High-pressure synthesis and characterization of the alkali metal borate HP-RbB₃O₅

The rubidium triborate HP-RbB₃O₅ (HP=high-pressure) was synthesized under highpressure =high-temperature conditions of 6 GPa and 1000 C in a Walker-type multianvil apparatus. The precursor was gained from a mixture of rubidium carbonate Rb₂CO₃ and boric acid H₃BO₃ heated at 850 C under normal pressure conditions. The single-crystal structure determination showed that HP-RbB₃O₅ is isotypic to HP-KB₃O₅, crystallizing monoclinically with eight formula units in the space group C2/c possessing the lattice parameters a = 982.3(2), b = 885.9(2), c = 919.9(2) pm, and β = 104.0(1) . The boron-oxygen framework consists of trigonal-planar BO₃ groups as well as corner- and edge-sharing BO₄ tetrahedra that are connected to a three-dimensional framework. Therein, the rubidium cations are surrounded by 10 oxygen anions. IR- and Raman-spectroscopic investigations were performed on single crystals of the compound.