Gradual Increase in Birefringence of Antimony Oxalates through Precise Tuning of the Sb 3+ /[C 2 O 4 ] 2- Ratio.

Birefringent crystals play a crucial role in modulating and controlling the polarization of light in the optical communication and laser industries. Recently, adopting appropriate strategies to enhance the birefringence of crystals has become a prominent area of research focus. Herein, four UV antimony oxalate birefringent crystals, namely, K 5 Sb 2 (C 2 O 4 ) 5.5 ·3H 2 O, BaSb(C 2 O 4 ) 2.5 ·3H 2 O, Na 4 Sb 2 O(C 2 O 4 ) 4 ·6H 2 O, and Na 3 Sb(C 2 O 4 ) 2 F 2 ·2H 2 O, have been successfully synthesized. These compounds feature a similar zero-dimensional (0D) cluster structure and share the same functional groups, including π-conjugated [C 2 O 4 ] 2- groups and Sb 3+ -based distorted polyhedra with stereochemically active lone pairs (SCALPs). Interestingly, we achieved a stepwise increase in birefringence by precisely controlling the Sb 3+ /[C 2 O 4 ] 2- ratio, ultimately resulting in the compound Na 3 Sb(C 2 O 4 ) 2 F 2 ·2H 2 O exhibiting a large birefringence (0.21@546 nm). Additionally, we confirmed that the synergistic effects between the π-conjugated [C 2 O 4 ] 2- groups and the distorted polyhedra based on Sb 3+ are responsible for the excellent optical properties observed in the reported compounds.