Gradual Increase in Birefringence of Antimony Oxalates through Precise Tuning of the Sb³⁺/[C₂O₄]^2– Ratio

Abstract

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, K5Sb2(C2O4)5.5·3H2O, BaSb­(C2O4)2.5·3H2O, Na4Sb2O­(C2O4)4·6H2O, and Na3Sb­(C2O4)2F2·2H2O, have been successfully synthesized. These compounds feature a similar zero-dimensional (0D) cluster structure and share the same functional groups, including π-conjugated [C2O4]2– groups and Sb3+-based distorted polyhedra with stereochemically active lone pairs (SCALPs). Interestingly, we achieved a stepwise increase in birefringence by precisely controlling the Sb3+/[C2O4]2– ratio, ultimately resulting in the compound Na3Sb­(C2O4)2F2·2H2O exhibiting a large birefringence (0.21@546 nm). Additionally, we confirmed that the synergistic effects between the π-conjugated [C2O4]2– groups and the distorted polyhedra based on Sb3+ are responsible for the excellent optical properties observed in the reported compounds.