High-Temperature, High-Pressure Hydrothermal Synthesis, Crystal Structure, Thermal Stability, and Solid State NMR Spectroscopy of an Aluminum Borate, Ba[AlB4O8(OH)]

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A new aluminum borate, Ba[AlB 4 O 8 (OH)], has been synthesized under high-temperature, high-pressure hydrothermal conditions at 550 °C and 1400 bar and its structure characterized by single-crystal X-ray diffraction, IR, and MAS 11 B, and 27 Al NMR spectroscopy. It crystallizes in the monoclinic space group P 2 1 / n with a = 7.0695(5) Å, b = 15.108(1) Å, c = 7.0746(5) Å, β = 93.593(2)°, and Z = 4. Its 2D layer structure is formed of corner-sharing B 4 O 8 (OH) clusters and AlO 4 tetrahedra with the charge-compensating Ba 2+ cations between the layers. While the same in the framework composition, the title compound and the hydrate, Ba[AlB 4 O 8 (OH)]·H 2 O, differ greatly in structure. Although the title compound contains an OH group, it is thermally stable up to 740 °C and then decomposes into Ba 2 Al 2 B 8 O 17, as indicated by high-temperature DSC/TG analysis and powder X-ray diffraction.