Cation Radius Engineering of d10 Metal Hybrid Halides toward Coordination Modulation for Giant Optical Anisotropy

Abstract Birefringent crystals are core materials for polarization modulation in optical devices, yet commercial crystals generally suffer from low birefringence values. In this work, three hybrid halide crystals, (C5H5NS)2ZnCl2, (C5H5NS)2CdCl2, and (C5H5NS)HgCl2, were successfully prepared by combining 4-mercaptopyridine unit with d10 metal chlorides to further modulate the metal coordination environments. For (C5H5NS)2ZnCl2 and (C5H5NS)2CdCl2, two π-conjugated units directly coordinate to one metal-centered tetrahedron, which further assembles into a one-dimensional structure via hydrogen-bond interactions. For (C5H5NS)HgCl2, two π-conjugated units coordinate to two metal-centered tetrahedra, forming the isolated (C5H5NS)2Hg2Cl4 dimer, and the complete crystal framework is stacked by intermolecular forces. The birefringence of the three compounds exhibits diversity with the variation of d10 metal ions. Notably, (C5H5NS)2CdCl2 sets records among cadmium-based hybrid halides with metal−halogen−organic functional groups. Structural analyses and theoretical calculations demonstrate that the birefringence of the three crystals is jointly contributed by the organic part and inorganic composition. This study verifies that modifying the coordination environment by tuning the ionic radii of metal center ions is an effective strategy for birefringence modulation and advances the research on hybrid halides for birefringent crystal applications.