Hydrothermal synthesis and characterization of a new boron-aluminium oxochloride

A new boron-aluminium oxochloride denoted as BAC(8) was synthesized hydrothermally in the system H₃BO₃-AlCl₃-CaO-H₂O, and the compound was characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), inductive coupled plasma analysis (ICP), infrared spectroscopy (IR), magic-angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy and thermal analysis. The effects of pH, temperature, cations and anions on the crystallization of BAC(8) are discussed. Chemical analysis shows that BAC(8) has an empirical molar composition of 1.0B₂O₃ : 1.0Al₂O₃ : 0.2HCl : 2.6H₂O. The basic building units for BAC(8) are triangular BO₃ and tetrahedral BO₄ with a BO₃/BO₄ ratio of 11.5, as well as octahedrally coordinated Al. Thermal analysis indicates that BAC(8) is stable up to 420°C, and at higher temperature HCl and H₂O evolve from the sample. Calcination at above 700°C causes BAC(8) to transform via an amorphous phase to the known crystalline material Al₄B₂O₉.