SYNTHESIS AND CRYSTAL-STRUCTURE OF BASRCUO2+X-BULLET-CO3

A new oxycarbonate BaSrCuO2.22.CO3 has been synthesised by heat treating a mixture of Ba(OH)2.8H2O, SrCO3, and CuO at 820-degrees-C in O2 for 60-70 h. The structural analysis was based on powder X-rav and neutron diffraction data. The compound was found to be tetragonal, a = 5.5899(2), c= 7.7153(3) angstrom. The structure can be described in the P4/mbm space group with 2 formulae per unit cell. The structural refinement was carried out by the Rietveld method. The atoms are in the following positions: Ba (0, 0, 0.2115), Sr (0, 0, 0.2423), Cu (0.5, 0, 0), C (0.5, 0, 0.5), O(1) (0.3434, 0.8434, 0.4553), O(2) (0.2509, 0.7509, 0.0187), O(3) (0.4672, 0.9672, 0.3286), O(4) (0.4140, 0.9140, 0.3771 ). The large cations are not long-range ordered. however: since the oxygen arrangement around them must be different, most of the atoms are highly disordered. The structure contains the following sequence along the c-axis: CuO2, Ba (or Sr) O, CO(x), Sr (or Ba) O, CuO2. The Cu cations have a square coordination with a fifth oxygen at 2.549 angstrom. The Ba and Sr cations are surrounded by ten atoms: 2 O(1), 4 O(2), and 4 O(3) (or O(4)). The C cations are three-coordinated for x = 0, a fourth oxygen atom occupies the empty O(1) positions and in this case the C cation are squarely coordinated. Because of the disorder between the Ba and the Sr cation, most of the anion positions are found to be split over two positions. The valence sum calculations allows one to propose different models for local ordering.