Syntheses, crystal structures and characterization of three alkaline metal borates

Abstract

Three alkaline metal borates Cs₂B₇O₉(OH)₅, K₂B₄O₅(OH)₄·H₂O, and K₂B₄O₅(OH)₄·3.6H₂O have been synthesized by a hydrothermal method. They were structurally characterized by single crystal X-ray diffraction. In the three compounds, the B–O anionic groups are all isolated, which are combined through the Cs/K–O polyhedra to form a three-dimensional (3D) network. The cationic framework presents different dimensions in Cs₂B₇O₉(OH)₅, K₂B₄O₅(OH)₄·H₂O and K₂B₄O₅(OH)₄·3.6H₂O. For Cs₂B₇O₉(OH)₅, which has been reported before, we obtained its large-scale crystal and the UV-vis diffuse reflectance spectral data of Cs₂B₇O₉(OH)₅ indicate that it has a UV cut-off edge of 209 nm which corresponds to an energy gap of 5.93 eV. Theoretical calculations were carried out to evaluate the band gaps of the title compounds, which illuminate that the replacement of K enlarges the band gap. Besides, the structural discrepancy among Cs₂B₇O₉(OH)₅, K₂B₄O₅(OH)₄·H₂O and K₂B₄O₅(OH)₄·3.6H₂O is compared to analyze the effect of the cation number on the polymerization degree of the B–O anionic groups. Density functional theory calculations were employed to evaluate the band structures and the density of states suggest that Cs₂B₇O₉(OH)₅ and K₂B₄O₅(OH)₄·H₂O possess wide band gaps of 5.23 and 5.24 eV, respectively. Meanwhile, the large-scale crystal of Cs₂B₇O₉(OH)₅ was obtained.