K ₃ Mg ₂ [K ₆ MgCl ₃ ][Ga ₂₀ S ₃₆ ]: A Diamond-like Framework Based on Mixed-Type Supertetrahedra for Excellent Nonlinear Optical Property

While individual supertetrahedra are well-established as building blocks for nonlinear optical (NLO) materials, compounds featuring mixed-type supertetrahedral frameworks remain notably rare. Herein, we report a salt-inclusion chalcogenide, K3Mg2[K6MgCl3][Ga20S36] (1), characterized by a diamond-like framework synergistically assembled from T2- and C2-type supertetrahedra. Notably, compound 1 exhibits a competitive second-harmonic generation (SHG) response of 1.1 × AgGaS2 (@1700 nm) originating from the ordered packing of the supertetrahedral units. Furthermore, the inclusion of cations and polycations in the supertetrahedral framework of 1 yields a wide band gap (3.20 eV), which results in a remarkably high laser-induced damage threshold (LIDT) of 8.0 × AgGaS2 at 1064 nm. This work provides a rational design strategy for the development of advanced NLO materials based on mixed-type supertetrahedral architectures.