• • KRBBO exhibits a birefringence of Δn = 0.08 at 546 nm, a 33–100% enhancement over the parent LiB3O5 (Δn = 0.040–0.060 at 1064 nm) and other [B3O7]-based borates, directly enabling shorter phase-matching wavelengths for deep-UV NLO applications.
• • The unprecedented 2∞[B3O5] layered structure in KRBBO arises from cation-modulated dimension reduction, which aligns [B3O7] units to maximize optical anisotropy; this structural motif is absent in all previously reported borates.
• • Cation substitution with mixed K/Rb (K0.7Rb1.3BaB6O11) and Li/Rb (Li2Rb2BaB18O30) alters the anionic framework dimensionality from 3D to 2D, providing a tunable handle for optimizing birefringence without sacrificing UV transparency.
• • The measured birefringence of KRBBO (0.08@546 nm) exceeds that of fluorooxoborates such as LiB6O9F (0.06@1064 nm) and Li2B6O9F2 (0.07@1064 nm), indicating that cation modulation is more effective than fluorine incorporation for enhancing birefringence in [B3O7]-based systems.