Unleashing the potential of π-conjugated [B2O3(OH)2] to enhance birefringence in DUV crystals

It is still an urgent problem to develop new DUV birefringent functional units and investigate the enhancement mechanism of birefringence to explore new DUV birefringent materials with outstanding comprehensive performance. In principle, increasing the polarizability anisotropy of the functional groups and maintaining a coplanar alignment are the key factors to achieve a large birefringence. Herein, by replacing [PO4] with [HPO3], the first borophosphite LiB3O3(OH)2(HPO3) was successfully designed based on optimizing the tetrahedra strategy. Meanwhile, by the cation-tuned modification strategy to manipulate the anionic group arrangement, K2NaB6P2O13(OH)5·2H2O was also successfully synthesized. Owing to the coplanar arrangement of π-conjugated groups, both of them exhibit large birefringence (0.09 and 0.08 @532 nm). Furthermore, reported compounds feature large bandgap (Eg = 7.81 and 7.27 eV) which further indicates that they are potential DUV birefringent crystals, and it also demonstrates the feasibility of the above strategies.