Na₄Sn₄(C₂O₄)₃F6 and NaSnC₂O₄FH₂O: 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)(3)F-6 and NaSnC2O4FH2O were gained by the combination of birefringence-active groups of Sn2+ with stereochemically active lone pairs and planar pi-conjugated [C2O4](2-) groups. These groups assemble into low-dimensional structures of 0D [C2O4F4](6-) clusters and 1D [SnC2O4F](infinity)(-) chains in Na4Sn4(C2O4)(3)F-6, and double [Sn-2(C2O4)(2)F-2](infinity)(2-) chains in NaSnC2O4FH2O, 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.