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 a t 980 degrees C and 1 atm oxygen pressure. The crystal structure has b een determined on a single crystal (R-1 = 0.027, wR(2) = 0.058). The s ymmetry 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-ax is, the structure is built up by the periodic stacking of three hexago nal close packed [BaO3] layers separated by a layer of composition [Ba 2O2] containing (O-2)(2-) peroxide ions. The detailed formula is then Ba5Ru2O9(O-2) if we emphasize the formation of peroxide ions in the ph ase. The ruthenium(v) ions occupy the octahedral sites formed between the [BaO3] layers (hexagonal perovskite slabs) and then constitute iso lated [Ru2O9] pairs. Ba5Ru2O9(O-2) belongs to a new family of compound s, generically formulated as [A(2)(O-2)][A(n)B(n-1)O(3n)] where n repr esents the number of [AO(3)] successive layers in the hexagonal perovs kite 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 b y 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 tern ary systems. The former proposed formula for the ternary barium sulfid es is discussed in the light of the framework of this new family.