Centrosymmetric Versus Noncentrosymmetric: Structural and Optical Studies on Inorganic‐Organic Hybrid Compounds of Bismuth Thiourea Iodide Resulting from Acid Effect

Abstract A series of crystals of bismuth thiourea iodide (Bi[CS(NH 2 ) 2 ][CS(NH 2 ) 2 H] 2 I 5 , Bi[CS(NH 2 ) 2 H] 3 I 6 , and Bi[CS(NH 2 ) 2 H] 6 I 9 ) have been successfully grown from hydroiodic acid solution of thiourea and bismuth oxide by slow evaporation technique. Interestingly, they exhibit distinct crystal structures resulting from the proton effect on the cation coordination. Single crystal X‐ray diffraction reveals that they belong to three different crystal systems, P 2 1 / c for Bi[CS(NH 2 ) 2 H] 3 I 6 , R c for Bi[CS(NH 2 ) 2 H] 6 I 9 , and only Bi[CS(NH 2 ) 2 ][CS(NH 2 ) 2 H] 2 I 5 into a noncentrosymmetric space group Pca 2 1 , with unit cell dimensions of a =15.755(3) Å, b =9.3257(19) Å, c =14.855(3) Å, and Z=4. It is observed that thiourea molecules in Bi[CS(NH 2 ) 2 ][CS(NH 2 ) 2 H] 2 I 5 adopt a novel coordination mode with central ions in which they can be classified into two parts: the ones which coordinate with the central metal ions and the protonated ones which are isolated in the crystal lattice. Meanwhile, all the thiourea molecules are protonated in Bi[CS(NH 2 ) 2 H] 3 I 6 , and the central Bi 3+ coordinates only with I − as the acidity increases. Furthermore, central Bi 3+ in Bi[CS(NH 2 ) 2 H] 6 I 9 employs the similar coordination mode with Bi[CS(NH 2 ) 2 H] 3 I 6 . However, more thiourea molecules are protonated (up to 6 protonated thiourea molecules vs . 3 ones in Bi[CS(NH 2 ) 2 H] 3 I 6 ) as the acidity keeps increasing. It is noteworthy that acidity is confirmed to be the key factor of crystallization and crystallographic structures. Thermal analysis shows that all compounds have high thermal stability up to 180 o C. In addition, optical characterizations such as FTIR, diffuse reflectance spectra and NLO test are also conducted.