Cation-Modulated Aligned Arrangement of [B3O7] Units in an Unprecedented 2∞[B3O5] Layer with Remarkable Birefringence
Birefringence is a critical linear optical parameter governing polarization modulation in birefringent crystals and phase-matching in nonlinear optical (NLO) crystals. Anionic group theory posits that the spatial arrangement of B–O functional motifs in borate frameworks can be tuned via cation modification, enabling the discovery of novel structures and optimized optical properties. Using LiB3O5 as the parent structure, we designed and synthesized two novel borates, Li2Rb2BaB18O30 (LRBBO) and K0.7Rb1.3BaB6O11 (KRBBO), through a dimension-reduction strategy induced by cation substitution. KRBBO features an unprecedented 2∞[B3O5] layered structure and exhibits a significantly enhanced birefringence of Δn = 0.08 at 546 nm, surpassing both the parent compound and other borates containing only [B3O7] groups. This work establishes an effective route for designing short-wavelength borate optical crystals with large birefringence and for enhancing the birefringence of [B3O7]-based NLO crystals to extend deep-ultraviolet phase-matching capabilities. The findings underscore the role of cation modulation in achieving aligned arrangement of anisotropic functional units, offering a viable strategy to overcome the intrinsic limitations of conventional borate NLO materials.