LiCs 3 AlB 7 O 14 : achieving enhanced optical anisotropy via [AlO 4 ] tetrahedron introduction to rearrange the anionic framework.

Borate has become a hot topic because of its rich structural chemistry and excellent properties for functional materials fields. The rearrangement of π-conjugated B-O units is key to enhancing the optical anisotropy, but it remains a challenge. Herein, by introducing [AlO 4 ] tetrahedra, a new congruent melting aluminoborate LiCs 3 AlB 7 O 14 with [B 7 O 14 ] clusters was discovered. This work confirms that the introduction of [AlO 4 ] tetrahedra can lead to the rearrangement of anionic framework of the borate system and thereby enhance the birefringence of LiCs 3 AlB 7 O 14 . The birefringence is about 4.1 times higher than that of its congener Li 4 Cs 3 B 7 O 14 with the same [B 7 O 14 ] clusters. Similarly, the effects of [AlO 4 ] tetrahedra on the rearrangement of the B-O anionic framework are also demonstrated in other known borates.