LiNa₂Ca₈B₁₂O₂₄F₆Cl and Li_1.2Na_2.8B₆O₁₁: A Case of Cation-Induced Birefringence Enhancement via Dimensional Changes of Highly Polymerized [B<sub>12</sub

Borates, due to their structural chemistry diversity and exceptional performance, are premier material systems for investigating UV optical crystals. The B-O anionic groups with high polymerization (B ≥ 6) are much less in the borate-based system, which is worthy of further research. Herein, cations with different radii and proportions are introduced to borate system, and two new highly polymerized borates, LiNa₂Ca₈B₁₂O₂₄F₆Cl (LNCBFC) and Li_1.2Na_2.8B₆O₁₁ (LNBO) were designed and synthesized successfully. LNCBFC possesses commonly isolated high-symmetry [B₁₂O₂₄] groups, while the structure of LNBO contains an unprecedented ¹_∞[B₁₂O₂₂] chain constructed by [B₁₂O₂₄] groups. Owing to the orientation of the functional motifs in the chain structure, LNBO displays an enhanced birefringence, which is about 25 × higher than that of LNCBFC and retains a short UV cutoff edge (< 200 nm). Even more significantly, a discussion of the cationic modulation of [B₁₂O₂₄]-based compounds and the patterns of isolated [B_nO_2n] motifs consisting of B-O rings was carried out by reviewing previous studies and existing borates. This work puts forward a decent structure design and property regulation strategy for highly polymerized borates.