Sn2PO4I: An Excellent Birefringent Material with Giant Optical Anisotropy in Non π‐Conjugated Phosphate

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. Sn2PO4I and Sn2BO3I are designed and synthesized successfully based on the unique two‐dimensional layered structure in KBe2BO3F2 (KBBF), in which the birefringence of Sn2PO4I is the largest among the reported borates and phosphates.