Structural chemistry of antimony(<scp>iii</scp>) 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 <i>via</i> 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.