Na₄Sn₄(C₂O₄)₃F₆ and NaSnC₂O₄F·H₂O: two tin(II) fluoride oxalates display large birefringence

Birefringent materials play an important role in laser techniques as an essential part of optical devices. Therefore, the exploration of high-performance birefringent materials has been a central focus of researchers. Herein, two tin(II) fluoride oxalates Na₄Sn₄(C₂O₄)₃F₆ and NaSnC₂O₄F·H₂O were gained by the combination of birefringence-active groups of Sn²⁺ with stereochemically active lone pairs and planar π-conjugated [C₂O₄]^2- groups. These groups assemble into low-dimensional structures of 0D [C₂O₄F₄]^6- clusters and 1D [SnC₂O₄F]_∞^- chains in Na₄Sn₄(C₂O₄)₃F₆, and double [Sn₂(C₂O₄)₂F₂]_∞^2- chains in NaSnC₂O₄F·H₂O, which gives rise to the large birefringence of 0.160@546 nm and 0.189@546 nm, respectively. Detailed structure-property analysis and theoretical calculations indicate that strong optical anisotropy can be induced by the rational arrangement of the Sn²⁺-polyhedra and [C₂O₄]^2- groups.