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.