Preparation, thermal stability and crystal structure of a new ruthenium(v) oxide containing peroxide ions: Ba5Ru2O9(O-2). Structural relationships to the hexagonal-type perovskite

The compound Ba5Ru2O11 has been synthesized by solid state reactions at 980 degrees C and 1 atm oxygen pressure. The crystal structure has been determined on a single crystal (R-1 = 0.027, wR(2) = 0.058). The symmetry is hexagonal (P6(3)/mmc) with the cell parameters a = 5.9470(5) Angstrom, c = 18.0428(10) Angstrom, Z = 2. Perpendicular to the c-axis, the structure is built up by the periodic stacking of three hexagonal close packed [BaO3] layers separated by a layer of composition [Ba2O2] containing (O-2)(2-) peroxide ions. The detailed formula is then Ba5Ru2O9(O-2) if we emphasize the formation of peroxide ions in the phase. The ruthenium(v) ions occupy the octahedral sites formed between the [BaO3] layers (hexagonal perovskite slabs) and then constitute isolated [Ru2O9] pairs. Ba5Ru2O9(O-2) belongs to a new family of compounds, generically formulated as [A(2)(O-2)][A(n)B(n-1)O(3n)] where n represents the number of [AO(3)] successive layers in the hexagonal perovskite slabs. Each member of the series is an intergrowth of [A(2)(O-2)] and hexagonal perovskite layers; n = 3 corresponds to Ba5Ru2O9(O-2). The isostructural phase has been observed when ruthenium is replaced by niobium. The n = 4 member and the intergrowth between the n = 3 and n = 4 members of this family has been observed in the Ba-Nb(Ta)-S ternary systems. The former proposed formula for the ternary barium sulfides is discussed in the light of the framework of this new family.