Birefringence is a performance parameter reflecting the optical anisotropy of crystals, which plays a crucial role in the application of birefringent crystals and nonlinear optical crystals. Especially, birefringent crystals can modulate the polarization state of laser, and they can be processed into various optical devices such as optical isolators and prism polarizers. During the application, the birefringent crystals with large birefringence can not only promote the miniaturization of the fabricated devices, but also effectively modulate polarized light. Currently, the preparation of birefringent crystals with high optical anisotropy is a research hotspot in the field of optoelectronic functional materials.

Sn²⁺, Sb³⁺-centered polyhedra usually possess large distortion and polarizability anisotropy, which is beneficial to generating a large birefringence. Recently, the Research Center for Crystal Materials of Xinjiang Technical Institute of Physics and Chemistry, Chinese Academy of Sciences (XTIPC, CAS) systematically investigated the research progress of Sn^Ⅱ, Sb^Ⅲ-based birefringent crystals. For the convenience of discussion, the summarized Sn^Ⅱ, Sb^Ⅲ-based crystals are classified as follows according to the factors affecting birefringence: (1) Sn^Ⅱ, Sb^Ⅲ cooperated with π-conjugated units to influence birefringence; (2) Sn^Ⅱ, Sb^Ⅲ cooperated with non-π-conjugated units to influencebirefringence; (3) Sn^Ⅱ, Sb^Ⅲ cooperated with π and non-π-conjugated units to influence birefringence. The review aims to focus on the crystal chemistry, optical properties, and the structure-properties relationship of the Sn^Ⅱ, Sb^Ⅲ-based crystals with theoretical or experimental birefringence reported, meanwhile, we also elucidate the influence of stereochemically active lone pair on birefringence. Furthermore, the development trend of such birefringent crystals is also discussed, which will provide theoretical guidance for the design of high-performance birefringent crystals.

The work was published in Angew. Chem. Int. Ed. with the title of “Recent Development of Sn^II, Sb^III-based Birefringent Material: Crystal Chemistry and Investigation of Birefringence”. Prof. Shujuan Han from Research Center for Crystal Materials, XTIPC, CAS was the first author. Prof. Shilie Pan and Prof. Shujuan Han from XTIPC, CAS were the corresponding authors. This work was financially supported by National Natural Science Foundation of China, Chinese Academy of Sciences, Autonomous Region, et. al. 

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