Halogen-Substitution-Modulated Lone-Pair Orientation in Sb(H2PO2)2X (X = Cl, F) for Enhanced UV Birefringence

Balancing large birefringence with ultraviolet (UV) transparency remains a central challenge in the development of birefringent crystals. Herein, we report two Sb(III)-based hypophosphites, Sb(H2PO2)2Cl and Sb(H2PO2)2F, and comparatively investigate their crystal structures and optical properties. Single-crystal X-ray diffraction analysis shows that Sb(H2PO2)2Cl crystallizes in a wave-like two-dimensional (2D) layered structure, whereas Sb(H2PO2)2F adopts a 1D chain structure. Optical measurements reveal that Sb(H2PO2)2F exhibits a markedly enhanced birefringence of 0.205 at 546 nm, compared with 0.055 for Sb(H2PO2)2Cl, in good agreement with DFT-calculated values of 0.211 and 0.041, respectively. Meanwhile, Sb(H2PO2)2F retains a wide band gap of 4.81 eV and a short UV cutoff edge of 258 nm, indicating a favorable balance between optical anisotropy and UV transparency. Further structure–property analysis suggests that the enhanced birefringence of the fluoride is closely associated with the more favorable orientational efficiency of the stereochemically active lone pair on Sb3+. These results demonstrate that halogen substitution is an effective approach for tuning optical anisotropy in UV-transparent Sb(III)-based hypophosphites.