Hg₄InS₂Cl₅: Achieving Giant Optical Anisotropy by Introducing Well‐Aligned Linear [Hg₂S₂] Units

Abstract Optical anisotropy, manifested as birefringence, plays an important role in the development of future electronic and optoelectronic devices. Designing optical materials with giant birefringence usually relies on the merit selection and arrangement of microscopic units. Anionic clusters with linear coordination geometries often exhibit larger polarizability anisotropy, making them recognized birefringence gain modules. Nonetheless, the rarity of linear units has hindered the advancement of research in this region. In this study, a new Hg‐based compound, Hg 4 InS 2 Cl 5 , containing a well‐aligned linear [Hg 2 S 2 ] functional unit has been synthesized. It exhibits the largest experimental birefringence (0.35@546 nm) and the widest bandgap of 3.10 eV among Hg‐based thiohalides. Density functional theory (DFT) calculation shows that the [Hg 2 S 2 ] unit has a substantial polarizability anisotropy (𝛿 = 188.05) among known Hg‐based anionic clusters, leading to a significant enhancement of birefringence along the direction of linear unit arrangement. This research highlights the potential of [Hg 2 S 2 ] units in improving birefringence and provides insights for the future development of advanced infrared optical materials.