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

Abstract 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 KBe 2 BO 3 F 2 (KBBF), two new compounds, Sn 2 PO 4 I and Sn 2 BO 3 I, were designed and synthesized successfully, maintaining the layer structural feature and enhancing the optical anisotropy of crystals. In particular, the birefringence of Sn 2 PO 4 I is larger than or equal to 0.664 @546 nm, which is largest among the reported borates and phosphates, even surpassing commercial birefringent crystals YVO 4 and TiO 2 . 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.