Edge-Sharing BO ₄ Tetrahedra in the Structure of Hydrothermally Synthesized Barium Borate: α-Ba ₃ [B ₁₀ O ₁₇ (OH) ₂ ]

Two polymorphs of a new barium borate, α- and β-Ba ₃ [B ₁₀ O ₁₇ (OH) ₂ ], have been synthesized through hydrothermal reactions at 500 °C and 1000 bar and their structures determined by single-crystal X-ray diffraction. The 3D framework structure of the α-form is formed of two different fundamental building blocks (FBB) with the descriptors 4Δ6□:[]-<3□>-<2Δ□>|-<3□>-<2Δ□>| and 2Δ3□:<Δ2□>-<Δ2□>. The former FBB is unique and contains unusual edge-sharing BO ₄ tetrahedra. The β-form has a double layer structure which is formed of two stereoisomers of a pentaborate polyanion with the descriptor 2Δ3□:<Δ2□>-<Δ2□>. Within a double layer, one sheet composed of FBBs in l configuration is connected by sharing tetrahedral vertices to the other sheet of FBBs in d configuration. The α-form is the first compound with edge-sharing BO ₄ tetrahedra synthesized in aqueous solution under hydrothermal conditions.