Rearrangement of [B 2 O 5 ] Dimers within [B 7 O 14 ] Clusters Enables Enhanced Optical Anisotropy in Li 3 Cs 6 Al 2 B 14 O 28 F.

Borates with tunable structure and property currently provide a new rich source for solid-state chemistry and materials science. Realization of property improvement via simple structural regulation is a rising hot spot of borate-based research. Herein, a new aluminoborate fluoride, Li 3 Cs 6 Al 2 B 14 O 28 F, with [B 7 O 14 ] clusters was discovered, and it was found to melt congruently. The optimally aligned [B 2 O 5 ] dimers within [B 7 O 14 ] clusters make Li 3 Cs 6 Al 2 B 14 O 28 F an enhanced birefringence, which is about 4.3× higher than its congener compound Li 4 Cs 3 B 7 O 14 with same [B 7 O 14 ] clusters. Structural analysis and additional theoretical calculations have revealed the origin of enhanced optical anisotropy.