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 [BO4] and [SO4] 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” [BO2(OH)] groups to modify the common [B(SO4)4] supertetrahedra for the first time and ultimately produced two borosulfates, Na3B3S2O11(OH)2 and K3B3S2O11(OH)2, striking the balance. Isolated fundamental building block (FBB) [B3O3(OH)2(SO4)2] was observed in the two compounds. Additionally, three conventional borosulfates featuring isolated [B(SO4)4] supertetrahedra, Na4KB(SO4)4, Na4RbB(SO4)4, and Na3Cs2B(SO4)4, were also obtained for comparison. Structural analysis and theoretical calculations reveal that the directional alignment of [BO2(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 [BO2(OH)] groups in optimizing the optical anisotropy of borosulfates, providing a design paradigm for exploring advanced DUV birefringent materials in borosulfates.