CaYIIIS4 (III = Ga, In): Two Olivine-Type Compounds Exhibiting Exceptional Birefringence Properties

Birefringent crystals capable of modulating light polarization represent a frontier of the current research. While numerous oxide crystals are well-established for UV and visible applications, their counterparts operating effectively in the infrared spectral region remain scarce. Here, two novel olivine-type sulfides, CaYIIIS4 (III = Ga, In), have been successfully synthesized, crystallizing in the orthorhombic Pnma space group. Their structure comprises a three-dimensional [CaYS10] framework formed by interconnected CaS6 and YS6 octahedra, with GaS4/InS4 tetrahedra embedded within the network. Characterization reveals that these compounds exhibit wide band gaps (3.2 and 2.9 eV), broad optical transmission extending to ∼ 20 μm, and significant birefringence (0.16 at 532 nm), establishing their promise as infrared birefringent crystals. Structure–property analysis demonstrates that distorted structural motifs and mixed-dimensional architectures synergistically enhance material birefringence. These studies will facilitate the exploration of new IR birefringent crystals.