Layered ruthenium hexagonal perovskites: The new series [Ba2Br2−2x(CO3)x][Ban+1RunO3n+3] with n=2, 3, 4, 5

Single crystals of the title compounds were prepared by solid state reactions from barium carbonate and ruthenium metal using a BaBr2 flux and investigated by X-ray diffraction method using Mo(Kα) radiation and a Charge Coupled Device (CCD) detector. A structural model for the term n=2, Ba5Ru2Br2O9 (1) was established in the hexagonal symmetry, space group P63/mmc, a=5.8344(2) Å, c=25.637(2) Å, Z=2. Combined refinement and maximum-entropy method (MEM) unambiguously show the presence of CO32− ions in the three other compounds (2, 3, 4). Their crystal structures were solved and refined in the trigonal symmetry, space group image, a=5.8381(1) Å, c=15.3083(6) Å for the term n=3, Ba6Ru3Br1.54(CO3)0.23O12 (2), and space group image, a=5.7992(1) Å, c=52.866(2) Å and a=5.7900(1) Å, c=59.819(2) Å for the terms n=4, Ba7Ru4Br1.46(CO3)0.27O15 (3), and n=5, Ba8Ru5Br1.64(CO3)0.18O18 (4), respectively. The structures are formed by the periodic stacking along [0 0 1] of (n+1) hexagonal close-packed [BaO3] layers separated by a double layer of composition [Ba2Br2−2x(CO3)x]. The ruthenium atoms occupy the n octahedral interstices created in the hexagonal perovskite slabs and constitute isolated dimers Ru2O9 of face-shared octahedra (FSO) in 1 and isolated trimers Ru3O12 of FSO in 2. In 3 and 4, the Ru2O9 units are connected by corners either directly (3) or through a slab of isolated RuO6 octahedra (4) to form a bidimensional arrangement of RuO6 octahedra. These four oxybromocarbonates belong to the family of compounds formulated [Ba2Br2−2x(CO3)x][Ban+1RunO3n+3] where n represents the thickness of the octahedral string in hexagonal perovskite slabs. These compounds are compared to the oxychloride series.