LiNa 2 Ca 8 B 12 O 24 F 6 Cl and Li 1.2 Na 2.8 B 6 O 11 : A Case of Cation-Induced Birefringence Enhancement via Dimensional Changes of Highly Polymerized [B 12 O 24 ] Motifs.

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, LiNa 2 Ca 8 B 12 O 24 F 6 Cl (LNCBFC) and Li 1.2 Na 2.8 B 6 O 11 (LNBO) were designed and synthesized successfully. LNCBFC possesses commonly isolated high-symmetry [B 12 O 24 ] groups, while the structure of LNBO contains an unprecedented 1 ∞ [B 12 O 22 ] chain constructed by [B 12 O 24 ] 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 [B 12 O 24 ]-based compounds and the patterns of isolated [B n O 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.