LaAgSiS₄: Increasing Optical Birefringence in Rare Earth Chalcogenide by Addition of Planar [AgS₃] Groups

Optoelectronic materials with excellent birefringent properties are of significant importance in the fields of optical communications and laser technology. Recently, rare earth (RE) chalcogenides with anisotropic [RES<i>_n</i>] groups have been proven to be high-performance infrared birefringent materials. Herein, we demonstrate that the addition of planar groups can further increase the birefringence in RE chalcogenides, as realized by incorporating planar [AgS₃] groups into the RE-I-IV-S₄ family for the first time. The newly obtained LaAgSiS₄ compound shows higher polarity anisotropy than its homologue LaLiSiS₄ and LaKSiS₄, which resulted in a larger birefringence (0.12@600 nm) at least twice as large as that of the latter two compounds (0.05/0.06@600 nm). The structure-property relationship of LaAgSiS₄ was investigated through structural analysis and first-principles calculations. The results indicate that the increased optical birefringence in LaAgSiS₄ originates from the synergic effects of the distorted [LaS_n] polyhedra and planar [AgS₃] triangles. This work provides an effective strategy for enhancing optical birefringence in IR chalcogenides.