LiCsAlBO: achieving enhanced optical anisotropy [AlO] 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] tetrahedra, a new congruent melting aluminoborate LiCsAlBO with [BO] clusters was discovered. This work confirms that the introduction of [AlO] tetrahedra can lead to the rearrangement of anionic framework of the borate system and thereby enhance the birefringence of LiCsAlBO. The birefringence is about 4.1 times higher than that of its congener LiCsBO with the same [BO] clusters. Similarly, the effects of [AlO] tetrahedra on the rearrangement of the B-O anionic framework are also demonstrated in other known borates.