LaAgSiS<sub>4</sub>: Increasing Optical Birefringence\nin Rare Earth Chalcogenide by Addition of Planar [AgS<sub>3</sub>]\nGroups

Optoelectronic materials with excellent birefringent\nproperties\nare of significant importance in the fields of optical communications\nand laser technology. Recently, rare earth (RE) chalcogenides with\nanisotropic [RES<i><sub>n</sub></i>] groups have been proven\nto be high-performance infrared birefringent materials. Herein, we\ndemonstrate that the addition of planar groups can further increase\nthe birefringence in RE chalcogenides, as realized by incorporating\nplanar [AgS<sub>3</sub>] groups into the RE-I-IV-S<sub>4</sub> family\nfor the first time. The newly obtained LaAgSiS<sub>4</sub> compound\nshows higher polarity anisotropy than its homologue LaLiSiS<sub>4</sub> and LaKSiS<sub>4</sub>, which resulted in a larger birefringence\n(0.12@600 nm) at least twice as large as that of the latter two compounds\n(0.05/0.06@600 nm). The structure–property relationship of\nLaAgSiS<sub>4</sub> was investigated through structural analysis and\nfirst-principles calculations. The results indicate that the increased\noptical birefringence in LaAgSiS<sub>4</sub> originates from the synergic\neffects of the distorted [LaS<sub>n</sub>] polyhedra and planar [AgS<sub>3</sub>] triangles. This work provides an effective strategy for\nenhancing optical birefringence in IR chalcogenides.