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 (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 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-62m to Pmc2 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 Δn = 0.008) of La 2 Ca 3 Sn 3 S 12 to the strong phase-matching (PM) SHG effect (3.0 × AgGaS 2 and Δn = 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.