Na 4 Sn 4 (C 2 O 4 ) 3 F 6 and NaSnC 2 O 4 F·H 2 O: 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 Na 4 Sn 4 (C 2 O 4 ) 3 F 6 and NaSnC 2 O 4 F·H 2 O were gained by the combination of birefringence-active groups of Sn 2+ with stereochemically active lone pairs and planar π-conjugated [C 2 O 4 ] 2- groups. These groups assemble into low-dimensional structures of 0D [C 2 O 4 F 4 ] 6- clusters and 1D [SnC 2 O 4 F] ∞ - chains in Na 4 Sn 4 (C 2 O 4 ) 3 F 6 , and double [Sn 2 (C 2 O 4 ) 2 F 2 ] ∞ 2- chains in NaSnC 2 O 4 F·H 2 O, 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 Sn 2+ -polyhedra and [C 2 O 4 ] 2- groups.