Chain‐like [S x ] ( x =2–6) Units Realizing Giant Birefringence with Transparency in the Near‐Infrared for Optoelectronic Materials

Abstract Birefringent crystals are requisite optical devices in laser and modern opto‐electronic fields. Development of excellent birefringent materials is still challenging. Herein, the linear or chain‐like [S x ] ( x =2–6) species were theoretically proved to be the origin of the large birefringence, and could be regarded as birefringent genes. Besides, the metal polysulfide family was first proposed to be rich birefringent materials source, among which Cs 2 S 6 realizes giant birefringence 0.58@1064 nm together with a wide band gap of 1.70 eV (based on the generalized gradient approximation). Moreover, the first dual‐anion group polysulfide Na 4 Ba 3 (S 2 ) 4 S 3 was obtained, showing wide infrared transmission range (0.5–6.2 μm), wide band gap (2.3 eV), and large birefringence (0.37 at 1064 nm). This work provides a new guiding thought for exploring large birefringence crystals in the future.