High-Temperature, High-Pressure Hydrothermal Synthesis and Characterization of an Acentric Borate Fluoride: Ba₂B₅O₉F·0.5H₂O

A new borate fluoride, Ba₂B₅O₉F·0.5H₂O, has been synthesized by high-temperature, high-pressure hydrothermal method, characterized by a combination of techniques and its structure determined by single-crystal X-ray diffraction. The compound crystallizes in the noncentrosymmetric space group P4n2 (No. 118) and powder SHG measurements were performed to confirm the absence of a center of symmetry. Its crystal structure is formed of a new fundamental building block which shares oxygen atoms with neighboring blocks to form a 3D borate framework with 12- and 8-ring channels where the Ba²⁺ cations, F^- anions, and water molecules are located. The structure is compared with those of minerals and synthetic borate fluoride and chlorides with similar framework compositions. The ¹¹B MAS NMR experimental results are in accord with those from crystal structure analysis and the resonances in the spectrum are assigned. The presence of water was confirmed by IR spectroscopy, and its content and the thermal decomposition products were determined by thermogravimetric analysis and powder X-ray diffraction.