Assembly of Highly Polarizable “Linkers” with Heteroleptic Groups for Inducing Large Optical Anisotropy in Ba₂SnIIGe₃O₈S and Ba₃SnIIGe₃O₂S₈

Birefringence, which is determined by optical anisotropy, is essential for optoelectronic functional materials. However, achieving large birefringence in infrared (IR) chalcogenides remains a huge challenge due to the weak anisotropic polarizabilities and imperfect arrangement of single-anion tetrahedra. Herein, two new SnII-based oxychalcogenides, Ba2SnIIGe3O8S and Ba3SnIIGe3O2S8, were designed and synthesized by introducing the "linkers" with lone-pair electrons in oxychalcogenides. In the structures, the heteroleptic functional building units, [Ge3O8S] and [Ge3O2S8], are linked by the Sn2+-based groups to form two-dimensional {[SnGe3O8S]4-}∞ layers and one-dimensional {[SnGe3O2S8]6-}∞ chains, respectively, which promotes the large birefringence of 0.168@1064 nm for Ba2SnIIGe3O8S and 0.225@1064 nm for Ba3SnIIGe3O2S8. Remarkably, Ba3SnIIGe3O2S8 features the largest birefringence at 1064 nm among the reported oxychalcogenides containing [GeOxS4-x] (x = 1, 2, and 3) groups. Ultraviolet-visible and near-infrared diffuse reflectance and IR spectra measurements show that they have large band gaps (2.65-3.62 eV) and broad IR transmission windows. These results demonstrate the potential application of Ba2SnIIGe3O8S and Ba3SnIIGe3O2S8 in the IR region as birefringent materials and provide an effective method for the synthesis of chalcogenides with large birefringence.