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

A new aluminum borate, Ba­[AlB₄O₈(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 ¹¹B, and ²⁷Al NMR spectroscopy. It crystallizes in the monoclinic space group P2₁/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 B₄O₈(OH) clusters and AlO₄ tetrahedra with the charge-compensating Ba²⁺ cations between the layers. While the same in the framework composition, the title compound and the hydrate, Ba­[AlB₄O₈(OH)]·H₂O, differ greatly in structure. Although the title compound contains an OH group, it is thermally stable up to 740 °C and then decomposes into Ba₂Al₂B₈O₁₇, as indicated by high-temperature DSC/TG analysis and powder X-ray diffraction.