Significant Enhancement of Birefringence Resulting from the Introduction of [BO ₂ (OH)] Groups into Borosulfates

Deep-ultraviolet (DUV) birefringent crystals are essential for modern optical technologies, yet achieving a balance between large birefringence and a wide bandgap remains challenging. Although conventional borosulfates with alternating [BO 4 ] and [SO 4 ] tetrahedra connected by sharing corners possess a wide bandgap, they suffer from small birefringence due to weak optical anisotropy resulting from the presence of only tetrahedral groups. This work introduced advantageous “genes” [BO 2 (OH)] groups to modify the common [B(SO 4 ) 4 ] supertetrahedra for the first time and ultimately produced two borosulfates, Na 3 B 3 S 2 O 11 (OH) 2 and K 3 B 3 S 2 O 11 (OH) 2, striking the balance. Isolated fundamental building block (FBB) [B 3 O 3 (OH) 2 (SO 4 ) 2 ] was observed in the two compounds. Additionally, three conventional borosulfates featuring isolated [B(SO 4 ) 4 ] supertetrahedra, Na 4 KB(SO 4 ) 4, Na 4 RbB(SO 4 ) 4, and Na 3 Cs 2 B(SO 4 ) 4, were also obtained for comparison. Structural analysis and theoretical calculations reveal that the directional alignment of [BO 2 (OH)] groups significantly enhances the birefringence of the borosulfates. The calculated birefringence at 1064 nm increases by approximately 10-fold, specifically from 0.0035–0.0047 for the three conventional borosulfates to 0.043–0.044 for the two hydroxylated ones. This work demonstrates the effectiveness of employing [BO 2 (OH)] groups in optimizing the optical anisotropy of borosulfates, providing a design paradigm for exploring advanced DUV birefringent materials in borosulfates.