Na₂B₆SO₁₃ with unprecedented [B₆SO₁₃]_∞ double chains and largest birefringence among borosulfates induced by the uniform arrangement of [B₃O₇] units

Birefringence is crucial for birefringent crystals to modulate polarized light and nonlinear optical crystals to enable phase-matching. It is closely related to the arrangement of advantageous functional groups with large microcosmic polarizability anisotropy in the structure. Currently, designing crystals with large birefringence by modulating the arrangement of advantageous functional groups is still a challenge. In this work, the arrangement of the advantageous [B₃O₇] groups is optimized by introducing rigid tetrahedral [SO₄] groups, resulting in a novel boron-rich borosulfate, Na₂B₆SO₁₃, which exhibits unprecedented [B₆SO₁₃]_∞ double chain and an enhanced birefringence (Δn = 0.07 at 589.3 nm) against the borate parent LiB₃O₅. Theoretical calculations confirm that the birefringence is mainly attributable to the uniform alignment of [B₃O₇] groups. Na₂B₆SO₁₃ also possesses a deep ultraviolet (DUV) cutoff edge (λ < 185 nm). This work provides a feasible method to design DUV birefringent crystals with large birefringence by inducing the arrangement of advantageous functional groups.