Structural Chemistry of Antimony(III) Oxalates with Lone Pairs: Polyhedral Distortion [SbO4F2], π-Conjugation [C2O4], and Birefringence Tuning

The development of novel advanced ultraviolet (UV) birefringent materials has remained a significant research focus, owing to their requirement for considerable optical anisotropy to enable effective light polarization modulation in laser technologies and scientific applications. In this work, we report the structural chemistry and optical properties of three antimony(III) oxalate crystals: NH4Sb(C2O4)F2·H2O (1), [C(NH2)3]3Sb(C2O4)2F2·2H2O (2) and (NH4)4Sb2(C2O4)3F4·2H2O (3); they have been synthesized via an aqueous solution evaporation method. Their crystal structures feature distinct architectures driven by Sb3+ lone pair effects: one-dimensional [Sb(C2O4)F2]∞- chains and zero-dimensional [Sb(C2O4)2F2]3- and [Sb2(C2O4)3F4]4- clusters. In these structures, although the arrangement angles of [C2O4]2- are less than 20°, changes in the overall structure result in variations in birefringence. Such well-ordered structures enable significant birefringence values of 0.381, 0.251, and 0.066 (at 546 nm) to be exhibited by compounds 1, 2, and 3. Also, the UV cutoff edges of compounds 1 and 2 are about 266 nm and 267 nm, which indicate their potential as UV-transparent birefringent materials. This study emphasizes that the optical properties of materials depend on the superposition of polarizabilities from stereochemically distorted polyhedra and π-conjugated planar units within the overall structure.