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

Abstract

AbstractStructural 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 (Ln3+) and alkaline earth (Ae2+: Sr, Ca) metals) affords the successful synthesis of 12 NLO thiostannates including Ln2Sr3Sn3S12 (Pmc21) and Ln2Ca3Sn3S12 (P‐62m); whereas 17 rigid GeS4 or SiS4 tetrahedra‐constructed Ln2Ae3Ge3S12 and Ln2Ae3Si3S12 crystallize in the centrosymmetric (CS) Pnma. This unprecedented CS to noncentrosymmetric (NCS) structural transformation (Pnma to P‐62m to Pmc21) in the Ln2Ae3MIV3S12 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 × AgGaS2 and Δn = 0.008) of La2Ca3Sn3S12 to the strong phase‐matching (PM) SHG effect (3.0 × AgGaS2 and Δn = 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.