Heavy‐Halide Coordination and Lattice‐Confined Molecular Alignment Enable Giant Birefringence in Hybrid Metal Halides

ABSTRACT High‐birefringence materials are essential for polarization optics, yet it remains difficult to translate anisotropic molecular building blocks into a strong directionally accumulated crystal‐scale optical anisotropy. Here, we establish a hybrid metal halide engineering strategy that couples heavy‐halide coordination with lattice‐confined molecular alignment to amplify birefringence in organic–inorganic hybrid crystals. Replacing ZnCl 2 with CdI 2 in 4‐cyanopyridine (C 6 H 4 N 2 , 4‐CP) adducts converts the discrete zero‐dimensional (0D) complex (C 6 H 4 N 2 ) 2 ZnCl 2 (CPZC) into the 1D perovskite‐like compound (C 6 H 4 N 2 ) 2 CdI 2 (CPCI). This coordination‐driven structural evolution simultaneously introduces a highly polarizable iodide‐rich inorganic backbone, a distorted octahedral coordination environment, and parallel alignment of the anisotropic 4‐cyanopyridine ligands. As a result, the birefringence increases from 0.106 for CPZC to 0.607 for CPCI at 546 nm. This value positions CPCI on par with the strongest hybrid perovskite‐type birefringent crystals reported to date. Crystallographic analysis and theoretical calculations consistently show that the giant birefringence of CPCI originates from the cooperative interplay of heavy‐halide‐enhanced local polarizability and lattice‐enforced orientational coherence. These results define a transferable crystal‐engineering principle for developing high‐performance hybrid birefringent materials for next‐generation polarization photonic applications.