Barium fluoroiodate crystals with a large band gap and birefringence
Two new alkaline-earth-metal fluoroiodate compounds, BaI2O5F2 and BaIO2F3, were successfully synthesized by replacing the O atoms in the iodates with F atoms via the hydrothermal method. They crystallize in centrosymmetric space groups P21/c and Cmca, respectively. Both compounds possess large birefringences (Δn) of 0.174 at 1064 nm for BaI2O5F2 and 0.133 at 1064 nm for BaIO2F3. Structurally, BaI2O5F2 has a large birefringence due to the highly polarizable [IO3F]2− with high anisotropic polarizability; for BaIO2F3, owing to the regulation of cationic polyhedra, the arrangement of [IO2F2]− anionic groups is well-ordered, and it exhibits large anisotropy. Also, both compounds exhibit a short UV cutoff edge, 230 and 234 nm, respectively. These results demonstrate that the two new alkaline-earth-metal fluoroiodate compounds will have potential applications as UV birefringent materials.
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