Nonbonding Electron Inversion-Driven Structural Engineering: Synergistic Enhancement of Linear and Nonlinear Optical Properties.

The simultaneous optimization of large birefringence (Δn, a linear optical property) and strong second-harmonic generation (SHG, a nonlinear optical (NLO) property) in a single crystal remains a significant challenge due to the inherently distinct structural requirements for these properties. Although nonbonding electrons have been extensively studied in oxides and chalcogenides, research has predominantly focused on their role along polar axes, leaving their influence along axes in tetrahedral stacking largely unexplored. Herein, we propose a nonbonding electron-inversion strategy to overcome phase-matching limitations in defect diamond-like structures. By incorporating T2-[GaS] supertetrahedral motifs, we successfully synthesized [BaCl][CdGaS] (space group: I ), which exhibits a 219% enhancement in Δn compared to the nonphase-matching parent structure CdGaS. The weakly bound nonbonding electrons, governed by atomic potentials, demonstrate strong SHG responses under optical fields. The compound [BaCl][CdGaS] not only achieves a broad transmission range (0.28-18.6 µm) and a high laser-induced damage threshold (40.1 × AgGaS) but also optimally balances a wide bandgap (E = 3.58 eV) and a large SHG response (1.4 × AgGaS), representing one of the best-performing Cd-based materials to date. This work introduces the first phase-matching design strategy based on nonbonding electron-driven structure-property relationships, providing critical insights for the rational design of high-performance NLO materials.