Birefringent crystals can manipulate polarized light for lasers and precision instrumentation, and the excellent properties of typical ultraviolet birefringent crystals, e.g., BBO and CaCO3, benefit from triangular planar π-conjugated groups such as (BO3)3- and (CO3)3-. In order to expand the applicable wavelength range of birefringent crystals to the mid-infrared region, d10 configuration metal (Zn, Cd, Hg) iodides were combined with planar π-conjugated benzoxazolium (C9H10ON)+ groups, and three new hybrid birefringent crystals, i.e., (ZnI4)(C9H10NO)2 (I), (CdI4)(C9H10NO)2 (II), and (HgI3)(C9H10NO) (III), were successfully synthesized by solution methods. Attributable to the synergistic enhancement of conjugated (C9H10ON)+ and near-parallel alignment of the triangular HgI3- units, compound III exhibits an obviously larger birefringence of 0.154 at 1060 nm than compounds I and II constructed from tetrahedrally coordinated ZnI42-/CdI42-. Further, the origin of anisotropic optical properties was elucidated via first-principles calculations. This study establishes that integrating the planar HgI3- unit through an organic-inorganic hybrid approach can effectively enhance birefringence, providing a feasible design strategy for mid-infrared birefringent crystals.
Li et al. (2026) studied this question.