[Ge S (S )] , A NLO-Active Unit Leading to an Asymmetric Structure Discovered in Li Cs Ge S (S )Cl : An Experimental and Theoretical study.

Metal-centered MS tetrahedra are fundamentally important building blocks for the structural construction of infrared (IR) nonlinear optical (NLO) mateirals, because they have a large effect on properties like second-harmonic generation, birefringence, band gap, etc. Therefore, rational design and selection of these tetrahedra could effectively help synthesize new materials and give satisfactory properties. In this work, we reported a new NLO-active [Ge S (S )] unit composed of two corner-shared GeS tetrahedra with a nonpolar covalent S-S bond, discovered in a new compound of Li Cs Ge S (S )Cl . To the best of our knowledge, this [Ge S (S )] unit has never been discovered. [Ge S (S )] units have intrinsic asymmetric features and lead to the whole structure being noncentrosymmetric (NCS). Further, real-space atom-cutting methods reveal that they make almost all the contribution to the second-harmonic generation response (SHG) for Li Cs Ge S (S )Cl . All of the above indicate that the [Ge S (S )] unit is a new functional unit and a good choice for designing new IR NLO materials.