz. The dielectric constant of the crystalline sample remains constant for the entire frequency range studied. 1. Introduction Many optically active amino acids show highly efficient optical second-harmonic generation (SHG) and are promising candidates for coherent blue-green laser generation and fre- quency doubling applications. 1 Moreover, amino acids such as L-alanine, L-histidine, and L-threonine have special features such as (i) molecular chirality, (ii) wide transparency in the visible and UV ranges, and (iii) zwitterionic character. The first feature forces the molecule to crystallize in a noncentrosymmetric space group, which is an essential criterion for a SHG material and the last feature paved the way for possessing high electrooptic parameters and good mechanical and thermal strength of the crystals. 2 Among these compounds, L-threonine is an important amino acid which has an SHG efficiency higher than that of many amino acids and their family crystals. 3 The compound crystallizes in the noncentric space group P2 1 2 1 2 1 (D 2 4 ) of an orthorhombic system with the unit cell parameters a ) 13.611 Å, b ) 7.738 Å, and c ) 5.144 Å. 4 In our earlier works, 5,6 we have reported the growth aspects and optical studies of L-threonine single crystals. Rodrigues et al. 7 reported the phase-matching properties of L-threonine single crystals. In the present investigation, we report the influence of pH on optical quality linear and nonlinear optical behavior of crystals grown from aqueous solutions containing different hydrogen ion concentrations of L-threonine. In addition, the morphology and dielectric properties of the crystal have also been discussed in this paper, as they are not found in any of the literature. 2. Experimental Section 2.1. Crystal Growth. High-purity L-threonine (99.9%) was used for the crystal growth experiments. The solubility of L-threonine was found to be very high in water and sparingly soluble in nonpolar solvents. Hence, L-threonine single crystals were grown from aqueous solutions of various pH values. The pH of the mother solution was adjusted and altered to pH 4.4, 5.87, and 6.70 with the help of dilute acetic acid. Seed crystals were taken by slow evaporation of solutions with pH 4.4, 5.87, and 6.7. For the bulk growth of L-threonine single crystals at different pH values, a slow cooling method was employed. Precautionary measures were taken to avoid microbial contamination of the solution by adding 10% ethyl alcohol in the growth vessel. Mother solutions of different pH values were saturated at 318 K and allowed to cool at the rate of 0.1 K/day. Crystals of various pH values have been harvested at different growth periods, and hence, the size of the crystal varies. This has been clearly shown in Figure 1. Crystals of pH 6.70, 4.40, and 5.87 have been produced at growth runs of 45, 30, and 20 days, respectively. The crystal grown at the characteristic pH (PI), called the isoelectric point (pH ) PI ) 5.87), 8 of L-threonine was found to be highly transparent when compared to crystals grown at pH 4.4 and 6.7. The transparency variation with pH can be seen from the photograph shown in Figure 1. 2.2. Characterization Studies. Crystals of L-threonine grown at different pH values were characterized by various studies. Optical transmittance was studied at room temperature using a Varian Cary 5E UV-vis-near-IR spectrophotometer. The morphology and faces of the crystal planes were identified by a single-crystal X-ray diffrac- tometer. The NLO efficiencies of the crystals grown at various pH values were found out using a Kurtz-Perry powder SHG test using an Nd:YAG laser (1064 nm). 9 Dielectric studies was also carried out using a Keithley 3330 Model LCZ meter to measure the dielectric constant as well as the dielectric loss of the title crystal. 3. Results and Discussion 3.1. Crystal Morphology. In general, the pH of the solution changes the morphology of the crystal. However, in our case * To whom correspondence should be addressed. Tel: +91-44-2854 4894. E-mail: professormohan@yahoo.co.in. † Presidency College. ‡ E-mail: rameshvandhai@yahoo.co.in. § Anna University. Figure 1. Photograph of L-threonine crystals grown at different pH values. CRYSTAL GROWTH & DESIGN 2006 VOL. 6, NO. 6 1308 - 1310 10.1021/cg050438g CCC: $33.50 © 2006 American Chemical Society Published on Web 04/27/2006