Rational design of large birefringence infrared oxychalcogenides via heteroanionic octahedron engineering.

To overcome low birefringence in IR chalcogenides caused by symmetric tetrahedra, we designed Ba3SnGe2O8S and Sr2SnGeO4S2. Notably, Sr2SnGeO4S2 features the first distorted [SnO2S4] heteroanionic octahedra reported in oxychalcogenides, enabling a remarkably enhanced birefringence of 0.134@1064 nm versus 0.020@1064 nm in Ba3SnGe2O8S, alongside large band gaps and broad IR transmission.