Flexible Anionic Groups‐Activated Structure Dissymmetry for Strong Nonlinearity in Ln2Ae3MIV3S12 Family

Abstract Structural dissymmetry and strong second‐harmonic generation (SHG) responses are key conditions for nonlinear optical (NLO) crystals, and targeted combinatorial screening of suitable anionic groups has become extremely effective. Herein, optimal combination of flexible SnS n ( n = 5, 6) groups and highly electropositive cations (lanthanides ( Ln 3+ ) and alkaline earth (Ae 2+ : Sr, Ca) metals) affords the successful synthesis of 12 NLO thiostannates including Ln 2 Sr 3 Sn 3 S 12 ( Pmc 2 1 ) and Ln 2 Ca 3 Sn 3 S 12 ( P‐ 62 m ); whereas 17 rigid GeS 4 or SiS 4 tetrahedra‐constructed Ln 2 Ae 3 Ge 3 S 12 and Ln 2 Ae 3 Si 3 S 12 crystallize in the centrosymmetric (CS) Pnma . This unprecedented CS to noncentrosymmetric (NCS) structural transformation ( Pnma to P‐ 62 m to Pmc 2 1 ) in the Ln 2 Ae 3 M IV 3 S 12 family indicates that chemical substitution of the tetrahedral GeS 4 /SiS 4 units with SnS n breaks the original symmetry to form the requisite NCS structures. Remarkably, strong polarization anisotropy and hyperpolarizability of the Sn (4+) S 5 unit afford huge performance improvement from the nonphase‐matching (NPM) SHG response (1.4 × AgGaS 2 and Δ n = 0.008) of La 2 Ca 3 Sn 3 S 12 to the strong phase‐matching (PM) SHG effect (3.0 × AgGaS 2 and Δ n = 0.086) of La 2 Sr 3 Sn 3 S 12 . Therefore, Sn (4+) S 5 is proven to be a promising “NLO‐active unit.” This study verifies that the coupling of flexible SnS n building blocks into structures opens a feasible path for designing targeted NCS crystals with strong nonlinearity and optical anisotropy.