Sb 4 O 5 I 2 : Enhancing Birefringence through Optimization of Sb/I Ratio for Alignment of Stereochemically Active Lone Pairs.

Birefringent crystals are the key components of functional optics, contributing significantly to scientific and technological advancements. To enhance birefringence, the presence of stereochemically active lone pairs offers a unique opportunity. In fact, strengthening the stereochemical activity and aligning uniformly lone pairs face tough challenges. Herein, an anisotropic layered crystal, Sb 4 O 5 I 2 , is discovered to exhibit enhanced birefringence. The influence of crystal symmetry on the birefringence of Sb 4 O 5 X 2 (X = Cl, Br, or I) is found to be minor. Instead, the asymmetric nature of ABUCBs (i.e., cis -X 3 [SbO 3 ] 6- and cis -X 3 [SbO 4 ] 8- ) plays a crucial role in enhancing the optical anisotropy. And the orientation of these ABUCBs is equally important. We demonstrate that by adjusting the Sb/I ratio from 5:1 to 2:1, all of the intralayer Sb atoms in Sb 5 O 7 I- P 6 3 are forced onto the surface position. This structural adjustment leads to strengthened ionic bonding interactions, enhanced activity of the lone pairs, and uniform alignments of the ABUCBs in Sb 4 O 5 I 2 . Consequently, this results in a 6-fold increase in birefringence.