(C₇H₁₀N)₃Bi₂I₉·I₂: Synergistic Cooperation of π-Conjugated Cations and Distorted [BiI₆] Octahedra Enabling a Large Infrared Birefringence

Birefringent crystals can modulate the polarization of light and have important applications in optoelectronic and photonic fields in the infrared optical communication band. However, existing birefringent crystals exhibit a significant decrease in birefringence (Δ<i>n</i> ≤ 0.26) in the infrared waveband due to the dispersion effect. Herein, we designed a new organic-inorganic hybrid halide (C₇H₁₀N)₃Bi₂I₉·I₂, which possesses a layered structure with π-conjugation cations. Though the structure has a stereochemical inert lone pair Bi³⁺, the compound exhibits a large birefringence of 0.39@589 nm (sodium D-line), which is consistent with the theoretical calculations of 0.386@589 nm. Remarkably, first-principles calculations reveal that it still maintains a large birefringence of 0.42@1064 nm and 0.32@1550 nm. This can be attributed to the highly distorted nature of the π-conjugated 1-ethylpyridinium cation and the significant distortion of the BiI₆ octahedra. This work highlights the persistently neglected potential of organic-inorganic hybrid halide materials as birefringent crystals.