Flexible Anionic Groups-Activated Structure Dissymmetry for Strong Nonlinearity in Ln 2 Ae 3 M IV 3 S 12 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 = 5, 6) groups and highly electropositive cations (lanthanides (Ln ) and alkaline earth (Ae : Sr, Ca) metals) affords the successful synthesis of 12 NLO thiostannates including Ln Sr Sn S (Pmc2 ) and Ln Ca Sn S (P-62m); whereas 17 rigid GeS or SiS tetrahedra-constructed Ln Ae Ge S and Ln Ae Si S crystallize in the centrosymmetric (CS) Pnma. This unprecedented CS to noncentrosymmetric (NCS) structural transformation (Pnma to P-62m to Pmc2 ) in the Ln Ae M S family indicates that chemical substitution of the tetrahedral GeS /SiS units with SnS breaks the original symmetry to form the requisite NCS structures. Remarkably, strong polarization anisotropy and hyperpolarizability of the Sn S unit afford huge performance improvement from the nonphase-matching (NPM) SHG response (1.4 × AgGaS and Δn = 0.008) of La Ca Sn S to the strong phase-matching (PM) SHG effect (3.0 × AgGaS and Δn = 0.086) of La Sr Sn S . Therefore, Sn S is proven to be a promising "NLO-active unit." This study verifies that the coupling of flexible SnS building blocks into structures opens a feasible path for designing targeted NCS crystals with strong nonlinearity and optical anisotropy.