Flexible Anionic Groups-Activated Structure Dissymmetry for Strong Nonlinearity in Ln₂ Ae₃ M^IV ₃ S₁₂ Family

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 (Ln³⁺ ) and alkaline earth (Ae²⁺ : Sr, Ca) metals) affords the successful synthesis of 12 NLO thiostannates including Ln₂ Sr₃ Sn₃ S₁₂ (Pmc2₁ ) and Ln₂ Ca₃ Sn₃ S₁₂ (P-62m); whereas 17 rigid GeS₄ or SiS₄ tetrahedra-constructed Ln₂ Ae₃ Ge₃ S₁₂ and Ln₂ Ae₃ Si₃ S₁₂ crystallize in the centrosymmetric (CS) Pnma. This unprecedented CS to noncentrosymmetric (NCS) structural transformation (Pnma to P-62m to Pmc2₁ ) in the Ln₂ Ae₃ M^IV ₃ S₁₂ family indicates that chemical substitution of the tetrahedral GeS₄ /SiS₄ units with SnS_n breaks the original symmetry to form the requisite NCS structures. Remarkably, strong polarization anisotropy and hyperpolarizability of the Sn⁽⁴⁺⁾ S₅ unit afford huge performance improvement from the nonphase-matching (NPM) SHG response (1.4 × AgGaS₂ and Δn = 0.008) of La₂ Ca₃ Sn₃ S₁₂ to the strong phase-matching (PM) SHG effect (3.0 × AgGaS₂ and Δn = 0.086) of La₂ Sr₃ Sn₃ S₁₂ . Therefore, Sn⁽⁴⁺⁾ S₅ 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.