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

Two antimony-based oxalofluoride crystals, (NH₄)₂Sb­(C₂O₄)_1.5F₂·H₂O (<b>1</b>) and (NH₄)­Sb­(C₂O₄)­F₂·H₂O (<b>2</b>), 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.