(C(NH₂)₃)₂(I₂O₅F)(IO₃)(H₂O) and C(NH₂)₃IO₂F₂: Two Guanidine Fluorooxoiodates with Wide Band Gap and Large Birefringence

Enhancing anisotropy through an effective synergistic arrangement of anionic and cationic groups is crucial for improving the birefringence optical properties of materials. In this work, by transforming I–O into I–F through the fluorination strategy, two metal-free guanidine fluorooxoiodates (C­(NH₂)₃)₂(I₂O₅F)­(IO₃)­(H₂O) and C­(NH₂)₃IO₂F₂ and one guanidine iodate C­(NH₂)₃IO₃ were successfully synthesized using the hydrothermal method. An unprecedented dimer [I₂O₅F] formed by [IO₃F] and [IO₃] in (C­(NH₂)₃)₂(I₂O₅F)­(IO₃)­(H₂O) was found, which greatly enriches the structural diversity of fluorooxoiodates. All three compounds feature a relatively large birefringence (Δn = 0.068, 0.110 and 0.075 at 546 nm) and a short ultraviolet cutoff edge. The theoretical calculation was carried out to understand the electronic structures and linear optical properties.