Cation Modulation on the Crystal Structure and Band Gap of Fluorooxoborates A3B3O3F6 (A = Alkali and Mixed Alkali Metal)

By using cation modulation strategy, a series of fluorooxoborates A3B3O3F6 (A = alkali and mixed alkali metal) were designed and successfully obtained via open high-temperature solid-state method for the first time, and single-crystal X-ray diffractions were used to determine their structures. The isolated B3O3F6 benzene-like ring structure in the title compounds is beneficial to obtain short ultraviolet absorption edge (<190 nm), which was confirmed by the UV–vis–NIR diffuse reflectance spectra. Owing to the cation size effect, there are some subtle differences in the alignment of the anionic groups. In addition, theoretical calculations were carried out to understand their electronic structures and optical properties.