BaGeFQ (Q = S, Se) and BaGeFSe: new F-based chalcohalides with enhanced birefringence.

Three new F-based chalcohalides, BaGeFQ (Q = S, Se) and BaGeFSe, have been successfully synthesized. On going from BaGeFSe (Δ = 0.063@1064 nm) to BaGeFQ (Q = S, Se) (Δ = 0.109 and 0.103@1064 nm, respectively), the birefringence doubled. The structure-property relationship study shows that the enhanced birefringence originates from the modulation of the configuration of the ionic lattices and highly polymerized covalent lattices. This provides not only promising IR birefringent crystals, BaGeFQ, but also some insights into the design of IR birefringent materials.