Achieving Phase‐Matching in Nonlinear Optical Materials CsM ₂ In ₂ S ₆ (M = Cd/In, Hg/In) by the Incorporation of Unprecedented Trigonal Planar MS ₃ Motifs

Abstract 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 MS ₃ (M = Cd/In, Hg/In) motifs into the nearly optically isotropic tetrahedral units, resulting in two novel chalcogenides CsM ₂ In ₂ S ₆ (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 MS ₃ 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.