Planar [HgCl ₃ ] and V‑Shaped [HgCl ₂ ] Units Synergistically Creating a Prominent Halide Birefringent Crystal β ‐CsHg ₂ Cl ₅ 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.