From (C6H5N2)2CdCl4 to (C6H5N2)2ZnCl4, Chirality Transformation to Realize a Nonlinear Optical Organic–Inorganic Hybrid Halide with Balanced Comprehensive Performance

Nonlinear optical (NLO) crystals are critical for modern optical devices. In this study, two new hybrid metal halides, (C₆H₅N₂)₂CdCl₄ (1) and (C₆H₅N₂)₂ZnCl₄ (2), were synthesized by a mild solution method. 1 and 2 crystallize in different chiral space groups of P4₃2₁2 and P2₁2₁2₁, respectively. Their crystal structures are composed of [MX₄]^2- (M = Cd, Zn) tetrahedrons and (C₆H₅N₂)⁺ cations with different arrangements. They both have wide band gaps (3.98 and 3.83 eV). Attractively, 2 exhibits a SHG (second-harmonic generation) effect of 1.2 × KDP, large birefringence (0.22@546 nm), and a high laser damage threshold (35 × AgGaS₂), and can be grown up to centimeter level. Theoretical calculations show that these properties result from the ordered arrangement of organic cations and the synergistic effect of [MX₄] tetrahedra, providing a new example to develop high-performance NLO materials by cation regulation.