{[Ge2S7]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 KInGeS4 to NCS KInGe2S6, in which the insertion of {[Ge2S7]6–}∞ chains disrupts the inversion symmetry in {[InGe2S6]−}∞ layers, and triggers a topological reorganization from layered architecture into a 3D framework. This architectural evolution endows the [InS4] tetrahedra with the highest degree of geometric distortion and the largest hyperpolarizability among known [InS4]-based functional building units. Crucially, KInGe2S6 exhibits a balanced set of favorable NLO performances, including a wide bandgap (Eg = 3.2 eV), phase matchable second-harmonic generation (0.4 × AgGaS2), moderate birefringence (Δn = 0.08@546 nm), and a high laser-induced damage threshold (7.18 × AgGaS2). First-principles calculations corroborate that the NLO effect originates primarily from the cooperative alignment of distorted [MS4] (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.