Cation-modulated aligned arrangement of [B3O7] units in an unprecedented 2∞[B3O5] layer with remarkable birefringence

Birefringence, as a crucial linear optical property of optical crystals, is pivotal for polarization modulation in birefringent crystals and phase-matching in nonlinear optical (NLO) crystals. Based on anionic group theory, the arrangement of advantageous B–O functional motifs in borate anionic frameworks can be adjusted by rational application of modification strategies, which can effectively promote the exploration of novel structures and optimization of optical properties. In this work, two novel borates containing distinct assembly modes of [B₃O₇] groups, Li₂Rb₂BaB₁₈O₃₀ (LRBBO) and K_0.7Rb_1.3BaB₆O₁₁ (KRBBO), were designed and synthesized based on the dimension reduction strategy induced by cation modification using LiB₃O₅ as the parent structure. Notably, KRBBO features an unprecedented ²_∞[B₃O₅] layered structure and exhibits significantly enhanced birefringence (Δn = 0.08@546 nm) compared to both the parent compound and other compounds consisting of only [B₃O₇] groups. This work provides an effective strategy for designing novel short-wavelength borate optical crystals with large birefringence and enhancing the birefringence of [B₃O₇]-based NLO crystals to expand the deep-ultraviolet phase-matching capability.