Lattice Confinement‐Stabilized Triple‐Connected Sn(II) Chalcogenides for Strong Second‐Harmonic Generation and Broadband‐Tunable Photoluminescence

Abstract Divalent tin (Sn 2+ ) compounds possess unique electronic configurations that make them promising candidates for advanced optoelectronic materials. However, their practical development is challenged by the inherent instability of Sn 2+ , which readily oxidizes to Sn 4+ . Here, a confinement‐engineered strategy is reported to stabilize Sn 2+ within a prismatic crystal field in the lattice, and eleven new chalcogenides A II 0.5 B II 3 C III 3 Q VI 8 (where A II = Sn, Pb; B II = Mg, Mn; C III = Al, Ga; Q VI = S, Se), including five Sn 2+ ‐based compounds, are rationally designed and synthesized. The compounds crystallize in the P space group, featuring rare [A II Q 6 ] triangular prisms, [B II Q 6 ] octahedra, and [C III Q 4 ] tetrahedra interconnected through simultaneous vertex‐, edge‐, face‐sharing triple connectivity, which confers exceptional structural stability and flexibility. Optically, these compounds exhibit broadband‐tunable photoluminescence, spanning from visible to near‐infrared regions (400–1000 nm) excited by ultraviolet sources. Moreover, the representative Sn 0.5 Mg 3 Ga 3 S 8 exhibits a strong second‐harmonic generation response of 1.5 × AgGaS 2 , and a high laser‐induced damage threshold of 3.0 × AgGaS 2 . Theoretical calculations reveal that the optical properties stem from the unique [SnS 6 ] and/or [GaS 4 ] units. The findings highlight the dual functionality of Sn 2+ ‐based chalcogenides and open an avenue for the design of advanced functional materials through confinement engineering within the rigid‐yet‐flexible frameworks.