Defect ordering in YBa2Cu3O6.5 and YBa2Cu3O6.6: Synthesis and characterization by neutron and electron diffraction

Polycrystalline samples of YBa2Cu3O6.5 and YBa2Cu3O6.6 were prepared by oxygen titration of YBa2 Cu3O6.0 at 450°C followed by slow cooling to room temperature. Both samples showed evidence for thea′ = 2a supercell in individual grains by electron diffraction as reported previously. In addition the superlattice was observed in neutron powder diffraction indicating that the bulk material is also well ordered. In this study the YBa2Cu3O6.6 phase showed longer correlation lengths for ordering along botha* andb* than YBa2Cu3O6.5. For the former compound the powder-averaged, sample-averageda* correlation distance is 26A˚from neutron diffraction. Analysis of electron diffraction profiles on selected single crystals give correlation lengths alonga*, b*, andc* of 100, 200, and 50A˚, respectively. Dark field imaging discloses the presence of striped, ordered domains elongated alongb* with a distribution of sizes. Both neutron diffraction and dark field imaging indicate that the volume fraction of the ordered domains is about 50%. A correlation is noted between the Meissner Effect and the extent of defect ordering in the bulk samples of the two phases.