Achieving Phase-Matching in Nonlinear Optical Materials CsM2In2S6 (M = Cd/In, Hg/In) by the Incorporation of Unprecedented Trigonal Planar MS3 Motifs.

The trigonal planar unit possesses significant hyperpolarizability and polarizability anisotropy, which makes it useful for optimizing nonlinear optical (NLO) materials, however, chalcogenide with this unit has seldom been reported. In this work, a novel approach is introduced by integrating the unprecedented trigonal planar MS₃ (M = Cd/In, Hg/In) motifs into the nearly optically isotropic tetrahedral units, resulting in two novel chalcogenides CsM₂In₂S₆ (M = Cd/In, 1; Hg/In, 2). Notably, structures 1 and 2 feature nearly planar triangular units at the center, encircled by three trimers, further interconnecting each other to create 3D frameworks. Importantly, phases 1 and 2 display phase-matching (PM) capabilities, primarily attributed to incorporating trigonal planar MS₃ units that additionally enhance polarizability anisotropy. Furthermore, compounds 1 and 2 demonstrate moderate second-harmonic generation (SHG) signals (0.70 and 0.84 × AgGaS2@1.7 µm). This study pioneers an efficient strategy for the design of infrared NLO crystals with PM capabilities.