Synthesis, Crystal Growth, Linear, and Nonlinear Optical Properties of Water-Grown Giant Optical Anisotropic Thiocyanates ABi(SCN) ₄ (A = Rb, Cs)

Thiocyanates are materials that constitute the thiocyanate anion, ([SCN])−. The linear shape of SCN– raises the chemical flexibility of the thiocyanates. More importantly, the thiocyanate crystals can be obtained in water under mild conditions. In this work, a thiocyanate system of ABi(SCN)4 (A = Rb, Cs) was studied as potential nonlinear optical (NLO) materials. CsBi(SCN)4 was grown as large millimeter-sized crystals in water at room temperature. The crystal structure of CsBi(SCN)4 was determined by single-crystal X-ray diffraction. CsBi(SCN)4 crystallizes in the orthorhombic space group P21212 (No. 18) with unit cell parameters of a = 11.1943(4) Å, b= 7.8431(3) Å, and c = 6.5510(2) Å. CsBi(SCN)4 is isostructural to previously reported RbBi(SCN)4. CsBi(SCN)4 features a three-dimensional (3D) framework, which consists of [Bi(SCN)4] units and [Cs(SCN)4] units. The UV–Vis measurement found that CsBi(SCN)4 exhibits a moderate band gap of 2.6(1) eV, which was verified by density functional theory (DFT) calculations, indicating an indirect band gap of 2.85 eV. Theoretical studies indicated that RbBi(SCN)4 and CsBi(SCN)4 possess large birefringence of 0.48@1064 and 0.66@546 nm, respectively. Theoretical studies verified that the Bi atoms and SCN– anions significantly contributed to the optical properties of CsBi(SCN)4. CsBi(SCN)4 exhibited excellent chemical stability, which remained unchanged in air for 180 days and was verified by powder X-ray diffraction. Type-I phase-matching behavior, moderate SHG response, large birefringence, easy growth of large crystals, and high chemical stability make CsBi(SCN)4 attractive as a nonlinear optical material.