NEUTRON POWDER DIFFRACTION STUDY OF THE CRYSTAL-STRUCTURE OF YSR2COCU2O7 AND Y1-XCAXSR2COCU2O7

The structures of YSr2CoCu2O7 and Y1-xCaxSr2CoCu2O7 have been analyzed by neutron powder diffraction techniques. Both materials crystallize with the symmetry of space group Ima2. The lattice parameters are a = 22.7900(5), b = 5.4516(1), c = 5.4097(1) angstrom for YSr2CoCu2O7, and a = 22.8266(7), b = 5.4312(2), c = 5.4048(2) angstrom for Y1-xCaxSr2CoCu2O7. In the undoped compound, the Co ions exclusively substitute for the copper ions located on the chain sites of the 123 parent structure (YBa2Cu3O6+x). The coordination of Co is tetrahedral. The CoO4 tetrahedra form chains running with a zig-zag configuration along the c-axis of the structure. The oxygen atoms of the CoO layers were found to be disordered over two positions. A re-analysis of the compound YSr2GaCu2O7 showed that this type of disordering is much less pronounced in this material. The disorder of the CoO4 tetrahedra results in the coexistence of two types of chains. In the structure of the doped compound Y1-xCaxSr2CoCu2O7, calcium substitutes for yttrium, but cobalt replaces copper both on the chain sites and in the CuO2 planar sites, thus explaining the lack of superconductivity in this material. The oxygen atoms on the Co layers are more disordered than in the undoped material.