CRYSTAL-STRUCTURE OF THE NEW METALLIC COPPER-OXIDE LA2SR6CU8O18-DELTA

The structure of metallic copper oxide La2Sr6Cu8O18-delta (delta almost-equal-to 0.4) has been determined by electron diffraction and neutron powder diffraction. The correct space group was found to be P 4/mbm with lattice parameters a=10. 8 502 (3) angstom almost-equal-to 2 square root 2a(p) and c=3. 7658 (1) angstrom almost-equal-to a(p), where a(p) is the lattice parameter of the cubic perovskite. The oxygen vacancies are ordered both in the basal plane, along [001], and in the z=1/2 plane, along [110]. The idealised framework of [Cu8O18] consists of square planar CuO4 groups and CUO5 square pyramids by sharing their corner oxygen, leading to the formation of hexagonal tunnels along the c-axis. La3+ and Sr2+ ions are located in these tunnels in a disordered manner.