Barium fluoroiodate crystals with a large band gap and birefringence

Abstract

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.