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 KBe₂ BO₃ F₂ (KBBF), two new compounds, Sn₂ PO₄ I and Sn₂ BO₃ I, were designed and synthesized successfully, maintaining the layer structural feature and enhancing the optical anisotropy of crystals. In particular, the birefringence of Sn₂ PO₄ 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 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.