Ba₂GeF₂Q₃ (Q = S, Se) and Ba₃GeF₂Se₄: new F-based chalcohalides with enhanced birefringence

Three new F-based chalcohalides, Ba₂GeF₂Q₃ (Q = S, Se) and Ba₃GeF₂Se₄, have been successfully synthesized. On going from Ba₃GeF₂Se₄ (Δn = 0.063@1064 nm) to Ba₂GeF₂Q₃ (Q = S, Se) (Δn = 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, Ba₂GeF₂Q₃, but also some insights into the design of IR birefringent materials.