Birefringence is a key parameter for opto-electronic functional crystals, especially the birefringent crystals, which possess the capacity of modulating the polarization of light in modern opto-electronic areas. Currently, several birefringent crystals used in the region from ultraviolet (UV) to infrared (IR) have been commercialized, including MgF₂, α-BaB₂O₄, CaCO₃, YVO₄, TiO₂, LiNbO₃ and so on. Although α-BaB₂O₄ crystal has excellent comprehensive performance, the birefringence is smaller than those of YVO₄ and rutile, which hinders the miniaturization of optical components and makes fabrication challenges. Hence, it is of scientific and technological importance to design new UV birefringent crystals with strong optical anisotropy for compact optical components.

Recently, a research group at Xinjiang Technical Institute of Physics & Chemistry, Chinese Academy of Science, has designed and synthesized two new crystals, Sn₂PO₄I and Sn₂BO₃I based on the unique two-dimensional layered structure in KBe₂BO₃F₂ (KBBF). These crystals not only maintain the layer structural feature of KBBF, but also enhance the optical anisotropy of crystals. Especially, 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. Through the theoretical calculation, the optimally aligned stereochemical activity lone pairs of Sn²⁺ polyhedra result in the giant birefringence. 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 paper was published in Angew. Chem. Int. Ed. with the title of “Sn₂PO₄I: An Excellent Birefringent Material with Giant Optical Anisotropy in Non π-Conjugated Phosphate”. This work was financially supported by National Natural Science Foundation of China, West Light Foundation of CAS, the Science and Technology Service Network Initiative of CAS, et. al.