OXYGEN EXCESS AND SUPERCONDUCTIVITY AT 45 K IN LA2CACU2O6+Y

Preparation of single-phase stoichiometric La2CaCu2O6+y from oxide precursors is reported along with high-resolution neutron powder diffraction studies. To date this is the only route that allows the stoichiometric phase to be obtained. An air-heated sample having y=0.0378(8) displays a transition onset at 45 K to diamagnetic susceptibility. Nevertheless, the maximum amount of superconducting phase inferred from these flux exclusion experiments is only 1% in volume. It is also found that the diamagnetic signal is not substantially modified by changing the annealing atmosphere at normal pressures. Neutron diffraction data show a high atomic ordering of La and Ca ions with a strong preference (75%) of Ca ions for sites eight-fold coordinated located between the Cu-O2 planes. The other 25% is occupied by La ions, around which the excess oxygen is located with partial occupancies, yielding a higher coordination number for some of these La ions. Comparison of this structure with that of the nonsuperconducting oxide La1.9Ca1.1Cu2O6+y, suggests that the observed small superconductivity islands are related to clustered oxygen excess intercalated between the two Cu-O2 planes, along with the La ions. The small overall concentration of defects observed here, and thus the small number of holes, is responsible for the absence of bulk superconductivity in La2CaCu2O6+y. © 1990.