STRUCTURAL STUDIES OF HIGH-PRESSURE BA-RH-O PHASES

Three different phases in the Ba-Rb-O system were prepared under high- pressure/high-temperature conditions. The black crystalline product wa s studied by X-ray diffraction techniques and the space groups for sev eral phases were obtained. The first phase, the 4H polytype BaRbO3, ha s already been reported in the literature, but in addition two new pha ses were isolated and their structures determined. The second high-pre ssure Ba-Rb-O product, previously reported to be an 18R polytype, was observed to be isostructural with BaIrO3. This phase was found to be n on-stoichiometric with a deficiency in rhodium in octahedral sites, wi th a stoichiometry of BaRh0.92O3. The space group is C2/m and the unit cell parameters are a = 10.005(2), b = 5.764(1), c = 14.937(3) angstr om, beta = 102.99-degrees and Z = 12. The third phase isolated in this study was observed to possess a hollandite-type structure with space group I2/m, where the RbO6 octahedra share edges and vertices. Its uni t cell parameters are a = 9.435(2), b = 3.1041(5), c = 10.399(2) angst rom, beta = 94.45(1)-degrees and Z = 1. Refinement of the structure ge nerated the composition Ba1.72RbO16.