High-pressure Synthesis and Characterization of the Alkali Metal Borate HP-RbB₃O₅

The rubidium triborate HP-RbB 3 O 5 (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 Rb 2 CO 3 and boric acid H 3 BO 3 heated at 850 °C under normal pressure conditions. The single-crystal structure determination showed that HP-RbB 3 O 5 is isotypic to HP-KB 3 O 5 , 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 BO 3 groups as well as corner- and edge-sharing BO 4 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.