Preparation and characterization of a bis thiourea sodium iodide (BTSI) A potential NLO crystal A. Subashini 1 • K. Rajarajan 2 • Suresh Sagadevan 3 • Preeti Singh 4 • Jiban Podder 5 Received: 7 January 2017 / Accepted: 8 November 2017 Ó Akade ´miai Kiado ´, Budapest, Hungary 2017 Abstract A semi-organic nonlinear optical single crystal of bis thiourea sodium iodide (BTSI) has been successfully grown from aqueous solution using the slow evaporation solvent technique (SEST) at room temperature. Obtained crystals using the SEST method were characterized by using different characterization techniques. Structural studies of the grown crystals have been carried out by single-crystal XRD to confirm the crystal system and functional groups by FT-IR spectroscopy. Single-crystal XRD reveals orthorhombic structure of semi-organic BTSI single crystals and its unit cell parameters. Metal complex coordination of the single crystal is studied by FT-IR spectroscopy. The optical absorption study revealed excellent optical transparency of BTSI crystal in the entire visible region with a sharp lower cutoff wavelength 298 nm. The energy band gap of BTSI is found to be 4.16 eV. Thermal stability and thermal decomposition of BTSI single crystals were investigated by TGA–DTA and DSC analysis. The surface appearance of BTSI crystals by scanning electron microscopy reveals the formation of layer growth pattern. The structural perfection and growth features of the grown crystal were analyzed by wet chemical etching studies. The above studies reveal the effect of incorporation of sodium iodide into the lattice of thiourea crystals. The as-grown BTSI single crystals can be used as a potential candidate for NLO material as well as in electronic and optoelectronic devices. Keywords Single-crystal XRD Á FT-IR Á Band gap Á Etching Á SEM Á Thermal analyses Introduction Recently, the strategy to design and develop new materials with suitable physiochemical properties is a prime factor to the research and scientists. Understanding the way in which molecules interact with each other through intermolecular forces is essential for new compounds and materials. Such new novel materials were obtained from semi-organics as it includes the properties of both organic and inorganic materials. Hence, more emphasis has been given by sci- entists to develop semi-organic crystals. Semi-organic complexes offer higher environmental stability combined with greater diversity of tunable electronic properties by virtue of the coordinated metal center [1]. Coordination compounds formed by thiourea with transition metal ions and halogen ion received renewed attention due to the following reasons (1) the nonlinear optical (NLO) proper- ties of these compounds and (2) thiourea convenient preparation of semiconducting materials through thermal decomposition of such complexes [2–6]. Thiourea, a cen- trosymmetric molecule, which on coordination with a metal ion gives a non-centrosymmetric material [7]. Thiourea molecule is an interesting inorganic matrix & Suresh Sagadevan sureshsagadevan@gmail.com 1 Department of Physics, Velammal Institute of Technology, Chennai 601 204, India 2 Department of Physics, Rajeswari Vedachalam Government Arts College, Chengalpet 603 001, India 3 Department of Physics, AMET University, Chennai 603 112, India 4 Bio/Polymers Research Laboratory, Department of Chemistry, Jamia Millia Islamia, New Delhi, India 5 Department of Chemical and Biological Engineering, University of Saskatchewan, Saskatoon, SK S7N 5A9, Canada 123 J Therm Anal Calorim DOI 10.1007/s10973-017-6829-8