Optically Anisotropic Mixed-Metal Fluoroiodate Ba₂[GaF₅(IO₃F)] with a Wide Optical Transparent Window and a Moderate Birefringence

Abstract

An optically anisotropic alkali-earth-metal gallium fluoroiodate, Ba 2 [GaF 5 (IO 3 F)] (1), was ingeniously obtained by integrating fluoride and fluoroiodate functional units under moderate hydrothermal conditions. It features a three-dimensional (3D) structure constructed by the highly polarizable fluoroiodate unit [IO 3 F] and the fluoride groups [GaOF 5 ] and [BaO 3 F x ] (x = 6, 7). The compound is stable at temperatures up to 500 °C. With the synergistic interaction between [IO 3 F] and the fluoride groups, the mixed-metal fluoroiodate induces a short ultraviolet cutoff edge at about 230 nm, a medium measured birefringence of 0.068 @ 550 nm, and a wide optical transparent window (0.34−11.9 μm), indicating that 1 has potential applications as a birefringent material from near-UV to mid-infrared. Theoretical calculations prove that the optical characteristics of the compound are mainly attributed to [IO 3 F] and the fluoride functional groups. This work demonstrates that the presence of various specific functional groups in compounds will help to develop promising inorganic functional materials possessing good optical performance.

Ba2Ga2F6(IO3)(PO4): the first fluoride-containing iodate-phosphate with a 1D [Ga2F6(IO3)(PO4)]4− helix chain

“…Therefore, applying the formula R = d × Δ n , the experimental birefringence of Ba 2 Ga 2 F 6 (IO 3 )(PO 4 ) was determined to be 0.072@ 550 nm. Such experimental birefringence is slightly larger than those of Ba 2 [GaF 5 (IO 3 F)] (0.068@550 nm) 33 and Pb 2 (IO 3 )(PO 4 ) (0.059@546 nm). 42…”

“…Consequently, the structural feature is a 0D unit, such as the 0D [GaF 5 (IO 3 F)] 4− polyanion group in Ba 2 [GaF 5 (IO 3 F)]. 33 (2) When the F : I ratio is less than 2, IO 3 − (IO 3 F 2− ) can bridge adjacent 0D basic building units, forming 1D or 2D infinite structures. For example, in LiGaF 2 (IO 3 ) 2 , the 0D [GaF 2 (IO 3 ) 4 ] 3− group is connected by IO 3 groups to form a 2D [GaF 2 (IO 3 ) 2 ] layer.…”

“…The decomposition process of Ba 2 Ga 2 F 6 (IO 3 )(PO 4 ) is consistent with that of previously reported barium gallium iodate fluorides. 31,33…”

“…28 Recently, Wang and others also reported a wide bandgap iodate birefringent crystal, Ba 2 [GaF 5 (IO 3 F)] (0.068@550 nm; 4.52 eV). 33 The excellent optical properties of these crystals can be attributed to the synergistic effect between the weakly distorted Ga–O/F octahedra and the highly distorted IO 3 or IO 3 F groups, which encouraged us to explore other novel metal gallium iodate fluorides. On the other hand, recent research results indicate that introducing oxyanions with wide HOMO–LUMO gaps, such as BO 3 3− , NO 3 − , SO 4 2− , PO 4 3− , etc ., into the iodates can yield novel mixed-anion iodates.…”