{[Ge ₂ S ₇ ] ^6– } _∞ Chains-Mediated Framework Reorganization Triggers Dimensional Upgrade and Infrared Nonlinear Optical Response

Noncentrosymmetry is the prerequisite for second-order nonlinear optical (NLO) activity, yet rationally converting centrosymmetric (CS) lattice into noncentrosymmetric (NCS) framework while simultaneously enhancing structural robustness remains nontrivial. Herein, we demonstrate a symmetry-breaking structural transformation from CS KInGeS 4 to NCS KInGe 2 S 6, in which the insertion of {[Ge 2 S 7 ] 6– } ∞ chains disrupts the inversion symmetry in {[InGe 2 S 6 ] − } ∞ layers, and triggers a topological reorganization from layered architecture into a 3 D framework. This architectural evolution endows the [InS 4 ] tetrahedra with the highest degree of geometric distortion and the largest hyperpolarizability among known [InS 4 ]-based functional building units. Crucially, KInGe 2 S 6 exhibits a balanced set of favorable NLO performances, including a wide bandgap ( E g = 3.2 eV), phase matchable second-harmonic generation (0.4 × AgGaS 2 ), moderate birefringence (Δ n = 0.08@546 nm), and a high laser-induced damage threshold (7.18 × AgGaS 2 ). First-principles calculations corroborate that the NLO effect originates primarily from the cooperative alignment of distorted [MS 4 ] (M = In, Ge) tetrahedra. This work presents a strategy for developing high-performance mid-infrared NLO crystals through the synergistic control of dimensionality engineering and symmetry breaking.