Recent advances in non-linear optical materials (NLO) have invoked a large revival of interest in this area of research on account of their widespread industrial requirements. Among them metal organic compounds are of particular interest for the design of new NLO chromophores with large NLO susceptibilities. Hyperpolarizibility (P) value can be drastically varied with electronic configuration of metal ions. Thiourea itself is in a centrosymmetric Pbnm space group and SHG inactive. But the architecture of introducing an inorganic component into the organic crystal to break the centrosymmetry is noticeable and yields non-centrosymmetric complexes, which possess NLO properties. The role of bromine ion is to control the crystal structure by bringing an extensive bonding between amino hydrogen and bromine ion leading to the growth of three-dimensional crystals. The resonance hybrid of thiourea also leads to the formation of three-dimensional network. Bromine can also improve the thermal stability of the compound. Hence, Br in the title crystal is effectual for second order non-linearity and production of second harmonic generation. An attempt has been made to give a complete vibrational picture for the title compound based on group theory for the first time . Crystals of KTTB were grown in saturated aqueous solution by slow evaporation technique. The synthesized salt was dissolved in deionized water and a mixture of acetone and water (1:1) at an optimized pH condition of 4.25. Transparent, colorless, good quality single crystals of KTTB were harvested within 2 to 3 weeks. The compositional analysis has been carried out using EDAX technique and the results were found to be in good agreement with calculated values. From the X-ray diffraction analysis, it has been found that the title compound crystallizes in tetragonal system and belongs to non-centrosymmetric space group P41. The relatively low angular spread of around 300-arc sec of the diffraction curve in high resolution X-ray diffraction shows that the crystalline perfection is reasonably good. FTIR and polarized Raman measurements in different scattering geometries have been carried out and the results are discussed in detail. The absorption of the crystal is less than 1 unit and its lower cut off wavelength is 350 nm. This can be attributed to the metal to ligand charge transfer or metal mediated ligand p - p* transition. The relative efficiency of KTTB with that of KDP has been measured by Kurtz and Perry technique. It is found that the efficiency of the title crystal is 60 percent to that of KDP. This proposes the title crystal for capable applications in opto-electronics technology. Group theoretical considerations show that the 204 optical modes can be divided into 144 internal modes and 60 external modes. Thiourea molecule has a sulphur atom and two nitrogen atoms capable of being coordinated to a metal atom. Coordination is expected to affect the vibrational frequencies of thiourea, especially those of NH₂ stretching and C=S stretching vibrations. This is due to the formation S-M (sulphur to metal bond) in K(Tu)4Br and it is expected to increase the contribution of high polar structure of the thiourea molecules resulting in the increase of the double bond nature of the C-N bond and a greater single bond character for the carbon to sulphur bond. In the crystal structure, thiourea molecules are planar and there exist extensive intra/inter molecular hydrogen bonds. The role of hydrogen bonding in determining the non-linear optical property is discussed in detail Frequencies and intensities of bands attributed to the different modes slightly change when polarization vary in Raman spectra, which supports that the title crystal is non-centrosymmetric, which arises essentially due to the anisotropic nature of polarizability with respect to the normal coordinates of the molecular vibrations and also due to the planar nature of thiourea molecules. Thus the title crystal has good propensity for second harmonic generation. Further studies on heavy ion irradiation are in progress.

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1. Studies on the first order hyperpolarizability and Terahertz generation in an organic non-linear optical crystal 3-nitroaniline