Metal-centered MS4 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 [Ge2 S5 (S2 )]4- unit composed of two corner-shared GeS4 tetrahedra with a nonpolar covalent S-S bond, discovered in a new compound of Li2 Cs4 Ge2 S5 (S2 )Cl2 . To the best of our knowledge, this [Ge2 S5 (S2 )]4- unit has never been discovered. [Ge2 S5 (S2 )]4- 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 Li2 Cs4 Ge2 S5 (S2 )Cl2 . All of the above indicate that the [Ge2 S5 (S2 )]4- unit is a new functional unit and a good choice for designing new IR NLO materials.
Keywords: IR; functional units; nonlinear optical materials; solid-state chemistry; thiogermanate.
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