Planar [HgCl3] and V‑Shaped [HgCl2] Units Synergistically Creating a Prominent Halide Birefringent Crystal β‐CsHg2Cl5 with Wide‐Spectrum

Abstract In the field of modern optics, it is of great significance to develop large birefringent crystals with a wide transparent range. In this study, a new polymorphic phase β ‐CsHg 2 Cl 5 ( P 2 1 /m ) is obtained through facile hydrothermal reaction. The structure of β ‐CsHg 2 Cl 5 is constructed by V‑shaped [HgCl 2 ], planar [HgCl 3 ] units, and Cs + cations, forming a 3D structure. Particularly, β ‐CsHg 2 Cl 5 shows a wide transparent window in the ultraviolet to far‐infrared spectral range (0.29‐25 µm). More attractively, β ‐CsHg 2 Cl 5 exhibits a large experimental refractive index difference value (∆n, 0.29@546 nm). Among the compounds with a wide transmission range (>0.5−20 µm), it has the largest birefringence. Theoretical calculations indicate that the well‐arranged [HgCl 2 ] (75.56%) and [HgCl 3 ] (24.44%) units are primarily responsible for this remarkable optical anisotropy. In addition, the synergy between isolated [HgCl 2 ] and [HgCl 3 ] functional units with birefringent activity is discovered for the first time. Meanwhile, theoretical calculations performed on α ‐CsHg 2 Cl 5 demonstrate a larger birefringence value of 0.31@546 nm. Where, the well‐arranged [(Hg 2 Cl 5 ) − ] ∞ chain accounts for this significant optical anisotropy, and the [HgCl 3 ] and the [HgCl 4 ] units contribute 64.38% and 35.62%, respectively. This provides new insights for developing high‐performance metal halide birefringent materials.