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.