Hydrogen Bond-Assisted Structural Regulation Enables Large Birefringence in Two Antimony Oxalofluoride Crystals

Two antimony-based oxalofluoride crystals, (NH4)2Sb(C2O4)1.5F2·H2O (1) and (NH4)Sb(C2O4)F2·H2O (2), were synthesized via a solvent evaporation method. Both compounds feature a [SbO4F2]7– pentagonal pyramid and planar π-conjugated [C2O4]2– groups, interconnected through hydrogen-bond networks. They exhibit large birefringence values of 0.24 and 0.22 at 546 nm, respectively, along with wide optical band gaps of 3.93 and 3.75 eV. Combined experimental and theoretical analyses reveal that the pronounced optical anisotropy originates from the synergistic effect of stereochemically active lone-pair electrons, π-conjugated anions, and dense hydrogen bonding. This work provides a viable strategy for designing UV birefringent materials through cooperative structural modulation at the molecular level.