Rearrangement of [B2O5] Dimers within [B7O14] Clusters Enables Enhanced Optical Anisotropy in Li3Cs6Al2B14O28F

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, Li3Cs6Al2B14O28F, with [B7O14] clusters was discovered, and it was found to melt congruently. The optimally aligned [B2O5] dimers within [B7O14] clusters make Li3Cs6Al2B14O28F an enhanced birefringence, which is about 4.3× higher than its congener compound Li4Cs3B7O14 with same [B7O14] clusters. Structural analysis and additional theoretical calculations have revealed the origin of enhanced optical anisotropy.