Designing Polar Covalent Hybrid Cadmium‐Based Chalcohalides Ultraviolet Nonlinear Optical Crystal with Strong Optical Anisotropy via Double‐Site Dual FBUs Tailoring

Abstract The pressing demand for both established and innovative technologies to expand laser wavelengths has rendered high‐performance nonlinear optical (NLO) crystals with large optical anisotropy indispensable. Here, centrosymmetric [SHC(NH 2 ) 2 ] 2 CdBr 4 ( 1 ) and pseudo ‐2D layered [SC(NH 2 ) 2 ] 2 CdBr 2 ( 2 ), as well as pseudo ‐3D noncentrosymmetric [SC(NH 2 ) 2 ] 2 CdCl 2 (3) are successfully synthesized through the introduction of π ‐conjugated SC(NH 2 ) 2 groups. Compared to ionic compound 1 containing full‐halogen coordination tetrahedra, covalent compounds 2 and 3 featuring novel polar [SC(NH 2 ) 2 ] 2 CdX 2 (X = Br, Cl) tetrahedral units demonstrate enhanced bandgaps (>4 eV) and birefringences (>0.3@546 nm) due to the unique coordination environment. Remarkably, 3 exhibits a strong second‐harmonic generation (SHG) response (2.1 × KH 2 PO 4 (KDP)), high laser‐induced damage thresholds (30 × AgGaS 2 (AGS), and excellent water stability. The birefringence of 3 is the largest among the hybrid halides NLO crystals containing d 10 metal cations. Detailed theoretical calculations confirm that such a modified double‐site dual functional building units (FBUs) substitution is an effective strategy for designing superior optical materials with large birefringence and strong SHG response, paving the way for the development of high‐performance devices in related fields.