STRUCTURE AND OXYGEN-EXCHANGE IN SUPERCONDUCTING ND2CE0.67SR0.33BACU3/9-DELTA

Oxidation-reduction of the title compound, a member of the A2B2Cu3O9-delta family [1] was studied by thermogravimetry and neutron diffraction. A superconducting material, obtained by annealing under 13 atm oxygen, has oxygen content delta = 0.08 (average copper valence 2.18). It reversibly loses oxygen by heating under variable oxygen partial pressures. A sample equilibrated under 0.02 atm oxygen has delta = 0.29, average copper valence 2.04 and does not superconduct. Neutron diffraction shows that the exchangeable oxygen is located on Cu(1) copper layers similar to the Cu chains in YBa2Cu3O7-delta, but without oxygen ordering, hence without forming long-range Cu-O chains. The bond distances and bond valences are very similar to those found in the intermediate (delta almost-equal-to 0.4) phase with T(c) almost-equal-to 60 K in the YBa2Cu3O7-delta system. The enthalpy of oxidation is approximately 40 kJ/mol.