Flexible Anionic Groups-Activated Structure Dissymmetry for Strong Nonlinearity in Ln₂Ae₃M^IV₃S₁₂ 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 SnSn (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) (Pmc2(1)) and Ln(2)Ca(3)Sn(3)S(12) (P-62m); whereas 17 rigid GeS4 or SiS4 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-62m to Pmc2(1)) in the Ln(2)Ae(3)M(3)(IV)S(12) family indicates that chemical substitution of the tetrahedral GeS4/SiS4 units with SnSn 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 x AgGaS2 and Delta n = 0.008) of La2Ca3Sn3S12 to the strong phase-matching (PM) SHG effect (3.0 x AgGaS2 and Delta n = 0.086) of La2Sr3Sn3S12. Therefore, Sn(4+)S5 is proven to be a promising "NLO-active unit." This study verifies that the coupling of flexible SnSn building blocks into structures opens a feasible path for designing targeted NCS crystals with strong nonlinearity and optical anisotropy.