(C(NH 2 ) 3 ) 2 (I 2 O 5 F)(IO 3 )(H 2 O) and C(NH 2 ) 3 IO 2 F 2 : 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(NH 2 ) 3 ) 2 (I 2 O 5 F)(IO 3 )(H 2 O) and C(NH 2 ) 3 IO 2 F 2 and one guanidine iodate C(NH 2 ) 3 IO 3 were successfully synthesized using the hydrothermal method. An unprecedented dimer [I 2 O 5 F] formed by [IO 3 F] and [IO 3 ] in (C(NH 2 ) 3 ) 2 (I 2 O 5 F)(IO 3 )(H 2 O) was found, which greatly enriches the structural diversity of fluorooxoiodates. All three compounds feature a relatively large birefringence (Δ n = 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.