High-Temperature, High-Pressure Hydrothermal Synthesis of Ba3[B6O10(OH)2](CO3) and Ba6[B12O21(OH)2](CO3)2, Two Barium Borate Carbonates with 2D Layer and 3D Framework Structures.

Two new barium borate carbonates, Ba3[B6O10(OH)2](CO3) (1) and Ba6[B12O21(OH)2](CO3)2 (2), have been synthesized by high-temperature, high-pressure hydrothermal methods at 460 °C and 600 bar and structurally characterized by single-crystal X-ray diffraction, TGA, IR, and MAS 11B NMR spectroscopy. The descriptors of the fundamental building blocks (FBB) of both structures are 2Δ4□:⟨Δ2□⟩=⟨4□⟩=⟨Δ2□⟩. The FBB of 1 has a chair conformation, and the FBBs of 2 have both chair and boat conformations. Compound 1 adopts a noncentrosymmetric 2D layer structure with the Ba2+ cations and CO32- anions between the layers, whereas compound 2 has a centrosymmetric 3D framework structure containing 9- and 10-ring channels with the Ba2+ cations and CO32- anions at the center or on the edges of the channels. The two structures are two of the few examples where the carbonate groups are isolated and occupy their own independent sites.