The excellent birefringent materials are needed for optical systems. Herein, we reported a new compound, the first tin borate chloride, Sn2B5O9Cl (SBOC) with a large birefringence (0.168 at 546 nm) measured by the polarizing microscope. Its birefringence is 16 times that of the isostructural Ba2B5O9Cl (BBOC) compound (0.010@ at 546 nm). The results show that the birefringence enhancement originates mainly from the Sn2+ polyhedra. We propose that the birefringence can be enlarged by substituting the alkaline‐earth metal cation by the Sn2+ cation in the isostructural borate with small birefringence. This strategy will guide the discovery of large birefringent materials in the future.

Exploring non π‐conjugated phosphate birefringent crystal with a large birefringence has been a great challenge. Herein, based on the unique two‐dimensional layered structure in KBe2BO3F2 (KBBF), two new compounds, Sn2PO4I and Sn2BO3I, were designed and synthesized successfully, maintaining the layer structural feature and enhancing the optical anisotropy of crystals. In particular, the birefringence of Sn2PO4I is larger than or equal to 0.664 @546 nm, which is largest among the reported borates and phosphates, even surpassing commercial birefringent crystals YVO4 and TiO2. This work indicates that a breakthrough in birefringence of inorganic compound was achieved. Also, it provides a guiding idea for exploring large birefringence materials in the future.

The simple binary fluoride α‐SnF2 is shown to be an excellent birefringent material with outstanding birefringence, about 14 times that of MgF2. Furthermore, it exhibits a shorter UV cutoff edge and easy crystal growth at ambient temperature compared to YVO4 and TiO2. A novel theoretical calculation mode was established to analyze the stereochemical‐activity lone‐pair (SCALP) contribution to the birefringence (SCB) based on the SCALP′s strength and arrangement, and it was found that the large birefringence of α‐SnF2 mainly benefits from well‐aligned [SnF5] polyhedra with a strong SCALP. The exploration of the α‐SnF2 birefringent crystal points out the direction of the future search for excellent birefringent materials.