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 (Δn ≤ 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.
Zhao et al. (Tue,) studied this question.