Rational Crystal Structure Design and Nonlinear-Optical Properties of Noncentrosymmetric RbCu 2 NbS 4 .

Crystal structure design based on known materials is an efficient strategy for exploring new compounds with evident second-harmonic-generation (SHG) effects. By the introduction of Rb + ions into the 3D framework of Cu 3 NbS 4 , the new compound RbCu 2 NbS 4 (space group Ama 2) composed of [ Cu 2 2 ∞ Nb S 4 ] - layers is obtained. The band gap of RbCu 2 NbS 4 is 2.1 eV, indicating that this compound is a semiconductor. Band-structure calculations indicate that the electronic transition mainly occurs from the S 3p/Cu 3d to Nb 4d states. The intercalation of Rb + ions induces a high degree of local distortion and symmetry reduction, which results in a dipole moment of 11.7 Debye for RbCu 2 NbS 4 . RbCu 2 NbS 4 shows a moderate SHG response of 0.33 × AgGaS 2 (particle size of 20-41 μm).