Cation-Modulated Activation of Linear Units in an Ordered Arrangement Achieves the Largest Optical Anisotropy in Wide-Bandgap Metal Chalcogenides

Abstract Optical materials with large birefringence are crucial for promoting the miniaturization of optical devices. The commercial birefringent materials generally have small birefringences. For researchers, the large optical anisotropy of 2D materials is difficult to apply to micro-optical devices, and organic–inorganic hybrid materials are usually applicable in the ultraviolet–visible light range and have poor thermal stability, which affects their applications in the infrared region and high-power optical systems, leaving a blank in infrared birefringent materials. Inspired by chemical polarization engineering, we regulated the structure through cation substitution to stabilize the linear HgS2 unit and predicted that the formation of α-SrHgS2, containing linear HgS2 units, is beneficial for large birefringence. Here, two SrHgS2 polymorphs were predicted and synthesized, revealing the relationship between the birefringence of the linear unit and the dihedral angle. The α-phase was computationally verified to have a wide bandgap and good infrared transmittance, with a large birefringence of 0.41 @532 nm and remaining >0.33 in the far-infrared region. Additionally, it has the largest birefringence among all known metal chalcogenides with the bandgap >3.0 eV. This work provides valuable ideas for exploring infrared birefringent materials for high-power optical systems.