Exceptional Optical Anisotropy Enhancement Achieved Through Dual‐Ions Cosubstitution Strategy in Novel Hybrid Bismuth Halides

Abstract Guided by a superb dual‐ions cosubstitution strategy, two novel, highly optically anisotropic hybrid bismuth halides are designed and synthesized. The first compound, Gu 3 Bi 2 NO 3 Cl 8 (Gu = C(NH 2 ) 3 ), is developed using the 2D perovskite halide Cs 3 Bi 2 Cl 9 as the maternal structure. This involved substituting all Cs + cations with organic Gu + and replacing some Cl − anions with [NO 3 ] − . Further substitution of Cl − with additional [NO 3 ] − resulted in the formation of nitrate‐rich Gu 2 Bi(NO 3 ) 3 Cl 2 crystal, exhibiting a 3.4‐fold increase in [NO 3 ] − per unit volume. Both compounds have a structurally 0D nature, comprising bismuth‐centered polyhedra formed by coordinated chlorides and monodentate/bidentate nitrate moieties, with Gu + serving as a separator and linker. Notably, the presence of superb optically anisotropic dual‐ions, i.e., planar Gu + and [NO 3 ] − , enables these crystals to possess sharply enhanced optical anisotropy, with birefringence values more than 1 order of magnitude higher than that of the initial crystal Cs 3 Bi 2 Cl 9 (0.162/0.186vs 0.011 at 546 nm). The discovery and characterization of Gu 3 Bi 2 NO 3 Cl 8 and Gu 2 Bi(NO 3 ) 3 Cl 2 crystals provide new insights into achieving expected modifications in optical properties through the utilization of a dual‐ions cosubstitution strategy.