Bridging Motifs Induced Ordered Arrangement Contributing Large Birefringence for Hg ₄ BiQ ₂ Cl ₅ (Q═S, Se)

ABSTRACT Chalcogenide birefringent materials are key for polarization applications in the middle and far‐infrared region, yet their performance remains constrained by bottlenecks birefringence and bandgap. Due to their remarkable polarizability anisotropy, the linear [HgQ ₂ ] (Q═S, Se) units are recognized as promising birefringence functional motifs. Therefore, Hg ₄ BiQ ₂ Cl ₅ (Q═S ( 1 ), Se ( 2 )) containing [HgQ ₂ ] units were synthesized and studied. Their structural difference caused by the congener substitution induces the bridging motifs transformed from extremely distorted [HgS ₂ Cl ₂ ] tetrahedron to [HgSe ₂ Cl] planar triangle, resulting in structural dimensionality transition from {[Hg ₇ S ₄ Cl ₂ ]} _n layers in 1 to {[Hg ₈ Se ₄ Cl ₂ ] ²⁺ } _n chains in 2 . This structure reorganization optimizes the coplanar alignment of the {[Hg ₃ Q ₂ ]} _n chains, leading to a remarkable enhancement in optical anisotropy. In both structures, the {[BiCl ₅ ] ²⁻ } _n chains constructed by vertex‐shared [BiCl ₆ ] octahedra occupy the voids within the frameworks. Experimental and theoretical calculation results reveal that 2 exhibits a large birefringence of 0.45@ 546 nm that surpasses all known commercial birefringent crystals, and also one of the largest ones for diverse chalcogenides. This study demonstrates that regulating structural dimensionality through bridging motifs can effectively optimize the spatial arrangement of linear functional motifs, providing a clear structural design paradigm for the development of high‐performance birefringent crystals.