Gradual Increase in Birefringence of Antimony Oxalates through Precise Tuning of the Sb/[CO] 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, KSb(CO)·3HO, BaSb(CO)·3HO, NaSbO(CO)·6HO, and NaSb(CO)F·2HO, have been successfully synthesized. These compounds feature a similar zero-dimensional (0D) cluster structure and share the same functional groups, including π-conjugated [CO] groups and Sb-based distorted polyhedra with stereochemically active lone pairs (SCALPs). Interestingly, we achieved a stepwise increase in birefringence by precisely controlling the Sb/[CO] ratio, ultimately resulting in the compound NaSb(CO)F·2HO exhibiting a large birefringence (0.21@546 nm). Additionally, we confirmed that the synergistic effects between the π-conjugated [CO] groups and the distorted polyhedra based on Sb are responsible for the excellent optical properties observed in the reported compounds.